Formation mechanisms of white etching cracks and white etching area under rolling contact fatigue

Abstract

The formation of white etching cracks in the 1 mm zone beneath the contact surface in steel rolling element bearings causes a premature wear failure mode called white structure flaking. The formation drivers of white etching cracks are contested, as are the initiation and propagation mechanisms of the cracks. Hydrogen diffusion into bearing steel sourced from the hydrocarbon lubricant or water contamination and transient operating conditions have been suggested as formation drivers. Extensive work has been conducted at Southampton to further understanding of white structure flaking and this paper summarises these evidences and the conclusions made. Serial sectioning has been used to map subsurface wear volumes of wind turbine gearbox bearings from service and large-scale test rigs, test specimens/bearings from laboratory under hydrogen charged conditions and non-hydrogen charged conditions. The process involves polishing of cross sections of test specimens/bearings at $3-5 mm material removal intervals typically over hundreds of slices, and this was used to map white etching cracks in their entirety for the first time. Serial sectioning has allowed a comprehensive investigation of the initiation and propagation mechanisms of white etching cracks and thresholds for their formation with respects to concentration of diffusible hydrogen, contact pressure and number of rolling cycles. From these studies it has been found that white etching cracks can form by subsurface crack initiation at inclusions under hydrogen charged and non-hydrogen charged conditions; hence it has been confirmed that this is one mechanism of WEC formation. Small/short sized sulfide inclusions, globular manganese sulfide þ oxide inclusions and small globular oxide inclusions between $1 mm and 20 mm in diameter/length predominated as crack initiators. In addition, detailed focused ion beam/transmission electron microscopic studies have been conducted to enhance the understanding of butterfly crack and white etching area formation mechanisms.

The Evolution of White Etching Cracks (WECs) in Rolling Contact Fatigue-Tested 100Cr6 Steel

“…The proposed initiation and propagation mechanisms for WSF/WECs are: (1) surface initiation through two opposing mechanisms, (1) shear stress-induced fatigue microcracks [17] and (2) localised high circumferential tensile stress spontaneously induced cleavage-like axial cracks that initiate independently [5,17,18], at defects such as inclusions [17][18][19][20][21] or due to corrosion, machining defects or electrical erosion pits [21]; (2) subsurface initiation by non-metallic inclusions (NMIs) [5][6][7][22][23][24][25][26], perhaps in some cases due to tensile stresses [27]; (3) adiabatic shear banding independent or including defects through impact events, cracks forming after microstructural changes occur [2,28]; (4) self-charging of lubricants triggering localised transient current flow causing local electromagnetic induction that crosses the contact surface leading to electrothermal mechanisms triggering subsequent WEA microstructural change [29,30]; (5) a multistage initiation of WECs as a result of migration of carbon under shear stress and high localised energy [31].…”

“…This supports evidence from previous studies conducted on WTGBs by authors of this manuscript [7] where predominantly small/short sized (3-20 μm) sulphides, globular oxides and globular MnS-oxide inclusions were recorded and judged likely initiators of WECs. It is proposed that small NMI-initiated WECs coalesce to form larger networks that eventually branch to the contact surface causing WSF or axial cracking [1,[4][5][6][7][8][23][24][25].…”

Understanding white etching cracks in rolling element bearings: State of art and multiple driver transposition on a twin-disc machine

“…8,33,46,47 Therefore, hydrogen is often considered as one of the main WEC root causes even though there is no consensus regarding the underlying mechanisms of hydrogen absorption into the bearing steel. 2,4,6,8,15,16,20,24,38,44,45,48,49 Hydrogen may be present in different states within steel: (1) highly diffusive and mobile, (2) trapped to a certain energy bond or (3) in a molecular form. Mobile hydrogen is generally identified as the main contributor to hydrogen embrittlement, even in concentrations as low as 1 ppm.…”

“…7,10,13,14 Due to the lack of common evident denominator and of systematic laboratory reproduction, no consensus on the formation mechanism nor durable countermeasure has been confirmed yet. 2,14–17 Some studies suggest that some black oxide surface treatments as well as nickel or tungsten coatings of bearing components significantly delay WEC-related failures. 2,14,18–20 Nevertheless, there are countless variances of these passivation layers, some being efficient, some not, and that they yet cannot be deemed a durable countermeasure as they may eventually wear off.…”

The role of electric current in the formation of white-etching-cracks

“…These regions are called white-etching-areas (WEAs) according to their bright appearance after etching in the light microscope. The coupling of WECs/WEAs is primarily reported to occur in bearings and rails but also concerns many further widespread engineering applications such as washing machines, transmissions, dryers and many more [1,2]. Material failure by WECs is thus a ubiquitous phenomenon causing enormous economic costs world-wide.…”