Formation Of White Etching Areas Research Articles

Microstructure alterations and white etching area formation in bearing steels under high-frequency dynamic loading

This article discusses the effects of various steels and lubricants on subsurface microstructural changes during a high-frequency dynamic pin-on-disc test under pure sliding and boundary lubrication conditions. Steel materials chosen for this study are 55SiMo8 steel, 100Cr6 steel, and 8 %Cr steel. Fully formulated polyalkylene glycols-based oil, synthetic hydrocarbon oil, and per-fluoro polyether base oil were selected as the lubricants. In 8 %Cr, the average roundness of large carbides was reduced by 33 % due to the deformation of carbides that occurred during severe cyclic loading. The outcomes showed that the material effect has a greater impact on the generation of white etching areas than the effect of lubricants under high-frequency dynamic loading.

Investigation on butterfly white etching area formation mechanism and crack source at different stages in M50 bearing steel

Rolling contact fatigue tests with different stresses and cycles have been conducted to study the formation mechanism and crack source of butterfly white etching area (WEA) in different stages of M50 bearing steel. The results suggest that the structural transformation is prior to cracks. The initial microstructure is the nanocrystalline α-Fe formed by dislocation assisted carbon migration. With the extension of cycles, unlike the previous studies, the butterfly-origin carbide is refined into Mo2C and M7C3 nanocrystalline by the accumulated dislocation clusters. Subsequently, the increase of interfacial energy drives the metal elements diffusion and thus the formation of amorphous structures. A new mechanism called diffusion-assisted dislocation has been proposed for butterfly WEA formation.

Effect of DLC film on the mitigation of white etching areas (WEAs) under dynamic loading

The premature failures in wind turbine gearbox bearings are primarily caused by the decay of subsurface microstructure called white etching areas and white etching cracks. Severe operating conditions of wind turbine gearbox bearings necessitated for tribological coatings to enhance the system performance. The current study investigated the performance of hydrogenated diamond-like‑carbon (DLC) coating against white etching areas formation under sliding-dynamic conditions. The dynamic loading experiments were carried out in the dynamic pin-on-disc tribometer with Polyalphaolefin base stock oil under a boundary lubrication regime. Prior to the dynamic load tests, detailed tribological characterisation of the deposited DLC coating was carried out. The post-test analysis results were compared with the uncoated AISI 52100 bearing steel tribopair to evaluate the performance of DLC-coated bearing steel against white etching areas formation under given conditions. The results obtained show the capability of the proposed coating to delay the formation of white etching areas in the bearing steel.

Influence of additive-derived reaction layers on white etching crack failure of SAE 52100 bearing steel under rolling contact loading

It is a common observation that rolling bearings fail due to the formation of white etching areas and white etching cracks (WEA/WEC). Branching crack networks beneath the surface and an altered microstructure known as WEA are characteristics of the failure mode. Therefore, the WEA-related cracks are known as WEC. In order to shed more light on this so far not fully understood phenomenon, the present study focussed on the role of specific oil additives and their influence on the formation of reaction layers and how these possibly affect the WEA/WEC formation. For this purpose, comprehensive tests were conducted on a three-ring-on-roller test rig using two different fully formulated oil lubricants with samples of SAE 52100 bearing steel. A subsequent carried out analysis by wavelength dispersive X-ray spectroscopy (WDX) and transmission electron microscopy (TEM) revealed the differences in the formed additive-derived reaction layers. While a known WEA/WEC-critical oil formulation, containing the two additives zinc dithiophosphate (ZDDP) and overbased calcium sulfonate (OBCaSul), did not result in any significant formation of reaction layers during mixed / boundary lubrication at an oil influx temperature of 95 °C, an oil formulation containing barium and phosphorus performed excellent in this respect. This finding is related to the observation that provocation of WEA/WEC failures in our experiment was always unsuccessful when the barium- and phosphorus-containing oil was used. The use of barium phosphate containing lubricants could therefore be an effective measure to prevent or significantly delay WEA/WEC damage to bearings.

Multiscale modelling and simulation of subsurface carbide deformation during the formation of white etching areas

One of the main failure indications in rolling bearings subjected to rolling contact fatigue (RCF) is premature subsurface cracking accompanied by white etching areas (WEAs). The cracks that are linked with WEAs are called white etching cracks (WECs). The formation mechanisms of WEAs/WECs are still highly debatable according to international state of art. Often SAE 52100 steel is used for bearing rings. Its microstructure consists of a martensitic or bainitic matrix and homogeneously distributed chromium rich cementite precipitations. There are two main hypotheses based on what forms first, the cracks or WEAs (hypothesis 1 & 2 respectively). There are many numerical studies that examine non-metallic inclusions as a source of cracks that leads to WEAs (hypothesis 1), but none that considers carbides as the source of severe plastic deformation subjected to RCF that leads to WEAs and, eventually WECs (hypothesis 2). In this study, the testing was conducted on a ZF-type modified RCF test rig with hydrogen charged samples to reproduce WEAs/WECs formation. Later the influence of the microstructural features of the carbides on the steel matrix, during the formation of WEAs was investigated by microstructural investigations and multiscale finite element modelling. It was found that carbides can contribute to severe local plastic deformation which leads to formation of WEAs. Thus, it also supported the likelihood of hypothesis 2. Additionally, the surface traction and microstructural characteristics of carbides are correlated with the wide range of WEAs formation depths and timescales.

Hydrogen-accelerated white etching area formation in bearings under rolling contact fatigue

The presence of hydrogen can dramatically facilitate microstructural alterations in components subjected to rolling contact fatigue (RCF) potentially leading to premature failure. A dislocation-assisted carbon migration model is developed to describe the formation of hydrogen-influenced microstructural alterations such as white etching areas; the model incorporates rolling parameters such as maximum contact stress, number of cycles, rotational speed, and temperature. Kinetic Monte Carlo is adopted to describe hydrogen-dislocation interactions which alters dislocation mobility, accelerating white etching area formation whilst reducing fatigue life. The results are experimentally validated by microstructural characterisation and RCF testing.

Experimental investigation on the performance of AISI 440C martensitic stainless steel against the formation of white etching areas under sliding dynamic loading

ABSTRACT Bearings fail prematurely in transient operating conditions, as a result of flaking that causes changes in subsurface microstructure. In this type of failure, the formation of White Etching Cracks (WECs) is preceded by the formation of White Etching Areas (WEAs). Typically the WEAs are formed due to the dissolution of cementite into the matrix in AISI 52100 bearing steel. This feasibility work investigates the performance of AISI 440C steel against WEAs formation using dynamic load Pin-on-Disc tribometer. Experiments were performed at 4.5 Hz loading frequency, Hertzian pressure 2.02 GPa and sliding velocity of 0.2 ms-1 under the boundary lubrication using Poly Alpha Olefins (PAO). From post-test analysis, it is found that AISI 440C steel trapped more hydrogen than AISI 52100 bearing steel. The comprehensive analysis of materials and lubricants revealed that the performance of AISI 440C stainless steel against WEAs formation is better than that of AISI 52100 bearing steel.

Effect of Retained Austenite on White Etching Crack Behavior of Carburized AISI 8620 Steel Under Boundary Lubrication

The formation of white etching cracks (WECs) is a dominant failure mode in wind turbine gearbox bearings that can significantly shorten their operating life. Although the phenomenon of WECs has been communicated in the field for more than a decade, the driving mechanisms are still debated, and the impact of proposed mitigation techniques is not quantified. Leading hypotheses to inhibit the formation of WECs center on material solutions, including the use of steel with high levels of retained austenite (RA). The present work aims to explore the impact of RA on the formation of WECs within AISI 8620 steel under boundary lubrication. A three ring-on-roller benchtop test rig was used to replicate WECs in samples with different levels of RA. While varying levels of RA had a minimal effect on time until failure, a significant effect on crack morphology was observed. Additionally, potential underlying mechanisms of White Etching Area formation were elucidated. Under the current test conditions, the microstructural alterations adjacent to the cracks in the lower RA samples were more developed compared to those of the higher RA samples. Additionally, the WEC networks in the high RA samples contained significantly more crack branches than those of the low RA samples.

Hydrogen assisted rolling contact fatigue due to lubricant degradation and formation of white etching areas

RCF tests were conducted on bearings lubricated with three different lubricants: a gearbox oil, a MAC fluid and PFPE oil. The decomposition of the MAC fluid and PFPE was investigated by FTIR and NMR. Gearbox oil tests showed flaking and networks of subsurface cracks with partial intercrystalline crack growth. Cross-sectional analysis revealed the formation of WEAs. FEM correlated the damage with stress fields. The results of the MAC lubricated tests showed surface flaking and intercrystalline crack growth in the subsurface. The PFPE lubricated bearings showed no signs of hydrogen induced damage.Post-experimental hydrogen analysis of the bearings indicated an increased hydrogen concentration in the samples lubricated with the gearbox oil and MAC compared to those lubricated with PFPE.

Formation of white etching areas in SAE 52100 bearing steel under rolling contact fatigue – Influence of diffusible hydrogen

White etching cracks (WECs) have been studied over the last decades as a common cause for premature failure of roller bearings in various applications. As a consequence of the non-directional and branched crack networks associated with an altered microstructure named as white etching area (WEA) the damage pattern appears as cracking or flaking of the bearing components. In most published literature dealing with WEA/WEC, it is assumed that WEA/WEC formation should only occur under the condition of an additional load superposed to the bearing operation. The formation mechanisms of this damage are still under debate, though. In this work, the influence of diffusible hydrogen on the formation of WEA/WEC has been investigated systematically using hydrogen pre-charged rollers made from the bearing steel SAE 52100 (DIN 100Cr6) prior to testing. These rollers were tested on a four-wheel test rig using different hydrogen concentration, Hertzian pressure and number of load cycles. As a result of testing, thresholds for WEA/WEC formation were found for different hydrogen concentrations, Hertzian pressures and running time of the rollers. Further investigation with manually interrupted tests revealed the formation of the damage being beneath the raceway surface of the rollers. Detailed microstructure studies using comprehensive analysis in the scanning electron microscope including EDX and EBSD as well as plasma focused ion beam (PFIB) have been conducted to give detailed information of the altered microstructure.

Review of the damage mechanism in wind turbine gearbox bearings under rolling contact fatigue

Wind turbine gearbox bearings fail with the service life is much shorter than the designed life. Gearbox bearings are subjected to rolling contact fatigue (RCF) and they are observed to fail due to axial cracking, surface flaking, and the formation of white etching areas (WEAs). The current study reviewed these three typical failure modes. The underlying dominant mechanisms were discussed with emphasis on the formation mechanism of WEAs. Although numerous studies have been carried out, the formation of WEAs remains unclear. The prevailing mechanism of the rubbing of crack faces that generates WEAs was questioned by the authors. WEAs were compared with adiabatic shear bands (ASBs) generated in the high strain rate deformation in terms of microstructural compositions, grain refinement, and formation mechanism. Results indicate that a number of similarities exist between them. However, substantial evidence is required to verify whether or not WEAs and ASBs are the same matters.

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.

Observations of the effect of varying Hoop stress on fatigue failure and the formation of white etching areas in hydrogen infused 100Cr6 steel rings

White etching cracks (WECs) in wind turbine gearbox bearings have been studied previously. Rolling contact fatigue (RCF) tests are conducted on 100Cr6 bearing steel rings, in this study, to generate WECs like those found in wind turbine bearings. This research studies the effect of two different levels of tensile Hoop stresses on the failure life and formation of WECs in the rings. The rings are pre-charged with hydrogen before RCF tests are conducted. It is found that these rings experience incremental fatigue failure, followed by a sudden rapid failure. The fractography, Reflecting Light Microscopy (RLM) and Scanning Electron Microscopy (SEM) results are presented in this paper.

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

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 µm 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 + oxide inclusions and small globular oxide inclusions between ∼1 µm and 20 µm 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.

White structure flaking (WSF) in wind turbine gearbox bearings: effects of ‘butterflies’ and white etching cracks (WECs)

The actual service life of wind turbine gearboxes is often well below the desired 20 years. One of the prevalent failure modes in gearbox bearing raceways is white structure flaking (WSF) by the formation of butterflies and white etching cracks with associated microstructural change called white etching areas. Despite these failures having been observed for two decades in various industries, the detailed reasons and mechanisms for their formation are not fully understood. In this review, white etching area formation mechanisms are discussed, specifically grain refinement, and effects of carbon/carbide in a range of bearing steels of widely differing carbon content. The review also highlights the severe transient, cyclic loading and tribochemical operating conditions of gearbox bearings and explains how these may act as drivers to produce WSF. Much previous research has focused on the detrimental effects of hydrogen, but other work suggests that hydrogen is not the only cause for WSF. Possible methods for preventing WSF are discussed, with attention paid to special steels such as high chromium steels, low carbon stainless nitrogen alloy steels and carbonitrided steels. Beneficial compressive residual stresses, surface coatings and enhanced lubrication and additive packages are shown to offer degrees of prevention, although the mechanisms leading to improvements are not fully understood.

Microstructural Changes and Crack Initiation with White Etching Area Formation under Rolling/Sliding Contact in Bearing Steel

Microstructural changes during the formation of White Etching Area (WEA) were investigated using a rolling/sliding contact fatigue test machine. In particular, interfaces between WEA and retained austenite or spherodized carbide in a high carbon chromium bearing steel have been observed. Fatigue tests were carried out by means of a disk on roller-type equipment. It was found that acicular structures are formed at an initial stage of the WEA formation process. Plastic deformations of spherodized carbides were caused by shear stress in the initial stage of this process as well. Additionally, micro voids were found near the interfaces. Furthermore, amorphous-like structures were found in the WEA. It was suggested that WEA formation is caused by the local plastic deformation of the material. The shear stress was caused by slipping between the roller and the disk. As a result, it was suggested that martensite forms an amorphous-like phase and then changes into a WEA. Cracks developed in the amorphous-like structure/the granular area interface in the WEA and in the Matrix/WEA interface.

Surface fatigue failure of nitride-hardened aluminium-chromium-molybdenum steel rollers under pure rolling and sliding-rolling contacts

To study surface failures of nitride-hardened gears, rolling fatigue tests were performed using nitride-hardened steel rollers as gear-tooth surface models. Surface failure of nitride-hardened rollers was by subsurface-initiated spalling under all contact conditions, nitriding depths and Hertzian stresses used. Cracks leading to spalling initiated at the case/core boundaries. White etching areas (WEA) were observed along spalling cracks. Electron probe microanalysis revealed that the WEA material was carbide. It is considered that there is a relation between the initiation of spalling cracks and the formation of WEA.