Analysis of fracture toughness in high Co-Ni secondary hardening steel using FEM

The microstructure and mechanical properties of ultrahigh strength steels (300M, Aermet100 and M54) were analyzed. The experiment results showed that the steels with austenite layers at the boundary of martensite laths had higher KIC than those without austenite layers. Finite element method (FEM) was used to establish models based on the actual microstructure of the steels. Extended finite element method (XFEM) was used to study the crack path and crack growth rate. Contour integral method (CIM) was used to study the best thickness of austenite layer, the effect of crack propagation direction and the value of KIC. According to the simulation results, 10-15 nm was the best thickness of austenite layer for the fracture toughness of the steels with martensite as matrix. The simulation results of normalized value of KIC were close to the experimental results.