Effect of Carbon Density at Grain Boundary on Delayed Fracture Properties of Martensitic Steels

Abstract

It is well known that carbon addition enhances the strength of steel, but it also increases the sensitivity to delayed fracture. The origin of increased sensitivity to delayed fracture at a higher carbon content is not clarified yet. In this report, high-strength steels with different carbon contents were fabricated, and their delayed fracture properties were firstly evaluated. As the amount of carbon in the material increased, the amount of carbon concentration at the grain boundaries and the dislocation density also increased, and the material became more sensitive to delayed fracture. Then, a grain boundary (GB) model was created based on the samples used in the experiments, and the hydrogen effect on the cohesive energy of GB was evaluated using first principles calculations. The cohesive energy without carbon atoms was 3.47 J/m² in the presence of hydrogen, while it with one additional carbon atom was 3.70 J/m². This result was inconsistent to the experiment. Therefore, based on the results of dislocation density measurements, we assumed that more vacancies are generated in materials with a higher carbon content. By adding vacancy at GB, the cohesive energy decreased to 1.79 J/m², and showed the same tendency as in the experiment. This suggests that not only hydrogen and carbon, but also vacancies, are responsible for the decrease in delayed fracture resistance.