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PWL#056 - Oxidation Resistance, Remaining Life Assessment, Ultrasonic Impact Treatment, Gas Filler April 01, 2008 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#056 - Oxidation Resistance, Remaining Life Assessment, Welding Copper to Stainless Steel, Ultrasonic Impact Treatment, Gaseous Filler Metal, Brazing Aluminum, Copper, Stainless Steel, Cast Iron and more... DON'T USE REPLY to send 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. 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: Oxidation Resistance 3 - How to do it well: welding Stainless to Copper 4 - Gaseous Filler Metals for a novel Brazing Process. 5 - Online Press: recent Welding related Articles and Video 6 - Terms and Definitions Reminder 7 - Article - Remaining Life Assessment 8 - Site Updating: Brazing Aluminum, Copper, Stainless, Cast Iron 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Ultrasonic Impact Treatment 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This 56th Issue of Practical Welding Letter opens with a short note on Oxidation Resistance and on the means to avoid its attacks. Finding the most suitable protective system for every application can be a long and painful journey, but many resources are available to limit the choices and find a good fix. Then we answer a question on welding Copper to Stainless without knowing anything about the application, so that we don't know how much our answer may help. We read of a novel brazing process. It is interesting although quite remote from practical applications, for the time being at least. The next article is about a weighty subject, likely to involve savings or expenses in the millions. To those having to deal with this argument, namely the Assessment of Remaining Life, it is worth every effort to learn all that is available and to become an expert in this field. To those not involved it is interesting to have at least an idea of what it is all about. A certain endeavor was made to enrich our website with new meaningful themes.
This month, in the Update section, we announce the publication of four new pages on Brazing of Aluminum, Copper, Stainless Steel and Cast Iron. There is a nice little tool built to provide a peening action to relieve weld stresses and to impart residual compressive stresses on weldments, especially of big constructions to improve resistance to fatigue damage. Recently the company is busy developing suitable adaptations to process underwater weldments. It is called Ultrasonic Impact Treatment and it may be helpful for some applications. The other usual departments are found at their place. We would like to get your comments and feedback. Don't use Reply, use Contact Us instead. 2 - Article: Oxidation Resistance The surface attack by oxygen of metals exposed to elevated temperatures is called oxidation. This process was briefly introduced in the article on Heat Resistance published (2) in Issue 55 of Practical Welding Letter for March 2008. Click on PWL#055 to read it. Unless checked by suitable means, this naturally occurring process is going to continue until the progressive destruction of properties reaches a point beyond usability. Although a few types of stainless steels capable of withstanding this attack are available, less expensive solutions are sought for applications at intermediate temperatures where the working stresses can be sustained by regular steel (either carbon steel or low alloy steel) so long as an adequate protection against surface oxidation is provided. This kind of solutions can be possibly found within a group of coatings presenting a range of materials and of application processes. As usual with engineering applications the best solution will be the one that assures the requirements of adequate protection and durability at the minimum possible cost, including maintenance costs along the projected useful life of the application. One of the least expensive oxidation protective systems is a paint based on aluminum pigment. It has to be specified for oxidation resistance as no organic components should be present for this application. Different ways like paint spray and dip immersion are used to cover the metal that must be absolutely clean. Aluminum was also applied in the past by metallizing, whereby an aluminum wire was melted in a special oxyacetylene torch and sprayed onto the surface to be coated. Another inorganic system resistant to quite high temperatures is a proprietary product, called SermeTel W by Sermatech, used also on certain components of turbine engines. See: Quite demanding environmental conditions must be met during application to assure best properties. For evaluation, test items should be processed only by authorized shops. More elaborate coatings are applied by plasma spray, giving higher quality coatings than previously available metallizing. Besides aluminum, different and more complex oxidation resistant coatings can be applied. Some systems, called thermal barriers involve ceramics. Special vacuum applied plasma spray systems are used to protect nickel base alloy blades at the highest temperatures of present gas turbines. See an article on Vacuum Plasma Spray, published (2) in Issue 44 of Practical welding Letter for April 2007. Click on PWL#044 to see the article. A number of proprietary diffusion processes by pack cementation are known and used, that provide, on the surface of metals, atoms of selected elements like aluminum, silicon, chrome and a few others to be diffused inward, in suitable, long thermal cycles, usually in a hydrogen atmosphere. The box containing the items and the pack is placed in a furnace where the materials are heated for a long time at the set temperature. Diffusion of the added elements takes place during the process. Pack cementation is a relatively inexpensive simple method to apply to mass production items. More modern and expensive processes of ion implantation or physical or chemical vapor deposition in vacuum are used for special applications. 3 - How to do it well: Welding Stainless to Copper Q: I would like to have information on how to effectively weld stainless to copper. In particular the pieces have cylindrical shapes. The 2 parts to be joined are cylindrical (equal dimensions) ranging from 1 to 10 mm. Materials are copper and stainless (different grades). A: It would be inappropriate to give a general answer. Depending on the actual shape of the joints, on the exact materials and on the application there are possibly a few techniques suitable to do the job. If you mean end to end joining I would start by trying friction welding: except for the smallest sizes where there might be some difficulty, for the others it should be OK. 4 - Gaseous Filler Metals for a novel Brazing Process A new hybrid process called gas/solid Transient Liquid Phase (TLP) bonding was the object of a fundamental study. The filler pure metal (Antimony (Sb) for bonding iron or steel, or Zinc (Zn) for bonding iron or nickel) was vaporized under vacuum. The vaporized phase (35 mbar) at elevated temperature (1100 0C) was made to deposit in the joint, in what amounts to a form of Physical Vapor Deposition (PVD). The deposited vapor adsorbed to the base metal formed by metallurgical reaction a liquid eutectic transition phase which upon alloying with the base metal solidified isothermally as required for TLP bonding. The special conditions (kinetic and thermodynamic) that must be met for this process to occur are present in the systems studied. The joint clearances were between 20 and 70 microns. The joint took quite a long time to form, by the production in place of an intermetallic compound. The almost complete filling of the joint gap and the highest mechanical properties of the joints investigated occurred at the maximum time allowed, 10 hours in this research. This study concludes that the process investigated provides the opportunity to use PVD techniques for joining assemblies, making use of the gaseous transport of the filler metal for applications where existing techniques are inadequate. Interested readers are urged to seek the original publication in the Welding Research Supplement of the Welding Journal for December 2007 (Volume 86) at page 373-s. See 5 - Online Press: recent Welding related Articles and Video Hybrid Welding Technology From TWI: From the Fabricator: From AWS: 6 - Terms and Definitions Reminder Heat Input is the heat energy supplied to the workpiece by a welding process to produce the weld. Longitudinal Weld Test Specimen has its major axis parallel to, and generally coincident with, that of the weld to be tested. Open Circuit Voltage is the tension that can be measured between the power source output terminals when no current is flowing. Recycled Flux is the portion of Submerged Arc Welding flux that was deposited along the seam, remained unmelted during welding and is collected for reuse. Scarf Groove is a joint shape with parallel bevels in both abutting elements. Transverse Weld Test Specimen has its major axis perpendicular to that of the weld to be tested, approximately at mid length. Weld Gauge is a tool used to check shape and size of weld beads. Weld Tab is a piece of material located temporarily at either end of a joint to provide a place where to start or end a weld. 7 - Article - Remaining Life Assessment As utilities approach the limit of their design life, managements are required to take decisions of heavy financial consequence. The extension of service life beyond original criteria permits operation cost reductions, if acceptable safety conditions are present. Central to those decisions are the evaluation of the present condition of the plants and the analysis of the effects of continued operation for extended periods. Trying to obtain the correct answers has spurred the development of Remaining Life Assessment Technology as a method based on metallurgical analysis and on fracture mechanics. Academic Institutions and commercial companies are now involved in detailed studies relative to separate facilities or to complete plants, with the purpose to outline the most economic and practical strategy to deal effectively with aging equipment. If in the past single sub-assemblies or units found defective because of accumulating damage were scrapped and replaced by new ones, now the trend is to determine how long could the damaged item still operate without danger: the Remaining Life Assessment is the tool that helps avoid the premature scrapping of the item. Within this technology appropriate inspection schedules, maintenance routines and operational guidelines are established which provide demonstrated assurance for safe functioning along extended periods. Inspections are articulated into progressive levels that refer to increasingly deeper analysis and understanding of the possible failure causes and mechanisms. Metallurgical examinations of essential components provide positive evidence of the present condition of the inspected items. Destructive tests are performed on selected specimens extracted from replaceable parts. Complex relations are established and measured against known and proved models. The conclusion is a motivated recommendation as to the continuing use of critical items, with the additional instruction to examine again the same feature after a new period of service, to check and refine the tools that permitted to reach the decision. Read more on this subject from the following sources: Remaining Life Assessment of Steam Turbine and Hot Gas Expander Components Remaining Life Assessment of Grade 91 Superheater [...] Development of Crack Propagation Remaining Life Evaluating the Condition & Remaining Life of Older Power Plants In order to advance the science of Plant Life Management
8 - Site Updating: Brazing Aluminum, Copper, Stainless Steel, Cast Iron This time we want to make a splash! Not just one or two but four (4) new Pages of the Month are hereby added to our Website. Each one of the following base materials has its own problems and special precautions to be dealt with for applying the processes successfully. We hope that these pages provide useful insight in the application of brazing to the given base metals and that they can be sought for if information is needed to exploit successfully the advantages of those processes. To be alerted of new website pages consult periodically the Site Map and/or our Blog that you can see also by including it in your RSS reader, under the NavBar in any page. See also our New Page on Metals Knowledge for assembling at no cost your Encyclopedia Online, a rich collection of valuable information on Metals, from expert Internet sources. 9 - Short Items 9.1 - Differential Thermal Analysis (DTA) measures the differences in energies released or absorbed, and the changes in heat capacity of materials as a function of temperature. The instrument that provides these data is built to record temperature differences between a sample and a reference as the two specimens are heated or cooled identically at a controlled rate. It is used to detect phase transformations and structural changes occurring during heating or cooling. 9.2 - S-N curve or diagram used to report on fatigue tests is a plot of stress (S) against the number of cycles to failure (N). The stress values are usually nominal, without adjustment for stress concentration. The diagram indicates the S-N relationship for a specified value of the mean stress (Sm) or the stress ratio (A the ratio of the alternating stress amplitude to the mean stress, A = Sa/Sm or R the ratio of the minimum stress to the maximum stress, R = Smin/Smax.) and a specified probability of survival. A log scale is used for the number of cycles. A linear scale is used most often for the stress ordinate. 9.3 - Thermo-Gravimetric Analysis (TGA) is an analytical technique used to determine a material’s thermal stability and its fraction of volatile components by monitoring the change in mass that occurs as a specimen is heated in air or in an inert atmosphere. 9.4 - Tumbling is a process used to remove sand, scale, or fins from castings or forgings by rotating a barrel, containing the workpieces, and partly filled with metal slugs or abrasives. It may be done dry, or within an aqueous solution. 9.5 - Vacuum Induction Melting (VIM) is a refining process whereby the metal is melted by induction heating in a crucible inside a vacuum chamber and then poured under vacuum into a mold. 9.6 - X-Ray Diffraction (XRD) is a group of analytical techniques which reveal information about the crystal structure, chemical composition, and physical properties of materials and thin films. These techniques are based on observing the scattered intensity of an x-ray beam hitting a sample as a function of various incident and scattered angles, polarization, and energy. 10 - Explorations: beyond the Welder Graphene Regrowing Human Limbs Inspiration by the Sea (stainless steel art) Space Junk Selling a Service... The Most Overlooked Opportunity on the Net. 11 - Contribution: Ultrasonic Impact Treatment It is widely known that impact peening modifies metal surfaces by introducing residual compressive stresses. These have favorable effects especially in situations where fatigue cracks are likely to form and develop. Small items are generally treated by shot peening performed under specified controls in large cabinets equipped to do the job. Although modifications exist for portable units made for treating large structures, it seems that these are not much in use, possibly because the application is cumbersome. A small hand held tool produces the peening action by using floating pins put in motion by ultrasonic energy from a small generator. The method is based on conversion of harmonic oscillations of the ultrasonic transducer into peening impulses of ultrasonic frequency. The tool is brought to welded structures and directed to treat welded beads in all positions, even overhead. The action provides reduction in residual weld stress and strain of up to 70% of the initial state but the real advantage is the introduction of compressive stresses in the surface layers of the weld, an important benefit for all structures subjected to cyclic loading and fatigue stressing. All concerned with welded structures could possibly profit from exploring the benefits obtainable by applying UIT technology. Read more on this argument in the article Visit also the website 12 - Testimonials First Name: Paul Elia,
On Mon Mar 17 23:00:20 2008, the following results were submitted from the "Form 5" on welding-advisers.com: First Name: Luiz Best Regards, Luiz Henrique 13 - Correspondence: a few Comments From time to time I get mail that is more extensive than what it seems when I first read it. A welder asks advice about a job he never did before. He gives all details and explains which good equipment he can use for this job. The letter is remarkable in that I can feel the keen interest he shows for his job, the pride he fosters for his profession, the loyalty he keeps to his employer and the sincere intention he has to make good welds. I have great respect for such a worker and I am sure he is an asset of the company he works for even if they may not know or not value him right. What strikes me the most in this tale is that Management shines for its absence. Nobody inquires if the welder's equipment is adequate, if he has all he needs, if he could benefit from some training, if the requirements are spelled out clearly and completely. All responsibility, in such situations, rests on the welder's shoulders, even if he is not aware of not being equipped to take it upon himself. Management will come to him with blame and complaints if the work does not come out perfect. He should go back to his boss and ask to be instructed exactly how he should do the job: he would do whatever he will be asked for, to the best of his own knowledge and skill but with no liability on the outcome. Management should know its own limits and know what can and what cannot be expected from the best of the employees. 14 - Bulletin Board 14.1 - Welding Aluminum 2008
14.2 - The European Welding Federation (EWF) and the Italian Institute of Welding (IIS) invite all people involved in welding and joining technology to take part in EUROJOIN 7 / GNS5. 14.3 - If you own a local small business you might benefit from some help. It simply makes sense for local small business owners like you to use SiteSell Services to build your site. Explore
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