Effect of Rolling after Heat Treatment on Hydrogen Embrittlement Susceptibility for High Strength Steel Fasteners

Steel fasteners comprising two different metallurgical structures were investigated for hydrogen embrittlement (HE) susceptibility by incremental step load testing. The metallurgical structures examined consisted of tempered martensite obtained by quenching and tempering and lower bainite obtained by austempering. It has been shown that lower bainite exhibits marginally lower HE susceptibility when tested under moderate hydrogen charging conditions (e.g., -1.0 V). At the most severe hydrogen charging potential of -1.2 V, both microstructures are equally embrittled. The current paper examines the effect of the sequence of the fabrication process, specifically the effect of rolling the threads before and after heat treatment (i.e., quenching and tempering or austempering). The results show irrespective of the metallurgical structure, rolling the threads after heat treatment causes a significant decrease in HE susceptibility. These findings are attributed to the presence of high dislocation density when thread rolling is performed on hardened parts as a final manufacturing step.