Dimensional stability is a bearing condition of great concern. This means heat treating is conducted to prevent excessive diametral growth in the bearing due to retained austenite transforming to martensite at elevated or subzero service temperatures, or shrinkage due to continued tempering of the martensite at high service temperatures.
Because the entire microstructure of a through-hardening bearing steel is intended to be uniform, it has a homogeneously distributed retained austenite content. If the retained austenite transforms to martensite in service, clearances between bearing rolling elements and raceways can be lost, and the bearing can “lock up,” leading to damage or failure of the machine. This is especially critical for larger diameter bearings, because the size change (diametral growth) is proportionally larger and consumes more of the clearances built into the bearing.
To prevent this, the amount of retained austenite must be carefully measured and controlled. Percent retained austenite in a bearing steel often is accurately measured using x-ray diffraction techniques.The heat treatment of 52100 bearing steel must be controlled to produce a level of retained austenite that the bearing can tolerate in service. This often is considered to be <10% for most applications (nonstabilized) and <3% (stabilized) for superprecision applications.
Figure 2 shows that austenitizing temperature and tempering temperature can be tailored to produce low amounts of retained austenite in the finished bearing. Indeed, the 232 °C (450 °F) A tempering temperature of 230 °C (450 °F) is particularly effective in transforming any retained austenite to tempered martensite. The international standard DIN 623 Rolling Bearings provides a guide for tempering 52100 to achieve adequate dimensional stability at specific service temperatures.
For more information, click on the link below (subscription required), then scroll down the Chapter Contents on the left of the page, and click on Dimensional Stability in Through-Hardening Bearing Steels.
https://dl.asminternational.org/handbooks/book/9/chapter/110017/Heat-Treatment-of-Bearings
