Mechanical properties and microstructure evolution of 1800 MPa grade low alloy ultrahigh strength steel during quenching and tempering process

The characterization techniques were employed like transmission electron microscope, X-ray diffraction and microstructural characterization to investigate microstructural evolution and impact of precipitate-phase precipitation on strength and toughness of a self-developed 32Si₂CrNi₂MoVNb steel during the quenching and tempering process. Research outputs indicated that the steel microstructure under the quenching state could be composed of martensite with a high dislocation density, a small amount of residual austenite, and many dispersed spherical MC carbides. In details, after tempering at 200 °C, fine needle-shaped ε-carbides would precipitate, which may improve yield strength and toughness of the steel. However, as compared to that after tempering at 200 °C, the average length of needle-shaped ε-carbides was found to increase to 144.1 ± 4 from 134.1 ± 3 nm after tempering at 340 °C. As a result, the yield strength may increase to 1505 ± 40 MPa, and the impact absorption energy (V-notch) may also decrease. Moreover, after tempering at 450 °C, those ε-carbides in the steel may transform into coarse rod-shaped cementite, and dislocation recoveries at such high tempering temperature may lead to decrease of strength and toughness of the steel. Finally, the following properties could be obtained: a yield strength of 1440 ± 35 MPa, an ultimate tensile strength of 1864 ± 50 MPa and an impact absorption energy of 45.9 ± 4 J, by means of rational composition design and microstructural control.