From the composition, original structure to quenching and tempering process, many factors affect the content of retained austenite after quenching and tempering.
Quenching cooling rate. Without non martensitic transformation, the slower the cooling rate is, the more the retained austenite content is. For example, the retained austenite content of GCr15 steel quenched at 850 ℃ in water, kerosene and engine oil is 8%, 10% and 14%, respectively. The same effect can be achieved by increasing the size of the parts. For example, the diameter of GCr15 steel austenitized at 850 ℃ is 5-8mm, and the retained austenite content of 50 mm sample after quenching is 15% and 19% respectively.
Quenching and cooling process. In the process of quenching and cooling, if it stays at a certain temperature below m point, the amount of retained austenite will increase due to the thermal stabilization of austenite. In addition, after cooling to room temperature, the longer the residence time at room temperature before cryogenic, the more retained austenite after cryogenic.
Cooling temperature and additional cooling. Bearing parts are usually tempered after oil quenching and air cooling to room temperature or after hot water cleaning. The higher the room temperature is, the more amount of retained austenite will be. In order to reduce the amount of retained austenite, additional cooling can be carried out after quenching and cooling. The commonly used method is to use the ultra-low temperature industrial refrigerator. The temperature can be – 150 ~ – 20 ℃.
From the aspect of dimensional stability, the less the content of retained austenite is, the better the stability is. However, in consideration of the favorable effect of participating in the Olympic Games on the mechanical properties, a certain amount of retained austenite should be retained on the premise of meeting the requirements of dimensional stability. For specific bearing parts, the amount of retained austenite volume shall be determined according to the specific working condition and performance requirements of the bearing, mainly considering the dimensional accuracy requirements of the bearing.
For instrument bearings, home appliance bearings, office machinery bearings and other micro and small bearings, the load borne by the bearing studio is light and stable, and the failure mode is mainly due to the loss of dimensional accuracy, or vibration, noise exceeding the standard, and rarely due to fatigue peeling or other forms of failure. For this kind of bearing, the dimensional accuracy of the bearing is required to be high. After quenching and tempering, the retained austenite should be controlled at a lower level as far as possible. The less the better, the higher the stability. During quenching, lower heating temperature and shorter holding time shall be adopted. After quenching, additional industrial refrigerator cooling treatment shall be carried out, and tempering shall be carried out in time. The tempering time shall be long enough to ensure the full transformation and stability of retained austenite.