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Case Depth Measurement by Cross-Section and Hardness Traverse

Case depth — the depth from a hardened surface to a specified minimum hardness in the sub-surface — is the primary specification driving induction hardening and carburizing work. UTEC Industrial provides in-house induction hardening, through-hardening, and quench-and-temper heat treating services for industrial components in the Pacific Northwest, with integrated CNC machining and reverse-engineering capability. A crane wheel specified with "0.25–0.50 inch case depth at 50 HRC minimum" or an industrial shaft with "0.125 inch effective case at 55 HRC" is telling the heat treater and the customer's incoming inspector exactly what must be verified before the part ships. Measuring that case depth reliably — not just estimating it from process parameters or eyeballing a macro-etched cross-section — requires a defined procedure: sectioning the part at a representative location, preparing the cross-section for hardness testing, performing a microhardness traverse from surface to core, and determining the depth at which the hardness drops to the specified cut-off. This article covers the cross-section hardness traverse method per ASTM E384 and SAE J423, the distinction between effective and total case depth, the practical challenges in measuring case depth on hardened industrial parts, and the documentation that ships with case-depth-verified heat treatment work.

What is case depth, and why are there multiple definitions?​

Case depth refers to the depth of the hardened layer at the surface of a heat-treated part, measured from the surface inward to a specified hardness cut-off. The complication is that "case depth" alone is ambiguous — several different definitions are in common use, each giving a different numerical result for the same physical hardness gradient. Effective case depth (sometimes called "functional case depth") is defined as the depth at which hardness drops to a specified minimum value, typically 50 HRC (or 513 HV, the Vickers equivalent) for induction-hardened steel; SAE J423 defines the 50 HRC cut-off as the standard for most commercial induction-hardened work. Total case depth is the depth at which the hardness traverse shows no further drop — essentially, the depth at which the hardness reaches the core hardness of the un-hardened base metal. The total case depth is always greater than the effective case depth, typically by a factor of 1.5 to 2 depending on the transition-zone steepness. Case depth to 550 HV and other specific hardness cut-offs appear on some drawings, particularly for carburized gears where the AGMA standards specify case depth at a hardness cut-off appropriate to the loading. Specifying case depth on a drawing without identifying the definition is a common source of miscommunication: "case depth 0.125 inch" could mean any of the definitions above, producing a 3-to-1 ratio of measured values depending on interpretation. The clear specification should always state "effective case depth 0.125 inch to 50 HRC per SAE J423" or equivalent language that leaves no ambiguity about the definition used (SAE J423; ASM Handbook, Vol. 4C, ASM International, 2014; ASTM E384; AGMA standards for gear case depth specification).

What is the cross-section hardness traverse method, and how is it performed?​

The cross-section hardness traverse is the destructive-but-authoritative method for measuring case depth. The procedure: (1) cut a section from the hardened part at a representative location — for a crane wheel, typically a radial slice through the tread; for a shaft, a transverse section at the midpoint of the hardened band; (2) mount the section in a thermoset or thermoplastic compound to protect the surface edge during subsequent preparation; (3) grind and polish the cross-section to a flat, scratch-free finish suitable for hardness indentation, typically through progressive SiC abrasive papers from 120 grit to 1200 grit, followed by diamond polishing to 1 micron; (4) optionally etch the polished surface with 2–5% nital to reveal the hardened zone visually (the etched structure shows the martensitic case as a light-colored band against a darker core background); (5) perform hardness measurements on a traverse from the hardened surface inward, with indentations spaced at regular intervals, typically 0.010–0.025 inch (0.25–0.63 mm) apart. The hardness values are plotted against depth from surface, producing the hardness gradient curve. The effective case depth is read from the curve at the intersection with the specified hardness cut-off (typically 50 HRC); the total case depth is read at the point where the curve levels off at core hardness. For shallow induction-hardened cases (less than 0.060 inch), the test should use a microhardness method (Vickers or Knoop per ASTM E384) to permit closer indentation spacing without plastic-zone interference between indents; for deeper cases (greater than 0.125 inch), Rockwell C indentation can be used directly, with minimum 3x-indent-diagonal spacing between consecutive indents per ASTM E18 (ASTM E384; SAE J423; ASTM E18; ASM Handbook, Vol. 4C, ASM International, 2014).

What is microhardness testing, and when is it required for case depth measurement?​

Microhardness testing uses lower indenter loads (typically 100 gram-force to 1 kilogram-force) than standard macrohardness methods (Rockwell C uses 150 kgf; Brinell typically 3,000 kgf). The smaller indentations — a 100 gf Vickers indent is typically 25–50 microns across, compared to 200–400 microns for a Rockwell C indent — allow closer indent spacing (down to 0.002 inch / 50 microns) without interaction between neighboring plastic zones, and therefore allow finer resolution of the case-depth profile. ASTM E384 is the authoritative standard for microhardness testing, covering both Vickers (square diamond pyramid) and Knoop (elongated diamond) indenter geometries. Vickers is the preferred choice for case depth work because its geometry produces a more consistent response across hardness ranges, and Vickers values convert readily to Rockwell C via standard conversion tables (ASTM E140). The load selection for microhardness case depth testing is typically 100 gf or 500 gf for through-hardened or induction-hardened steel, depending on the expected hardness range and the available indent spacing. For very shallow cases — surface hardening to depths below 0.030 inch (0.76 mm), which is below the resolution practical for Rockwell C testing — microhardness is the only viable method. For the typical induction-hardened crane wheel or industrial shaft with case depth in the 0.125–0.500 inch range, either microhardness or macrohardness Rockwell C is acceptable; the choice depends on specification requirements and on the resolution desired in the transition zone (ASTM E384; ASTM E140; ASM Handbook, Vol. 4C, ASM International, 2014; Metallography and Microstructures, ASM Handbook, Vol. 9, ASM International, 2004).

How does the shape of the hardness gradient curve indicate process quality?​

The shape of the hardness traverse curve is diagnostic of the induction hardening process itself. A properly executed induction hardening cycle produces a hardness profile with these characteristics: (1) high surface hardness (typically 50–58 HRC for 4140-grade crane wheel tread, 58–62 HRC for higher-carbon shafts or rollers) that holds roughly constant from surface to within the depth where the case begins to transition; (2) a transition zone of 0.020–0.050 inch thickness where hardness drops linearly from case hardness to core hardness; (3) a core hardness consistent with the base-metal condition before induction hardening — for 4140 in the quench-and-tempered 28–32 HRC condition, the traverse should level off at 28–32 HRC beyond the transition zone. A curve that shows an inflection or "shoulder" in the transition zone typically indicates incomplete austenitization during induction heating — the surface reached full austenite temperature but an intermediate depth reached only the inter-critical range (between Ac1 and Ac3), producing a mixed microstructure that shows partial hardening. A curve with a "dip" below core hardness at some depth may indicate tempering-back: during induction hardening, heat conducted from the surface into the sub-surface can raise a band below the case to tempering temperatures, producing a locally softer region before the core hardness is reached. A curve with core hardness above the specified base-metal condition indicates the core was partially hardened by the cycle, which for some base-metal conditions is acceptable and for others is not. Reading the traverse curve against the expected shape for the specific grade and cycle reveals process anomalies before they show up as failures in service (ASM Handbook, Vol. 4C, ASM International, 2014; Rudnev, V., et al., Handbook of Induction Heating, 2nd ed., CRC Press, 2017).

How is case depth measurement traceably documented for heat-treated parts?​

The documentation for a case depth measurement record should include: (1) identification of the specific part, including serial number, drawing number, and position of the tested section on the part (e.g., "crane wheel tread, centerline, radial section at 0° reference mark"); (2) the sectioning procedure — where the section was cut, the cutting method (abrasive saw, waterjet, or other), and any steps taken to prevent sectioning-induced heat damage to the hardened zone; (3) the preparation procedure — mounting compound, grinding sequence, polishing steps, and etchant used if any; (4) the hardness test equipment — machine model class (microhardness or macrohardness), indenter type (Vickers, Knoop, Rockwell C), load (in gf or kgf), and calibration status of the machine; (5) the traverse protocol — starting distance from the hardened surface, indent spacing, total number of indents, and any repeat indents or confirming measurements; (6) the raw data — each indent's depth-from-surface and measured hardness, ideally with the indent diagonal measurement (for Vickers) or dial reading (for Rockwell); (7) the derived case depth values — effective case depth to the specified cut-off, total case depth, and any other case-depth metrics required by the specification; (8) conformance statement — whether the measured values are within drawing tolerance. The traceability chain from the measured indent to the reported case depth is reviewable by the customer's incoming inspector or by a third-party auditor, and is particularly important for aerospace, defense, and critical-application work. UTEC Industrial includes case depth verification documentation with every induction-hardened part where case depth is a drawing acceptance criterion, tied to the part's other heat treatment documentation (cycle record, hardness verification on the hardened surface, chemistry certificate) into a single documentation package delivered with the part (ASTM E384; SAE J423; ASM Handbook, Vol. 4C, ASM International, 2014).

What are the non-destructive alternatives to cross-section case depth measurement?​

Cross-section measurement is destructive — the tested section is cut from the part, consumed in preparation, and not available for service. For production work where every part cannot be destructively tested, non-destructive case depth methods are used on a sampling basis or on finished parts. The principal non-destructive methods: (1) Eddy current testing, which measures electrical conductivity and magnetic permeability changes between the hardened case and the softer core — calibrated against cross-sectioned reference standards, eddy current can estimate case depth with ±10–15% accuracy on typical induction-hardened parts. (2) Ultrasonic backscatter, which measures the depth of the austenitized zone during induction hardening in real time and provides in-process case depth estimation — used mostly in high-volume production lines rather than in single-part custom work. (3) Barkhausen noise measurement, which responds to residual stress and microstructural differences in the hardened layer — useful for tempered-back detection and other process anomalies but less directly quantitative for case depth. (4) Surface-only hardness testing (portable Rockwell or Leeb on the hardened surface) combined with process-parameter confirmation — not strictly a case depth measurement, but verifies that the surface reached the specified hardness, which combined with the consistent process parameters from the induction control record gives high confidence in the case depth profile. For sampling-based destructive verification, one part from a production run is cross-sectioned and measured in full, with that result establishing the depth profile that the process produced; subsequent parts are verified by surface hardness testing and process-record review, under the assumption that consistent process parameters produce consistent case depth. For single-part custom work, where there is no production run to sample from, either a destructive verification is performed on a sacrificial companion piece, or the acceptance is based on surface hardness plus process-record review alone. The specific verification plan should be agreed between the heat treater and the customer before the job runs (ASTM E1444 — eddy current; ASM Handbook, Vol. 17: Nondestructive Evaluation and Quality Control, ASM International, 1989; Rudnev et al., Handbook of Induction Heating, 2nd ed., CRC Press, 2017).

How does specified case depth tolerance affect the induction hardening process design?​

Case depth is a direct output of the induction hardening process parameters: induction frequency, coil power, dwell time, and quench severity. Tighter case depth tolerance on a drawing drives tighter process parameter control in the shop. For a typical tolerance of ±0.050 inch on a 0.250-inch case, the induction parameters can be run at nominal settings with normal process variation, and the resulting cases will consistently meet tolerance. For a tight tolerance of ±0.015 inch on the same 0.250-inch case, the shop must control frequency and power more precisely, run tighter dwell-time control, and may need to add process monitoring (power-time recording, for example) to document that each part received the specified energy dose. The practical consequence: case depth tolerances much tighter than ±0.025 inch on cases of 0.125–0.500 inch depth drive the cost of induction hardening noticeably higher because they reduce the tolerable process variation and may require dedicated setups rather than shared coil-and-fixture arrangements. Before specifying a tight case depth tolerance, the designer should verify that the tolerance is actually required for the service function — in many applications, a looser tolerance (±0.050 inch or so) is functionally acceptable and significantly less costly. For crane wheels, industrial rollers, and similar components with conventional service-class hardness and case depth specifications, the tolerance typically runs ±0.050 inch on a 0.250–0.500 inch case. For precision aerospace and defense applications, tighter tolerances are normal and the cost structure reflects that. UTEC Industrial's induction hardening station is configured for the typical industrial service-class tolerances, and specifies any job with tighter tolerance requirements as a separate cost consideration (ASM Handbook, Vol. 4C, ASM International, 2014; Rudnev, V., et al., Handbook of Induction Heating, 2nd ed., CRC Press, 2017; SAE J423).

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References​

  • ASM International. (2014). ASM Handbook, Volume 4C: Induction Heating and Heat Treatment. ASM International.
  • ASM International. (2004). ASM Handbook, Volume 9: Metallography and Microstructures. ASM International.
  • ASM International. (1989). ASM Handbook, Volume 17: Nondestructive Evaluation and Quality Control. ASM International.
  • SAE J423: Methods of Measuring Case Depth. SAE International.
  • ASTM E384: Standard Test Method for Microindentation Hardness of Materials. ASTM International.
  • ASTM E18: Standard Test Methods for Rockwell Hardness of Metallic Materials. ASTM International.
  • ASTM E140: Standard Hardness Conversion Tables for Metals. ASTM International.
  • ASTM E1444: Standard Practice for Magnetic Particle Testing. ASTM International.
  • Rudnev, V., Loveless, D., and Cook, R.L. (2017). Handbook of Induction Heating (2nd ed.). CRC Press / Taylor & Francis.

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