When bearing steels are in the soft (unhardened) state, metalworkers refer to their structure as the pearlite state. To harden bearing steel, it is necessary to heat it to a very high temperature and then cool it very quickly.
When heat treating bearing steels occurs in a furnace at 1,750°F, the structure changes from pearlite to what is known as austenite. After quenching (very rapid cooling), the bearing steel structure changes from austenite to martensite.
Once transformed into martensite through the heat treatment of bearing steels, the material becomes very hard. However, it is not considered “thermally stabilized” at this time. This is because not all austenite transforms into martensite during the quenching process. This phenomenon is called “retained austenite”.
If the steel is not thermally stabilized, the retained austenite will transform to martensite over a long period of time (perhaps years). This transformation of the heat treatment of the bearing steel is accompanied by an increase in volume called metallurgical growth (not to be confused with thermal growth). Metallurgical growth will cause a change in the size and shape of any steel part, such as bearings, even at room temperature.
While not a problem with low precision commercial grade bearings, in high precision miniature bearings (ABEC 5P, 7P, 9P) this lack of dimensional stability can cause problems.
To eliminate this unwanted metallurgical growth, the steel must undergo thermal stabilization. This is achieved through repeated cycles of cooling to -120 F and tempering to transform a large percentage of the retained austenite into martensite.
