Hydrogen Embrittlement Failure of a Galvanized Washer

A galvanized washer used for a locomotive impeller broke into three pieces after an accumulative service of 4–5 h. The washer is fabricated from 42CrMo steel and the fracture surfaces reveal intergranular fracture morphology with microvoid coalescence observed on the facets. Microstructure observation indicates the presence of a severely banded microstructure mainly consisting of untempered martensite and bainite. The average hardness of the failed washer is HRC 59.1. The hardness value is much higher than specified (HBW 260-300) and is in the range of hydrogen embrittlement susceptibility. The significant number of elongated MnS inclusions acting as traps of hydrogen are present in the martensite regions. The delayed fracture associated with the predominance of intergranular fracture micromechanism and the high hardness level and the presence of microstructure susceptible for the hydrogen embrittlement strongly suggest the hydrogen embrittlement being the mostly possible failure mechanism. The likely sources for hydrogen entrapment are the electro-galvanizing process and acid-pickling before galvanizing. The hydrogen was retained in the washer due to absence of baking treatment to remove it or insufficient baking. The incorrect heat treatment process before galvanizing resulting in the high hardness level of the washer is mainly responsible for the occurrence of hydrogen embrittlement on the washer.