Skip to main content

Microhardness Indentation for Case Depth Profiling (ASTM E384 Procedure)

Microhardness indentation per ASTM E384 is the authoritative method for measuring case depth profiles when the hardened zone is too shallow, or the hardness gradient too steep, for Rockwell C macrohardness to resolve reliably. 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. This article covers the procedural essentials — Vickers and Knoop indenter selection, load choice in the 100 to 500 gf range, indent spacing governed by the 2.5× diagonal rule, traverse protocol from surface through core, and interpolation of the effective case depth at a 50 HRC converted cut-off per ASTM E140. The method underlies every case profile documented on induction-hardened or carburized steel parts where drawing tolerance demands more resolution than a macrohardness traverse can provide.

Why use microhardness rather than Rockwell C for case profile measurement?​

Rockwell C uses a 150 kgf major load with a 120° diamond cone indenter, producing an indentation roughly 0.012 to 0.020 in. (0.30 to 0.50 mm) across on hardened steel and requiring a minimum 3× diagonal spacing of about 0.040 to 0.060 in. between indents per ASTM E18. On a shallow case — for example, a 0.040 in. induction-hardened layer — that spacing permits at most one or two indents inside the case before the traverse crosses into the transition zone, which is not enough resolution to define the hardness gradient. Microhardness methods per ASTM E384 use loads in the 10 gf to 1 kgf range, producing indentations measured in tens of microns rather than hundreds of microns, and permit indent spacing on the order of 0.002 to 0.005 in. (0.05 to 0.13 mm). This fine spacing resolves the full curve from surface to core in 10 to 30 indentations on a typical case, which is what makes meaningful interpolation of the effective case depth possible. For deep cases exceeding 0.125 in. (3 mm), Rockwell C can give acceptable resolution with wider indent spacing, and is faster to run; for shallow cases and for any case where the transition zone is narrow, microhardness is the default method and is specified by name in most drawings that call out tight case depth tolerance (ASTM E384; ASTM E18; ASM Handbook, Vol. 4C, ASM International, 2014).

What indenter geometries and load ranges does ASTM E384 cover?​

ASTM E384 covers two indenter geometries: the Vickers diamond pyramid (HV), which is a square-based pyramid with 136° face angles, and the Knoop diamond (HK), which is an elongated four-sided pyramid with a long-to-short diagonal ratio of approximately 7:1. The Vickers indentation is geometrically similar across loads and hardness values, which makes Vickers readings comparable to one another and convertible to Rockwell via ASTM E140. The Knoop indentation's elongated shape reduces its depth-to-diagonal ratio by roughly a factor of three relative to Vickers at equal load, which means Knoop indentations do not penetrate as deeply for a given diagonal length — useful when the layer under test is thin and the Vickers depth-to-layer-thickness ratio would violate the 10× rule (the tested layer must be at least 10 times the indent depth, per ASTM E384 and metallographic practice). Standard loads for case depth profiling are 100 gf (HV0.1 or HK0.1), 300 gf (HV0.3), 500 gf (HV0.5), and 1,000 gf (HV1). HV0.5 is a common default for induction-hardened steel in the 50 to 60 HRC range, producing a diagonal around 20 to 25 microns that is large enough to read precisely with a calibrated optical system and small enough to permit 0.003 in. indent spacing. HV1 is used when the specification requires direct comparison to ISO 2639 CHD at 550 HV1, which explicitly cites the 1 kgf load. Load selection for a given job balances indent visibility, required spacing resolution, and the governing standard's load call-out (ASTM E384; ISO 2639; ASTM E140).

What indent spacing is required to avoid strain-field interference?​

ASTM E384 specifies that the center-to-center distance between adjacent indentations must be at least 2.5 times the diagonal of the larger of the two indentations, and the distance from an indent center to an edge or a previous indent must be at least 2.5 times the diagonal. This rule exists because each indentation produces a plastic deformation zone extending several indent diagonals into the surrounding material, and a subsequent indent falling inside that zone reads a work-hardened value higher than the true material hardness. On a Vickers HV0.5 indent with a 22 micron diagonal in 55 HRC steel, the 2.5× rule places consecutive indent centers at a minimum 55 microns apart — roughly 0.002 in. — which sets the practical upper limit on traverse resolution. For Knoop indentations, the 2.5× rule is applied to the short diagonal rather than the long diagonal, because the plastic zone around a Knoop indent is not symmetric and the short diagonal governs the interference distance. Staggering indents laterally across the traverse line (offsetting alternate indents by one indent diagonal perpendicular to the traverse direction) does not relax the spacing rule because the plastic zone is approximately spherical, but staggering does allow the traverse to pack indents more densely along the surface-to-core axis while maintaining the 2.5× center-to-center distance in three-dimensional space. Violations of the spacing rule show up in the data as an upward drift in measured hardness as the traverse proceeds — an artifact rather than a real gradient — and must be identified and discarded during data review (ASTM E384; ASM Handbook, Vol. 8: Mechanical Testing and Evaluation, ASM International, 2000).

What is the traverse protocol from surface to core?​

The traverse begins at a fixed distance from the hardened surface — typically 0.003 to 0.005 in. (0.08 to 0.13 mm) in from the edge of the mounted cross-section, to avoid edge-rounding artifacts from specimen preparation per ASTM E3. Indent spacing is close near the surface where the case hardness plateau is being established (typically 0.003 to 0.005 in.), continues at moderate spacing through the transition zone where the gradient is steepest (typically 0.005 in.), and widens beyond the expected total case depth where the curve approaches core hardness (0.010 to 0.020 in.). For a typical induction-hardened 4140 crane wheel with 0.25 in. case depth, this produces on the order of 30 to 50 indentations spanning from the tread surface to roughly 0.375 in. below — enough to define surface hardness plateau, transition zone shape, and core hardness clearly. For a carburized 8620 part with 0.050 in. effective case, the full traverse might run 20 to 30 indentations spanning 0.100 in. of depth. Each indent is located precisely by the hardness machine's stage coordinates, and its depth from the hardened surface is recorded alongside the hardness reading. The data is tabulated with depth, load, diagonal dimensions (both for Vickers, or long and short for Knoop), and calculated hardness, then plotted as hardness versus depth. The curve is examined for the expected shape — plateau at case hardness, monotonic drop through the transition, plateau at core hardness — and for any anomalies suggesting process issues or measurement artifacts (ASTM E384; ASTM E3; SAE J423; ASM Handbook, Vol. 4C, ASM International, 2014).

How is the effective case depth interpolated from the hardness versus depth curve?​

The effective case depth is the depth at which the traverse curve crosses the specified hardness cut-off — typically 50 HRC for North American induction-hardened work, which converts to approximately 513 HV per ASTM E140. Since the traverse indents fall at discrete depths and the hardness values are discrete Vickers readings, the cut-off depth must be interpolated between adjacent data points. The standard approach is linear interpolation: identify the two consecutive indents bracketing the cut-off hardness, then solve for the depth at which a straight line between them reaches the cut-off value. If, for example, the indent at 0.048 in. reads 528 HV and the indent at 0.053 in. reads 498 HV, the 513 HV cut-off falls at 0.048 + (528−513)/(528−498) × (0.053−0.048) = 0.048 + 0.0025 = 0.0505 in. More sophisticated curve-fitting — polynomial or sigmoid functions fitted to the full traverse — can give smoother interpolation but is not required by SAE J423 or ASTM E384 and may over-fit measurement noise; linear interpolation between the two bracketing indents is the standard method. The reported ECD is then stated with the cut-off used, the measurement load, and the interpolation approach — for example, "ECD 0.050 in. at 50 HRC (513 HV0.5 converted per ASTM E140), linear interpolation between adjacent indents." The total case depth is read from the same curve at the depth where successive indents first match core hardness within measurement scatter (ASTM E384; ASTM E140; SAE J423).

What typical case profile shapes emerge from different processes?​

A correctly executed induction hardening cycle on medium-carbon alloy steel produces a profile with a high, flat surface hardness plateau, a steep linear transition zone, and a flat core hardness plateau. For 4140 induction-hardened to 50 to 58 HRC surface with a 0.25 in. case depth and a prior quench-and-tempered core at 28 to 32 HRC, the plateau-transition-plateau structure is distinct and the ECD reads cleanly from the curve. Carburized 8620 produces a different characteristic shape: surface hardness in the 58 to 62 HRC range, a relatively shallow effective case (0.030 to 0.060 in.) depending on carburizing time, a longer and more gradual transition as carbon diffuses into the core, and a softer core of 30 to 40 HRC reflecting the lower carbon content of the base metal. Nitrided parts show a sharp, thin surface spike — often 700 HV or higher over a 0.005 to 0.020 in. layer — with a rapid drop to core hardness; the nitrided profile shape is different enough from carburized and induction-hardened profiles that ISO 3754 defines the nitriding hardness depth relative to core-plus-50-HV rather than an absolute cut-off. Deviations from the expected shape — a hardness dip below the surface plateau, an inflection in the transition zone, or a core hardness above the pre-processing baseline — are diagnostic of process issues (tempering-back, incomplete austenitization, inadvertent through-hardening) and should be noted in the case depth report. UTEC Industrial's crane-wheel induction-hardening work routinely produces case profiles documented by this microhardness method, with the traverse curve and the interpolated ECD included in the heat treatment documentation that ships with each wheel (ASTM E384; ASM Handbook, Vol. 4C, ASM International, 2014; SAE J423; ISO 3754).

What documentation records should accompany a microhardness case depth report?​

A complete microhardness case depth report includes: (1) part identification (serial number, drawing number, lot) and the location of the tested cross-section on the part; (2) specimen preparation record per ASTM E3 — mounting compound, grinding sequence, polishing steps, etchant if any; (3) hardness machine identification and calibration status, including the date of the last calibration and traceability of the calibration reference blocks; (4) indenter type (Vickers or Knoop), load, and dwell time used for each indent; (5) raw tabulated data — indent number, depth from surface, diagonal measurements (both diagonals for Vickers, long and short for Knoop), and calculated hardness value; (6) the plotted hardness-versus-depth curve; (7) the interpolated effective case depth at the specified cut-off, the total case depth, and any other derived metrics required by the drawing; (8) the conversion pathway if a Rockwell-defined cut-off (50 HRC) is being compared to a Vickers-measured curve via ASTM E140; (9) the conformance statement against the drawing tolerance. This documentation supports third-party audit, incoming inspection by the customer, and retained file review under quality-system retention schedules. For case-depth-controlled parts used in crane wheels, mill rollers, shafts, and similar industrial applications, the report is typically bundled with the rest of the heat treatment documentation package — cycle chart, surface hardness verification, chemistry certificate — and shipped with the part (ASTM E384; ASTM E3; ASTM E140; SAE J423; ASM Handbook, Vol. 4C, ASM International, 2014).

Related Articles

References​

  • ASM International. (2014). ASM Handbook, Volume 4C: Induction Heating and Heat Treatment. ASM International.
  • ASM International. (2000). ASM Handbook, Volume 8: Mechanical Testing and Evaluation. ASM International.
  • SAE J423: Methods of Measuring Case Depth. SAE International.
  • ISO 2639: Steels — Determination and verification of the depth of carburized and hardened cases. International Organization for Standardization.
  • ISO 3754: Steel — Determination of effective depth of hardening after flame or induction hardening. International Organization for Standardization.
  • ASTM E3: Standard Guide for Preparation of Metallographic Specimens. 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 E384: Standard Test Method for Microindentation Hardness of Materials. ASTM International.

Need In-House Heat Treating for Heavy Industrial Parts?​

UTEC Industrial operates a 6' × 10' × 17' car-bottom furnace (1,800 °F, 50-ton capacity), in-house induction hardening with per-part hardness verification, and automated vibratory stress relief at our Spokane, WA facility. Weldment stress relief, annealing, quench and temper, and induction hardening — all under one roof, with full documentation on every job.

Request a Quote →

Questions? Call (509) 922-1832 or email sales@utec.co