Lincoln Electric Named Official Sponsor Of Exxon Flyin' Tiger Aircraft
 Reaching an altitude of 20,000 feet in six minutes and 40 seconds after take off may not be for everyone, but for Bruce Bohannon, the pilot and owner of the Exxon Flyin' Tiger, it is just one in many record-breaking flights his aircraft has accomplished. The Exxon Flyin' Tiger of Angleton, Texas, currently holds 25 altitude and time to climb world records. The Lincoln Electric Company has recently become an official sponsor of this specially modified kit plane. According to the National Aeronautics Association's (NAA) World & U.S. Aviation & Space Records publication, the Exxon Flyin' Tiger currently is the world's fastest climbing piston aircraft in the unlimited class to 20,000, 30,000 and 40,000 feet. In the C1-B weight class, which includes aircraft from 1,100 to 2,200 pounds, the plane holds every altitude and time to climb record. Time to climb is defined as how quickly the plane reaches a specified altitude from the ground. Bohannon performs these record-breaking flights in conjunction with Experimental Aircraft Association (EAA) events such as the annual AirVenture in Oshkosh, Wis., and the Sun ‘N Fun fly-in in Lakeland, Fla.
According to Bohannon, for working on an aircraft this special, he only wants the best equipment and that's why he turned to Lincoln Electric. "For a high performance plane, you don't want to be second guessing the weld quality, especially at high altitudes where temperatures can be 70 degrees below zero and the engine is being pushed to the limit," said Bohannon. Previously, Bohannon and his crew chief Gary Hunter were using an oxyacetylene process to weld the mostly aluminum body of the Exxon Flyin' Tiger, but recently have made the switch to a TIG process and Lincoln's Invertec® V205-T AC/DC. "TIG offers much higher quality, less corrosion and a strong weld," noted Bohannon. "When you are pushing a plane to its limits, quality can't be compromised." For years now, Bohannon has used Lincoln equipment around his farm and air field for repairs and building farm implements. He recently began using the Power MIG 255. "It's fantastically easy to use and reliable. This is the first MIG machine I've ever owned and it is so much easier compared to Stick."
But the equipment is not the only thing that has impressed Bohannon about Lincoln. "The company's service has been absolutely impeccable," said Bohannon. "Delivery has always been on time and I appreciated all the help provided by Lincoln's Scott Skrjanc in specifying machines that would work best for us." What's next for the Exxon Flyin' Tiger? Bohannon has two goals in mind. The first is to be the highest flying piston airplane in American history. That record is currently held by a World War II B-29 Bomber at 47,910 feet. The second goal is to hold the unlimited class' time to climb record in the 10,000 feet category. http://www.lincolnelectric.com |  |
Monday, November 12, 2007
Lincoln Electric Named Official Sponsor Of Exxon Flyin' Tiger Aircraft
During these attempts, an NAA official is present to certify the results. A video camera in the plane's cockpit is positioned at the instrument panel to verify altitude. An official clock on the panel records the time a specific altitude is reached.
Shop Gains Productive Edge with Robotic Welding
Shop Gains Productive Edge with Robotic Welding
|
F.R. Custom Metal Fabricating Ltd. made a sound investment in the future with the recent purchase of its first robotic welding cell. Faced with a contract to produce a large volume of pallet truck handles, the fabricating shop found it increasingly difficult to maintain the skilled workforce of welders needed to produce consistent, high quality parts at the necessary volume. By adding a two-station Lincoln Electric automated welding system, the firm is now able to meet production and quality goals without trying to find employees willing to work overtime.
Robotics Help Custom Fabricator Tackle a Production JobF.R., a custom fabricator founded 28 years ago by its President, Alfred Nonnewitz, traditionally has specialized in lower volume, higher quality fabrications, particularly those involving tubular components. For the last three or four years, the company has been producing pallet truck handles on a contract that calls for increasingly higher volumes, now generally between 700 and 1,000 parts per week, according to Bill Nonnewitz, the founder's son.
 |
Robotics Not Just for Larger Shops
After meeting with representatives of The Lincoln Electric Company's Automated Systems Group (ASG), Alfred and Bill Nonnewitz concluded that the benefits of automating the handle-welding operation would be worth the investment.
The result is a complete two-station work cell that incorporates a six-axis Fanuc ARC Mate™100i robot equipped with a SYSTEM R-J3™ controller, a Lincoln Power Wave® 450 power source, and a Binzel ROBO-WH-455-22 water-cooled torch.
Safety was built into the welding cell with the addition of safety zone rings (limit switches) that prevent the robot and welding torch from going to the station where the operator is working on the next setup until the area is cleared. In addition, each station has a safety mat that prevents the robot from coming to that station if the operator is standing on the mat. Further safety protection is provided by a breakaway nozzle that minimizes damage in case the torch hits any part of the workstation or the parts being welded.
All components are mild steel, with the handle and brackets being 16 gage and the tube being 14 gage. Welds are made with .035" diameter Lincoln SuperArc® L-56™ (AWS ER70S-6) electrode, which includes extra deoxidizers that provide cleaning action and improve weld quality. Since the handles are used to move pallet trucks that weigh up to 5000 lbs., weld integrity is critical to meeting the stresses they must withstand. All welds are fillets, but the welds that join the round handle to the square tube are especially critical. Bill Nonnewitz explains, "The welding power source is capable of making a full regulated controlled weld without spatter or burn-through. Because we only have 1/16" on a side, the welds can't extend out very much. They have to be good, and the penetration has to be consistent."
A Packaged Approach to Automated Welding
Jim Grant explains that the entire workstation is modular and can be moved with a forklift. "It's essentially a plug-and-play robot that was placed on the floor as a complete unit, pre-programmed for the parts they make. Everything is mounted on a platform that keeps the units in proper relationship to one another and allows the cell to be moved easily to another location if needed."
The Lincoln PowerWave® 450 is a sophisticated power source that gives the company significant advantages, with cost savings that are expected to more than offset a slightly higher initial investment. First is the unit's capability for Pulsed MIG welding, an advanced welding mode that combines the best qualities of all other metal transfer modes without their disadvantages. Its lower heat input prevents burn-through on thin metals. Spatter and fume are reduced as well, which improves the working environment and reduces costs. Since only one wire size is required, wire and gas costs are also lower, and the process delivers higher deposition rates than other processes, so productivity is greater. In addition, the power source can be updated easily with new software, an important advantage that protects the company's investment.
The Fanuc ARC Mate™100i robot is designed with integral utilities to improve reliability and setup time. The wire feed motor cable, gas line and air line are all inside the robot arm. Its high motion speeds help improve productivity, and advanced servo-control features allow faster and smoother point-to-point motion.
The FANUC Robotics System R-J3 controller is completely integrated with the robot itself, which allows flexible work cell layout and easy installation. It utilizes the most advanced software with features that include welding restart, scratch start, and on-the-fly weld adjustment. Mirror image utility allows the programmer to shift an existing program and also to invert it, which is extremely useful with F.R. Custom Metal's two-station mirror-image cell. The program also allows starting and stopping the programmed welding sequence from the Teach Pendant, which features commands in English for ease of use.
The Binzel ROBO-WH-455-22W torch includes a wire-cutting feature that helps speed up maintenance and maintain productivity. Lincoln Electric's Jim Grant explains, "If the operator has to pull the nozzle out for cleaning or adjustment, it is designed to cut the wire cleanly. There is a key that releases the whole bottom nozzle assembly and cuts the wire, so you can make any adjustments on the bench instead of trying to make them while the nozzle is on the robot or having to cut the wire to remove the nozzle."
While no comparative figures were kept, the improvements in product quality and productivity have been significant since the company added the robotic cell, and the labor shortages it experienced earlier have been minimized. Bill Nonnewitz says that, since installing the robotic system a few months ago, the company has already seen significant savings, both in reduced overtime costs and in rework and scrap losses.
http://www.lincolnelectric.com
Sunday, November 11, 2007
Extensive Use of Wire Welding Speeds Productivity on Cheyenne Plains Gas Pipeline Project
Extensive Use of Wire Welding Speeds Productivity on Cheyenne Plains Gas Pipeline Project
First Major X-80 Grade Pipe Project in U.S. Utilizes New Lincoln Wire
|
 |
During the summer and fall of 2004, the plains of the west were busy with the hustle and bustle of cranes, trucks, heavy equipment and supplies to lay pipe for the Cheyenne Plains Gas Pipeline Project. The 380-mile, 36-inch natural gas pipeline runs from the Cheyenne hub in Colorado to existing pipelines near Greensburg, Kan. Once operational in early 2005, the pipeline will export 560 million cubic feet of natural gas per day from Wyoming to growing markets in the mid-continental U.S. and further east.
The $425 million pipeline, owned by El Paso Corporation, was being constructed in three spreads which were connected in the final phase of the project. Spread one was contracted to Associated Pipelines while spreads two and three were handled by U.S. Pipeline, Inc.
The Cheyenne Plains Gas Pipeline Project was the first major pipeline in the U.S. to use X-80 grade pipe. Already a standard in other parts of the world, the X-80 pipe provides higher strength with a thinner wall.
As on any pipeline project, welding plays a critical role in the construction process. But what made this project unique was the extensive use of wire welding to provide high productivity. A new consumable from the Lincoln Electric Company, the Pipeliner® G80M gas-shielded flux-cored wire, was selected by the evaluation team at U.S. Pipeline, Inc. for use on the pipe tie-ins at connection points and road crossings near populated areas. These welds, completed manually by independent pipeline contractors, connect the mainline pipe to the thicker walled pipe of the tie-ins.
For the mainline pipe, a CRC-Evans® automated welding system was used in combination with Lincoln’s premium SuperArc® L-56 copper-coated gas metal arc wire.
 |
An Overview of the Pipe Installation Process
More than 140,000 tons of pipe and 25,000 individual sections were used to complete this project. Each section of pipe is 78-feet long and per industry standards, was buried at least 30 inches below the ground through a trenching process.
Much like an assembly line where each worker is responsible for a certain portion of the job, construction crews in each spread followed after each other along the length of the pipe to complete specialized tasks. Crews at the front staked the area and prepared the right-of-way. Those following behind aligned the pipe, welded and inspected the pipe and then lowered it into the trench. Finally, the construction crews at the rear were responsible for conducting hydrostatic pressure testing, backfilling the trench and restoring the land as close as possible to its original condition.
Welding Connection Points and Road Crossings
After a lengthy process of evaluating solid and flux-cored electrodes for the job, U.S. Pipeline, Inc. selected a .045-inch diameter Pipeliner G80M wire as the consumable of choice for the vertical up welding of the pipe tie-in fill and cap passes. The Cheyenne Plains Pipeline Project marked the first time this new consumable was used in the field and for many of the pipeline contractors, it was also the first time they had made the switch from a traditional Stick process to complete the tie-ins for the job.
“We chose the Lincoln Pipeliner G80M wire for this project for a number of reasons. First, it met the mechanical requirements of the job and also offered a crisper arc,” said Dana Bratcher, Welding Foreman, U.S. Pipeline, Inc. “In addition, we felt that the Lincoln product flowed better and was more durable for outdoor use. We also were attracted to the fact that the pipe supplied from the manufacturer was welded using a Lincoln consumable.”
This electrode, specifically designed for pipeline welding, is easier for the operator to use and provides a smooth arc, lower spatter levels and less frequent clogging of gun nozzles when compared to other flux-cored wire electrodes.
All tie-in welds were performed manually because of the specialized skills needed to handle fit-up issues between the thinner mainline pipe and thicker tie-ins. Each manual welding team consists of two welders, one on each side of the pipe performing one-half of the welding pass – root, hot, fill and cap. Each welder also had an assistant who performed tasks such as preheating the joints, setting the clamp to align the two lengths of pipe, setting up the welding equipment and completing the finish wire brushing on the joint.
Every tie-in weld was inspected with a radiographic process and throughout the job the weld quality had been excellent. “We had a low repair rate with the Lincoln wire. It was consistent and worked wonderfully,” noted Bratcher. “Our welding operators liked the fact that they were able to see how the puddle flowed.”
 |
The use of Pipeliner G80M wire to weld the spiral seam pipe’s fill and cap passes provided significantly increased productivity and high quality welds. “The wire is about three to five times faster than stick welding,” said Ray Edwards, an independent pipeline welder from the Pipeliners Union 798 and one of the welders on Spread Two of the project. “The same length of weld bead that would take up to five minutes with stick welding is now taking me about one minute.”
In addition, Lincoln’s Shield-Arc 70+ stick electrode was used to complete the tack welds and vertical down root pass on these tie-ins. This rod was chosen for its ability to accomplish the out-of-position welding required for this job.
On-Site Support and Equipment
For the pipe tie-in welding, Lincoln fully supported the welding efforts from the initial qualifying of the consumables to on-site assistance to ensure the independent contractors weren’t subject to any downtime.
According to Dave Thomas, District Manager in Lincoln’s Tulsa Office, he and representatives from the Cleveland headquarters were present while the Pipeliner G80M consumable was being tested by for the job against competitive consumables. “We watched as U.S. Pipeline officials welded with each of the consumables. The Pipeliner wire product was much more proficient in operator appeal and also created a better looking weld. In addition, it also came out on top after destructive tests on the plate in the lab,” said Thomas.
 |
Thomas also described how once the G80M product was selected, Lincoln representatives qualified the procedures for the wire and were close at hand to answer questions and provide technical assistance as U.S. Pipeline qualified their operators.
During initial start-up of the job in the field, Lincoln representatives were at the site three to four days per week, and at least one day a week on an ongoing basis throughout the project’s duration.
“The Lincoln consumables ran flawlessly on the job with great operator appeal and ease of use,” commented Steve Duren, Technical Sales Representative from Lincoln Electric’s Denver office. “The only issues we addressed with consumables were questions about techniques and proper drag and push angles. On a project of this magnitude, having so few issues was exceptionally good.”
David Fullen, Lincoln Electric Denver District Manager echoed these same words about the Pipeliner G80M electrode. “For many of the welding operators on the job, this was one of the first times they had wire welded. What’s remarkable is that they adapted so well and never had any complaints with the wire. The integration of the wire welding has gone smoothly and the job finished ahead of schedule. Most of our time spent on the job site was addressing equipment issues.”
Although a competitive brand of welding equipment was specified for the job, its alarming failure rate and resulting downtime prompted Lincoln to step in and provide equipment.
“We received an urgent call about equipment when the contractors were struggling with the competitive wire feeders. We were able to answer the call and overnight equipment to the field,” said Fullen. “We sent Lincoln LN-15 wire feeders with accompanying Magnum welding guns to the job which remedied the situation. We also spent time on site providing technical assistance, setting up the equipment and installing wire feed CV modules. We were dedicated to keeping the customer up and running.”
“Throughout this project I received great service from Lincoln,” explained Bratcher. “With a phone call, I would have what I needed either hand delivered or on site the next day.”
For this project, Lincoln Electric partnered with distributor Airgas Intermountain to supply select consumables and equipment to the site as well as an 85 percent/15 percent argon blend shielding gas. According to Mark Duncan, General Manager-East for Airgas Intermountain, Airgas provided additional on-site support to augment Lincoln’s efforts. “We have a strong relation with the Lincoln Denver office and worked as a team. I couldn’t have asked for better support from them,” said Duncan. “I also tip my hat to our field person, Tom McClelland, who did a fantastic job providing technical assistance, process help and service for the equipment when necessary.”
 |
Welding Pipe Mainline
For welding the mainline pipe of the Cheyenne Plains Gas Pipeline, Lincoln consumables also played a prominent role. A CRC-Evans® automated pipe welding system was utilized to automatically weld the circumference of the pipe using a gas metal arc process for high productivity.
The system was used in combination with Lincoln’s premium SuperArc® L-56 copper-coated gas metal arc wire for welding the pipe’s outside diameter, consisting of hot, fill and split cap passes. This .040-inch diameter wire, provided specifically for this project, offers superior feedability and excellent arc characteristics, a result of Lincoln’s manufacturing which uses a strictly controlled chemistry process. All mainline welds are inspected using automatic ultrasonic tests.
“From a business perspective, it was a good decision for us to work with a domestic consumable supplier such as Lincoln Electric,” said Brian Laing, President of CRC-Evans. “From a technical standpoint, the mechanical property requirements for the project were easily met by the SuperArc L-56 wire. Our direct customers, the construction contractors, and ultimately the pipeline owner, were satisfied with the overall quality of the welding and productivity achieved, thanks in part to the Lincoln electrode selected.”
According to Peter Nicholson, Manager of Pipeline Products at The Lincoln Electric Company, providing consumables for the Cheyenne Plains Gas Pipeline required a coordinated effort between many Lincoln entities including its International Division, the Cleveland headquarters, and Lincoln’s Houston district office.
“Six months prior to the start of pipeline construction, consumables were tested in the Houston office to ensure the weld procedures with the consumable complied with the strict industry standards for this job. Lincoln engineers worked closely with CRC-Evans personnel to ensure the right consumable for the job was selected,” said Nicholson.
View/Print/Download PDF version of this story
Want to see results like this? We can help. Ask Lincoln How!http://www.lincolnelectric.com/knowledge/articles/content/cheyennepipeline.asp
Multiple-Wire Welding Creates High Productivity and High Quality
Multiple-Wire Welding Creates High Productivity and High Quality Welding Techniques Produce Faster Travel Speeds and Improve Mechanical Properties
In comparison to two-wire submerged arc welding (SAW) systems, Berg's techniques have created welding travel speeds that are three times greater for welding the ID and four times greater for OD welding on 0.820" thick pipes. In addition, total heat input has been decreased by 25 percent while overall mechanical weld properties have been improved by these systems. Overall, employing multiple-wire welding has enabled Berg Steel to produce 40 foot long 0.820" thick pipes at a rate of 14 pipes per hour. During 2001, Berg manufactured for the Gulfstream Project, a natural gas pipeline that originates near Pascagoula, Mississippi and crosses the Gulf of Mexico to Manatee County, Florida. Once onshore, the pipeline stretches across south and central Florida to Palm Beach County. This natural gas pipeline will serve Florida utilities and power generation facilities, generating 1.1 billion cubic feet per day of additional natural gas - enough to supply electricity for 4.5 million homes. For Phase One of this project, the offshore portion, Berg Steel Pipe along with its parent company, Europipe GmbH in Germany, supplied 443 miles of pipe in only six months. In Phase Two, Berg will produce 292 more miles for the onshore pipeline.
Transformation "Welding technology has allowed us to transform a facility that was built in 1980 to produce 5,000 tons of pipe per month to a facility that produced more than 40,000 tons per month during the Gulfstream Project," said John Burton, General Manager of Production, Berg Steel Pipe Corporation. "We needed to add the multiple-wire processes because we don't have enough square footage to add more welding stations. We have to make use of the technology on the market to be the most efficient plant possible in the space we have available." Berg is able to service customers that range from oil and gas transmission companies to pipe distributors. The 275-employee company also manufactures structural piling and is even qualified to produce linepipe to arctic specifications with weld metal Charpy requirements to -50° F. Berg Steel Pipe Corporation can supply to this broad range of customers because of the flexibility of its three-roll bending type forming process, which offers a unique advantage to do quick changeovers so the company can accept both small and large projects. API Grade B through X80
Preparation of Plates Once the plate is formed into the proper tubular shape, the longitudinal weld is completed in three passes, beginning with a continuous tack weld. For the Gulfstream Project, the longitudinal welding procedures were qualified to both ASME Section IX and Berg's internal standards. Tack Welding
In 1999, Berg had a single, DC 1500 power source. For every diameter change in pipe, the system would require 45 minutes to change out the cage that positioned the plate edges for welding. To eliminate this, Berg turned to the expertise of The Lincoln Electric Company. Because of a long, 21-year relationship with Lincoln, Berg looks to the company for innovative solutions to welding challenges and ways to provide higher productivity in its welding operations. At Lincoln's suggestion, Berg installed a continuous tack welding system utilizing two, DC 1000 power sources and a NA-5 control. This new, CNC-controlled tack welder has hydraulic cylinders, which automatically reposition themselves when the pipe diameter changes. This system has reduced changeover time to approximately five minutes. This new system has also eliminated some of the problems with closing the bevel for welding that Berg Pipe was experiencing with its former system. These two DC 1000s create a continuous seam tack using an open arc gas-metal arc welding (GMAW) process at high speeds of 260 inches per minute. Commonly referred to as "buried arc", this process is characterized by low voltage, short arc length, and very high travel speeds allowing for a deep penetrating weld at reduced heat inputs.
While some competitors use intermittent tack welds, Berg Steel Pipe Corporation feels there are benefits to a continuous seam technique to close the formed pipe cylinder for welding. "Intermittent tack welds require the use of a backing flux or a copper backing bar during subsequent submerged arc welding, while ours does not," noted Fred Hafner, PE, Chief Metallurgist/QA Manager for Berg Steel Pipe Corporation. "This means that we achieve higher productivity since the backing provided by the continuous seam provides the attributes for fast travel speeds and deep penetration in ID welding." Using a buried arc process allows for the elimination of weld spatter typical of globular transfer because the arc is "buried" in the weld puddle. This process also requires only a CO2 shielding gas, eliminating the need for more expensive argon gas. For the tack welding of 0.820" thick pipe, Berg Steel Pipe Corporation uses Lincoln L-50™ 5/32" diameter wire electrode at 1,500 amps. Inner Diameter Welding During inside welding, the pipe is moving forward while all the welding equipment is fixed and mounted to a boom that permits travel through the length of the 40-foot pipe. The arcs are buried underneath the granulated flux. The four-wire process on the ID weld creates faster travel speeds - up to 68 ipm on the Gulfstream pipe- and the mechanical properties of the weld improve because there is less heat input delivered per arc. Depending on customer specifications, different types of wires are used at this station. For the Gulfstream Project, 1/8" Lincoln L-61 with a 761 pipe flux are the consumables of choice. This special 761 flux is a special adaptation of the standard Lincoln 761 flux with different-sized particles to create better fluidity, bead shape and edge tie-in to fill in the weld groove on the inside of the pipe.
Outer Diameter There are three main advantages to using a five-wire process: 1) higher weld speeds - the process is four times the speed of the standard, two-wire process; 2) reduced heat input, which reduces grain coarsening in the heat affected zone; and 3) impurities such as slag or porosity have time to escape prior to solidification because of the large weld puddle. The Berg OD systems use a DC lead wire to ensure complete penetration while the trail arcs are AC for fill and travel speed. At one welding cell, Berg uses two Lincoln DC-1000 units in parallel with Lincoln NA-3 controls. At another welding cell, two Lincoln DC-1500 units are used with similar NA-3 controls. In all cases, the AC trailing wire is produced with AC-1200 units from Lincoln and NA-4 controls. Laser seam tracking on the OD ensures that the weld is positioned correctly in the weld groove. Since the arc is buried under a pile of flux and alignment can't be seen, this equipment facilitates positioning of the weld head. For the 0.820" (20.83 mm) thick Gulfstream pipe, the 3/16" DC lead wire operates at 1,550 amps with a weld speed of 90 ipm. Welding consumables were a combination of L-61 and L-70 wire and 995 flux. (L-70 wires are used when higher weld metal Charpy impacts are required - the addition of 0.5 percent Mo L-70 wire results in higher acicular ferrite contents in the weld, and this improves the weld metal Charpy impact energy.) On pipe thinner than 14.3 mm (0.56”), a four-wire process is used while all five-wires are utilized for heavier wall thicknesses. As customers requirements dictate changing wire or flux, Berg Steel Pipe Corporation has called on Lincoln Electric to help run trials at the company's Cleveland headquarters where a five-wire welding station is located for this purpose. In this way, valuable time on Berg's machines doesn't have to be spent on product development. Finishing Berg Pipe Corporation then sizes the pipe ends between the opposing hourglass rolls to ensure uniform diameters and roundness essential for efficient field constructions. Next, the company simultaneously bevels both ends of the pipe for field girth welding. A stenciling machine labels and barcodes each pipe with its unique identification before shipping via barge or rail to Gulfstream. Inspection According to Burton, welding the pipe correctly the first time is critical to achieving high production rates. "In our pipe mill, a pipe that needs rework has to displace a new pipe in the production process," said Burton. "Removing or holding up a new pipe from production to correct a problem can be very costly." Service
Berg Steel Pipe plans to continue its long relationship with Lincoln. "As the company grows and moves into new areas, Lincoln will be directly involved in all phases," explained Burton. Currently, the company is looking to develop seam welding procedures and process controls for Grade X80 pipe. Berg recognizes that Lincoln's Surface Tension Transfer® (or STT®) process will have advantages for field welding of this grade and will help popularize this grade for new pipelines. "Berg is one of the most progressive companies that I have worked with," noted Troy Gurkin, Lincoln Electric area sales representative. "The company is always looking to evaluate new products which can help increase efficiency."
Future http://www.lincolnelectric.com/knowledge/articles/content/bergsteel.asp | | |||||||||||||||
| | ||||||||||||||||
Since installing the new system, Berg has been extremely pleased with the results. "Our tacking system is very reliable and helped relieve the problem of the arc outage we were experiencing," said Burton. "We also employ the use of CNC controls which can store data on a particular wall thickness so that we can provide rapid changeovers by simply calling up previously-stored information." 
For the ID weld, a V-shaped guide wheel rides in the weld seam groove to ensure alignment of the weld head in the weld groove. The ID weld penetrates into the previously laid continuous tack weld that is now serving as weld backing at this stage. 
Challenges and Solutions for the Retention of Field Production Rates in High Strength Pipe
Challenges and Solutions for the Retention of Field Production Rates in High Strength Pipe
|
 |
Abstract
New, high strength steels are offering many advantages to the pipeline industry. These steels allow for the use of thinner pipe at higher operating pressures. These steels can also drive down total construction costs. However, as pipeline owners and designers look to these new steels, they are presenting a challenge for the welding and fabricating industry that must respond with cost-effective ways of joining them. To complicate the matter, these high performance steels have surpassed conventionally fabricated weld metal in terms of hydrogen crack resistance and fracture toughness.
The latest evolution in these high strength pipe steels is X-80, carrying a specified minimum yield of 80 ksi. With even more advanced steels expected soon, unique opportunities are being created for the arc welding industry to introduce new welding consumables and processes capable of welding these high strength steels
The welding industry’s priority is quality. All pipeline fabricators need to provide repeatable, high quality welds – the first time.
 |
Since SMAW has been adequate to produce safe, economic pipelines, the industry has been slow to adopt the more recent advances in welding process development. SMAW i.e. Cellulosic electrodes has remained the most common process for welding pipelines and has a greater ability than other processes to accommodate non-ideal conditions such as poor fit-up to a change in terrain. However, as the strength of the steel increases there has become a greater risk of failure, either due to hydrogen cracking or reduced toughness. It is this risk of hydrogen cracking that is the main issue with the cellulosic SMAW electrodes in higher strength applications.
Hydrogen dissolves in the molten puddle during welding. Upon cooling, diffusable, as opposed to chemically bound, hydrogen can cause porosity during solidification and cracking in the finished weldment. This moisture originates primarily from the moisture and organic components
Attempts to adapt welding processes and procedures to X-80 and higher strength steels must be assessed. To take advantage of the pipe strength, the weld metal must either match the strength of the pipe or exceed it. The welding process and consumables must be controlled to minimize the risk of brittle fracture in the weld during pipeline fabrication and the resulting risk of hydrogen cracking. In addition, the demands of higher productivity
Introduction
Low hydrogen manual processes suitable for the pipeline girth welding have been available for some time, but despite their availability and advantages in terms of reduced crack susceptibility, they have rarely been used in production up to the present time.
With the advent of the new high strength steels such as X-80, any large pipeline project will, consider if not use, these grades of pipe steel, and therefore contractors and engineers must accept the advancements in the welding processes.
 |
New, high strength steels are offering many advantages to the pipeline industry. These steels allow for the use of thinner pipe at higher operating pressures. These steels can also drive down total construction costs. However, as pipeline owners and designers look to these new steels, they are presenting a challenge for the welding and fabricating industry that must respond with cost-effective ways of joining them. To complicate the matter, these high performance steels have surpassed conventionally fabricated weld metal in terms of hydrogen crack resistance and fracture toughness.
Manufactured using a combinations of heat treatment and mechanical treatment that produce higher strength without significant higher alloy. Such advancements in thermomechanical processing in steels (TMCP) make it possible to achieve the balance in strength and toughness, but with a greater resistance to heat affected zone cracking.
With all the positive factors regarding the higher yield materials the biggest challenge is how to weld these steels. The pipe steel is no longer the limiting metallurgical factor. For example, steels are no longer as sensitive to hydrogen cracking as are conventional weld materials. This is why it is important to examine hydrogen cracking and potential solutions in relation to the weld metal.
 |
Hydrogen Cracking
It is the main issue with cellulosic SMAW electrodes in higher strength applications. This hydrogen originates primarily from the burning of the electrode coating, which contains moisture and organic components Hydrogen dissolves in the molten puddle during welding. Upon cooling, diffusable, as opposed to chemically bound, hydrogen can cause porosity during solidification and cracking in the finished weldment. That is why it is critical that hydrogen levels be minimized for the weld to be considered sound.
With respect to the hydrogen issue, the challenge is to minimize the risk of weld metal cracking by controlling the factors know to be influential.
1. Minimize the amount of hydrogen that is available through judicious selection of consumable and/or welding process controls.
2. Minimize stresses, both residual and applied.
3. Minimize weld metal strength, thus controlling the susceptibility of the microstructure. Some industries have successfully employed undermatching strength weld metals, contingent upon design requirements.
It should be noted that with respect to hydrogen, the risk of cracking might be minimized, but it is impossible to eliminate. Since all steel microstructures are susceptible somewhat to cracking, it simply becomes a question of controlling hydrogen and stress levels simultaneously.
A report published in 1996 by The Welding Institute titled “ Evaluation of Low Hydrogen Processes For Pipeline Construction in High Strength Steel”, (PR-164-9330) which investigated suitable processes for the welding of X80 grade pipe reported,
“ The most successful root welding performance was obtained using the Lincoln Electric STT® power source and the LA90 electrode wire. The STT power source provided very precise control of short circuiting metal transfer, which resulted in good handling characteristics, well-fused beads, minimal spatter and lower fume emissions. The TWI welder involved in the trials was depositing satisfactory root beads within two hours of being introduced to the welding machine. The root welding speeds were comparable with cellulosic."
 |
What is the Proper Way of Joining Pipe?
It depends on many variables, including fit-up, accessibility, terrain, if it’s a tie-in or crossing, whether it is a repair weld, the welder skill and a number of other factors.
Each possible welding process has its own merits, and there is no single clear-cut answer for every application. Attempts to adapt welding processes for higher strength steels must be considered. In addition, the demands of higher productivity and quality are always present.
 |
Root Pass Welding
Traditionally the welding of the root and hot pass have been completed using a high hydrogen cellulosic electrode.
However on X-80 type material, restrictions do apply. Deposition of small weld passes at high travel speeds are possible with sufficient preheat temperature control.
Preheat is required to slow the rate at which the weld cools, thus allowing hydrogen to diffuse, in addition, the use of preheat can also help control residual stresses in the weld zone.
While this approach sacrifices nothing in terms of deposition rate compared with current practices on lower yield pipe, the need for potentially high levels of preheat can limit overall productivity.
It is currently not advisable to weld with cellulosic electrodes on X-80 that is more than 10mm in thickness.
The use of basic coated “low hydrogen” electrodes for the root pass weld as productivity issues. Albeit they can produce one-tenth the level of hydrogen compared to a cellulosic electrode, the major disadvantage is that the process is slow compared to any other process, apart from Gas Tungsten Arc Welding (GTAW).
Whilst SMAW as been used, and can be used to create acceptable quality welded joints in the high yield steels, productivity issues make SMAW less attractive than welding with a continuous wire process. Since there is a strength ceiling for the cellulosic electrodes because of the high hydrogen levels, and the poor productivity levels with the basic low hydrogen electrodes, alternatives are required.
Welding Productivity
For field welding of pipelines, it is the elapsed time to remove the pipe clamp, which is the critical indicator of welding productivity. The elapsed time will depend on a number of factors including the strength of the weld deposit, the dimension of the welded ligament and the level of hydrogen present.
The root pass welding speed determines the advancement of the construction, and therefore it is the focal point that lends itself best to technological advances. Mechanized systems can weld faster than manual or semi-automatic, provided the requisite care is taken in joint preparation and fit-up to ensure proper process control.
 |
Preparation and Presentation
In Manual and Semi – Automatic welding, the journeymen or pipe fitter has a tolerance in which the welder can compensate for variation in the joint fit – up. In automatic welding that tolerance is more critical, therefore the bevel geometry, and the line – up and spacing needs to be addressed mechanically.
Preparation – Pipe Facing Machines (PFM’s). The roundness and consistency of the joint are more important when making welds using an automated system. Therefore, the use of a PFM in the field is critical to the success and quality of weld, regardless of whether an internal or external root pass machine is used.
Presentation – After preparation two approaches maybe adopted. One approach is to have the root pass welded in the short arc mode on the inside of the pipe with multiple torches. Production rates can be high, but the process requires great care in joint preparation and fit-up. If this is not perfect, then there is a greater chance that many poor quality welds will be made in a very short period of time.
 |
A second option is to weld the root pass from the outside utilizing an internal line-up spacer clamp, shown above. The sequence of operation is somewhat different to conventional internal clamps were as the Internal line – up / Spacer clamp, is first located into the pipe, and the first set of internal chucks are expanded into a hard chuck. This aligns the clamp with the ID of the pipe, and takes out any ovality.
The second step is to bring into close proximity the next section of pipe. The second set of chucks are then engaged, which grabs the piece of pipe in a soft chuck. Once in a soft chuck, the clamp is spaced in, which forces the two pipes together.
With one set of the chucks been in a soft state, they allow the pipe to move, so that the two faces of the pipe are butted up tight together. The second set of chucks are then actuated into a hard chuck, which again takes out any ovality. The final stage is to space the clamp out, giving the journeymen/fitter a predetermined uniform gap around the pipe.
Gas Metal Arc Semi-Automatic / Automatic welding processes provide an opportunity for increased productivity and deposition rates by virtue of fewer stop/starts. However, traditionally the short-circuit mode as been identified as having a risk of lack of fusion.
|
Normal short arc welding is considered to be low heat input. However, the current, which is proportional to wire feed speed, can be increased to give higher input and increased penetration. This can result in a fine balance between too little and too much, and requires considerable operator skill so to avoid defects like internal “whiskers”. There needs to be enough heat to fuse the inside edges but not so much as to “blow through”. The operator has to ride the puddle with the arc to achieve the correct penetration and does not have much room for deviation.
These defects can easily be detected by industry standard non-destructive evaluation, such as radiography and ultrasonic inspection. On the other hand, GMAW does have its advantages. Since hydrogen levels with GMAW are typically low, pre-heat requirements are minimal and cracking is rarely an issue.
 |
Controlled Short Circuit Transfer (STT)
One area of recent innovation that as provided operational advantage on high strength pipe, is on external open gap root pass utilizing the controlled short circuit transfer mode
The controlled short circuit transfer mode is different in several ways from the conventional CV process. The STT power source operates neither in constant current (CC) nor constant voltage (CV), rather it is a wide – band width, current – controlled machine wherein the power to the arc is based on the instantaneous arc requirements.
In principle, it is a power source, which has the capability of delivering and changing electrode current in the order of microseconds.
 |
How Does It Work.The electrode current supplied by the SurfaceTension Transfer power source is guided by the state of the arc voltage.
- Background current (T0 – T1). This is the current level of the arc prior to shorting to the weld pool. It is a steady current level between 50 and 100A.
- Ball time (T1 – T2). When the electrodes initially shorts, the “arc voltage” detector provides a signal that the “arc” is shorted. The background is further reduced to 10A for approximately 0.75 m/s.
- Pinch mode (T2 - T3). Following the ball time, a high current is applied to the shorted electrode in the form of an increasing, dual – slope ramp. This accelerates the transfer of the molten metal from the electrode to the weld pool by applying electric pinch forces.
- dv/dt calculation (T2- T3). This calculation is included within the pinch mode. It is the calculation of the rate of change of the shorted electrode vs. time, when this calculation indicates that a specific dv/dt value has been attained, indicating that fuse separation is about to occur, the current is then reduced to 50 A in microseconds. (Note this event occurs before the shorted electrode separates. T4 indicates the separation has occurred, but at a low current.)
- Plasma boost (T5 – T6). This mode follows immediately the separation of the electrode form the weld pool. It is a period of high arc current where the electrode is quickly “melted back.”
- Plasma (T6 – T7). This is the period of the cycle where the arc current is reduced from plasma boost to the background current level.
 |
The process can be applied to either semi-automatic or automatic applications. It operates in the short circuiting region with various shielding gas blends including 100% carbon dioxide for mild steel, as well as various blends of argon/oxygen, argon/carbon dioxide and argon/helium for stainless steel.
Fill / Cap Pass Welding
Cellulosic covered electrodes may still be used if care is taken in controlling preheat and interpass temperatures, with pipe wall thickness up to and including 10mm on X-80. It is not currently advisable to attempt to weld on thickness greater than 10mm because increased restraint and cooling rates are expected to increase the risk of weld metal hydrogen cracking.
Low hydrogen vertical-up electrodes are not normally used for transmission pipeline production welds, due to the low productivity.
 |
It is now become more common, especially in Europe, to use the basic Low Hydrogen vertical Down electrodes for the weld of high yield pipe.
LH-D electrodes are designed for any vertical down welding application where low-hydrogen weld metal is demanded, such as high strength pipe welding.
Although these are designed specifically for vertical down welding of pipe joints, the LH-D are useful in any vertical down application where a low-hydrogen deposit is required.
The low-temperature impact properties are superior to cellulosic electrodes that possess the same level of strength. As downhill electrodes, they have significantly greater deposition rates than both vertical - up electrodes and downhill cellulosic electrodes of equal diameter; this increases the potential for greater productivity, while meeting high strength requirements.
 |
Semi-Automatic processes can be categorized into, gas shielded and shelf shield.
Gas shield GMAW would typically be fully mechanized, because the welding area needs to be protected from the elements. If the contractor is prepared to protect the welding area, it is more feasible to use a fully automatic set-up.
This leaves FCAW-S (self-shielded) as the only serious semi – automatic welding for consideration, the obvious advantage is that you gain from the reduction in stop starts, without the need for ancillary shielding.
Mechanized Processes
GMAW and FCAW-G are the most common means of mechanized welding for fill and cap passes on pipe. The current techniques are based on either short arc or pulsed spray transfer with high wire feed speeds.
The advancement and development of welding power source technology could conceivably yield significant productivity improvement
Along with the improvements and welding characteristics of solid or cored electrodes, it will be possible to improve the welding performance and deposition.
 |
Conclusions
The direction of the welding community must follow two clearly define channels.
- Enhancement of current welding processes: by advancing the designs of current basic low hydrogen manual and semi-automatic consumables, to better control hydrogen, strength and toughness. Although this methodology may prove successful for the X-80 applications, the increasing demand for greater hydrogen control and higher toughness, will likely drive the industry to new technological advances in consumables and processes.
- Innovation in welding consumables and welding power source design. Pipe contractors and fabricators have much to benefit by adopting these new technologies.
- The development of the Controlled Short Circuit Transfer utilizes the latest technology to enable previously unattainable control over several arc variables.
- Advances in pipe spacing, clamping, Bug and Band systems and multiple arc equipment can save time, increase quality and reduce costs for pipeline projects.
- Advancements in solid and cored electrodes will limit hydrogen and will enable manufacturing flexibility.
With quality and productivity in mind, Lincoln Electric is in the forefront of pipe welding processes and is constantly seeking solutions for these high strength steel challenges.
Reference:
TWI: Evaluation of Low Hydrogen Processes for pipeline Construction in High Strength Steel – PR-164-9330
Elliot K. Stava: 1993 The Surface Tension Transfer® power source, A New, Low –Spatter Arc Welding Machine. Welding Journal 72(1): 25-29Common Mistakes Made in the Design of Aluminum Weldments
Common Mistakes Made in the Design of Aluminum Weldments
By Frank G. Armao, Senior Application Engineer, The Lincoln Electric Company, Cleveland, Ohio
|
 |
| Figure 1 |
As a rule, designers of metallic structures have learned to design using steel. When designing with aluminum, however, the engineer must not base the design on prior experiences with steel or any other material. The alloy selection, proper joint design and the choice of an optimal welding process may all be a function of the base material. While aluminum obviously obeys the same laws of mechanics as all other materials, it must be approached differently than steel when welded. Aluminum structures are not necessarily more difficult to design or weld than steel structures, they are just different.
Don’t Just Choose the Strongest Alloy
Aluminum is often chosen as a structural material for applications in which weight savings are important. Very often, the designer will choose the very strongest alloy available. This is a poor design practice for several reasons. First, the critical design limitation for many structures often is deflection, not strength. In such cases, the modulus of elasticity, not the tensile properties, will govern the design. The modulus of most aluminum alloys, weak and strong alike, is approximately the same (one-third the modulus of elasticity of steel), so no benefit accrues from using the strongest alloy. Second, and most importantly, many of the strongest aluminum alloys are not weldable using conventional techniques.
When we speak about aluminum alloys being "weldable" or "non-weldable," we are usually referring to the alloy’s ability to be welded without hot cracking. Alloys that are extremely susceptible to hot cracking are not considered appropriate for structural (load-carrying) applications, and are generally put in the non-weldable category. Hot cracking in aluminum alloys is primarily due to the chemistry of the alloy and the weld bead. For virtually every alloying addition, the cracking sensitivity varies as alloy content increases as shown in Figure 1. Weldable alloys have a composition that falls either well above or well below the maximum cracking sensitivity. In some cases, such as that of 6061, which is very crack-sensitive if welded without filler material, the weld cracking sensitivity can be reduced to acceptable levels with the addition of a high silicon or high magnesium filler metal. The additional silicon or magnesium pushes the solidifying weld metal below the cracking sensitivity level. In other alloys, such as 7075, it is not possible to design a weld filler alloy that results in a crack-resistant chemistry. These are considered to be non-weldable.
Alloys are broken into two groups: heat-treatable alloys and non-heat-treatable alloys. A relative assessment of weldability is also given for each of these.
The non-heat-treatable alloys are composed of the 1XXX, 3XXX, 4XXX, and 5XXX series. It is not possible to strengthen these alloys by heat treatment. They can only be strengthened by cold working (also called strain hardening). The 1XXX alloys, such as 1100, 1188, or 1350, are essentially pure aluminum (99+% purity). They are relatively soft and weak, with good corrosion resistance, and are usually used where high electrical conductivity is required, such as for bus bars or as electrical conductors. They are also used in certain applications that require a high degree of resistance to corrosion. All of these alloys are readily weldable.
The 3XXX series of alloys have various levels of manganese (Mn) added to strengthen them and improve their response to cold work. They are of moderate strength, have good corrosion resistance, and are readily weldable. They are used for air conditioning and refrigeration systems, non-structural building trim, and other applications.
The 4XXX series of alloys have silicon (Si) added as an alloying element to reduce the melting point and increase their fluidity in the molten state. These alloys are used for welding and brazing filler materials and for sand and die castings. They are the least crack-sensitive of all the aluminum alloys.
The 5XXX series of alloys have magnesium (Mg) added in order to increase their strength and ability to work-harden. They are generally very corrosion resistant and have the highest strengths of any of the non-heat-treatable alloys. Increasing magnesium content in these alloys results in increasing strength levels. These alloys are commonly available in the form of sheet, plate and strip, and are the most common structural aluminum alloys. They are generally not available as extruded sections, because they are expensive to extrude. They are readily weldable, in most cases, with or without filler metal. However, there is an Al-Mg cracking peak at approximately 2.5% Mg, so care must be used in welding alloys such as 5052. It should not be welded autogenously (i.e., without adding filler metal). Weld filler metal with a high Mg content, such as 5356, should be used to reduce the crack sensitivity.
The heat-treatable alloys are contained in the 2XXX, 6XXX, and 7XXX alloy families. The 2XXX family of alloys are high strength Al-Cu alloys used mainly for aerospace applications. In some environments, they can exhibit poor corrosion resistance. In general, most alloys in this series are considered non-weldable. A prime example of a non-weldable alloy in this series, which is attractive to designers because of its high strength, is alloy 2024. This alloy is commonly used in airframes, where it is almost always riveted. It is extremely crack-sensitive and almost impossible to weld successfully using standard techniques.
Only two common structural alloys in the 2XXX series are weldable: 2219 and 2519. Alloy 2219 is very easily weldable and has been extensively welded in fabricating the external tanks for the U.S. space shuttle. This alloy gets its good weldability because of its higher copper content, approximately 6%. A closely related alloy, which is also very weldable, is 2519. It was developed for fabrication of armored vehicles. Although there are detailed exceptions to this rule, the designer should probably consider all other alloys in the 2XXX series to be non-weldable.
The 6XXX series of alloys are the alloys probably most often encountered in structural work. They are relatively strong (although not as strong as the 2XXX or 7XXX series) and have good corrosion resistance. They are most often supplied as extrusions. In fact, if the designer specifies an extrusion, it will almost certainly be supplied as a 6XXX alloy. 6XXX alloys may also be supplied as sheet, plate and bar, and are the most common heat treatable structural alloys. Although all alloys in this series tend to be crack-sensitive, they are all considered weldable and are, in fact, welded every day. However, the correct weld filler metal must be used to eliminate cracking. Additionally, these alloys will usually crack if they are welded either without, or with insufficient, filler metal additions.
The 7XXX alloys are the ones that usually trip designers up. They are the very high strength Al-Zn or Al-Zn-Mg-Cu alloys that are often used in aerospace fabrication, and are supplied in the form of sheet, plate, forgings, and bar, as well as extrusions. With the few exceptions noted below, the designer should assume that the 7XXX alloys are non-weldable. The most common of these alloys is 7075, which should never be welded for structural applications. In addition, these alloys often suffer from poor corrosion performance in many environments.
A few of the 7XXX series defy the general rule and are weldable. These are alloys 7003 and 7005, which are often seen as extrusions, and 7039, which is most often seen as sheet or plate. Some common uses of these alloys today are bicycle frames and baseball bats, both of which are welded. These alloys are easily welded and can sometimes offer strength advantages in the as-welded condition over the 6XXX and 5XXX alloys.
There is one other exception to the general rule that 2XXX and 7XXX alloys are unweldable. There are a number of thick cast and/or wrought plate alloys designed as mold plate material for the injection molding industry. These alloys, which include Alca Plus, Alca Max, and QC-7, are all very close in chemistry to 7075 or 2618. The designer should absolutely avoid structural welds on these alloys. However, welding is often performed on these alloys to correct machining mistakes, die erosion, etc. This is acceptable because there are only low stresses on such welds and, in fact, the weld is often in compression.
This discussion has tried to make a few points:
- First, when designing a structure of any kind, don’t scroll through the nearest list of aluminum alloys and pick the strongest.
- Realize that some alloys, often the stronger ones, are non-weldable. Make sure the selected alloy is readily weldable.
- Recognize that some alloys or alloy families are more suitable for some applications than others.
One more caveat: when welding aluminum, the designer must not assume that the properties of the starting material and the properties of the weld are equivalent.
Why Isn’t the Weld as Strong as the Original Base Metal?
A designer of steel structures generally assumes that a weld is as strong as the parent material, and the welding engineer who is responsible for fabricating the structure expects to make a weld which is as strong as the steel being used. It would be tempting to assume that the situation is the same when designing and fabricating aluminum structures, but it isn’t. In most cases, a weld in an aluminum alloy is weaker, often to a significant degree, than the alloy being welded.
Non-Heat-Treatable Alloys
 |
| Figure 2 |
 |
| Figure 3 |
An obvious question is whether anything can be done to restore material properties after welding a strain-hardened material. Unfortunately, the answer is almost always no. The only way to harden these materials is through mechanical deformation, and this is almost never practical for welded structures.
Heat-Treatable Alloys
 |
| Figure 4 |
However, most alloys are given an additional heat treatment to obtain the highest mechanical properties. This heat treatment consists of holding the material at approximately 400°F (205°C) for a few hours. During this time, the alloying additions that were dissolved in the prior heat treatment precipitate in a controlled manner, which strengthens the alloy. Material in this condition is designated as T6 (artificially aged) temper, the most common heat-treated alloy temper.
Again, the complete temper designation system is actually much more complex than this, but understanding the T4 and T6 tempers will help to overcome some of the most common mistakes made when designing aluminum weldments. It is important to note that heat treatable alloys can also be strain-hardened after heat treatment, and this can further complicate the temper designation.
Remember that the aging treatment is performed at approximately 400°F (205°C). Any arc welding process gets the HAZ much hotter than this. Therefore, welding constitutes an additional heat treatment for the HAZ. Some alloys experience an additional solution heat treatment, while other alloys become overaged in the HAZ. This results in degradation of material properties, especially if the as-welded properties are compared to T6 properties. For example, the minimum specified tensile strength in ASTM B209 for 6061 – T6 is 40 ksi (275 MPa). Most fabrication codes require a minimum as-welded tensile strength of 24 ksi (165 MPa), which is a significant degradation.
As when designing for the non-heat-treatable alloys, the designer must not use the parent material properties in design. Realistic as-welded properties must be used. It is difficult to generalize what these properties are. They change from alloy to alloy and depend strongly on the starting temper of the alloy. Most design codes contain as-welded properties for aluminum alloys and these should be used.
With heat-treatable alloys, however, there are some ways to recover some of the material properties of the parent. Figure 4 shows a plot of tensile stress versus distance from the weld for 6061, revealing curves for both T4 and T6 material in both the as-welded (AW) and post-weld-aged (PWA) conditions. The PWA condition represents a weld that is subsequently aged for one hour at approximately 400°F (205°C). Post weld aging improves the mechanical properties for both T4 and T6 starting materials. In fact, often times it is better to weld in the T4 condition and post weld age after the welding process.
There is one final alternative to discuss. If after welding, the structure is given a complete heat treatment (i.e., solution treat at 1000°F [540°C], quench, age at 400°F [205°C]), all of the material properties (even in the weld) will be recovered and T6 properties will be obtained. This practice is frequently followed on small structures such as bicycle frames, but it is impractical for larger structures. Furthermore, the quenching usually causes enough distortion of the structure that a straightening operation is necessary before aging.
Conclusions
In the design of welded aluminum structures, too often the differences between steel and aluminum are not taken into account. To recap, common mistakes include:
- Not all aluminum alloys are weldable. In general, the least weldable alloys are also the strongest alloys.
- The weld will rarely be as strong as the parent material.
- The HAZ will have O temper annealed properties for non-heat-treatable alloys regardless of the initial material temper.
- For the heat treatable alloys, the as-welded properties will be significantly lower than the properties of the T6 alloy temper.
- Post-weld heat treatment can help to restore the mechanical properties of welds in heat treatable alloys.
Controlling Welding Fume, A Total Systems Approach
Controlling Welding Fume, A Total Systems Approach
|
Introduction
Operators are exposed to fume and gases when welding, and exposures vary depending upon the process and specific working conditions. Fabricators are under continual pressure to reduce worker exposure to potentially harmful substances in the workplace, including welding fume. This article will address the following:
- How welding fume is generated
- Coordinating factors that affect fume generation and exposure to fume such as welding design, process, equipment, consumables, gases, work management and ventilation
- Highlights of fume extraction technology
- The current U.S. regulatory climate with regard to welding fume
- Current published exposure limits for typical components of fume
What Is Welding Fume?
Although many people think of gases and vapors from gasoline or other chemicals as "fume," technically, fume is comprised of very small, solid particles. Since Arc welding usually produces only small concentrations of gases, exposure to gases is seldom a concern except in confined areas. Therefore, the issue of secondary gas production will not be specifically discussed here.
Arc welding creates fume as some of the metal boils from the tip of the electrode and from the surface of molten droplets as they cross the arc. This metal vapor combines with oxygen in the air and solidifies to form tiny fume particles. These particles are visible because of their quantity, but each particle is only between 0.2 and 1.0 micron in size. Since fume primarily comes from the electrode, it consists of oxides of its metals, alloys and flux compounds. In steel welding, therefore, fume is primarily iron oxide and oxides of alloys such as manganese and chromium. With plated or coated metals, some of the fume comes from the weld pool as well. This adds oxides of metals from the base material into the fume such as zinc oxide from welding galvanized steels.
A Total Systems Approach
There are many ways to reduce exposure to welding fume. Each solution addresses part of the welding system. Each solution, however, has its advantages and disadvantages, and should be considered in the context of the total system. Likewise, a solution cannot work without proper implementation. The most successful solutions rely on a coordinated effort between managers, engineers, welding supervisors, vendors and especially welders themselves.
Although "fume extraction" may be the first solution that comes to mind, other options should be considered as well. Approaches to controlling welding fume actually fall into two broad categories:
- Reducing fume generation
- Limiting operator exposure to fume
http://www.lincolnelectric.com
Increasing Productivity with a FCAW Wire Optimized for Your Application
Increasing Productivity with a FCAW Wire Optimized for Your Application
Ken Lee, Application Engineer, The Lincoln Electric Company
|
If welding with a solid wire is satisfactory, why use a higher priced flux-cored wire? A flux-cored wire is optimized to obtain performance not possible with a solid wire. For many welding applications like vertical-up welding, flat welding, welding over galvanized, or welding hard-to-weld steels, a flux-cored wire can do it better and faster.
Although gas metal arc welding (GMAW) with a solid mild steel wire is popular, easy-to-use, and effective for many applications, it does have limitations and drawbacks. For example, GMAW is slow for out-of-position welding. It is either limited to short-circuit transfer, which is restricted by many welding codes due to the tendency for lack-of-fusion, or pulse transfer, requiring a special welding power source. It also requires very clean steel.
The ability to add a variety of materials to the core of the welding wire allows many performance enhancements to be made. Slag formers are added to shield the weld pool and shape and support the weld. Iron powder is used to increase deposition rates. Powdered alloys are added to produce low-alloy deposits or improving the mechanical properties. Scavengers and fluxing agents are used to refine the weld metal.
Flux-cored arc welding gas-shielded (FCAW-G) wires were introduced to the market around 1957. The flux-cored arc welding self-shielded (FCAW-S) wires were introduced to the market later, around 1961.
The core ingredients for FCAW-G wires have been formulated to obtain performance impossible to achieve with a solid GMAW wire. As all of shielding is provided by the shielding gas, the core materials may be carefully selected to maximize a certain area of welding performance, such as obtaining smooth spray-type transfer with 100% carbon dioxide shielding gas and welding speeds twice as fast in the vertical position.
The FCAW-S wires on the other hand, the core materials must provide all of the shielding. The core materials generate its own shielding gases, slag formers, and compounds to refine the weld pool. The benefits of self-shielded flux-cored wires lie in its simplicity. They may be used outdoors in heavy winds without tenting and the additional equipment required for gas shielding.
Now, we're going to discuss several popular types of flux-cored wires and how they can increase welding productivity.
For semi-automatic out-of-position welding, E71T-1 wires offer unsurpassed performance. Its fast freezing rutile slag provides the highest deposition rates in the vertical-up position, up to 7 pounds per hour, unmatched by any other semi-automatic arc welding process. In addition, the E71T-1 wires also offer an exceptionally smooth welding arc and minimal spatter, even with 100% carbon dioxide shielding gas. Argon/carbon dioxide blends are used for the smoothest arc and best out-of-position performance. These are reasons why E71T-1 is the world's most popular flux-cored wire. It is a top choice for shipbuilding, structural steel, and general steel fabrication applications.
For semi-automatic out-of-position welding without shielding gas, E71T-8 wires offer the highest deposition rates. Lincoln Electric's NR®-232 can deposit 4.5 lbs./hr. in the vertical-up position, 50% faster than other E71T-8 wires. Since this wire is self-shielded, it is widely used outdoors and in field erection of structural steel.
For semi-automatic welding in the flat position, the fastest way to join thick steel plate is with an E70T-4. It offers the highest semi-automatic deposition rates, up to 40 pounds per hour. This wire is widely used to join thick steels where there is no Charpy impact toughness requirement. This wire is also self-shielded, allowing it to be easily used outdoors.
The highest deposition rate gas-shielded flux-cored wire is E70T-1. Compared to E70T-4, they offer slightly lower deposition rates of up to 30 pounds per hour, but they offer a smoother welding arc and Charpy impact toughness properties. It offers higher deposition rates than GMAW, handles dirtier plates, and uses lower cost 100% carbon dioxide shielding gas. E70T-1s are widely used in structural steel fabrication shops.
For welding coated and galvanized sheet steels, E71T-14 is the wire of choice. The self-shielded E71T-14 wire has core materials which explode in the arc, volatizing the steel coating, minimizing cracking and porosity. The result is higher quality welds and fast welding speeds. E71T-14 wires are widely used in the automotive industry for fabricating galvanized steels.
What is the fastest way to weld hard-to-weld steels? E70T-5 gas shielded wire offer excellent crack resistance on hard-to-weld steels, such as T-1 quench and tempered steels, abrasion resistant steels, and free machining steels. E70T-5 has a basic slag system, similar to 7018 stick electrode, which removes phosphorus and sulfur from the weld metal, which can cause cracking, porosity, and poor toughness. E70T-5s have lowest diffusible hydrogen levels among the flux-cored wires, resulting in excellent resistance to delayed hydrogen cracking, as. In addition, they offer exceptional Charpy impact toughness properties.
Flux-cored wires offer higher productivity for many mild steel semi-automatic welding applications:
E71T-1 (FCAW-G): Highest deposition rates out-of-position.
E71T-8 (FCAW-S): Highest deposition rates out-of-position without a shielding gas.
E70T-4 (FCAW-S): Highest deposition rates in the flat position.
E70T-1 (FCAW-G): Highest deposition rates in the flat position with Charpy properties.
E71T-14 (FCAW-S): Fastest travel speed on galvanized and coated steels.
E70T-5 (FCAW-G): Fastest way to weld hard-to-weld steels.
Why be limited by a solid wire when a flux-cored wire can do it better and faster? Select the flux-cored wire optimized for your welding application. Use it and increase your productivity and lower your welding costs.
For more on Lincoln Flux-Cored Self-Shielded Electrodes, Click Here
For more on Lincoln Flux-Cored Gas-Shielded Electrodes, Click Here
For more on Lincoln Metal-Cored Gas-Shielded Electrodes, Click Here
http://www.lincolnelectric.com/
Inverter Based Welding Power Supplies for Welding Aluminum
Inverter Based Welding Power Supplies for Welding Aluminum
By Frank G. Armao, The Lincoln Electric Company
|
The world is changing. That's no surprise to anyone who is even remotely conscious of their surroundings. However, it's tempting to look at long- established technologies, such as welding, and believe that there is little or no technology development taking place at this late date. However, the person who took that view would be wrong.. In fact, the design and capabilities of welding power supplies has changed , and is continuing to change, rapidly. One of the technologies driving this change is the development and popularization of power supplies based on inverter technology. This technology is particularly well suited to welding aluminum alloys, especially thin aluminum alloys.
What's New?
In the past, welding power supplies have been based on transformers. The power supply took in 60 Hertz 230, 460 or 575 volt power. A metallic transformer changed it from the relatively high input voltage to 60 Hertz current at a lower voltage. This low voltage current was then rectified by some sort of rectifier bridge to get direct current (DC) welding output. Control of this output was usually performed by some sort of relatively slow magnetic amplifiers.
Transformers are relatively inefficient operating at 50 or 60 Hertz. A lot of heat is generated in the transformer, and the transformer must be relatively large and heavy. A significant part of the power cost goes into heating the transformer and the surrounding air. Most such welding power supplies weigh around 400 pounds and have a shape something like a 32 inch cube. Additionally, if 60 Hertz is used, control signals are limited to being issued at no more than 120 per second, so it's impossible to pulse the welding current any faster than this.
In inverter controlled power supplies, the same incoming 60 Hertz power is used. However, instead of being fed directly into a transformer, it is first rectified to 60 Hertz DC. Then it is fed into the inverter section of the power supply where it is switched on and off by solid state switches at frequencies as high as 20,000 Hertz. This pulsed, high voltage , high frequency DC is then fed to the main power transformer, where it is transformed into low voltage 20,000 Hertz DC suitable for welding. Finally it is put through a filtering and rectifying circuit . Output control is performed by solid state controls which modulate the switching rate of the switching transistors.
What advantages does this new inverter controlled design offer? First, the main power transformer, which operates at 20,000Hertz is vastly more efficient than 60Hertz transformers, which means it can be much smaller. Remember, transformer - based machines typically weigh 400 pounds plus and are a 32 inch cube. The accompanying photo shows the Lincoln line of inverter - based gas tungsten arc welding (GTAW) power supplies. The machine in the center, the V205, weighs 33 pounds and is 9 inches wide, 19 inches deep and 15 inches high. The other two machines are DC only inverters and are even lighter and smaller. So there is a huge advantage in weight and portability in favor of the inverter - based machines.
There is another advantage of the inverter power supplies - power cost. The inverter equipment is much more efficient than transformer equipment. For instance, the current draw at 205 amperes for the Lincoln V205 is 29 amperes on 230Volt single phase power. The current draw of an older transformer welder is typically 50 to 60 amperes on 230 Volt single phase power when welding at similar currents. While the cost savings in switching to inverters is often overstated, under normal circumstances, it is safe to say that annual power savings are approximately 10% of the power supply purchase price.
The other significant advantage of inverter power supplies is that, by "choppingup" the incoming AC so finely, we end up with a very steady DC, without the typical 60 Hertz ripple. This results in a much smoother, more stable DC welding arc.
So far, we've only discussed inverters that supply direct current. For quite a few years, this was all that was available. Inverters that supplied AC output simply did not exist. Then, someone had the idea of packaging two inverters inside one case. By having them run at different polarities and alternately switching them on and off, a pseudo AC output was generated. Some inverters still generate AC in this manner. There are also more sophisticated methods of generating AC today, but for the purposes of this article, it's easier to think of generating the AC from two inverters at opposite polarities.
The ability to generate AC is what really makes the inverter shine for welding aluminum using GTAW. The fact that the arc voltage never truly goes through zero means that the AC arc is much more stable than previously. Most inverter - based GTAW power supplies do not need the high frequency to be on continuously for stability. In fact, the Lincoln V205 has no provision for using continuous high frequency. It will automatically be extinguished as soon as the arc starts.. The elimination of continuous high frequency drastically reduces the amount of RFI generated by the power supply.
Second, the fact that we can send control signals at 20 kilohertz means that we can vary the frequency of the AC welding output. Older machines were 60 Hertz AC output only. The V205 can put out AC at anywhere 20 and 150 Hertz. Higher frequencies can be beneficial in welding thin materials. As the frequency is raised, the arc cone, and the weld, become narrower, resulting in deeper penetration.
It was realized many years ago that in GTAW, weld penetration comes from the electrode negative part of the AC cycle. During the part of the cycle when the electrode is positive, weld penetration is reduced and more heat goes into the tungsten electrode. However, during the electrode positive part of the cycle, the arc actually acts to remove the oxides from the surface of the aluminum, making welding easier. It is for this reason that, although most other materials are GTA welded using direct current, aluminum is usually welded using AC. Very early GTAW power supplies supplied a simple sine wave output where equal amounts of electrode positive and electrode negative were generated. However, this was inefficient. We didn't need that much electrode positive to get adequate cleaning. Later power supplies allowed us to vary the proportion of electrode negative to electrode positive. It was found that approximately 65% electrode negative and 35% electrode positive gave adequate arc cleaning and good penetration. However, a lot of the arc energy was still going to heat the tungsten electrode., so that large diameter tungsten electrodes were required.
The inverter power supplies provide adequate arc cleaning with as little as 15% electrode positive. Reducing the amount of electrode positive makes the process more efficient, increases weld penetration, and reduces the amount of heat going into the tungsten electrode, which means smaller diameter, pointed electrodes can be used. This further concentrates and narrows the weld.
Finally, the newer inverter power supplies are software programmable. This makes it much easier to change power supply characteristics. The accompanying photo shows another Lincoln power supply, the Invertec® V350 Pro. This power supply is primarily designed as an inverter - based machine for gas metal arc welding (GMAW). It contains quite a number of different programs for steady state, pulsed GMAW and non - traditional control algorithms for GMAW. A good number of the pulsed GMAW programs where the pulsing parameters are optimized for specific filler materials and wire sizes. However, because of the software programming, it is also ready to use as a power supply for shielded metal arc welding or gas tungsten arc welding. It can also be reprogrammed in the field in a short time. Along with all of this, the power supply weighs 79 pounds and can put out as much as 425 amperes.
The future is here.
http://www.lincolnelectric.com
Kicking The Stick Habit...Cored Electrodes Add Welding Versatility
|
Cored electrodes can provide a broad range of metallurgical and physical characteristics that may be difficult or impossible to achieve with traditional stick welding. The varied demands of maintenance welding make cored electrodes the ideal choice for many applications. Here is a look at some of the potential choices available to industry.
While traditional stick or shielded metal arc welding (SMAW) will always have its place, continued advances in cored electrodes make them an attractive solution to an ever-broadening range of welding requirements. Cored electrodes tend to provide higher deposition rates than those of other processes without developing excessive electrical resistance heating, even with relatively small diameter electrodes.
A cored electrode is a continuously fed tubular metal sheath with a core of powdered flux and/or alloying ingredients. These may include fluxing elements, deoxidizing and denitriding agents, and alloying materials, as well as elements that increase toughness and strength, improve corrosion resistance, and stabilize the arc. Typical core materials may include aluminum, calcium, carbon, chromium, iron, manganese, and other elements and materials. While flux-cored electrodes are more widely used, metal cored products are useful for adjusting the filler metal composition when welding alloy steels. The powders in metal cored electrodes generally aremetal and alloy powders, rather than compounds, producing only small islands of slag on the face of the weld. By contrast, flux-cored electrodes produce an extensive slag cover during welding, which supports and shapes the bead.
Flux-Cored Arc Welding
A flux contained within the tubular electrode produces the shielding and forms a slag for flux-cored arc welding (FCAW). Additional shielding may be provided by an externally supplied gas or gas mixture, in which case the process is referred to as gas-shielded (FCAW-GS). Otherwise, it is termed self-shielded (FCAW-SS). Electrode and flux composition are tailored to specific applications. Electrodes used with external gas shielding generally differ from those that are self-shielded. Within each category are several classes that together cover a broad range of welding applications.
Self-Shielded Flux-Cored Electrodes
With self-shielded FCAW, the heat of the arc causes decomposition and some vaporization of the electrode's flux core, which partially protects the molten metal (Fig. 1). Because the flux ingredients introduce deoxidizing and denitriding agents, self-shielded FCAW is ideal for many types of field welding, especially under windy conditions. By contrast, the gas-shielded process may require the use of tents or other physical shields to protect the gas shielding from winds over about five mph. The self-shielded process also is more portable, since it requires only a wire feeder and constant-voltage power source. Gas tanks, regulators and hoses are unnecessary. This simplicity extends to the torch head, which allows the operator greater visibility of the work.
For high deposition welding applications, long electrode extensions (up to 3-3/4 in.) are used frequently. This preheats the electrode and decreases the welding current to produce ashallow penetrating welding bead, which is suitable for bridging gaps caused by poor fit-up on heavy, complex, or tubular pieces.
Various electrode types are designed for high-speed, single-pass welding, general purpose welding, structural fabrication, and high-strength pipe welding. Electrode diameters available range from as small as .030 in. to 5/32 in., depending upon the classification and application. Some are most suitable for use in flat and horizontal positions, while others can accommodate most or all positions. Typical applications vary from automatic welds on sheet metal to heavy steel structures in bridge construction, with almost everything in between.
Manufacturers have developed a broad range of electrodes to meet special conditions. Some can weld steel as thin as 20 gauge, while others can deposit 40 pounds of weld metal per hour. Within each classification, there may be several electrodes that meet the category's overall requirements, but each provides a different set of characteristics. Most manufacturers of consumables have ample test data and application information and should be consulted when selecting electrodes to meet specific conditions. Some manufacturers will actually design custom-filled FCAW electrodes for special applications.
Specific elements can be used to clean and desulphurize weld deposits by including them within the tubular electrodes, although they could not be incorporated as alloying elements during the processing of solid steel electrodes. This versatility with cored electrode composition and design results in welding consumables with excellent resistance to weld cracking on hard-to-weld steels such as high sulfur and high carbon grades.
Gas-Shielded Flux-Cored Electrodes
Gas-shielded FCAW (Fig. 2) is similar to self-shielded but uses protective gas shielding in addition to the flux core. Shielding gases typically include CO2 or a mixture of argon and CO2, such as 75% Argon/25% CO2, 90% Argon/10% CO2, or others. The exact choice and blend of shielding gas will vary depending upon the electrode composition and desired arc characteristics.
With gas shielding, typically short electrode extensions (1/2" to 1-1/4") are used for most wire diameters. The use of larger electrode diameters (greater than 3/32") and the use of 100% CO2 shielding gas can result in welds having deep penetration. This is desirable for some welding applications to reduce required weld size or weld joint volume. Gas-shielded FCAW electrodes are most popular for automatic, semi-automatic and robotic welding of mild and low alloy steels. Representative applications include bridges, mining machinery, offshore drilling rigs, ships, structural and general fabrication.
Electrodes for gas-shielded FCAW are commonly available in mild steel and low alloy grades, in diameters that range from .035 in. to 1/8 in., depending upon the electrode type. Some are most suitable for downhand welding, while others can be used for out-of-position welding, and each produces a different set of physical, metallurgical and welding characteristics. A recent development is the emergence of low smoke or low fume flux-cored electrodes, particularly for use with gas shielding, to reduce welding fumes.
Self-Shielded Flux-Cored Electrodes
Designed to provide their own shielding as well as various other functions, these electrodes have benefitted in several ways from metallurgical improvements. In about the mid -1960s , controlled amounts of aluminum were first used in these electrodes as an element to combine with nitrogen. It was found that adding other alloying elements, such as carbon and nickel, could improve ductility and increase notch toughness.
Such advances have led to the development of an entire series of self-shielded flux-cored electrodes. Lincoln Electric®, for example, offers more than 25 such electrodes in its product line. Some have been used in extremely demanding applications, such as offshore oil platforms, where Charpy V-notch and CTOD (cracked tip opening displacement) test requirements are stringent. Other developments include electrodes using new alloy types that will promote the welding of higher strength steel, electrodes with improved usability on galvanized steel, and specialty electrodes for the welding of cross-country pipe.
Gas-Shielded Flux-Cored Electrodes
Although not well-suited for outdoor use because wind can cause loss of shielding, these electrodes offer many advantages for other applications. The T-1 or rutile-bearing (containing titanium dioxide, a desirable slag ingredient) electrodes in this class are easy to use, have excellent spray transfer and may have lower fume generation. Out-of-position usability is high, with deposition rates of 6-1/2 to 7 pounds per hour easily achievable.
As with other cored electrodes, a wide range of choices provides great versatility. For example, rutile-bearing electrodes for both downhand and out-of-position welding are available in mild steel as well as a variety of alloy types. Another example is a family of patented extra-low hydrogen electrodes that incorporate hydrogen scavenger technology. These electrodes were developed to reduce the tendency for hydrogen cracking during the welding of high-strength steel.
Operator appeal is a constant goal. One area where gains are being made is in T-5 electrodes. While these fluoride-type electrodes provide greater crack resistance and meet certain Charpy V-notch requirements, they have always been more difficult to use, especially for out-of-position welds. Recent developments are targeted at creating T-5 electrodes that have the operator-friendly characteristics and positional versatility of the T-1 rutile-type electrodes.
Metal-Cored Electrodes
A metal-cored electrode is a composite electrode, generally consisting of a mild steel jacket with a core of specifically selected iron and other metal powders and alloys. Stabilizers and arc enhancers can be added easily, providing a wider operating window for the welder. Metal-cored electrodes are gas-shielded types that are an alternative to solid alloy electrodes. Versatility is possible with these electrodes because of the infinite alloy compositions that can be made easily by electrode manufacturers.
Special alloy combinations can be achieved that would be difficult or impractical with solid electrodes, including special types for welding higher strength steels. Versions that meet military and other specifications requiring low diffusible hydrogen are available. Metal-cored electrodes are being produced with 12% chromium for production welding of catalytic converters and other automotive exhaust components of 409 stainless steel. Another type incorporates chemistry that reduces surface tension of the weld puddle, for improved wetting action that produces a wider weld, useful in applications such as stitching parts together.
When a job calls for special electrodes, metal-cored electrodes are a more economical alternative to solid electrodes. Because the manufacturing process involves blending metal powders instead of creating a special melt of steel, small quantities are easier to produce, and minimum order quantities are much lower. As a result, metal-cored electrodes can be produced with shorter turnaround times and at lower cost than special-ordered solid electrodes.
Arc Enhancement and Wave Form Control
When metal-cored electrodes are combined with pulsed waveform control technology, several benefits occur. Using a metal-cored electrode in the pulse mode, as opposed to straight spray-type transfer, reduces the volume of smoke as well as the amount of spatter, particularly at low settings. Cored electrodes operate extremely well in a pulsed environment, especially with machines like Lincoln's Power Wave® 455M. These are inverter-based welding systems designed to combine ease of use and welding performance. They incorporate specific programs for pulsed welding of metal-cored electrodes as well as many other program selections. The Power Wave® 455M, in fact, can be reprogrammed by using software to create the ultimate arc for any application, at any time.
A typical application is the manufacture of automotive and truck mufflers from Type 409 stainless steel. These can be welded with an electrode consisting of a mild steel sheath and core materials that are blended to provide the proper amount of chromium and other elements for the stainless steel composition. 
When compared to the globular transfer normally used in this type of application, the use of pulsed wave form control will reduce heat and spatter. This leads to longer welding gun life and reduced maintenance because of less damage to fixturing, especially in high volume applications. With waveform control, pulsed operation provides better weld quality with the thinner materials (typically 16 gauge), high travel speeds and less than perfect fitup often encountered in this type of application.
Development Continues
Future development of cored electrodes will continue to provide solutions to welding problems as well as improving quality and increasing productivity. Another focus is the area of operator appeal, both through making electrodes easier to use and in reducing weld fumes through new, low-fume electrodes.
Higher strength is a goal for all processes, along with improved ductility and toughness. Special electrodes continue to be developed for high-strength steel applications such as in the offshore industry, but greater versatility and improved operator appeal are still a major focus of most development efforts.
By Robert Munz, Senior Project Engineer, Cored Electrode Products, Consumable Research & Development, The Lincoln Electric Company, June, 1998.
Pulsed MIG Welding Provides Increased Savings and Quality
Pulsed MIG Welding Provides Increased Savings and Quality
|
Some of the latest technology power sources on the market today are those that provide pulsing capabilities. Most likely, you have heard how these sophisticated machines make welding easier for the operator and provide a high quality weld. But, did you know that these machines actually provide a cost savings? Although you may pay a little more for these power sources initially, the advantages that they provide will decrease overall welding costs and provide a payback of your investment in the long run. Advantages provided by pulsing machines include:
- Wire and gas savings. Pulsed MIG machines offer a wider operating range because they extend the low and high range of each wire diameter. For instance, before the operator would have to stock .035”, .045” and .052” wire diameters for varying applications, but with Pulsed MIG, .045” can be extended on the low end and top end range so that it can be used for a variety of applications. What this means is that rather than having two or three different sized wires, an operator would only require one. Having one wire type minimizes inventory costs and reduces changeover times. The same is true with shielding gas – one gas can reach both the low and high ranges of the application. In addition, the different types of spare parts (gun, gun tips, liners, etc.) are decreased for additional cost savings.
- Spatter and fume reduction. Compared to Conventional MIG, Pulsing offers reduced spatter and fume. Reduction in spatter translates into significant cost savings because more of the melted wire is applied to the weld joint, not as surface spatter on the product and surrounding fixtures. This also means less clean-up time. A reduction in the welding fumes creates a safer and healthier environment for the entire plant or shop.
- Heat reduction. Pulsing offers controlled heat input leading to less distortion and improved overall quality and appearance which means fewer production problems. This is especially important with stainless, nickel and other alloys that are sensitive to heat input.
- Improved productivity. Pulsed MIG offers high deposition rates. In addition, since the new machines are simpler and adaptive, it is easier to weld with pulsed MIG than other transfer methods, less time is spent training.
- Better quality. All these advantages of Pulsed MIG outlined above result in overall better quality of the finished and a more stable arc. In addition, operators are receiving a better quality working environment since they are not dealing with fume, spatter and extra clean-up or grinding time. One more benefit is that synergic power sources allow for these high quality welds to be achieved by those with relatively less training.
In simple terms, pulsed MIG is a non-contact transfer method between the electrode and the weld puddle. This means that at no time does the electrode ever touch the puddle. This is accomplished through high-speed manipulation of the electrical output of the welding machine. It is designed to be a spatterless process that will run at a lower heat input than spray or globular transfer methods.
The pulsed MIG process works by forming one droplet of molten metal at the end of the electrode per pulse. Then, just the right amount of current is added to push that one droplet across the arc and into the puddle. The transfer of these droplets occurs through the arc, one droplet per pulse.
To understand this process in detail, let’s take a look at a waveform. Unlike CV (constant voltage) where current is represented by a straight line, pulsed MIG drops the current at times when extra power is not needed, therefore cooling off the process. It is this “cooling off” period that allows pulsed MIG to weld better on thin materials, control distortion and run at lower wire feed speeds.
During the process, the current rises to a peak when the droplet is formed. Then, in the background current phase, the current is lowered to reduce the overall heat input. It is the height and the width of the peak that is important for proper transfer.
Pulsed MIG Compared to Other Transfer Methods
How does pulsed MIG compare to other welding transfer modes? We will examine each with their advantages and disadvantages.
Short Circuit
In short circuit, the wire touches the work piece and shorts to itself. This is the coldest form of welding that still offers good fusion. Short circuit allows operators to weld on both thick and thin material in all positions. It also has the benefit of a small, quickly solidifying puddle. Its disadvantages include limited wire feed speed, and deposition rates. There is also a danger of “cold lapping” on thicker metals. This is where there is not enough energy in the puddle to fuse properly. Short circuit also produces an increased amount of spatter over the other transfer methods.
Globular
The globular transfer mode is basically uncontrolled short circuit. It is characterized by a large volume of weld metal coming off the electrode. These large droplets are pinched at the arc and drop into the puddle. This method of transfer produces a tremendous amount of spatter as well as high heat input. Also, globular is limited to flat and horizontal fillet welds. Less fusion is often common because the spatter disrupts the weld puddle. Also, because globular transfer uses more wire, it is generally considered less efficient.
On the positive side, globular transfer runs at high wire feed speeds and amperages for good penetration on thick metals. Also, it can be used with inexpensive, CO2 shielding gas. It is used mainly when appearance is not an issue.
Spray Arc
Spray arc propels small molten droplet of the electrode to the work. It is a pure CV process that must produce enough current to send a constant stream of metal off the electrode. Its advantages include high deposition rates, good penetration, strong fusion, good weld appearance with little spatter.
Its disadvantages include high heat input, a limited range of welding positions and proneness to burnthrough on thin materials.
Pulsed MIG
Pulsed MIG is an advanced form of welding that takes the best of all the other forms of transfer while minimizing or eliminating their disadvantages. Unlike short circuit, pulsed MIG does not create spatter or run the risk of cold lapping. The welding positions in pulsed MIG are not limited as they are with globular or spray and its wire use is definitely more efficient. By cooling off the spray arc process, pulsed MIG is able to expand its welding range and its lower heat input does not make burnthrough on thin metals a problem.
Pulsed MIG is one of the best welding processes for a wide variety of applications and metal types. [GRAPHIC 2]
Customization of the Waveform
To take the pulsed MIG process a step further, Lincoln Electric offers complete customization of the welding waveform through its state-of-the-art Waveform Control Technology™. This technology allows the power source to be finely tailored to the wire and process. The power source rapidly adjusts the pulse waveform for superior welding performance. It does this by providing a fast or slow front edge on the pulse to transfer the droplet at the proper rate, the back edge then falls at a controlled rate to add 
the heat needed to wet the droplet to the puddle. With Waveform Control Technology™, built-in templates are set up in the power source for standard usage on a variety of materials. Variables such as ramp rate, peak time, tailout, step off, among others are controlled in a precise manner so that when there is a process set-up change, there is a corresponding change in waveform configuration. And, with Lincoln’s Wave Designer™ Software, welding engineers can use their PCs to further tailor and tweak the welding arc.
Equipment Selection
Pulsed MIG welding has evolved quite a bit since it was first introduced to the marketplace. In the 1980s, it was a highly complex process that could only be performed by the most skilled welders. That was because the operator would have to know exactly how to set the machine for the correct wire feed speed to perform this type of welding. Today, this is all done for him or her as part of the synergic control. When the operator adjusts wire feed speed, the synergic operation adjusts the waveshape and frequency automatically.
The synergic operation of the machine makes it easy to use, even for the beginning welder, with a single knob that controls all operations. In addition, its sophisticated internal electronics are even “adaptive” to adjust for variations in stickout, gap or the torch angle.
Here are some tips to help operators choose which equipment to use for Pulsed MIG:
- Choose equipment capable of operating over the new expanded range of welding processes
- Look for advanced synergic controls
- Consider investing in dual procedure guns
- Carefully choose welding gun size
- Look for work voltage sensing if welding far from the power source
Set-Up Tips
Set-up for Pulsed MIG machines is a little different than Conventional MIG machines. Take care to ensure that the appropriate guidelines are followed for safe operation.
- Higher pulse currents require a better ground
- Cable lengths should be minimized to reduce inductance
Conclusion
Cost savings, better quality, improved productivity and easier operation…all these factors make Pulsed MIG an option that should not be overlooked. Although the high price tag may scare you, carefully weigh the initial investment with the benefits that will be derived over the long term. Take advantage of the new technological advantages provided by Pulsed MIG – one machine to handle virtually any application, flawlessly.
http://www.lincolnelectric.com/
Selecting a MIG Wire to Boost Your Bottom Line
Selecting a MIG Wire to Boost Your Bottom Line
|
 |
Lisa Byall, Product Manager
Chris Hsu, Engineering Manager
The Lincoln Electric Company
Offshore competition, the increased cost of raw materials, higher energy rates and elevated transportation costs…these are just a few of the issues today which are forcing manufacturers to take a hard look at ways to reduce costs and ultimately improve the bottom line.
Because welding can be a significant fabrication activity for many companies, it is usually one of the first manufacturing processes to receive careful scrutiny when cost cutting is the goal. Unfortunately, many manufacturers think that switching to a lower cost MIG wire will be the silver bullet to boost the balance sheet. In reality, an inferior MIG wire could translate into additional dollars spent in pre- and post-weld operations such as cutting, forming, surface and joint preparation, pre-heating, cleaning, tacking, grinding and painting.
 |
In fact, when examining the total cost of welding, the cost of the wire is often as low as as approximately four percent of actual welding costs, while the bulk of costs are overhead and labor. So, saving a penny on the price of the wire in the end may actually cost a company much more in productivity-robbing activities that otherwise could be avoided.
Selecting a quality MIG wire is critical. It can be more forgiving and produce a sound weld even under less than perfect conditions. Take, for instance, a plate with surface contaminants. The right MIG wire for that application may make some pre-welding operations obsolete. And, as more companies move to robotics, a quality MIG wire will provide accuracy in wire placement and consistency in the weld, making rework less of an issue.
 |
Common Mistakes in Choosing MIG Wire and How To Avoid Them
The most common types of MIG wire for welding mild steel are ER70S-3 and ER70S-6. These wires are designed to meet minimum tensile strength requirements of 70,000 psi. But which one is best for a particular application?
ER70S-3 is typically used on clean, oil-free and rust-free base material. It is also the best choice for avoiding silicon islands, which can sometimes form on the top of the weld, giving it a “glassy” look. Paint applied over a silicon island may later flake off. In addition, with multiple pass welding, a silicon island may be detected on an x-ray as a slag inclusion. Such defects may require costly rework.
Choose an ER70S-6 wire for welding on plate that has mill scale or surface contaminants, since this wire incorporates the proper deoxidizer to combat these issues. A deoxidizer absorbs oxygen so that it vaporizes into the arc or forms as scale oxides. ER70S-6 is also better for creating a smooth transition from the weld to the base metal, also known as wash-in or tie-in. Better wash-in may be a requirement in applications subject to fatigue. ER70S-6 wire can provide better wetting at the weld toe when compared to an ER70S-3 wire.
 |
Beyond Mild Steel
HSLA (high strength low alloy) steels are becoming increasingly popular for fabrication in many industries. In addition, AHSS (advanced high strength steel) is gaining momentum in the automotive industry where weight reduction becomes a priority.
However, studies have shown that the increase of base metal strength in AHSS makes the weldment strength more susceptible to weld defects. Defects and discontinuities in the weld and heat affected zone previously tolerable in low carbon steel applications can result in rejections in AHSS welded structures. It is essential to select premium welding consumables and to optimize welding procedures together with a quality assurance program to weld AHSS.
It is especially important to with HSLA (high strength low alloy) and AHSS (advanced high strength steel) welding to pay careful attention to electrode matching, depending on specific application requirements and conditions. These applications typically are less forgiving to weld defects.
“Matching” weld metal has minimum specified yield and tensile strengths equal to or higher than the minimum specified strength properties of the base metal. In the case of the overmatching weld metal, however, the base metal is the controlling variable. For this situation, it is important to check the capacity of the base metal to ensure that the
connection has the required strength. Always consult with your designer, weld engineer and electrode supplier before making a final selection.
The Effect of Wire Diameter
Consistent wire diameter is critical to ensure proper current passage from the contact tip to the wire. Undersized wire causes arcing between the wire and the inside diameter of the tip, which erodes the I.D. of the tip and eventually fuses the tip to the wire. Oversized wire causes excessive feeding force, tip blockage, wire slippage and downtime.
During wire manufacturing, care must be taken to avoid abrupt diameter and cast changes where wire ends are joined together by butt welds. These manufacturing butt weld locations within your wire spool or reel can often cause significant variation in the wire diameter or cast on lesser quality MIG wires. Wire diameter variation over time, even within the AWS specification range, can also affect weld deposition by as much as eight percent. One way to manufacturers can ensure wire diameter consistency and increase your weld quality is to inspect 100% of the wire using laser micrometer inspection methods.
The Importance of Chemical Composition
Be sure to select a wire with consistent chemical composition. Why? Consistent chemical composition results in more consistent performance. Consistent performance will result in greater, more stable quality control. Your operators and weld engineers will more likely be able to set and forget their procedures, rather than re-adjusting to accommodate wire with wildly fluctuating chemical composition.
Here’s something to consider: There are two methods of alloy analysis and control recognized by AWS A5.01. The first, and most common, uses lot control by heat number. A typical heat certification relies on a small sample taken from a heat of molten steel. The resulting heat certification indicates that the chemical composition of the heat of steel is within AWS specifications for its intended use. The problem is that this small mill test sample represents the chemical composition of a huge quantity – often 250,000 lbs. – of molten steel.
Also, during the continuous casting of steel, segregation of elements occurs in the ladle from bottom to top as the heat is being cast. Typically, the end of the heat (top of the ladle) will contain steel that has an accumulation of residuals and elements that are not indicative of the rest of the heat. Because a heat certification is an average of the start, middle and end of the heat, there is some probability that material in the heat may contain steel that does not meet AWS requirements. In addition, as different orders are melted at the mill, materials with different chemical compositions can get mixed together. This transition material can alter – sometimes significantly – the nature of the steel.
The second method of alloy analysis and control is by controlled chemical composition. In this scenario, every coil of incoming rod (typically 2,500 to 4,500 pounds of raw steel wire) is tested twice by the electrode manufacturer for chemical composition before being put into production. In this way, the properties in specific coils of steel are matched with qualities that are desirable in specific electrodes and the steel is put into electrode production accordingly. When compared with the heat certification method, this method carries the capacity to allow for additional consistency in chemical composition.
It is also important to note that while AWS provides chemical composition requirements for the finished product, there is no system of monitoring or policing compliance. For some applications, meeting the requirements of the industry for which the application is being produced may be more important than conforming to AWS standards. These industry standards include American Bureau of Shipping (ABS), U.S. Military Requirements (MIL), Lloyds, Bureau Veritas and American Society of Mechanical Engineers (ASME).
 |
Packaging: More Important than You Might Think.
With the breadth of packaging options on the market today, selecting the right packaging for MIG wire is an important cost consideration. For example, bulk packaging of 250 pounds and up in drums, reels or boxes can offer many cost advantages. Typically, because there is less handling by the electrode manufacturer, these packages are offered at a lower price per pound.
Especially desirable in robotic applications or heavy-duty semi-automatic fabrication shops, bulk packages make it possible to reduce the number of wire package changeovers per shift or per week. On the other hand, bulk packages can mean increased inventory costs and lost floor space for some shops. Here’s one rough rule of thumb: Some in the industry feel the best price to inventory cost relationship is achieved when the package is depleted and replaced approximately every 30-45 days.
Also, look at shop conditions when considering packaging options. While one company may see cost advantages by purchasing a bulk reel, another manufacturer with dusty and humid shop conditions and/or floor space constraints might be motivated to choose a box or drum which offers full enclosure. In addition, remember that exposed wire is electrically hot when welding. For safety reasons, some shops may prefer enclosed packaging. And while a open reel may be less expensive than a drum or box package, the moving parts of a dereeler can pose a safety hazard. Plus, the dereeler will require ongoing maintenance and upkeep – other cost factors to consider.
The cost of disposal is another issue for manufacturers. To save even more, choose a fully recyclable cardboard box that can be crushed down and shredded instead of a wooden reel or metal-rimmed drum. Also, the use of recyclable boxes will aid a company’s compliance in regards to ISO 14001, the latest supplier yardstick in the automotive and other industries.
 |
Items such as lifting straps make it easier for operators to handle the packaging. Also, a wood pallet under the packaging allows for convenient moving via forklift. In contrast, integral paper pallets can be more easily damaged by a forklift. Lastly, select packaging which suits the plant layout. For instance, if some of the welding stations are located on a mezzanine level, it may be more difficult to lift and use some package types into these tight spaces.
Switching to MIG
Up to this point we have been discussing how to get the most from MIG wire. But what if a manufacturer is currently using a stick electrode, cored wire, submerged arc or spot resistance welding? Can a switch to MIG provide benefit in these types of application?
From automotive parts to fabricated structures, shipbuilding, metal buildings and sheet metal applications, all have often reaped benefits from the switch to MIG wire. Advantages include slag free welding with less clean up, even in multiple pass operations. In addition, MIG requires lower operator skills levels than stick or TIG.
Using two MIG wires, called the Tandem MIG™ process, provides lower heat input than submerged arc as well as lower distortion. It is also very versatile and can be used on a wide range of materials from high strength/low alloy metals to advanced high strength steels (AHSS).
Depending on equipment and procedures, MIG has the potential for all-position welding, meaning less fixturing, or positioner, costs. It also has a lower heat input, with the exception of MIG in the spray arc weld mode, for less distortion and burnthough in the finished weld. Other advantages include high electrode efficiency of 97 to 98 percent. In comparison, SMAW offers an efficiency of only 60 to 70 percent because of factors such as spatter, slag coating burn off and stub loss.
Also, solid MIG wire typically has better wire placement than cored wire. Wire placement is the ability of the wire to exit the contact tip in the same location every time for accurate weld placement. This can be an important consideration, especially in automated applications. When comparing MIG wire placement in the joint, look for wire with a consistent cast as a further aid to accurate wire placement.
 |
Conclusion
When looking to reduce overall welding costs, look beyond the price of the wire. Saving a couple of pennies in the short-run, may cost you hundreds of dollars in lost productivity in the long run. Be sure to choose the right wire for a particular application, ensure its chemical composition, and purchase the best packaging option for the plant where it will be utilized. MIG wire quality matters in your overall cost structure. Choose Wisely.
For more on Lincoln's Quality Manufacturing System, click here.
http://www.lincolnelectric.com
Selecting Filler Metals: Low Hydrogen
Key Concepts in Welding Engineeringby R. Scott Funderburk
This is part two in a series on selecting filler metals. When selecting filler metals, the specifier may elect to require "low hydrogen electrodes." Such electrodes may be required to minimize the possibility of hydrogen related cracking. In some cases the engineer may specify low hydrogen electrodes because he believes these electrodes will also provide weld deposits exhibiting a high minimum level of notch toughness. While this may be true, it can not be assumed. This article will address specifying filler metals that resist hydrogen related cracking while also providing good mechanical properties.
The term "low hydrogen" has been around for about 60 years. It was first introduced to differentiate this classification of shielded metal arc welding (SMAW) electrode (e.g., E7018) from other non-low hydrogen SMAW electrodes (e.g., E6010). They were created to avoid hydrogen cracking on high strength steels, such as armor plate.1
 |
| Figure 1. "Fish-eyes" on an all-weld-metal tensile specimen fracture surface. |
Confusion About the Term
Although so-called "low hydrogen electrodes" have been around for many years, there is some confusion about what is meant by the term. Many codes and specifications use the designation, however, neither "low hydrogen" nor "low hydrogen electrodes" are listed in the American Welding Society’s (AWS) Standard Welding Terms & Definitions (AWS A3.0-94)2. This may come as a surprise to some, especially to engineers that have been specifying that "only low hydrogen electrodes shall be permitted," or "all welds shall be low hydrogen", or that "all welding processes shall be low hydrogen." Without a formal definition, the term "low hydrogen" can be understood differently by engineers, contractors, or inspectors, which can lead to confusion and conflicts.
"Low Hydrogen Electrode" Means SMAW Electrode
| EXX15-x |
| EXX16-x |
| EXX18-x |
| Table 1. AWS SMAW Electrodes with Low Hydrogen Coverings |
Can Hydrogen Affect Mechanical Properties?
The influence of hydrogen can be observed in mechanical testing; however, its effects on the test results are limited. A high hydrogen content in a tensile specimen can produce "fish-eyes" on the fracture surface as seen in Figure 1.
Additionally, the presence of hydrogen can reduce ductility (as expressed by elongation and reduction in area). Hydrogen, however, does not typically influence the impact toughness, ultimate tensile strength or yield strength results. It is only in severe cases that it can influence the ultimate tensile strength.
Since low hydrogen SMAW electrodes like E7018 are also required to have a minimum specified level of Charpy V-notch (CVN) impact energy, low hydrogen is sometimes equated with a minimum CVN level. This has led some people to specify low hydrogen when the real desire is for notch toughness. The better approach is to specify notch toughness requirements since there is no automatic link between low diffusible hydrogen content in the weld and CVN values. Actually, some deposits with high hydrogen levels can deliver relatively high levels of notch toughness. For example, the E6010 classification (non-low hydrogen, 30-50 mL/100g) has a minimum CVN requirement of 20 ft-lbs at minus 20°F.
Use of the Term in Codes and Specifications
Some codes and specifications refer to hydrogen control in terms of either (1) requiring low hydrogen SMAW electrodes or (2) placing specific limits on diffusible hydrogen. The Structural Welding Code – Steel (AWS D1.1-2000)5 has provisions related to hydrogen in the preheat table (Table 3.2). In the table, Category "A" is applicable to "shielded metal arc welding with other than low hydrogen electrodes." The minimum preheat temperatures listed in Category "A" are higher than Category "B" because Category "B" is for "shielded metal arc welding with low hydrogen electrodes, submerged arc welding, gas metal arc welding, flux cored arc welding."
| Diffusible Hydrogen, mL/100g | |
| H8 | 8 |
| H4 | 4 |
| H2 | 2 |
| Table 2. Optional Hydrogen Designators | |
In the Interim Guidelines: Evaluation, Repair, Modification and Design of Welded Steel Moment Frame Structures6 published by the Federal Emergency Management Agency (FEMA), a comparison between low hydrogen SMAW electrodes and FCAW and SAW is made. This document states, "All of the electrodes that are employed for flux cored arc welding (both gas shielded and self shielded), as well as submerged arc welding, are considered low hydrogen." Implied is the assumption that FCAW and SAW will provide weld deposits with diffusible hydrogen levels similar to SMAW electrodes with low hydrogen coverings.
Weld Deposit Hydrogen Levels
As mentioned above, no definition exists for a "low hydrogen weld deposit." The word "low" is an imprecise description. The preferred method of controlling the level of hydrogen in a weld deposit is to use the optional hydrogen designators as defined by the American Welding Society. These designators are in the form of a suffix on the electrode classification (e.g., H8, H4, and H2). The filler metal manufacturer may choose to add the hydrogen designator to the electrode classification if the filler metal meets the diffusible hydrogen requirements in the applicable AWS A5.x filler metal specification. Following are examples of the designator requirements:
To avoid hydrogen induced cracking, the hydrogen level in the material must be held to a certain maximum level. This level is a function of the microstructure susceptibility, constraint (or restraint), and residual stresses. Microstructure susceptibility to hydrogen induced cracking often increases with increasing steel strength. Therefore, for higher strength steels lower levels of hydrogen are required. To simply state "use low hydrogen" is not enough. For example, "low" for a 50 ksi steel may not be "low" for a 100 ksi steel. Rather than require that "only low hydrogen electrodes can be used," engineers and fabricators are should use statements such as, "only electrodes or electrode-flux combinations capable of depositing weld metal with a maximum diffusible hydrogen content of 8 mL/100g (H8) are permitted."
Codes That Use Hydrogen Designators
The AWS D1.1 Structural Welding Code also has several provisions that utilize hydrogen designators (e.g., H8). For example, Category "D" in the minimum preheat and interpass temperature table (Table 3.2) allows only "…electrodes or electrode-flux combinations capable of depositing weld metal with a maximum diffusible hydrogen content of 8 mL/100 g (H8)." This is a good example of properly using the H-designators.
The AWS D1.1 Code also has an alternate method to determine the minimum preheat temperature (Annex XI) that uses three levels of diffusible hydrogen. In Annex XI, category H1 is called an "extra low hydrogen" at less than 5 mL/100g. Category H2 is labeled as "low hydrogen" at less than 10 mL/100g. The third category, H3, is a hydrogen level that is not controlled. Although category H2 is labeled "low hydrogen," this does not define low hydrogen electrode as less than 10 mL/100g. The actual diffusible hydrogen value can also be used to calculate the minimum preheat temperature with this method instead of using the H1, H2 and H3 categories.
The Fracture Control Plan of the AWS Bridge Welding Code7 (AWS D1.5-95) is another fine example of hydrogen control. This code requires the following for welding Fracture Critical Members:
- H16, H8 or H4, when the minimum specified yield strength is 50 ksi or less.
- H8 or H4, when the minimum specified yield strength is greater the 50 ksi.
Furthermore, SMAW electrodes can be used for tack welding without preheat if the electrode has an H4 designator, according to AWS D1.5.
Other agencies such as the United States Military8 and the American Bureau of Shipping9 also set limits on the diffusible hydrogen levels. Both use limits of 15, 10 and 5 mL/100g, and the military specification has a stricter limit of 2 mL/100g for some applications. Today, a logarithmic system (i.e., H16, H8, H4, and H2) is preferred in the United States.
Other Issues
Using an H8, or even an H4, electrode with controlled diffusible hydrogen alone provides no guarantee of eliminating problems related to hydrogen during or after welding. In addition to the electrode, several other factors can influence the diffusible hydrogen level and the potential for cracking. These should be considered as well.
- base metal surface condition (contamination from oils, grease, dirt, moisture, acid, rust and other hydrogen containing materials can increase hydrogen levels);
- relative atmospheric humidity (humid conditions generally lead to higher hydrogen levels);
- welding shielding gas (higher moisture content results in higher hydrogen levels);
- consumable storage conditions (improper or prolonged storage can lead to higher hydrogen levels);
- welding procedures (electrical stickout, arc voltage, wire feed speed and other parameters can influence diffusible hydrogen).
Conclusions
- A "low hydrogen electrode" refers only to a SMAW electrode that has a coating moisture of less than 0.6%.
- The maximum diffusible hydrogen level associated with low hydrogen SMAW electrodes has been a point of confusion because SMAW electrodes with low hydrogen coatings are not tied to any specific hydrogen level.
- "Low hydrogen" should not be specified in order to achieve specific impact properties. If notch toughness is required, then it should be listed separately from the hydrogen limits (if any).
- Job specifications should be written clearly and precisely regarding the use of "low hydrogen." The intent of the specifier should be listed in such a way that the contractor will understand what is required.
- If a contractor has any questions regarding in the intent of the engineer, or if the specifications are not clear, the contractor should seek clarification before welding. For example, if "use low hydrogen electrodes only" is listed on the contract, then the contractor may want to ask: "Is only SMAW allowed, or can other processes also be used?"
- Supplemental hydrogen designators (e.g., H8 and H4) are the preferred way to define a specific level of diffusible hydrogen in the weld deposit and should be used when needed.
- Finally, there are applications where low hydrogen electrodes are not required or where non-low hydrogen SMAW electrodes, like E6010, are preferred. Therefore, utilizing the blanket statement "use low hydrogen" should be avoided.
 | Stick Electrodes: Low Hydrogen Group |
References
- Robert O’Con. "Welding with Low Hydrogen Electrodes: A Look at the Past with Tips for Today." Practical Welding Today. March/April 2000, pp. 33-35.
- American Welding Society. Standard Welding Terms and Definitions. (ANSI/AWS A3.0-94), 1994.
- American Welding Society. Specification for Carbon Steel Electrodes for Shielded Metal Arc Welding. (ANSI/AWS A5.1-91), 1994.
- American Welding Society. Standard Methods for Determination of Diffusible Hydrogen Content of Martensitic, Bainitic, and Ferritic Steel Weld Metal Produced by Arc Welding. (AWS A4.3-93), 1993, p. 16.
- American Welding Society. Structural Welding Code – Steel. (AWS D1.1:2000), 2000.
- Federal Emergency Management Agency. Interim Guidelines: Evaluation, Repair, Modification and Design of Welded Steel Moment Frame Structures. (FEMA 267), August 1995, p. 8-11.
- American Welding Society. Bridge Welding Code. (AWS D1.5-95), 1995.
- United States Military. Military Specification – Electrodes – Welding, Flux Cored, Ordinary Strength and Low Alloy Steel, (MIL-E-24403/1D), November 14, 1985.
- American Bureau of Shipping. Rule Requirements for Materials and Welding, Part 2, 1997.
Tandem MIG™ Process for Increased Production
The dual-wire Tandem MIG™ process continues to gain popularity as a means to increase production in automated arc welding applications. The process follows trends established early in the welding industry of reducing welding costs by developing dual-wire processes for greater productivity. Early developments in multiple wire welding focused on the submerged arc process. The availability of high-powered inverter power sources and Waveform Control Technology™ has enabled dual-wire welding using the MIG (GMAW) process.
Since the introduction of Tandem MIG in the early 1990s, the estimated installed base of dual wire MIG (GMAW) systems has grown to over 1,000 units worldwide. The majority of the systems installed have replaced single-wire processes that had been pushed to the extreme high end of the useable operating range in an attempt to improve productivity and lower cost by depositing as much metal as possible in the shortest time frame. Tandem MIG extends the welding productivity range beyond that possible with conventional single-wire processes. A comparison of weld metal deposit rates of popular single wire processes to that of Tandem MIG demonstrates the possible production gains associated with the Tandem MIG process.
 |
Process Fundamentals:
The Tandem MIG process employs two electrically isolated wire electrodes positioned in line, one behind the other, in the direction of welding. The first electrode is referred to as the lead electrode and the second electrode in line is referred to as the trail electrode. The spacing between the two wires is usually less than ½ inch so that both welding arcs are delivering to a common weld puddle. The function of the lead wire is to generate the majority of the base plate penetration, while the trail wire performs the function of controlling the weld puddle for bead contour, edge wetting and adding to the overall weld metal deposit rate.
The process works best with a large diameter lead wire and a small diameter trail wire. The larger lead wire may represent as much as 65% of the total deposition rate, while providing greater penetration. The smaller, trail welding wire is focused on the trail edge of the weld puddle. The trail wire is typically smaller in diameter and therefore draws less current. This helps to control the shared weld puddle and aids in keeping it cool.
A common compromise is to specify the lead and trail welding wires to be the same diameter to satisfy inventory constraints or because the direction of welding must be reversed somewhere on the weldment. Satisfactory operation may be achieved with this compromise but the maximum travel speed is limited and the robustness of the process is reduced.
Tandem MIG depends on specialized power source control software that facilitates the stable operation of two independent welding arcs working in close proximity. The power source must be controlled to stabilize the disruptive electromagnetic influences that cause severe instability when two unregulated direct currents (DC) welding arcs are operated close together.
 |
In the standard operating mode, the lead arc is programmed for a Tandem MIG DC positive constant voltage mode, and the trail for operation in a Tandem MIG DC positive pulse mode. The constant voltage lead arc is desirable to maximize penetration and travel speed. The lower heat input pulsed trail arc is instrumental in minimizing possible electromagnetic arc interference between the two arcs, as well as cooling and controlling the common molten weld puddle that is generated.
The combination of the constant voltage (CV) lead and Tandem MIG pulse trail configuration provides a wide operating range. The lead and trail arc procedures may be changed independently to achieve a balance between penetration and fill.
 |
A second potential configuration calls for operating both the lead and trail welding wires in a Tandem MIG pulse mode. This configuration is typically used to manage total process heat input on thin gauge material and other heat sensitive applications. This configuration requires synchronization of the pulse frequency of both electrodes so that the peak of each pulse on one of the arcs occurs during the background of the other arc.
Sychronization implies that both the lead and trail arcs must be operated at the same frequency (or frequencies that are an integer multiple of each other). The requirement imposes strict operation constraints and the process must be carefully applied.
- Again, the lead and trail procedures must be programmed to operate at the same frequencies or at integer multiples of each other.
- To increase or decrease deposition, the lead and trail wire feed speeds must be varied together. This reduces the degree of independence between penetration and fill.
- Arc voltage regulation cannot be achieved by dynamically varying the frequency.
Equipment configuration:
The Tandem MIG equipment is configured to provide individual parameter control for each of two separate, and electrically isolated, welding arcs. This requires a pairing of all equipment; two specially designed high-speed inverter power sources, two wire drives, two separate welding wire pay-off sources and a Tandem MIG welding torch. The power sources used for this process rely on fast digital control and Tandem MIG specific software. Welding parameters are set at the power source via digital communication from either a Programmable Logic Controller (PLC) associated with a dedicated hard automation work cell or by a robot controller.
The Tandem MIG torch is a critical component of the system, engineered with specific contact tip alignment and spacing to achieve proper arc control. Due to the need to withstand the demands of high amperage, high duty cycle production runs the torches are generally rated in terms of the total current flowing in both wires. This rating is typically in the 600-1200 amp range. Additionally, the maximum current for each wire is specified. This rating is typically in the 400-800 amp range. An example of a hard automation equipment configuration would be:
 |  |
Process Benefits:
The increased production benefits of the Tandem MIG process have been used to:
- Justify the cost of automation equipment
- Improve profitability of existing automation
- Reduce the initial capital expense costs of new production lines by reducing the number of weld stations required
- Shorten payback periods associated with new welding automation.
The Tandem MIG process has a wide operating range that can be generally segmented into two catagrories addressing high speed sheet metal welding and heavy plate welding. On sheet metal, the process is often operated at travel speeds in excess of 100 IPM on thin gauge material (.040 - .100 inch). On heavy gage material, weld metal deposition rates exceeding 35 lbs./hr. are possible.
 |
High travel speed applications:
The ability to distribute the total welding current across two separate welding wires provides unique benefits for high-speed welding. When pushed to increase travel speeds on thin gauge metal components in industries such as automotive, tank and general sheet metal fabrication, welding operations are faced with one or two quality issues, either burn through or lack of weld metal follow characteristics.
The Tandem process addresses both of these speed-limiting issues. The ability to distribute the necessary welding current over two welding wires allows the lead wire to generate needed penetration while the trail wire rides on the back edge of the weld puddle creating added fill. Also, the trail wire acts as an additional force that pushes the puddle for better follow and wetting capabilities. This trail arc behavior in the shared weld puddle provides excellent gap filling characteristics. Improved gap filling capabilities are of particular value to industries processing high volumes of stamped or formed parts.
 |
| Tandem MIG gap filling capabilities. Welds performed at 100-ipm travel speed on .100 inch material Truck side panels being welded with robotic Tandem MIG. |
The system utilizes robot touch sensing software to locate the weld joint and through-the-arc seam tracking (T.A.S.T.) software option for real-time tracking. The Tandem system welding at an average speed of 60 ipm was installed to replace an older single wire robotic system that was that was averaging 24 ipm. Overall welding speed was increased 150 %.
High deposit rate applications:
As illustrated in the previous graph, the Tandem MIG process can on average represent a 30-80% increase in deposition potential when compared to conventional single-wire processes
 |
| Truck bolster plates being welded with automated Tandem MIG. The 8-foot long bolsters require a 5/16 inch fillet weld placed on both sides of a 3/8 inch vertical support member. Tandem MIG was able to increase production from 5-6 units per day to 25 units per day. Process uses two .045 dia welding wires depositing 28 lbs. per hour. Production was increased over 300%. |
The Tandem MIG process typically employs small diameter electrodes. As higher welding currents are applied to the small diameter electrodes (.035- .062 inch) the electrode melt-off rate rises exponentially. The resulting electrode melt-off rate for a given current draw is higher for Tandem MIG than that of a single large diameter electrode. This higher melt-off rate potential and lower amperage draw provides unique benefits for the heavy plate fabricating industry. The high deposit rate obviously provides the means for improved production throughput. The lower heat input can be used effectively to reduce plate distortion and time between passes when controlling inter-pass temperature on mult-pass welds. The process is capable of producing x-ray quality welds with excellent mechanical properties.
Return on investment:
The Tandem MIG process is designed for use on automated welding cells or automated lines. Common host automation is either a hard-automated work cell that has dedicated motion functions or a robotic cell with flexible, programmable motion. Investment in these high-volume production lines is generally a significant capital expenditure that requires detailed analysis and cost justification. Part floor-to-floor time, including welding speed as a critical component, plays an important role in determining if a project can be cost justified. When compared to single-wire processes, Tandem MIG higher travel speed capabilities can assist in cost justifying greater capital expenditures as well as accelerate equipment payback periods.
Tandem MIG has assisted in reducing the cost of new production lines by meeting production needs with fewer welding stations. This is particularly true for high-volume production lines producing automotive components or similar parts where tooling and part handling equipment constitutes a sizeable portion of the initial installation costs. The cost of hydraulic tooling and handling equipment may be reduced by using fewer weld stations based on the higher per-station throughput of Tandem MIG. Additionally, the expense of up-keep and maintenance of duplicate tooling sets to insure consistent part dimensions is minimized.
Production cells welding large components must be cost justified in a different way based more on welding time and not part count. The heavy equipment industry, which was the first to embrace the Tandem MIG process, typically utilizes large robotic work cells that include expensive positioners to handle the large and heavy weldments that often take two or more hours to weld. Most weldments must be placed in the flat or horizontal position. This requires the use of large positioners and makes the use of multiple robots per cell difficult. Tandem MIG has repeatedly been used to replace single-wire robotic systems welding at averaging deposit rates in the range of 15-20 lbs./hr. with Tandem MIG operating in the 28-34 lbs./hr range.
The increased weld metal deposit rate has been used to justify the cost of purchasing new more technically sophisticated workstations.
Tandem MIG continues to benefit a number of industries, from companies welding thin sheet metal automotive components to companies performing multi-pass welding of large earth moving equipment and offshore drilling rigs.
 |  |
Technology Gets to the Root of Pipe Welding
Open root welds on pipes can be made three to four times faster than GTAW by using the Surface Tension Transfer® process. When integrated with an internal spacer clamp into a new automatic orbital pipe welding system, even faster production is possible, with no lack of fusion.
Pipe welding codes, whether for applications in the field or in the plant, require high-quality root pass welding. To ensure that the joints will not leak, especially for steam or pressurized applications, a weld must penetrate completely through the pipe.
In the past, pipe welding was done by one of three methods, each of which has its advantages and disadvantages. These are the methods that have been used.
Gas tungsten arc welding (GTAW) is popularly known as TIG. Travel speeds are slow, heat input is usually high, and it requires high operator skill level.
Gas metal arc welding (GMAW) - also known as MIG - is a much faster process than GTAW. However, because operator skill level is hight and heat input difficult to control, fusion may not always be 100 percent.
Shielded metal arc welding (SMAW), also known as stick, can be cost effective in terms of equipment but requires high operator skill. Frequent starts and stops are another potential problem.
 |
| Smoke and Spatter are reduced when pipe joints are welded by means of the Surface Tension Transfer (STT®) process. |
By contrast, the Surface Tension Transfer (STT) process makes it possible to complete open root welds three or four times faster than GTAW, with low heat input and no lack of fusion. The STT process uses high frequency inverter technology with advanced waveform control to produce a high-quality weld with less spatter and smoke. For pipe welding, the process also makes it easier to perform open gap root pass welding, with better back beads and edge fusion. It is easier to operate than other processes, yet produces consistent, X-ray quality welds. The STT process results in a complete back bead without shrinkage from the 12 to 6 o'clock weld positions. Also, because current control is independent of wire feed speed, the process allows greater flexibility under all conditions.
Controlling Spatter and Smoke
STT is a proprietary Lincoln Electric process that makes use of Wave Form Control Technology™ to control current precisely and rapidly during the entire welding cycle. It is unique in that it is neither constant current (CC) nor constant voltage (CV). Instead, the power source adjusts current automatically to the instantaneous heat requirements of the arc.
Spatter and smoke are reduced with this process, whether the arc shielding gas is 100 percent CO2, blends of argon and CO2 or helium mixtures for use with stainless steel. Reducing spatter minimizes final weld surface preparation and allows the operator more welding time before the gun nozzle must be cleaned of accumulated spatter.
 |
| In open root pass pipe welding, the STT process controls the wave form of the welidng current for excellent penetration control, fusion, and back bead. |
Reduced spatter also translates into significant cost savings because more of the electrode is applied to the weld joint, not as spatter on the pipe and surrounding fixtures. Further cost savings are realized because larger diameter wire can be used.
At the start of the cycle, when the electrode shorts, the current is reduced immediately, eliminating the incipient short. This low-level current is maintained for a short time so that the surface tension forces can begin transferring the drop to the puddle, forming a solid mechanical bridge. A high level of pinch current is then applied to accelerate the transfer of the drop. The necking down or squeezing of the shorted electrode is monitored. When a specific value is reached, the pinch current is reduced quickly to a low value before the fuse separates. When a short breaks, it does so at a low current, which produces very little spatter.
Next, the arc is reestablished and a high current known as peak current is applied. This momentary pulse of current establishes the arc length and causes the arc to broaden and melt a wide surface area, which eliminates cold lapping and promotes good fusion.
 |
| Spacer clamp and welding |
Better Pipe Welding Results
The constant voltage GMAW process normally used for pipe welding does not control the current directly. Instead it controls the average voltage. This can cause the weld puddle temperature or fluidity to be too high, and the internal bead may be flat or shrink back into the root. This is known as "suck back." Also, when using conventional short arc GMAW, the operator must concentrate the arc on the lip or leading edge of the puddle to ensure proper penetration and fusion. If the arc is too far back on the puddle, penetration will be incomplete. If the arc is too far ahead, the electrode shoots through the gap and causes whiskers to form inside the pipe.
Because Surface Tension Transfer controls the welding current independently of wire feed speed, the process makes it easy to control the temperature or fluidity of the puddle to ensure proper penetration and fusion. This is what makes it so attractive for open root pipe welding applications. In the 5G position, the operator simply has to stay in the puddle. Experienced pipe welders almost always find the process a welcome improvement, both in ease of welding and comfort. They particularly appreciate the reduction in spatter when welding in the 6 o'clock position.
 |
As the decision process evolves, the vendor and the fabricator will continue working together to determine the appropriate system accessories, including safety devices, the optimal layout for the robotic cell, manpower and training requirements, and service and maintenance requirements (internal vs. outside vendor support).
The STT process is gaining acceptance in pipe welding and similar applications, which require precise control of heat input as well as smoke and spatter reduction. Since the heat is controlled directly, the internal backbead profile is also controlled. Welders find that not only are open root welds easier to make, but their mechanical and metallurgical properties are excellent. Superior welding bead profiles can be attained with improved properties in the heat affected zone. Moreover, open root welds are made without the use of ceramic or copper internal backup. In the case of copper, corrosion is thus eliminated by avoiding the possibility of copper inclusions.
The process is effective for welding mild and high-strength steels, as well as stainless steel and related alloys. On steel, it offers the advantages of low hydrogen and 100 percent CO2 shielding with low spatter. When welding duplex stainless, critical pitting temperature is significantly better with STT than with GTAW, and travel speeds three or four times that of GTAW can be obtained, with much less skill.
By Elliott K. Stava, Technical Advisor-Advanced Welding Technology, The Lincoln Electric Company
http://www.lincolnelectric.com
The ABC's of Nondestructive Weld Examination
An understanding of the benefits and drawbacks of each form of nondestructive examination can help you choose the best method for your application
By Charles Hayes
The philosophy that often guides the fabrication of welded assemblies and structures is "to assure weld quality." However, the term "weld quality" is relative. The application determines what is good or bad. Generally, any weld is of good quality if it meets appearance requirements and will continue indefinitely to do the job for which it is intended. The first step in assuring weld quality is to determine the degree required by the application. A standard should be established based on the service requirements.
Standards designed to impart weld quality may differ from job to job, but the use of appropriate weld techniques can provide assurance that the applicable standards are being met. Whatever the standard of quality, all welds should be inspected, even if the inspection involves nothing more than the welder looking after his own work after each weld pass. A good-looking weld surface appearance is many times considered indicative of high weld quality. However, surface appearance alone does not assure good workmanship or internal quality.
Nondestructive examination (NDE) methods of inspection make it possible to verify compliance to the standards on an ongoing basis by examining the surface and subsurface of the weld and surrounding base material. Five basic methods are commonly used to examine finished welds: visual, liquid penetrant, magnetic particle, ultra-sonic and radiographic (X-ray). The growing use of computerization with some methods provides added image enhancement, and allows real-time or near real-time viewing, comparative inspections and archival capabilities. A review of each method will help in deciding which process or combination of processes to use for a specific job and in performing the examination most effectively.
Visual Inspection (VT)
Visual inspection is often the most cost-effective method, but it must take place prior to, during and after welding. Many standards require its use before other methods, because there is no point in submitting an obviously bad weld to sophisticated inspection techniques. The ANSI/AWS D1.1, Structural Welding Code - Steel, states, "Welds subject to nondestructive examination shall have been found acceptable by visual inspection." Visual inspection requires little equipment. Aside from good eyesight and sufficient light, all it takes is a pocket rule, a weld size gauge, a magnifying glass, and possibly a straight edge and square for checking straightness, alignment and perpendicularity.
Before the first welding arc is struck, materials should be examined to see if they meet specifications for quality, type, size, cleanliness and freedom from defects. Grease, paint, oil, oxide film or heavy scale should be removed. The pieces to be joined should be checked for flatness, straightness and dimensional accuracy. Likewise, alignment, fit-up and joint preparation should be examined. Finally, process and procedure variables should be verified, including electrode size and type, equipment settings and provisions for preheat or postheat. All of these precautions apply regardless of the inspection method being used.
During fabrication, visual examination of a weld bead and the end crater may reveal problems such as cracks, inadequate penetration, and gas or slag inclusions. Among the weld defects that can be recognized visually are cracking, surface slag inclusions, surface porosity and undercut.
On simple welds, inspecting at the beginning of each operation and periodically as work progresses may be adequate. Where more than one layer of metal filler is being deposited, however, it may be desirable to inspect each layer before depositing the next. The root pass of a multipass is most critical to weld soundness. It is especially susceptible to cracking, and because it solidifies quickly, it may trap gas and slag. On subsequent passes, conditions caused by the shape of the weld bead or changes in the joint configuration can cause further cracking, as well as undercut and slag trapping. Repair costs can be minimized if visual inspection detects these flaws before welding progresses.
Visual inspection at an early stage of production can also prevent underwelding and overwelding. Welds that are smaller than called for in the specifications cannot be tolerated. Beads that are too large increase costs unnecessarily and can cause distortion through added shrinkage stress.
After welding, visual inspection can detect a variety of surface flaws, including cracks, porosity and unfilled craters, regardless of subsequent inspection procedures. Dimensional variances, warpage and appearance flaws, as well as weld size characteristics, can be evaluated.
Before checking for surface flaws, welds must be cleaned of slag. Shotblasting should not be done before examination, because the peening action may seal fine cracks and make them invisible. The AWS D1.1 Structural Welding Code, for example, does not allow peening "on the root or surface layer of the weld or the base metal at the edges of the weld."
Visual inspection can only locate defects in the weld surface. Specifications or applicable codes may require that the internal portion of the weld and adjoining metal zones also be examined. Nondestructive examinations may be used to determine the presence of a flaw, but they cannot measure its influence on the serviceability of the product unless they are based on a correlation between the flaw and some characteristic that affects service. Otherwise, destructive tests are the only sure way to determine weld serviceability.
Radiographic Inspection
Radiography (X-ray) is one of the most important, versatile and widely accepted of all the nondestructive examination methods - Fig. 1. X-ray is used to determine internal soundness of the welds. The term "X-ray quality," widely used to indicate high quality in welds, arises from this inspection method.
Radiography is based on the ability of X-rays and gamma rays to pass through metal and other materials opaque to ordinary light, and produce photographic records of the transmitted radiant energy. All materials will absorb known amounts of this radiant energy and, therefore, X-rays and gamma rays can be used to show discontinuities and inclusions within the opaque material. The permanent film record of the internal conditions will show the basic information by which weld soundness and be determined.
X-rays are produced by high-voltage generators. As the high voltage applied to an X-ray tube is increased, the wavelength of the emitted X-ray becomes shorter , providing more penetrating power. Gamma rays are produced by the atomic disintegration of radioisotopes. The radioactive isotopes most widely used in industrial radiography are Cobalt 60 and Iridium 192. Gamma rays emitted from these isotopes are similar to X-rays, except their wavelengths are usually shorter. This allows them to penetrate to greater depths than X-rays of the same power, however, exposure times are considerably longer due to the longer intensity.
When X-rays or gamma rays are directed at a section of weldment , not all of the radiation passes are through the metal. Different materials, depending on their density, thickness and atomic number, will absorb different wavelengths of radiant energy.
The degree to which the different materials absorb these rays determines the intensity of the rays penetrating through the material. When variations of these rays are recorded, a means of seeing inside the material is available. The image on a developed photo-sensitized film is known as a radiograph. Thicker areas of the specimen or higher density material (tungsten inclusion), will absorb more radiation and their corresponding areas on the radiograph will be lighter - Fig 2.
Whether in the shop or in the field, the reliability and interpretive value of radiographic images are a function of their sharpness and contrast. The ability of an observer to detect a flaw depends on the sharpness of its image and its contrast with the background. To be sure that a radiographic exposure produces acceptable results, a gauge known as an Image Quality Indicator (IQI) is placed on the part so that its image will be produced on the radiograph.
IQI's used to determine radiographic quality are also called penetrameters. A standard hole-type penetrameter is a rectangular piece of metal with three drilled holes of set diameters. The thickness of the piece of metal is a percentage of the thickness of the specimen being radiographed. The diameter of each hole is different and is a given multiple of the penetrameter thickness. Wire-type penetrameters are also widely used, especially outside the United States. They consist of several pieces of wire, each of a different diameter. Sensitivity is determined by the smallest diameter of wire that can be clearly seen on the radiograph.
A penetrameter is not an indicator or gauge to measure the size of a discontinuity or the minimum detectable flaw size. It is an indicator of the quality of the radiographic technique.
Radiographic images are not always easy to interpret. Film handling marks and streaks, fog and spots caused by developing errors may make it difficult to identify defects. Such film artifacts may mask weld discontinuities.
Surface defects will show up on the film and must be recognized. Because the angle of exposure will also influence the radiograph, it is difficult or impossible to analyze fillet welds by this method. Because a radiograph compresses all the defects that occur throughout the thickness of the weld into one plane, it tends to give an exaggerated impression of scattered type defects such as porosity or inclusions.
An X-ray image of the interior of the weld may be viewed on a fluorescent screen, as well as on developed film. This makes it possible to inspect parts faster and at a lower cost, but the image definition is poorer. Computerization has made it possible to overcome many of the shortcomings of radiographic imaging by linking the fluorescent screen with a video camera. Instead of waiting for film to be developed, the images can be viewed in real time. This can improve quality and reduce costs on production applications such as pipe welding, where a problem can be identified and corrected quickly.
By digitizing the image and loading it into a computer, the image can be enhanced and analyzed to a degree never before possible. Multiple images can be superimposed. Pixel values can be adjusted to change shading and contrast, bringing out small flaws and discontinuities that would not show up on film. Colors can be assigned to the various shades of gray to further enhance the image and make flaws stand out better. The process of digitizing an image taken from the fluorescent screen - having that image computer enhanced and transferred to a viewing monitor - takes only a few seconds. However, because there is a time delay, we can no longer consider this "real time." It is called "radioscopy imagery."
Existing films can be digitized to achieve the same results and improve the analysis process. Another advantage is the ability to archive images on laser optical disks, which take up far less space than vaults of old films and are much easier to recall when needed.
Industrial radiography, then, is an inspection method using X-rays and gamma rays as a penetrating medium, and densitized film as a recording medium, to obtain a photographic record of internal quality. Generally, defects in welds consist either of a void in the weld metal itself or an inclusion that differs in density from the surrounding weld metal.
Radiographic equipment produces radiation that can be harmful to body tissue in excessive amounts, so all safety precautions should be followed closely. All instructions should be followed carefully to achieve satisfactory results. Only personnel who are trained in radiation safety and qualified as industrial radiographers should be permitted to do radiographic testing.
http://www.lincolnelectric.com
The Benefits of Lincoln Electric’s Micro-Start TIG Technology
The innovative Micro-Start TIG technology was developed with the user in mind. Numerous interviews were conducted with TIG operators – from those with an advanced skill level to the beginner. Lincoln asked these welding operators about their most common problems and set out to provide a technological advantage that would overcome those problems. What resulted was the Precision TIG™ with its Micro-Start TIG technology. This machine will help any TIG operator create their best possible weld – time and time again. Among SCR TIG machines, Micro-Start produces best-in-class DC welding.
Having a technology that addresses TIG welding problems is of critical importance today as more and more manufacturers are turning to new materials and exotic alloys that are thinner and in many cases, harder to weld. Any industry that welds thin materials, including aerospace and marine, can take advantage of Micro-Start TIG to provide precise control and top quality welds.
So what are these common problems that Lincoln’s Micro-Start TIG technology overcomes? Basically, they fall into four categories: 1) low-end performance; 2) low-end starting; 3) minimum starting amperages; and 4) crater fill.
Problem: Poor Low-End Performance
In many traditional, low amperage TIG applications operators have trouble maintaining a smooth, stable arc. When using a SCR (silicon controlled rectifier) machine to weld at low amperages, the SCR conductions in the machine are "phased back" to very short duration spikes of output. This 
results in a great deal of ripple in the output current as these minimal firings produce gaps between the spikes of current. Even with the normal output choke filtering, the choke cannot store enough energy between SCR firings to stabilize the arc. This ripple effect leads to arc instability and sporatic, high-frequency re-initiations, thus leading to inconsistency in the welds.
Trying to correct this problem, many operators traditionally purchased more expensive, conventional TIG machines. They believe these machines with larger chokes will better filter the arc current to produce more stability and better low end welding performance. But even the larger choke cannot adequately filter out the low current ripple.
The Micro-Start TIG Solution
Micro-Start TIG technology employs an independent power supply capable of welding without SCR assistance at low amperages – SCRs only fire to raise the current and supplement the 2 amp welding supply. This gives Micro-Start TIG very stable low current welding and provides it with the ability to eliminate erratic high frequency and weld thin materials in a consistent, high quality manner. Lincoln is the first manufacturer to offer a background circuit from which an operator can weld and smoothly transition to, or from, higher outputs.
Micro-Start TIG is capable of independently welding off of its electronic power supply when the amperage is down to the minimum rated 2 amps. As the operator depresses the foot Amptrol™ to increase the current, the main welding circuit (i.e. transformer and SCR bridge) turns on and provides amperage. The technology assists the transformer SCR choke circuit with its special electronic welding circuit instead of completely relying on chokes to smooth the arc as do conventional machines. The result is a very stable and smooth output at low amperage levels.
With Micro-Start TIG, operators don’t have to buy more expensive machines to get low end welding capabilities – Micro-Start technology is able to provide inverter-like performance using a lower cost, conventional machine.
Problem: Low-End Starting
Today’s TIG machines establish an arc by using high frequency to ionize a path from the tungsten to the workpiece. Though high frequency is necessary to establish the arc, in most machines it remains on for a long duration and with a high intensity, thus creating “tracking” marks on the weld surface. For critical welding applications such as aerospace or nuclear qualified welds, these tracking marks can cause micro-cracking and lead to weld imperfections. Even in non-critical applications, high frequency can create starting with a great deal of objectionable arc wander.
Another problem with conventional machines is that they can’t start at very low current (typically below 5 amps). This is because at the minimal firing of the SCRs, the output choke cannot store enough energy to maintain the current at a welding voltage to initiate and sustain the arc without re-initiating high frequency.
To improve starting, many competitive TIG machines use a Hot Start feature. Hot Start utilizes SCR conduction spikes of high current at sufficient voltage and duration to heat the tungsten and establish an ion path quickly from the tungsten to the work piece in order to reduce duration of 
high frequency. For example, if the operator sets the machine for 5 amps, the machine may spike up over 100 amps for a significant period of time during starting. But this method is too problematic because on thin material, a Hot Start will erode the workpiece and burn away the base metal. Some operators have even resorted to starting on copper blocks or welding coupons before moving the arc onto the weldment to combat negative effects of high frequency and hot-starting. This method allows time for the arc to stabilize and prevents damaging the workpiece.
Operators may often manually “Hot Start” by pushing down the TIG machine’s foot pedal to a higher starting amperage. But with this approach, the machine never starts at a low enough amperage resulting in potential burn-through or erosion of the welded work piece. It also does not produce consistency since operators have to “guess” where to start.
The Micro-Start TIG Solution
With Micro-Start TIG technology, Lincoln Electric has devised a way to get the arc established quicker, smoother and with more stability using the electronic 2 amp welding power supply to supplement a SCR starting pulse height and duration appropriate for the welding level. An improved control circuit lets this new technology utilize a shorter, less intense pulse ignition to light the arc without “popping” or creating “burnthough”, thus allowing the high frequency to turn off virtually when the arc first strikes.
In fact, most users can’t even detect that the high frequency is on. This quick start is short and will not allow enough heat input to burn any material away. But the start offers enough energy to heat the tungsten and establish a plasma flow to the work piece.
Micro-Start TIG also allows operators to adjust the minimum amperage of the machine. This allows the operator to adjust the low end of the machine to match the specific operating amperage range for the tungsten diameter being used, as well as his or her own low current skill level.
Problem: Minimum Starting Amperages
Most conventional machines allow operators to set only the maximum amperage, but will not allow for a minimum to be set. This means that if a tungsten and/or operator cannot start at the minimum output of the machine, than the foot pedal control must be “punched” to a higher level to achieve a start. This makes it difficult to achieve consistent starting, as well as repeatable crater filling.
Micro-Start TIG Solution
Lincoln offers the only machine with a minimum output control which lets the operator adjust the minimum amperage of the machine at minimum foot pedal depression to match the tungsten operating range or operator skill level. For example, if the operator is using a 3/32 diameter tungsten, its typical operating range is 10 to 150 amps. The operator can now set the minimum amperage of the machine not to go below 10 amps at foot pedal minimum to promote stability in welding and starting. Likewise for someone using a .020 or .040 tungsten – the minimum amperage can be turned down to 2 amps since this tungsten can run stable in this range. This minimum output control allows independent preset of minimum current level between 2 and 60 amps. This provides an optimum resolution range for remote control (foot pedal) between the minimum and maximum preset settings.
Problem: Crater Fill
One of the most frequent complaints conventional TIG operators voiced was the problem associated with lowering the current to fill a crater at the end of the weld. Traditional machines use a current sensing threshold approach, which means that when the operator ramps down and the arc becomes unstable, the machine detects the arc is in danger of extinguishing and initiates the high frequency again. With the current threshold method, the high frequency comes on at typically about 3 amps. The high frequency coming back on creates a wandering or “dancing” arc that leaves track marks on the weld allowing for contamination, microscopic cracking and surface imperfections.
Micro-Start TIG Solution
Lincoln’s Micro-Start TIG technology uses a voltage sensing method. This is a more “intelligent” sensing method that knows whether the operator is trying to maintain an arc. High frequency will only come back on if the sensed output voltage is greater than 35v (well above normal welding voltage). Therefore, the machine will provide a low current ramp down during cratering without unintended high frequency. In other words, during welding high frequency will not come back on again after the start of the sequence.
Traditionally, power sources are not sophisticated enough to sense whether or not an operator is actually welding – and when the operator wants to weld at low amperages. With Micro-Start TIG technology, once the high frequency initiates the DC arc, high frequency is no longer needed because of the low amperage stability of the background circuit.
Conclusion
With Lincoln’s Micro-Start TIG technology, an operator at virtually any TIG skill level will be able to make repeatable, high-quality starts, welds and finishes. This is because the new technology makes it easy to overcome the most common TIG welding issues with machines that overcome the most common performance limitations.
**RealPlayer can be downloaded for free from www.real.com
http://www.lincolnelectric.com
Welding's Wave of the Future
Tighter control of welding process variables, ease of equipment use with a high degree of process versatility, and improved weld consistency are just a few of the benefits of applying Waveform Control Technology™ to a wide range of welding requirements.
In the past, most of the guesswork in welding occurred when trying to match welding parameters to the specific material being welded. When most welding was done by the SMAW ("stick" ) process, the operator only had to worry about one variable, the welding current. As more productive wire welding processes were introduced, they brought with them the need to monitor both current and voltage. With hundreds of different material types, including high- strength, low-alloy steels, plus an ever-growing choice of electrodes, it has become extremely difficult for the operator to set specific welding procedures.
Add to this the need to control welding results more closely than ever, to achieve consistency in the strength and metallurgy of welded joints that are being held to ever-tighter standards, and it becomes obvious that much better control of the welding arc is a necessity.
To achieve these results, it became necessary to develop new power sources that would do the thinking for the operator and actually control the electrode current throughout the welding cycle. These advanced power sources blend the sophistication of computers with the power of inverter technology. They control the waveshape of the current to deliver the exact characteristics needed at any given instant in the welding process, through what has come to be known as Waveform Control Technology.
The concept of precision waveform control began in 1985 at The Lincoln Electric Company of Cleveland, OH. This breakthrough led the company's team of engineers to develop a series of inverter-based welding power sources, including the firm's most recent introduction, the Power Wave® 455 (Fig. 1). These machines are at the forefront of welding technology for pulse GMAW (Gas Metal Arc Welding). They use Lincoln Electric's patented Waveform Control Technology to control every aspect of the welding output, manipulating waveforms by sophisticated internal control software. In addition, other machine control variables are automatically coordinated, to simplify the process and improve overall quality. The operator only needs to select one variable, wire feed speed, to achieve complete control of the system.
