After the workpiece is quenched and cooled to room temperature, it is immediately placed in an environment lower than room temperature for a certain period of time, and taken out and placed in room temperature. This treatment lower than room temperature is called cold treatment. Industrial freezers are often used for bearing steel cold treatment.
1.Function of cold treatment
Precision bearings have higher requirements for dimensional stability; after ring quenching (especially step quenching), there are still more retained austenite in the internal structure, which is an unstable structure. In the process of storage and use of bearings, it will constantly change, so as to change the accuracy of bearings. Therefore, the cold treatment process can reduce the retained austenite in the structure and slightly improve the hardness of the parts.
2.Cold treatment process
Stay at room temperature after quenching: after quenching, make sure that the inner and outer rings are evenly cooled to room temperature before cold treatment, otherwise it is easy to crack. After cold to room temperature, immediately cold treatment (generally no more than 30min), otherwise the transformation from austenite to martensite will be stopped.
Cold treatment temperature: the temperature of cold treatment is mainly based on the martensitic transformation termination temperature MF of the steel, in addition, the influence of cold treatment on mechanical properties and technological factors should be considered.
Cold treatment and heat preservation: Although a large number of martensite transformation is completed in the period from cold to a certain temperature, a certain heat preservation time, generally 1-1.5H, is required to make a batch of ferrule surface and center reach the cold treatment temperature uniformly.
Tempering after cold treatment: put the ferrule in the air after cold treatment, and conduct tempering in time after the temperature rises to room temperature slowly. Temperature rise shall not be too fast, or it is easy to crack; tempering shall be carried out in time, or the large residual stress inside the ferrule will cause the ferrule to crack, generally no more than 2H.