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Plasma Nitriding Treatment of High Alloy Steel For Bearing Parts

Bearings that operate under high temperature and high speed conditions for a long time need to meet high reliability requirements. Therefore, the cleanliness of the materials used in this type of bearing is much better than that of ordinary bearing steel. This kind of clean steel can resist peeling caused by the expansion of internal non-metallic inclusions, and can obtain a longer life.

Unfortunately, bearings are not always lubricated with clean oil, and bearing operating conditions can be very harsh under contaminated lubrication conditions. In this harsh working condition, before internal spalling occurs, contaminants will cause depressions in the bearing raceway, and the depressions will provide expansion points for the spalling. Under such severe conditions, the bearing life is lower than the life of the internal spalling extended mode, and it will be affected by various factors such as the amount of dirt, the size and hardness of the dirt.

When the bearing fails due to external dirt, the occurrence of the failure is always sudden and unpredictable. Since an engine failure during flight will cause an emergency landing, this failure mode is particularly dangerous for the safety of aero engine bearings. Therefore, due to economic factors, it is necessary to contaminate long-life bearings under lubrication and extend the bearing replacement cycle…

There are various technologies that enable bearings to effectively resist failure caused by external dirt in the lubricating oil, including improved heat treatment technology. One example is the surface hardening process, which can form a hardened layer on the surface of the bearing raceway. Plasma nitriding, also known as ion nitriding, is a type of low-temperature nitriding treatment. Therefore, compared with the difficulty of gas nitriding of high alloy steel, plasma nitriding becomes relatively easy due to the sputtering effect.

NTN uses M50 or M50NiL alloy steel to manufacture bearings for high temperature and high speed working conditions. Plasma nitriding process is suitable for this type of high alloy steel. The chemical composition of M50 or M50NiL alloy steel is shown in the following table:

Material	C	Si	Mn	Ni	Cr	Mo	V
M50	0.8	0.2	0.3	0.1	4.0	4.3	1.0
M50NiL	0.13	0.2	0.2	3.4	4.1	4.3	1.2

The test results of plasma nitriding process show that:

When the sum of carbon and nitrogen content (mass fraction) in the steel exceeds 1.7%, it is easy to produce intercrystalline precipitates, which will adversely affect the quality of the bearing parts;

In the case of suppressing the generation of intergranular precipitates, nitriding treatment alone cannot obtain sufficient thickness of the nitriding layer, and it is necessary to increase the diffusion treatment;

Even when the diffusion temperature is lower than the tempering temperature, prolonging the diffusion time will result in a decrease in the hardness of the base steel; therefore, the diffusion annealing process of the workpiece should not only obtain sufficient nitride layer thickness, but also not cause the related work of reducing the hardness of the base steel. Under the circumstances;

Plasma nitriding and diffusion treatment can improve the resistance to peeling damage of steel;

After proper plasma nitriding and diffusion treatment, the outer surface and surface hardness of M50 and M50NiL samples have been improved. Compared with the M50 and M50NiL samples without plasma nitriding and diffusion treatment, the plasma nitriding sample significantly improves the damage resistance, and the sample with indentation on the rolling surface also shows a longer life.