Application framework for selection and method discussions; not a customer test report.
Purpose
Surface and core structures in gears, shafts and heat-treated parts.
Preparation route
Section the region of interest, retaining the surface-to-core direction. Support edges, prepare the surface and arrange microscopy and a hardness traverse as required.
Checkpoints
Check edge rounding, loss of the surface layer and unclear indentation positions. Interpret structure and hardness against the agreed material and process requirements.
Equipment stages
Cutting → Mounting → Grinding and polishing → appropriate microscopy and testing.
Record sampling position, consumables, actual settings and images. Evaluate using the agreed method.
Typical inspection scenarios
20CrMnTi carburized and hardened gear
- Challenge
- Case depth must be measured precisely at multiple locations along the flank and root, but the root fillet is prone to edge rounding and collapse, distorting depth readings; core and case structures differ greatly in hardness, so one specimen must serve both.
- Preparation & method
- A cross-section is taken from a single tooth along the face width with a precision cut-off machine, then cold mounted in low-shrinkage epoxy to protect the edges. Grind with SiC papers from P400 to P2000, rotating 90° between steps; rough polish with 3 μm diamond suspension, finish with 1 μm and a final 0.05 μm colloidal silica polish. After etching with 4% nital, measure the effective case depth by the microhardness profile method (550 HV limit) and evaluate surface martensite, retained austenite and core structure.
- Reference standards
- GB/T 9450 Determination and verification of carburized and hardened case depth of steel parts; GB/T 13298 Metallographic examination methods for metals; GB/T 6394 Determination of average grain size of metals
42CrMo induction-hardened crankshaft journal
- Challenge
- The hardened layer varies in depth around the journal circumference and the fillet transition, requiring multi-position evaluation; the hardness gradient between the martensitic case and the tempered sorbite core is steep, so the effective case depth must be defined by the hardness method; the crankshaft is large and valuable, limiting sampling locations and size.
- Preparation & method
- Cut small transverse specimens from the worn zone and the fillet of the journal on a wet abrasive cut-off machine (alumina wheel) with ample coolant to prevent tempering softening. Cold mount in low-shrinkage epoxy to protect the surface edges. Grind with SiC papers from P400 to P2000 and polish with 3 μm then 1 μm diamond. After etching with 4% nital, first estimate the case depth metallographically from the martensite boundary; then run a radial micro-Vickers hardness profile (HV0.3–0.5) and define the effective case depth at the 550 HV limit (or 0.8× surface hardness), rating the case martensite level and core structure per JB/T 9204.
- Reference standards
- GB/T 5617 Determination of effective case depth of steel parts after induction or flame hardening; JB/T 9204 Metallographic examination of induction-hardened steel parts; 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.