4.2. Comparisons of the cooling process
The cooling curves at 5 mm from the quenched surface of the samples during different multi-stage quenching processes are shown in Fig. 15. For convenience of observation, the curves were divided into three parts. It can be observed from Fig. 15(a) that the temperature drops rapidly in the initial 10 s, and the sam- ple C has a clearly higher cooling rate than others. This means that if the heat transfer coefficient is constant, the high pressure with low flux density has a better cooling effect in the initial stage. The overlapping curves of sample A and B split apart in a short time, because the temperature of the quenched surface drops ex- tremely fast and makes the second stage start soon. In the second stage, the heat transfer coefficient of sample B is larger, so curve B becomes lower and gradually closes to curve C. During 15–500 s in the quenching process, as shown in Fig. 15(b), the decline rates of the three curves appear diminishing trend. Because test A has a longer duration of the second stage (about 20 s), the rapid decline rate of curve A maintains longer. Finally, when all the three tests reach the third stage, the gap between them becomes constant, as shown in Fig. 15(c).