Investigation into the thermal stability of a novel hot-work die steel 5CrNiMoVNb

Abstract A novel hot-work die steel 5CrNiMoVNb is developed by optimizing the alloy composition of 5CrNiMoV steel. Thermal stability tests were carried out to compare the hardness evolution of the two steel types. The hardness reduction of 5CrNiMoVNb at 600 and 650°C was only 4.3HRC and 9.6HRC, while that of 5CrNiMoV steel at the same condition was as large as 6.5HRC and 17.5HRC, respectively, which suggests that the thermal stability of the 5CrNiMoVNb steel is more excellent. The thermal stability mechanism of 5CrNiMoVNb was studied based on microstructure analyses and thermodynamic calculations. This suggests that high tempering temperatures cause the coarsening of some carbides and suppress the recovery and recrystallization of the martensite matrix, which is the main reason for the slight decrease in the thermal stability. For the adding of the medium and strong carbide-forming elements, the carbides in 5CrNiMoVNb steel are mainly MC and M23C6 with low coarsening rate coefficient, and the content of these two carbides is almost constant below 670°C. The fine MC and M23C6 carbides showed strong pinning and dragging effects on the dislocations and suppressed martensite recovery and recrystallization. Therefore, the novel hot-work die steel showed excellent tempering softening resistance and thermal stability than 5CrNiMoV steel.