Abstract
Recent studies on hydrogen embrittlement (HE) of austenitic stainless steels have been reviewed with a brief summary of fundamentals of hydrogen behaviors. The hydrogen states are characterized by high solubility and low diffusivity, occasionally leading to a high local concentration of hydrogen. The high concentration of hydrogen leads to the formation of hydrides, which transform into martensites on increasing hydrogen concentration or degassing. The susceptibility to HE is critically dependent on the stability of austenite, i. e. the composition of steels, and the test temperature. The effects of inhomogeneity of alloy distributions and of microstructures have been presented.
The mechanism of HE has been examined from fractographic features, microstructures around the crack path and the amount of transformed martensite. Contradictory observations have been reported on the role of α' martensite in the crack propagation. It is noticed that apparent correlation of the embrittlement with the amount of α' does not necessarily imply the essential function of α' serving as the crack path. The instability of austenite is associated with the extent of stacking fault the formation of which precedes martensitic transformation. A possibility is suggested that hydrogen-enhanced creation of strain-induced lattice defects results in both HE and the instability of austenite.
