Metallurgical failure analysis

Metallurgical failure analysis is the process by which a metallurgist determines the mechanism that has caused a metal component to fail. Typical failure modes involve various types of corrosion and mechanical damage. It has been estimated that the direct annual cost of corrosion alone in the United States was $276 billion, approximately 3.1% of GDP, in 1998. Corrosion costs have continued to skyrocket and total corrosion costs now are greater than $1 trillion annually in the United States as of 2012.

Metal components fail as a result of the environmental conditions to which they are exposed to as well as the mechanical stresses that they experience. Often a combination of both environmental conditions and stress will cause failure. Metal components are designed to withstand the environment and stresses that they will be subjected to. The design of a metal component involves not only a specific elemental composition but also specific manufacturing process such as heat treatments, machining processes, etc. The huge arrays of different metals that result all have unique physical properties. Specific properties are designed into metal components to make them more robust to various environmental conditions. These differences in physical properties will exhibit unique failure modes. A metallurgical failure analysistakes into account as much of this information as possible during analysis. The end goal of failure analysis is to provide a determination of the root cause and a solution to any underlying problems to prevent future failures.[1]

Analysis of a failed part can be done using destructive testing or non-destructive testing (NDT). Destructive testing involves removing a metal component from service and sectioning the component for analysis. Destructive testing gives the failure analyst the ability to conduct the analysis in a laboratory setting and perform tests on the material that will ultimately destroy the component. Non-destructive testing is a test method that allows certain physical properties of metal to be examined without taking the samples completely out of service. NDT is generally used to detect failures in components before the component fails catastrophically.

Metallurgical failure modes

There is no standardized list of metallurgical failure modes and different metallurgists might use a different name for the same failure mode. The failure mode terms listed below are those accepted by ASTM,[2] ASM,[3] and/or NACE[4] as distinct metallurgical failure mechanisms.

Caused by corrosion and stress

• Stress corrosion cracking[5]
• Corrosion fatigue
• Caustic cracking (ASTM term)
• Caustic embrittlement (ASM term)
• Stress corrosion (NACE term)
• Sulfide stress cracking (ASM, NACE term)
• Stress-accelerated Corrosion (NACE term)
• Hydrogen stress cracking (ASM term)
• Hydrogen-assisted stress corrosion cracking (ASM term)

Caused by stress

• Fatigue (ASTM, ASM term)
• Mechanical overload
• Creep
• Rupture
• Cracking (NACE term)
• Embrittlement

Caused by corrosion

• Erosion corrosion
• Oxygen pitting
• Hydrogen embrittlement
• Hydrogen-induced cracking (ASM term)
• Corrosion embrittlement (ASM term)
• Hydrogen disintegration (NACE term)
• Hydrogen-assisted cracking (ASM term)
• Hydrogen blistering
• Corrosion

See also

• Forensic engineering

References

1. http://www.g2mtlabs.com/failure-analysis/what-is-failure-analysis/ G2MT Labs - "What is Failure Analysis?"
2. “Standard Terms Relating to Corrosion and Corrosion Testing” (G 15), Annual Book of ASTM Standards, ASTM, Philadelphia, PA.
3. ASM-International Metals Handbook, Ninth Edition, Corrosion, ASM-International, Metals Park, OH
4. NACE-International NACE Basic Corrosion Course, NACE-International, Houston, TX
5. M&M Engineering Conduit Fall 2007 “Chloride Pitting and Stress Corrosion Cracking of Stainless Steel Alloys,” "Archived copy" (PDF). Archived from the original (PDF) on 2011-07-14. Retrieved 2010-08-20.CS1 maint: archived copy as title (link)