Cracks Found in Brass Fitting of an HVAC System
“Call a metallurgist!” When components are made of metal, this is typically the first reaction. It’s a fact of life – things break. Learn more about when to call a metallurgist and what’s involved.
Introduction
Although often a good starting point – in many cases it’s prudent to include a mechanical or structural engineer in a failure investigation. If welds are involved in the failure, a trifecta root cause analysis team is completed with a competent welding consultant.
Metallurgy, as the term suggests, involves the study of metals. Metals are 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., all strongly influence the performance of metals. A metallurgist investigates and assesses these influences.
When components fail to perform their intended function (typically – 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 contributing factors include misuse, age, and maintenance issues.
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.
Load and Stress Factors
The ability of a material to safely transfer a load is governed by its strength, geometric and thickness properties. The mechanical and thermal demand put on a component in 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. The design engineer determines if the material/structure possesses the capacity to withstand the intended in-service conditions. Often, not all of these conditions are known. Another factor in performing a failure analysis includes a comparison between the as-built component with the as-designed component. Changes are 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.
Weld Factors
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 from welding is introduced locally, harmful residual stresses are created. There are numerous 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.
Conclusion
If you unfortunately experience the premature failure of a component made of metal, by all means contact a metallurgist. However, for a more comprehensive assessment, consider the involvement of an interdisciplinary team in the investigation.
Such 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 failure.
Once we identify component failure modes, we:
- Assess remaining life
- Recommend inspection methods/ intervals
- Develop repairs to components and modify equipment designs to reduce the likelihood for future failures
@ O’Donnell Consulting Engineers
