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PWL#066 - Weld Failures, Distortion, GMAW filler for AHSS, Welding Positions, Medical Devices February 02, 2009 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#066 - Welding Failures Investigations, Preventing or Limiting Distortion, GMAW filler metals for Advanced High Strength Steels, Test Welding Positions, Visual Weld Inspection, Welding Nitinol, Materials and Processes for Medical Devices and more... DON'T USE REPLY to send us 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. You are urged to pass-along this publication to your friends, if you like it, and if you want to 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. Note: References to articles or other documents are given here in one of two forms. If the links are "live" (usually underlined or otherwise highlighted) they are operated with a click of the mouse. 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 been selected 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.
1 - Introduction 2 - Article - Welding Failures Investigations 3 - How to do it well: Preventing Distortion 4 - Filler Metals for GMAW of AHSS 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - Welding Test Positions 8 - Site Updating: Visual Weld Inspection, Welding Nitinol 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contributions: Medical Devices Materials 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This 66th issue of Practical Welding Letter opens with a short note on the preliminary part of process failure investigations that every shop running into manufacturing difficulties should be able to perform in house. The hint to such a subject came to me after receiving a few questions ("why did it happen to me?") where the presentation of the facts was lacking essential elements that could have thrown some light on the causes of the mishap. Then, answering a specific question, I recall a few basic principles that govern the amount and the direction of welding distortions. As a complement of information to the subject of Advanced High Strength Steels that was presented in the last issue of this publication and also in the last Mid Month Bulletin, I call the readers' attention to the difficulties and the problems intrinsic in the selection of proper filler metal for GMA Welding of the above steels. The Pages of this Month that were added to the website refer to Visual Weld Inspection and to Welding Nitinol, this last one on a material with amazing properties. In the Contributions section there is a short review of the last supplement on Materials and Processes for Medical Devices, a subjects that commands increasing attention from medical personnel, high performance material suppliers and engineers of specialized manufacturing companies. The other columns can be found at their usual place. Use the Contact Us form to let us have your comments and feedback. (Don't use Replay). 2 - Article - Welding Failures Investigations When you run into a production welding failure you may need to check carefully all the factors likely to influence the faulty outcome of your efforts. Of course you know your job, but some minor inadvertent changes can produce mighty unwanted effects. Before rushing for expert advice, there are a few checks you should make to find out if any obvious causes contributed to the mishap. The complete metallurgical investigation of any failure is a complex project that demands knowledge, experience and a whole lot of testing equipment in the hands of professional metallurgists. Not always is a complete investigation required though, especially if the immediate task is only to repair the failure. It is essential however to achieve the most complete information on the faulty process if it is required to find out the probable causes in order to correct them and restart production. For a limited purpose a simple analysis may hint at what went wrong and suggest how to overcome the hindrances. In any case it is always advisable to collect information and to look for suspicious signs. Here I would like to suggest a few general recommendations that might help to give an idea of what happened. First look for changes. If you know that until some time before, there were no hints of difficulties, and production went on normally, then look for anything, including negligible details, that may have since changed. Suppliers, materials, consumables, equipment, routine maintenance, tools, fixtures, workers, helpers, spares, parameters, location in the shop, time of the day, weather, temperature and any further detail not included in this list. Don't overlook anything at this stage. If you find a difference, don't dismiss it as unimportant. Weld again with and without the change, see if there are any influences on the results. Be suspicious with material changes, even if nominally the new one (possibly cheaper) should be exactly the same as that used before. Subtle changes can bring about large differences. Then check the equipment working condition. Maintenance people should see worn out or partly damaged items that must be replaced. If measuring instruments were not calibrated recently, they should be calibrated now, otherwise there is no way to know if the applied parameters are correct. Check again your Welding Procedure Specifications. If you lack such a document, this is the time to establish one, to have all details written and fixed. Maybe someone decided to cut corners, to skip essential preparation steps or checks, in order to save time, to increase production. If it is about new production, never attempted before, then one should be even more suspicious. Maybe the design is questionable. Someone should ask the engineer if the same implementation has been in use elsewhere. Maybe process development should be attempted before going into production. Finally, if you had personal experience of failures that you were able to analyze and correct, you are invited to tell about them to your fellow readers, we all want to learn. Send your contribution to Welding Talk. 3 - How to do it well: Preventing Distortion Q: I have a question regarding welding angular distortions in butt-welded stainless steel plates. I have done some experiments for plates with dimensions 150*200*2 mm, but something strange has happened. Some of my plates have distorted upward but the others have distorted downward. The welding conditions have been the same for all of them. A: Distortion is the consequence of residual stresses set in by welding. Especially for manual welding, it is almost impossible to determine if welding conditions were or not exactly the same. Taking into account that distortion will be always greater in stainless steel than in regular carbon steel, to reduce or prevent distortion in the simple setup described in the question above one should consider the following recommendations. Use a joint configuration based on welding from both sides (X-joint), requiring less filler material and less heat input, instead of a simpler design welded from one side only (V-joint). The recommended configuration above is also more symmetrical than the discarded one, and therefore it is likely to introduce more balanced residual tensile stresses. Reduce as much as possible the root opening and, if applicable, the bevel angle, again to reduce filler material and heat input. Restrain the plates in a heavy fixture and introduce compressive stresses by peening the joint with a hammer while the weld bead is still hot. Preset the plates at an angle before welding (open like a book downward, more than needed to have the pages flat in the same plane). Residual stresses will pull the plates back in the same plane after cooling down. Try to use block and back-step progressions to balance and reduce residual stresses and resulting distortions. 4 - Filler Metals for GMAW of AHSS Readers may recall that in the last issue of this publication, PWL#065, a general introduction was presented of the new classes of Advanced High Strength Steels, specifically developed for the automotive industry, in view of improving collision safety (crashworthiness) while reducing vehicle weight for improved fuel efficiency. In the Mid Month Bulletin PWL#065B, selected references were given to online publications on the same subject. Welding can influence remarkably the basic mechanical properties and collision performance of these steels. In particular, depending on material composition, thickness and welding parameters, the heat affected zone hardness of fusion welds can increase or drop without connection to base material strength. The selection of suitable filler metals for Gas Metal Arc Welding (GMAW) of AHSS is not a simple proposition. Filler metal of type ER70S-6 exceeds mild steel strength but may be much weaker than AHSS base metals. Higher strength electrodes like ER100S-G, closer to AHSS properties, may need special techniques to control heat input. Otherwise the weld metal may result too hard and brittle. The research indicated hereafter can throw some light upon the selection options available to designers. The problems are connected also to base metal properties and to welding parameters. Two filler metals were employed: a nominally low strength electrode, ER70S-6 and a nominally high strength electrode ER100S-G. It was found that, for each condition of cooling rate and joint geometry, filler metal strength did not affect the static or dynamic tensile properties (joint strength or ductility) of the AHSS base metals used in the program. The article on the referenced Research is titled: Furthermore, when preparing for production, one has to consider that other problems too may affect the outcome of welding AHSS, in particular thinner sections and increased spring back relative to mild steel, affecting the tendency to burn through and tolerances of alignment. Therefore testing and qualification of procedures cannot be disregarded. 5 - Online Press: recent Welding related Articles A Shielding Gas Primer for GTAW TWI - Connect - November/December 2008 TWI - Welding costs 12 Tips for improving Welding Safety Challenges and Solutions 6 - Terms and Definitions Reminder Flare-Groove Weld is the joint done in the groove between two curved surfaces or between a flat and a curved surface of two members whose flat portions are making contact. Getter is a reactive material, added on purpose within a furnace, for its capacity to absorb traces of oxygen and permit successful processing. Goggles are a kind of protective dark spectacles used to reduce the intense light of oxyacetylene flame. Laser Beam Splitter is an optical device used for dividing a laser beam in two for separate manipulation of the two beams after the split. Multiple Welding Position is that continuously changing encountered when welding in a vertical plane the circumferential joint of a fixed horizontal pipe. Static Electrode Force is the measurable compression load exerted by electrodes of a resistance welding machine on the workpiece, in the absence of current and of other movements. Welding Test Positions represents the orientation of a weld joint set up for welder qualification testing. Wire Feed Speed is the length of wire supplied per unit time in any welding or other process that consumes wire continuously. The measured length of wire running for exactly ten seconds, when multiplied by six, gives the wire feed speed expressed in length (meter or feet) per minute. 7 - Article - Welding Test Positions A few queries were addressed to this website concerning welding positions. According to the American Welding Society the welding position describes the relationship between the weld pool, joint, joint members and welding heat source during welding. The following exposition refers to arc welding with covered electrode. Other welding methods may need adjustments in the interpretation of results. The following welding positions are established for the tests performed by welders, towards their qualification. See the following Tables.
The flat position is used to weld from the upper side of the joint at a point where the weld axis is approximately horizontal (+ or - 15 degrees), and the weld face lies in an approximately horizontal plane. It is the easiest welding position even with relatively high current and should be always preferred whenever possible. The joint and the members to be welded lay flat. The force of gravity keeps the molten metal in the puddle until it solidifies. EN code: PA. AWS code: 1G. The horizontal position is that where the welding axis is approximately horizontal. For a tube or pipe standing vertical, the circumferential weld is performed in horizontal position. This horizontal welding test position for a tube or for butt weld plates is called PC for EN code, 2G for AWS code. The horizontal fixed position called also multiple welding position refers to the circumferential weld on a non rotating pipe or tube lying horizontally. The welding position changes continually and requires specific skill for successful completion of the joint. Horizontal rolled position is equivalent, for a horizontal tube or pipe, to the flat position, in that the weld pool is stable and in the best location. This is also known as flat testing weld position for a tube and is called 1G. The Inclined position indicates a weld axis anywhere between 15 degrees from the horizontal up to 80 degrees. It maybe a challenge for the welder to keep the weld pool from running away and liquid slag from being embedded into the solidifying mass. The inclined test position for a tube or pipe, not rotated during welding is again called the multiple welding test position 6G. The pipe axis for the EN code has an inclination of 45 degrees. Position HL045 refers to upward progression, JL045 to downward progression. If the pipe or tube is not rotated and the inclination is fixed but a restriction ring is placed near the weld, this is called the Multiple Welding Test Position with Restriction Ring and is called position 6GR. The Overhead position is performed from the underside of the joint. This is possibly the most difficult welding position, requiring a careful balance between gravity force and molten liquid viscosity. The current must be reduced to limit the mass of liquid metal at any given time and the scale should be of the fast freezing type. For butt welding overhead symbol is PE for EN code, 4G for AWS code. For fillet welds overhead symbol is PD for EN code and 4F for AWS code. The Vertical position has the weld axis approximately vertical. Actually it could be between 80 degrees and vertical. The welding progression can be from bottom to top (vertical up) or from top to bottom (vertical down). The vertical down is faster, and is sometimes preferred by skilled welders even when they could use flat position. Furthermore, for thin sheets it may have the advantage of reduced risk of burn through. Vertical welding can be used for welding rotating tubes, either down or up, depending on rotation direction and on location of the arc. Vertical up requires skillful manipulation of the electrode from side to side for depositing a thin layer upon which new layers will be deposited. When welding of fixed tube or pipe, the arc will move through all positions in succession. The skill required must adapt the welding technique in real time to the actual position. In practice, for large pipes, the circumference is divided in four or six sectors, caring to weld in succession opposite pairs to reduce residual stresses. For more information see the article 8 - Site Updating: Visual Weld Inspection, Welding Nitinol Two Pages of the Month were recently released in our website. The first one, on Visual Weld Inspection, explains the difference between two different aspects of this inspection. Good preparation and wide experience are among the important characteristics of Visual Inspectors. The second page, on Welding Nitinol introduces the amazing capabilities of this material and explains how welding is performed. To stay updated on new subjects as they are added to the website, browse through the frequently revised Site Map or subscribe to our Welding Blog, both showing the new and the revised pages. See details under the NavBar, in the left column of every website (www.welding-advisers.com) page. Let us have your comments! DON'T USE REPLY, click on Contact Us instead. 9 - Short Items 9.1 - Failure is a general term meaning that a fabricated part or assembly is not fit for its intended use because of faulty manufacture or because of damage occurred in service. 9.2 - Piercing is a general term to indicate cutting (shearing or punching) openings, such as holes and slots, in sheet material, plate, or parts. The removed material is collected if useful or scrapped if useless. 9.3 - Principal Stress (normal) is the maximum or minimum value of the normal stress at a point in a plane where the shear stress is zero. There are three principal stresses on three mutually perpendicular planes. The state of stress at a point is uniaxial if two of the three principal stresses are zero, biaxial, if only one of the three principal stresses is zero, and triaxial if none of the principal stresses is zero. 9.4 - Temper Embrittlement of low-alloy steels is caused by holding within or cooling slowly through a temperature just below the transformation range. This is generally 300 to 600 0C, or Embrittlement is the result of segregation at grain boundaries of impurities like arsenic, antimony, phosphorus and tin. It is usually found as an increase in ductile to brittle transition temperature. 9.5 - Terne is a lead alloy containing 3 to 15% Tin, used as a hot dip coating for protecting steel sheet or plate from corrosion, and to enhance its ability to be formed, soldered or painted. 9.6 - Thermomechanical Working (or treatment) is performed by processes combining controlled thermal and deformation treatments for obtaining improvements in strength without loss of toughness. 10 - Explorations: beyond the Welder Apollo 11 Anniversary Stirs Memories Spent Nuclear Fuel Understanding Consciousness 9 Ways NASA Can Tackle Climate Change Science minister wants focus on fewer disciplines
11 - Contributions: Medical Devices Materials Advanced Materials and Processes, the monthly magazine of ASM International, that organizes international conferences on Materials and Processes for Medical Devices, publishes regularly an insert dedicated to new applications and success stories. Medical devices enjoyed accelerated development in the last two decades, when considerable research was concentrated in solving specific problems of the medical profession through suitable or improved implants made with old or new materials. In the January 2009 issue, classic and new biocompatible materials, offered by specialized companies, are reviewed. Among the materials, the following are listed:
Furthermore some news are presented reporting on experimental materials, composites or nanostructured with special coatings for building an interface free of protein buildup between human tissue and medical devices. Also zirconia toughened alumina (ZTA) reinforced with platelets is introduced. The new material is reportedly strong and tough with improved resistance to advancements of microcracks. Other news report on improved methods developed for producing titanium powders less expensively, to ease the production of sintered products assuring higher resistance against progression of corrosion. Nanocrystalline hydroxyapatite granules in collagen biopolymer encourage bone ingrowth, important when superior bone generation is required. A special Welding process that was developed to join nitinol tube to stainless helps to build a cost effective catheter device. Finally a cobalt-chrome alloy was selected for a spinal implant of new design. Whoever is involved in this modern quest for ever improved medical devices, should know where to find information and whom to ask. 12 - Testimonials Date: 02 Jan 2009, 10:18:23 AM On Sat Jan 03 01:40:09 2009, the following results were submitted from the "Form 5" on welding-advisers.com: Name: Hassan Kheir 13 - Correspondence: a few Comments 13.1 - We have no difficulty in referring readers to the pages of the website showing the answers to their questions. It seems however that readers did not take seriously the recommendation included in the Contact Us page. There we invite everyone to save time by performing an easy check as follows. Before sending in your contact Questions, please Questions are welcome... but prior screening is even better! 13.2 - I received a letter from a Manufacturing Engineer who "was made redundant" and therefore was not interested anymore in continuing getting this publication, (I don't see the connection...) despite the fact that it comes at no cost. I expressed my sorrow and my sympathy at the sad news, and the view that Manufacturing Engineers will still be required so that a new job might become available in the near future. Nevertheless, to all unemployed people who may read this page, but also to all those who retired or are retiring some time in the future, I would like to recommend out of personal experience, to explore the avenue of starting a small business on the Internet. The amazing thing is that anyone can do just that. I told about that in my page on Create your new Job that I invite all to look at. I think that it might be helpful. 14 - Bulletin Board 14.1 - WESTEC 14.2 - 12th Aluminum Welding Conference 14.3 - Build your own University
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