Steels for bearings

During rolling contact fatigue of ball bearings structural changes may occur below the raceway, in the region of maximum shear stress. A great number of 6309 type bearings were tested at two stress levels for varying numbers of revolutions, and the resulting structural changes in the inner rings were studied by optical and electron microscopy. The present observations have been compared to previously reported observations, which are often described in unsystematic terminologies and sometimes apparently contradictory. A unified terminology is worked out, on the basis of which, the structural changes are described. The following features occur, in chronological order. i) A ferritic phase, containing an inhomogeneously distributed excess carbon content, corresponding to that of the parent martensite. A mixture of this phase with residual martensite constitutes the well known dark etching region. ii) Disc-shaped regions of ferrite, thermodynamically stable, about 0.1 μm thick, inclined by about 30 deg to the raceway, and sandwiched between carbide rich discs. The latter are constituted by very small carbide particles, and are not necessarily compact. iii) A second set of larger disc-shaped regions about 10 μm thick, of plastically deformed ferrite in thermodynamic equilibrium, forming an angle of about 80 deg to the raceway. Transformation mechanisms are proposed. Particular attention is paid to short range carbon diffusion, induced by the cyclic stresses.

Martensite decay during rolling contact fatigue in ball bearings

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Tribological Aspects of Rolling Bearing Failures

Starting about 1920 it becomes easier to track the growth of bearing materials technology. Until 1955, with few exceptions, comparatively little progress was made in this area. AISI 52100 and some carburising grades (AISI 4320, AISI 9310) were adequate for most applications. The catalyst to quantum advances in high-performance rolling-element bearing steels was the advent of the aircraft gas turbine engine. With improved bearing manufacturing and steel processing together with lubrication technology, the potential improvements in bearing life can as much as 80 times that attainable in the late 1950s or as much as 400 times that attainable in 1940. This paper summarises the chemical, metallurgical and physical aspects of bearing steels and their effect on rolling bearing life and reliability: the single most important variable that has significantly increased bearing life and reliability is vacuum processing of bearing steel. Differences between through hardened, case carburised and corrosion resis...

/pdf/rolling-bearing-steels-a-technical-and-historical-57754bo3is.pdf

H. Muro

Papers

Microstructural, microhardness and residual stress changes due to rolling contact