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Materials for bearing rings and rolling elements

Standard bearing steel

The steel used for standard SKF super-precision bearings is an extremely clean, through-hardening carbon chromium steel (100Cr6), containing approximately 1% carbon and 1,5% chromium, in accordance with ISO 683-17:1999. 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.

SKF super-precision bearings are heat stabilized up to 150 °C (300 °F). But other factors like cage material, seal material or lubricant might limit the permissible operating temperature.

For information about material properties, refer to table 1.

NitroMax steel (high-nitrogen stainless steel)
NitroMax is a new generation of ultra clean, high nitrogen stainless steel. When compared to standard carbon chromium bearing steel (100Cr6), NitroMax steel provides the following:
  • enhanced fatigue/wear resistance under poor lubrication conditions
    (k < 1)
  • higher degree of fracture toughness
  • superior corrosion resistance

Each of these characteristics is beneficial when speed is higher than A = 1 to 1,15 × 106 mm/min.

Enhanced fatigue/wear resistance enables the bearings to operate longer under all lubrication conditions and particularly those of thin-film operation that result from kinematic lubricant starvation at very high speeds.

Increased fracture toughness reduces the risk of inner ring fracture due to increased tensile hoop stresses caused by centrifugal forces when operating at very high speeds.

Compared with bearings made of carbon chromium steel, this ultra-clean, high nitrogen content steel can significantly extend bearing service life when operating under full-film lubrication conditions (k ≥ 1). Under thin-film lubrication conditions, this life extending effect is even more significant diagram 1.

NitroMax steel is superior not only to conventional carbon chromium bearing steels but also to other high-nitrogen stainless steels. To illustrate why this is the case, it is necessary to understand the way that nitrogen influences the microstructure of the steel and how this is optimized during heat treatment.

When carbon chromium steel is heat treated, the process produces large, brittle chromium and chromium-molybdenum carbides that deplete the surrounding steel matrix of chromium and molybdenum, thereby reducing its corrosion pitting resistance. On the other hand, when NitroMax steel is hardened and tempered, small, fine chromium nitrides are formed (fig. 1). This occurs because when the nitrogen partly replaces the carbon in the steel alloy, a much higher content of chromium is dissolved in the steel matrix. The resulting, smaller chromium-depleted zones around the nitrides make NitroMax steel much more corrosion resistant (fig. 2).

The enhanced fatigue strength of NitroMax steel is associated with its coherent microstructure and fine distribution of chromium nitride precipitates with few, if any, undissolved secondary carbides in the microstructure. The fineness of the NitroMax structure compares favourably to the standard bearing steel 100Cr6, which helps in explaining the superior performance of the NitroMax steel structure. High impact toughness, dimensional stability, and hardness (> 58 HRC) result from the final quenching and tempering stages of heat treatment.

Another benefit of NitroMax steel is that it has a lower coefficient of thermal expansion than 100Cr6. This benefit, when paired with the extremely low coefficient of thermal expansion of ceramic rolling elements, used as standard in SKF bearings with NitroMax steel rings, enables bearings combining the two materials to be less sensitive to temperature differences between the inner and outer rings. The level of preload therefore remains much more stable even over the extremes of operating conditions, resulting in reduced frictional losses, lower operating temperatures and extended grease service life.


The ceramic material used for rolling elements in SKF super-precision bearings 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 especially for high-speed bearings:

  • high hardness
  • high modulus of elasticity
  • low density
  • low coefficient of thermal expansion 
  • high electrical resistivity
  • low dielectric constant
  • no response to magnetic fields

For information about material properties, refer to table 1.

Bearings with steel rings and ceramic balls are called hybrid bearings.

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