Effect of Mo-Related Precipitation Behavior on the Strengthening and Thermal Stability of 4Cr5Mo2V Die Steel

The effect of molybdenum content (1.25, 2.25, 3.0 wt.%) on the microstructure, mechanical properties, and thermal stability of 4Cr5Mo2V steel has been investigated. The increase of Mo content promotes the transition of M₂₃C₆ to M₆C and inhibits the nucleation and growth of M₂₃C₆, as supported by the transmission electron microscope observation presented in this paper. The result shows that the mechanical properties and the thermal stability at 620 °C are both strongly dependent on the Mo content. The yield/tensile strengths at room temperature of 1.25, 2.25, and 3.0% Mo steels are 1028/1287, 1080/1347, and 1114/1402 MPa; meanwhile, the elongation is 23, 26, and 27%, respectively. And the yield/tensile strengths at 620 °C of the 1.25, 2.25, and 3.0% Mo steels are 285/661, 379/714 and 335/690 MPa; meanwhile, the elongation is 23, 20, and 22%, respectively. The M₆C-type nano-carbides in the three steels enhance the tensile strength through the Orowan bypass strengthening mechanism, providing increments of tensile strength of 145 MPa (1.25 Mo), 167 MPa (2.25 Mo), and 182 MPa (3.0 Mo), respectively. It is obvious that the coarsening kinetics of solute atoms in three steels is controlled by volume diffusion during the tempering at 620 °C. The Avrami exponent n of the 1.25, 2.25, and 3.0% Mo steels at 620 °C is 0.42, 0.41, and 0.37, respectively. The calculations were in consistent with the finding that the average precipitation rate and coarsening rate of carbides in 3.0 Mo steel are the lowest.