In the quenched low-alloy high-strength wear-resistant steel, the strain energy and interface energy of quenched martensite are in a very high state, carbon is highly supersaturated in quenched martensite, in addition, the quenched residual internal stress in the structure is also very high, so martensite is usually in an unstable state. According to the principle of thermodynamics, the supersaturated solid solution carbon atoms of martensite are desolved spontaneously, so that martensite itself can be spontaneously transformed into a relatively stable and strong structure. However, the spontaneous adjustment rate of this process at low temperature is too small, so it is difficult to complete spontaneously at room temperature. Therefore, the adjustment of carbon atom solid solution must be accelerated with the help of thermal activation of tempering heating in order to meet the demand of production. Therefore, we believe that tempering treatment is an indispensable process after quenching treatment in production.
In order to investigate the effect of tempering process on the mechanical properties and microstructure of experimental steel, the air-cooled and furnace cooled samples kept at 900 ° C for 2h were tempered at 250-600 ° C (step 25 ° C) for 2 hours respectively. The test results of mechanical properties are shown in the figure.
It can be seen from figure a that the impact energy of air-cooled samples is distributed between 10.18-20.31j and that of furnace cooled samples is distributed between 18.02-28.32j. During tempering, the impact energy of both samples shows two valley values at 350 ° C and 500 ° C, corresponding to the first and second types of return fire brittleness respectively, Attention should be paid to avoid when designing process parameters. The hardness results of different tempering temperatures in Figure b show that the hardness of air cooling is slightly higher than that of furnace cooling. When tempering above 500 ° C, the microstructure changes to tempered sorbite and the hardness decreases obviously. Therefore, the experimental steel should be tempered at low temperature.