The heat treatment operation of continuously cooling the quenched workpiece from normal temperature to a lower temperature to transform the retained austenite into martensite is called cold treatment. The purpose of cold treatment is to improve the hardness and wear resistance of steel, and stabilize the size of the workpiece. It is mainly used for bearings, tools and some carburized parts.
- Cold treatment process of bearing steel
The cold treatment of bearing steel often uses a freezer and dry ice alcohol solution. Precision bearings have higher requirements for dimensional stability; after the ring is quenched (especially staged quenching), the internal structure still retains more retained austenite. This retained austenite is an unstable structure. When storing and using the bearing In the process, changes will continue to change the accuracy of the bearing. For this reason, the cold treatment process is adopted, which can reduce the retained austenite in the structure and slightly increase the hardness of the parts.
After quenching and cooling the workpiece to room temperature, it is immediately placed in an environment below room temperature for a certain period of time, and then taken out and placed at room temperature. This treatment below room temperature is called cold treatment. The cold treatment of bearing steel often uses a freezer and dry ice alcohol solution. Stay at room temperature after quenching: After quenching, the material must be cooled uniformly inside and outside to room temperature and then cold treated, otherwise it will be easy to crack. Cold treatment immediately after cooling to room temperature (generally no more than 30min), otherwise it will stop the transformation of austenite to martensite change.
2, cold treatment temperature
The temperature of the cold treatment is mainly based on the martensitic transformation termination temperature Mf of the steel. In addition, the influence of the cold treatment on the mechanical properties and the manufacturability should also be considered. For GCr15 steel, the cold treatment is -70℃; when the precision requirements of ferrules or equipment are limited, the cold treatment temperature can be -40～-70℃; for ultra-precision bearings, it can be between -70℃～-80℃ Perform cold treatment in between. The temperature of the subcooling affects the impact fatigue and contact life of the bearing.
3, cold treatment heat preservation
Although the transformation of a large amount of martensite is completed when it is cooled to a certain temperature, in order to make the surface and core of a batch of ferrules reach the cold treatment temperature uniformly, a certain holding time is required, generally 1～1.5h.
- Tempering after cold treatment
The ferrule is placed in the air after cold treatment, and its temperature is slowly raised to room temperature and then tempered in time. The temperature rise should not be too fast, otherwise it is easy to crack; the tempering is timely, otherwise the larger residual stress inside the ferrule will cause the ferrule to crack, generally not more than 2h.
- cold treatment time
For the length of holding time of cold treatment, some scholars believe that the amount of retained austenite transformed into martensite depends only on the temperature reached by cooling, and the retention time at low temperature does not matter, because retained austenite-martensite transforms at low temperature The cooling is completed in an instant, so there is no need to keep it at a low temperature, just let the workpiece cool from the outside to the inside. It is also believed that the cold treatment time should be determined according to the thermal conductivity of the workpiece, the volume, the time required for cooling through, and the stability of the transformation of retained austenite. The retained austenite in the steel is fully transformed and is more conducive to the formation of carbide particles. After the transformation is completed, the hardness of the parts will not change significantly. Therefore, the holding time of cold treatment also needs to be reasonably controlled according to the actual situation.
In fact, there are many studies on cold treatment of bearing steel, including GCr15 steel, aviation high temperature bearing steel, carburized bearing steel, etc., except for the reduction in toughness (decreased impact toughness, 21% reduction), almost all of the research All show the many advantages of cold treatment: reduced Paralympics; increased hardness; increased wear resistance; increased tensile strength; fine carbide precipitation, increased fatigue life, etc.