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Fastener Technology International December 2012 / January 2013 : Page 28

Liquid Metal Embrittlement ...continued that the bolt flange be mated to a tin (Sn)-coated brass washer ( Figure 3 ) at assembly. Fig. 6 6 — Spectral analysis con-firming presence of tin (Sn)-rich areas at exposed faceted fracture surfaces and in grain boundaries. Fig. 3 6 — Tin (Sn)-plated brass washer showing mechanical abrasion and exposure of brass be -neath plating in contact with flange area on bolt. Fig. 4 6 — Underside of flange exhibiting radial and circumferential cracks with evidence of washer remnants in cracks. A metallurgical investigation followed and revealed the presence of a tin-rich phase ( Figure 5 and Figure 6 ), which was confirmed under the scanning electron microscope to be LME with a brittle intergranular fracture mode (as seen in Figure 7 ). The tin was found at the grain boundaries, both those already exposed by fracture and internally which were not exposed. EDS analysis was used to confirm the presence of tin-rich areas. 20.0 kV x 300 SE 6 100 µm Fig. 7 — SEM of fracture face illustrating predominately intergranular fracture with coated areas overlying grain boundaries. Unexpected species present is tin (Sn). Summing Up Liquid metal embrittlement is one of the least expected failure modes experienced by products in service applications. When it occurs, it is often catastrophic, but is correctable by proper matching of materials with their environment as well as controlling the stress state and hardness of the material. FTI References: 1 2 (a) Optical Microscopy Image de Rosset, William S., Use of Liquid Metal Embrittlement (LME) for Controlled Fracture, Army Research Laboratory, ARL-TR-4976, Sept. 2009. C. J. McMahon, Jr., Brittle Fracture of Grain Boundaries,Interface Science 12, 141-146, 2004. 3 Bogner, B., G. Rorvik, and L. Marken, Bolt Failures–Case Histories from the Norwegian Petroleum Industry, Microscopy and Microanalysis, Volume 11 Supplement S02, August 2005. 4 Wikipedia ( 5 Kolman, D. G., Environmentally Induced Crackng, Liquid Metal Embrittlement, ASM Harndbook,Volume 13A, Corrosion: Fundamentals, Testing and Protection, ASM International, 2003., pp 381 392. 6 ATRONA Test Labs, Inc. (, with permission. 20.0 kV x 200 BSECOMP 40Pa 200 µm (b) Scanning Electron (Right) Microscopy Image Fig. 5 6 — Fractured washer segment-suspect areas appear white. 28 Fastener Technology International/December 2012 “Vacuum Heat Treatment”, written by Dan Herring , “The Heat Treat Doctor” ® , and monthly editorial contributor to this maga-zine, is now available. The new book focuses on principles, practices and applications, and provides the heat treating industry with a comprehensive resource on the subject of vac-uum technology. It provides the reader with practical advice, a diverse set of application examples and a wide range of technical and engineering information necessary to make in-formed decisions about how to heat treat, and what equipment features are necessary to do New Vacuum Heat the job. Purchase the book by visiting: Treatment book.

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