Abstract
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 tempered martensite, austempered, and quench and partitioned microstructures at hardness levels between 52-54 HRC. The quenched and tempered (Q&T) martensite conditions were produced with two different prior austenite grain sizes. The quenched and partitioned (Q&P) microstructures consisted of lath martensite, retained austenite, martensite-austenite (M/A) constituents, and intercritical ferrite in some conditions, while the austempered microstructure consisted of bainite and M/A constituents. The Q&P process generally promoted higher strength and uniform elongation than the other conditions, though also had 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, which resulted in a hydrogen level of 1-1.5 ppm in all conditions. The Q&T conditions exhibited better slow strain rate performance and notch tensile strength after hydrogen pre-charging than the other conditions. In the Q&P conditions, a higher HE susceptibility is correlated to higher quench temperatures, which had lower austenite stability and more fresh, non-tempered martensite in the initial microstructure in the slow strain rate tensile tests. The austempered condition was also relatively susceptible to HE for the same reasons. However, the HE susceptibility was comparable for all of the quenched and partitioned conditions and the austempered condition in the circular notch tensile tests. Prior austenite grain size refinement in the quenched and tempered conditions did not result in improved HE performance.