Bearing Steel Heat Treatment
When bearing steels are in their soft (unhardened) state, metallurgists refer to their structure as being in the pearlite state. In order to harden the bearing steel it must be heated to a very high temperature and then cooled very rapidly.
When heat treatment of bearing steels happens in a furnace of 1,750°F, the structure transforms from pearlite to what is known as austenite. After quenching (very rapid cooling), the structure of the bearing steelthen transforms from austenite to martensite.
Once transformed to martensite through heat treatment of bearing steels, the material becomes very hard. However, at this point it is not considered “thermally stabilized”. This is because not all of the 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 over an extended period of time (possibly years) transform into martensite. This bearing steel heat treatment transformation is accompanied by an increase in volume that is called metallurgical growth (not to be confused with thermal growth). Metallurgical Growth will cause a change in dimension and form of any steel parts such as bearings’ even at room temperature.
While not a problem with low precision commodity type bearings, in high precision (ABEC 5P, 7P, 9P) miniature bearings this lack of dimensional stability can cause problems.
In order to eliminate this unwanted metallurgical growth, the steel must be subjected to thermal stabilization. This is accomplished by repeated cycles of chilling at -120 F and tempering to transform a large percentage of the retained austenite to martensite.
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