In the research work from 1945 to 1946, the effect of cooling rate from 180 ℃ to 20 ℃ was studied. The research on the specimen with special shape shows that, in the above temperature range, what kind of cooling conditions will usually form cracks, and what kind of cooling conditions will not form cracks.
The test piece is made of P18 steel. The shape of the gasket is keyway, and the groove is made into sharp angle which can cause stress concentration. Four cooling schedules from 100 ℃ to – 79 ℃ have been studied, that is, cooling at different rates from 100 ℃ to 20 ℃, that is, from 20 ℃ to – 79 ℃.
The test piece was heated to 1280 ℃ and quenched in hot oil at 100 ℃. Then the specimens were cooled in air to room temperature (slow cooling) or acetone (fast cooling). In this way, the cooling rate in the range of 100 ~ 20 ℃ can be adjusted. The cooling rate from 20 ℃ to – 79 ℃ can be changed by the following methods: when cooling slowly, the test piece and the cooler are cooled to – 79 ℃ (the cooler is filled with acetone and cooled with dry ice); when the cooling speed is to be fast, the test piece is immersed in the cooler whose temperature has reached – 79 ℃.
It can be seen from table 1 that rapid cooling in the range of 100 ~ 20 ℃ is dangerous because it will cause cracks. When the specimen is cooled rapidly in this range, there are no exceptional cracks in the stress concentration area and the whole surface in all cases. On the contrary, when it is cooled slowly in this range, no cracks will occur in all cases (independent of the cooling rate from 20 to – 79). It can be seen that, as long as the hardening conditions are observed (graded hardening procedure, after the prop is taken out of the hot oil tank, it must be slowly cooled to room temperature before cold treatment), even if it is rapidly cooled at sub zero temperature, it will not cause cracks due to cold treatment.