White-Etching Matter in Bearing Steel. Part I: Controlled Cracking of 52100 Steel
- PDF / 2,039,188 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 71 Downloads / 192 Views
BEARINGS are subjected to complex and repeated contact loads, which induce damage that accumulates and ultimately leads to failure or removal from service when monitoring systems identify precursors to failure. However, it is not clear whether some of the damage that is detected in post-service examinations is the cause or consequence of failure. In order to gain confidence in the mechanisms of failure, it is necessary to be able to reproduce damage phenomena in controlled experiments, where the ambiguity of interpretation is reduced. There is a particular kind of microstructural damage in the form of regions of the structure, which appear white in etched metallographic samples, that is a cause of controversy in the context of failure mechanisms in large bearings used in wind turbines.[1] The hard regions of white-etching matter are sometimes identified with the cause of failure, and the mechanism by which this structure forms has been associated with diverse phenomena such as severe localized deformation, adiabatic shear, carbide dissolution, and low-temperature recrystallization.[1–4] An alternative interpretation is that the process begins with the initiation of a microcrack,[5] which then develops the white-etching matter by the rubbing together of the crack surfaces, causing intense, localized mechanical deformation.[4] The design of better bearing
W. SOLANO-ALVAREZ, Ph.D. Student, and H.K.D.H. BHADESHIA, TATA Steel Professor of Metallurgy and Director of the SKF University Technology Centre, are with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, U.K. Contact e-mail: [email protected] Manuscript submitted December 11, 2013. Article published online July 8, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
steels depends on a clarification of these issues. For example, the idea that white-etching matter formation is the cause of failure would require different issues to be addressed, whereas if cracking precedes white-etching matter formation, then the problem can be mitigated by enhancing the resistance to cracking.[5] One way of establishing the mechanism is to subject samples, in which microscopic cracks are introduced deliberately, to rolling contact loading. Such experiments would reveal whether white-etching matter development in the pre-cracked samples occurs at the internal crack faces with greater vigor than in specimens that do not contain cracks. They could also help establish whether damage development is a function of the orientation of the crack faces relative to the principal stresses during rolling contact loading. However, to conduct such experiments, it is necessary to create populations of cracks which permit the steel to survive rolling contact loads for a significant number of cycles, sufficient to stimulate white-etching matter, but less than that required to cause gross fracture over the period of the test. The cracks that are relevant are classified as ‘‘very short,’’ typically 1 to 10 lm in length.[6] The purpose of the present work was, therefore, to desig
Data Loading...
