The effect of low-temperature tempering on hydrogen diffusion and trapping in direct-quenched martensitic steel

Low-temperature tempering (LTT) is a common heat treatment for martensitic ultrahigh-strength steels, aimed at improving mechanical properties and enhancing resistance against hydrogen embrittlement (HE). This study investigates the effects of LTT on hydrogen (H) diffusion and trapping in a 500 HBW direct-quenched (DQ) steel with lath-martensitic microstructure. Four materials are tested: initial DQ steel auto-tempered during quenching, and the same tempered with a 1-hour holding time at 50 °C, 150 °C, and 250 °C. The given tempering temperatures are used to quantify the effects of carbon segregation (50 – 150 °C) and transformation of retained austenite to cementite (250 °C). Electrochemical hydrogen permeation and melt extraction tests were carried out to explore the impact of LTT on H diffusion and total H concentration. In addition, HE susceptibility was investigated using a novel tuning-fork test (TFT) with an incremental step loading technique. As tempering temperature increases, H concentration decreases while H diffusion accelerates. The changes in H concentration and diffusivity are correlated with the microstructure, focusing on the effects of Cottrell atmospheres and dislocation mobility, which can contribute to improved HE resistance after 250 °C tempering.