Isothermal and thermomechanical low-cycle fatigue of the hot work steel X38CrMoV5-3 with and without overaging heat treatment

In this work, the martensitic hot work steel X38CrMoV5-3 is investigated by isothermal and thermomechanical fatigue tests conducted between 20 and 650 °C with mechanical strain amplitudes from 0.4 to 1%. The fatigue behavior of the material hardened and tempered to 54 HRC is compared to conditions subjected to different overaging heat treatments, resulting in reduced hardness levels. Time- and temperature-dependent deformation including thermal and cyclic softening as well as stress relaxation and rate-dependent effects are analyzed with pronounced changes above 500 °C depending on heat treatment. Overaging affects fatigue life positively or negatively, depending on temperature and strain amplitude. At lower temperatures, materials with high hardness tend to exhibit partially brittle fracture surfaces, while at high temperatures a creep-enhanced low-deformation ductile fracture and oxidation is observed. In-phase thermomechanical fatigue conditions are more detrimental than out-of-phase TMF conditions. Classical fatigue damage parameters are not capable of adequately capturing the effects of temperature, phase relationship and heat treatment on fatigue life.