Formation Of White Etching Cracks Research Articles

Multi-Parametric Investigations on White Etching Crack Formation in Deep Grove Ball Bearings

Research on White Etching Cracks (WEC) in multiple bearing applications has identified various drivers that cause them. Lubricants and electricity combined with contact mechanics have been proven to catalyze WEC significantly. However, none of these factors solely cause WEC on its own; instead, combinations of factors discretize whether WEC appears or not. Hence, the WEC phenomenon appears to be multidimensional, making WEC still unpredictable. The current paper is about a systematic study using a Deep Grove Ball Bearing test rig to investigate how lubricant chemicals, combined with electricity and variations in oil flow and pressure, lead to WEC formation. It becomes evident that even under critical conditions for WEC formation, increasing oil flow and decreasing contact pressure can prevent WEC.

The Influence of Peripheral Components in Test Rig Creation of White Etching Cracks

White Etching Cracks (WEC) have become a subject of extensive research in material science, chemistry and lubrication, and even operational mathematics by AI learning. Initially reported in the 1960s and considered an exotic anomaly, the failures gained importance with the global rise of wind energy power and the automotive industry. Unexpectedly high failure rates in various bearing applications have led to the need for a deeper understanding and prevention of WEC. It has come a long way from materials inspection, to parametrically studying WECs on test rigs, to the understanding that WEC is a stand-alone phenomenon and sparingly related to common failures in bearing technology. It has been commonly accepted that WEC drivers have multiple dimensions, e.g., material, contact mechanics, chemistry, and electricity. The impact of these factors on WEC failures is frequently studied using test rigs at the component level, such as the FE8 test rig. The FE8 has been utilized in numerous investigations due to its ability to replicate WEC failures without requiring artificial electricity or hydrogen charging by using specific lubricant chemistry and operating conditions. However, through intensive testing, it was observed in this study that a standard material in an FE8 rig component demonstrated a profound influence on WEC formation. This paper presents the details of the testing and analysis, aiming to investigate the mechanisms of interactions between the hose material and the low reference lubricant. The results demonstrate that the chemistry of the component material plays an important role in WEC formation. This finding may have significant impact in WEC studies, especially when the FE8 rig is used.

Process of White Etching Cracks Formation in Carburized Bearing Steel under Rolling Contact Fatigue

Process of White Etching Cracks Formation in Carburized Bearing Steel under Rolling Contact Fatigue

Influence of MnS inclusion characteristics on generation of white etching cracks in 100Cr6 bearing steel

Wind turbine gearbox (WTGB) bearings suffer from premature failures through white etching cracks (WECs). To investigate causes of this failure mechanism, we tested two types of 100Cr6 bearing steel components. We carried out rolling contact fatigue experiments on standard washers and material extracted from a WTGB bearing. These differed strongly in morphology and orientation of non-metallic inclusions (NMIs) with respect to the raceway, especially of MnS-type. While the standard washer did not exhibit WECs formation, the WTGB bearing did - even under moderate conditions (1.9 GPa Hertzian pressure, 107 cycles). We therefore conclude that MnS-inclusions are a key factor in initiation of WECs in WTGB bearings.

A Study on the Influence of Electrical Discharges on the Formation of White Etching Cracks in Oil-Lubricated Rolling Contacts and Their Detection Using Electrostatic Sensing Technique

In bearing applications, the presence of stray and parasitic currents in combination with lubricants has been studied for almost a century and has been found to cause fluting and corrugation damages under high current densities. However, recent research has suggested that at low current densities (<1 mA/mm2) under specific operating conditions, electrical discharges can substantially reduce bearing life due to the formation of white etching cracks (WECs). To date, limited studies have investigated the critical operating and electrical conditions for WEC formation and demonstrated effective fault detection techniques. This study uses a novel monitoring technique known as the electrostatic sensing technique to detect, monitor and characterise electrical discharges in an oil-lubricated steel–steel rolling contact on a TE74 twin-roller machine. The findings demonstrate that WECs can be formed under the influence of electrical discharges in less than 50 h, and the electrostatic sensors are effective for the early detection of critical electrical discharges related to WEC-induced failures.

Uncovering the white etching area and crack formation mechanism in bearing steel

The microstructure of a damaged bearing from the field was characterized in this work with the intention to better understand microstructural features behind formation of White Etching Cracks (WEC) in bearings. Microstructural characterization of the altered white etching area (WEA) involved conventional electron backscattered diffraction (EBSD), followed by transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD). In addition, automated crystallographic orientation mapping in TEM was performed on lamellae from selected regions of the WEA extracted via focus ion beam milling. The results revealed that the orientation of detectable grains within WEA is similar to that of the vicinal bulk material. WEA consists of small spherical grains (average 30 nm) and the orientation of the grains varied significantly in the deformed zone, suggesting that recrystallization had occurred. The interface between bulk material and the deformed zone is very sharp. Furthermore, needle-like grains, most likely originating from the zone undergoing only modest levels of severe plastic deformation, occurred in WEA. The occurrence of different grain sizes in WEA and incomplete plastic deformation strongly support the hypothesis of WEC formation via severe plastic deformation followed by recrystallization.

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 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.

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.

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.

An interdisciplinary framework to predict premature roller element bearing failures in wind turbine gearboxes

Roller element bearings present in the intermediate and high-speed stages of wind turbine gearboxes operate in dynamic working conditions and in some cases may fail within 30% or less of their designed lifetime. Upon investigation, it has been identified that these premature failures happen due to a peculiar failure mode associated with formation of white etching cracks (WEC). This continues to be a great challenge for the wind energy operators as it leads to an increase of maintenance and operation costs in addition to long wind turbine downtime. Therefore, the industry is in dire need of a lifetime prediction methodology that could take in multi-scale inputs ranging from bearing loads at the system level down to the level of bearing material properties at the microscopic level. This work summarizes the overall approach of a project that aims towards an integrated framework which links load data from the bearings and microstructure related non-metallic inclusion statistics from bearing steels, to predict a material based probability of failure. The interlink between both aspects is a numerical rolling contact fatigue (RCF) framework based on finite element analysis, which includes multi-scale data as an input to calculate rolling contact fatigue damage. The outcome will help the wind industry to better predict bearing failures.

Predicting white etching matter formation in bearing steels using a fretting damage parameter

White etching cracks (WECs) are deleterious artifacts found in fatigue damaged bearings. The formation of WECs has been attributed to the frictional energy dissipation at subsurface cracks during cyclic rolling contact. This study introduces a novel Ruiz fretting damage parameter (FDP) implementation to predict the frictional energy dissipation at subsurface cracks. A parametric study using a finite element model explores the role of crack length, orientation, depth, and coefficient of friction (COF) on FDP. The FDP is maximized at a normalized length of 0.50a and depth of 0.78a, between the orientation of ±30°, at a COF of μ = 0.3, in agreement with experimental observations. These results provide strong evidence of the FDP as a predictor of WEC formation.

Post-testing measurement of freely movable and diffusible hydrogen in context of WEC formation at cylindrical roller thrust bearings from 100Cr6

Sub-surface crack networks in areas of altered microstructure are a common cause for bearing failures. Due to its appearance under light microscopy, the damage pattern is referred to as White Etching Cracks (WEC). The root causes leading to the formation of WEC are still under debate. Nevertheless, it has already been shown that atomic hydrogen can have an accelerating effect on the formation and propagation of WEC. In addition to hydrogen pre-charging, hydrogen can be released and absorbed during rolling/sliding due to the decomposing of the lubricant and water. The current work focuses on the analysis of the hydrogen content of cylindrical roller thrust bearings after testing in a FE8 type test rig using two different lubricants. Within the framework of this work, two different hydrogen analysis methods were used and assessed regarding their applicability. The results show that the so-called Hydrogen Collecting Analysis (HCA) is more suitable to investigate the correlation between lubricant chemistry and hydrogen content in the test bearings than the Local Hydrogen Analysis (LHA). The measurements with the HCA show a continuously increasing freely movable and diffusible hydrogen content under tribological conditions, which leads to the formation of WEC. Comparative tests with an oil without hydrogen showed that the tendency of the system to fail as a result of WEC can be reduced by using a lubricant without hydride compounds.

Mechanism of cementite decomposition in 100Cr6 bearing steels during high pressure torsion

Severe plastic deformation leads to cementite decomposition in pearlitic and martensitic alloys, resulting in high-strength nanocrystalline ferrite. This effect can be employed to strengthen pearlitic wires but it can also be associated with material failure by white etching cracks (WECs) that are primarily known to concern bearings or rails. We investigate the decomposition of spheroidal cementite in the martensitic bearing steel 100Cr6 with 62 Rockwell hardness during high pressure torsion at 9.5 GPa applied pressure. The hard martensitic matrix and the even harder spheroidal cementite precipitates behave plastically very differently. The enforced macroscopic plastic deformation is almost entirely carried by the matrix. Plastic material flow of the matrix around the spheroidal cementite leads to wear of the spheroidal cementite as indicated by continuously increasing levels of chromium in the matrix. Plastic deformation of spheroidal cementite via dislocation gliding supposedly accelerates this process as slip steps generated thereby are preferential sites of wear at the matrix/cementite interface. Larger spheroidal cementite precipitates are more prone to plastic deformation and to decomposition than smaller ones. The mechanism likely holds true in general for multiphase materials with large strain difference between phases subjected to high pressure torsion. Although the formation of WECs in 100Cr6 bearings might be slowed down by reducing the size of the spheroidal cementite precipitates, it is unlikely that this could entirely prevent this failure mechanism.

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

ABSTRACT Material failure by white-etching-cracks (WECs) can cause enormous economic costs. The formation of WECs emerges from the decomposition of the original, usually cementite-containing, microstructure. As small amounts of electric current can trigger this failure mechanism, we investigate the contribution of electric current to cementite decomposition. We applied ∼700 A/cm2 for two weeks at 60°C to a pearlitic Fe-0.74C (wt%) specimen. The comparison of the microstructure before and after showed no differences. Theoretical considerations support the conclusion that at this low temperature such electric current densities cannot directly cause cementite decomposition. Electric current could play an indirect role in the formation of WECs, however, by generating hydrogen from the lubricant which is known to accelerate WECs formation.

Mechanism of fine granular area and white etching crack formation in AISI 52100 bearing steel

Fine Granular Area (FGA) and White Etching Cracks (WEC) are two types of fatigue damage phenomena which are characterised by the development of cracks accompanied by a local microstructural transformation. The resulting microstructures share many properties, chiefly a strong reduction in grain size. In this study, the thickness of the transformed zones along the crack formed during FGA and WEC experiments is investigated. It is shown that in WEC, similar to earlier results on the FGA, the size of the transformed zones increases with increasing crack length. For further analysis of the more complex WEC case, an approach is presented that uses the results of a critical plane analysis for the calculation of key figures in fracture mechanics. The results are shown to be very similar to those of the FGA. Drawing on these and earlier results, it is proposed that a common mechanism of WEC and FGA formation exists. In addition, some important differences of the two phenomena are explained within the framework of the common mechanism.

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.