{"id":367,"date":"2022-05-27T04:14:00","date_gmt":"2022-05-27T04:14:00","guid":{"rendered":"https:\/\/mewelding.com\/?p=367"},"modified":"2024-03-18T09:57:32","modified_gmt":"2024-03-18T09:57:32","slug":"weld-failure-analysis","status":"publish","type":"post","link":"https:\/\/mewelding.com\/weld-failure-analysis\/","title":{"rendered":"Weld Failure Analysis"},"content":{"rendered":"\n
Failure of a weld can have serious consequences. A weld failure in the backyard of house may not have much consequences. However, when the weld lies in a large sized pressure retaining equipment, weld failure can cost heavily on life and equipment. Such failures have to be analysed by an expert or a team of experts, who carry out an investigation into the causes behind the failure.<\/p>\n\n\n\n
In the non-critical welds too, it is important to understand the causes of failure so that a repetition of weld failure can be avoided.<\/p>\n\n\n\n
Such an investigation must be thorough, and scientific, so that the right causes can be captured. This is required so that failures can be prevented from re-occurring. In order to do a sound investigation, the expert must have good knowledge on welding techniques, NDE techniques, and metallurgy.<\/p>\n\n\n\n
The expert must take into account all information available, right from design, fabrication sequence of the component, and operating parameters of the component. Any factor that could have remotely<\/em> contributed to failure must be taken into account.<\/p>\n\n\n\n An investigation in weld failure must take into account the metallurgical make-up of the weld, the service life of the component, conditions that might have led to the failure, and the mode of failure. Often the information revealed by such an investigation leads to largescale changes in the design, fabrication sequence, operating parameters of the equipment. Such information leads to advancement of knowledge. The study therefore, must be done capably \u2013 as this allows us to build a more reliable product in future.<\/p>\n\n\n\n In this article, we shall discuss how to objectively approach such an investigation. How do we go about doing it, how to establish findings and results?<\/p>\n\n\n\n Any investigation focusses on the following factors while going about analysing a weld failure: Initial observations, background data, laboratory studies, and welding failure analysis. This helps us in determining the causes of failure and interplay of factors, if any. Let us see these factors in brief.<\/strong><\/p>\n\n\n\n When a welding failure occurs, the expert investigating it must first inspect the site of failure and record his initial observations. This inspection should be made as early as possible after the failure has occurred. The observation can be in the form of notes, and photographs of all parts the captures the texture and location of failure surfaces.<\/p>\n\n\n\n The initial observations must also include interviews with the witnesses.<\/p>\n\n\n\n The investigator then collects an in-depth back ground information about the failed weldment. This information may include design aspects of the joint, methods of fabrication, weld procedures, welding consumables used to make the joint, post weld heat treatment, repairs, operating parameters of the part, normal loading, possible abuse \u2013 inadvertent or otherwise, maintenance schedule, etc.<\/p>\n\n\n\n An effort should be made to collect all information that might point to the causes of failure.For example, the fabrication sequence may have recorded a non-conformance which might not have contributed to the failure, but should be recorded nonetheless to draw up a holistic picture.<\/p>\n\n\n\n The investigator then subjects the failed surfaces to a few tests such as chemical composition, macrograph or micrograph of the failed surface. The chemistry check allows him to ascertain whether the part actually possess the composition indicated by the welding procedures. A micrograph tells about the microstructure of the part. This allows the expert to correlate he microstructure on the failed part with that recorded during fabrication sequence.<\/p>\n\n\n\n The laboratory tests add to the whole picture, and give more information to the investigator so that he can form a holistic view.<\/p>\n\n\n\n Following the compilation of background data and laboratory tests, the investigator then makes assumptions about the causes that might<\/em> have caused the failure. These assumptions may be made by taking into account not just the pieces of information available, but also the pieces that are not<\/em> available. The assumptions must then be tallied with the available evidence to determine the cause of failure.<\/p>\n\n\n\n Failure in welds can occur due to one of the following factors:<\/strong><\/p>\n\n\n\n Let us examine each of these three factors in some detail.<\/p>\n\n\n\n We shall examine a few commonly occurring types of failures in weldments such as in-service cracking, brittle fracture<\/p>\n\n\n\n The weldment may develop cracks during its\u2019 service due to a variety of reasons. These reasons could be stress corrosion, cold-cracking, or fatigue failure, or graphitization, etc. As we know, a weldment contains a relatively hard micro-structure in the heat affected zone. Moreover, residual stresses are found in any weld. The residual stress, hard microstructure in HAZ, and the aforementioned factors may combine to produce cracking during the service of the part.<\/p>\n\n\n\n Failure of a part may occur due to many other reasons that are not attributable to welding. An overall analysis of failure would be much wider in scope. In the below paragraphs, we shall limit ourselves to analysis of failure in weldments<\/em>.<\/p>\n\n\n\n Below, we shall see the four types of failures that can occur in a weldment during its\u2019 service: brittle fracture, fatigue fracture, lamellar tearing, and stress corrosion cracking.<\/p>\n\n\n\n Fractures are ordinarily of two types: Brittle fracture and ductile fracture.<\/p>\n\n\n\n Ductile fracture is accompanied by a deformation of the structure. That is, the material yields first \u2013 before undergoing fracture, similar in the manner to the fracture of a mild steel tensile specimen in the tensile test. Yielding or stretching is accompanied by a reduction in cross section area of the part as well.<\/p>\n\n\n\n The figure below depicts a ductile fracture surface.<\/p>\n\n\n\nAreas Of Focus<\/h2>\n\n\n\n
Initial Observations<\/h3>\n\n\n\n
Background Data<\/h3>\n\n\n\n
Laboratory Studies<\/h3>\n\n\n\n
Weld Failure assumptions<\/h3>\n\n\n\n
Causes Of Weld Failure<\/strong><\/h2>\n\n\n\n
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In-Service Cracking<\/strong><\/h2>\n\n\n\n
Brittle Fracture<\/h2>\n\n\n\n