White Etching Matter Research Articles

Effects of Water Contamination on Micropitting and Rolling Contact Fatigue of Bearing Steels

Abstract In this research, the effects of water contamination in oil were investigated on two kinds of failures that occur in bearing steel: micropitting and rolling contact fatigue. Whereas the presence of water in the oil had little effect on the generation of surface wear in these experiments, increases in the occurrences of micropitting and rolling contact fatigue were observed due to the presence of either dissolved or free water in lubricants. Additionally, the presence of white etching matter at crack interfaces was observed and evaluated. The experimental results showed that water content can be considered as a significant factor that accelerates the formation of micropitting and fatigue cracks in rolling bearings.

Microstructural characterization of white etching cracks in bearings after long-term operation in wind turbines

White etching cracks and other microstructural features have been investigated in two 100Cr6 steel bearings failed after several years of service in wind turbines. It is found that cracks in these bearings propagate predominantly through prior austenite grains in the tempered martensite structure. Both sharp and gradual transitions between the tempered martensite structure and white etching matter adjacent to the cracks are observed. Furthermore, it is suggested that structural refinement of carbide particles present in the microstructure of the wind turbine bearings occurred at a late stage of white etching matter formation.

Redistribution of carbon caused by butterfly defects in bearing steels

Butterfly defects initiate from inclusions in the subsurface of steel bearing components subject to rolling contact. The white etching matter (WEM) microstructure is a characteristic of butterflies and is related to the dissolution of carbides and thus generally believed to be enriched with carbon, in supersaturated solid solution, relative to the parent microstructure. Here, several butterflies are investigated using wavelength dispersive spectroscopy (WDS), soft x-ray emission spectroscopy (SXES) and electron microscopy (EM). Contrary to established thinking, in all cases investigated the butterfly-neighbouring WEM was found to be depleted in carbon, relative to parent material, by around 27% (measured in counts). Furthermore, the carbon level was shown to be lower than the matrix itself, suggesting that solute carbon is also expelled from the WEM during its formation, possibly due to the low level of solubility of carbon in ferrite. This was observed in both AISI 52100 and 18NiCrMo14-6 bearing steels. In spite of this depletion, nano-indentation found that WEM in both alloys was ~17% harder than the parent material. This may explain the strings of micro-voids observed near the WEM-parent interface, which appear to play a role in the growth of the butterfly cracks. It is suggested that the increased hardness of the WEM is mainly due to microstructural changes, rather than changes in solute carbon concentration.

The heterogenous distribution of white etching matter (WEM) around subsurface cracks in bearing steels

White etching cracks (WECs) initiate subsurface in bearings and can propagate to cause premature failure. These cracks are bordered by an altered microstructure known as white etching matter (WEM), which is thought to form via a crack-rubbing mechanism. However, WEM is often observed bordering a single side of a meandering crack. In search of a microstructural difference to justify the observed WEM asymmetry, regions of untransformed material, adjacent to cracking in a bearing inner ring, which had undergone hydrogen charging prior to testing, were studied using correlative electron microscopy (EM), electron backscattered diffraction (EBSD), electron probe microanalysis (EPMA) and nano-indentation techniques. The investigations found no significant differences between the untransformed material neighbouring the cracks and the parent material; both were found to have similar: grain size and shape; crystallographic texture; carbon concentration; carbide population and hardness, which questions why only one rubbed surface of the crack has formed WEM. The initiation of WEM ahead of the crack is suggested, however more investigations are needed to build another WEM formation model. EPMA characterisation revealed evidence of carbide dissolution in the WEM. Despite this, significant variation in the carbon concentration of the WEM was found; ranging from +13% enrichment to 42% depletion (in counts), relative to the parent material.

Microstructure deformation and white etching matter formation along cracks

Rolling elements made of 52100 steel were subjected to high austenitizing temperatures to embrittle the steel. The embrittled rolling elements were intentionally given a rougher surface finish and were subjected to bearing tests under rolling-sliding conditions. Post-test material investigations revealed cracks in the raceways and rollers. Detailed electron microscopy investigations showed highly deformed microstructure in the vicinity of the cracks and white etching matter (WEM) along the cracks in the subsurface regions. Intergranularly oriented cracks and associated WEM were observed in the lower subsurface areas of the rollers. In contrast to the cracks formed closer to the surface, the martensitic microstructure in the vicinity of the intergranular cracks did not exhibit the microstructure deformation. The highly deformed microstructure and associated cracking are result of the high surface roughness of the rollers combined with the sliding conditions. The formation of the white etching matter, which was composed of a nanograin structure, is attributed to the crack faces rubbing. These results advance the understanding of damage mechanisms in bearings and the associated microstructural changes in the steel.

Evolution of white-etching cracks and associated microstructural alterations during bearing tests

ABSTRACTThe effect of hydrogen and contact pressure on the sequence of microstructural changes during bearing operation has been investigated. The continuation of bearing testing in the presence of microcracks stimulates the formation of (1) white-etching matter (WEM) on the crack surfaces that endure severe surface rubbing, (2) slip bands in the regions that experience higher stress levels due to microcrack formation and (3) engineering bands within the WEM. Engineering bands are a new observation whereby ferrite crystals cross a multitude of fine grains (WEM). The presence of hydrogen and high contact pressure during the bearing tests led to early bearing failure, but with the formation of large crack networks that coexist with slip bands, precursors to WEM and conventional WEM.

White etching matter promoted by intergranular embrittlement

To determine the role of brittleness in producing “white etching matter,” rolling elements were intentionally case-carburize heat treated to produce an embrittled case. The intergranular embrittlement was confirmed by characterizing the fracture surfaces of the steel. Post rolling contact fatigue investigations showed subsurface cracks, forming preferentially along the grain boundaries, and white etching matter was determined to be present along these intergranular cracks. The white etching matter was characterized by electron microscopy. These results provide novel insights into cause-effect relationships and evolution mechanisms associated with white etching matter.

Energy dissipation at rubbing crack faces in rolling contact fatigue as the mechanism of white etching area formation

Subsurface cracks decorated with white etching areas are one of the failure marks in bearings subjected to rolling contact fatigue. In the literature, multiple mechanisms were proposed to explain this phenomenon, one of which stated that the white etching matter formed around a subsurface crack is a symptom of crack faces rubbing which leads to local changes in the material microstructure. The current work presents a theoretical study on white etching area formation by the modelling of frictional interaction at crack faces in mode-II, caused by cyclic shearing and compression – the conditions typical for rolling contact. Based on energy considerations, the feasibility of white etching area formation along subsurface cracks in bearings is theoretically investigated.

Soft novel form of white-etching matter and ductile failure of carbide-free bainitic steels under rolling contact stresses

There has been a great deal of work on the formation of hard white-etching regions in conventional bearing steels such as 1C-1.5Cr wt% when subjected repeatedly to rolling contact stresses. The regions are a consequence of localised mechanical attrition across microcrack faces and mixing, which refine the local structure and force cementite to dissolve. This white-etching matter is often associated with brittle phenomena because the hardness can exceed 1100 HV. In contrast, carbide-free mixtures of bainitic ferrite and retained austenite when subjected to the same loading have been unexpectedly found not to develop the characteristic patches of hard material and to show instead signs of ductility in the attrited regions. The work presented here shows that the white-etching areas that develop in carbide-free bainite are softer than their surroundings, whether they are in hard nanostructured bainite destined for bearing applications or when the steel is designed for the manufacture of rails. Advanced characterisation tools were used for the first time to understand carbon redistribution during its formation. The deep interest about soft white-etching matter originates from the idea that it could lead to the reduction in premature failure of wind turbine gearbox bearings and white-etching layer grinding of rails.

Elucidating white-etching matter through high-strain rate tensile testing

A form of damage in bearing steels subjected to rolling contact fatigue is the formation of localised regions of white material just below the contact surface. These ‘white-etching regions’ are strikingly visible signs of damage during metallographic examination. One mechanism proposed to explain their formation is adiabatic shear localisation. Experiments are reported here using high-strain rate (250 s−1) tensile testing to show that this is not the case.

Critical Assessment 13: Elimination of white etching matter in bearing steels

One form of damage due to rolling contact fatigue is the formation of localised regions of extremely hard material forming within the body of a bearing. These regions have a relatively homogeneous structure and hence etch mildly with respect to the surrounding unaffected matrix. They, therefore, appear white in a darker background when examined using optical microscopy. We assess here the cause of this damage and propose solutions that are backed by evidence that already is available. The issue is important because of the spate of unexpected failures in large wind turbine bearings and of generic importance in determining the life of more well behaved bearing applications.

White-Etching Matter in Bearing Steel. Part II: Distinguishing Cause and Effect in Bearing Steel Failure

The premature failure of large bearings of the type used in wind turbines, possibly through a mechanism called “white-structure flaking”, has triggered many studies of microstructural damage associated with “white-etching areas” created during rolling contact fatigue, although whether they are symptoms or causes of failure is less clear. Therefore, some special experiments have been conducted to prove that white-etching areas are the consequence, and not the cause, of damage. By artificially introducing a fine dispersion of microcracks in the steel through heat treatment and then subjecting the sample to rolling contact fatigue, manifestations of hard white-etching matter have been created to a much greater extent than samples similarly tested without initial cracks. A wide variety of characterization tools has been used to corroborate that the white areas thus created have the same properties as reported observations on real bearings. Evidence suggests that the formation mechanism of the white-etching regions involves the rubbing and beating of the free surfaces of cracks, debonded inclusions, and voids under repeated rolling contact. It follows that the focus in avoiding early failure should be in enhancing the toughness of the bearing steel in order to avoid the initial microscopic feature event.