Rolling Contact Fatigue Tests Research Articles

Rolling contact fatigue property of gradient rare earth addition M50 bearing steel with surface nano-grains and its inhibited martensite decay

A gradient nanostructured surface layer of ∼600 μm in thickness was prepared on M50 steel with rare earth additions by a surface mechanical rolling treatment. The initial microstructure was refined into nanosized grains (∼29 nm in diameter) in the top surface layer, causing a significant increase of surface hardness. Meanwhile, the grain size increases, and the microhardness decreases gradually to the matrix values with increasing depth. Rolling contact fatigue tests showed that the fatigue lives are significantly increased by the gradient nanostructured surface layer. Analysis of failure mechanism revealed that both softening in the near surface layer and hardening in the subsurface occur under rolling contact testing. And surface spalling is the predominant failure mode, of which the cracks initiate from primary carbides in the top surface layer owing to surface softening. In gradient nanostructured samples, the nano-grains inhibit carbon migration by impeding dislocation movement in the near surface layer, so that martensite decay is suppressed. Consequently, the crack initiation is inhibited, and the fatigue property is enhanced.

Observation of wear around surface crack of PEEK thrust bearing under rolling contact fatigue in water

Abstract Rolling bearings are important mechanical elements for machines. Poly-ether-ether-ketone (PEEK) polymer is a suitable material for bearings in water and corrosive conditions because of its excellent water and chemical resistance. Some study had been examining rolling contact fatigue (RCF) tests in water; however, the competition between cracks propagation and wear on the surface of PEEK bearing in water was not clear. In the present study, in order to evaluate the crack propagation and wear process, RCF test with PEEK thrust bearings at 3750 [N] in water was carried out. The lengths of the surface cracks and the profiles of the grooves were observed at different number of cycles. In addition, so as to identify the cracks which cause the flaking, start-stop test was carried out. We found the surface crack, which was initially observed at 4.62×105 cycles, were gradually worn down and completely disappeared at 6.32×105 cycles. We concluded that rolling contact fatigue caused some surface cracks of PEEK to wear down in water condition.

Rolling Contact Fatigue Life and Categorization of Failure in Experiment of Flat PEEK Race-Alumina Ball Bearings in Water Lubrication under 1700 N at 1000 rpm

Abstract The rolling contact fatigue tests of flat polyether-ether-ketone (PEEK) race-alumina balls bearings in water lubrication were done. From the experiments under 1700 N at 1000 rpm, three types of failures were obtained: a crater-shaped flaking, a rhombus-shaped flaking and a large cracking failures. To research the relationships between the failures and the lives, the surface properties near the failures were measured and observed. No relationships between the surface roughness and the life were found. On the other hand, the contact width of the shorter life specimen was larger than that of the longer life specimen. Two types of the flaking holes were compared to study the differences. The crater-shaped flaking hole exceeded the contact track, but the rhombus-shaped flaking hole existed inside the contact track. In addition, the crater-shaped flaking hole was shallower than the rhombus-shaped flaking hole. This means that the flaking mechanisms were different between them, i. e. the differences of the flaking mechanisms caused the differences of the life.

Discovery of white etching areas in high nitrogen bearing steel X30CrMoN15-1: A novel finding in rolling contact fatigue analysis

White etching areas (WEA) and white etching cracks (WEC) are frequently linked to premature bearing failure in conventional high carbon bearing steels like 100Cr6 (SAE 52100). In contrast, no WEA/WEC has yet been reported for the high nitrogen bearing steel X30CrMoN15-1 (SAE AMS 5898). Thus, the present study proves for the first time that X30CrMoN15-1 is also susceptible to develop WEA/WEC under rolling contact fatigue (RCF) when pre-charged with hydrogen. RCF tests conducted in parallel without hydrogen pre-charging resulted in RCF damage only, which identifies hydrogen as an active agent for WEA/WEC formation in X30CrMoN15-1. These findings correspond to the fact that hydrogen diffusion during RCF is often considered to cause or accelerate the formation of WEA/WEC. Additionally, it is observed that the M2(C, N) and M23C6 precipitates of the martensitic microstructure of the X30CrMoN15-1 do not entirely decompose during the WEA formation process as observed for M3C precipitates in 100Cr6. In conclusion, the results for X30CrMoN15-1 strongly suggest that the formation of WEA is driven by a hydrogen-activated local severe plastic deformation process, which initiates continuous dynamic recrystallisation, leading to the characteristic nano-ferritic grains observed in WEA. Also, the highly stable and self-regenerating passive chromium-oxide layer of X30CrMoN15-1 mitigates the risk of WEA/WEC failure during typical RCF operation by hindering the formation and adsorption of ionic hydrogen. Hence, this study emphasises the importance of protecting the base material against hydrogen ingress to delay WEA/WEC formation.

Microstructural decay of matrix and precipitates during rolling contact fatigue in a martensitic dual-hardening bearing steel

We investigate the microstructural degradation during rolling contact fatigue (RCF) in a martensitic dual-hardening bearing steel. The dual-hardening steel makes use of both carbide precipitation and intermetallic precipitation hardening. The microstructural degradation leading to fatigue failure is studied using electron microscopy, atom probe tomography, and synchrotron X-ray diffraction (SXRD). The initial microstructure of the steel consists of tempered martensite with a fine dispersion of secondary M7C3, and NiAl precipitates. During RCF testing at 2.2 GPa contact pressure, ferrite microbands develop and the partial dissolution of NiAl and M7C3 precipitates occur within the ferrite microbands. For the RCF testing at higher contact pressure of 2.8 GPa, nanosized ferrite grains develop in the ferrite microbands. The SXRD analysis reveals a decrease in dislocation density in the sub-surface region experiencing microstructural degradation. This is believed to be associated with the rearrangement of dislocations into low energy configuration cells. We conclude this manuscript by proposing a microstructure decay mechanism for martensitic dual-hardening bearing steel that provides insights in the fatigue initiation process.

Analysis of rolling contact and tooth root bending fatigue in a new high-strength steel: Experiments and micromechanical modelling

To find materials that meet the future requirements of increasing load density in wind turbine gear structures, the commonly used reference material, 18CrNiMo7-6, and another high-strength steel were analysed. Their fatigue properties were studied through rolling contact fatigue (RCF) and gear tooth root bending fatigue (TRBF) tests. The reference steel exhibited better fatigue performance under RCF conditions compared to the new steel; however, it was outperformed by the new steel under TRBF conditions. This study aims to understand gear contact fatigue at the microscopic level, moving beyond the macroscopic focus that dominates current research literature. To establish the causal link between the varied fatigue performance observed in these materials, we proposed a multiscale modelling workflow based on crystal plasticity. This crystal plasticity framework is combined with the fatigue indicator parameter (FIP) to explore the fatigue resistance of materials subjected to RCF and TRBF conditions. Emphasis has been placed on the role of retained austenite (RA) in fatigue performance. Utilizing representative elementary volumes (REVs) generated based on statistically representative crystallographic features and measured size distribution of RA, we examined the effects of various RA levels on fatigue resistance. Our findings reveals that the fatigue damage accumulation is significantly influenced by the level of RA. Different fatigue damage accumulation behaviours were observed under RCF and TRBF conditions. These insights offer new perspectives on RA’s role in fatigue resistance and highlight its complex influence under varied loading conditions.

The effect of fine particle peening on aviation gear performance, part II: Rolling contact fatigue

AbstractFine particle peening (FPP) technology offers an effective method to achieve outstanding surface integrity and fatigue performance in aviation gears. However, the lack of sufficient research on the correlation between process parameters, surface integrity parameters, and fatigue life undermines the full potential of FPP in strengthening aviation gears. Therefore, we have developed an elastic–plastic contact fatigue life model to predict the fatigue life of AISI 9310 gear steel after different strengthening processes. This model can consider surface integrity parameters such as surface micro‐topography, residual stress gradient, and hardness gradient. To validate the model, we conducted rolling contact fatigue tests and compared the results with the predictions from our model. The influence of process parameters on the rolling contact fatigue of AISI 9310 gear steel was further investigated. A process window for FPP was established, and an optimization method was proposed in order to provide guidance for selecting process parameters for FPP on high‐performance aviation gears.

Effect of surface rolling process on rolling contact fatigue behavior and failure mechanism of powder metallurgy steel

The effect of surface rolling process on the surface microstructure and rolling contact fatigue of the Fe-2Cu-0.6 C powder metallurgy steel was investigated in the present investigation. Results indicated that a densified and hardened layer with a densification depth of 330 μm and a surface hardness of 330 HV0.1 was generated in the surface layer after rolling process. Ferrite grain and pearlite lamellae became typical fibrous structures on the top surface. The microstructure of pearlite was presented by two typical features. One was that the discrete cementite plates were severely curled and wavy. The other one was that the cementite plates remained parallel to each other, but were a reticular structure at a macroscopic level. The rolling contact fatigue (RCF) test showed that the RCF life of the rolled sample was superior. The L10, L50, L63.2 life increased by 182%, 105%, 92%, respectively. Surface initiated spalling was the main fatigue failure mode. Subsurface equivalent contact stress and local equivalent stress were calculated. The results showed that surface cracks were due to the repetitive tensile stress, which caused pits. Subsurface cracks nucleated and propagated in the region where the local equivalent stress was higher than the matrix yield strength, which caused spalling as connecting with surface cracks. The surface densified layer plays an important role in improving the rolling fatigue resistance of the material.

Analysis of the Influence of Contact Stress on the Fatigue of AD180 High-Carbon Semi-Steel Roll

In this study, to investigate the problem of contact fatigue and the damage mechanism of an AD180 high-carbon semi-steel roll, rolling contact fatigue tests were conducted using specimens cut from the periphery of a roll ring. These specimens were characterized under different contact stresses using SEM, a profile system, an optical microscope, and a Vickers hardness tester. The results indicates that the main forms of fatigue damage of an AD180 high-carbon semi-steel roll are peeling, pitting corrosion, and plowing. Moreover, the surface of the roll exhibits delamination and plastic deformation characteristics under high contact stress. Meanwhile, the size and depth of peeling, as well as the amount of pitting corrosion, increase with the contact stress. Peeling is mainly caused by a crack that originates at the edge of the specimen surface and propagates along the pearlite structure and the interface between pearlite and cementite. High contact stress can lead to an increase in the crack propagation depth and angle, resulting in the formation of larger peeling. Under cyclic loading, the near-surface microstructure of the specimen hardens due to grain refinement and dislocation strengthening, and the depth of the hardened layer increases with the increase in contact stress. When the contact stress reaches 1400 MPa, the near surface structure of the specimen changes from pearlite to troostite.

Effect of Case Depth and Hardness Distribution on the Rolling Contact Fatigue Performance of G20CrNi2MoA Carburized Steel

This study aims to investigate the effects of the case depth and hardness distribution of carburizing on the rolling contact fatigue (RCF) properties of G20CrNi2MoA steel under deeper carburizing conditions. Four groups of specimens of different case depths are prepared and subjected to RCF tests. A 2D finite element model based on continuum damage mechanics is developed to further analyze these two factors. As the depth of the carburized case increases, the RCF life first increases and then decreases. The optimum case depth is approximately one‐quarter of the thickness of the specimen. A gentler hardness distribution due to a change in the diffusion process parameters ensures a minimal increase (8%) in the RCF performance. The proper case depth has a more notable effect on the RCF life compared with those of the gentle or steep hardness distribution. It is expected that the results can help to promote a better understanding of the heat treatment process of carburized bearings and ensure more reliable carburized bearings are produced.

Microstructure effects on improving rolling contact fatigue by a double-quenching heat treatment process for GCr15 steel

The heat treatment process is important in improving the rolling contact fatigue (RCF) life of GCr15 bearing steel by changing the microstructure. The conventional heat treatment process of GCr15 bearing only has one quenching (CQ) process, and the corresponding RCF life cannot satisfy the requirements of many applications. A double-quenching (DQ) heat treatment with two quenching processes was proposed, and it can improve the RCF life. However, the improving mechanisms are not clarified. Therefore, the authors applied the CQ and the DQ heat treatment processes on GCr15 bearing steel and performed the ball-on-disk RCF tests for the specimens heat treated by the different processes. Moreover, comparisons of the microstructure and mechanical behavior between specimens were made. The results show that the DQ heat treatment decreases the average grain size, enhances the retained austenite content, and has no special effect on the carbide distribution. It also increases the homogeneity of grains, hardness, and retained austenite distribution. Moreover, the combined effect of grain size and retained austenite evolutions is the main mechanism that decides the RCF life improvement.

Micromechanical response of dual-hardening martensitic bearing steel before and after rolling contact fatigue

Material decay in bearing steels under rolling contact fatigue (RCF) leads to fatigue initiation and failure. This study examines the local structure-property relationship in decayed material through in-situ compression testing of micropillars prepared from a dual-hardening martensitic bearing steel (Hybrid 60) before and after RCF testing. The results demonstrate a pronounced enhancement in local yield strength for decayed regions (2200–2340 MPa) as compared to non-decayed regions (1755–1780 MPa). The higher initial stress for dislocations glide in the decayed regions and their discontinuous yield behavior are attributed to the presence of ferrite microbands. Crystal plasticity simulations corroborated these findings, showingincreased critical resolved shear stress (CRSS) and reduced strain hardening in decayed samples.

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.

Two Contributions to Rolling Contact Fatigue Testing Considering Different Diameters of Rail and Wheel Discs

Scaled rolling contact fatigue tests, used to practically simulate the wear of the wheel and rail material under laboratory conditions, are typically classified into two categories. Tests in the first category use twin-disc stands, while the second group of test rigs use two discs of different diameters considering the rail disc as the larger one. The latter setup is closer to the real situation, but problems can occur with high contact pressures and tractions. The focus of this paper is on two main contributions. Firstly, a case study based on finite element analysis is presented, allowing the optimization of the specimen geometry for high contact pressures. Accumulated plastic deformation caused by cycling is responsible for abrupt lateral deformation, which requires the use of an appropriate cyclic plasticity model in the finite element analysis. In the second part of the study, two laser profilers are used to measure the dimensions of the specimen in real time during the rolling contact fatigue test. The proposed technique allows the changes in the specimen dimensions to be characterized during the test itself, and therefore does not require the test to be interrupted. By using real-time values of the specimen’s dimensional contours, it is possible to calculate an instantaneous value of the slip ratio or the contact path width.

Prediction of surface damage behavior in rolling contacts using torsional fatigue results and surface roughness modeling

This paper presents a novel approach to predict the effect of surface roughness on surface damage in rolling contacts. Rolling contact fatigue (RCF) tests were conducted on AISI 4130 case carburized steel specimens at two different roughness levels, Ra=0.3μm and 0.1μm, under boundary lubrication regime. The RCF experiments were performed using a Thrust Bearing Surface Pitting Rig employing a ball-on-flat configuration. Surface damage was assessed at different combinations of contact pressure and fatigue cycles using optical surface profilometry. Subsequently, the surface damage stress-life (S–N) results were generated at both roughness levels. Torsional fatigue experiments were also performed on the same material. The torsional fatigue S–N results were combined with stress concentrations at the RCF test surfaces due to roughness to predict the surface damage S–N results. A dry circular contact model incorporating the surface roughness was developed and utilized to compute the stress concentration at the contact. The S–N results based on the von-Mises stresses were compared for RCF and torsional fatigue. The predicted results corroborated well with the experimental findings. The approach developed in this investigation illustrates the fundamental relationship between RCF and torsional fatigue.

Rolling contact fatigue failure mechanism of bearing steel on different surface roughness levels under heavy load

It is inherent knowledge that rolling contact fatigue (RCF) failure of subsurface-initiated spalls usually occur under optimum surface under heavy load. However, our previous work has revealed surface-initiated RCF failure mechanism under heavy load and initial high roughness surface. The formation of surface crack caused by RCF is a complicated phenomenon that is accompanied by surface morphology and lubrication condition, as well as stress distribution and microstructure evolution. To clarify the main failure mechanism, this study carried out systematic RCF tests with different surface roughness specimens and conducted characterization of superficial microstructure for specimens of pre-service and post-service by Focused Ion Beam (FIB). The results clearly suggest precursor of collapsed morphology and nanocrystalline layer are the main factor that causes the RCF life with high roughness is lower than that with low roughness. While the spalling failure initiating from low roughness surface under heavy load are strongly relied on surface plastic deformation. Two main plastic deformation mechanisms are conductive to formation of surface cracks: the micro-plastic deformation and the local severe plastic flow. A newly failure mechanism that spalling failure initiates from surface has been proposed to link surface roughness and microstructure evolution.

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.

White etching area damage induced by shear localization in rolling contact fatigue of bearing steel

AbstractWhite etching area (WEA) is a primary damage in rolling contact fatigue (RCF) of bearing steels. In spite of extensive investigations, there is a large discrepancy in the existing mechanisms of WEA formation. We attempt to unify the mechanisms from the perspective of shear localization and plastic damage accumulation based on ductile damage. RCF tests were conducted to generate WEAs with various microstructures and compositions. A thermodynamically consistent model of the ductile damage evolution from an inclusion was established by developing the phase field damage coupled with the crystal elastic‐viscoplastic constitutive relationship under RCF. The model was implemented into the FE framework through a user materials subroutine. The results indicated that the WEA is the shear band resulting from shear localization. The large inhomogeneity and scatter in WEA's microstructure are due to the influence of the crystal orientation. The development and orientation of SBs predicted by the model and the experimental observation of the WEA are in good agreement. The large micro‐shear strain in the WEA provides the driving force for mechanically controlled austenite phase transformation. The shear band center, which has the largest strain and the least stress, is where cracks initiate. This demonstrates that contrary to earlier reports that WEA is induced by previously formed crack faces friction, cracks actually initiate from the interior of the WEA.

Effect of Number of Cycles on Surface Roughness of PPS Thrust Bearings Under Rolling Contact Fatigue in Water

Rolling Contact Fatigue (RCF) caused crack propagation, flaking, and wear. We focused on the wear in the present study because in recent years it was found that PPS thrust bearings under the RCF in water caused wear. The RCF test was performed under a thrust load of 2300 N in water to investigate the effect of the number of cycles on surface roughness in detail. We obtained the experimental results that Ra and Rz values decrease with the number of cycles. This means the bottom surface is worn down at the early stage of wear fatigue.

Fundamental relationship between rolling contact fatigue driven surface damage and torsional fatigue

This paper presents fundamental findings on equivalence of rolling contact fatigue (RCF) driven surface damage and torsional fatigue modes of failure. Fatigue experiments were conducted for both modes of failure using AISI 4130 case carburized steel specimens. A Thrust Bearing Surface Pitting Rig (TBSPR) was designed and developed to perform RCF tests on flat test specimens using thrust ball bearings. The experiments were conducted in boundary lubrication regime, simulating the contact conditions in tribological machine components. The current investigation presents a novel approach where the effect of contact pressure and number of fatigue cycles on the initiation of surface damage was characterized by surface profilometry of the wear track after each test to generate a stress-life (S-N) map. The S-N map provides unique insights to early detection of RCF driven fatigue damage. The S-N map was utilized to generate the surface damage S-N curve. Torsional fatigue experiments were conducted on the same material, and the S-N curves of both failure modes were compared. The comparison shows key similarities between the two S-N curves, as they differ by a simple scaling factor. This investigation demonstrates that RCF driven surface damage characteristics can be predicted by conducting much simpler, and faster, torsion fatigue tests. This paper also presents a novel methodology to determine contact pressures to avoid incipient micropitting/microdenting damage.