Microstructure testing of carburized layers in alloy steels verifies the effective case depth, hardness gradient, carbide distribution, retained austenite content, and core structure that determine the fatigue life and wear resistance of carburized components. This article explains the sampling strategy, evaluation items, standard test methods, and acceptance criteria commonly applied by accredited metallurgical laboratories to gears, shafts, and bearing parts. It also summarizes the international standards (ISO, ASTM, GB, JIS) that govern carburized layer inspection.
Technical Overview
Carburizing is a case-hardening heat treatment in which carbon is introduced into the surface of a low-carbon alloy steel at approximately 900–950 °C, followed by quenching and low-temperature tempering at 150–200 °C. The result is a hardened case with a high-carbon tempered martensitic structure and a tough, lower-hardness core. Because the carbon and hardness gradients, phase constitution, and defect population of the case directly control contact fatigue, bending fatigue, and wear performance, microstructure inspection of the carburized layer is an essential qualification step for safety-critical parts.
A typical qualified carburized case consists of fine high-carbon tempered martensite, a controlled amount of retained austenite, and fine, dispersed globular or granular carbides. The transition zone gradually shifts toward the core structure of low-carbon martensite, bainite, and a limited fraction of ferrite, depending on the hardenability of the steel. Common microstructural defects include continuous or semi-continuous carbide networks along prior austenite grain boundaries, excessive retained austenite, surface decarburization, internal oxidation with an associated non-martensitic surface layer, and coarse prior austenite grains.
- Typical carburizing grade families: 20CrMnTi, 20CrMo, 20CrMnMo, 20CrNiMo, 18CrNiMo7-6, and corresponding JIS grades such as SCM420 and SCr420.
- Key inspection standards: GB/T 9450 / ISO 2639 (effective case depth of carburized and hardened cases), GB/T 9451 / ISO 4970 (total case depth), GB/T 13298 (metallographic inspection method), GB/T 4340.1 / ISO 6507-1 / ASTM E384 (Vickers hardness), ASTM E1077 (decarburization), ASTM E975 (retained austenite by XRD), ASTM E112 / GB/T 6394 (grain size), and JIS G 0557 (case depth measurement).
- Typical acceptance targets: surface hardness 58–63 HRC, core hardness about 30–45 HRC, and effective case depth per drawing specification, commonly 0.6–2.0 mm for gear teeth.
Testing Samples
Gears and Drivetrain Components
Automotive and industrial transmission gears are the most frequently inspected carburized parts. Cross-sections are normally taken through a tooth so that the case depth, carbide rating, and retained austenite can be evaluated at the pitch line, the tooth flank, and the tooth root fillet, where bending fatigue initiates. Tooth-by-tooth evaluation is often required because case depth varies with geometry and gas circulation during carburizing.
Shafts, Pins, and Bearing Components
Camshafts, CV joints, piston pins, roller bearings, and raceways are sectioned perpendicular to the functional surface. For cylindrical parts, at least two positions 180° apart are examined to detect circumferential non-uniformity caused by part loading or atmosphere circulation in the furnace.
Process Coupons and Witness Blocks
Heat treatment facilities carburize sacrificial coupons of the same steel grade alongside production loads. These coupons are fully consumable in testing, which makes them ideal for routine process monitoring, cycle validation, and carbon-potential calibration without destroying finished components.
Failed Parts for Root-Cause Analysis
When a carburized component fails in service by pitting, spalling, tooth fracture, or case crushing, metallurgical sectioning of the fracture origin reveals whether anomalous case depth, surface decarburization, carbide networks, or an excessively soft non-martensitic layer contributed to the failure.
Testing Items
A comprehensive inspection program for a carburized alloy steel layer generally covers the following items, selected according to the product drawing and applicable technical specification:
- Effective case depth (CHD), defined as the distance from the surface to the point of a specified hardness limit, typically 550 HV1 for carburized and hardened cases.
- Total case depth, defined as the distance from the surface to the point where the structure or hardness is no longer distinguishable from the core.
- Surface hardness and core hardness, reported in HRC after conversion per GB/T 1172 or ASTM E140.
- Case microstructure rating: morphology and fineness of tempered martensite.
- Carbide evaluation: amount, size, shape, and distribution, with special attention to network or massive carbides at grain boundaries.
- Retained austenite content and distribution, either by image comparison rating charts or quantitative metallography and X-ray diffraction.
- Internal oxidation depth and the associated non-martensitic transformation products (ferrite, bainite) beneath the surface.
- Surface decarburization depth and severity.
- Core microstructure, including the permissible ferrite fraction and the absence of undesirable transformation products.
- Prior austenite grain size in the case and core.
- Carbon concentration profile from the surface into the core, confirming surface carbon content typically in the range of 0.75–1.05%.
Testing Methods
Specimen Preparation and Etching
Cross-sections are cut strictly perpendicular to the carburized surface using abrasive cutting with abundant coolant to avoid tempering or burning the case. Specimens are mounted in epoxy, preferably with edge-retention filler or a protective nickel plating to prevent edge rounding during grinding. Grinding proceeds through successive grit sizes followed by diamond polishing to 1 µm, after which the section is etched for a few seconds in 2–4% nital to reveal martensite, carbides, ferrite, and grain-boundary features.
Optical Microscopy Evaluation
The polished and etched cross-section is examined at magnifications from 100× to 1000× using a calibrated metallurgical microscope in accordance with GB/T 13298. The case, transition zone, and core are photographed, and each microstructural feature—carbides, retained austenite, non-martensitic layer, decarburization, and core ferrite—is rated against the comparison charts of the applicable product or industry standard. A measuring reticle or image analysis software is used to determine depths of internal oxidation, decarburization, and total case depth.
Vickers Microhardness Traverse for Effective Case Depth
Effective case depth is determined per GB/T 9450 / ISO 2639 by a microhardness traverse using a Vickers indenter at a test force of 9.807 N (HV1). Indentations are placed along one or two parallel traverses starting approximately 0.3 mm below the surface and extending beyond the expected case depth, with the spacing between indentation centers at least 2.5 times the diagonal length of the indentations. Hardness is plotted against distance from the surface, and CHD is obtained at the intersection with the 550 HV1 limit by graphical interpolation or curve fitting. The reported value includes the test force and the hardness criterion, for example CHD = 1.05 mm (550 HV1).
Advanced and Complementary Techniques
Scanning electron microscopy (SEM) resolves fine carbides, grain-boundary films, and micro-cracks that are ambiguous under the light microscope. Electron backscatter diffraction (EBSD) and X-ray diffraction per ASTM E975 quantify retained austenite accurately, typically with acceptance limits in the range of 10–30% depending on the specification. Carbon profiles are measured by glow discharge spectroscopy (GDS) or by stepwise grinding with optical emission spectrometry (OES) at each exposed depth. Prior austenite grain boundaries are revealed by saturated aqueous picric acid etching, and grain size is rated per ASTM E112 / GB/T 6394.
Applications
Carburized layer microstructure testing supports product qualification, heat treatment process control, and failure analysis across industries in which surface-hardened alloy steel components carry cyclic contact and bending loads. Representative applications include:
- Automotive transmission and differential gears, CV joints, and piston pins.
- Wind turbine and heavy-duty industrial gearboxes with large-module carburized gears and planet carriers.
- Railway axle boxes, bearings, and traction gear sets.
- Construction machinery final drives, swing reducers, and track components.
- Aerospace actuator gears, accessory drives, and landing gear pins.
- Oil and gas downhole tools, pump shafts, and mud motor rotors.
- Heat treatment supplier audits and first-article inspection of outsourced carburizing services.
FAQ
What hardness limit defines the effective case depth of a carburized layer?
For carburized and hardened cases, GB/T 9450 and ISO 2639 define effective case depth at the 550 HV1 limit, measured with a 9.807 N Vickers indenter. Some customer specifications instead specify 600 HV1 or another limit; in that case the criterion must be stated together with the reported depth, because the numeric value of CHD changes with the chosen hardness threshold.
How is retained austenite evaluated and what level is acceptable?
Retained austenite is first screened by comparison with standard rating charts under the optical microscope, then quantified when required by X-ray diffraction per ASTM E975 or by SEM-based image analysis. Most gear and bearing specifications accept retained austenite below roughly 30% by volume; excessive amounts lower surface hardness and can cause dimensional growth or grinding burn during finishing.
Why are network carbides rejected?
Continuous or semi-continuous carbide networks form along prior austenite grain boundaries when the surface carbon potential is too high or diffusion time is insufficient. These brittle networks act as crack initiation sites under rolling contact, causing pitting, spalling, and case exfoliation, so specifications usually limit carbides to fine, discrete particles or allow only interrupted, low-severity networks.
What causes the soft non-martensitic layer at the surface?
During gas carburizing, oxygen bearing species penetrate along grain boundaries and selectively oxidize chromium, manganese, and silicon. This internal oxidation depletes the alloy elements needed for hardenability, so on quenching a thin ferritic or bainitic layer forms instead of martensite. Specifications typically cap its depth at a few hundredths of a millimeter, and low-pressure or vacuum carburizing is often adopted to eliminate it.
Is the test destructive, and how should parts be sampled?
Yes, examination requires sectioning through the case, so testing consumes the sampled component. For routine control, consumable witness coupons carburized with the production load are preferred; finished parts are sectioned during first-article inspection, periodic audits, and failure investigations, with sample locations chosen at the most highly stressed functional surfaces.