Microstructure and mechanical properties of a Cr–Ni–W–Mo steel processed by thermo-mechanical controlled processing

Experiments were conducted to evaluate the microstructure and tensile properties of a medium carbon Cr–Ni–W–Mo steel processed by thermo-mechanical controlled processing (TMCP) with cooling at different conditions in water, oil, air or lime followed by low tempering. Compared to normal heat-treatment processing, TMCP with water-cooling after deformation enhances the yield strength and tensile strength of the steel by ~ 323 MPa and ~ 251 MPa, respectively, due to higher dislocation strengthening and grain boundary strengthening. Meanwhile, it increases the elongation by ~ 1.76% attributed to the increase in volume percentage of the retained austenite and the refined laths of tempered martensite. Slowing the cooling rate after deformation during TMCP leads to a decrease in the strength. This results from the coupling effects by the reduction in dislocation density and volume fraction of tempered martensite together with the coarseness in martensite sizes. However, cooling rate decreasing has less influences on ductility because the improved elongation from the increase in the volume fractions of both retained austenite and lower bainite together with dislocation density decreasing is compensated by the reduced elongation from coarsened grains.