What is the Effect of Sulfide on Bearing Life?

The influence of sulfide inclusions on bearing life has been controversial, which is the reason for the different requirements of sulfur content in the standards of various countries. The impact of sulfide inclusions on bearing life should be considered from the following two aspects:

First, a separate sulfide inclusions destroy the continuity of bearing steel, reducing the life of the bearing. Second, because the sulfide has a low melting point, when the steel in the process of solidification, its attachment to the surface of the polygonal oxide inclusions, especially in the corner. The formation of oxide sulfide symbiotic inclusions. Therefore, with the increase of sulfur content in steel, it increases the number of sulfide inclusions, so that the oxide inclusions are surrounded by sulfide opportunities to increase the number of symbiotic inclusions. When the bearing steel is hot worked, this symbiotic inclusions and the matrix form a smooth inner surface, the tendency and size of stress concentration is lower than the brittle irregular oxide inclusions. In addition, the oxygen content in bearing steel also decreases with the increase of sulfur content, so that the number of brittle oxide inclusions decreases and get favorable results for bearing life, because oxide inclusions are one of the main causes of bearing fatigue damage. Therefore, to form more oxide-sulfide inclusions, it is required that the steel contains a certain amount of sulfur content corresponding to the oxygen content. There are test results reported: when the steel sulfur content from 0.008 percent ~ 0.010 percent to 0.013 percent ~ 0.018 percent , can make contact fatigue life Lio more than double.

When symbiotic oxide-sulfide inclusions occur, the coefficient of thermal expansion of the presence of such two-phase inclusions will determine the magnitude of residual stresses present in the matrix according to their relative amounts. Even if the volume of the sulfide is smaller than that of the oxide, the residual stress in the matrix is reduced in magnitude compared to that of the pure oxide inclusions. The residual stress itself is not sufficient to cause fatigue. However, if the presence of oxide inclusions is encountered, the stress is intensified and the oxide is surrounded by this microscopic stress, together with some applied stress, when the stress reaches a certain level, it will lead to a source of cracking. Therefore, the external stress is the direct cause of fatigue damage, while the oxide inclusions itself is the internal cause, the superposition of these two stress fields will make the bearing material fatigue faster. When the oxide inclusions and sulfides are coeval, the residual stress is low, which will prevent fatigue cracking, and the material will be able to withstand the applied stress, thus increasing the service life of the bearing.

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