| Summary: | The effect of tempering on the mechanical properties, structure, and dispersion of secondary phase particles is studied in 0.4%C-2%Si-1%Cr-1%Mo-VNb steel. This steel austenitized at 900 °C with subsequent water quenching exhibits a yield stress of 1445 MPa and a lath martensite structure with MX particles of ~40 nm located in matrix and boundary M<sub>6</sub>C carbides of ~210 nm. Tempering in the temperature interval of 200–400 °C provides a yield stress of 1625 MPa due to the precipitation of ε-carbide and cementite within laths. The yield stress decreases to 1415 and 1310 MPa after tempering at 500 and 650 °C, respectively, due to the replacement of matrix carbides by boundary M<sub>23</sub>C<sub>6</sub> carbide. A Charpy V-notch impact energy of ~12 J/cm<sup>2</sup> is almost independent from tempering temperatures of up to 400 °C and increases up to ~33 J/cm<sup>2</sup> after tempering at 650 °C due to decreased yield stresses and increased plasticity.
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