According to the results of phase transformation point measurement, thermal expansion curve, metallographic structure observation and microhardness measurement, the self-designed CCT curve is obtained (as shown in the figure). In the temperature time (logarithm) curve in the figure, the key transformation temperatures (AEL, AC3, MS and BS) of the experimental steel and the microhardness at different cooling rates are reflected. The following conclusions can be drawn from the CCT curve:
1) For hypoeutectoid steel such as experimental steel, the quenching and holding temperature shall be selected above AC3 point. If the measurement result of AC3 point of experimental steel is 843 ° C, the subsequent design quenching and holding temperature shall be selected between 850-1000 ° C.
2) The experimental steel has good hardenability. Martensitic transformation occurs in the cooling rate range of 40 ° C / s-0.05 ° C / s. a certain amount of bainite structure can be obtained in the cooling rate range of 5-0.05 ° C / s. The bainite transformation point can be determined in the cooling rate range of 0.2-0.05 ° C / s. Therefore, when formulating the heat treatment process, the cooling rate should not be too large, so that the structure becomes full martensite, resulting in too low toughness and too small, so that the hardness of the experimental steel decreases a lot. The cooling rate of each cooling medium in the laboratory is roughly as follows: water cooling above 20 ° C / s, oil cooling from 15 ° C / s to 8 ° C / s, air cooling from 5 ° C / s to 2 ° C / s, air cooling from 1 ° C / s to 0.5 ° C / s, furnace cooling from 0.1 ° C / s to 0.05 ° C / s. therefore, the cooling rate of the experimental steel should not be higher than that of oil cooling.
3) In order to improve the initial properties of the experimental steel and obtain tempered martensite, the low-temperature tempering process shall be adopted. The tempering temperature shall be selected near the MS point. The measurement result of the MS point of the experimental steel is 281 ° C, so the subsequent design tempering temperature shall be selected between 250-400 ° C.
In conclusion, the heat treatment process of experimental steel should be: 850-1000 ° C insulation, oil cooling / air cooling / furnace cooling, 250-400 ° C tempering.