Cold treatment of bearing rings

After the workpiece is quenched and cooled to room temperature, it is immediately placed in the environment below room temperature for a certain period of time, and then taken out and placed in room temperature. This kind of treatment below room temperature is called cold treatment. Chiller and dry ice alcohol solution are often used in bearing steel cold treatment.

1. Function of cold treatment

Precision bearings require high dimensional stability; After ring quenching (especially step quenching), the internal structure still retains more retained austenite. This retained austenite is an unstable structure, which will constantly change in the process of storage and use of bearings, thus changing 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 inside and outside of the ring are evenly cooled to room temperature before cold treatment, otherwise it is easy to crack. Cold treatment immediately after cooling to room temperature (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 processability should be considered.

For GCr15 steel, cold treatment temperature is - 70 ℃; The temperature of cold treatment can be from - 40 ℃ to - 70 ℃ when the precision requirement of ferrule or equipment is limited; Ultra precision bearing can be cold treated between - 70 ℃ and - 80 ℃. The temperature of supercooling affects the impact fatigue and contact life of bearings.

Cold treatment and heat preservation: Although the transformation of a large number of martensite is completed when it is cooled to a certain temperature, in order to make the surface and core of a batch of rings reach the cold treatment temperature evenly, it needs a certain heat preservation time, generally 1 ~ 1.5h.

Tempering after cold treatment: put the rings in the air after cold treatment, and temper in time after the temperature rises to room temperature slowly. The temperature rise should not be too fast, otherwise it is easy to crack; Tempering in time, otherwise the larger residual stress inside the ring will lead to the ring cracking, generally not more than 2 hours.