Microstructural evolution and mechanical property changes of a new nitrogen-alloyed Cr–Mo–V hot-working die steel during tempering

Highlights: • Microstructural evolution of nitrogen-alloyed Cr-Mo-V die steel during tempering was analyzed by 3DAP and HRTEM. • Quantification of contribution from various strengthening factors to die steel was analyzed. • Mechanisms contributing to the improvement of tempering stability by trace nitrogen were clarified firstly. In-depth understanding of the impact mechanism of nitrogen on tempering behavior of hot-working die steel is still limited. In present work, the microstructural evolution and mechanical property change of new nitrogen-alloyed Cr–Mo–V hot-working die steel during long-term tempering were investigated by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), eletron backscattered diffraction technique (EBSD) and 3D atom probe tomography (3DAP). The impact mechanism of nitrogen on tempering stability were discussed in detail. The results reveal that whether nitrogen is beneficial to the tempering stability of steel varies with quenching temperature. Trace nitrogen exerts a significant effect on the allowed quenching temperature and metastable M₃C carbides, affecting the coarsening rate and transformation of M₂₃C₆ carbides during tempering process. Tempering stability was improved by nitrogen mainly by enhancing precipitation hardening and grain refinement strengthening, which account for about 70% of total yield strength. Improving effect of nitrogen in precipitation hardening is more distinct with the increase of tempering time. These results are helpful for understanding the mechanisms of nitrogen in steels worked at high temperatures.