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.

A gear manufacturer experienced service problems with various gears and pinions that had worn prematurely or had fractured. All gears and pinions were forged from 1.60Mn-5Cr steel and were case hardened by pack carburizing. Gear Failure: One of the gears showed severe wear on the side of the teeth that came into contact with the opposing gear during engagement. The microstructure at the periphery of a worn tooth at its unworn side consisted of coarse acicular martensite with a large percentage of retained austenite. Pinion Failure: The teeth of the pinion exhibited severe spalling; the microstructure at the surface consisted of coarse acicular martensite with retained austenite. Also, a coarse network of precipitated carbide particles showed that the carburization of the case had appreciably exceeded the most favorable carbon content. This evidence supported the following conclusions: 1) High wear rate on the gears was caused by spalling of the coarse-grain surface layer. The underlying cause of the wear was overheating during the carburization. 2) Pinion failure resulted from overheating combined with excessive case carbon content. Thus, no recommendations were made.

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.

A carbon steel ball-peen hammer ejected a chip that struck the user's eye. Failure occurred when two hammers were struck together during an attempt to free a universal joint from an automotive drive shaft. Two samples were cut from the face of the hammer one through the chipped area on the chamfer and the other from the undamaged area on the chamfer. The shape and texture of the fracture surfaces were typical of spalling. The fracture was conchoidal and exhibited a complete lack of plastic deformation. White etching bands that intersected the face and chamfer were revealed during metallographic examination. Fracture occurred through a white band. Failure was attributed to formation of envelopes of untempered martensite under the chamfer that ruptured explosively during service.

A crane long-travel worm drive shaft was found to be chipped during unpacking after delivery. Chemical analysis showed that the steel (EN36A with a case depth of 1 mm, or 0.04 inch did not meet specifications. Magnetic particle inspection revealed a crack on the side of the shaft opposite the chip. Metallographic examination indicated that the case depth was approximately 2 mm (0.08 in.) and that a repair weld of an earlier chip had been made in the cracked area. The chipping was attributed to excessive case depth and rough handling. It was recommended that the shaft be returned to the manufacturer and a replacement requested.

An AISI 4320 H transfer gear shaft that was part of a transmission sustained severe surface damage after 12 h of dynamometer testing at various gearing and torque loads. The damage was characterized by generalized wear and spalling. Examination of a cross section of the shaft that intersected undamaged, burnished, and surface-spalled zones revealed no anomalies in the chemistry, microstructure, or hardness that could have caused the damage. The physical evidence suggested that the operable mechanism was contact fatigue caused by misalignment of the shaft in the assembly.

A spiral bevel gear and pinion set that showed "excessive wear on the pinion teeth" was submitted for analysis. This gear set was the primary drive unit for the differential and axle shafts of an exceptionally-large front-end loader in the experimental stages of development. There was no evidence of tooth bending fatigue on either part. Several cracks were associated with the spalling surfaces on the concave sides of the 4820H NiMo alloy steel pinion teeth. The gear teeth showed no indication of fatigue. The primary mode of failure was rolling contact fatigue of the concave (drive) active tooth profile. The spalled area was a consequence of this action. The pitting low on the profile appeared to have originated after the shift of the pinion tooth away from the gear center. The shift of the pinion was most often due to a bearing displacement or malfunction. The cause of this failure was continuous high overload that may also have contributed to the bearing displacement.

Rolling-contact fatigue (RCF) is a surface damage process due to the repeated application of stresses when the surfaces of two bodies roll on each other. This article briefly describes the various surface cracks caused by manufacturing processing faults or blunt impact loads on ceramic balls surfaces. It discusses the propagation of fatigue cracks involved in rolling contacts. The characteristics of various types of RCF test machines are summarized. The article concludes with a discussion on the various failure modes of silicon nitride in rolling contact. These include the spalling fatigue failure, the delamination failure, and the rolling-contact wear.

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.

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.

Two 38 mm (1.5 in.) diam threaded stud bolts that were part of a steel mold die assembly from a plastics molding operation were examined to determine their serviceability. Chemical analysis showed the material to be a plain carbon steel that approximated 1045. Visual examination revealed evidence of severe hammer blows to the clevis and boss areas and a gap between the die and the underside of the boss. Magnetic particle inspection showed cracks at the thread roots that, when examined metallographically, were found to contain MnS stringers. The cracking of the threads was attributed to a poor stud bolt design, which allowed a high stress concentration to occur at the base of the threads upon application of a lateral load. It was recommended that bolts of a new design that incorporated a stress-relieving groove be used. Threading of the bolt to eliminate the gap between the lower face of the boss and the die and an improved method of inserting or removing the bolt to avoid hammering (use of a wrench on a square or hexagonal boss) were also recommended.

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.

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.

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.

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.

Rolling-element bearings use rolling elements interposed between two raceways, and relative motion is permitted by the rotation of these elements. This article presents an overview of bearing materials, bearing-load ratings, and an examination of failed bearings. Rolling-element bearings are designed on the principle of rolling contact rather than sliding contact; frictional effects, although low, are not negligible, and lubrication is essential. The article lists the typical characteristics and causes of several types of failures. It describes failure by wear, failure by fretting, failure by corrosion, failure by plastic flow, failure by rolling-contact fatigue, and failure by damage. The article discusses the effects of fabrication practices, heat treatment and hardness of bearing components, and lubrication of rolling-element bearings with a few examples.

This article briefly reviews the analysis methods for spalling of striking tools with emphasis on field tests conducted by A.H. Burn and on the laboratory tests of H.O. McIntire and G.K. Manning and of J.W. Lodge. It focuses on the metallography and fractography of spalling. The macrostructure and microstructure of spall cavities are described, along with some aspects of the numerous specifications for striking/struck tools. The article also describes the availability of spall-resistant metals and the safety aspects of striking/struck tools in railway applications.

Gears can fail in many different ways, and except for an increase in noise level and vibration, there is often no indication of difficulty until total failure occurs. This article reviews the major types of gears and the basic principles of gear-tooth contact. It discusses the loading conditions and stresses that effect gear strength and durability. The article provides information on different gear materials, the common types and causes of gear failures, and the procedures employed to analyze them. Finally, it presents a chosen few examples to illustrate a systematic approach to the failure examination.

A major cause of failure in components subjected to rolling or rolling/sliding contacts is contact fatigue. This article focuses on the rolling contact fatigue (RCF) performance and failure modes of overlay coatings such as those deposited by physical vapor deposition, chemical vapor deposition, and thermal spraying (TS). It provides a background to RCF in bearing steels in order to develop an understanding of failure modes in overlay coatings. The article describes the underpinning failure mechanisms of TiN and diamond-like carbon coatings. It presents an insight into the design considerations of coating-substrate material properties, coating thickness, and coating processes to combat RCF failure in TS coatings.