Effect of tempering process on the cryogenic impact toughness of 13Cr4NiMo martensitic stainless steel

The effects of tempering process on the cryogenic impact toughness of 13Cr4NiMo martensitic stainless steel (13-4MSS) were investigated by experiments and crystal plasticity finite element modeling (CPFEM). The tempering structure mainly includes tempered martensite lath, reversed austenite and M23C6 carbide. The tempered martensite lath is refined by the tiny new martensite transformed by reversed austenite, and the refinement degree increases with the tempering temperature. The nucleation of reversed austenite depends on the new martensite and carbide generated during tempering, and more reversed austenite is obtained by double tempering. The cryogenic impact toughness of the material changes nonlinearly with the increase of tempering temperature. It indicates that fine martensitic lath and inverse austenite are beneficial to improve the cryogenic impact toughness of the material, while carbide particles play the opposite role. The results of CPFEM show that reversed austenite coordinates the impact force of the surrounding martensite through large plastic deformation, and eases the stress concentration. Reversed austenite even occurs DIMT behavior under large plastic deformation to further reduce the stress concentration and prevent the initiation and propagation of cracks. As a hard particle, carbide causes the stress concentration, leading to cracks initiation and propagation along the martensite boundary.