Abstract
The notable degradation of mechanical properties of high-strength steels in the presence of hydrogen occurs when hydrogen attains a certain critical concentration in the steel. In this paper, we determine the hydrogen critical concentration and its influence on the mechanical performance of a 600-HBW hot-rolled martensitic steel by means of constant extension rate tests (CERT) and constant load tests (CLT) followed with thermal desorption spectroscopy hydrogen measurements. The steel manifests sensitivity to hydrogen with a tensile strength reduction up to 25% of ultimate tensile strength (UTS) at critical hydrogen concentrations determined to be about 1.1 wt.ppm and 50% of UTS at hydrogen concentrations of 2 wt.ppm. No further strength degradation was observed for hydrogen concentrations up to 4.8 wt.ppm. Under CLT, the steel does not show sensitivity to hydrogen at applied loads below 50% of UTS under continuous electrochemical hydrogen charging up to 85 h. The results are discussed in terms of the interplay and contributions of local hydrogen concentrations and local stress states, accompanied with the presence of total average hydrogen reducing the general plasticity of the steel. In addition, the effects of varying strain rates on the determination of critical concentration and mechanical performance of the studied steel are discussed, as well as the possible mechanisms of the critical behaviour.