BUTTERFLY FORMATION MECHANISMS IN ROLLING CONTACT FATIGUE

摘要

Rolling bearings in field applications are regularly operated in the very high cycle regime of fatigue. The established Lundberg-Palmgren bearing life theory is based on damage initiation at volume defects of the hardened steel microstructure within a certain depth below the raceway surface, defined by the distribution of the orthogonal shear stress. Clarification of butterfly crack generation at non-metallic inclusions and subsequent wing growth within the matrix is therefore of fundamental interest. The mechanism, by which butterflies develop at artificially introduced alumina inclusions, is presented. It is found that oxide inclusions, due to poor bonding to the steel matrix induce a minute rubbing action against the steel matrix under the influence of the subsurface cyclic stress field. Thus, repeated atomic scale material transfer from the matrix to the inclusion surface occurs. The deposited material, initially possibly amorphous, constitutes the butterfly wing. It exhibits an ultra-fine equiaxed ferrite grain size. All carbon is accommodated in the deposited wing that etches 'white' (uniformly). A crack-like feature, the original gap between the inclusion and the steel matrix, is now found in-between the white etching wing and the undamaged steel matrix. In this work, also another mechanism is introduced, by which butterflies are formed in components exposed to rolling contact fatigue. As known, for instance, from brittle precipitates in tensile tested aluminium alloys, MnS inclusions embedded in the steel matrix may fracture under rolling contact loading. The initiated crack can further open up in the direction of the major axis of the sulphide inclusion and exceed into the bulk material. Cyclic shear along such cracks induce severe micro-plastic damage in the adjacent microstructure, by which the associated white-etching butterfly wing is built up. The stages of butterfly formation are documented by metallography and scanning electron microscopy. Fractured inclusions are not present in the as-delivered bearing condition. The Hertzian pressure required for MnS cracking and subsequent butterfly development during overrolling is estimated to be about 1400 MPa.