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One Minute Mentor: Martensitic heat treating
June 12, 2019
Source: ASM International
The graph shows the tempering response of hardened 52100 bearing steel. In North America, the 1C-1.5Cr through-hardening steel was given the designation of 52100. One of the main reasons this steel became popular as a bearing steel is its consistent response to salt-bath and furnace heat treatment and oil quenching (assuming the incoming raw material conditions met hardness, microstructure, and grain size specifications).
The standard heat treating cycle for martensitic hardening of 52100 bearing steel is:
Austenitize 815–855C (1500–1570F) 15–40 min + 30 min for each additional inch of thickness, endo or neutral atm with 0.9–1 C potential;
Quench ≤150C (300F) agitated oil (martensitic hardening); ≥230C (445F) molten salt (bainitic hardening)
Temper 150–230C (300–445F) 2–4 h (58–64 HRC).
As-quenched hardness after heat treating is typically 64–67 HRC. Figure 1 shows hardness as a function of tempering temperature.
The goal of tempering 52100 bearing steel is to achieve a measure of toughness in the steel while maintaining high as-tempered hardness. A bearing must have a minimum hardness of 58 HRC to achieve the expected rolling contact fatigue performance in service. This requirement dictates a maximum tempering temperature of 260°C (500°F).
Another goal of tempering bearing steels is to achieve the required level of dimensional stability as described in specification DIN-623. This topic is discussed more thoroughly in an upcoming section.
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 Martensitic Heat Treating.
Materials Processing and Treatment | Heat Treating