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PWL #033-PQR, Weld Ti to SS, Creep Resistant Filler, Weld for Gas Turbines, SAW tips, Electrolysers May 01, 2006 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. Procedure Qualification Record, Welding Ti to SS, New Creep Resistant Filler Metal, Welding and other Processes for Gas Turbines, Submerged Arc Welding tips, Electrolysers 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.
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Practical HARDNESS TESTING Made Simple.
1 - Introduction 2 - Article: Procedure Qualification Record 3 - How to do it well: Welding Titanium to Stainless Steel 4 - Filler Metal for new Creep Resistant Steels 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Gas Turbines Welding and other Processes 8 - Site Updating: Submerged Arc Welding Tips 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Electrolysers 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This 33rd issue of Practical Welding Letter continues the subject that we started last month on welding documentation, with a presentation of PQR, the Procedure Qualification Record. Although the details may vary among different Codes and Specifications, the document format is widely accepted as a required proof that the procedure to be employed was tested and found suitable to produce acceptable welds. Questions on welding dissimilar metals come up time and again. Now it is for welding Titanium to Stainless Steel. Are we going to disappoint somebody? The process of developing new steels suitable for specific purposes and to adapt filler metals to weld them successfully is quite interesting. We were fortunate enough to fall upon a report that throws light on the efforts being dedicated to that research. Welding and related processes have an important part in manufacturing and repair of Gas Turbines that use advanced and specialized metals. An overview is presented. This months update offers Tips for Submerged Arc Welding, a high productivity process that has many benefits when used properly. We are glad to include a Contribution on Electrolysers, that provide Hydrogen and Oxygen from water at the point of consumption in many industrial processes. Other departments present their information in the usual form. In the Correspondence Note, we would like to urge our readers to do a short research before submitting too general questions. We indicate how. For your feedback and comments please use the Contact Us form, from the Contact Page (Opens a new page), reached also when one clicks on the button of the same name in the NavBar at top left of each website page. 2 - Article: Procedure Qualification Record This kind of documents (PQR) was treated briefly in the last issue of PWL ANSI/AWS B2.1:2005 ANSI/AWS B2.2-91
The PQR document, listing the actual test results, represents essentially the needed demonstration that the implementation of the Procedure, to which the PQR refers, is indeed capable of producing welds that, when tested, meet Code or Specification requirements. This proof is taken to mean that the said Procedure is capable to produce in manufacturing sound welds of adequate properties. The manufacturers of welded structures are responsible for the procedures they use. Even for weldments not covered by Codes or Standards, it is advisable to prepare the forms to prove that the processes employed were qualified to meet requirements specified in an internally issued engineering document. The purpose is to be ready against future possible Welding Liability issues. It is understood that the test weldment does not simulate in every characteristic the actual manufacturing conditions of the real job. To make up for the differences, the instructions given to the welders should include such additional details as necessary to assure the adequacy of the production to the service conditions accounted for in the design process. The tests for Procedure Qualification are themselves the object of detailed Code or Specification demands as to type, number and results required. They are selected to provide information on strength, hardness, ductility, toughness and additional properties. In particular specified impact properties, as assessed by the Charpy V-notch test, may need being met at given low temperatures to guarantee adequate ductility in severe service conditions. To prepare for the execution of a job, the responsible technical authority of the manufacturer has to write a preliminary Procedure including at a minimum the essential elements of description specified by the applicable Code or Specification including, if applicable, preheat and post weld heat treatments. The materials (base and filler metal) and thickness, the form and dimensions of the test coupon, to be welded according to the said Procedure, have to be defined, together with the directions for sectioning out of it all the test specimens required. There must be correlation between the main characteristics of the job and the type and details of the test coupon. The type, size and number of test specimens to be taken from the coupon are detailed in the binding document (Code or Specification). Other than that it is not required that the test coupon reproduce in every aspect the form and joint type of the projected construction. Before sectioning, the test coupon is usually subjected to regular non destructive testing, and the results are recorded in the PQR. It is not uncommon that the first test of the provisional Procedure reveal some weak points representing marginal or low results. This should be a welcome sign providing the opportunity to modify the Procedure in order to obtain improved fully qualified results. Unfortunately the delay in Qualification may have heavy consequences on the production schedule. Once the results are demonstrated to meet requirements, the PQR can be signed by the person in charge of qualification, and then the official Welding Procedure Specification can be finalized, signed for the record and issued for production. Contrary to what one might think, a single PQR may refer to several WPS, by identifying the differences in thickness or other characteristics pertinent to each one of them. In the same way a few PQRs may be needed to cover a single WPS. Far from being a standard routine to be performed to satisfy the customer, this matter requires a good deal of sound engineering judgement to evaluate the important properties for each application depending on service conditions. As already remarked in the previous article (see above), there are Specifications that accept the use of prequalified Procedures, disposing for the need of actual qualification in well defined cases. When applicable, the prequalified Procedure Specification must be issued and signed as the binding instruction document. However thorough evaluation of the application should be conducted, and strict adherence to the procedure involved. Any deviation from its details would preclude the possibility to use it, by voiding the qualification. We should remind that dedicated software programs exist, designed to ease and speed up the writing, editing, filing and retrieving of routine documents. The best among them include updated reference to the most used code requirements so that they work as handy check up lists. See Welding Software (Opens a new page).
3 - How to do it well: Welding Titanium to Stainless Q: How is Titanium welded to Stainless Steel? A: Titanium and Titanium alloys cannot be fusion welded to Stainless Steel. However special solid state processes that do not resort to fusion, can be used to join the two materials for preparing Transition Elements. Among these processes are friction welding, explosive welding, ultrasonic welding, magnetic pulse welding, coextrusion or roll welding and other methods. Regular fusion welding processes can then be used to weld each end of the transition element to the part of the same material needed to accomplish the required assembly.
4 - Filler Metal for new Creep Resistant Steels. The field of designing and constructing boilers and pressure vessels for use at high temperatures is quite developed under the regulation of accepted codes and specifications. Elevated temperature creep resistance is the most important property for all these materials. Currently available steels with a long history of successful applications include those well known low alloy steels based on various proportions of chromium and molybdenum (0.5-0.5, 1-0.5, 1.25-0.5, 2.25-1, In an effort to improve thermal efficiency of fossil fired power plants and reduce the impact of noxious gas emission, fundamental studies are being carried all over the world with the purpose of increasing the materials capacity of operating with steam at higher pressures and temperatures. Parallel development efforts are dedicated to provide suitable filler metals that will permit the exploitation of the improved steels without impairing their properties in the welded joints. Besides creep resistance, mostly for the steels with higher temperature utilization potential, scaling resistance is a concern, normally addressed with increased chromium content to about 10-12%. This composition modification brings about a whole new series of martensitic steels, whose weldability problems must be controlled by developing suitable Post Weld Heat Treatments. Acceptable treatments, adapted to different welding processes (GTAW, SMAW and SAW) and to thickness ranges, are tested for the impact properties obtained, as required by current codes. Where possible, consumable developers strive to stay as near as they can to the base metal alloy content, provided that filler metal wire manufacturing is still feasible. Sometimes, if the composition includes elements like titanium that will not survive the passage into the welding arc, trials are performed to check the substitution of niobium in its place. Martensitic grades require well conceived welding procedures with preheat, interpass and post weld treatment strictly controlled. The studies concern also the case for the formation of dissimilar joints, where the two sides are different materials. Diffusion of elements like carbon and chrome can cause microstructural changes whose consequences must be evaluated. To get a taste for the kind of development work on materials and consumables being dedicated to the technological progress connected with improved power plants for the immediate future, interested readers are urged to read the complete report at 5 - Online Press: recent Welding related Articles From The Fabricator: Random radiography From AWS: From TWI: From NIST:
6 - Terms and Definitions Reminder Acceptable Weld must meet applicable requirements. No need to do it any better. It would only be more costly (overwelding) and possibly objectionable. "As Welded" referred to a weldment means: in the condition existing at the end of the welding process, without any additional post processing. Backstep Sequence means a succession of short weld passes made in the opposite direction from that of welding each stretch. Cluster Porosity represents a group of randomly concentrated pores as seen in radiographs or in macro sections. Defect is any flaw that exceeds the limiting requirements, causing rejection or needing repair if possible. Groove Weld is a weld between two edges or surfaces, in the groove or space between the parts. Mixing Chamber with reference to a welding torch is the space where two gases are mixed together before exiting through the nozzle. Throat of a fillet weld is the dimension between the root and the face.
7 - Article: Gas Turbines Welding and other Processes Gas Turbines are complex power plants that transform the chemical energy of combustibles into mechanical energy. In their known aircraft version they power modern airplanes to assure global mobility. In other outfits, not so visible to the general public, they power electrical production units, industrial and naval application with remarkable thermal efficiency. They include a compressor coupled to a turbine, with a combustion chamber between them. Static elements are mounted on casings, and one or more rotors are mounted on a shaft supported by bearings. The rotor, the moving part, holds disks fitted with blades, the cold section pertaining to the compressor and the hot section forming the turbine. Welding processes are used by Original Manufacturers in the production of gas turbines, and by service and overhaul stations to repair or recondition damaged items. The cold part, that at the last stages reaches quite high a temperature due to air compression, is characterized by material exhibiting elevated specific strength (ratio of strength at high temperature to density or specific weight), selected to minimize total weight. The hot part elements, where the temperature reached by gas combustion can be very high even for high temperature resistant materials, are generally of heat resisting materials with suitable surface protection. The properties required are oxidation resistance for the static parts, creep resistance and stress rupture resistance for the moving parts, subjected to elevated centrifugal stresses. Combustion chambers, diffusion nozzles and afterburners, made of heat resisting materials like nickel and/or cobalt base alloys are manually Gas Tungsten Arc welded. In certain engines the main shaft can be built by welding together two or three elements by Electron Beam Welding or by Friction Welding. Orbital GTAW is used to weld stainless steel engine piping sections together and to produce manifolds. Besides welding, other processes involving heat are used to manufacture and/or repair certain items. In particular furnace brazing (in vacuum or protective atmosphere) is used to assemble complex diffusers and fabricated compressor blades. Hot diffusion processes are used for surface protection. First stage fan rotating vanes forged from titanium alloys include a reinforcement element called "mid-span" designed to help keeping the operating airfoils in place in their correct shape, by making contact with the same parts of other neighboring rotating vanes without introducing the additional constraints of a rigid monolithic part. The edges of those mid-spans are subject to abrasion and wear because in operation the ones hit against the others. Therefore the edges are protected by a layer of hard and wear resistant material deposited by one of a series of high speed thermal spray processes. Large rotating vanes used for the front three stages of fans, are reconditioned by electron beam welding when their trailing edges become worn and damaged. A new sheet metal patch of the same material is joined at the trailing edge to add new service life to those expensive parts. The added patch is then machined to reproduce the required airfoil section and restore the vane to a condition as good as new. The main shaft of powerful engines is a critical part whose front portion that includes splines becomes occasionally worn out and damaged so much that has to be discarded. A large friction welding machine is sometimes used to salvage such expensive items by cutting the worn out section and welding in place a new piece, to be then machined to regular dimensions and tolerances. This process is ideal in that fusion is avoided, its heat input is limited and there are not harmful consequences on the properties of the reconditioned part. Damaged static parts like engine casings, gear box casings and compressor vanes can be sometimes repaired if the heat developed by welding does not reduce the mechanical properties of the original materials. One should remark that critical parts like compressor and turbine discs, which may be life limited, are not repaired by any thermal treatment but simply discarded when they reach the limit of their useful life, monitored through a complex system of individual marking and cumulative service records. Also creep resistant turbine blades, which are usually surface protected by a diffusion coating, display a delicate balanced microstructure that can only be disturbed by heat intensive repair processes. Nonetheless limited surface repair by laser fusion for the deposition of additional material has been attempted.
8 - Site Updating: Submerged Arc Welding Tips What is the Page of This Month about? It is about Tips on one of the most productive welding processes, namely Submerged Arc Welding (SAW). Is there a need for this new page? For those who use SAW day in day out there may be, to help them to think out of the box from time to time. The purpose is to try to profit from new combinations of materials and parameters. For those who may not be familiar with the process, the page may urge them to test more carefully if introducing SAW in their shop, or transferring to SAW some of their usual operations, might benefit them with increased productivity. Less passes, more welds in the same time, increased deposition rate, less skillful workforce, mechanical semi-automatic set-ups, all these are but some of the advantages and benefits of SAW, that it may be helpful to explore in some depth. To read the page click on Submerged Arc Welding Tips (Opens a new page). Bookmark the Welding Advisers website, come back to browse the Site Map, subscribe to our RSS feed following the instructions under the NavBar of each page, and you will always be updated on additions and revisions.
9 - Short Items 9.1 - Case Hardening is a generic term covering different thermal processes applicable to steel, that change the chemical composition of the surface layer by enriching it with carbon, nitrogen, or a mixture of the two. A concentration gradient is then created by diffusion. The process commonly used is carburizing followed by quench hardening and tempering. Other processes are cyaniding, nitriding and carbonitriding. Special furnaces are used. Common applications are gear and shaft hardening. 9.2 - Dendrite is a word of Greek origin used to designate a crystal that has a treelike branching pattern. Dendritic structure is most evident in cast or weld metals slowly cooled through the solidification range. 9.3 - Grain refinement is a procedure performed on molten metal by adding certain materials that produce a finer grain size in the final structure. The process modifies the transformations occurring when cooling from liquid to solid to cause more, smaller grains to form. 9.4 - Hardfacing consists in applying a hard, wear resistant material to the surface of a component by welding, spraying, or other processes to reduce wear or loss of material by the damaging action of fretting parts or media on the surface. 9.5 - Solidification is the transformation from the liquid to the solid state occurring upon cooling. Depending on the composition of the liquid phase and on the rate of cooling, different phases and structures are expected to form, accompanied by segregation (non uniform distribution of alloying elements). 9.6 - Wetting Agent is a substance that reduces the surface tension of a liquid, thereby causing it to spread more readily on a solid surface. This activity is most important in fluxing for brazing and soldering. 10 - Explorations: beyond the Welder Click on A Waste Treatment Project of huge Proportions Sonnets By William Shakespeare From Scientific American Global Health Watch
11 - Contribution: Electrolysers "Electrolysers generate high purity hydrogen and oxygen gases through electrolytic dissociation of water molecules; for this process the generator needs demineralized water and electricity. To use the gas immediately, as required for many applications, the gas is produced at the specified operating pressure with appropriate working characteristics. In applications which need higher purity, additional purifiers can be added or for larger flow rates, the purifier can be built in to the generator itself. The electrolysers have safety and ecological features offering emission free, low pressure alternatives to compressed gas cylinders. Low operating costs, together with other user friendly characteristics are found in the best products adopting separation technology. One should stress that Hydrogen is more stable than Acetylene if compressed. Actually compressed Acetylene may dissociate explosively. Therefore acetylene is stored dissolved in acetone in cylinders filled with porous material. By producing Hydrogen industrially at low pressure there is no need to store high pressure or liquefied cryogenic gas. Applications include: Braze Welding, Silver Soldering, Heat Treating. These generators can be used in all applications in which hydrogen and oxygen are needed to produce a flame for torch uses and also for welding, heating, braze-welding, metal spraying application, cutting and so on. Hydrogen can be used to weld aluminum, magnesium and lead but not steel because its flame has too low a heat value to produce good fusion. Special safety issues must be addressed in any practical application to avoid the risk of explosion. Hydrogen alone or mixed with other noble gases can also be used to feed furnaces with protective atmospheres for metal treatment applications. Special provisions must be in place at start up and shut down to avoid the risk of explosion of ignited hydrogen-air mixtures. The above mentioned applications are just some of the possible uses for electrolysers. In fact, this technology is quickly establishing itself as an ever increasing choice for industrial applications. Electrolysers may offer RHA (remote help assistance) option for remote surveillance through modem connection, providing on-line control capability."
The note above was kindly contributed by Stephen Robertson from
12 - Testimonials From: Abdelaziz Hagar (e-mail address withheld for security) Thank u for good information. [...] Abdelaziz Hagar Sudan University Khartoum - Sudan
From: "DDFC Development" (e-mail address withheld for security) I am really glad to join this site.
From: Tim Prusak (e-mail address withheld for security) You'll find that I e-mailed you this question twice and I apologize!
13 - Correspondence: a few Comments We usually enjoy receiving queries on welding and metallurgy problems, and try to provide pertinent answers intended to be of practical help. However, as already remarked in the past, many questions are formulated in so general terms that it seems the inquirer did not know how and what to ask. See the following few examples:
May we suggest to our readers to focus on details before sending in their questions?
And please give us some feedback on how much the answer did or did not help. Use the Contact Us button in the NavBar of every page.
14 - Bulletin Board 14.1 - The Cleveland Clinic, ASM International, and the Nano-Network will partner to host the Materials, Medicine, and Nanotechnology Summit. The anchor event will be held Oct. 2-5 at the InterContinental Hotel and MBNA Conference Center in Cleveland. 14.2 - Seattle 2006: Three materials conferences convene together, May 15-18, 2006, Seattle. The 2006 International Thermal Spray Conference & Exposition (ITSC), the ASM International AeroMat Conference & Exposition, and the ASM International Surface Engineering Congress & Exhibition (ISEC). 14.3 - The Association for Iron and Steel Technology presents the International Conference on New Developments in Long and Forged Products: Metallurgy and Applications. June 4-7, 2006, Winter Park, Colo. 14.4 - If you still think that building a website is only for the elect few, we recommend our Website Host who distributes valuable educational resources at no cost, and provides us and all their customers with faultless, helpful service and assistance. We urge our readers, who are in a hurry, to visit the condensed 30-Second Nutshell Version of CTPM. It is worth your attention See you next time... Copyright (c) 2006 by Elia E. Levi and welding-advisers.com, all rights reserved
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