After quenching at the most appropriate temperature, the tool should be cooled to the temperature of the surrounding medium before cold treatment.
In order to obtain the best effect of cold treatment, the following conditions must be observed: for high speed steel, the time interval between complete cooling to room temperature after quenching and before cold treatment shall not exceed 30 minutes; for X12 steel which can be secondary hardened after quenching, the above interval shall not exceed 80 hours. A longer time in air can cause austenite stabilization. The effect of re cooling treatment is to prevent austenite from transforming into martenite, which substantially reduces the effect of cold treatment.
Small and medium-sized workpieces can be put into the cooling liquid or freezer with the temperature of – 79 ~ – 80 ℃. During cold treatment of large workpieces, the temperature of coolant or freezer should not be lower than – 40 ~ – 50 ℃.
The holding time of cold treatment of workpiece is determined by the time required for cooling to – 75 ~ – 80 ℃.
Prolonging the holding time at sub zero temperature can not actually make more transformation of austenite. In the process of cold treatment, the important time is only to make the whole volume of the workpiece reach the working temperature (- 75 ~ – 80 ℃). Practice has proved that it takes 20-30 minutes for the whole volume of medium-sized workpiece to be cooled to – 75 ~ – 80 ℃ coolant or the same temperature of freezer.