618 Bearing materials

Bearing inner and outer raceway members and their rolling elements, be they balls or rollers, can be made from either a case hardening alloy steel or a through hardened alloy steel.

a) The case hardened steel is usually a low alloy nickel chromium or nickel-chromium molybdenum steel, in which the surface only is hardened to provide a wear resistance outer layer while the soft, more ductile core enables the bearing to withstand extreme shock and overloading.

b) The through hardened steel is generally a high carbon chromium steel, usually about 1.0% carbon for adequate strength, together with 1.5% chromium to increase hardenability. (This is the ability of the steel to be hardened all the way through to a 60-66 Rockwell C scale.)

The summary of the effects of the alloying elements is as follows:

Nickel increases the tensile strength and toughness and also acts as a grain refiner. Chromium considerably hardens and raises the strength with some loss in ductibility, whilst molybdenum reduces the tendency to temper-brittleness in low nickel low chromium steel.

Bearing inner and outer raceways are machined from a rod or seamless tube. The balls are produced by closed die forging of blanks cut from bar stock, are rough machined, then hardened and tempered until they are finally ground and lapped to size.

Some bearing manufacturers use case-hardened steel in preference to through-hardened steel because it is claimed that these steels have hard fatigue resistant surfaces and a tough crack-resistant core. Therefore these steels are able to withstand impact loading and prevent fatigue cracks spreading through the core.

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