Comprehensive effect of nitrogen on Cr-Mo-V hot-working die steel with enhanced strength and toughness

Abstract This study demonstrated the possibility of maximizing the role of vanadium in Cr-Mo-V die steel using trace amounts of nitrogen. Extraordinary strength and toughness were achieved after suitable treatments owing to an ideal combination of the various hardening mechanisms, particularly grain refinement strengthening and precipitation hardening. The comprehensive effect of nitrogen in this steel type was investigated using transmission electron microscopy and three-dimensional atom probe tomography (3DAP), and the applicability and validity of conventional strengthening theories were further discussed. Finally, a theoretical model was built to estimate the C:N ratio of V(C, N). The results show that nitrogen addition increases the undissolved V(C, N) and exerts significant effect on the microstructure characteristics such as grain refinement and tempering precipitate composition. Nitrogen migrates along the (100) plane of V(C, N) to the (110) plane of the matrix during the tempering process. Nitrogen alloying is an effective method for improving the properties of Cr-Mo-V die steels, and inspires a new method of maximizing the vanadium attributes in Cr-Mo-V die steel via trace nitrogen. These results are important not only for understanding the strengthening mechanisms of die steel, but also for future high-performance steel development.