Page 83 of Ball and Roller Bearings by NTN Corporation Japan

bearing materials 13 bearing materials 13.1 raceway and rolling element materials while the contact surfaces of a bearing s raceways and rolling elements are subjected to repeated heavy stress they still must maintain high precision and rotational accuracy to accomplish this the raceways and rolling elements must be made of a material that has high hardness is resistant to rolling fatigue is wear resistant and has good dimensional stability the most common cause of fatigue cracking in bearings is the inclusion of non-metallic impurities in the steel by using pure materials low in these non-metallic impurities the rolling fatigue life of the bearing is lengthened for all ntn bearings steel low in oxygen content and non-metallic impurities then refined by a vacuum degassing process as well as outside hearth smelting is used for bearings requiring especially high reliability and long life steels of even higher in purity such as vacuum smelted steel vim var cevm and electro-slag melted steel esr are used 1 high/mid carbon alloy steel in general steel varieties which can be hardened not just on the surface but also deep hardened by the socalled through hardening method are used for the raceways and rolling elements of bearings foremost among these is high carbon chromium bearing steel which is widely used for large type bearings and bearings with large cross sectional dimensions induction hardened bearing steel incorporating manganese or molybdenum is used also in use is mid-carbon chromium steel incorporating silicone and manganese which gives it hardening properties comparable to high carbon chromium steel 2 case hardened carburized steel because of its combination of a hard surface layer which has been carburized and hardened to an appropriate depth and a relatively pliable inner core case hardened steel has excellent efficiency against shock loads ntn uses case hardened steel for almost all of its tapered roller bearings in terms of case hardened steel for ntn s other bearings chromium steel and chrome molybdenum steel are used for small to medium sized bearings and nickel chrome molybdenum steel is used for large sized bearings 3 heat resistant bearing steel when bearings made of ordinary high carbon chromium steel which have undergone standard heat treatment are used at temperatures above 120°c for long durations unacceptably large dimensional changes can occur for this reason a dimension stabilizing treatment ts treatment has been devised for very high temperature applications through application of this dimension stabilizing treatment shortening of rolling fatigue life due to decreases in bearing hardness at high temperatures can be avoided refer to page insert a-17 3.4.2 a-81 for standard high temperature bearings used at temperatures from 150°c ­ 200°c the addition of silicone to the steel improves heat resistance and results in a bearing with excellent rolling fatigue life with minimal dimensional change or softening at high temperatures a variety of heat resistant steels are also incorporated in bearings to minimize softening and dimensional changes when used at high temperatures two of these are high speed molybdenum steel and high speed tungsten steel for bearings requiring heat resistance in high speed applications there is also heat resistant case hardening molybdenum steel 4 corrosion resistant bearing steel for applications requiring high corrosion resistance stainless steel is used to achieve this corrosion resistance a large proportion of the alloying element chrome is added to martensite stainless steel 5 induction hardened steel besides the use of surface hardening steel induction hardening is also utilized for bearing raceway surfaces and for this purpose mid-carbon steel is used for its lower carbon content instead of through hardened steel for induction hardening of the deep layers required for larger bearings and bearings with large surface dimensions mid-carbon steel is fortified with chrome and molybdenum 6 other bearing materials for ultra high speed applications and applications requiring very high level corrosion resistance ceramic bearing materials such as si3n4 are also available.

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