Bearings made of M50 have been used in jet-engine applications for many years. M50 is a secondary-hardening alloy typically tempered multiple times to achieve high hardness and dimensional stability. High-alloy, high-carbon steels are stabilized by tempering at temperatures 10 to 15 °C (20 to 30 °F) higher than their peak secondary-hardening temperature. Austenite is stabilized by alloy in solution. Carbides that form during secondary hardening deplete alloy in solution making the austenite susceptible to transformation during cooling and during additional tempering cycles
Figure 17 shows the dimensional stability of M50 aged at room temperature as a function of tempering temperature and number of tempering treatments. Tempering M50 five times shows an improvement over triple tempering.
Thermal cycling is another test used to quantify the stability of bearings. M50 thermal cycled from 20 to 260°C (68 to 500°F) 20 times, results in 23 and 15 micro-in./in. of growth for triple tempering at 510 and 525°C (950 and 975°F), respectively, but no difference between three and five tempers at 540°C (1000°F), for which the growth is only 2 to 3 micro-in./in. of growth (Ref 10).
For more information, click on the link below (subscription required), then scroll down the Chapter Contents column on the left of the page, and click on “Special Purpose Through-Hardening Bearing Steels.”
https://dl.asminternational.org/handbooks/book/9/chapter/110017/Heat-Treatment-of-Bearing
