Effect of Quenching in Dual-phase Region on Microstructure and Mechanical Properties of 7Ni Steel

The influence of quenching temperature and holding time in dual-phase region on microstructure and mechanical properties of 7Ni steel was studied by X-ray diffractometer, scanning electron microscopy and thermal dilatometer. Based on the thermal expansion curve, the production and transformation rate of reversed austenite of 7Ni steel under each stage of the QLT treatment was quantitatively measured, and the transformation process of reversed austenite was revealed. The results indicate that the transformation of reversed austenite during QLT treatment successively goes through four stages: quenching temperature holding, quenching cooling, tempering temperature holding and tempering cooling. Appropriately improving the quenching temperature in dual-phase region and extending the holding time is beneficial to the transformation of reversed austenite during tempering, and also promotes its uniform distribution, which consequently reduces the strength and enhances the ductility as well as toughness of 7Ni steel. Under the quenching in dual-phase region treatment at 660 degrees C for 30min, the comprehensive mechanical properties of the 7Ni steel are the best, which completely meet the mechanical property requirements of 9Ni steel stipulated by ASTM A553.