Hydrogen Embrittlement Resistance of High Strength 9260 Bar Steel Heat Treated by Quenching and Partitioning

Hydrogen embrittlement (HE) continues to be a limiting factor in implementing high strength steel alloys in applications such as fasteners. In this work, 9260 bar steel was heat treated to produce quench and tempered (Q&T) martensite and quench and partitioned (Q&P) microstructures at hardness levels between 52 and 54 HRC. The Q&P microstructures consisted of lath martensite, retained austenite, martensite-austenite (M/A) constituents, and intercritical ferrite in some conditions. The Q&P process promoted higher strength and uniform elongation than the Q&T martensite, though also exhibited a relatively low degree of post-uniform elongation except in the lowest quench temperature condition. Slow strain rate tensile and circular notch tensile tests were performed on all conditions after electrochemical hydrogen charging at hydrogen levels of 1–1.5 ppm. The Q&T condition exhibited a better slow strain rate performance and notch tensile strength after hydrogen pre-charging than the Q&P conditions. In the Q&P conditions subjected to slow strain rate tensile tests, a higher HE susceptibility is correlated with higher quench temperatures, which had lower austenite stability and more fresh, non-tempered martensite in the initial microstructure. However, the HE susceptibility was comparable for all of the Q&P conditions in the notch tensile tests.