Effect of tempering on microstructure and mechanical properties of 3Mn-Si-Ni martensitic steel

The dependence of microstructures and mechanical properties on tempering temperature (from 180 to 650 degrees C) in a designed 3Mn-Si-Ni martensitic steel was systematically analyzed. Microstructure was characterized using scanning and transmission electron microscopy; mechanical properties were measured using uniaxial tensile test and Charpy V-notch impact test. After tempering at different temperatures, recovery, partial recrystallization, carbides precipitation and decomposition of residual austenite were observed. After tempering at 230 degrees C, an excellent combination of strength (1550 MPa) and toughness (91.5 J) was achieved, due to high dislocation density and e-carbides precipitation. However, with an increase in tempering temperature from 320 to 550 degrees C, tempered martensite embrittlement was observed, where impact energy was similar to 10 J. It was ascribed to cementite formation instead of transition carbides and decomposition of residual austenite. With an increase in tempering temperature up to 650 degrees C, high fracture impact toughness of 75 J was obtained with deteriorated tensile strength of 850 MPa due to strong recovery and partial recrystallization.