MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TEMPERED 3Cr-3Mo SECONDARY HARDENING STEEL

The microstructure and mechanical properties of a secondary hardening steel containing 3 % Cr and 3 % Mo in quenched and tempered condition were investigated. It is found that increasing the tempering temperature gives rise to successive precipitation of M 3C, M 2C and M 7C 3 type carbides, resulting in a peak secondary hardening intensity at 575 ℃. When tempered at 400~575 ℃, the steel has high tensile strength of about 1600 MPa and low impact toughness of 30 J/cm 2. Drastic declines in hardness and strength as well as an increase in impact toughness are observed when the tempering temperature is increased to above 600 ℃. Tempering at 640 ℃ results in a microstructure with fine M 2C carbides in the α-matrix, which ensures a combination of high strength of 1100 MPa and good impact toughness of 55 J/cm 2.