Nitrogen Effects on Microstructural Evolution, Fracture Behavior of Cr–Mo–V Hot‐Working Die Steel during High‐Temperature Deformation at 700 °C

Abstract

Herein, nitrogen effects on microstructural evolution, fracture behavior of Cr–Mo–V hot‐working die steel during high‐temperature deformation at 700 °C are systematically investigated. The results reveal that increasing N/C ratio can enhance the amounts and stability of undissolved V(C, N) and enable austenitizing temperature of steel increase from 1030 to 1080–1100 °C. Adding both nitrogen and nitrogen‐substituted carbon can increase the ultimate tensile strength at 700 °C from 380 to 480–490 MPa, but has the inverse effect on resistance to crack initiation. Nitrogen addition promotes the M23C6 nucleation locating on the boundary and decreases resistance to crack initiation. The mechanisms of nitrogen‐substituted carbon contributing to the improvement of resistance to crack initiation and high‐temperature strength can be ascribed to several reasons. First, allowed higher austenitizing temperatures and lower carbides coarsening rates delay matrix recovery and voids formation, ensuring grain boundary strength. Next, the amounts and coarsening rate of undesirable M23C6 located on the grain boundaries are effectively decreased, reducing the crack initiation. Then, increasing desirable V(C, N) can act as strong obstacles to the crack propagation. These results contribute to the further interpretation of nitrogen mechanisms in steels worked at high temperature.