Effect of Precipitation on Hardening and Toughening of Nitrogen‐Alloyed H13 Steel

Abstract

The optimum hardening and toughening heat treatment of a new type of nitrogen‐alloyed H13 hot‐work die steel is investigated using hardness and toughness measurements and optical microscopy. The effect of precipitation upon hardening and toughening of nitrogen‐alloyed H13 steel is also analyzed with transmission electron microscopy, X‐ray diffraction and the Thermo‐Calc thermodynamic software. The results show that nitrogen alloying improves the hardness of H13 die steel in NADCA#207‐2003 by 3–5 HRC with no reduction in toughness after a special heat treatment process. The tempered stability of H13 die steel is also improved by nitrogen addition, and thermodynamic analysis suggested that the nitrogen in H13 steel has two strengthening modes. First, nitrogen promotes the precipitation of vanadium as V(C, N), which is finer and more stable than VC and exerts a stronger pinning effect on restraining the growth of austenitic grains during the heating stage of the quenching process (1050 °C). The contribution of V(C, N) toward grain refinement is estimated to be about 3.2× that of VC in the tested steel. Second, during the tempering process most of the N atoms in V(C, N) are solid‐dissolved into the matrix, inducing crystal lattice distortions and enhancing the solid solution strengthening effect.