Microstructure Evolution and Strength–Toughness Trade-Off in 1Cr12Ni2WMoVNb Martensitic Stainless Steel During Tempering

The influence of tempering temperature on the microstructure evolution and mechanical properties of 1Cr12Ni2WMoVNb martensitic stainless steel was systematically investigated in the range of 200–650 °C. Microhardness, tensile strength, and impact toughness were evaluated, combined with analyses using confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), and phase characterization techniques. The results show that hardness and tensile strength initially decrease, then increase to a peak at approximately 550 °C due to secondary hardening, and finally decline significantly. In contrast, impact toughness exhibits an inverse trend, revealing a clear strength–toughness trade-off. Microstructural analysis indicates that this behavior is governed by the competition among multiple strengthening mechanisms. At intermediate temperatures, the precipitation of fine and dispersed M23C6 carbides leads to strong dislocation pinning and pronounced precipitation strengthening, resulting in peak strength but reduced plastic deformation capability. At higher temperatures, carbide coarsening and dislocation recovery weaken strengthening while restoring plasticity, thereby improving toughness. These findings clarify the temperature-dependent competition among strengthening mechanisms governing the strength–toughness evolution during tempering.