Enhanced Mechanical Properties and Ultrafine Microstructure of 18CrNiMo7-6 Steel Prepared by Quenching and Partitioning Combined Cryogenic Treatment

In this work, a novel heat treatment of quenching and partition (Q-P) combined cryogenic treatment (CT) was designed, to improve the mechanical properties of a 18CrNiMo7-6 steel. Based on the microstructure characterization and mechanical property testing, the effects of Q-P combined with CT on 18CrNiMo7-6 steel could be included as follows: (1) the grain size of 18CrNiMo7-6 steel after Q-P and CT could be refined from 0.31 to 0.20 μm; (2) the volume fraction of retained austenite in 18CrNiMo7-6 steel increased up to 10% and then decreased up to 6% for the transformation of unstable austenite to martensite during CT; (3) Q-P combined with CT could increase ultimate tensile strength of 18CrNiMo7-6 steel from 1560 to 1602 MPa, increase yield strength of 18CrNiMo7-6 steel from 1015 to 1280 MPa and increase total elongation of 18CrNiMo7-6 steel from 11.6 to 21.1%. Consequently, the Q–P combined with CT treatment is an effective method to refine the microstructure and improve the mechanical properties of 18CrNiMo7-6 steel.