Cold treatment of bearing rings

1. Effect of cold treatment

Precision bearings require high dimensional stability; After ring quenching (especially step quenching), the internal structure still retains more residual austenite, which is an unstable structure. During the storage and use of the bearing, it will constantly change, thus changing the accuracy of the bearing. Therefore, the cold treatment process is adopted, which can reduce the residual 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 inside and outside of the ferrule are uniformly cooled to room temperature and then conduct cold treatment, otherwise it is easy to crack. After cooling to room temperature, conduct cold treatment immediately (generally not 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 martensite transformation termination temperature Mf of the steel. In addition, the influence of cold treatment on mechanical properties and processability should be considered.

For GCr15 steel, - 70 ℃ is selected for cold treatment; When ferrules or equipment with low accuracy requirements are limited, the cold treatment temperature can be - 40~- 70 ℃; Ultra precision bearings can be cold treated between - 70 ℃ and - 80 ℃. The supercooled temperature affects the impact fatigue and contact life of cylindrical roller bearings.

Cold treatment and heat preservation: although the transformation of a large amount of martensite is completed in the process of cooling to a certain temperature, in order to make the surface and center of a batch of ferrules reach the cold treatment temperature uniformly, a certain heat preservation time is required, generally 1~1.5h

Tempering after cold treatment: the ferrule is placed in the air after cold treatment, and its temperature slowly rises to room temperature before tempering. Temperature rise shall not be too fast, otherwise it is easy to crack; Tempering shall be timely, otherwise the large residual stress inside the ferrule will cause the ferrule to crack, generally not more than 2h.