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PWL #026-GMA Welding Pipes, Aluminum to Stainless, Filler for Zirconium Welding, Cryogenic Materials October 01, 2005 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. GMA Welding Pipes, Welding Aluminum to Stainless, Filler Metals for Zirconium Welding, Cryogenic Materials, Clanging Sounds from the Roof 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.
Date: October 2005 - Practical Welding Letter - Issue No. 26
1 - Introduction 2 - Article: Parameters for GMA Welding Pipes (IV) 3 - How to do it well: Welding Aluminum to Stainless 4 - Filler Metals for Zirconium Welding 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Cryogenic materials 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution:Snapping Sounds from the Roof 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction Hello, here we are again, with this 26th issue of Practical Welding Letter, with articles and news and links for our readers. We hope you enjoy what you read here as we struggle to provide useful and practical information on some of the most requested welding subjects. Please pause for a moment and forward this publication to those of your friends who could appreciate its content. Thanks. Our article which follows this Introduction, is based, as three other articles that were published in previous issues, on the work of Ed Craig. It provides important information on GMA Welding of Pipes and Tubes. There is probably much to do to improve quality and economics of Pipe Welding. We would be delighted to be of substantial help. Welding of Aluminum to Stainless Steel continues to be a subject that strikes the imagination of many persons. Unfortunately severe limitations restrict the possibilities to a handful of solutions. If there is a real need for a dissimilar joint, one should at least do it well. Zirconium is possibly not a very common material. But some of its outstanding properties, in particular corrosion resistance, make it, together with titanium, the preferred selection for certain classes of applications. Besides that it is most used in nuclear reactors. Filler materials are well classified and the instructions for their selection are quite simple, limited to compatibility with base metals. For the successful use of cryogenic materials, capable of low temperature service, one should have some fundamental knowledge of the special requirements involved and of the classic testing methods employed for determining suitability of welding procedures. For the Site Update we present a new page, this month devoted to Pipe and Tube Welding. Besides the article described above we thought that the subject is so important as to deserve a dedicated page in the Website. See section 8 further down for a direct link to the new page. The information collected should prove useful to those involved in joining of tubular sections. If more detailed specific inquiries are needed please send us Your Questions and Feedback. The Contribution originated from a specific question of a reader. We hope that the information supplied solved the reported inconvenience. We would like to get comments on that. The other regular departments are in place as usual. Would you recommend our site and this publication to some of your friends? Thanks.
2 - Article: Parameters for GMA Welding Pipes (IV) This is the fourth and last article based on lessons from Ed Craig. We believe we are doing a service to our readers interested in learning from this pioneer of Gas Metal Arc Welding, who kindly gave us permission to spread from his knowledge and experience. We drew freely from Chapter Twelve, from page 435 to page 512 of his book: "A Management and Engineers Guide to MIG Welding Quality-Costs-Training". The first remark of Ed Craig, from his extensive knowledge, is that a deep lack of expertise at all levels afflicts the pipe welding industry, causing considerable losses in productivity and quality. If wrong parameters are adopted, and excessive current is selected without giving adequate attention to the soundness of the results, lack of weld fusion can be expected. The importance of logically designed procedure development and qualification (capable of revealing such internal defects) cannot be overemphasized. Moreover one should be wary of x-ray inspection, because it may sometimes miss defective pipe welds, especially in the hands of radiographers with limited experience. See on this subject a study on "The Reliability of Radiography of Thick Section Welds" Ed Craig insists that all involved should understand process, consumables and equipment. It should be appreciated that GMAW process depends on more parameters (including wire size, wire feed speed, voltage, weld travel rate, groove size, weave technique and speed, wire stick out and arc length) than do SMAW (stick) or GTAW (Tig) and is therefore more difficult to execute correctly. In particular when using short circuit transfer mode for the root pass of a rotating pipe, in order to attain consistent short circuit welding fusion, one must maintain control on the rapidly solidifying narrow weld pool, keep the tip of the wire at the leading edge of the weld pool and use controlled, consistent weave speed across the groove. The real importance of the lessons given by Ed Craig in this Chapter, are not a static list of parameters to choose from, but the clear and patient explanation of which were the results and why, of carefully planned tests. Change from short circuit to spray type and examine the results, change rotation direction and torch position, change weave technique, speed and pointing direction of torch, and again look for the results. Inadequate results merely indicate that the parameters were not optimized. Ed Craig comments negatively on some Code requirements and restrictions that impose unwarranted limitations on GMAW parameter selection. In the years passed since the publication of the book it may be that some more accepted GMAW expertise has percolated into Code recommendations. It is also affirmed that if Fitness for Purpose calculations were available for pipe welding for different applications, it would appear that a certain level of weld defects (to be determined by research) could be accepted without compromising weld performance. This opinion derives from the knowledge that many miles of defective welded pipes are known to have performed satisfactorily for years. For the root pass, practical joint weld preparation dimensions, in particular of land (the short straight portion of the joint up to the bevel) and gap, are proposed for GMAW short circuit technique. Important considerations for the root pass recommend a low energy weld capable of bridging the gap without melt trough. The importance of the weld direction is stressed. For rotated pipe welding it should be different for root and filler passes. Root pass should be welded at the equivalent of vertical down, corresponding to counter clockwise rotation for a torch kept between one and two o'clock (on a circle representing the pipe section). Fill passes should be welded at the equivalent of vertical up, corresponding to clockwise rotation, which will maximize weld fusion potential. The same favorable influence would be obtained by increasing the total bevel angle from 60 to 80 degrees. The real lesson to be learnt from Chapter Twelve, is a step by step procedure development plan called Weld Process Control Program that we will not attempt to summarize here. It is sketched in our new Website page described hereafter. We believe that this procedure should be studied and adopted by anyone involved with GMAW process, especially for Pipe welding. As indicated further down in Department 8 on Site Update, also the new Page of the Month deals with Pipe and Tube Welding. Some of the information is reported here too, but more references and links are given there. To see the page click on Pipe Welding.
3 - How to do it well: Welding Aluminum to Stainless Q: How is Aluminum welded to Stainless Steel? A: It must be realized that fusion welding is generally not suitable for welding together dissimilar materials like aluminum and stainless steels. That is because of widely different melting temperatures, no mutual solubility in molten state, and because of differences in thermal conductivity and in thermal expansion that cause stresses and cracks. During welding, low temperature melting phases and several brittle intermetallic phases are generated that compromise the integrity of the weld. Also not every aluminum type and not every stainless steel type can be considered for being joined together. However a highly localized fusion welding process of elevated power density like Electron Beam Welding in vacuum may be sometimes used, provided that a third transition metal, compatible with both base metals, is used in between. In the specific case Silver might be used as a transition element, or to bridge the gap. Solid state welding is applicable in certain combinations, providing acceptable joints can be realized that meet requirements. One of the most used of these processes is friction welding. Cleaning of the surfaces is of the utmost importance because contaminants entrapped in the joint risk to undermine its properties. For joining large parts a suitable transition hybrid element (part of which is aluminum, the other part being stainless) can be prepared, welded by friction. The ends of the transition element can then be welded to the main structure parts by more conventional procedures between similar base metals. Besides that, if alternative solution can be considered, brazing or adhesive bonding, if appropriate, are applicable.
4 - Filler Metals for Zirconium Welding Zirconium is a hard, silvery metal that is very resistant to corrosion and that is used in nuclear reactors since it does not absorb neutrons. It is a reactive metal with properties similar to those of Titanium, Niobium (Columbium) and Tantalum that are the only other metals to which it can be welded, besides itself. It is available in most wrought forms as commercial grade, containing some Hafnium (from 1 to 4.5%), very difficult to eliminate, and as nuclear grade, called Zircaloy, with minimum Hafnium traces. In preparation for welding it should be cleaned as much as possible to avoid contamination. Chemical cleaning practice can be performed per ASTM B 614. Commercial zirconium alloys are specified in ASTM B 550/B 550 M, while nuclear grades are given in ASTM B 351/B 351 M. Non contaminating welding processes in vacuum or protective atmosphere are selected for fusion welding. For resistance and friction welding no shielding is required. GTAW (Gas Tungsten Arc Welding) is probably the most used process, with Direct Current Straight Polarity (Electrode Negative), generally with high frequency arc starting. Thoriated tungsten (EWTh2) is employed. Filler metal composition is selected to match the base metal. ERZr2 welds commercially pure zirconium R60702, type ERZr3 welds grade R60704, type ERZr4 welds grades R60705 and R60706. Nuclear grades are welded with filler metal of similar composition. The oxide of zirconium is used to make heat-resistant crucibles, foundry bricks, ceramics and abrasives. It is also used as thermal barrier in Thermal Spray Applications. See hereafter 9.6. Zirconium based ceramic fiber insulation products are offered under the commercial name Zircar, for lining high temperature furnaces. (See the September 2005 issue of Advanced Materials and Processes of ASM International, page 37). A Zirconium and Niobium alloy (Zr-2.5 Nb) that is surface oxidized to develop zirconia ceramic, was developed under the name Oxinium for prosthetic implants. (See AM&P Sept, 2005 page 39). The study of a related mineral crystal called zircon (ZrSiO4) helped determine that the earth was already cool perhaps as early as 4.4 billion years ago. See "A Cool Early Earth?" in Scientific American for October 2005.
5 - Online Press: recent Welding related Articles From AWS: From The Fabricator: Get your Resistance Weld Schedule in Order From Design News: From TWI:
6 - Terms and Definitions Reminder Adaptive Control welding is performed with systems that automatically, through specific sensors, find out changes in conditions, and consequently promote suitable actions intended to take care of the modified status. Examples are the correction of welding parameters because of locally altered response, or keeping track of sudden joint differences. Consumable Insert is filler metal, usually of a well defined shape and of carefully selected composition, to be put in place before welding, i.e. between two pipe sections, to be fused in place without additional filler material, in order to provide a sound root weld. Ductility is not an intrinsic property of materials. It is an expression of the tendency of a metal to deform plastically under stress, before fracture, while absorbing energy. It is indicated by the quantitative results of elongation and reduction of area in a standard tensile test (the higher the values, the higher is its ductility). But it is important to note that under particular conditions of low temperature, of high strain (sudden impact), of stress concentrators (like notches) or of multi axial stresses, even a normally tough metal that usually produces ductile fracture, can get shattered suddenly with brittle failure. Land (also called Root Face) is the height of the straight portion of a butt joint, bevels excluded. A complete joint design has to establish its size and tolerance. Positioner is any kind of mechanical device that is designed to support a weldment and to present it to the torch or other accessory in the most favorable position for welding. In certain cases the weldment has to be moved uniformly and continuously to present new sections of the joint and proceed with the welding. Runoff Weld Tab is a piece of metal extraneous to the weldment, positioned beyond the end of a joint, to provide a place where to terminate the weld without worrying on the lower quality of the weld stop, because the tab is discarded. See also Starting Weld Tab. Side Bend Test is a bend test performed on a transversal section of a welded specimen in such a way that the weld side is stressed on the convex portion of the bend. Starting Weld Tab is a piece of metal extraneous to the weldment, positioned before the beginning of a joint, to provide a place where to initiate the weld without worrying on the lower quality of the weld start, because the tab is discarded. See also Runoff Weld Tab.
7 - Article: Materials and Welding for Cryogenic Service Vessels and pipelines designed to operate at very low temperatures(cryogenic) must be fabricated from materials and with techniques suitable to the service conditions. The problems connected with the behavior of normal steels at low temperatures were first brought to attention at the end of World War II, when eight ships of the Liberty class where involved in spectacular disasters by their sudden brittle fracture in two parts under conditions of cold weather and high seas. A link to an article on this subject is given in our page on Arc Welding Processes. The investigations that followed were instrumental in discovering a new property, unheard of up to that time, called Fracture Toughness. Furthermore it was found that, in certain materials, ductility is strongly dependent upon temperature. A special Drop-Weight Nil-Ductility-Test was introduced to help rating materials and processes and to determine the temperature above which a dynamic crack is arrested. In normal steels perfectly ductile at room temperature, a gradual passage to brittle fracture susceptibility upon impact was found, strongly dependent on decreasing temperatures. The temperature range where this phenomenon occurs has been called the Ductile to Brittle Transition Temperature (DBTT). (See hereafter at 9.3). Structures made for service at low temperatures must therefore be made of materials that maintain adequate ductility at sub freezing conditions. Cryogenic alloys include austenitic stainless steels, manganese stainless steels, 9 Nickel steels, maraging steels, titanium, aluminum and nickel alloys. A low cost way to measure the resistance of a weldment to impact fracture is performed by means of an Impact Toughness Testing on a specimen of specified geometry like the Charpy V-notch (CVN) test (ASTM E23). The amount of energy absorbed at any specified temperature during fracture is measured and recorded. This test does not measure an inherent material property but results in a relative measure of impact toughness. The results permit to rate different materials and procedures in a database to be used for design and for inspection. The specimen is oriented to have the notch and the expected plane of fracture run longitudinally through the weld metal. The test is conducted on welded specimens at the specified low temperature. The CVN absorbed energy result should be at least as specified by the requirements. Cryogenic Tanks are used for Liquefied Natural Gas (LNG), Cryogenic Ammonia and other products at cryogenic temperatures for storage and/or transportation. A list of Standards for cryogenic equipment is available online at
8 - Site Updating The Page of this Month added to our Website deals with welding of Pipes and Tubes. It is an important subject that reflects huge capital expenses for very large industry segments like power generation, oil drilling and transportation, distillation and chemical plants to name a few. The welding community is committed to provide the required quantity and quality of welds in time at a reasonable price, not a simple proposition at all. Our page provides some links to Standards and Codes, to useful information and to welding instructions that may help in developing the required procedures. To see the new page click on Pipe and Tube Welding. To see what is new in the site go to the Site Map.
9 - Short Items 9.1 - Automated welding. Implementing some form of automation in any repetitive production welding process is meant to increase productivity and decrease costs by dispensing with the employment of operators. It may be as simple as loading elements and unloading welded parts. Or it may be designed to perform actual welding functions. In general Systems are designed around a specific assembly, and may need partial manual intervention or perform their task unattended. Certain mechanized systems may include multiple torches or otherwise perform simultaneously different welding operations. Flexible automation is capable of being reconfigured to suit different but similar assemblies by the use of reprogrammable welding robots. Besides gains in productivity, automation may present an opportunity to improve also quality by providing incentives and need to limit tolerances and increase precision. Careful planning and a cooperative attitude of all involved is essential to the success of the application. 9.2 - Computational Fluid Dynamics (CFD) is a mathematical procedure to perform numerical simulations that can be used to describe graphically the evolution of transient heat transfer processes and to predict temperature distributions in space, in the presence of moving fluids. It is being applied to the study of Heat Treatment Processes with the purpose of improving the performance of furnaces and of controlling the conditions for obtaining successful heat treatments. With improvement of methods, accumulation of experience and confirmation of predictions proposed by simulations, it is quite probable that this important design tool will find increasingly accepted use. Three practical application examples are included in an article at page 44 of the last issue of Heat Treating Progress for August 2005. 9.3 - Ductile to Brittle Transition Temperature (DBTT) is an empirically determined condition, affecting certain metals, whereby the ductile properties are seen to progressively deteriorate under decreasing temperatures. Ductility is the capability of certain metals to undergo substantial deformation before rupturing, with considerable absorption of energy. Brittle behavior, on the contrary, is characterized by sudden fracture with no or minimal deformation and energy absorption. Some metals that are tough at normal temperature but fragile at low temperatures are said to undergo a Ductile to Brittle Transition in fracture behavior. Other causes besides low temperature may promote brittle fracture: high strain rate, tri-axial stress state and the presence of stress concentrators act all to promote brittle fracture. Note that intermediate fracture conditions do appear, where fracture surfaces present both a ductile region and a brittle one. 9.4 - Heating with Microwaves is a process that normally cannot be applied to metals because metals reflect microwaves. It has been recently reported however (Heat Treating Progress, August 2005, pg. 72) that a patent was awarded for doing just that, by generating a microwave absorbing plasma to surround the parts to be heated. As the plasma absorbs microwave energy, the parts are quickly heated to elevated temperatures. Plasma generation is performed at atmospheric pressure (without vacuum chambers). When the process is commercialized it will provide quick and easy new ways to heat metal parts for every conceivable process. A more detailed article on the same subject, including also the reference to the original SAE Paper, describes results on sintering of powder metallurgy preforms as well. The article is published in the September 2005 issue of Advanced Materials and Processes of ASM International, at page 52. Readers may recall that plasma was recently proposed in a different context as a "window" for performing Electron Beam Welding in air. See 9.1 in PWL#019. 9.5 - Material Characterization: The definition selected for the ASM-International Materials Characterization Handbook is as follows: "Characterization describes those features of composition and structure (including defects) of a material that are significant for a particular preparation, study of properties, or use, and suffice for reproduction of the material." This definition limits the characterization methods included in the Handbook to those that provide information about composition, structure, and defects and excludes those methods that yield information primarily related to materials properties, such as thermal, electrical, and mechanical properties. Metals Handbook Ninth Edition, Volume 10: Materials Characterization. 9.6 - Vacuum or Low Pressure Plasma Spraying is carried out in a chamber which has been evacuated to a low partial pressure of oxygen. It is then usually partially backfilled with argon to avoid the possibility of forming a glow discharge. For high temperature resistant nickel base single crystal cast, internally air cooled turbine blades, a thin ceramic thermal barrier layer is believed to keep the metal temperature at a safe level while permitting a gas temperature increase of 50 to 200 0C that improves substantially (by 6 to 12%) the gas turbine efficiency. The thermal barrier is usually composed of a first layer, bond coat, of Nickel and/or Cobalt, Chromium, Aluminum and Yttrium vacuum plasma sprayed (called MCrAlY), to minimize thermal mismatch, upon which a mixture of oxides (Yttria stabilized Zirconia) are deposited by a number of processes.
10 - Explorations: beyond the Welder Making Music by Numbers online http://tones.wolfram.com/generate/ The storm Watchers Plan To Return to the Moon by 2018 Herschel space telescope
11 - Contribution: Snapping Sounds from the Roof The following originated from a question of one of our readers. Question: What corrective measures would remedy that activity? The event occurs when the roof heats up in the morning and then cools down at days' end. Answer: When cold, the metal lays in a stable position. To simplify we will call it the concave position. Upon heating up the sheet metal tends to expand, but it is restrained mechanically, by welding or otherwise, at the borders that block its expansion. The thermal stresses generated by heating will cause its central area, that is not limited in transversal movement, to bulge, in order to reach an equilibrium position, that we will call convex.
The situation where a mechanical system is stable with minimum internal stresses, in two different positions, is called a bi-stable equilibrium. The passage from concave to convex position occurs suddenly, upon heating, when the internal stresses are just right. The reverse movement occurs upon cooling. That is the movement generating the snapping sounds. To avoid the snapping activity one has to permit the free thermal expansion of the sheet, by freeing at least two out of the four sides. Upon expanding freely, no stresses will build up causing the sheets to bulge and no sounds will be heard. In order to avoid compromising stability one should provide suitable restraints in other directions while permitting free expansion and contraction along the plane of the sheet itself.
12 - Testimonials From: "Lui, Kilun" 'Kilun.Lui@kocheng.com' Elia: Best regards,
From: "End, Fernando" 'fernando.end@epcos.com' Dear Elia, Thank you for your concern. In fact your advise until know were really very important for us, Fernando
13 - Correspondence: a few Comments Most of our readers using the form to contact us skip the introductory questions. Often the technical questions they pose are not complete, it is not clear to which material and process they refer, and the purpose of the question is left vague. I believe that a helpful answer can be given only if one understands the application and the problems. When I answer by requesting the skipped data, most of times the inquirers do not come back, as if their question lost interest, suddenly. I can only say that this behavior looks strange to me. I would always like to know if the advice was considered and, if it was applied, did it solve the problem. Unfortunately only seldom I get some feedback. Anyhow I receive sometimes thanks and comments that repay me of my efforts. Please feel free to continue to send Your Questions and Feedback but complete the form and explain your question.
14 - Bulletin Board 14.1 - The Global Powder Metallurgy Database is accessible online to everyone at 14.2 - The International 2005 SAMPE Fall Technical Conference (37th ISTC) is scheduled for Oct. 31-Nov. 3 in Seattle, Washington, USA. 14.3 - Our Website Host, SiteSell, provides a system and a complete set of tools permitting to anyone without previous Internet experience to launch a rewarding personal initiative. We are delighted of going with them. If you have a passion that you would like to spread, if you can think of something new, then we strongly recommend that you explore the following clicks: Passion, and Nutshell.
See you next time Copyright (c) 2005, by Elia E. Levi and welding-advisers.com, all rights reserved
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