Effect of microstructure on hydrogen embrittlement resistance of 1400 MPa grade fastener 42CrMoVNb steel

The hydrogen embrittlement (HE) susceptibility of ultra-high strength fastener steels is a critical factor for their practical applications. To explore the influence of microstructure on HE for 1400MPa grade fastener steel, two heat treatment processes, austempering and quenching and tempering (QT), were carried out for 42CrMoVNb steel. The microstructures were observed and analyzed by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Slow strain rate test (SSRT) revealed that the austempered 42CrMoVNb steel exhibited significantly superior resistance to HE compared with the tempered condition. The microstructure of the austempered steel consisted of lower bainite and martensite, with carbides distributed within the bainite laths and absent at the original austenite grain boundaries. Additionally, the lower dislocation density in this microstructure further enhances its resistance to HE. These findings suggest that austempering treatment can effectively reduce the HE susceptibility of ultra-high strength fastener steels while maintaining the required strength level of fasteners.