Ferritic nitriding, or ferritic nitrocarburizing (FNC), has limited application in most bearing systems because stress fields exceed the depth affected by the process, typically much less than 0.75 mm (0.030 in.).
However, duplex hardening of M-50 and M50NiL provides a hard, wear-resistant surface that exhibits high damage tolerance and damage resistance to hard-particle contamination. Duplex hardening is defined as conventional martensitic hardening (M-50), or carburizing and martensitic hardening (M-50NiL), followed by nitriding.
The nitriding process for duplex hardening typically is a plasma-assisted nitriding or gas nitriding process. The process is frequently carried out at a temperature of about 500 °C (930 °F) to ensure sufficient nitriding depth in a reasonable time. The nitriding temperature should be lower than the tempering temperatures of M50 and M50NiL so the hardness achieved following the conventional hardening thermal treatment is not compromised.
Both M-50 and M50NiL are typically tempered multiple times (three or more). M50 generally is tempered at a temperature between 540 and 550 °C (1000 and 1020 °F) and M50NiL at a slightly lower temperature between 525 and 540 °C (980 and 1000 °F).
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 “Duplex Hardening.”
https://dl.asminternational.org/handbooks/book/9/chapter/110017/Heat-Treatment-of-Bearings
