Rolling-contact fatigue (RCF) is a common failure mode in components subjected to rolling or rolling-sliding contact. This article provides a basic understanding of RCF and a broad overview of materials and manufacturing techniques commonly used in industry to improve component life. A brief discussion on coatings to improve surface-initiated fatigue and wear is included, due to the similarity to RCF and the increasing criticality of this failure mode. The article presents a working knowledge of Hertzian contact theory, describes the life prediction of rolling-element bearings, and provides information on physics and testing of rolling-contact fatigue. Processes commonly used to produce bearings for demanding applications are also covered.

An air blower in an electric power plant failed unexpectedly when a roller bearing in the drive motor fractured along its outer ring. Both rings, as well as the 18 rolling elements, were made from GCr15 bearing steel. The bearing also included a machined brass (MA/C3) cage and was packed with molybdenum disulfide (MoS 2 ) lithium grease. Metallurgical structures and chemical compositions of the bearing’s matrix materials were inspected using a microscope and photoelectric direct reading spectrometer. SEM/EDS was used to examine the local morphology and composition of fracture and contact surfaces. Chemical and thermal properties of the bearing grease were also examined. The investigation revealed that the failure was caused by wear due to dry friction and impact, both of which worsened as a result of high-temperature degradation of the bearing grease. Fatigue cracks initiated in the corners of the outer ring and grew large enough for a fracture to occur.

Explosive cladding is a viable method for cladding different materials together, but the complicated behavior of materials under ballistic impacts raises the probability of interfacial shear failure. To better understand the relationship between impact energy and interfacial shear, investigators conducted an extensive study on the shear strength of explosively cladded Inconel 625 and plain carbon steel samples. They found that by increasing impact energy, the adhesion strength of the resulting cladding can be improved. Beyond a certain point, however, additional impact energy reduces shear strength significantly, causing the cladding process to fail. The findings reveal the decisive role of plastic strain localization and the associated development of microcracks in cladding failures. An attempt is thus made to determine the optimum cladding parameters for the materials of interest.

An investigation was conducted to determine what caused a bearing sleeve in a locomotive turbocharger to fail. The sleeve, which is made of nitrided 38CrMoAl steel, fractured at the transition fillet between the cylinder and plate. Visual examination revealed significant wear on the external surface of the cylinder, with multiple origin fatigue fracture appearing to be the dominant fracture mechanism. Metallurgical examination indicated that the nitrided layer was not as deep as it was supposed to be and had worn away on the outer surface of the sleeve, exposing the soft matrix underneath. This led to further wear and an increase in friction between the sleeve and bearing bush. Fatigue crack initiation occurred at the root fillet because of stress concentration and large frictional forces. Insufficient nitriding depth facilitated the propagation of fatigue cracks.

Spalled fragments from the work rolls of a steel bar straightening machine were received for failure analysis. Visual inspection coupled with optical and scanning electron microscopy were used as the principal analytical techniques for the investigation. Fractographic analysis revealed the presence of a characteristic fatigue crack propagation pattern (beach marks) and radial chevron marks indicating the occurrence of final overload through a brittle intergranular fracture. The collected evidence suggests that surface-initiated cracks propagated by fatigue led to spalling, resulting in severe work roll damage as well as machine downtime and increased maintenance costs.

The case study presented in this article details the failure investigation of an M50 alloy steel bearing used in a jet engine gearbox drive assembly. It discusses the investigative steps and analytic tools used to determine the root cause, highlighting the importance of continuous, thorough questioning by the investigating activity. The combined analyses demonstrated that the bearing failed by a single event overload as evidenced by bulk deformation and traces of foreign material on the rolling elements. The anomalous transferred metal found on the rolling elements subsequently led to the discovery of overlooked debris in an engine chip detector, and thus resulted in a review of several maintenance practices.

An aero engine failed due to the misalignment of the ball bearing fitted on the main shaft of the engine. The aero engine incorporates two independent compressors: a six-stage axial flow LP compressor and a nine-stage axial flow HP compressor. The bearing under consideration is a HP location bearing and is fitted at the rear of the nine-stage compressor. It was supposed to operate for at least 5000 h, but failed catastrophically after 1300 h, rendering the engine unserviceable. Unusually high stresses caused by misalignment and uneven axial loading resulted in the generation of fatigue crack(s) in the inner race. When the crack reached the critical size, the collar of the race fractured, causing subsequent damage. The cage also failed due to excessive stresses in the axial direction, and its material was smeared on the steel balls and the outer race.

A ball bearing in a military jet engine sustained heavy damage and was analyzed to determine the cause. Almost all of the balls and a portion of the outer race were found to be flaking, but there were no signs of damage on the inner race and cage. Tests (chemistry, hardness, and microstructure) indicated that the bearing materials met the specification requirements. However, closer inspection revealed areas of discoloration, or nonuniform contact marks, on the ID surface of the inner ring. The unusual wear pattern suggested that the bearing was not properly mounted, thus subjecting it to uneven or eccentric loading. This explains the preferential nature of the flaking on the outer race and points to an assembly error as the root cause of failure.

A tri-lobe cylindrical roller bearing was submitted for investigation to determine the cause of uniformly spaced axial fluting damages on its rollers and outer raceway surfaces. The rollers and raceways were made from premium-melted M50 and M50NiL, aircraft quality steels often used in bearings to minimize the effects of orbital slippage and rolling-contact fatigue. The damaged areas were examined under a scanning electron microscope, which revealed a high density of microcraters, characteristic of local melting and material removal associated with bearing currents. Investigators also examined the effect of electrical discharge on crater dimensions and density and the role that thermoelectric voltage potentials may have played.

Several surgical tool failures were analyzed to understand why they occur and how to prevent them. The study included drills, catheters, and needles subjected to the rigors of biomedical applications such as corrosive environments, high stresses, sterilization, and improper cleaning procedures. Given the extreme conditions to which surgical tools can be exposed, and the potential for misuse, failures are inevitable and systematic methods for analyzing them are necessary to keep them in check.

Rolling contact fatigue is responsible for a large number of industrial equipment failures. It is also one of the main failure modes of components subjected to rolling contact loading such as bearings, cams, and gears. To better understand such failures, an investigation was conducted to assess the role of friction in subsurface fatigue cracking in rolling-sliding contact applications. Based on the results of stress calculations and x-ray diffraction testing of steel samples, friction contributes to subsurface damage primary through its effect on the distribution of orthogonal shear stress. Although friction influences other stress components, the effect is relatively insignificant by comparison. It is thus more appropriate to select orthogonal shear stress as the critical stress when assessing subsurface rolling contact fatigue in rolling-sliding systems.

Drastic reduction in the service life of a production gearbox was observed. Within the gearbox, the axial load on a bevel gear (8620 steel, OD 9.2 cm) was taken by a thrust-type roller bearing (3.8 cm ID, 5.6 cm OD) in which a ground surface on the back of the bevel gear served as a raceway. Spalling damage on the ground bearing raceway at five equally spaced zones was disclosed by inspection of the bevel gear. The bearing raceway was checked for runout by mounting the gear on an arbor. It was found that the raceway undulated to the extent of 0.008 mm total indicator reading and a spalled area was observed at each high point. The presence of numerous cracks that resembled grinding cracks was revealed both by magnetic-particle inspection and microscopic examination. Spalling was produced by nonuniform loading in conjunction with grinding cracks. As corrective measures, the spindle of the grinding machine was reconditioned to eliminate the undulations and retained austenite was minimized by careful heat treatment.

In this study, the failure modes of cartwheel and mechanical properties of materials have been analyzed. The results show that rim cracking is always initiated from stringer-type alumina cluster and driven by a combination effect of mechanical and thermal load. The strength, toughness, and ductility are mainly determined by the carbon content of wheel steels. The fatigue crack growth resistance is insensitive to composition and microstructure, while the fatigue crack initiation life increases with the decrease of austenite grain size and pearlite colony size. The dynamic fracture toughness, KID, is obviously lower than static fracture toughness, KIC, and has the same trend as KIC. The ratio of KID/sigma YD is the most reasonable parameter to evaluate the fracture resistance of wheel steels with different composition and yield strength. Decreasing carbon content is beneficial to the performance of cartwheel.

A failure analysis case study on railroad rails is presented. The work, performed under the sponsorship of the Department of Transportation, addresses the problem of shell and detail fracture formation in standard rails. Fractographic and metallographic results coupled with hardness and residual stress measurements are presented. These results suggest that the shell fractures form on the plane of maximum residual tensile stresses. The formation of the shells is aided by the presence of defects in the material in these planes of maximum residual stress. The detail fracture forms as a perturbation from the shell crack under cyclic loading and is constrained to develop as an embedded flaw in the early stages of growth because the crack is impeded at the gage side and surface of the rail head by compressive longitudinal stresses.

A portion of two large spur tooth bull gears made from 4147H Cr-Mo alloy steel that had spalling teeth was submitted for evaluation. The gears were taken from a final drive wheel reduction unit of a very large open-pit mining truck. The parts had met the material and initial heat treat hardening specifications. The mode of failure was tooth profile spalling. By definition, spalling originates at a case/core interface or at the juncture of a hardened/nonhardened area. The cause of this failure was either insufficient or no induction-hardened case along the active profile. The cause was activated by a nonfunctioning induction hardening coil that did not or was not allowed to harden the midprofile of several teeth.

Two intermediate impeller drive gears (made of AMS 6263 steel, gas carburized, hardened, and tempered) exhibited evidence of pitting and abnormal wear after production tests in test-stand engines. The gears were examined for hardness, case depth, and microstructure of case and core. It was found that gear 1 had a lower hardness than specified while the case hardness of gear 2 was found to be within limits. Both the pitting and the wear pattern were revealed to be more severe on gear 1 than on gear 2. Surface-contact fatigue (pitting) of gear 1 (cause of lower carbon content of the carburized case and hence lower hardness) was found to be the reason for failure. It was recommended that the depth of the carburized case on impeller drive gears be increased from 0.4 to 0.6 mm to 0.6 to 0.9 mm to improve load-carrying potential and wear resistance. A minimum case-hardness requirement was set at 81 HRA.

The contamination of lubrication with powdered stone resulted in progressive wear of the internal surfaces of a bearing. Because of the motion of rollers, the inner race exhibited an unusual cyclic washboard wear pattern. Because of a lack of bearing conformity, wear progressed into severe coarse-grain spalling.

Butterfly-shaped microstructural features in tempered martensite in an otherwise clean steel suggested that overloading led to premature spalling of a coal-crushing plant taper bearing. Extensive rolling contact fatigue occurred because of the overload condition. The crusher was designed to handle soft lignite coals but had been used to crush hard deep-mined anthracite coals.

Microstructural examinations on transverse cross sections of a steam reformer tube, showed the presence of large macrovoids elongated in the radial direction and emanating from the internal surface of the tube. The macrovoids were located at the interdendritic regions, and were partially filled by a Mn-Fe bearing chromium oxide film. The areas adjacent to the oxide film were chemically depleted in C, Cr and Mn and rich in Fe and Ni. Associated with this depletion were a large concentration of microvoids. It was suggested that the dissolution of carbides in areas surrounding the macrovoids and the concentration of stresses at their tips, caused extensive localized plastic deformation which led to the formation of microvoids and subsequently to the spalling of the oxide film. The non-protective character of the film induced a progressive deterioration of the grain boundaries properties. Grain boundary sliding and dislocation motion were enhanced, causing a local increase in the steady state strain rate and the premature failure of the tube.

Work rolls made of indefinite chill double-poured (ICDP) iron are commonly used in the finishing trains of hot-strip mills (HSMs). In actual service, spalling, apart from other surface degeneration modes, constitutes a major mechanism of premature roll failures. Although spalling can be a culmination of roll material quality and/or mill abuse, the microstructure of a broken roll can often unveil intrinsic inadequacies in roll material quality that possibly accentuate failure. This is particularly relevant in circumstances when rolls, despite operation under similar mill environment, exhibit variations in roll life. The paper provides an insight into the microstructural characteristics of spalled ICED HSM work rolls, which underwent failure under similar mill operating environment in an integrated steel plant under the Steel Authority of India Limited. Microstructural features influencing ICDP roll quality, viz. characteristics of graphite, carbides, martensite, etc., have been extensively studied through optical microscopy, quantitative image analysis (QIA), and electron-probe microanalysis (EPMA). These are discussed in the context of spalling propensity and roll life.