JMatPro thermodynamic software is used to simulate the property diagram of the experimental steel under the designed composition system. The results are shown in Figure 1. The austenite unidirectional zone is bounded by 778 ° C, which is also the boundary formed by M23C6. With the decrease of temperature, the austenite content decreases and the ferrite content increases; Reduce the temperature to 742 ° C and precipitate cementite. At this time, the content of austenite and ferrite is the same, both of which are 50%; When the temperature decreases to 709 ° C, austenite basically disappears, and another carbide M7C3 begins to form at 662 ° C.

In order to obtain more references, the property diagram of the experimental steel is simulated by thermo Calc thermodynamic software. The results are shown in Figure 2. It can be seen from the figure that the experimental steel begins to appear austenite phase at 707 ° C and reaches austenite single-phase zone at 774 ° C. This result is very close to the simulation result of JMatPro software.

For the experimental steel, due to the small specific volume of austenite, the volume will shrink during austenite transformation during heating, so an inflection point will be formed on the thermal expansion curve, that is, the austenite transformation temperature (ac1a). When the austenitization is complete, the experimental steel meets the conditions of thermal expansion and cold contraction, and the volume will increase when the temperature continues to rise, At this time, the second inflection point appears on the curve, that is, the end transformation temperature of austenite (AC3 point). During the cooling process, with a faster cooling rate, the observed key inflection point is the starting temperature of martensitic transformation (MS point). The determination of these three key transformation points has important guiding significance for the determination of quenching holding temperature and tempering temperature of sample steel.

In this experiment, the linear expansion (or shrinkage) and temperature curve of the experimental steel under heating and cooling conditions were measured by dil805 expander. During heating, it was heated to 1000 ° C at the speed of 1 ° C / s, kept for 10 minutes, and then cooled to room temperature at the cooling speed of 40 ° C / s. The ACL AC3 and MS point temperature, and the experimental results are shown in Fig. 3. It can be seen from Fig. 3B that the AC1 and AC3 temperatures of the experimental steel are 762 ° C and 843 ° C respectively, about 50-60 ° C higher than the 707-709 ° C and 774-778 ° C simulated by the thermodynamic software, which is due to the lag of phase transformation in the actual heating process and the error of the simulation software, However, the phase transformation law simulated by JMatPro and thermo Calc thermodynamic software is consistent with the experimental steel, which still has important reference significance. As shown in Figure 3B, the MS temperature measured by thermal expansion results is 281 ° C.