Influence of Microstructure Constituents on the Hydrogen‐Induced Mechanical Degradation in Ultra‐High Strength Sheet Steels

We investigate the influence of constituent phases on the hydrogen-induced mechanical degradation in two ultra-high strength sheet steels. In the complex phase (CP) steel, of which microstructure can be regarded as monolithic in terms of hydrogen behaviors, the rate of hydrogen uptake gradually decreases with the charging time, while the transformation-induced plasticity (TRIP) steel exhibits a persistent hydrogen absorption up to the charging time of 48 h, which is attributed to the presence of austenite. The mechanical degradation of TRIP steel goes beyond that of the CP steel at charging time over 12 h, coincident with the condition in which the austenite contributes to the hydrogen uptake. Both the ultra-high strength and the presence of austenite have unfavorable influence on the hydrogen embrittlement. Therefore, in the TRIP steel, it is necessary to evaluate quantitatively the critical hydrogen concentration over which more care should be taken to reduce the risk of hydrogen-induced mechanical degradation. Graphic abstract