Welding-failures:approach and study.SOLUTIONS with Effective, Powerful Advice
Welding-failures, service failures, weld defects, discontinuities, burst fractures, corrosion attack, overheat, wear damage, fatigue cracks, overload, run out service conditions, faulty design, metallurgical failures, wrong material selection, inadequate welding procedures, welding links, welding tips, joining questions needing answers: these are some of the items developed in this Site for the benefit of interested readers. What is in here for me? You know a success from a failure, don't you? The problem with Welding-failures is that they happen without warning, at the worst moment, in the most damaging way, with most costly repair procedures. That is common to all failures, probably. What can we learn from a failure? If we are keen and determined we can learn how to avoid it next time, how to improve what we do.
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It is amazing to find, in a respected engineering Forum, a question like this: "We recently found a failure in our equipment [a short description follows]. What could be the reason?" With all due respect, the question is not the right one. One should not attempt to guess Welding-failures. One should investigate. Only by looking at all evidence and clues, by tying all together, one arrives at the most probable cause of the failure. Welding-failures have a most important, if costly, function. That is to teach what is at fault and what needs correction and improvement. But only if we investigate making an effort to see the reality without being clouded by any theory. And if we know how to learn from experience. What are really Welding-failures? In our opinion Welding-failures occur only in service. Regarding production, the inability of a test piece to stand up to mechanical requirements, or of a workpiece to meet nondestructive inspection demands, should be called a defective condition, and should be addressed with a different approach. Investigation of the causes could be easier, and corrective action simpler to implement as all contributing elements are known. For eliminating production defects click on Welding Defects. The good news is that not all Welding-failures can be imputed to the welder, or to the shop, or to the procedure. The bad news is that you may have to prove it. So does it make sense to understand what an investigation is and how it is done? Don't you think you might profit from looking somewhat deeper into this subject? Tip!: When Welding-failures arise it is good practice to leave everything as is at the time of occurrence, until the expert arrives. Any needless manipulation of the fractured surface and surroundings could erase important clues that the investigator would have used to reach his conclusions.
We recommend the following Book to anybody interested in Why would you want an expert? Sometimes because of legal requirements, especially if life or property were endangered. Other times because of insurance claims. In every other case whenever the origins of the Welding-failures are not obvious. There is a need for a wealth of knowledge and experience that form the tools of the expert's trade. Would you like to know how an investigation is conducted? If yes then you can find here a few outlines for orientation. As for any metallurgical investigation one should first collect all the available evidence including witnesses reports, details on time and circumstances of failure occurrence, on operating conditions before failure, was there fire, explosion or any other accident connected. What were the operating fluids and their temperature and pressure, and other auxiliary materials, if any. Notes on shut down procedure for the facility and removal of heat or load or both should be included. Description of the appearance and first assessment of Welding-failures's importance, collateral damage if any, photographic documentation of the appearance of the cracked or fractured surface is part of the expert's work and should be left under his/her responsibility. Then one should collect all documentation available on the manufacture of the item involved, on base material and filler material specification and certificates, on the welding process used, on the welding procedure and inspection record, on the welder involved (welders identification code marked near the welds is required by many specifications), on all the tests and inspections that were performed, including radiographs of the failed area, if any exist: these records must be preserved for a specified number of years, according to binding specifications. Final load test report at the time of manufacture should also be made available. One should assess if the welded assembly was realized as prescribed by drawing and relevant specifications. Then one should review if design took into account the working stresses as applicable for the service involved. One should also investigate if working conditions were significantly different from those presumed in design. Having collected all available information the investigator is now ready to take a closer look of the Welding-failures. First a thorough visual inspection is needed, by the naked eye but also with some low power enlarging stereomicroscope. The important features must be documented by photographic imaging. Finally one should confirm by analysis and tests that the chemical composition of the material and its heat treatment condition were in effect as prescribed and requested by design. One should investigate if the physical condition of the surrounding area presents evidence of local damage by mishandling or by some other accident. The nature of many Welding-failures resulting in fractures, if ductile or brittle, if signs of fatigue propagation are present or of corrosion products, if there are wear marks, gross deformation, burst or other features can be determined by examining the fracture surface by the techniques of fractographic analysis. ASM Handbook Volume 11: Only when all studies of visual characteristics have been completed, time is ripe for metallographic sections. These sections, duly prepared with grinding and polishing, can be examined under the optical microscope, after etching to reveal the form and consistency of weld beads and the appearance of the heat affected zone, with possible defects and unwanted metallurgical phases being present. In certain cases where clear cut results cannot be obtained, the expert may have recourse to more sophisticated means like scanning electron microscope (SEM) and other specialized techniques.
In many cases any expert will profit from unbiased observations, especially when they appear as striking deviations from usual instructions or behavior, when they are proposed by a curious and thorough observer, familiar with the facility and its operation, no matter what his/her official rank or preparation. Of the common service Welding-failures we shall address burst fractures, overload failures, deformation, corrosion generated or heat dependent failures, wear dependent, metallurgical failures, faulty design failures, wrong material selection and inadequate welding procedures. Only expert metallurgists, familiar with this type of inquiries and knowledgeable with the disciplines involved can conduct a thorough investigation. Burst Welding-failures, usually of ductile character, can derive from overpressure, or overload, if the original thickness and soundness of the material at the failure place is still in good condition, not undermined by wear or corrosion. The actual position of the fracture in relation to weld location can hint at possible metallurgical problems to be addressed by metallographic examination. Deformation caused by faulty assembly could easily build up excessive stresses capable of inducing fatigue cracks and Welding-failures especially if combined with superficial stress raisers, in presence of vibrations or shocks. Corrosion diffused on the surface or concentrated in fissures can be evaluated if there is line of sight to the place. For looking at restricted areas one inserts borescopes, visual instruments including a light source for examining features at low power enlargement, in internal cavities or tubes. Intergranular corrosive attack can best be assessed by examining a metallographic section. Once assessed the presence of corrosion one should still determine the cause of its formation. Stress corrosion is a dangerous combined condition that could have been easily avoided if anticipated. NEW RESOURCES
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Overheat as a condition generating failure can have many outcomes, including distortion, intergranular fracture, oxidation. It is usually possible to find the traces of unwanted heat, like color, surface appearance, lower hardness, metallographic evidence. Wear or erosion on a visible surface can easily be assessed. The opposite mating surface also should be investigated to answer the question why the two surfaces came together in the first place. Cracks in welds can appear of two different kinds. Hot cracks form when the material solidifies, generally because of the presence of low melting constituents. Cold cracks are generated later, when the material is cold and under stress, sometimes by the presence of dissolved atomic hydrogen. They are easily differentiated under the microscope, but anyhow, hot or cold they should not be allowed in a sound structure, otherwise the structure is in danger if not already failed altogether. Arc strikes should have been detected and repaired in production. If they were not removed they act as stress raisers and also as crack origins. By retracing the fracture course one can easily find them as culprits. Arc strikes could also be generated by careless use of electric current application during magnetic particles inspection procedures. Poor weld contours, incomplete penetration and inadequate fusion have a manifest side and a hidden one. They can cause or contribute to a failure as stress raisers and least resistant path. A properly approved welding procedure should have detected these defects in time and have avoided occurrence of failure. When in presence of a brittle failure one should investigate if the material selected was adequate, and if all means of influencing the metallurgical structure, like preheating and post heating were applied correctly within a balanced welding procedure. Welding-failures can be attributed to faulty design only if it did not take into account the service conditions or if the details of the construction generated stress concentration conducive to brittle fracture. Tip!: One should be most careful with such a conclusion, making sure it is well documented and proven, because of the fierce resistance from the designer who obviously will not like it... An Article of ours on Failure Analysis was published on Nov. 9, 2004 in the online magazine The Fabricator. An Article on Thermal Fatigue was published in the December 2004 Issue of Practical Welding Letter #016. To read it click on PWL#016. An Article on Welding Failure Investigation was published (2) in Issue 66 of Practical Welding Letters for February 2009. Click on PWL#066 to see it. NEW RESOURCES
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It might shed new light even on a failure. Click here. To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics. For anyone of the following subjects, click on the underlined item.
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Welding-failures can be the dreaded outcome of one or more of very different causes: for future improvement it is imperative, even if not easy, to get to the roots... |
