Axles, wheels, gearboxes, brake discs and bogie weldments of rail vehicles are subjected to alternating loads and impacts for years, and their fatigue life depends heavily on the microstructure, surface hardened layers and weld quality. Axle surfaces are commonly medium-frequency induction hardened to improve bending fatigue strength, wheel rims are tread-quenched, and gears and high-strength fasteners are quenched-and-tempered or carburized to achieve the required strength-toughness balance. Metallographic examination is a fundamental tool for delivery inspection, type testing and failure analysis of these components.
Typical test objects
- LZ50 steel axles (medium-frequency induction hardened layer)
- CL60 solid rolled wheels (tread-quenched rim)
- 20CrNiMo and 18CrNiMo7-6 gearbox gears
- 42CrMo traction motor shafts and coupling forgings
- Vermicular graphite iron / cast steel brake discs
- Q355 bogie frame welded joints
- Class 10.9 high-strength connecting bolts
Inspection challenges
- The induction hardened layer varies in depth around the axle circumference and at the shoulder transition, requiring multi-position sampling; the effective case depth must be defined by the hardness method;
- The steep structural gradient between quenched martensite at the wheel rim and the pearlite-ferrite core tends to cause relief and residual scratches during grinding and polishing;
- A welded joint contains base metal, HAZ and weld metal with different etching sensitivity, so one preparation must reveal all zones clearly;
- Axle and wheel steels are extremely sensitive to non-metallic inclusions; polishing must avoid inclusion pull-out and comet tails that cause misjudgment;
- Sampling of large components is restricted, and cutting heat may mask the original structure, so low feed rates with ample cooling are required.
Recommended preparation and inspection route
According to GB/T 5617, cut transverse specimens at the axle body, shoulder and fillet transition on a wet abrasive cut-off machine with an alumina wheel at low feed and ample cooling; cold mount in low-shrinkage epoxy to protect the surface edge. Grind with SiC papers from P240 to P2000, rotating 90° between steps; rough polish with 9 μm and 3 μm diamond suspensions, finish with 1 μm, and use a short 0.05 μm colloidal silica polish for inclusion rating. After etching with 4% nital, observe the structure and grain boundaries at 100–500×, and rate inclusions in bright field against the GB/T 10561 standard charts; measure the radial hardness profile with a micro-Vickers tester at HV0.5 and define the effective case depth at the 550 HV limit. Welded joints additionally undergo macro-etching to verify fusion depth and detect lack of penetration.
Common standards
- GB/T 5617 Determination of effective depth of hardening after induction or flame hardening of steel
- GB/T 13298 Metallographic examination methods for metals
- GB/T 10561 Steel — Determination of content of non-metallic inclusions — Micrographic method using standard diagrams
- GB/T 6394 Determination of average grain size of metals
- ISO 4967 Steel — Determination of content of non-metallic inclusions — Micrographic method using standard diagrams
The above methods and parameters are application examples for technical communication rather than fixed process recipes; the actual preparation route, etchants and acceptance criteria should be confirmed against the material, dimensions and inspection purpose of each specific specimen.
Typical inspection scenarios
Effective case depth measurement of a medium-frequency induction-hardened LZ50 axle
- Challenge
- The induction hardened layer varies in depth along the axle body, shoulder and fillet transition, and the boundary between quenched martensite and the pearlite-ferrite core is indistinct at some positions, making the metallographic method alone unreliable; as the axle is a large safety-critical part with limited sampling, one specimen must serve both metallographic and hardness methods.
- Preparation & method
- Cut transverse specimens at the axle body and fillet transition on a wet abrasive cut-off machine with an alumina wheel, low feed and ample cooling to prevent tempering of the case. Cold mount in low-shrinkage epoxy to protect the surface edge. Grind with SiC papers from P240 to P2000, rotating 90° between steps, then polish with 9 μm and 3 μm diamond and finish with 1 μm. After etching with 4% nital, observe the case martensite, transition zone and core in bright field at 100–400× and preliminarily estimate the depth metallographically; then measure the radial hardness profile with a micro-Vickers tester at HV0.5, determine the effective case depth by the hardness method per GB/T 5617, and issue the verdict after rating the martensite level against the standard charts.
- Reference standards
- GB/T 5617 Determination of effective depth of hardening after induction or flame hardening of steel; GB/T 13298 Metallographic examination methods for metals; GB/T 10561 Steel — Determination of content of non-metallic inclusions — Micrographic method using standard diagrams
Metallographic examination of a Q355 bogie frame welded joint
- Challenge
- A welded joint consists of base metal, HAZ and weld metal with very different structures and etching sensitivity, and one preparation must reveal all zones clearly; lack of fusion, lack of penetration and slag inclusions may exist at the weld root, and polishing scratches or uneven etching can easily mask fine defects; coarse- and fine-grained HAZ regions adjoin each other, so grain size must be rated zone by zone.
- Preparation & method
- Cut a full-section specimen covering base metal, HAZ and weld metal normal to the weld on a wet cut-off machine with a diamond or alumina wheel and ample cooling; cold mount upright. Grind with SiC papers from P240 to P2000, polish with 9, 3 and 1 μm diamond, and finish with a short 0.05 μm colloidal silica polish. First macro-etch (hot or cold acid etching) to reveal weld macro-morphology and the fusion line, then etch with 4% nital per GB/T 13298 and observe weld columnar grains, the coarse-grained HAZ and base metal in bright field at 50–500×, using dark field for non-metallic inclusions if needed; measure micro-Vickers hardness at HV0.5 point by point across base metal, HAZ and weld, and judge the joint's structure and hardness against the specified limits.
- Reference standards
- GB/T 13298 Metallographic examination methods for metals; GB/T 6394 Determination of average grain size of metals; GB/T 4340.1 Metallic materials — Vickers hardness test — Part 1: Test method
LZ50 steel axle with mid-frequency induction-hardened layer
- Challenge
- The hardened layer varies in depth across the journal body, shoulder and fillet transition, requiring multi-position evaluation; the steep hardness gradient between the martensitic case and the core means the effective case depth must be defined by the hardness method; the axle is a large critical component, so sampling locations are limited and cutting heat could introduce tempering artefacts.
- Preparation & method
- Take transverse sections from the axle body, shoulder and fillet transition on a wet abrasive cut-off machine (alumina wheel) at low feed with ample coolant; cold mount in low-shrinkage epoxy to protect the hardened edges. Grind with SiC papers from P240 to P2000, rotating 90° between steps; rough polish with 9 μm and 3 μm diamond and finish with 1 μm. After a light 4% nital etch, examine the case martensite, transition zone and core at 200–500×; then run a radial micro-Vickers profile at HV0.5 and define the effective case depth at 0.8× surface hardness (or 550 HV) per GB/T 5617, rating the martensite level and core banding.
- Reference standards
- GB/T 5617 Determination of effective case depth of steel parts after induction or flame hardening; GB/T 13298 Metallographic method for microstructure inspection of metals; GB/T 4340.1 Metallic materials — Vickers hardness test
Q355 bogie frame weld joint
- Challenge
- The joint combines three zones with different properties — base metal, heat-affected zone and weld metal — that must be observed and hardness-mapped on the same section; the component is large so only small sections can be taken, and weld edges must be preserved; the heat-affected zone is narrow, so the hardness scan needs a fine step to capture hardening peaks.
- Preparation & method
- Cut small transverse sections perpendicular to the weld axis on a precision cut-off machine with ample coolant; cold mount to protect the edges. Grind with SiC papers from P240 to P2000 and polish with 3 μm then 1 μm diamond. Macro-etch with 10% nital to reveal the bead contour, measure penetration and reinforcement, and check for lack of fusion, pores and other macro flaws; re-polish and lightly etch with 4% nital, then examine the base metal, coarse- and fine-grained heat-affected zones, fusion line and columnar weld metal at 100–500×; run a micro-Vickers line scan (HV0.5) across the fusion line at 0.1–0.5 mm steps, checking for HAZ hardening peaks against process limits.
- Reference standards
- ISO 17639 Destructive tests on welds in metallic materials — Macroscopic and microscopic examination of welds; GB/T 2654 Hardness test methods for welded joints; GB/T 4340.1 Metallic materials — Vickers hardness test
Scenarios are method examples, not customer test reports; confirm the process per specimen.
MICROCRE learning notes · confirm the method for the specimen.