Effect of molybdenum content on the thermal stability, competitive carbide precipitation, and mechanical properties of 5% Cr hot-work die steels

Increasing Mo is known to improve the high-temperature strength and tempering softening resistance of Cr-Mo-V hot-work die steels, yet the mechanisms governing long-term microstructural stability and mechanical property retention remain insufficiently understood. In this study, the role of Mo was investigated using three 5% Cr hot-work die steels containing 1.4, 1.8, and 2.4 wt% Mo. Tempering softening resistance was evaluated at 580-620 °C for up to 96 h, and detailed mechanical and microstructural characterization focused on 620 °C using tensile and instrumented impact testing, electron backscatter diffraction, X-ray diffraction, and transmission electron microscopy. Increasing Mo markedly improved thermal stability and strength retention. After 48 h at 620 °C, the 2.4% Mo steel retained a room-temperature yield strength of 770 MPa, an ultimate tensile strength of 1009 MPa, and an elongation of 13.94%. High-resolution transmission electron microscopy revealed coherent M 23 C 6 /M 6 C and semi-coherent M 7 C 3 /M 6 C interfaces, supporting heterogeneous nucleation of M 6 C on Cr-rich carbides. Higher Mo promoted fine Mo-rich M 2 C/M 6 C precipitation, suppressed coarsening of Cr-rich carbides, and delayed martensitic recovery, increasing the apparent tempering activation energy from 262 to 384 kJ mol −1 . This stabilization preserved precipitation and dislocation strengthening, while the more strongly retained martensitic substructure restricted plastic deformation and delayed the increase in impact toughness. These results demonstrate that Mo governs long-term mechanical stability through competitive carbide precipitation and matrix recovery, thereby controlling the balance between strength retention and plastic deformation ability.