Effect of Mo Content on Cold and Thermal Fatigue Properties of4Cr5MoV Hot Work Die Steel

The surface and section crack morphology, microstructure and hardness changes of 4Cr5MoV hot work die steel containing different mass fraction (1.8%, 3.2%) of Mo were analyzed by self-constrained cold and thermal fatigue test from room temperature to 600℃. The effect of Mo content on cold and thermal fatigue properties of hot work die steel was studied. The results show that after 1 000 cold and hot fatigue cycles, the cracks on the surface of test steel containing 3.1wt% Mo were thin and sparse, and the crack number was significantly less than that of the test steel containing 1.8wt% Mo; the test steel containing 3.1wt% Mo had higher temper softening resistance, and high surface hardness could delay the initiation of cold and thermal fatigue cracks in the die steel. After 2 000 cold and thermal fatigue cycles, the crack propagation depth of the test steel containing 3.1wt% Mo was greater than that of the test steel containing 1.8wt% Mo, indicating the crack propagation rate was larger, which was related to the low impact toughness at room temperature and coarsening of a large amount of carbides of the test steel containing 3.1wt% Mo.