Analysis of a Coated Fastener That Failed by Liquid Metal Embrittlement

A failure analysis was conducted for a fractured 1038 stud that was quenched and tempered to a fully martensitic microstructure with a hardness of 33 to 38 Rockwell C. All surfaces of the studs were tin-zinc coated. The composition, microstructure and hardness of the fractured stud and two other companion studs were determined. Examination of the stud fracture surface showed that it was entirely coated with tin and zinc and numerous prior austenite grain boundaries were filled with the tin-zinc coating. During riveting and manufacturing experiments, the studs all survived after oven exposure at 188°C (370°F) whereas most and all studs failed after exposure at 216°C (420°F) and 221°C (430°F), respectively. These results are consistent with the known requirements of liquid metal embrittlement (LME) of steel by tin in the presence of the tensile stress due to riveting and grain boundary penetration by tin.