Cold treatment, also known as cold treatment, sub zero treatment and cryogenic treatment, is to continue cooling the steel parts quenched to room temperature at a temperature below MS, which is generally – 60 ~ – 190 ℃, so that the austenite transformation at room temperature has not completed transformation into martensite.
The purpose of cold treatment is:
1) to keep the precision of die parts and prevent the size change due to the change of retained austenite at room temperature;
2) to promote the transformation of non transformed austenite into martensite, so as to further improve the hardness and improve the wear resistance and service life of the parts.
The cold treatment process is determined according to the steel type used in the die parts. Generally, it is enough to take – 60-80 ℃. Excessive cooling can not make all transformation of austenite, but will increase the cost and have the possibility of cracking. It can be cooled to a lower temperature under special conditions, such as – 190 ℃. The common cooling methods are air cooling in industrial refrigerator or special cold treatment equipment; cooling in dry ice (solid C02) added with alcohol solution; this method can only be cooled to – 60 ℃, which is not easy to control temperature and consumes a lot of energy; some are in liquid nitrogen（ -At this time, in order to emphasize the deep cooling, it is different from the general cold treatment (f principle, also known as cryogenic treatment). This method is especially effective for some high chromium steel dies. During cold treatment, the transformation of retained austenite to martensite mainly occurs in the cooling process, and the intermediate retention will stabilize austenite and affect the complete transformation of martensite. After reaching the predetermined cold treatment temperature, the internal and external average time is estimated according to the size of the parts and the charging condition, and it is not necessary to extend the holding time intentionally. Cold treatment should be carried out immediately after quenching (i.e. continuous), but in order to save energy consumption, it is generally washed with cold water first, and then put into the cold treatment equipment or medium after cooling gradually. Cooling should be slow, too fast cooling rate is easy to cause cracking. After the cold treatment, take out the parts in the air and slowly rise to room temperature, and then tempering. In order to prevent cracking, low temperature tempering and then cold treatment are carried out, but the effect of cold treatment is not very good.
Cold treatment is mainly used for precision parts of cold working die.