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PWL#048 - Materials Selection, Robot Breakdown, U_water Filler Metal, Shielding Gases, Surface Engrg August 01, 2007 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#048 - Materials Selection for Product Design, Welding with Robots, Filler Metal for Underwater Welding, Shielding Gases, Surface Engineering, Welding Defects in Stainless Steels and more... 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 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.
1 - Introduction 2 - Article: Materials Selection for Product Design 3 - How to do it well: Welding with Robots 4 - Filler Metal for Underwater Welding 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - Shielding Gases 8 - Site Updating: Surface Engineering 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Welding Defects in Stainless Steels 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction We open this 48th issue of Practical Welding Letter with a short article on Materials Selection. It used to be an almost straight forward routine. Armed with the last edition of some Engineering Handbook, the Chief Engineer, a charismatic figure with loads of experience, would select from a restricted group of materials the most suitable for the project at hand. The lucky selection would have then appeared in the corner of the drawing, sometimes with an additional note, and the signature of the staff members and of the Chief Engineer would have sanctioned the choice. No more. A single person, however endowed with knowledge and experience, can no more be presumed to master the exploding variety of materials, processes, temper and finishes, of joining methods and of compatibility for harsh service conditions more and more difficult to meet. Even if all physical and mechanical properties were tabulated in well constructed databases, not an easy task, it would still be exceedingly hard to find the information needed. One needs a team work of specialists of just some important aspect of Materials Engineering and a logic procedure capable of singling out the best selection or a few possible options. Most of times, for the design of critical performance items, one has to design a special test sequence capable of confirming or of questioning the initial selection. All this new work is being attempted only in recent times and applied in the most advanced fields. Our article gives only a glimpse on what is being done and offers a few links for further investigation. Next we were prompted to offer our thoughts by the complaints of a reader who operates robots for welding but suffers form repeated breakdowns. Who is to blame? What can be done to enjoy the promise embedded in modern robot cells? Another reader asked for information on filler metal for underwater welding. We found a few answers, although there is no off the shelf prescription to cover all situations. The good welder will find, with study and hard work, what suits him/her best in any given case. Shielding gases for welding are complex physical systems whose characteristics influence heavily the results of welding operations. We hope to provide useful indications with the help of a few references that suggest recommended selections. Our Page of the Month deals with Surface Engineering, a collection of processes that help maintain the surface of any item sound and operational for a long service life. Wise selection and correct application through study and practice make the difference between long time service success and premature failure. In the Contribution section we point out a recently published thorough article that we recommend to interested readers on Weld Defects in Stainless Steels. It is suggested reading it, learning it and keeping it handy for quick reference. Other sections will be found as usual. What about letting us have your comments and feedback? Use the form, click on Contact Us. 2 - Article: Materials Selection for Product Design Mechanical Design requires the integration of many disciplines. Although traditionally the responsibility for it stood on the sturdy shoulders of an engineer with outstanding theoretical preparation and many years of experience, it is more and more difficult to find such a universal person familiar with so many aspects of the engineering profession. It is now not only impractical but even uneconomic to rely on a single professional however endowed with knowledge, wisdom and understanding. It is therefore necessary to rely on team work that has to be guided and controlled so that the focus will always be kept on the important requirements of the project taking care that every decision be explained and documented. An important part of the Design process concerns the Selection of Materials. From a larger publication called Design Guidelines of the Precision Metalforming Association (PMA), the chapter on Material Selection, limited to metals, presenting data and useful information is available from With the multiplication of options now available, material selection has become a more and more difficult task. However in the last few decades there is a concerted effort to establish procedures and databases suitable for software assisted help in the critical selection stages. The Department of Engineering of the University of Cambridge has been busy for many years in all activities designed to formalize the process of product design and material selection. From their site Furthermore a simple Tutorial with specific examples is offered A more formal and complete exposition of the suggested process is presented in the paper Materials Selection in Mechanical Design that can be downloaded from New software for material selection is now available for institutions needing access to the tools that simplify and speed up continuing design activities. There are other, unexpected and provocative ways to look at material selection. What about personality? Or character? Or perceptions? We recommend reading Materials and Product Design by Mike Ashby at http://stargate.uwaterloo.ca/~jzelek/teaching/syde361/designpaper.pdf 3 - How to do it well: Welding with Robots Q: - I have a setup of 6 Mig Welding Robots for welding Car Seats Frames. I have lots of down time because of Arc outs, changing tips. It gets worse during cold weather. What can be done to improve? A: - It is not uncommon for industry at large to experience breakdowns of robot lines. One way to overcome the difficulties is to grow in-house expertise of a few technicians. That is done either by sending them to follow special Mig (GMAW) courses or by hiring an instructor for the needed time. The personnel involved should master welding expertise before starting to program robot cells because a sound theoretical background in GMAW is essential. It is usually recommended to develop first a robust manual welding procedure for the robot parts, and then to prepare the robot program while adapting the optimized techniques used by the manual welder. This assertion, although quite true, should be improved by making sure that the basic manual welding cycle is cautiously but firmly modified in order to exploit the robot increased productivity capabilities relative to manual processing. In particular, while assuring quality as required, one should make skillful use of the parameters that robots can accommodate like larger wire size, higher current, higher weld speed and swift relocation between welds. Attention should be paid also to control of part distortion, minimizing spatter and establishing the most suitable sequence. It would be a costly mistake to entrust the job of running robot cells for GMAW to personnel skilled primarily or only in robot programming. To achieve the maximum benefit from robot installations, Management should understand the special requirements of robot cells in welding environments. Among these are parts design, part and gap tolerances, fixture design, weld process expertise and quality of the operating programs. Furthermore expert maintenance personnel should be readily available whenever needed.
4 - Filler Metal for Underwater Welding and Cutting One of our readers asked for information on specific underwater wet welding electrodes, and that set us in action to assemble the answers here. Filler Metal Electrodes suitable for underwater wet welding have much in common with regular electrodes used for surface welding (SMAW), with a few additional twists that make them capable of doing the job under water. A comprehensive treatment of the subject of Electrodes for Underwater Welding is found in Chapter 5 of the Book "A Welder's Mate" by David J. Keats that can be ordered online from our page Underwater Welding that includes a short explanation of the processes. The various functions of the electrode flux covering are explained to some length in the above chapter, the most important feature being of course that of providing a protective gas environment around the arc to allow the fusion of base and filler metal and to protect the solidifying material from external contamination. This is even more accentuated in underwater welding where water and its dissociation products must be kept out of the protective bubble. The following article discusses the flux composition for Covered Electrodes in general: The specific flux composition is left to the manufacturers who, while applying common knowledge principles, may adapt them to their preferences. David Keats in his book concedes that most of the electrodes offered commercially for underwater welding may be satisfactory for practical applications. The final selection, in his own words, appears to be a matter of personal ease of use. Surface tension, viscosity and melting point are characteristics that can be manipulated by blending in the flux the right amounts of the various materials. The correct combination of flux constituents with proper welding technique permits to deposit successful weld beads in the positions required and to control penetration and final aspect of bead surface. The display of the required mechanical properties, ductility in particular, is attained by the best electrodes available. In his developments David strove to provide electrodes usable by underwater welders with limited welding experience even in cases of low visibility. Electrodes for underwater use are waterproofed before being packed. However, good storage practices must always be applied per manufacturers' recommendations to avoid entrapping additional moisture in the flux. TWI has long experience with underwater wet welding. An article from a commercial company describes a program of qualification for Class A Welding according to AWS Specification D3.6 The following document, See also: The current specifications for covered electrodes are the following: ANSI/AWS A5.1/A5.1M:2004 BS EN ISO 2560:2005 BS EN 1011 5 - Online Press: recent Welding related Articles As advised in our last issue, our article See also from TWI: From TWI: From ESAB - Articles on Stainless Steels
6 - Terms and Definitions Reminder Arc Voltage is the voltage measured between the electrode and the work across the arc of a welding set-up. Austenite is an interstitial solid solution of one or more elements, usually Carbon, in face-centered cubic iron (gamma iron), normally stable at high temperatures unless stabilizing elements like nickel are present that keep the structure at room temperature. Cementite is the hard and brittle compound of iron and carbon, known as iron carbide and having the approximate chemical formula Fe3C. It is characterized by an orthorhombic crystal structure. Martensite designates diffusionless phase transformation microstructures occurring upon cooling quickly from elevated temperatures. If the solute atoms are interstitial like carbon in iron, the structure is hard and strained. If the solute atoms occupy substitutional positions like nickel in iron, martensite is soft and ductile. Plasma Spraying is a Thermal Spray process whose heat source is a non transferred plasma arc. The powder metal fed into the arc is fused and thrown with force against the substrate unto which it adheres. Resistance Seam Welding is a process, applied to overlapping sheet metal elements, that joins them together across the meeting surfaces by creating overlapping resistance weld nuggets under rotating electrodes pressing the sheets forcefully against each other, or by forging the resistance heated edges continuously as in formed and welded tubes. Short Circuit Metal Transfer is the low current method based on melting drops of the electrode during short circuits between electrode and work in GMAW. Tig (Tungsten Inert Gas) Welding is an old non standard name for Gas Tungsten Arc Welding (GTAW).
7 - Article - Shielding Gases Essential variables, as defined in AWS D1.1 - Structural Welding Code, must be specified in any Welding Procedure Specification (WPS) in order to maintain a stable and consistent welding process. For GTAW, PAW and GMAW, these variables include voltage, amperage, electrical stick-out, shielding gas and others. In particular, regarding Procedure Qualification Record (PQR), AWS D1.1 requires requalification when "A change in shielding gas from a single gas to any other single gas or mixture of gases, or in the specified nominal percentage composition of a gas mixture, or to no gas" is introduced. AWS A5.32 "Specification for Welding Shielding Gases", prescribes the requirements for shielding gases. In Europe the applicable specification is EN 439 "Welding Consumables - Shielding Gases for Arc Welding and Cutting". ANSI/AWS A5.32/A5.32M-97 (R2007) BS EN 439:1994 Shielding gases are used to avoid contact of molten and hot metal with the atmosphere while welding. Otherwise oxidation products and other discontinuities could degrade the quality of the process being performed. But the gas enveloping the arc column has other equally important effects on
Therefore while developing a WPS for a given job, attention should be given to the possible effects of changes in shielding gases on the results obtained. Originally only inert gases like Argon and Helium were used. Active gases were later on used either alone (like CO2) or as small additions (like O2) with remarkable effects. Ionization Potential and thermal conductivity of the gases influence arc start and stability. A thorough and informative article on Shielding Gases for GMAW is found in pages 12 to 15 of the publication "GMAW Welding Guide" from Lincoln Electric, that can be downloaded from Please note that this publication, in its 96 pages, includes much more information on GMAW, besides shielding gases. It is therefore a highly recommended reference for serious study. Another suggested reference on Shielding Gas Selection can be found in the page The page Shielding gases for arc welding and cutting gives guidance to the european classification of Shielding Gases per BS EN 439. A page giving some guidance for selecting Gas Selector Software was developed at TWI based on expert knowledge gathered from different sources. When presented in 2002, see A review of its usability from the part of an inquisitive researcher could provide the confidence needed for spreading the interest in its use. It may represent a significant progress when it becomes fully operational. 8 - Site Updating: Surface Engineering The Page of this Month concerns Surface Engineering, a most important subject that needs thorough attention to make sure that the necessary characteristics of resistance to environment and of durability are conferred to the items designed. The surface can be a weak point, but not necessarily. The designer can impart exceptional qualities to this interface, with the purpose of outperforming what was known and used before and of extending the useful service life of the item. See the new page by clicking on Surface Engineering. Look for new pages in the whole website by clicking on Site Map. Comments and feedback are welcome. Click on Contact Us.
9 - Short Items 9.1 - Intermetallic Compound is an intermediate phase in an alloy system with a narrow range of homogeneity. Atomic binding can range from metallic to ionic. 9.2 - Isotropy is the condition whereby properties have the same values in all directions. The opposite is Anisotropy, where properties change according to the direction in which they are measured. 9.3 - Knockout is the removal of sand cores from a casting or, for investment casting, the removal of the casting from the mold. 9.4 - McQuaid-Ehn Grain Size is a procedure used to determine the previous austenitic grain size developed in steels. It consists in carburizing under specified conditions followed by slow cooling. In the subsequent metallographic examination, the austenitic grain sizes are outlined by the white cementite network, permitting rating per ASTM. 9.5 - Notch Ductility is the measured percentage reduction in area after complete separation of the two sides in the tensile test of a notched specimen. 9.6 - Press Quenching is a quench process performed in a quench press with the austenitized (hot) part held between two dies. The pressure helps keep the part in shape while quenching with flowing oil. 10 - Explorations: beyond the Welder Fire Information - Science and Technology On Climate Change Readings on Perennial Grain Cropping Research Virtual World with Secondlife Writing Help Online
11 - Contribution: Welding Defects in Stainless Steels Anyone involved with welding Stainless Steels is advised to obtain a copy of the Article Avoiding Defects in Stainless Steel Welds by Richard D. Campbell, at page 56 of the May 2007 Issue of the Welding Journal. One can find, besides a short description of stainless steel types, a thorough list of the main defects that can occur in such welds, a note on how to recognize them and an explanation on what should be done to avoid them. A thorough reminder on Sensitization is included followed by the methods used to avoid it. Corrosion issues are dealt with, and practical advice is given on how to select suitable filler metals to resist corrosion. A discussion of distortion and its causes follows, with explanation of the effects of thermal conductivity and thermal expansion. Further attention is given to the problem of weld penetration and how to control it. One can find also reference to the sulfur content problem and its influence on penetration. Our readers will recall having read a complete article on this subject in section 11 of issue 45 of our Practical Welding Letter for May 2007. Finally Hot and Cold Cracking are addressed with practical indications of which welding conditions might affect the appearance of cracks. In the conclusion the precaution to maintain cleanliness in all stages to promote defect free welds in stainless steel is stressed again. We recommend this article to the attention of all concerned. 12 - Testimonials From: Eli Minoff (E-mail address removed for security) Excellent Issue.
Best regards,
From: C Sridhar (E-mail address removed for security) Thank you Elia Levi. 13 - Correspondence: a few Comments 13.1 - I was surprised to receive a message from a reader proposing to translate parts of my website into a foreign language for the purpose of benefiting some readers that could profit from a translation. I think it would be a nice initiative. 13.2 - Some of the readers' messages find their way into PWL because they originate an article or a note. I would appreciate real contributions from readers' experience. I hope soon to be able to include a new form here facilitating expression and correspondence. 13.3 - A word of caution. My answer to a reader bounced back undelivered. Either the e-mail address was incorrect or the inbox was full. I have no other way to reach that reader if not by mail or from my website. Please be careful when you give your address. And if you get no answer try again, possibly from another address.
14 - Bulletin Board 14.1 - Weld Cracking VI Conference October 16 - 17, 2007 Imperial Palace Hotel, Las Vegas, Nevada USA www.aws.org/conferences 14.2 - FABTECH International & AWS Welding Show Nov 11 - 14, 2007 McCormick Place, Chicago, Illinois USA www.aws.org/show 14.3 - What did they do with their Passion
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