Abstract: Changes of microstructure and reverse austenite content of 7Ni steel at different tempering temperatures during quenching, lamellarizing and tempering (QLT) were analyzed by means of OM, SEM, TEM and XRD, and the influence of tempering temperature on cryogenic strength and toughness of 7Ni steel was studied. The results show that with the increase of tempering temperature, the tensile strength of 7Ni steel increases gradually, while the low temperature toughness increases first and then decreases. In the process of tempering temperature increasing from 560 ℃ to 620 ℃, the martensite of the 7Ni steel changes from thick to uniform dispersion and fine, and the tensile strength of the steel gradually increases. When the tempering temperature is low, the martensite recovery in the steel is insufficient, the reverse austenite content is less, and the low temperature toughness is low. With the tempering temperature increasing, the reverse austenite content of the 7Ni steel increases continuously, but the stability decreases. A large number of unstable reversed austenite transforms at low temperature, which is detrimental to the improvement of low temperature toughness. The low temperature toughness of the 7Ni steel increases first and then decreases as the tempering temperature increases, and the best low temperature toughness is obtained at 580 ℃.

Key words: 7Ni steel, tempering, microstructure, cryogenic mechanical properties, reverse austenite