Effect of dispersed retained γ-Fe on brittle crack arrest toughness in 9% Ni steel in cryogenic temperatures

In structural steels, high crack arrest toughness is required to stop brittle cracks from propagating to prevent eventual fracture of the structure. It is known that Ni can reduce the risk of cleavage fracture of α-Fe, as it exists in a solid solution as a substitutional alloying element. Additionally, Ni can enhance the stability of γ-Fe between the martensite-laths, leading to improved toughness. Although there are several interpretations of the effect of retained γ on the fracture toughness, especially regarding dynamic crack propagation, there is little knowledge on the role of retained γ. In this study, the relationship between the amount of transformation and crack arrest toughness is evaluated by an experimental approach. As a result, it is revealed that the γ-α transformation at the propagating crack tip enhances the crack arrest properties. Furthermore, compared to static loading conditions, γ-Fe seems reluctant to transform to martensite as a result of an adiabatic temperature increase due to plastic work. Stress relaxation effects due to deformation-induced transformation of γ-Fe are the proposed mechanism of enhancement of the crack arrest properties.