Bearings materials are classified as through-hardened materials (used largely for ball bearings) and case-hardened materials (used largely for roller bearings). Critical applications require vacuum-processed steels.
Regardless of material type, a commonly accepted minimum hardness for bearing components is 58 Rc. Less hardness allows the races to Brinell. Since hardness decreases with increasing temperature, conventional bearing steels such as 440C and SAE 52100 cannot be used much above 350°F.
Through-hardened materials have received much attention in an effort to develop steels suitable for high-temperature bearings. Alloys with various proportions of molybdenum, tungsten, chromium, vanadium, aluminum, and silicon are used to produce hardness at high temperatures.
The ball-bearing industry has used SAE 52100 steel as a standard material since 1920. This is a high-carbon chromium steel which also contains small amounts of manganese and silicon. This air-melted alloy is clean, hard, and wear resistant.
For corrosive environments, 440C stainless steel should be considered. However, bearings made of 440C do not have as high a dynamic capacity as those made of SAE 52100.
Halmo and M-50 are usable up to about 600°F. M-50 is the more widely used. T-1 and M-10 are good to 800°F, M-1 and M-2 to 900°F, and WB-49 to 1,000°F. Oxidation resistances of M-1 and M-2 are marginal above 900°F.
For M-50 and M-1 the dynamic capacity can be exceeded at 600°F, but dynamic capacity of bearings made of WB-49 should be derated above that temperature.
In addition to operating temperature, criteria for choosing a material should include fabrication costs. Generally, bearings made from M-series materials cost about 50% more than those made from SAE 52100, because of grinding difficulties. At temperatures below 350°F, there appears to be no technical advantage of M-series materials over SAE 52100.
Carburized, or case-hardened materials, are characterized by a hardened surface (greater than 0.015 in. thick) and a soft core. At room temperatures, the surface hardness of these materials run around Rc 58 to 63, and their core hardnesses are Rc 25 to 48. These materials are generally limited to temperatures less than 350°F.
Two exceptions to the temperature rule are Timken CBS600 and CBS1000. If hot hardnesses of 58 Rc are required, CBS600 appears to be limited to 450°F and CBS1000 to 600°F.
No data are available to compare the dynamic capacities of carburized and through-hardened materials. Where shock and high vibrational loads are present, carburized materials may have an advantage because of their soft, ductile inner core.
Vacuum-melt steels provide increased reliability and basic dynamic load rating because they have a lower inclusion content than air-melted steels. However, the improvement in bearing life is not always commensurate with the improvement in cleanliness. Nonmetallic inclusions are present to some degree even in exceptionally clean steels and can be the nucleus of fatigue cracks.
Induction vacuum melting is a process in which a cold charge is melted in an induction furnace and then poured into ingots; the whole operation is performed in vacuum. Drawbacks of this process are high cost and variations in quality because of refractory problems.
A better way to make bearing steels is consumable-electrode vacuum melting. Electrodes made from air-melted heat are remelted under vacuum by electric arcs. The remelted product solidifies in a water-cooled copper mold under vacuum. This gives different solidification conditions than the induction-melting technique and produces a more consistent high-quality steel.