Cryogenic treatment is an optional part of the heat treating process. It is the deep-freezing of tool steel usually to –250 to –300 degrees F for 2-3 hours. The purpose of the cryogenic treatment is to stabilize the microstructure of the tool steel.
Just as retained austenite is transformed to martensite during the tempering process, the same change occurs during the cryogenic treatment. Some grades of tool steel with high alloy content and low tempering temperatures will not completely transform to a martensitic structure during a thermal tempering process.
A-2 and D-2 are two common grades that may benefit from a cryogenic treatment. These two grades are commonly tempered at temperatures as low as 300 degrees F. The complete transformation to martensite provides for a stable structure meaning that there would be minimal size change during further processing. One to two points higher hardness may be attained due to more of the alloy content being transformed in low temperature tempering grades.
Grades of tool steel with high tempering temperatures usually have more of the retained austenite transformed to martensite during the tempering process thus reducing the effectiveness of the cryogenic treatment. If a cryogenic treatment is performed on a grade of tool steel with a high tempering temperature then the tempering temperature should be lowered 50 to 100 degrees F. There usually is very little or no increase in hardness in these grades.
It is recommended that one thermal temper cycle is performed on the tool before a cryogenic treatment is performed. Since the newly hardened and quenched tool is brittle, tempering prior to cryogenic treatment will reduce the chance of cracking due to stress induced during the process. At least one thermal tempering cycle should follow the cryogenic treatment in order to temper the newly formed martensite resulting from the cryogenic treatment.
Sometimes, it may be necessary to weld tool steel. For the process and precautions check out our