Call a Metallurgist!
It’s a fact of life… things break. When those things are constructed out of metal, the first reaction of most is usually to “Call a metallurgist!” Although this is often a good starting point, in many cases also including a mechanical or structural engineer in a failure investigation is prudent. If a weld is involved in the failure, a root cause analysis team trifecta is completed with a competent welding consultant.
Metallurgy, as the term suggests, involves the study of metals. Metals can be rather complicated, and consideration of parameters such as metal makeup (chemistry and alloying), fabrication methods (wrought, forged, cast, 3D printed, etc.), the environment in which an item is used (temperature, radiation and chemical exposure, other), and the presence of welds, etc. can all strongly influence the performance of metals. A metallurgist can investigate and assess the influence of each of the above.
When metals fail to perform their intended function (usually the safe transfer of loads), blame generally falls first on the material. However inadequate design may be the culprit, in full or in part. Other contributors to failures may include: operations, age, and maintenance of the failed entity. Wait, what? – do metals age? In a sense, yes. When a metal is exposed to an aggressive environment/conditions, exposure time is usually a critical parameter to degradation. Examples include radiation effects, corrosion, and erosion) and vibration exposure.
The ability of a material to safely transfer a load is governed by its strength properties as well as its geometric properties (i.e. section properties) including thickness. These properties establish the capacity a component or structure has to transfer loads. The demand put on an in-service material with regard to load transfer is quantified by the “stress” level in the material. Stress is the internal force per unit area in a body resulting from loads applied to it. Such loads generally fall into one of two categories: structural or thermal. It is in the purview of the engineer to determine if the material/structure combination used for a component (i.e. its design) possesses sufficient capacity to withstand the demand placed on it by the intended in-service conditions.
An additional consideration for the engineer is the as-built component in comparison with the as-designed component. It may be necessary for changes to be made to a part during fabrication for any number of reasons. These include, fabrication methods/issues, human error, availability of parts and materials, etc. If such changes are made and not conveyed back to the designer for review, a failure may result.
A number of complexities are introduced when welds are added to a component or structure. High temperatures are needed to join metal parts together. But high temperatures generally cause microstructural changes to the metal. Material strength is related to microstructure and therefore strength can be affected. Furthermore, because heat for welding is introduced locally, deleterious residual stresses may be created by the welding process. There are many parameters involved in welding and most depend on the type and composition/chemistry of the materials being joined. When a failure involves a weld, an experienced welder/welding consultant or weld engineer is often needed to determine not only if the welding was a contributor, but also what specific weld-related parameter was the cause.
In conclusion, if you are unfortunate enough to experience the premature failure of a part/component made of metal, by all means contact a metallurgist. However, for a more comprehensive assessment, consider the involvement of an engineer and a weld consultant in the investigation.
Interdisciplinary teams are essential for performing most failure analyses, since the root cause is rarely the result of a single variable. Understanding not only the metallurgical properties but also the environmental effects and stresses to a system or component is important in determining the cause of a material failure.
Once we identify component failure modes, we assess component remaining life, recommend inspection methods/ intervals, develop repairs to components and modify equipment designs to reduce the likelihood for future failures and ensuring structural integrity.
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Metallurgy Related Services
@ O’Donnell Consulting Engineers