Abstract: Thermo-mechanical fatigue behavior of 4Cr5Mo2V hot work die steel was studied by changing the cycle lower limit temperature in different temperature ranges (100-600, 160-600, 200-600 and 300-600 ℃) using MTS thermo-mechanical fatigue hydraulic servo machine. The macro morphology and microstructure of the TMF specimens were analyzed by ultra depth of field microscope, optical microscope (OM) and scanning electron microscope (SEM). The results show that the thermo-mechanical fatigue lag loop of the tested steel has obvious asymmetry. The lower the lower limit temperature, the larger the area enclosed by the hysteresis loop, and the shorter its fatigue life. The larger the temperature range, the larger the width of the main crack, the more tortuous the propagation path and the more serious the damage. With the increase of lower limit temperature, the coarsening degree of martensite and carbide in the tested steel increases. When the temperature range is 300-600 ℃, the continuous action time of temperature cycle and mechanical cycle is longer, and the coarsening phenomenon of martensite laths and carbide precipitation is more serious.

Key words: hot work die steel, thermal-mechanical fatigue behavior, lower limit temperature, microstructure