Transformation Behavior of 9Ni Steel Under Continuous Cooling Conditions: Experiments and Simulation

To investigate the effect of cooling rate on the phase transformation behavior and mechanical properties of 9Ni steel, a 7 mm thick industrial 9Ni steel plate was selected as the research object in this study. The JMatPro software was employed to simulate and calculate key parameters, including the thermodynamic phase diagram, CCT curve, and mechanical properties. Meanwhile, static thermal simulation experiments at cooling rates ranging from 0.5 to 30 °C/s were conducted on a Gleeble-3500 thermal simulation testing machine. Microstructure characterization and property tests were carried out using a metallographic microscope, scanning electron microscope (SEM), and Vickers hardness tester, and the experimental CCT curve was subsequently plotted and compared with the simulation results. The results revealed that the microstructure of 9Ni steel changed regularly with the cooling rate. With the increase in cooling rate, the ferrite content decreased continuously, the bainite content increased initially and then decreased, and the martensite content increased continuously. At a cooling rate of 30 °C/s, the martensite content reached approximately 90%. The microhardness of 9Ni steel initially sharply increased and then stabilized with the increase in cooling rate, stabilizing at 359 HV1 at a cooling rate of 30 °C/s. The phase transformation law of the measured CCT curve is highly consistent with the simulation results, verifying the reliability and accuracy of JMatPro for predicting the phase transformation behavior and mechanical properties of 9Ni steel. This study provides a theoretical basis and data support for the precise optimization of the heat treatment process of 9Ni steel and has important practical significance for enhancing its service performance in cryogenic engineering applications.