Low-cycle fatigue behavior of 4Cr5MoSiV1 hot-work die steel at 700 ℃

4Cr5MoSiV1 hot-die steel exhibits excellent thermal fatigue and comprehensive mechanical properties, and it is widely used in hot forging die and hot extrusion die. Under actual service conditions, mold cavity temperature reaches 700 ℃ during mold operation. Mold cavity surface produces tension and compression strain owing to acute heat and cooling-constraints of subsurface layer, thereby resulting in local plastic deformation of mold and low-cycle fatigue. Therefore, low-cycle fatigue behavior of 4Cr5MoSiV1 steel at 700 ℃ is studied to provide reference data for component design and life prediction of 4Cr5MoSiV1 steel. The effect of total strain amplitude on low-cycle fatigue behavior of 4Cr5MoSiV1 steel at 700 °C has not been studied. The influence of total strain amplitude on the low-cycle fatigue behavior of 4Cr5MoSiV1 steel at 700 ℃ was studied using the low-cycle fatigue test with an axial strain amplitude control, including cyclic stress-response behavior, cyclic stress-strain behavior, cyclic hysteresis loop, and strain-fatigue life behavior. Results show that, with the increase of the total strain amplitude from 0.2% to 0.6%, the cyclic stress responses of 4Cr5MoSiV1 steel at 700 ℃ show the characteristics of cyclic hardening and recycling softening, and the maximum stress amplitude increases from 220 MPa to 308 MPa. As the total strain amplitude increases, the low-cycle fatigue life of 4Cr5MoSiV1 steel at 700 ℃ decreases from 6750 cycles to 210 cycles, and its transition life is approximately 1313 cycles. The results of fatigue fracture morphology analysis show that the crack mainly occurs on the surface of the sample during the high-temperature low-cycle fatigue. With the increase in the strain amplitude, the crack source gradually increases, the distance between fatigue stripes widens, and the fracture mode changes from ductile to brittle fracture. The results of TEM analysis show that the cyclic softening may be related to the change of lath structure to cellular structure, dislocation annihilation of matrix, carbide precipitation, and coarsening.