Through-Hardened vs Case-Hardened
Although most bearings, in general, are made of through-hardened 52100, tapered roller bearings in the US market have typically been manufactured with case-hardened steels. Case-hardened bearing components are typically used in bearing applications involving high loading conditions. A review of application criticality and imposed bearing stresses can dictate the usability of through-hardened ring material in tapered roller bearings.
Case-hardened steels exhibit higher fracture toughness than through-hardened steel (52100). Fracture toughness is the property that defines the stress to initiate rapid fracture in the presence of a local defect such as a spall or other surface deterioration. Sub-surface cracks (spalls) or surface initiated defects that lead to cracks, have a propensity to materialize into a catastrophic failure with through-hardened steel. Generally speaking, case-hardened steels tolerate substantial damage progression without catastrophic failure during the interval between the onset of a problem and when routine maintenance identifies the need for repair.
It is important to note that tests conducted to develop life factors for bearings based on steel material find 52100 steel to be higher or better than most case-carburized steels. It is only through advanced melting processes of the raw material that bearing life adjustment factors for materials increase significantly. Generally speaking, it is believed that a 52100 through-hardened bearing life factor would be between 3 – 4.5, based on using either an air melt or vacuum processed melting practices on 52100 steel. In the case-carburized bearing it is more difficult to identify the life factor. However, based on currently published data, on similar case-carburized steels and vacuum arc re-melting practices used for commercial steel we have a life adjustment factor range of 4.5 – 6.0. Based on this information we would estimate a reduction in fatigue life on the through-hardened product of 0 to a maximum of ½ of that of the case-carburized product.
The life adjustment factors are based on information available within the text, “STLE Life Factors for Roller Bearings” (E.V.Zaretsky). Without performing actual life testing or without valid endurance test results from suppliers comparing the two materials, the current life factor data provides the most accurate information regarding estimating bearing life.
Our testing has proven that we get a very wide variation on through-hardened part life and we believe that it is very much attributed to steel melting practices. But with that said, we have also had success with some suppliers with through-hardened parts when compared to a baseline case-carburized material.