Skip to main content

Effective Case Depth vs. Total Case Depth: Definitions and Specification

Effective case depth (ECD) and total case depth (TCD) are two different numbers that describe the same hardness gradient, and confusing them on a drawing or purchase order routinely produces parts that either fail incoming inspection or were over-processed at unnecessary cost. 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 defines both terms per SAE J423 and the relevant ISO standards, shows drawing callouts that avoid ambiguity, and summarizes the measurement standards that bound the reported values. Every case-depth-controlled drawing should specify which definition applies and at what hardness cut-off — anything less leaves the heat treater guessing what "0.080 in. case depth minimum" means.

What is the formal definition of effective case depth?​

Effective case depth (ECD, sometimes labeled CHD for carburized parts in ISO usage) is the perpendicular distance from the hardened surface to the point in the cross-section where hardness first drops to a specified minimum value. Per SAE J423, the default hardness cut-off for induction-hardened and carburized steel in North American practice is 50 HRC, with the Vickers equivalent commonly stated as 513 HV per the conversions in ASTM E140. ISO 2639 defines the carburized "case hardening depth" CHD at a 550 HV1 cut-off (equivalent to roughly 52 HRC), and ISO 3754 defines the nitrided "nitriding hardness depth" NHD relative to the core hardness plus 50 HV. ISO 18203 covers surface-hardened components more broadly, including induction- and flame-hardened parts, and defines the "surface hardening depth" SHD at a specified HV value chosen to reflect the part's design intent. The value on a drawing is always a depth coordinate from the surface inward — never a hardness or time — and must be paired with the hardness cut-off that defines it. Specifying "0.060 in. ECD" without stating the hardness at which the depth is measured is ambiguous and must be rejected by the heat treater or corrected by the designer before the cycle is run (SAE J423; ISO 2639; ISO 3754; ISO 18203; ASTM E140).

What is the formal definition of total case depth, and how does it differ?​

Total case depth (TCD) is the perpendicular distance from the hardened surface to the first hardness traverse point that is indistinguishable from core hardness — in practice, the depth at which the hardness gradient has levelled off and subsequent indentations produce readings within the core hardness scatter band. SAE J423 describes total case depth as the depth beyond which no further hardness change is observed as the traverse proceeds inward. Because the transition zone between the fully hardened case and the core spans a finite depth — typically 0.010 to 0.050 in. (0.25 to 1.3 mm) on induction-hardened steel, and 0.015 to 0.030 in. on carburized 8620 — the TCD is always greater than the ECD on the same part, commonly by a ratio of 1.3 to 2.0 depending on the steepness of the gradient. For a carburized 8620 part with ECD 0.050 in. at 50 HRC, the TCD might read 0.080 to 0.100 in. The two numbers describe different features of the same curve: ECD reports where the load-bearing hardness ends, and TCD reports where the carbon or austenitizing effect of the process has stopped influencing the microstructure. A drawing calling out only "case depth 0.080 in. min." cannot be measured objectively unless the definition is specified, because the heat treater cannot know whether the designer needs a 0.080 in. ECD (a deeper case) or 0.080 in. TCD (a shallower effective depth) (SAE J423; ISO 2639; ASM Handbook, Vol. 4C, ASM International, 2014).

Why does the distinction matter for drawing callouts?​

The practical consequence of confusing ECD and TCD on a drawing is a two-to-one mismatch between what the heat treater produces and what the receiving inspector verifies. Consider a drawing callout "case depth 0.080 in. minimum" on an induction-hardened shaft. If the heat treater reads this as total case depth and runs a cycle that produces a 0.080 in. TCD, the corresponding ECD at 50 HRC would be on the order of 0.050 to 0.060 in. — possibly below what the designer intended for load-bearing capacity. If instead the heat treater reads this as ECD at 50 HRC and runs the longer cycle needed to place the 50 HRC point at 0.080 in. depth, the TCD will extend to 0.120 to 0.150 in., consuming more energy and more material transformation than necessary, and potentially increasing distortion on thin-section parts. The unambiguous drawing callout format pairs the definition, the value, and the hardness cut-off: "ECD 0.050 min at 50 HRC per SAE J423" or "CHD 1.3 mm min at 550 HV1 per ISO 2639." The tolerance band should also be stated — for example, "ECD 0.050 to 0.080 at 50 HRC" — so the heat treater knows both the minimum and the upper limit. For heavy-industrial induction-hardening work, UTEC Industrial works with customers to convert legacy "case depth 0.125 in." callouts into unambiguous ECD-plus-cut-off language at the quote stage, which avoids dispute on incoming inspection and documents the agreed acceptance criterion in the heat treatment record (SAE J423; ISO 2639; ASM Handbook, Vol. 4C, ASM International, 2014).

What hardness cut-offs are standard for different process and material combinations?​

The 50 HRC cut-off is the North American default for induction-hardened and through-hardened steel, codified in SAE J423 and carried into most commercial drawings for shafts, rollers, pins, and crane wheels in medium-carbon alloy steels such as 4140, 4340, and 1045. The 550 HV cut-off specified in ISO 2639 is standard for carburized case-hardening steels internationally and is applied in much of the automotive and gear industry; 550 HV converts to approximately 52 HRC per ASTM E140. AGMA gear standards allow the specifier to select a cut-off appropriate to the gear's loading — often 50 HRC for industrial service gears and higher values for aerospace gears. Nitrided parts use a different convention: ISO 3754 defines the nitriding hardness depth NHD at a cut-off of core hardness plus 50 HV, because nitrided surfaces produce a different hardness profile shape (a thinner, more sharply defined case) and a fixed HV cut-off is not meaningful across the range of nitriding base materials (4140, Nitralloy, tool steels, stainless). For decarburized surfaces — the inverse problem — ASTM E1077 uses an optical metallographic method rather than a hardness traverse, recognizing that the depth being measured is defined by microstructural appearance more than by hardness numerics. The drawing should always state the applicable standard and cut-off together: "ECD at 50 HRC per SAE J423," "CHD at 550 HV1 per ISO 2639," or "NHD per ISO 3754" are all unambiguous (SAE J423; ISO 2639; ISO 3754; ISO 18203; ASTM E140; ASTM E1077).

How is the hardness cut-off converted between Rockwell and Vickers scales?​

The 50 HRC cut-off and the 513 HV cut-off are not independently specified values; they are conversions of each other via the ASTM E140 hardness conversion tables, which map between Rockwell, Brinell, Vickers, and Knoop scales for steel and nonferrous alloys. For the common case-depth hardness region, the conversions run approximately: 50 HRC = 513 HV = 481 HB (10 mm ball, 3,000 kgf), 52 HRC = 544 HV, 55 HRC = 595 HV, and 58 HRC = 653 HV. The ISO 2639 cut-off of 550 HV corresponds to approximately 52 HRC, which is modestly higher than the SAE J423 50 HRC cut-off and produces a slightly shallower reported CHD than an ECD measured at 50 HRC on the same part. When microhardness testing is used to measure the gradient — the normal practice for shallow cases per ASTM E384 — the raw data is Vickers HV, and the ECD to 50 HRC is obtained by interpolating the traverse curve to the Vickers value that converts to 50 HRC per ASTM E140. The conversion introduces a small uncertainty (on the order of ±1 HRC in the 50 HRC region) that the measurement protocol should acknowledge; for parts specified with tight case depth tolerance, the spec should state whether the cut-off is a Rockwell value converted to Vickers for microhardness measurement, or a direct Vickers value used without conversion. SAE J423 permits either approach as long as the measurement is documented (SAE J423; ASTM E140; ASTM E384).

Which measurement standards bound the reported case depth values?​

Three ASTM and SAE standards together bound the measurement of case depth in commercial practice. SAE J423 defines what is being measured — ECD, TCD, hardness cut-offs, and the traverse concept — and is the primary reference cited on most North American drawings for case-hardened steel. ASTM E384 defines how microhardness indentations are made: Vickers or Knoop geometry, load selection, indent spacing, and diagonal measurement procedure. For case depth traverses with indent spacing below roughly 0.010 in., ASTM E384 is the governing method; for broader traverses where Rockwell C macrohardness is used, ASTM E18 applies. ASTM E140 provides the conversion tables that let Vickers microhardness data be reported as an ECD at a Rockwell-defined cut-off. ASTM E1077 describes the optical metallographic method for estimating decarburization depth; it is analogous in principle to case depth measurement (a depth coordinate from the surface to a feature in the gradient) but uses microstructural appearance rather than hardness as the feature. ISO 2639, ISO 3754, and ISO 18203 are the international equivalents used on European-specification drawings and on parts exported to international customers. Specifying a case depth on a drawing should cite at minimum the depth definition standard (SAE J423 or the relevant ISO number) and the measurement method (ASTM E384 or ASTM E18), so that the heat treater and the customer's inspector are testing to the same procedure. Metallographic specimen preparation per ASTM E3 — mounting, grinding, and polishing the cross-section — is the implicit prerequisite for any of these hardness traverse methods (SAE J423; ASTM E384; ASTM E18; ASTM E140; ASTM E1077; ASTM E3; ISO 2639; ISO 3754; ISO 18203).

What should a complete, unambiguous case depth drawing callout look like?​

A complete case depth specification on a drawing includes five elements: (1) which case depth is being specified — ECD, TCD, CHD, NHD, or SHD; (2) the minimum value and, if applicable, the maximum value (tolerance band); (3) the hardness cut-off that defines the depth — 50 HRC, 550 HV1, core plus 50 HV, or similar; (4) the measurement standard that governs the procedure — SAE J423, ISO 2639, or equivalent; (5) the location on the part where the case depth applies — especially for parts with multiple hardened regions or non-uniform sections. An example complete callout for an induction-hardened shaft: "ECD 0.050 to 0.080 in. at 50 HRC per SAE J423, measured on a transverse section at the midpoint of the hardened band." An example for a carburized gear tooth: "CHD 1.0 to 1.3 mm at 550 HV1 per ISO 2639, measured on a section through the tooth flank at mid-height." Incomplete callouts that heat treaters see routinely — "case depth 0.080 min," "0.25 inch hardened layer," "hardened to depth as required" — must be clarified before the cycle is designed; running a job on an ambiguous spec exposes both the heat treater and the customer to rework and inspection dispute. Documenting the agreed interpretation in the purchase order and on the heat treatment record closes the loop and prevents the same ambiguity from surfacing on the next lot (SAE J423; ISO 2639; ASM Handbook, Vol. 4C, ASM International, 2014).

Related Articles

References​

  • ASM International. (2014). ASM Handbook, Volume 4C: Induction Heating and Heat Treatment. 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.
  • ISO 18203: Steel — Determination of the thickness of surface-hardened layers. 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.
  • ASTM E1077: Standard Test Methods for Estimating the Depth of Decarburization of Steel Specimens. 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