Materials for bearing rings and rolling elements
The most common steel for through-hardening is a carbon chromium steel, containing approximately 1% carbon and 1,5% chromium (such as ISO 683-17 - 100Cr6). 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 bearing steel provides an optimum balance between manufacturing and application performance. This steel normally undergoes a martensitic or bainitic heat treatment to obtain a hardness between 58 and 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 SKF Explorer bearings are made.
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 areas, which means that a combination of properties can be achieved in one component.
An example of this is the flanged wheel hub bearing unit (HBU), where the properties of the unhardened flange must resist structural fatigue, while the raceways are hardened to resist rolling contact fatigue.
Chromium-nickel and manganese-chromium alloyed steels in accordance with ISO 683-17:1999 with a carbon content of approximately 0,15% are the steels most commonly used for case-hardened SKF rolling bearing components.
In applications where there are high tensile interference fits and heavy shock loads, SKF recommends bearings with case-hardened rings and/or rolling elements.
The most common stainless steels used for SKF bearing rings and rolling elements are high chromium content steels like X65Cr13 in accordance with ISO 683-17:1999 and X105CrMo17 in accordance with EN 10088-1.
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, contact the SKF application engineering service.
Depending on the bearing type, standard bearings that are made of steels for through-hardening and surface-hardening have a recommended maximum operating temperature that ranges between 120 and 200 °C (250 to 390 °F). The maximum operating temperature is directly related to the heat treatment process.
For operating temperatures up to 250 °C (480 °F), a special heat treatment (stabilization) process can be applied. In this case, however, the process reduces the load carrying capacity of the bearing, which must be taken into consideration.
For applications that operate continuously at elevated temperatures, where bearings require materials of high hardness, bearings made of highly alloyed steels can be supplied on request. For additional information about high-temperature bearing steels, contact the SKF application engineering service.
The common ceramic used for SKF bearing rings and rolling elements is a bearing grade silicon nitride in accordance with ISO 26602:2009. 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:
- high hardness
high modulus of elasticity
- low density
- low coefficient of thermal expansion
- high electric resistivity
- low dielectric constant
- no response to magnetic fields
For information about material properties, refer to table 1.