Effects of cryogenic and tempered treatment on the hydrogen embrittlement susceptibility of TRIP-780 steels

Cryogenic and Tempered (CT) treatments were performed on commercial TRIP 780 steels in order to reduce the hydrogen embrittlement (HE) susceptibility. The HE behavior was assessed immediately after cathodically hydrogen charging on both CT treated and untreated samples. Slow strain-rate tensile (SSRT) tests were conducted to evaluate their HE performance. It is shown that samples with CT treatments behave higher resistance to HE comparing with their untreated counterparts. Meanwhile, microstructure characterization and magnetization measurements were adopted to reveal the evolution of retained austenite (Ar) and its stabilization due to CT treatment. Moreover, hydrogen-induced cracking (HIC) accompanied with martensite phase transformation in TRIP steel was studied by electron backscattering diffraction (EBSD) technique and it was proved that cracks initiated from the fresh untempered martensite inherited from phase transformation of unstable Ar upon straining. Finally, results in this study demonstrate the relationship between Ar stability and HE susceptibility, and provide a possible solution to reduce HE susceptibility in TRIP steels.