Determination of Carburized Case Depth in Alloy Steel

Determination of carburized case depth in alloy steel is a critical quality-control test that verifies whether carburized and hardened components—such as gears, shafts, and bearings—achieve the specified surface hardness, fatigue performance, and core toughness. By applying internationally recognized methods such as the microhardness traverse technique described in ISO 2639, GB/T 9450, and ASTM E384, accredited third-party laboratories can accurately measure effective case depth and total case depth. This article provides a comprehensive technical overview of sample preparation, measurement criteria, testing procedures, and typical applications of carburized layer depth measurement for alloy steels.

Technical Overview

Carburizing is a thermochemical heat treatment process in which carbon is diffused into the surface of a low-carbon alloy steel at elevated temperature (typically 880–950 °C), followed by quenching and tempering. The result is a component with a hard, wear-resistant, high-carbon martensitic surface layer (the case) and a tough, low-carbon core. Common carburizing grades include 20CrMnTi, 20CrMo, 20CrNiMo, SAE 8620, SAE 4320, and SAE 9310 alloy steels, which are widely used in power transmission and load-bearing components.

The depth of the carburized layer is one of the most important acceptance criteria for carburized parts, because both insufficient and excessive case depth can lead to premature failure. Insufficient depth reduces contact fatigue strength and allows subsurface yielding (case crushing), while excessive depth can embrittle tooth roots and thin sections, increasing the risk of spalling and fracture. Case depth determination is therefore specified in nearly every carburizing drawing and purchase specification.

Two principal depth values are defined in standards such as ISO 2639 and GB/T 9450:

• Effective case depth (ECD): the depth below the surface at which the hardness falls to a specified limit, most commonly 550 HV1 (approximately 52 HRC), or alternatively the depth at which the carbon content reaches a specified value such as 0.35–0.40%.
• Total case depth (TCD): the depth at which the microstructure or hardness becomes indistinguishable from that of the underlying core material, determined metallographically.

Because the hardness gradient near the criterion is gradual, precise sampling, indentation technique, and interpolation rules are essential for reproducible results. Accredited third-party laboratories performing these tests operate in accordance with ISO 2639, GB/T 9450, GB/T 9451, ASTM E384, SAE J423, and JIS G 0557.

Testing Samples

Finished or Semi-Finished Carburized Components

Typical submitted parts include transmission gears, pinions, camshafts, crankshafts, CV joints, slewing rings, bearing rollers, bushings, and chain sprockets. Parts should be submitted in the final heat-treated condition (carburized, quenched, and tempered), since any subsequent grinding or re-tempering alters the measured hardness profile.

Sacrificial Coupons and Witness Blocks

For batch control, carburizing is often monitored using witness coupons made from the same steel grade, placed in representative furnace positions alongside the workload. These coupons, typically 10–20 mm in diameter or square section, are sectioned and measured as an economical proxy for the actual parts.

Sectioned Teeth and Feature Samples

For gears, measurement is normally performed on a transverse section cut through the middle of a gear tooth. The test plane may be taken at the tooth flank (pitch circle), the tooth root fillet, or the tooth tip, depending on the drawing requirement, because local case depth varies with geometry and carbon potential exposure.

Failure Analysis and Dispute Samples

Fractured or disputed components are sectioned away from the fracture origin to characterize the as-delivered case depth, and near the origin to reveal any depth-related contribution to the failure.

Testing Items

A complete carburized case depth evaluation typically includes the following measurements and observations:

• Effective case depth, defined by the 550 HV1 hardness criterion or another contractually specified hardness limit.
• Total case depth, determined by metallographic observation of the case-to-core microstructural transition.
• Microhardness profile (hardness traverse curve) from the surface to the core, plotted at defined indentation intervals.
• Surface hardness, typically measured as 1–3 indentations averaged within 0.05–0.15 mm of the surface, expressed in HV1, HRC, or HR15N.
• Core hardness measured at a depth at least three times the expected case depth, or at mid-radius/centerline of the section.
• Case microstructure assessment: martensite level, retained austenite content rating, carbide network or precipitate rating per relevant rating charts.
• Surface condition checks: decarburization depth per GB/T 9451 or ASTM E1077, intergranular oxidation, and grinding damage.
• Optional carbon concentration gradient, determined chemically or spectrometrically, to define the depth at a specified carbon content.

Testing Methods

Microhardness Traverse Method (Effective Case Depth)

The microhardness traverse is the preferred and most widely accepted arbitration method, specified in ISO 2639 and GB/T 9450. A transverse section is cut perpendicular to the carburized surface using abrasive cutting with copious coolant to avoid tempering the specimen. The section is mounted, ground, and polished to a metallographic finish suitable for microindentation testing per ASTM E384.

Indentations are made with a Vickers indenter using a test force of 9.807 N (HV1) unless otherwise specified. The measurement procedure typically follows these rules:

• The first indentation is placed approximately 0.05–0.15 mm from the surface (excluding any visible grinding or decarburized zone), and subsequent indentations are spaced at least 2.5 diagonal lengths apart.
• Near the specified hardness limit, the indentation spacing is reduced (often to 0.05–0.1 mm) to improve interpolation accuracy.
• The effective case depth is the depth at which hardness equals the specified limit (commonly 550 HV1), obtained by linear interpolation between the two adjacent indentation points that bracket the limit.
• Two or more parallel traverses, or traverses on both sides of a tooth flank, are measured and the results averaged, with the spread reported.

The resulting hardness–depth curve is reported together with the interpolated effective depth, rounded to 0.05 or 0.1 mm as required by the specification.

Metallographic Method (Total Case Depth)

For total case depth, the polished section is etched with 2–4% nital and examined under an optical microscope. The case typically exhibits high-carbon tempered martensite, possibly with retained austenite and carbides, while the core shows low-carbon tempered martensite, bainite, or ferrite–pearlite depending on hardenability and quench severity. Total case depth is taken as the depth at which the structure becomes indistinguishable from the core structure. Microhardness spots or selective etching may be used to assist in locating the boundary in materials with a diffuse transition.

Chemical and Spectrometric Methods (Carbon Profile)

Where the case is defined by carbon content rather than hardness, the carbon gradient can be established by one of the following techniques:

• Sequential layer removal by turning or grinding, followed by combustion infrared analysis of the removed chips.
• Glow discharge optical emission spectroscopy (GDOES), which provides a rapid carbon concentration–depth profile.
• Electron probe microanalysis (EPMA) or energy-dispersive spectroscopy line scans on the polished cross section.

The case boundary is then defined at the specified carbon level, commonly 0.35% C or 0.40% C, per the applicable drawing or SAE J423 guidance.

Macroscopic and Reference Methods

Rapid shop-floor estimation of case depth may use macroscopic techniques such as the fracture test (comparing the fine-grained case fracture against a reference color chart) or the macro-etch test, in which a polished cross section is etched to darken the high-carbon case for visual depth measurement with a calibrated scale. These methods are useful for process control but are not accepted as arbitration methods; discrepancies are resolved by the microhardness traverse.

Applications

Carburized case depth determination supports quality assurance, process validation, and failure analysis across many industries:

• Automotive: manual and automatic transmission gears, differential pinions, ring gears, CV joint components, steering worms, and camshafts.
• Wind energy: intermediate-speed and high-speed gearbox gears and planet pins requiring strict fatigue margins.
• Heavy machinery and mining: bucket wheel reducer gears, drill bit bearings, track rollers, and crusher components.
• Rail transit: traction gearboxes, axle boxes, and coupling components.
• Bearings and power transmission: tapered roller bearing components, raceways, heavy-duty chains, and sprockets.
• Oil and gas, and aerospace ground equipment: actuators, gears, and pins where case specifications are governed by stringent customer standards.
• Heat treatment suppliers: furnace qualification, carbon potential audits, and periodic batch conformity testing of witness coupons.

FAQ

What is the difference between effective case depth and total case depth?

Effective case depth is a functional, hardness-based value—the depth at which hardness drops to a specified limit, most commonly 550 HV1 (about 52 HRC)—and is the usual acceptance criterion on drawings. Total case depth is a metallographic value marking where the carburized structure merges into the core, and it is always larger than the effective case depth, often by 0.2–0.6 mm depending on the hardenability of the steel and the sharpness of the carbon gradient.

Which test force should be used for the microhardness traverse?

ISO 2639 and GB/T 9450 specify a test force of 9.807 N (HV1) for the hardness traverse unless the specification states otherwise. Lower forces such as HV0.5 may be used for very shallow cases or near-surface gradients, but the force used must be reported because indentation size effects change the apparent hardness values and therefore the interpolated depth.

How is the depth measured on a gear tooth?

A transverse section is cut through the mid-length of a gear tooth, mounted and polished. Traverses are commonly run from the tooth flank at the pitch line and, when required, from the root fillet, because case depth at the tip, flank, and root differ due to geometry and process effects. The drawing usually specifies which locations and which minimum/maximum limits apply.

Why can two laboratories report slightly different case depths?

Small differences arise from indentation placement relative to the curved tooth surface, indentation spacing, the interpolation interval around the 550 HV1 criterion, surface condition (grinding stock, decarburization), and material homogeneity. Following the standardized spacing and interpolation rules of ISO 2639 minimizes scatter; typical reproducibility between competent laboratories is within about ±0.05 to ±0.10 mm.

How does retained austenite or carbide network affect the measurement?

Excessive retained austenite can locally depress hardness near the surface and distort the profile shape, while a continuous carbide network can raise near-surface hardness and embrittle the case. These structural features are therefore rated metallographically alongside the depth measurement, and specifications commonly limit retained austenite (for example to ratings of 3–4 or a volume percentage) and require no continuous carbide network at the case core boundary.

Can case depth be verified non-destructively?

Indirect, non-destructive estimation is possible on production parts using surface hardness checks, eddy current sorting against calibrated reference parts, or ultrasonic and magnetic techniques in controlled setups. However, all non-destructive results must be correlated with destructive sectioning on sample parts, and contractual disputes are settled by the destructive microhardness traverse method.

What standards govern carburized case depth determination?

The principal standards are ISO 2639 (effective depth of carburized and hardened cases), GB/T 9450 (Chinese equivalent covering the microhardness method), GB/T 9451 (decarburized layer depth), ASTM E384 (microindentation hardness), ASTM E1077 (decarburization estimation), SAE J423 (case depth measurement), and JIS G 0557 (case depth of carburized steel). Accredited third-party laboratories select the applicable standard based on the product specification and customer requirements.