Bearing Materials Key to Increased Reliability-NSK

The use of application-specific bearing steels and surface treatments permits the reliability of bearings to be increased significantly, which in turn contributes to the reduction of machinery and equipment TCO (total cost of ownership).

For high-performance bearings, the selection and optimisation of (steel) materials plays a central role in their development. For this reason, materials engineering is one of four core research and development technologies at NSK.

A combination of a special alloy and specific heat treatment was used to develop NSK’s Super Tough steel

Material purity

The fatigue life of alloyed bearing steels such as 100 Cr6 (or SUJ2 in the Japanese standard), for instance, depends principally on the inclusion content. Oxide or non-metallic inclusions in particular, promote negative effects under the raceway surface. As an example, it is known that aluminium oxide inclusions, which are formed by the process of oxidisation during the melt, can lead to a major reduction in bearing fatigue life.This effect is created because aluminium oxide inclusions are relatively hard and can break up when the steel is being processed, such as during forging. When break-up occurs, the inclusions shrink and weaken the microstructure.

A combination of a special alloy and specific heat treatment was used to develop NSK’s Super Tough steel

Material purity

The fatigue life of alloyed bearing steels such as 100 Cr6 (or SUJ2 in the Japanese standard), for instance, depends principally on the inclusion content. Oxide or non-metallic inclusions in particular, promote negative effects under the raceway surface. As an example, it is known that aluminium oxide inclusions, which are formed by the process of oxidisation during the melt, can lead to a major reduction in bearing fatigue life.This effect is created because aluminium oxide inclusions are relatively hard and can break up when the steel is being processed, such as during forging. When break-up occurs, the inclusions shrink and weaken the microstructure.

The difference is in the alloy

The third parameter in the quest for greater bearing reliability is the alloy. Alloys can prevent, or at least minimise, the formation of cracks in the bearing microstructure. Again in collaboration with steel manufacturers, NSK has developed various special alloys for this application profile.

Materials such as NSK’s Super Tough steel come from combining the optimum heat treatment with a special alloy (Image1). For instance, the hardening of steels using a process such as carbonitriding increases service life by a factor of two in comparison with the estimated service life under contaminated lubrication conditions. In environments where the lubricant has normal impurities, bearing service life can even be increased by a factor of 10. The reason for this improved performance is that surface-induced wear caused by insufficient lubrication or contamination of the lubricant is significantly reduced. In turn, any potential damage caused by ‘white etching cracks’ (WECs) is delayed.