关键词: QLT处理, 两相区淬火温度, 回火温度, 逆转变奥氏体, 强韧性

Abstract: 7Ni steel was manufactured by industrial production line, and the effect of quenching, lamellarizing and tempering (QLT) treatment on microstructure and properties of the 7Ni steel was studied. The properties of the 7Ni steel treated by QLT were tested, and the microstructure along thickness direction and reversed austenite content were characterized by SEM and XRD. The results show that the 7Ni steel obtain the optimum strength and toughness after austenitizing quenched at 820 ℃, intercritically quenched at 680-700 ℃ and tempered at 540 ℃, the mechanical properties meet the standard requirements. The yield strength is between 616 MPa and 639 MPa, the tensile strength is between 731 MPa and 752 MPa, the elongation after fracture is between 23.1% and 25.8%, the impact absorbed energy is between 226 J and 248 J at －196 ℃, and the drop hammer at －196 ℃ is qualified. In addition, the homogeneity of microstructure along thickness direction is excellent, and the microstructure is mainly composed of ferrite, fine tempered martensite and a small amount of reversed austenite.

Key words: quenching, lamellarizing and tempering (QLT) treatment, intercritical quenching temperature, tempering temperature, reversed austenite, strength and toughness