Materials for bearing rings and rolling elements

Bearing steels for through-hardening

The most common steel for through-hardening is a carbon chromium steel containing approximately 1% carbon and 1,5% chromium according to ISO 683-17:1999. Today, carbon-chromium steel is one of the oldest and most intensively investigated steels; due to the continuously increasing demands for extended bearing service life. The composition of this rolling bearing steel provides an optimum balance between manufacturing and application performance. This steel is normally given a martensitic or bainitic heat treatment during which it is hardened to the range of 58 to 65 HRC.

Within the last few years process developments have enabled more stringent cleanliness specifications to be realized, which has had a significant impact on the consistency and quality of SKF’s bearing steel. The reduction of oxygen and harmful non-metallic inclusions has led to significantly improved properties of rolling bearing steels – the steels from which the SKF Explorer class bearings are made.

Bearing steels for induction-hardening

Surface induction-hardening offers the possibility to selectively harden a component’s raceway, while leaving the remainder of the component unaffected by the hardening process. The steel grade and the manufacturing processes used prior to surface induction-hardening dictate the properties in the unaffected area, which means that a combination of properties can be achieved in one component.

An example of this is a flanged wheel hub bearing unit (HBU) where the properties of the unhardened flange are designed to resist structural fatigue, while the raceway is designed to resist rolling contact fatigue.

Bearing steels for case-hardening

Chromium-nickel and manganese-chromium alloyed steels according to ISO 683-17:1999 with a carbon content of approximately 0,15% are those steels for case-hardening most commonly used for SKF rolling bearings.

In applications where there are high tensile interference fits and heavy shock loads, bearings with case-hardened rings and/or rolling elements are recommended.

Stainless bearing steels

The most common stainless steels used for SKF bearing rings and rolling elements are the high chromium content steels X65Cr14 according to ISO 683-17:1999 and X105CrMo17 according to EN 10088-1:1995.

It should be noted that for some applications, corrosion resistant coatings might be an excellent alternative to stainless steel. For additional information about alternative coatings, please consult the SKF application engineering service.

High-temperature bearing steels

Depending on the bearing type, standard bearings made from steels for through-hardening and surface-hardening have a recommended maximum operating temperature, which differs between 120 and 200 °C. The maximum operating temperature is directly related to the heat treatment process used in manufacturing components.

For operating temperatures up to 250 °C; a special heat treatment (stabilization) can be applied. In this case a reduction of the load carrying capacity of the bearing has to be taken into account.

For bearings operated at elevated temperatures, higher than 250 °C, for extended periods, highly alloyed steels like the 80MoCrV42-16 manufactured to ISO 683-17:1999 should be used, because they retain their hardness and bearing performance characteristics even under extreme temperature conditions.

For additional information about high temperature bearing steels, please contact the SKF application engineering service.

Ceramics

The common ceramic used for SKF bearing rings and rolling elements is a bearing grade silicon nitride material. It consists of fine elongated grains of beta-silicon nitride in a glassy phase matrix. It provides a combination of favourable properties for rolling bearings, such as high hardness, low density, low thermal expansion, high electric resistivity, low dielectric constant and no response to magnetic fields (table).