Abstract
Quenched and tempered SCM435 steel round bar specimens with circumferential notch, whose tensile strength is 1100MPa class, were charged with hydrogen and subjected to various types of loading for evaluating hydrogen embrittlement susceptibility.,
They were followed by TDA (thermal desorption analysis) . As reported elsewhere, all the strength with different strain rates was not determined by the amount of diffusible hydrogen, which is the amount of desorbed hydrogen under 250°C in TDA. Applying Gaussian distribution function to TDA desorption curves for peak separation, the state of hydrogen that means the kinds and amount of hydrogen traps were evaluated. The state of hydrogen after CSRT (constant strain rate test) was different from that of SSRT (slow strain rate test) or CLT (constant load test) . In SSRT or CLT, the applied stress served to reduce the hydrogen trapped by dislocation and to increase the hydrogen trapped by grain boundary. This hydrogen along grain boundary can be rationalized the change in the nominal fracture strength. The more hydrogen is trapped by grain boundary, the lesser is the nominal strength accompanying grain boundary facet. In case of no or very few grain boundary facet, the strength is determined by the amounts of vacancy and vacancy cluster, as explained by the HESIV (hydrogen enhanced strain induced vacancy) theory.
