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PWL#104 - Spot Weld Lobes, Controlling Distortion, Filler Metals for Stainless Steel 316H April 02, 2012 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#104
PWL#104 - Spot Weld Lobes, Controlling Distortion, Filler Metals for Stainless Steel 316H for high temperature service, Ultrasonic Force Assisted Metal Transfer in GMAW, Stud Welding (R), Material Identification (R), Submerged Arc Welding (SAW) on Cr-Mo-Ni Steels and much more...
April 2012 - Practical Welding Letter - Issue No. 104
DON'T USE REPLY to send your messages! Use Contact Us instead. This publication brings to the readers practical answers to welding problems in an informal setting designed to be helpful and informative. We actively seek feedback to make it ever more useful and up to date. We encourage you to comment and to contribute your experience, if you think it may be useful to your fellow readers. You are urged to pass-along this publication to your friends, if you like it, and if you think it may help them. If you received this from a friend and if you like what you read, please subscribe free of charge and you will also receive a bonus book on Practical HARDNESS TESTING Made Simple. The addresses reported hereafter were live and correct at the time of their publication. There is no guarantee that they will always be so, because they are administered by the sources themselves and are under their control. Note: References to articles or other documents are given here in If they are URL's (Uniform Resource Locator), which is the analogue of an address, they begin with "http://..." or "www.". These are not live and must be copied and pasted entirely into the browser (after having selected them with the mouse or otherwise). If they are long they may be displayed in two or more lines. In that case one has to care that the URL be copied completely in a single line without any space, and Enter. If the information is important to you as we hope, you may save the selected pages in a suitable folder on your Computer for easy reference. You are welcome to forward this page to those of your friends who may profit of this information.
1 - Introduction 2 - Article - Spot Weld Lobes 3 - How to do it well: Controlling Distortion 4 - Filler Metals for Stainless Steel 316H for high temperature service 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - Ultrasonic Force Assisted Metal Transfer in GMAW 8 - Site Updating: Stud Welding (R), Material Identification (R) 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contributions: Submerged Arc Welding (SAW) on Cr-Mo-Ni Steels 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board (Sponsored Links)
1 - Introduction Hello to everybody. This 104th Issue of Practical Welding Letter is the first one built with the new and much improved SiteSell unique Block Builder 2 (in short BB2) editing tool, which will give users like me, much faster and easier capabilities. While designed to be transparent to readers, it will most probably help, once tested and used for a certain time, to present the text in more pleasing and useful formats likely to be appreciated even by those interested primarily in the content. Curious readers, included those without a cue on how Websites are built ready for the Internet, are urged to cast a glance on the presentation at SiteSell. This issue starts with an article explaining the use of certain diagrams, called Spot Weld Lobes, developed to ease finding correct parameters for preparing Resistance Welding Schedules. Those familiar with the job certainly know already how to use such aids, and may possibly contribute their experience to other readers who could profit from it. Then we discuss again the problems of distortion, affecting in certain measure most of the welds done in industrial settings. It is amazing to find out from the queries received, basic misunderstanding of the root causes, expressed generally as a request for a change of electrode or flux, as if any magic formula like that could help get rid of warping. A reminder that finding the correct filler metal for a certain austenitic stainless may not be as trivial as looking up in a table, comes from a renowned expert. A short note, based on one of his articles, may cast some light on a mysterious issue. Researchers amuse themselves by combining known technologies to test subtle influences of unusual means on classical processes. Such is the case where ultrasonic force is summoned to help in detaching droplets from the electrode of GMAW. Interesting results are found. The Pages of this Month are revised old pages referring to Stud Welding (R) and Material Identification (R). Unfortunately the revision itself is a time consuming operation. Many more pages are still in need of revision and update. I am glad to be able to publish an original article on special applications of Submerged Arc Welding (SAW) by my most competent friend Naddir M. Patel who kindly submitted his contribution to our readership. I am confident this is an informative lesson for attentive readers in search of answers on this subject. Regular columns are to be found at their usual place. 2 - Article - Spot Weld Lobes If you were given the task of developing a Spot Welding Schedule, would you know where to start from and how to go? Would you use Spot Weld Lobes? One way to express the acceptable parameter ranges is to create Weld Lobe Diagrams. Of course such useful aids are not available ready for use anywhere, but can be generated by a long and serious trial and error process. But what are Spot Weld Lobes? These are (ideally three dimensional) Cartesian diagrams, where any point in the designated volume identifies three values of Force, Current and Time, expected to generate acceptable spot welds, in a given situation completely describing equipment and joint configuration. These diagrams help to delineate the parameter limits of the relevant coordinates (Force, Current and Time), likely to provide acceptable spot welds. The need to correlate the values of the main parameters (Force, Current and Time) is caused by the mutual influence of each one on the other two, when the acceptability of any given triplet is decided by the success of the spot weld produced. Given a production Spot Welder, designated by its fabrication details and by the ranges of applicable parameters (Electrode Force, Current and Time), the conduct of any specific job is generally spelled out in its Welding Schedule. While the reference Tables, explained hereafter as a starting point for establishing welding schedules, show only a triplet of data or of ranges, suitably developed lobes modify and enlarge the selection of possible data to include many more, expected to provide acceptable spot welds in the majority of cases. In any practical case it is not needed to establish the largest and most complete Spot Weld Lobe but to get a sufficiently large window of possible values likely to be useful. In principle the validity of any triplet of values should be ascertained for every condition of the deteriorating electrodes during a normal production run. For each combination of material, condition (Strength or Hardness), metal thickness and configuration (number of sheets in the stack and position of spot welds relative to either edge), electrode (Material, Shape and Size), a unique Welding Schedule must be established, indicating the parameters that are expected to perform, most of times, acceptable welds. Other indications, like squeeze time1, hold time, current waveform and, if applicable, preheating and postheating, should also appear in the Schedule. A certain variability is tolerated for each parameter vs. the unique Schedule value, reflecting the knowledge that the sheet metal characteristics (chemical composition, mechanical properties, coating if any) may vary somewhat from lot to lot, and that the electrode surface condition is going to deteriorate after a number of spot welds, to the point that acceptable welds are no longer possible. Also subtler important data, like metal surface condition, cleanliness and fit-up, may vary from time to time, influencing the outcome in ways not readily foreseen and compensated for. For these reasons it is recognized that any suggested Schedule, available from a number of publications in suitable Tables, is only a starting point, for an industrial application, to be modified and tested in a logical way. The purpose of testing is to determine practical limits of variability of parameters assuring robust processing. The other purpose of such Tables is also to help in selecting a suitable spot welder for purchasing, well before production can be started. Keeping in mind that a whole set of parameters, used in the determination of those suggested values, have a known but not measurable influence on weld outcomes, one should not be surprised in finding substantial differences among different sources. A list, possibly incomplete, of additional parameters influencing the practical weld quality, would include electrode shape, size and condition, electrode alignment, part fit-up, electrical impedance of the equipment used, welding current profile and nature, if AC or DC. Furthermore a publication (quoted hereafter) alerts that the parameters suggested by the Resistance Welder Manufacturer Association (RWMA) chart may be found outside lobe boundaries. This is explained with the different steel making manufacturing processes used at the time the data were elaborated, and the superior quality of present time steel. An optimized Schedule is one with adjustable parameters within a normal range, (far from critical limits), that almost always guarantees acceptable results. To test the effects of change, one should keep constant one of the three (normally the electrode force), while modifying the others in small steps and keeping track of the results obtained. It is suggested to pick an Electrode Force by making sure that expulsion is avoided, and then to stick to it for most of the tests unless it is found to be too low. Then a series of tests should be performed, keeping the time constant and increasing the Current by not more than 2% at each step. A new series of tests with different Current settings is suggested with a one Time change of no more than 5%. In this way the number of tests and the results are multiplied. In practice it is usual to determine two dimensional diagrams of Time against Current, for a series of different Force values, kept constant for every set of tests, which amounts to producing slices of the three dimensional diagram. The results should be evaluated according to normal Specification requirements, such as minimum spot diameter, to be determined by metallographic examination of suitable cross section tests, and/or by tensile testing results reporting the break force value of standard shear test lap specimens. It should be understood that those parameter limits indicated above are of probabilistic nature (not deterministic), because they are affected by random, uncontrollable values of too many factors whose influence is complex and not easily quantified. A robust Weld Lobe Diagram delimits a volume (in 3D, an area in 2D) within which all values (of Force, Current and Time) have good probability of giving acceptable results. Standard procedures that can be followed to develop Spot Weld Lobes are found in:
For further reading on this subject see an article published at page 20 of the Welding Journal, March 2012. See also the article at page 469 of ASM Handbook Volume 6A, Welding Fundamentals and Processes. Note 1: A short article explaining what Squeeze Time is and how it is determined was published (3) in Issue 102 of Practical Welding Letter for February 2012. 3 - How to do it well: Controlling Distortion From time to time I get queries trying to locate parameter changes capable of reducing distortion while welding. The following, besides other queries, illustrate the kind of questions on this argument: 1) - "I need to do some extensive aluminum welding on a cast outboard block for modification purposes. I have a Lincoln square wave tig 255. I'm experienced in welding from thin Alu sheets to the much thicker ones and different parts etc. I've been told to watch for warping/distortion when welding Aluminum that extensively. my question now is what can I do to reduce these risks of distortion or warping?" 2) - "We have a storage tank (18000m3, floating roof, type of metal A283C, diameter of tank 44m approximately, height 13m). Thickness of bottom 9mm. Thickness of floating roof 7mm Thickness of shell (18,16,14,12,8,8,8)mm. When we weld the tank by using submerged arc welding technique (electrode spec. EH12K, DIA 3mm,4mm), we notice distortion in welding joint in bottom 9mm and floating roof 7mm and shell at thickness of 12,10&8 mm. Please advise me what is the proper electrode and flux to weld this tank without causing distortion in welding joint." My standard answer is as follows: "Unfortunately, distortion problems are among the most difficult and intractable of welding practice. Contrary to what you may think they will not go out by a simple change of electrode or flux. You may wish to see my page What this means in practice is that a complete description of the structure must be secured and that a logical sequence must be established to limit concentrating heat input in a limited area. This analysis is best done by an experienced welding engineer capable to calculate heat input based on used parameters and to exploit fabrication sequences so as to minimize residual stresses. In certain situations it may be possible to straighten out deformed plates or profiles.
4 - Filler Metals for Stainless Steel 316H My inexhaustible source of invaluable information is, today again, as quite often, the Q&A column on Stainless by Damian J. Kotecki on the Welding Journal. An inquirer had asked the Expert on the most suitable filler metal for welding thin-wall (1/4-in.) vessel out of 316H material to be used in service for certain testing at 1000°F. The customer had specified the following requirements for consumables: 0.04 to 0.05% Carbon and ferrite less than 5 FN. No specific reasons were offered for these requests. The manufacturer on his own was concerned with sensitization and had asked if E318 per AWS A5.4 or ER318 per AWS A5.9 would be a better selection. Damian J. Kotecki notes that the whole material 316 would have become sensitized at 1000 0F (538 0C) independent of the filler metal. But at high temperature, sensitization is not an issue, contrary to room temperature service where corrosion may develop. 318 filler metal contains some Niobium (good against sensitization, if this had been important), however this addition increases susceptibility to solidification cracking at low ferrite levels. In this respect 316H would be preferable to 318 for its improved resistance to cracking at low ferrite levels. But a better choice, called 16-8-2, is available, although it may take some search because somehow it is not very popular. With lower alloy content than the other two, it exhibits higher creep resistance and less formation of sigma phase at elevate temperatures. With reference to a modern version of the Shaeffler Diagram, namely the (Welding Research Council) WRC-1992 Diagram, the author shows that 16-8-2 (with chromium equivalent of 17) solidifies as primary ferrite with even less than 1 FN, while 316H (with chromium equivalent above 20) needs at least 3FN to solidify as primary ferrite. It is reassuring to know that knowledgeable experts are available to provide helpful solutions when needed. Contrary to their best interest, Managements prefer usually to muddle through, instead of facing squarely their lack of familiarity with new questions. Interested readers are urged to see the original article at page 14 of the Welding Journal, March 2012. 5 - Online Press: recent Welding related Articles Apple Refines Older 2008 Ultrasonic Bonding Techniques Steel stud's saga shows why VA Hospital's delays mount Advances in welding technology to lower costs The Netherlands: Damen Develops Single Weld Nozzle Spinning Method Bechtel: Ditching Waste, and Making It Safe
6 - Terms and Definitions Reminder High Pulse Time, in pulsed power welding, measures the duration of the high pulse current. Inter-pulse Time, in resistance welding, is that between successive pulses of current, within the same impulse. Lightly Coated Electrode is a filler metal electrode consisting in a metal wire with a light coating applied for stabilizing the arc. Push Welding is a resistance welding process variant where spot or projection welds are obtained by manually applying force to one electrode and using the workpiece or a support as the other electrode. Rotary Roughening is a surface conditioning method used for thermal spraying, consisting in forcefully pressing a rotating roughening tool against the workpiece surface. Spot Weld is made between overlapping sheet metal members, coalescence being produced by resistance or by local fusion Theoretical Throat is the perpendicular distance, assuming zero root opening, between the joint root and the hypotenuse of the largest right triangle that can be inscribed in a fillet joint. Welding Power Source is an implement that supplies electric energy in the form of current and voltage suitable for welding. 7 - Article - Ultrasonic Force Assisted Metal Transfer in GMAW The metal transfer modes available with Gas Metal Arc Welding (GMAW) are well known and understood. Each one of the common modes has advantages and limitations, expressed mainly in the process stability and weld quality for definite welding configurations. Short circuit metal transfer is performed with the least heat input and therefore it is mainly used for welding thin metals. Larger current values, above a so called transition current, generate globular drops and still higher cause spray transfer. An article recently published in the Welding Journal Research Supplement in the March 2012 Issue at page 91-s reports on a research effort intended to investigate the influence of superposing ultrasonic energy to increase the detaching force and to influence the molten metal droplets shape and frequency of detachment for improving weld quality. A description explains why in the regular GMAW process only increasing the current produces significant changes in the normal metal transfer modes, with results not always acceptable. It is true that by using pulsed current, with peaks higher than the transition current, one develops a higher value of electromagnetic force to detach the droplets without an excessive average current increase. The article points to certain limitations, like the need for reliable feedback signal, that may decrease the usefulness of pulsed current. Another method, consisting in applying a laser beam to help detaching the droplets is mentioned. The beauty of the proposed and tested new method, that employs ultrasonic energy for the same purpose, is that it does not increase the heat input, a most important characteristic to be held as low as possible when welding sensitive materials. The article continues with theoretical considerations, with a detailed description of the experimental setup and with exposition of test results. It was proved that the Ultrasonic assisted GMAW method improves the metal transfer characteristics with a higher metal transfer frequency, as compared to the regular GMAW. The article concludes with a program of further research work to study the effect of ultrasonic parameters on weld quality, microstructure and grain size. Interested readers are urged to seek the original article, to be found at the reference indicated above, or downloaded from 8 - Site Updating: Stud Welding (R), Material Identification (R) The Pages of this Month are Revised old pages. The first, on Stud Welding, re proposes the fundamentals of a well known and successfully applied variations of a process intended to weld short fasteners on the surface of load sustaining members. The details should be worked out after maximizing the knowledge and gathering experience by trying and learning. The other page on Material Identification explains how to proceed for identifying the essential types of materials, without which one cannot hope to perform successful welding. No shortcuts in the effort of detecting which is the metal to be welded can ever be justified. It is an essential part of the preparation, to be performed as thoroughly and as seriously as any other important item of welding practice. In case you are not absolutely certain of the Identification, don't weld or be prepared the most painful of surprises. Pages old and new can be found using the Site Map or the Index Page. Also check the new or revised additions in the Welding Blog. Let your friends and colleagues know of this website. Forward them this page and ask them to Subscribe. 9 - Short Items 9.1- Arc Gouging is an arc cutting process variation used to form a bevel or groove by melting and pushing away metal along a joint 9.2 - Barrel Finishing means improving the surface finish of small workpieces by processing them in rotating equipment along with abrasive particles suspended in a liquid. The barrel is normally loaded about 60% full with a mixture of parts, media, compound, corrosion inhibitors and water. 9.3 - Differential Heating or Cooling is a Heat process that intentionally produces a temperature gradient within an object such that, after cooling, a desired stress distribution or variation in properties is present within the object. Directional solidification is obtained by applying a strongly directional cooling gradient on cast parts like blades, to improve mechanical properties in the longitudinal direction. 9.4 - Freckling is a type of segregation revealed as dark spots on a macroetched specimen of a consumable-electrode vacuum-arc-remelted alloy. 9.5 - Gate, in foundry technology, is the portion of the runner in a mold through which molten metal enters the mold cavity. Sometimes used also to mean the entire network of connecting channels that bring molten metal into the mold cavity. 9.6 - Ironing is a forming operation used to increase the length of a tube or cup through reduction of wall thickness and outside diameter, the inner diameter remaining unchanged. 10 - Explorations: beyond the Welder Go check out what Scientists really look like Cameron Completes Titanic Solo Journey to the Ocean Floor Can Fast Reactors Speedily Solve Plutonium Problems? (3 pages) Evidence for Flowing Water on Mars Grows Stronger Small Reactors Make a Bid to Revive Nuclear Power
11 - Contributions: Submerged Arc Welding (SAW) on Cr-Mo-Ni steels Submerged Arc Welding (SAW) is a high deposition process that needs to be applied logically, with a well thought out plan of action. For Stainless Steel (SS) and any other alloy, one needs to correlate inter-pass temperature control limits with heat input calculations. If this is done, for nozzle to flange welds, or any weld groove under 10-12mm thickness, there will be no need to stop between welding runs and possibly risk air quenching and in effect poor Charpy V-Notch (CVN) values for Impact Tests. Like any welding process, the key to a successful welding procedure, one which is easily replicated on the shop floor, is proper and diligent planning. A one size fits all approach will result in defects and outright failures. With tighter material requirements coming up in the Oil and Gas and other industries, various alloys in the Nickel-Chromium-Molybdenum matrix, such as Inconel, are being specified. These high performance alloy-steels are very sensitive to welding parameters and susceptible to cracking in the Heat Affected Zone (HAZ). A small deviation in welding parameters can results in defects and discontinuities, as well as loss of impact values. Precise control of Heat Input, pre-heat temperature and inter-pass temperatures are critical to ensuring production of acceptable micro-structures and mechanical properties. Consistent temperatures, free from spikes due to ambient conditions, will ensure adequate weld strength. As cracking usually occurs at ambient temperatures, any measure that slows down the cooling of the weld, will ensure lower hydrogen levels. For the process designer, therefore, it is not just a matter of looking up the steel manufacturers’ specifications, but developing one that will suit your specific application. Thus if an optimal inter-pass temperature of 300-350 0F (166-177 0C)has been arrived at, the welding parameters need to be fine-tuned to ensure this temperature is not easily reached. In my experience I have found that welding P5 and P11 materials at a heat input between 0.9 and 1.1 KJ/mm results in the best mechanical properties. One needs to keep in mind that the Cr-Mo-Ni steels are sensitive to temperature fluctuations and overall heat input has a direct correlation with the formation of acicular ferrite. The following methodology can be used, for starters,
This is especially true if air arcing and grinding are being used. An increased, non-uniform groove size could cause the arc to wander. Thus the smallest possible arc-air electrode should be used to maintain the designed groove geometry. An electrode extension is not an essential variable in ASME IX specifications, but many welders have a habit of using extensions in excess of 1.5” (38mm), a situation ripe for arc wander. Other “Best Practices” could include ensuring that the CET (Carbon Equivalent) of the parent plate and the planned weld metal are as close to each other as possible. For certain alloy steels, the following would be applicable: CET = C + (Mn +Mo)/10 + (Cr + Cu)/20 + Ni/40 As CET can be co-related to the plate thickness, the hydrogen content of the weld metal and the heat input; one can back calculate the ideal chemistry of the parent plate and generate a customized specification for purchase. Similarly, choosing your SAW consumables (both cored/solid wire and fluxes), very carefully and with due diligence will ensure peak performance in mechanical properties. In a nutshell, Submerged arc welding is a very powerful process to be deployed for improving productivity. When using it to weld Cr-Mo-Ni alloys, attention to detail, when generating a Welding Procedure Specification (WPS) and while welding in the shop, is critical for ensuring a perfect, repeatable weld. Best practice would therefore involve providing welders with a business card sized instruction sheet that covers all parameters, specially the pre-heat and interpass temperatures and electrode extension.
PWL gratefully acknowledges the generous contribution of Naddir M. Patel on this important technical subject. Interested readers are urged to address their questions on this matter to the Author Naddir M. Patel from Raya Technical Services at the website. www.rayatechnicalservices.com 12 - Testimonials On Thu Mar 01 09:14:12 2012, the following results were submitted from the "Form 5" on welding-advisers.com: Name: Paul Ipolito
On Mon Mar 05 13:34:50 2012, the following results were submitted from the "Form 5" on welding-advisers.com: Name: Duane Wiegardt 13 - Correspondence: a few Comments 13.1 - This website intends to provide useful advice to those who need and who look for it. Here are two simple examples and the common answer valid for both: Q1: is it safe to weld on welding? Answer: If you care to explain more thoroughly your question, the answer will have a chance of being more useful. It did not surprise me that no details were ever offered in answer to my request. More often than not, even if I spend considerable time to explain why, missing essential details of the problem, the answer cannot be complete, I get no further feedback. This is most frustrating. I recently started asking my correspondents to fill in a simple feedback form at As a consequence of this regrettable habit, I am considering conditioning the right of asking any queries to the payment of a minimum fee, for the explicit purpose of weeding out unwanted, useless questions. 13.2 - With regard to the same subject stated above, I noticed that just discussing the problem with me, causes sometimes on the part of the inquirer a deeper investigation, that proves helpful to resolve the issue. This gives me some satisfaction, provided I am informed. 1 - "Thank you for your reply. Our senior engineer has resolved the issue we had question about". 2 - "I had a meeting scheduled today with an internal welding engineer but he had to postpone until Tuesday". [No further follow up as yet] 14 - Bulletin Board 14.1 - 6th International Quenching and Control of Distortion Conference 14.2 - The Energy Boom: Get on the Bandwagon 14.3 - You owe this to yourself: 14.4 - SiteSell announces the launch of the 14.5 - What people think of SiteSell? See
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