Effect of quenching and tempering on structure and mechanical properties of a low-alloy 0.25C steel

Relationships between the microstructure including the dispersed particles and the mechanical properties of a 0.25%C-1.6%Si-1.5Mn-0.5%Cr-0.3%Mo steel was examined after quenching and tempering at 200, 280, 400, and 500 degrees C. The lath martensite containing the dislocation and twin substructures provides a combination of 1350 MPa yield strength with 9% elongation-to-failure and Charpy V-notch impact toughness of about 60 J/cm2. The transition eta-Fe2C carbide precipitates at tempering temperature of 200-400 degrees C, whereas cementite precipitates at tempering temperature of 500 degrees C. An increase in tempering temperature in the range of 200-400 degrees C leads to insignificant decrease in the strength and an increase in the impact toughness, while further increase in tempering temperature to 500 degrees C results in remarkable decrease in the yield and ultimate tensile strengths to 1090 MPa and 1180 MPa, respectively, and an increase in the impact toughness to 116 J/cm2. An attractive combination of the yield strength and the impact toughness is attributed to the large contribution of the solid solution strengthening to the overall yield strength and the high value of the fracture stress.