Enhancing Hydrogen Embrittlement Resistance in Martensitic Steel through Partial Phase Transformation and Tempering

Abstract

Tempered martensite containing alloy carbide within the steel structure requires high-temperature tempering, posing a challenge for high-strength applications. In this study, to overcome such issues, a Partial Phase Transformation and Tempering (PPTT) heat treatment method utilizing the transformation bay phenomenon occurring in steel with a large addition of carbide-forming elements was proposed. The effect of this heat treatment method was proven through dilatometer experiments and microstructure observations. We confirmed that, given sufficiently fast cooling and heating rates, it is possible to create a structure combining Heavily Tempered Microstructure (HTM) that contain alloy carbide inside its structure and Low-temperature Tempered Martensite (LTM) with high strength using conventional steel composition through PPTT heat treatment. The resulting structure showed higher hydrogen embrittlement resistance than conventional tempered martensite when the HTM ratio was around 20%. To investigate investigate the mechanisms of enhancement of hydrogen embrittlement resistance, we performed thermal desorption analysis (TDA) and permeation tests, and identified that the alloy carbide contained in the HTM structure was the major cause. Furthermore, we studied the changes in HTM + LTM microstructure and its effect on hydrogen embrittlement resistance. Results showed that the lower the strength of LTM in the steel, the better the hydrogen embrittlement resistance at high hydrogen levels. Significant differences in HTM morphology were observed depending on the method of HTM creation (quenching vs. austempering). It was also found that the hydrogen trapped in HTM was not simply proportional to the HTM fraction but was greatly affected by morphology. This revealed that a microstructure with a small aspect ratio is advantageous to maximize the enhancement of hydrogen embrittlement resistance by HTM. This PPTT heat treatment concept was applied to fastener steel, and the alloying elements (Cu in this study) was adjusted for additional hydrogen trapping effects. As a result, we were able to produce steel with about twice the hydrogen embrittlement resistance under the same hydrogen injection conditions without a decrease in tensile strength. We proposed a factor, weakly trapped H content per prior austenite grain boundary, to explain this strength improvement.