White Etching Bands Research Articles

Refining the mechanistic understanding of microstructural decay during rolling contact fatigue in 52100 bearing steel tempered at high temperature

Subsurface rolling contact fatigue (RCF) failure occurs beneath heavily loaded hard contacts like gears, bearings, and cams. This study investigates microstructural decay beneath a RCF-tested surface in AISI/SAE 52100 bearing steel tempered at 240 ℃. RCF tests were conducted at 100 ℃ with a maximum Hertzian contact pressure of 4 GPa for four stress cycles. Microstructural characterization utilized scanning electron microscopy, electron backscatter diffraction, transmission Kikuchi diffraction, and transmission electron microscopy. Due to high tempering temperature, white etching bands (WEBs) were observed without preceding dark etching regions. The microstructural decay sequence involved: (1) formation of elongated ferrite and ferrite microbands, (2) complete dissolution of tempered carbides and partial dissolution of residual cementite, (3) formation of WEBs composed of nano-sized ferrite grains (100–300 nm) transformed from ferrite microbands, and (4) appearance of lenticular carbides. Within the WEBs, most nano-sized grains had high-angle grain boundaries, while the fraction of low-angle grain boundaries increased in later stages of RCF. Lenticular carbides formed alongside elongated ferrite and coalesced nano-sized ferritic grains.

Influence of White Etching Bands Formation on Integrity of Rolling Element Bearings

Influence of White Etching Bands Formation on Integrity of Rolling Element Bearings

Further Investigations and Parametric Analysis of Microstructural Alterations under Rolling Contact Fatigue.

Bearing elements under rolling contact fatigue (RCF) exhibit microstructural features, known as white etching bands (WEBs) and dark etching regions (DERs). The formation mechanism of these microstructural features has been questionable and therefore warranted this study to gain further understanding. Current research describes mechanistic investigations of standard AISI 52100 bearing steel balls subjected to RCF testing under tempering conditions. Subsurface analyses of RCF-tested samples at tempering conditions have indicated that the microstructural alterations are progressed with subsurface yielding and primarily dominated by thermal tempering. Furthermore, bearing balls are subjected to static load tests in order to evaluate the effect of lattice deformation. It is suggested from the comparative analyses that a complete rolling sequence with non-proportional stress history is essential for the initiation and progression of WEBs, supported by the combination of carbon flux, assisted by dislocation and thermally activated carbon diffusion. These novel findings will lead to developing a contemporary and new-fangled prognostic model applied to microstructural alterations.

Rolling Contact Fatigue-Related Microstructural Alterations in Bearing Steels: A Brief Review

Bearings are vital components that are widely used in modern machinery. Although usually manufactured with high-strength steels, bearings still suffer from rolling contact fatigue where unique microstructural alterations take place beneath the contact surface as a result of the complex stress state. Studying these microstructural alterations is a hot research topic with many efforts in recent decades. In this respect, the key information regarding four major types of microstructural alterations, white etching areas/white etching cracks, dark etching regions, white etching bands and light etching regions is reviewed regarding the phenomenology and formation mechanisms. Then, classical and state-of-the-art models are established to predict their formation and are summarised and evaluated. Based on the current research progress, several key questions and paradoxes for each type of microstructural alteration are raised, suggesting possible research directions in this field.

White etching bands formation mechanisms due to rolling contact fatigue

Subsurface micro-structural changes such as dark etching region (DER) and white etching bands (WEB) which develop in bearing steels due to cyclic stresses in rolling contacts, have been studied for decades and a number of theoretical models have been proposed to explain their formation mechanisms and predict their initiation. In WEB investigations, studies have generally focused only on one subtype of WEB, e.g. low angle bands (LAB) or high angle bands (HAB), while the most recent semi-empirical model has shown to be able to predict the formation of both LAB and HAB based on observed growth patterns of ferrite grains. Following from the modelling study, this paper presents a detailed mechanistic study, showing the evolution of ferrite grains (equiaxed and elongated grains) and carbide structures in WEB formed in inner rings of angular contact ball bearings at their different life stages through SEM, EBSD and nano-indentation analysis. The results strongly suggest both LAB and HAB initiate as equiaxed ferrite grains due to recrystallization arising from energy build-up in the initial microstructure that later develops to elongated ferrite grains through a grain rotation/coalescence recovery mechanism induced from plastic deformation. The formation of carbide structures in LAB is associated with the transformation of equiaxed to elongated grains, where carbides nucleate at the edges of the elongated grains rather than at the equiaxed grain band edges as being previously suggested in literature. The newly proposed formation mechanism links LAB and HAB based on experimental findings from detailed inspection of gradual microstructural alteration sequence of LAB and HAB in rolling contact fatigue (RCF) tested bearings.

Development of white etching bands under accelerated rolling contact fatigue

Bearing steel under severe loading condition undergoes substantial subsurface microstructural alterations known as Dark etching regions and white etching bands. White etching bands (WEBs) develop after hundreds of millions of stress cycles in bearing components and have been reported for several decades but the formation mechanism of white bands is not fully elucidated. Current research presents a systematic rolling contact fatigue (RCF) testing in a rotary tribometer under accelerated conditions, where rolling cycles are simulated in a 4-ball test configuration. The post RCF investigations have been carried out to understand the formation mechanism of WEBs in a ball-on-ball point contact load. WEBs have been characterised with the help of nanoindentation and Energy-dispersive X-ray spectroscopy analysis. The quantitative analysis of WEBs growth with subsurface stress field has revealed that the unique orientations of white bands are governed by the plane of maximum relative normal stress along the contact track. Moreover, the accelerated growth and reversal of WEBs sequence at elevated temperature have revealed that the WEBs formation is dependent on temperature/load combination. The observed growth of lenticular carbides in current research is also compared with dislocation gliding model and the role of carbon diffusion within WEBs is highlighted.

Semi-empirical model for predicting LAB and HAB formation in bearing steels

The formation of white etching bands (WEBs), including both low- (LAB) and high- (HAB) angle bands, in the subsurface of steel bearings due to rolling contact fatigue (RCF) has been investigated over decades to understand their characteristics and predict their formation during operation. A few studies recently have modelled the development of LABs theoretically based on thickening of lenticular carbides adjacent to the band controlled by dislocation-assisted carbon migration. To date, no study has been reported to predict the formation of HABs. Based on the percentage of white etched region formed in the subsurface of the bearings, this study presents a semi-empirical model that predicts the formation of both LABs and HABs in steel bearings for different contact pressures. The LAB semi-empirical model is compared with a theoretical model from literature and experimental data. According to this, the new model more realistically predicts the initiation of LABs than the carbide thickening model. This is achieved by involving the formation of ferrite (cellular and elongated) shown as white areas under optical images. Literature data have been used to evaluate the LAB and HAB models. The growth pattern obtained in this study provides strong evidence of a diffusion-based mechanism leading to WEB formation.

White Etching Bands Formation Mechanisms Due to Rolling Contact Fatigue

White etching bands (WEBs), known as subsurface micro-structural changes in bearing steels due to cyclic stresses in rolling contacts, have been studied for decades and a number of theoretical models have been proposed to explain their formation mechanisms and predict their initiation. While many studies focus only on one subtype of WEBs, e.g. low angle bands (LAB) or high angle bands (HAB), the most recent semi-empirical model has shown to be able to predict the formation of both LAB and HAB based on observed growth patterns of ferrite grains. Following from the modelling study, this paper presents a detailed mechanistic study, showing the evolution of ferrite grains (equiaxed and elongated grains) and carbide structures in WEBs formed in inner rings of angular contact ball bearings at their different life stages through SEM, EBSD and nano-indentation analysis. The results strongly suggest both LAB and HAB initiate as equiaxed ferrite grains due to recrystallization arising from energy build-up in the initial microstructure that later develops to elongated ferrite grains due to a grain rotation/coalescence recovery mechanism induced from plastic deformation. The formation of carbide structures in LAB is associated with the transformation of equiaxed to elongated grains, where carbides nucleate at the edges of the elongated grains rather than at the equiaxed grain band edges as being previously suggested in literature. The newly proposed formation mechanism links LAB and HAB based on experimental findings from detailed inspection of gradual microstructural alteration sequence of LAB and HAB in rolling contact fatigue (RCF) tested bearings.

Preliminary Research on Response of GCr15 Bearing Steel under Cyclic Compression.

During the bearing service, a series of microstructural evolutions will arise inside the material, such as the appearance of feature microstructures. The essential reason for the microstructural evolution is the cumulative effect of cyclic stress. The Hertz Contact formula is usually adopted to calculate the internal stress, and there is a correlation between the shape and distribution of the feature microstructure and the stress distribution. But it is insufficient to explain the relationship between the morphology of feature microstructures and the rolling direction, such as specific angles in butterfly and white etching bands. The rolling phenomenon will cause the asymmetry of stress distribution in the material, which is the source of the rolling friction coefficient. Moreover, slipping or microslip will produce additional stress components, which also cause the asymmetry of the stress field. However, there is no experimental or theoretical explanation for the relationship between the asymmetry of the stress field and the feature microstructure. According to the current theory, the appearance of feature microstructures is caused by stress with or without rolling. Therefore, it is of great significance to study the formation mechanism: whether feature microstructures will appear in the uniaxial cyclic compression stress field without rolling. In this paper, uniaxial cyclic compressive stress was loaded into a plate-ball system and a cylinder system. The characteristics of microstructural change of bearing steel (GCr15) were studied. It was found that the hardness of the material increased after the cyclic compressive load, and the inclusions interacted with the matrix material. In the local microregion a white etching area was found, although the scale is very small. No large-scale feature microstructures appeared. Other phenomena in the experiment are also described and analyzed. For example, the production of oil film in the contact area and the changing law of alternating load.

Re-investigation of dark etching regions and white etching bands in SAE 52100 bearing steel due to rolling contact fatigue

Microstructural alterations such as dark etching regions (DERs) and white etching bands (WEBs) have been known to manifest in the subsurface of steel bearings due to rolling contact fatigue (RCF) under medium-high stress cycles. Even though such manifestations have been reported substantially in literature for decades, their formation and their evolution mechanisms are not fully understood. As part of the re-investigation of DERs and WEBs, this paper presents the results from characterization of DER and WEBs, including both low angle bands (LABs) and high angle bands (HABs), formed in SAE 52100 bearings at different stages of bearing life. Angular contact ball bearings (ACBBs), subjected to RCF testing under different contact pressure, stress cycles and steel cleanliness, have been examined to reveal new information related to the microstructural alteration processes, e.g. the 3-D structure of the WEBs and their growth pattern. Contrary to some of the literature results, the experimental results have shown that the mean depths and density of WEBs (LABs and HABs), as opposed to DER, are correlated with the position and distribution of the principle shear stress. The results also show that HABs form as a consequence of LABs especially in densed LAB areas in the ACBBs studied here.

Further understanding of rolling contact fatigue in rolling element bearings - A review

Abstract Rolling bearings are one of the most widely used components in industrial machinery. If suitably mounted, loaded, lubricated and isolated from contamination, rolling contact fatigue (RCF) is believed to be the most probable mode of failure resulting in subsurface originated failures. Over the past several decades many researchers have studied the failure and microstructural alterations in bearings such as dark etching regions and white etching bands and how operating conditions such as pressure, temperature and running time impact them during RCF. This paper aims to provide an overview of such alterations, their properties, formation mechanisms and impact on bearing failure.

Influence of steel cleanliness and heat treatment conditions on rolling contact fatigue of 100Cr6

The aim of this work is to determine the rolling contact fatigue (RCF) strength of steel AISI 52100 (DIN 100Cr6) as a common bearing steel in different heat treated conditions. In order to study and evaluate the steel cleanliness and its influence on the fatigue behavior the results are compared to AISI L3 (DIN 102Cr6) as a cold-working tool steel containing higher amounts of non-metallic inclusions as a consequence of a different metallurgical processing. In this work experimental results obtained from both materials with martensitic and bainitic microstructures are presented. In addition, the inclusion content in each material was analyzed using extreme value statistics, and the endurance limits are predicted based on the maximum expected inclusion in the maximum loaded volume as a critical crack initiating defect. The predicted RCF values are in good agreement with the experimental results. Irrespective of the microstructure, the hardness after heat treating dominates the endurable Hertzian pressure. The second important parameter, which determines the endurance limit, is the cleanliness of the steel. Furthermore, the crack pattern in the surface and microstructure of the RCF specimens were analyzed by optical microscopy. The results show no microstructural modifications such as dark etching regions or white etching bands. However, the formation of butterflies was detected around non-metallic inclusions in both steels.

Evolution of White Etching Bands in 100Cr6 Bearing Steel under Rolling Contact-Fatigue

The formation of white etching bands (WEBs) occurs at the subsurface of rolling contact-fatigued bearing inner rings, exhibiting microstructural decay detrimental to bearing life. Despite the fact that WEBs have been observed in bearing steels for nearly 70 years, the understanding of WEB formation is still limited and mostly qualitative. Therefore, a systematic investigation is carried out in this research to reveal the evolution of WEBs with respect to the number of contact cycles. WEBs formed at different stages are reproduced by full-scale bearing RCF tests with predetermined numbers of cycles. Multi-scale characterisation techniques such as optical microscopy, micro-indentation, scanning and transmission electron microscopy and atomic force microscopy are conducted on the microstructural alterations to study the development and microstructure of WEBs. WEBs are found in the absence of dark etching regions which is attributed to the heat treatment. With an increasing number of cycles, WEBs grow in number density and in all three dimensions, and their formation is found to be controlled by the maximum shear stress component. Ferrite bands within WEBs that contain dislocation cells manifest accumulated plastic strain in the material. Based on the characterisation results, the evolution of plastic strain under RCF is quantified.

On the origin of microstructural banding in Ti-6Al4V wire-arc based high deposition rate additive manufacturing

Directed energy high deposition-rate additive manufacturing processes involve a larger melt pool diameter (∼5–10 mm) and layer height (1–2 mm) than powder bed technologies, which generally leads to greater microstructural heterogeneity and more severe Heat Affected Zone (HAZ) banding. While HAZ banding has been widely reported in AM, in this study the banding features seen in samples produced by Wire-Arc Additive Manufacturing (WAAM) have been more rigorously quantified than previously possible, using statistically reliable compositional and, purpose developed, microstructure analysis mapping tools, which has provided new insight into their nature and mechanisms of formation. In addition to HAZ banding, a segregation layer has also been discovered at the fusion boundary from each melt track. This transient segregation layer and the weak coring seen, for the first time in the AM deposits, can be attributed to the lower partition coefficient of Fe in titanium, as well as limited V and Al segregation. The detailed microstructure evolution occurring in the HAZ bands has been revisited, based on new evidence, and is shown to involve both dark and white etching bands. The lower temperature dark etching region is caused not just by an increase in the α lamellar spacing due to coarsening, but also by greater chemical partitioning with temperature rise. In addition, it is shown by thermal simulation that the thin white band occurs on re-heating to just below the β transus temperature, which is shifted upwards owing to the high heating rate in AM. This white band is associated with a morphological change to a fine α lamellar colony morphology, which exhibits less solute partitioning. The mechanisms involved are discussed. The rapid coarsening that occurs in the range of the β approach curve is attributed to interface migration from β re-growth, rather than conventional surface tension driven effects, whereas the fine colony microstructure is proposed to be caused by colony nucleation, in subsequent cooling, on a low volume fraction of residual α.

A unified theory for microstructural alterations in bearing steels under rolling contact fatigue

Three major types of microstructural alterations occurring under rolling contact fatigue, white etching areas (WEAs), dark etching regions (DERs) and white etching bands (WEBs), are modelled under a unified approach: dislocation-assisted carbon migration theory. Following our previous work on DERs and WEBs, a novel model is proposed to describe dislocation cell formation in WEAs. The proposed model yields predictions of WEAs appearance, agreeing with experimental observations. Bearing life can be estimated by the WEA appearance model. The three microstructural alterations models are combined and, for the first time, it becomes possible to predict the occurrence and the formation progress of WEAs, DERs and WEBs with a unified theory. Microstructural alteration maps are plotted as a function of number of cycles, temperature, contact pressure and stress cycle frequency. The models are validated by the experimental results reported over the last 50 years.

Magnetooptical Analysis of the Subsurface Region in a Bearing Ring Subjected to Rolling Contact Fatigue

ABSTRACT It has been known for decades that high contact load in bearings leads to fatigue alterations of subsurface material, which can be optically visualized on etched samples. In the literature, these alterations are termed dark etching regions and white etching bands, due to their peculiar patterns and contrast. In the current work, it is proposed to study these alterations by the magnetooptical Kerr effect, expecting that the material changes associated with rolling contact fatigue also affect the magnetic properties of bearing steel. The advantage of this method is that it has good spatial resolution, allowing recording of magnetic properties on microscopic scales. Samples containing dark etching regions and white etching bands were compared in the magnetooptical test with the reference sample containing no optically visible changes. It was found that the coercive force of material reduces as result of microstructural changes caused by rolling contact fatigue.

Microstructural changes in White Etching Cracks (WECs) and their relationship with those in Dark Etching Region (DER) and White Etching Bands (WEBs) due to Rolling Contact Fatigue (RCF)

Substantial microstructural changes, such as Dark Etching Region (DER), White Etching Bands (WEBs) and White Etching Cracks (WECs), can occur in typical bearing steels such as AISI 52100 and 4320 due to Rolling Contact Fatigue (RCF). Although it has been reported that DER and WEBs typically appear in bearings over extended rolling cycles (>100×106) as a result of material deterioration under high-stress RCF while WECs are found to occur in much earlier of bearing’s life (<20% the calculated L10 life), differences and relationship between DER/WEBs and WECs are not fully investigated or understood. This study investigated the microstructural alterations in WECs in an AISI 52100 through-hardened martensitic bearing using a combination of characterisation methods and the results are directly compared with those observed in DER and WEBs due to high-stress RCF (Šmeļova et al., 2017). The results show that, although the features maybe initiated by different causes, there are many similarities between the microstructural alterations in WECs and DER/WEBs, such as the carbide disintegration, and carbon and chromium movement that plays a role in the formation and development of microstructural alterations in all three features.

Microstructural Alterations in Bearing Steels under Rolling Contact Fatigue: Part 2—Diffusion-Based Modeling Approach

The microstructure of rolling element bearings can experience significant transformation when subjected to repetitive contact cycling. In Part 1 of this article, a detailed overview of the known microstructure alteration phenomena was presented and the different mechanisms for describing them proposed by various investigators were critically reviewed. It is agreed generally that the primary path of these structural changes is the decay of martensite due to carbon diffusion, leading to the formation of ferrite and lenticular carbides. Furthermore, these altered regions in the material microstructure can be stress concentration regions leading to crack initiation and propagation. In this article, a J2-based elastic–plastic Voronoi finite element model (previously developed by the authors) is coupled with a carbon diffusion–based model. Using Fick's law for stress-assisted diffusion, the dispersion of carbon in the bearing microstructure is evaluated. A backward Euler finite difference scheme is employed to solve the partial differential equations. The model can accurately predict the onset of martensitic decay and formation of the white etching bands along with their distinctive orientation. A comparison of the numerical results shows good corroboration with experimental observations.

Identification of Material Constitutive Laws Representative of Machining Conditions for Two Titanium Alloys: Ti6Al4V and Ti555-3

Determining a material constitutive law that is representative of the extreme conditions found in the cutting zone during machining operations is a very challenging problem. In this study, dynamic shear tests, which reproduce, as faithfully as possible, these conditions in terms of strain, strain rate, and temperature, have been developed using hat-shaped specimens. The objective was to identify the parameters of a Johnson–Cook material behavior model by an inverse method for two titanium alloys: Ti6Al4V and Ti555-3. In order to be as representative as possible of the experimental results, the parameters of the Johnson–Cook model were not considered to be constant over the total range of the strain rate and temperature investigated. This reflects a change in the mechanisms governing the deformation. The shear zones observed in hat-shaped specimens were analyzed and compared to those produced in chips during conventional machining for both materials. It is concluded that the observed shear bands can be classified as white-etching bands only for the Ti555-3 alloy. These white bands are assumed to form more easily in the Ti555-3 alloy due to its predominately β phase microstructure compared to the Ti6Al4V alloy with a α + β microstructure.

Microstructural Alterations in Bearing Steels under Rolling Contact Fatigue Part 1—Historical Overview

Microstructural alterations in bearing steels during rolling contact cycling have been reported in the literature for more than half a century. These structural changes are primarily caused by the decay of parent martensite and have been designated as white and dark etching regions due to their preferential etching characteristics. One of the most striking features of the white etching bands is their repeatable directionality, which has puzzled investigators for decades. Despite numerous attempts, a satisfactory explanation for the orientation of these bands is still not available. In this article (Part 1), an overview of the phenomenon is presented with detailed discussion of various experimental observations from the literature. The article also examines the previous approaches adopted to explain white etching bands and address their limitations. In Part II of the article, a J2-based elastic–plastic finite element model coupled with a carbon diffusion model is developed that directionally predicts the occurrence and orientation of the white etching bands.