Thermal stability and softening mechanism of a new type of high nitrogen hot-work die steel 3Cr5Mo2SiVN

A new type of high nitrogen hot-work die steel 3Cr5Mo2SiVN based on the idea of substituting C with N, proper Mo addition, and proper Si and V subtraction was designed, exhibiting higher microstructure and hardness stability than the frequently-used H13 steel. The results showed that the softening mechanism consisted of the coarsening and decomposition of laths, the formation and coarsening of precipitates, the annihilation and rearrangement of dislocations and the formation of sub-grains. The superior thermal stability of 3Cr5Mo2SiVN steel could be attributed to the low precipitate and dislocation recovery. First, the higher activation energy inhibited the diffusion of alloying elements, thus delaying the formation and coarsening of precipitates. Second, the undissolved M(C, N) exhibited a higher solid solution temperature than the undissolved MC, and M3C had a stronger heat absorption ability. In addition, the nano-carbonitride M(C, N) exhibited lower lattice disregistry and interface energy than the nano-carbide MC. Hence, the undissolved M(C, N), M3C and nano-carbonitride M (C, N) showed a higher precipitate stability. Third, in terms of dislocation evolution, the higher stability of lattice constant and solid solution of interstitial atoms as well as the smaller carbide size and lower precipitation content significantly inhibited the dislocation recovery. The preservation of prior austenite grain boundaries and lath boundaries effectively hindered the dislocation migration. After isothermal tempering at 580, 600 and 620 degrees C for 96 h, the hardness decreased to 41.1, 35.4 and 31.7 HRC with softening rate 4.6, 8.5 and 10.0 for 3Cr5Mo2SiVN steel, and 35.2, 29.3 and 28.1 HRC with softening rate 7.2, 11.2 and 11.5 for H13 steel. The difference in hardness and softening rate at 600 degrees C was the most prominent, attaining respective values of 6.1 HRC and 2.7, respectively.