Issue 38

S. Averbeck et alii, Frattura ed Integrità Strutturale, 38 (2016) 12-18; DOI: 10.3221/IGF-ESIS.38.02 12 Focussed on Multiaxial Fatigue and Fracture A study of white etching crack formation by compression-torsion experiments S. Averbeck, E. Kerscher Materials Testing (AWP), University of Kaiserslautern, Gottlieb-Daimler-Strasse, 67663 Kaiserslautern, Germany stefan.averbeck@mv.uni-kl.de , kerscher@mv.uni-kl.de A BSTRACT . In this study, an attempt was made to recreate the bearing damage phenomenon “White Etching Cracks” with a simplified testing setup. Rolling contact fatigue conditions were simulated with in-phase and out-of- phase cyclic compression-torsion experiments on 100Cr6 steel specimens. The results are compared in terms of microstructural change. Focused Ion Beam and metallographic analysis reveal that a fine-grained, white etching zone formed in the vicinity of the fatigue cracks of specimens tested with the in-phase load pattern. In contrast, no such structures were found after testing the out-of-phase load pattern. The properties of the white etching zone are characterised in more detail and compared with White Etching Cracks. K EYWORDS . White Etching Cracks; Multiaxial fatigue; Bearing steel. Citation: Averbeck, S., Kerscher., E., A study of white etching crack formation by compression-torsion experiments, Frattura ed Integrità Strutturale, 38 (2016) 12-18. Received: 28.04.2016 Accepted: 15.06.2016 Published: 01.10.2016 Copyright: © 2016 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION n the past two decades, increasing attention has been focused on the damage mechanism “White Etching Cracks” (WEC). WECs can lead to very early bearing failure (usually 5-20% of L 10 ) under operating conditions that would usually allow the bearing to achieve its calculated service life [1]. Despite intensive research and a variety of root cause hypotheses, no comprehensive explanation for WEC formation exists yet. The phenomenon seems to be influenced by multiple parameters which are often interdependent. The different hypotheses and influencing factors have been reviewed by Gegner [2], Evans [1] and, more recently, by Stadler et al. [3]. The characteristic feature of White Etching Cracks is the appearance of the crack faces, which appear white when etched with 3% nitric acid. This etching behaviour is attributed to an extreme grain refinement compared to the original martensitic structure. The average grain size is reduced from ~1µm to 10-100nm [2, 4]. The reduction in grain size is coupled with an increase in hardness from about 700HV to 1000HV and above [1, 5], in part due to the Hall-Petch effect, in part due to the dissolution of carbides in the white etching area [4]. The lattice structure is body-centred cubic, which is carbon-supersaturated ferrite [1, 2], although amorphous structures have also been reported [6]. WEC have a tendency to branch and form networks or groups [1, 7]. This heavy branching of the cracks and their three-dimensional interconnections are responsible for their detrimental effect on bearing life, as this structure easily leads to spalling damage. WECs are found almost exclusively near or at the surface of bearing components, although they can propagate very deeply into the material [8]. It is therefore assumed that there is a link to rolling contact fatigue (RCF) mechanisms, which I