采用室温至600℃的自约束冷热疲劳试验方法对含不同质量分数（1.8%，3.1%）钼的4Cr5MoV热作模具钢进行了试验，分析了表面与截面裂纹形貌、显微组织以及硬度变化，研究了钼含量对该热作模具钢冷热疲劳性能的影响。结果表明：经1 000次冷热疲劳循环后，含质量分数3.1%钼的试验钢表面产生细小稀疏裂纹，并且裂纹数量明显少于含质量分数1.8%钼的试验钢，含质量分数3.1%钼的试验钢具有更高的抗回火软化性，高的表面硬度可以延缓模具钢中冷热疲劳裂纹的萌生；经2 000次冷热疲劳循环后，含质量分数3.1%钼的试验钢截面裂纹扩展深度大于含质量分数1.8%钼的试验钢，其裂纹扩展速率较大，这与含质量分数3.1%钼的试验钢具有较低的室温冲击韧性和大量碳化物的粗化有关

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.