White Etching Cracks Research Articles

A Validated Computational Study of Lubricants under White Etching Crack Conditions Exposed to Electrical Fields

The problem of White Etching Cracks (WEC) leading to failures in the life cycle of industrial and automotive drive trains has existed for more than 30 years. Many parameters leading to WEC, such as the presence of electricity, unfavorable lubricants, and other factors, have been identified by the use of test rigs. However, since (a) within lifecycle operation these impacts appear incidentally, and (b) the identified impacts stimulate each other as stated in WEC research, the appearance of WEC in a drive train is impossible to predict for any given application. This difficulty causes WEC to be a serious problem. Obtaining reliable failure prediction rates using construction guidelines is still impossible, resulting in warranty claims. The aim of this paper is, first, to establish a routine to determine if and how lubricants could be described numerically with respect to their susceptibility toward electrical fields and to see how this factor could be added in a later stage to construction guidelines. Second, this paper aims to give advice to the applicant in terms of countermeasures. Currently, as predictors are missing, extensive and time-consuming testing is required. Contradictory test results in the field of application, indicate the need for fundamental parameters to determine in which circumstances the application is exposed to WEC risk and to suggest countermeasures. The current study presents a simulation method based solely on the chemical structure of lubricant components, investigating the appearance of WEC and their response to increasing electrical fields. The results show a clear pattern in WEC criticality with respect to lubricants: if two or more components present in the lubricant create clusters through their dipolar interaction, an apparent WEC risk could be accurately predicted apart from test rig results. These clusters are charged like one big particle in an early stage of electrical field exposure. As a result, the surface area increases, facilitating a higher uptake of charge. The incidental breakdown of this charge is assumed. As the charge of a cluster is higher than that of single molecules, WEC critical lubricants are subjected to this pattern. The study validates these results by comparing them using lubricants known to be critical with respect to WEC, suggesting further tests in the near future. Despite the fact that some lubricants seem more critical than others related to WEC, each lubricant clearly might lead to WEC under given conditions. Thus, constructing guidelines for a prediction routine is essential.

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.

Lubricant performance against white etching areas (WEAs) formation in AISI 52100 bearing steel under cyclic compressive loading

Premature bearing failures commonly occur in applications that operate under extreme boundary conditions. The lubrication engineers and tribologists are confronting a key challenge in these bearing failures associated with microstructure decay, such as White Etching Cracks (WECs) and White Etching Areas (WEAs). Lubricant degradation is one of the critical factors for the subsurface decay in the bearing steel. This work evaluates the performance of mineral oil (heavy paraffin oil) and synthetic poly alkyl glycol (PAG) against WEAs formation under pure sliding with cyclic compressive loading. The performance of lubricants was evaluated using Infrared Fourier Transform (FTIR) and Electron Spin Resonance (ESR) spectroscopy. The outcomes reveal that the free radical formation rate is higher for paraffin than PAG. This study found that WEAs formation in the bearing steel is delayed in the PAG tested samples compared to paraffin.

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.

Nano- and Micro-Tribological Investigations of Boundary Layers on Axial Bearing Washers Tested under WEC Critical Conditions

The formation of boundary layers on bearing surfaces due to the operational conditions has a significant influence on bearing lifetimes and frictional properties. Zinc dialkyldithiophosphate is an anti-wear additive widely used in oil and grease formulations that forms beneficial surface boundary layers. Under certain circumstances, this additive can cause early bearing failure due to white etching cracks (WEC) formation. By influencing chemical reactions and diffusion processes, the boundary films are suspected to be a reason for the emergence of WECs. The properties of these layers under WEC critical and uncritical conditions are of interest. To gain knowledge of these layers, nano- and micro-tribological tests were performed. One possibility is to measure the hardness by nanoindentation and scratching on and into the layers by nano scratch tests. Another way is to perform local resolved micro-pin-on-disk tests. Additionally, ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) was performed on the bearing surfaces to conclude the chemical compositions of the boundary layers. In the focus was, if the measured properties of the layers can be correlated to the bearing’s early failures due to WEC, frictional properties of the whole system, and the wear on the surfaces.

On micro‐defect induced cracking in very high cycle fatigue regime

AbstractIt is known that fine granular area morphology is often formed around interior micro‐defect in the case of very high cycle fatigue failure of high strength steel. The mechanism of “fragmentation of martensitic laths and formation of dislocation cells” (Zhu's model), which revealed a microstructure‐dependent crack initiation and early growth for micro‐defect induced internal fatigue cracking, was discussed in several lathy martensitic or similar steels, verified under environmental fatigue, and applicable to white etching crack in rotating contact fatigue. A machine learning‐Z parameter integrated approach was developed for fatigue life, which is promising for design against fatigue of engineering structures for long life.

Formation of White Etching Cracks under electrical current flow - Influence of load, slip and polarity

White Etching Cracks (WEC) are currently discussed as a common cause for premature failure of roller bearings in various applications. The formation mechanism of WEC is still under debate in the literature. However, it is emphasized that varying additional loads like electrical current or hydrogen, have an amplifying effect on the formation of WEC. In this work, the formation of WEC under the influence of electrical current was investigated. The testing was conducted on a three-ring on roller test rig using rollers made from the steel AISI/SAE 52100. These rollers were tested utilising different electrical polarities, current intensities, Hertzian pressures and slide-roll-ratios. As a result of the testing, possible WEC formation values for the tested electrical intensities and Hertzian pressures were found. Furthermore, no additional slip has to be present for the WEC formation under the influence of electrical current. Detailed microstructure analysis using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) have been conducted to investigate the effect of electrical current, polarisation and slide-roll- ratio on the microstructural alterations. The analyses showed that in the investigated regions different reaction layers are formed depending on the electrical polarity. Furthermore, the formation of the nanocrystalline structure can be attributed to high local plastic deformation.

A Study on Decisive Early Stages in White Etching Crack Formation Induced by Lubrication

The reliability of rolling bearings is affected by white etching crack (WEC) or white structure flaking (WSF) failures, causing tremendous commercial burdens for bearing manufacturers and operators. The research for the underlying failure mechanism has attracted interest from a large scientific community over decades. Despite the significant amount of efforts, a root cause of white etching cracking is still missing. Amongst other factors, lubricant chemistry is considered to be essential in WEC formation. The authors aim to elucidate this key parameter by provoking white etching crack formation on a FE8 bearing test rig using a well-described set of chemicals in high- and low-reference lubricants. Scanning electron microscopy and energy dispersive X-ray analysis prove the presence of a patchy tribofilm on the surface of bearing washers, leading most likely to a higher frictional torque at the early stages of operation when the low reference oil is used. Secondary neutral mass spectrometry (SNMS) shows a hydrogen containing tribofilm in the shallow subsurface of about 30 nm depth, suggesting that hydrogen proliferating into bearing material may subsequently facilitate crack propagation via dislocation pileups, leading to premature bearing failure.

Simulation of the Fatigue Crack Initiation in SAE 52100 Martensitic Hardened Bearing Steel during Rolling Contact

An investigation on the White Etching Crack (WEC) phenomenon as a severe damage mode in bearing applications led to the observation that in a latent pre-damage state period, visible alterations appear on the surface of the raceway. A detailed inspection of the microstructure underneath the alterations reveals the existence of plenty of nano-sized pores in a depth range of 80 µm to 200 µm. The depth of the maximum Hertzian stress is calculated to be at 127 µm subsurface. The present study investigates the effect of these nanopores on the fatigue crack initiation in SAE 52100 martensitic hardened bearing steel. In this sense, two micro-models by means of the Finite Element Method (FEM) are developed for both a sample with and a sample without pores. The number of cycles required for the crack initiation for both samples is calculated, using the physical-based Tanaka–Mura model. It is shown that pores reduce the number of cycles in bearing application to come to an earlier transition from microstructural short cracks (MSC) to long crack (LC) propagation significantly.

A Study on Early Stages of White Etching Crack Formation under Full Lubrication Conditions

The appearance of White Etching Cracks (WEC), not covered by the ISO 281 modified failure rate calculation, leads to difficulties in predicting bearing reliability. This uncertainty in bearing applications leads to a worldwide activity in order to understand and prevent this situation since the WEC failure mode deviates from the traditional Rolling Contact Fatigue (RCF) mode. Plenty of factors have been found to influence this phenomenon over the years, however the precise initiation of the WEC is still under debate. In order to understand the initiation and analyze the temporal evolution, interrupted tests on the same material were performed under conditions that were known to lead to WEC formation and RCF. To avoid the added complexity of boundary lubrication, a Deep Groove Ball Bearing (DGBB) test rig under full lubrication (Elastohydrodynamic Lubrication, EHL) was chosen. Within a standard operating mode, named Mode 1 (RCF), the bearings are solely subjected to a radial load. By suspending the tests at different time steps, a continuous progress of changes in the subsurface material structure seen as equiaxed grains with low dislocation densities, identified as ferrite, is observed. The bearings did not fail up to load cycles of 109. In contrast, a Mode 2 Electrical Charged Contact Fatigue (ECCF) test provoked the early formation of cracks and crack networks, first without WEA, then later with WEA. It became obvious when comparing Mode 1 (RCF) with Mode 2 (ECCF) that Mode 2 (ECCF) achieves far fewer load cycles until failure occurs.

Electrical Discharges in Oil-Lubricated Rolling Contacts and Their Detection Using Electrostatic Sensing Technique

The reliability of rolling element bearings has been substantially undermined by the presence of parasitic and stray currents. Electrical discharges can occur between the raceway and the rolling elements and it has been previously shown that these discharges at relatively high current density levels can result in fluting and corrugation damages. Recent publications have shown that for a bearing operating at specific mechanical conditions (load, temperature, speed, and slip), electrical discharges at low current densities (<1 mA/mm2) may substantially reduce bearing life due to the formation of white etching cracks (WECs) in bearing components, often in junction with lubricants. To date, limited studies have been conducted to understand the electrical discharges at relatively low current densities (<1 mA/mm2), partially due to the lack of robust techniques for in-situ quantification of discharges. This study, using voltage measurement and electrostatic sensors, investigates discharges in an oil-lubricated steel-steel rolling contact on a TE74 twin-roller machine under a wide range of electrical and mechanical conditions. The results show that the discharges events between the rollers are influenced by temperature, load, and speed due to changes in the lubricant film thickness and contact area, and the sensors are effective in detecting, characterizing and quantifying the discharges. Hence, these sensors can be effectively used to study the influence of discharges on WEC formation.

Ultrasonic detection of white etching defect based on convolution neural network

&lt;sec&gt;Unlike classical defects formed by rolling contact fatigue, white etching defect (WED) including white etching area and white etching crack will cause surface to spall in the early stage and the service life to shorten seriously. Located in the subsurface of bearings, the tiny size WED is difficult to detect by conventional ultrasonic methods. The root cause of WED generation remains unclear. It is time consuming and expensive to prepare samples during the evolution of such defects. For characterizing the WED at early stage, five evolving states concerning the existing microscopic information are established in this paper. The immersion ultrasonic inspection process is simulated based on &lt;i&gt;k&lt;/i&gt;-space pseudo spectrum method.&lt;/sec&gt;&lt;sec&gt;For the later evolutionary stage with crack, the bearing can be simplified into a homogeneous three-layer model by ignoring the internal grain structure. The crack depth is obtained by using the ultrasonic reflection coefficient amplitude spectrum (URCAS), with an error of 1.5%. For other states without crack, the spectrum characteristic is no longer evident with slight acoustic impedance difference between layers. The polycrystalline structure on a microscale is thus realized based on Voronoi diagram, from which the grain induced backscattering can be used to amplify the microstructure variations at different stages. The backscattering signal is influenced by the grain size and detection frequency from the simulation. Since a direct comparison of backscattering information among evolutionary stages is difficult, the five different evolutionary stages of WED are recognized with the help of deep learning. The received waveform is transformed into a time-frequency map by short-time Fourier transform. Based on RESNET network structure, the results show that the train accuracy and validation accuracy reach 92% and 97% respectively. This study provides a sound way to characterize WED, which is conducive to early failure prediction and residual life evaluation.&lt;/sec&gt;

Uncovering the White Etching Area and Crack Formation Mechanism in Bearing Steel

Uncovering the White Etching Area and Crack Formation Mechanism in Bearing Steel

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.

Correlation between grain size and carbon content in white etching areas in bearings

Premature failure of bearings during rolling contact fatigue is often associated with the formation of white etching cracks (WECs). Crack surface rubbing of WECs transforms the original bainitic/martensitic microstructure into white etching areas (WEAs), comprised of nanocrystalline ferrite. The grain size and carbon content vary within the WEA. Here, we show by atom probe tomography and scanning electron microscopy, that there is an inversely proportional relationship between grain size and carbon content in WEAs formed in 100Cr6 bearings that failed by WECs in service. We explain this phenomenon by the reduction of grain boundary energy through carbon segregation. Depending on the carbon content, this reduces the driving force for recrystallization and grain coarsening, thereby stabilizing the nanocrystalline microstructure. No such effect is observed for the substitutional element chromium. The smallest grain size (< 10 nm) is found directly next to decomposing cementite precipitates, which act as carbon sources, leading to carbon contents as high as ~9.5 at% in ferrite. Correspondingly, the WEA segments with the lowest carbon contents exhibit the largest grain sizes. Increasing carbon contents in sub regions of WEAs do not only lead to smaller grain sizes but also to higher average carbon contents at the grain boundaries as well as in the grain interior. Our results show that the mechanisms of ferrite microstructure stabilization through carbon grain boundary segregation shown in model experiments are also valid for the microstructure alterations associated with WEC failure occurring in practical bearing applications of the technical alloy 100Cr6.

Influence of High Electrical Currents on WEC Formation in Rolling Bearings

Premature bearing failures due to white etching cracks (WECs) occur when the rolling contact is subjected to a so-called additional load such as an electrical current flow through the bearing in addition to the pure rolling load. Tests with oil-lubricated roller bearings and greased ball bearings showed that high electrical currents are a very critical WEC trigger. The influence of current intensity, electrical polarity, and load type (stationary ring load or rotating ring load) on WEC formation was proven by the performed bearing tests, whereas the influence of lubricant was relatively low. Based on the findings, the failure hypothesis “energetic WEC fatigue” was extended for high electrical currents.

Interlaboratory comparison: Round robin test for the damage reproduction of white etching crack in cylindrical roller thrust bearings

White Etching Cracks (WEC) is a damage pattern in roller bearings characterized by sub-surface crack networks within local microstructural changes. Nowadays, neither the causes of occurrence are clarified, nor are there commercial calculation methods for damage prediction. The comparability of current research results is hampered by the use of different testing methods and their transferability is, therefore, limited. Until today, there is no generally accepted test method for the risk assessment of predefined tribological systems regarding WEC occurrence. To establish such a test method, the statistically valid transferability of experimental results with a commercially available test rig is necessary. The present work, therefore, concentrates on the transferability of the WEC occurrence using cylindrical roller thrust bearings, a standardized test rig, and a detailed test guideline. The statistical validation of the WEC occurrence and therefore of the proposed reference test method was carried out using an interlaboratory comparison of 20 tests in 10 laboratories. Subsequent metallographic examinations confirmed that all failures were caused by surface anomalies related to WEC. Furthermore, the WEC's position and orientation were systematically and extensively analyzed. The high reproducibility (Weibull coefficient of correlation of 0.975) and low variance (Weibull exponent of 6.4) show the suitability of the test method for risk assessment of predefined tribological systems.

Early stage white etching crack identification using artificial neural networks

White Etching Cracks (WEC) in gearbox bearings is a major concern in the wind turbine industry, which can lead to a premature failure of the gearbox. Though many hypotheses regarding the generation of WEC have been proposed over the decades, the answer is still disputable. To trace back the failures to earlier stages before they occur, an innovative sensor-set has been utilized on a test rig to monitor the influencing factors that lead to WEC. This paperwork seeks to recognize abnormal patterns from recorded sensor data and derive statements of sensible sensor combinations in WEC early detection. A Long Short Term Memory (LSTM) network-based autoencoder is proposed for the anomaly detection (AD) task. Employing an auto-associative sequence-to-sequence predictor, a model is trained to reconstruct the normal time series data without WEC. The reconstruction error of testing time series data is evaluated for the determination of its anomaly. The results show that the specified LSTM autoencoder framework can qualitatively distinguish anomalies from collected multivariate time series data. Moreover, the anomaly score evaluated via reconstruction-error-based metrics can discriminate normal and abnormal behaviors in the study. This investigation’s results entail a significant step towards early WEC risk detection and more cost-efficient wind turbine technology if this approach can be further applied on stream data with plausible thresholds in monitoring system.

A proposed criteria to identify wind turbine drivetrain bearing loads that induce roller slip based white-Etching cracks

In this article, the type of roller slip behavior that may result in the formation of white-etching cracks (WECs) in wind turbine gearbox bearings is identified. A new hypothesis based on the inner raceway normal contact load magnitude at the time of roller slip is proposed as the probable cause of WECs. For this purpose, the maximum normal contact loads are identified when roller slip occurs in high-speed shaft bearings at different mean wind speeds. Subsequently, the annual probability of occurrence of the maximum normal loads are obtained. The probability of maximum load under slip exceeding a limit probability is hypothesized as a probable cause for WEC. In order to apply the proposed hypothesis, two different wind turbines high-speed shaft bearings are used: the cylindrical roller bearing of the General Electric 1.5 SLE turbine and the tapered roller bearing of Vestas V52 turbine. Both the chosen bearings are on the generator side of the high-speed shaft. For both turbines, measurement data together with analytical models are used for identifying the slip and the maximum normal contact loads. We propose forecasting the probability of exceedance of a threshold maximum normal contact load level during slip to identify the possibility for inducing WECs.

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.