The aim of this work is to study the susceptibility of 42CrMo4 tempered martensitic steel to hydrogen embrittlement (HE). Tempering temperatures ranging between 500 and 700ºC were applied in order to analyse the relationship between microstructural variations and the deleterious effect of hydrogen in the mechanical properties of the steel. The influence of hydrogen on the mechanical behaviour of the steel was investigated by means of tensile tests on smooth and circumferentially-notched round-bar specimens. Firstly, the specimens were pre-charged with gaseous hydrogen in a pressurized reactor at 19.5 MPa and 450ºC, during 21 h. Afterwards, the amount of hydrogen introduced in the steel was determined in a hydrogen analyser. In this way, it was possible to correlate the amount of hydrogen present in the specimens during the tensile tests with the loss of tensile properties. Finally, scanning electron microscopy was employed to study both, the steel microstructures, and the fracture micromechanisms that took place during the mechanical tests.
It was observed that hydrogen solubility decreases as the tempering temperature is increased, due to the fact that hydrogen microstructural trapping is more important in distorted and high energy martensitic microstructures. Moreover, hydrogen embrittlement was greater in the grades with higher hardness (tempered at the lowest temperatures). The negative effect of hydrogen on the steel tensile properties was barely appreciated using smooth tensile specimens but it was remarkable in notched specimens (higher triaxiality), where a change in the fracture micromechanism, from ductile (microvoid coalescence, MVC) in the absence of hydrogen, to intermediate (plasticity-related hydrogen induced cracking, PRHIC) and/or brittle (intergranular fracture) in the presence of hydrogen, was clearly noticed.