Figure 8 shows a cooling time and transformation (CTT) diagram for a 52100 Jominy test-bar sample austenitized at 845°C (1550°F). The bar cools at different rates, which results in different microstructural constituents and amounts as the cooling curves intersect and traverse the different phase transformation domains.
The expected result of the cooling rate of the second point on the Jominy bar is a microstructure consisting of martensite plus some nonmartensitic transformation products, such as upper bainite, which is a softer constituent than martensite. The softer constituents serve as points of weakness in the martensitic matrix, making them objectionable in a bearing steel.
Inhomogeneities such as nonmetallic inclusions and voids, and constituents like upper bainite reduce rolling contact fatigue life of through-hardening bearing steels.
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 Quench problems that can affect through-hardening bearing steels. Then scroll to Figure 8.
https://dl.asminternational.org/handbooks/book/9/chapter/110017/Heat-Treatment-of-Bearings
