WEB Influences on the Formation and Depth Distribution of ButterfliesThursday (24.09.2020) 12:05 - 12:20 S: Structural Materials 1 Part of:
Small non-metallic inclusions (NMIs) are unavoidable in technical steels. They act as stress raisers and can influence the fatigue behavior of rolling element bearings. So-called butterflies are white-etching areas with a characteristic two-wing shape. They typically originate from relatively small NMIs after a certain time of operation. A high number of butterflies is often observed even in bearings that have not failed.
Under moderate contact pressures (e.g. p < 2500 MPa) the depth distribution of butterflies closely corresponds to the level of the cyclic subsurface stresses. The highest density of butterflies is observed in the depths with the highest subsurface stresses.
Under increased contact pressures (e.g. p = 4000 MPa) plastic deformation of the steel occurs in the depths with the highest subsurface stresses and the formation of butterflies in these depths is suppressed.
However, even at low contact pressures (e.g. p = 1400 MPa) a suppressed formation of butterflies in the depths with the highest subsurface stresses can be observed when the bearing is run under WEC-critical conditions. In this case, no global plastic deformation occurs, and correspondingly no compressive residual stresses develop.
A hypothesis for the reason behind the butterfly suppression under normal as well as under WEC-critical bearing operation conditions is suggested. According to the hypothesis the carbon depletion of the nanosized grain structure in white-etching areas may lead to microstructural alterations and consequently to a suppressed wing formation. In this process the carbon diffusion in dislocation cores plays a significant role. Hence, the carbon depletion is triggered by the dislocation density in the surrounding material.