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PWL#064 - Welding Stainless Castings,Copper to Aluminum,Electrode Development,HY Steels,Coat Removal December 01, 2008 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. December 2008 PWL#064 - Welding Stainless Steel Castings, Welding Copper to Aluminum, Electrode Development to reduce Mn fumes, Welding HY Steels, Coating Removal by Abrasive Water Jet 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 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.
1 - Introduction 2 - Article - Welding of Stainless Steel Castings 3 - How to do it well: Welding Copper to Aluminum 4 - Filler Metals: Electrode Developments 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - Welding of HY Steels 8 - Site Updating: Magnetic Pulse Welding, Weldability Testing 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contributions: Coating Removal 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board 1 - Introduction The program outlined in the last issue, of splitting this newsletter in two different formats, is being delayed because of technical problems. As soon as it is ready for launch, the readers will be advised of the change. This 64th issue begins with an article outlaying the problems to be addressed when approaching welding of stainless steel Castings. Depending on the main microstructure present, attention must be addressed to somewhat different hindrances likely to undermine the success of welding operations. Although it is practically impossible to cover all the problems in a short note, it is hoped that a general indication of the complexity involved is emerging, with the need to study in depth any specific case. New people are asking old questions as it appears from the queries being addressed to this website. Welding of incompatible materials surfaces again, this time with the combination of copper and aluminum. A reality check is always in proper place. Electrode development is a difficult subject. Fortunately, even if the market drive is weak, there are powerful knowledge centers that are ready to sponsor research efforts addressing low appeal issues like the care for welders' health. A short note presents a recent article summarizing such a commendable endeavor that missed the successful conclusion, although useful experience was attained in the process. Welding HY Steels used to be a hot issue, until economic forces promoted the development of new families of materials, now called HSLA Steels, for the fabrication of ship hulls and submarines with less expensive procedures. The new pages added to our website deal with a solid state process, Magnetic Pulse Welding, to be remembered even if only for subcontracting, and with development tools to be used for Weldability Testing in difficult cases. A promising new system for stripping of thermal sprayed coatings that have to be removed for repair, was described in a recent interesting article mentioned hereafter. Those who need such a process are encouraged to explore it more in depth. Other columns are where you would expect them. Section 5 in particular offers, to those eager to learn more on welding, a few links to valuable information. Your feedback is welcome. Click on Contact Us. 2 - Article - Welding of Stainless Steel Castings Stainless Steel Castings are selected primarily to withstand corrosion or to resist high temperatures. The main alloying elements are Chromium and Nickel in various proportions depending on the end use. In general Carbon is relatively low for corrosion resistant alloys and quite high, up to 0.75%, for heat resistant alloys. Stainless steels are cast for economic reasons, as the process is suitable for quite large parts. Many of the alloys that where developed are unique for casting, as deformation processes like forging would be difficult to perform. The large primary grains obtained in the cast microstructure are favorable for resisting creep at high temperatures. Fusion welding is required for repairing casting defects like shrinkage cavities, hot tears and cold shuts or for fabricating larger structures by joining the castings to other elements, cast of wrought, even of dissimilar metals if compatible. Most of the available fusion welding processes are suitable and usually quite economic for application. The selection of filler metals is such that a similar composition is selected when available, except that high nickel alloys may be preferred in certain cases. The common segregation of elements during dendritic solidification is rather advantageous in that some phases may increase creep resistance. Martensitic castings of CA-15 and CA-40 may be susceptible to quench cracking unless preheating to 200 to 315 0C (390 to 600 0F) is applied to reduce quench severity. Post Weld Heat Treatment (PWHT) is performed at 610 to 760 0C (1130 to 1400 0F) to restore corrosion resistance properties. On the contrary, corrosion resistant alloys, mainly duplex or austenitic, are not preheated to avoid sensitization or precipitation of chromium carbides, and interpass temperature is held low, even by water cooling. PWHT is needed. For each alloy the exact treatment temperatures should be checked with the supplier and adapted to the thickness of the part. It is reported that relatively high residual elements like sulfur, phosphorus and possibly silicon have an adverse effect on crack sensitivity especially when carbon is low. Of more consequence may be the presence of brittle phases (chi, sigma, Laves) that tend to form after long times at elevated temperatures. This is particularly important when considering welding repair on structures that were subject in service to high temperature for extensive periods of time. Welding should always be performed with low heat input. For joining these castings to steel parts, buttering is always required using a consumable less crack sensitive. On this subject see an interesting contribution of a reader in our page on 3 - How to do it well: Welding Copper to Aluminum Q: How can one weld Copper to Aluminum? A: Copper and Aluminum are incompatible materials that cannot be fusion welded together. They can however be welded by solid state processes that do not heat the materials to melting temperatures. Among these processes are friction welding, friction stir welding, magnetic pulse welding, ultrasonic welding, cold welding, explosion welding. After having prepared bimetal transition parts, welded by a suitable solid state process, one can proceed with regular welding or brazing of additional components by matching the materials by type (copper to the copper side of the transition element, aluminum to the aluminum side). 4 - Filler Metals: Electrode Developments Manganese fume emissions during welding operations are known to be a welders' health hazard of major concern. Techniques are implemented intended to remove the fumes from being inhaled by welders, through the use of special fume extractors (aspirators). Additionally, efforts were initiated by the Navy Metalworking Center with the purpose of developing modified, low fuming, flux cored arc welding electrodes capable of emitting 50% less Mn fumes than current baseline electrodes while still meeting the mechanical properties requirements specified by NAVSEA MIL-101TM, for the weld metal, in particular minimum yield strength of 88 ksi (606 MPa). The selected electrode size was 0.045" (1.14 mm), the base metal was HSLA-100 plate which is widely used in the construction of aircraft carriers and considered a difficult plate material to use for qualifying electrodes. Two consumable manufacturers took part in the development, each selecting his preferred approach to meet the goals. One manufacturer succeeded in reducing Mn fume emissions by limiting the Mn content of the electrode, however the mechanical properties were impaired so that minimum requirements were not attained. The other manufacturer obtained partial success but was unable to meet all the requirements. The efforts clarified the effects of Mn content on the mechanical properties of the weld metal. Despite missing the prefixed goal, the test results will prove valuable in further research. Interested readers can learn all of the details on the above development and testing program in the original article published at page 25 of the November 2008 issue of the Welding Journal. 5 - Online Press: recent Welding related Articles Note: Readers interested in learning essential information on welding are urged to download the documents from the following links (copy and paste them in your browser and ENTER) and to save them in a suitable folder for further reference. Design - Part 5 Connect - Issue 155 July/August 2008 (8 pages) Connect - Issue 156 September/October 2008 (8 pages) Alloy 625 Pipe Welding Welding News 3 - 2008 (6 pages)
6 - Terms and Definitions Reminder Electrode Holder is a mechanical device for hand holding, manipulating and providing electrical contact to shielded electrodes used in SMAW. Flush is said of two elements whose top surfaces are at the same level. Groove Angle is the total angle included between the bevels of a groove joint. Inertia Drive Friction Welding is a type of Friction Welding where the rotary motion is stored in a flywheel and then applied to the spindle holding the part. The kinetic energy is then converted to frictional heat under axial force. The weld is complete when the flywheel stops. Machine Welding is performed by tooling, equipment, power sources etc. that provide also relative motions where applicable, under constant human control. Nano Technology describes research in areas of chemistry, medicine and bulk or surface physics, based on devices of the magnitude of single atoms, within the size-scale of 1 to 100 nanometers (Nm). A nanometer is a billionth meter (10 -9 m). Pulse in resistance welding represents a current of controlled duration, whose multiples are used as necessary to describe a welding schedule. Slugging is the forbidden and unacceptable, dangerous practice of hiding separate pieces of metal in the joint, to speed up apparent production at the expense of sound welding. Generally revealed by radiographic inspection. 7 - Article - Welding of HY Steels HY Steels are a family of steels that owe their name to High Yield. This was the main property designed into the materials as they were developed in the form of thick plates, in the sixties of the past century, at the request of the US Navy, mainly for fabrication of ship hulls and submarines. Within this class, that includes HY-80, HY-100, HY-130 and HY-180, which are high strength and toughness, quenched and tempered, martensitic steels, the single materials are identified by the numbers that represent their yield strength, expressed in ksi (kilo or 1000 pounds per square inch). These weldable steels of high impact resistance require strict welding conditions to realize their characteristic potential also in the welds and in their heat affected zone (HAZ). In particular low hydrogen electrodes should be used, with due precautions in keeping them dry and heating them before use in suitable ovens to drive away any moisture. This is because the welds may be prone to Hydrogen Induced Cracking (HIC). Furthermore welding should be performed with minimal heat input, in small stringers without weaving. While the lower yield steels of this class are welded by SMAW with low hydrogen electrodes of Type E-10018 or E-11018, the higher yield ones are best welded by GTAW and Plasma arc welding with special filler metals of extremely low impurity levels (oxygen, hydrogen, nitrogen, carbon, sulfur and phosphorus). Preheating is required for the thicker plates to slow down the cooling rate after welding. This helps in avoiding HAZ and weld cracking, reducing the amount of untempered martensite, provides the conditions for hydrogen to escape, and limits residual stresses. Procedures should follow consumable supplier recommendations and be qualified by passing successfully all destructive tests required. It was soon realized that the strict conformance to precise practices, while achieving acceptable properties, resulted in low productivity and high fabrication costs, especially for large structures. Therefore renewed efforts, again initiated by the requirements of the US Navy, resulted in the development of a new family of materials called High Strength Low Alloy (HSLA) steels that allowed more economic fabrication because their modified microstructure is more tolerant of less strict welding conditions. Welding of these steels is performed, by design, with the same filler metals that were used for HY Steels. 8 - Site Updating: Magnetic Pulse Welding, Weldability Testing The Pages of this Month touch to a process and to a testing subject. The process, Magnetic Pulse Welding, has interesting properties likely to benefit mass production industries like automotive because of the impressive list of advantages that it offers. In particular it is suitable for dissimilar metals that are incompatible for fusion welding. One should keep it in mind because it is the perfect process for the right applications. Look for it. Click on Magnetic Pulse Welding for finding the details. Weldability Testing is not done every day in the shop. But when it is found that good welds are difficult to come about, one should know that there are tools available that, in the hands of knowledgeable people, can help in developing better welding procedures. Click on Weldability Testing for finding how this can help you. To remain updated with new subjects, browse through the Site Map or subscribe to our Welding Blog, showing the new and the revised pages. For letting us have your comments DON'T USE REPLY, click on Contact Us instead. 9 - Short Items 9.1 - Cavitation in a liquid is the formation and collapse of cavities or bubbles that contain vapor or gas or both, that originates from a decrease in the static pressure in the liquid. The exposure of solid bodies to cavitation results in degradation including loss of material, surface deformation, changes in properties, dimensions and appearance. 9.2 - Dilatometers are instruments for measuring the linear expansion or contraction in a metal resulting from changes in such factors as temperature and microstructure or physical state. 9.3 - Expander is a forming tool or equipment used to shape sheet metal tubular parts by radially moving segments to enlarge the diameter of selected sections to required form and dimensions. 9.4 - Feeding means providing material. In casting, molten metal has to reach any solidifying region to fill the mold cavity and to compensate for any shrinkage during solidification. In welding, a consumable electrode wire must be replenished in the torch to maintain a stable arc. 9.5 - Master Alloy, rich in definite addition elements, is added in calculated proportions to a metal melt to raise the percentage of desired constituents to the required level. 9.6 - Pack Carburizing is a method of surface enriching in carbon the surface of steel in which parts are packed in a steel box with a carburizing compound and heated in a furnace to elevated temperatures for relatively long times. 10 - Explorations: beyond the Welder Sandia, SES win Popular Mechanics Breakthrough Innovator Award Water on Mars Phishing and Anti-Phishing Life through the Microscope (Click NEXT after viewing each photo) The Cormorant Story (video)
11 - Contributions: Coating Removal An article, published at page 76 of the November 2008 Issue of Advanced Materials & Processes, the ASM International publication, introduces the readers to a new process for effective stripping of adherent thermal sprayed coatings. Coating removal from damaged components is necessary before repairing and refurbishing them. A case in point is that of expensive gas turbine engines components which, after extensive operation, need repair of the metal or of the coating. In fact, even if the metal substrate was in perfect condition, no new coating can be applied by thermal spray or by any other process upon an old coating contaminated by use, if perfect adhesion has to be assured. After explaining the drawbacks resulting from the use of classic methods, namely Acid Stripping (that may cause Inter Granular Attack) or grit blasting with aluminum oxide that removes coatings unevenly and incompletely while contaminating and distorting the surface, the authors introduce the modern process. It is based on Abrasive Waterjet Processing where speed, feeds, pressures and material flow are under control for every approved removal procedure. Two additional elements are essential in assuring the success of the operation. One is due to the fact that the iterative abrasive waterjet operation is performed by a five axes computer numerically controlled (CNC) machine tool, that guarantees high precision tolerance and repeatability. The other, interestingly, is the use of an X-Ray Fluorescence instrument to measure the content of elements in the coating as this is removed. In a specific example the content of the element yttrium (present in Thermal Barrier coatings but absent in substrate metals) is declining as the base metal is approached. See my page on Material Identification for details on this method of analysis. Therefore the instrument permits full control of the residual coating present at any stage of the removal process. The article informs that the process has been scrutinized, qualified and approved by different entities in the past five years on various parts. Interested readers are urged to obtain the above mentioned article for seeing the pictures and the micrographs attached and for learning additional details. 12 - Testimonials On Sun Nov 02 01:43:19 2008, the following results were submitted from the
"Form 5" on welding-advisers.com
On Wed Nov 05 04:06:16 2008, the following results were submitted from the
"Form 5" on welding-advisers.com: 13 - Correspondence: a few Comments 13.1 - I have a lot of inquirers who send a one time shot. No follow up, no details. My question is: why should they ask if the answer does not matter? Did they enjoy the answer when they get it? I would like to convey the notion that such a behavior is incomprehensible and quite frustrating. Readers are welcome to ask, but they should do so with respect of my time and for a real interest or need. 13.2 - An interesting query was for a standard repair procedure for a construction that had been fabricated wrong to the drawing. It needed to be cut at various locations and re-welded. As no details were given as to Code requirements if any, it could not be determined if this procedure could be forbidden by the original purchase order. However it is quite improbable that a general welding procedure (one size fits all), could be found for such a case. 13.3 - When you Contact Us, check that your e-mail is indeed correct and that your inbox is not full. Otherwise we are unable to let you have our answers.
14 - Bulletin Board 14.1 - 1st International Conference on Multifunctional, Hybrid and Nanomaterials (Hybrid Materials 2009) 14.2 - IBSC, 4th International Brazing and Soldering Conference
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