Effects of two-step austenitizing processes on hydrogen evolution, permeation, and cracking behaviors of ultra-high-strength martensitic steel

Abstract

This study examined the effects of microstructural modifications by two-step austenitizing, involving successive martensite-austenite reversion processes, on the hydrogen embrittlement (HE) of ultra-high-strength steel with a tensile strength of more than 1500 MPa. To achieve a deeper understanding of the hydrogen evolution, permeation, and cracking behaviors of steel samples consisting of martensitic structures with different prior austenite grain (PAG) sizes and/or cementite (Fe3C) particles, a series of experimental methods were employed: electrochemical impedance and permeation test, slow-strain rate test (SSRT), and hydrogen microprinting test (HMT). The test results revealed that the undissolved Fe3C remained in the martensite, even after austenitizing at above the A3 temperature, promoted hydrogen evolution on the surface. On the other hand, a reduction in the PAG size with a higher kernel average misorientation (KAM) value, realized by two-step austenitizing processes, led to a decrease in the hydrogen diffusion kinetics and an increase in hydrogen solubility (i.e., total concentration of diffusible hydrogen (Ctot)). Nevertheless, the steel sample developed by two-step austenitizing showed the lowest HE-index (i.e., degree of hydrogen-induced mechanical degradation) from SSRT. Based on HMT using Ag decoration locating hydrogen trapping, the contradictory results between the electrochemical permeation and SSRT indicated that the local concentration of hydrogen trapped preferentially at the PAG boundaries could be more critical to failure by HE rather than the Ctot in steel.