Modelling dislocation assisted tempering during rolling contact fatigue in bearing steels

Strain-induced martensite decay in bearing steels under rolling contact fatigue: Modelling and atomic-scale characterisation

“…As for the microstructural parameters, the average initial half width of the precipitates (r p0 ) for the steel investigated in this research was estimated to be $7 nm, and the initial volume fraction of the precipitates (f p0 ) was estimated to be $6.7 vol% [24]. Hence, the spacing between the precipitates can be approximated via l p ¼ 2rp0 fp0 [19]. The total carbon content (C 0 ) of the system was calculated by Kang et al [19] to be 3.1 at%, which can be converted to carbon concentration per unit volume through…”

“…Hence, the spacing between the precipitates can be approximated via l p ¼ 2rp0 fp0 [19]. The total carbon content (C 0 ) of the system was calculated by Kang et al [19] to be 3.1 at%, which can be converted to carbon concentration per unit volume through…”

“…2 (d), a temper carbide is taken as a thin plate with a half width r p . This morphology was detected by transmission electron microscopy [24,26] and was used in previous modelling work on bearing steels [19]. The carbide precipitate can be thickened from its both sides, and thereby at the proceeding interface on each side, a carbon flux equilibrium can be obtained as: dr p dt…”

“…Besides, Bush et al [5] found threshold contact pressures below which DERs were never formed, indicating the essential role played by stress. Moreover, the depth range where a DER is formed is considered to be related to the stress state of Hertzian contact, although there is still in disagreement as to the stress component responsible for such phenomenon with some authors [15,19] arguing it is the maximum shear stress, others [13,20] that it is the 45 shear stress, the orthogonal shear stress [5,6] or the vonMises stress [9]. Nevertheless, it was experimentally proved [15] that increasing either contact pressure or operation temperature can accelerate DER formation.…”

“…As for the carbon distribution in DERs, Swahn et al [8] postulated that the excess carbon resides at dislocations and the cyclic stress somehow promotes the carbon migration, but this requires an unrealistically high dislocation density of 10 17 m À2 contradicting the observation of hardness decrease in DERs, which apparently implies a carbon depletion in the matrix. Recently, Kang et al [19] postulated a more plausible mechanism, stating that during DER formation the excess carbon migrates to pre-existing carbide precipitates with the assistance of gliding dislocations thickening them; the model managed to explain the material softening in DERs but failed to accurately describe the effects of temperature, rotational speed and contact pressure. In this context, a dislocation-assisted carbon migration theory was suggested by a recent study [21] to describe the precipitation of lenticular carbides (LCs) in rolling contact fatigued bearing steels, which can also be applied to describe the thickening of pre-existing carbide precipitates in DERs.…”