Effects of hydrogen on the fracture toughness of 42CrMo4 steel quenched and tempered at different temperatures

Highlights: • A trend between dislocation density and hydrogen diffusion coefficient was obtained. • Only diffusible hydrogen promotes hydrogen embrittlement. • Maximum embrittlement occurred in the grades with higher yield strength. • Hydrogen embrittlement increases under low displacement rates. • Changes in the failure micromechanisms were observed due to hydrogen embrittlement. -- Abstract: The aim of this paper is to study the effect of hydrogen on the fracture toughness of 42CrMo4 steel quenched and tempered at 500, 550, 600, 650 and 700 °C. The influence of hydrogen was assessed by means of fracture toughness tests carried out on compact CT specimens, pre-charged with gaseous hydrogen in a pressurized reactor at 19.5 MPa and 450 °C for 21 h. Thermal desorption analysis (TDA) and finite element simulations (FEM) were employed to study the hydrogen diffusivity and solubility of the different steel grades. Additionally, X-ray diffraction (XRD) was used to determine the dislocation densities present in these steel microstructures and scanning electron microscopy (SEM) to analyse them and to identify the fracture micromechanisms that took place during the fracture toughness tests. According to the obtained results, dislocation density, hydrogen solubility and residual hydrogen strongly trapped in the steel microstructure were seen to decrease with increasing tempering temperature, following the apparent hydrogen diffusion coefficient the opposite trend. It was also observed that hydrogen embrittlement was much greater in the steel grades tempered at the lowest temperatures (with higher yield strength), and in tests performed at the lowest displacement rates. Moreover, a change in the fracture micromechanism was detected, from ductile (microvoids coalescence, MVC) in the absence of hydrogen, to intermediate (plasticity-related hydrogen induced cracking, PRHIC) in the case of the hydrogen pre-charged steels with relatively low yield strengths under low displacement rates and, finally, to a fully brittle behaviour (mostly intergranular fracture, IG) in the case of steels with the highest strengths, tested at low displacement rates. It was finally demonstrated that only diffusible hydrogen, hydrogen atoms able to move at room temperature, is responsible for hydrogen embrittlement.