Rolling bearing failure analysis is a systematic laboratory investigation that determines why a bearing ceased to perform, whether through surface distress, subsurface fatigue, or bulk-material degradation. The test object is a failed rolling bearing component — rings, rolling elements, or cages — together with lubricant residues and fracture surfaces. The analytical workflow combines visual and stereomicroscopic inspection, metallographic sectioning and microstructure examination, Scanning Electron Microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) fractography, and hardness verification. Together these modules identify the dominant failure mechanism, distinguish fatigue spalling from wear, corrosion, overload, or manufacturing defects, and trace the root cause to load, lubrication, mounting, or material condition. The resulting evidence supports warranty decisions, design revisions, and maintenance-interval adjustment.
Principle & mechanism
The method rests on linking observable damage features to the stress and metallurgical history of the component. Rolling contact generates cyclic Hertzian stresses concentrated near the surface, and subsurface shear stresses peak at a shallow depth governed by contact geometry and load. Repeated cycling initiates microcracks at stress raisers such as non-metallic inclusions, carbide clusters, or decarburized layers; crack propagation then coalesces into pitting or spalling. Metallography reveals the microstructural evidence of this history — retained austenite transformation, martensite alteration, white-etching areas, and inclusion distribution — while SEM resolves crack morphology, fracture origin, and micro-adhesion features invisible under optical microscopy. EDS adds local chemistry, distinguishing oxide films, corrosive residues, and foreign particles. Interpretation follows the logic that each failure mechanism leaves a characteristic damage signature; matching observed features against established morphology catalogs converts physical evidence into a root-cause conclusion rather than an assumption.
Failure Modes and Sample Preparation
Common mechanisms investigated include contact fatigue spalling, abrasive and adhesive wear, false brinelling from vibration at standstill, corrosion and fretting corrosion, electrical erosion damage from current passage, overload fracture, and lubricant breakdown with surface glazing. Before sectioning, the specimen receives full photographic documentation under a stereomicroscope, with unmixed lubricant sampled for later analysis. Contaminants and corrosion products must never be cleaned away before chemical capture, since wiping destroys diagnostic residues. Sectioning employs abrasive cutting with abundant coolant, positioned so the cut face passes through the damage zone while avoiding burn or deformation artifacts; the extracted section is then marked for orientation. Fracture surfaces intended for SEM are preserved dry, protected from touching, and stored in desiccated containers, because oxide growth or organic contamination degrades fractographic detail. Mounting in resin follows, with the running track of interest oriented for longitudinal or transverse viewing according to whether surface-parallel or through-thickness features carry the diagnostic weight.
Metallographic Examination Procedure
Mounted specimens proceed through grinding with successively finer silicon-carbide papers, followed by polishing with diamond suspensions down to a one-micron or finer finish. Bearing steels are typically etched with nital to reveal the tempered martensite structure, carbide distribution, and any decarburization or transformation bands. Examination under an inverted metallurgical microscope begins at low magnification to map the running track, locating spalls, indentations, and discoloration relative to the load zone. Higher magnifications then assess inclusion content and morphology, carbide networking, grain-boundary condition, and heat-treatment quality such as retained austenite levels estimated from etching response. Depth measurements of decarburized layers, white-etching bands, and case-hardened zones are recorded on calibrated micrographs. Subsurface crack profiles beneath spalls deserve particular attention: their angle, depth, and branching pattern distinguish classical inclusion-initiated fatigue from surface-initiated damage caused by debris denting or poor lubrication, which directly narrows the root-cause hypothesis.
SEM and EDS Fractography Analysis
Fracture surfaces and damaged raceway zones are examined in a scanning electron microscope at accelerating voltages commonly in the 5–20 kV range, selected to balance image resolution against beam penetration. Secondary-electron imaging resolves fine topographic detail — fatigue striations, dimples from ductile overload, intergranular facets from brittleness or hydrogen influence, and smeared adhered layers from sliding contact. Backscattered-electron imaging gives compositional contrast, highlighting embedded foreign particles and oxide films. EDS point and mapping analysis then assigns local chemistry to each feature: elevated oxygen indicates corrosion or oxidative wear; specific metallic residues reveal debris origin or material transfer; chlorine or sulfur peaks point to aggressive lubricant additives or corrosive media. For electrical erosion, characteristic melt craters and craters with re-solidified rims confirm current passage. The analysis sequence always proceeds from low to high magnification so that the overall crack pattern and origin location are established before micro-feature interpretation begins, preventing misreading of secondary damage as the primary cause.
Hardness and Microstructure Verification
Hardness testing verifies whether the steel condition fell within the specified range implied by its grade and heat treatment. Ring and roller surfaces are checked with Rockwell C or Vickers methods on prepared flats; case-hardened components additionally require microhardness traverses using a Vickers or Knoop indenter at small loads to plot the hardness-depth profile through the case into the core. A softened surface layer signals tempering overheating, grinding burn, or retained-austenite transformation; an abnormally shallow case reduces subsurface fatigue resistance. Microstructure cross-checks the hardness data: coarse carbide networks, incomplete hardening, or excessive decarburization each explain deviations and point to manufacturing deficiency rather than service conditions. Microhardness mapping around damage zones further reveals localized changes such as white-etching layers, which are markedly harder and more brittle than the matrix. Results are interpreted jointly with the metallographic and fractographic findings, because hardness data alone cannot discriminate among mechanisms — it confirms or eliminates material-condition hypotheses raised by the morphological evidence.
Acceptance Criteria and Report Interpretation
No single pass-or-fail criterion governs failure analysis; acceptance is judged against the bearing specification, drawing requirements, and applicable product standards for material, hardness range, case depth, and microstructure rating. The report should state the damage morphology with supporting micrographs and SEM images, the identified failure mechanism ranked by evidence strength, contributing factors, and a root-cause classification — design, manufacturing, installation, lubrication, operation, or maintenance related. Hardness values and profiles are compared with specification limits; inclusion ratings are compared with cleanliness classes where required. Interpretation must separate primary from secondary damage: post-failure debris denting and corrosion frequently overprint the original evidence. Conclusions are phrased with stated confidence, and where evidence is incomplete, the report lists the ambiguity rather than forcing a single cause. Actionable output — revised fit clearances, lubricant changes, mounting procedure corrections, or material substitution — should follow logically from the cited mechanism so the client can close the corrective-action loop.
Frequently Asked Questions
What does a rolling bearing failure analysis report contain?
The report documents sample condition on arrival, stereomicroscopic and photographic evidence of damage, metallographic microstructures, SEM fractography with EDS findings, and hardness results. It concludes with the failure mechanism, ranked contributing factors, and a root-cause classification. Clients use it for warranty claims, design revisions, material specification checks, and corrective maintenance decisions.
How should a failed rolling bearing be submitted and discussed with the laboratory?
Submit the complete bearing, including rings, rolling elements, cage, and any retained lubricant, without cleaning or disassembling. Protect fracture surfaces from contact and moisture. Communicate service history, load and speed conditions, lubrication practice, and when the failure was detected, since this context directs which metallographic and SEM examinations carry priority.
What factors affect the cost of rolling bearing failure analysis?
Cost depends on the number of components examined, the extent of sectioning and metallographic preparation, whether SEM with EDS mapping is required, and the volume of hardness profiling. Lubricant residue analysis and multiple failure zones or competing hypotheses also increase examination scope and reporting effort.