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Specifying Heat Treatment on Engineering Drawings: What to Call Out and Why

A heat treatment specification on an engineering drawing is the contract between the design authority and the heat treater: it defines what process must be performed, what properties the finished part must have, and how those properties will be verified. 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 clear, complete specification produces the intended part; an ambiguous or incomplete specification produces either a clarification request that delays the job or a non-conforming part that has to be reworked. The most common heat treatment errors in industrial manufacturing trace back to drawing specifications that omit critical parameters — the steel grade, the required process, the hardness tolerance, the test location, or the minimum tempering temperature. This article covers what every heat treatment specification should contain, the typical error patterns, and the practical rules that produce unambiguous callouts.

What elements must every heat treatment specification include?​

A complete heat treatment specification on an engineering drawing contains six elements at minimum, regardless of the process. First, the steel grade or aluminum alloy designation, by its standard identifier (AISI, SAE, ASTM, or AMS) — not a generic category like "alloy steel" or "carbon steel." Second, the heat treatment process name using standard terminology: full anneal, normalize, quench-and-temper, induction harden, stress relieve, solution treat and age, and so on. Third, the target hardness range with both upper and lower limits, in the hardness scale that will be used for verification (HRC, HB, HV) — with specific range tolerances, not single values. Fourth, the test location on the part where hardness is measured, with a sketch when the location is not obvious (surface, core, rim, hub, tooth flank, case depth). Fifth, any additional process constraints needed to avoid errors — minimum tempering temperature for alloy steel to avoid the embrittlement range, maximum austenitizing temperature to avoid grain coarsening, specific quench medium when it matters. Sixth, the documentation and verification requirements — certification of process, hardness test report, material traceability if applicable. Beyond these six core elements, specifications for code-regulated work (aerospace per AMS, pressure vessels per ASME, structural per AWS) add the governing specification reference and any unique code-required parameters. A specification that includes all six elements produces an unambiguous, executable cycle; a specification that omits any of them creates the potential for a non-conforming outcome (ASM Handbook, Vol. 4A, ASM International, 2013; AMS 2759; ASME Section VIII Div 1).

How should material grade be specified?​

Material grade must be specified by its standard designation with the governing specification reference. Acceptable examples: "AISI 4140 per ASTM A29/A29M" — unambiguous, machinable, heat treatable per the grade's published parameters. "AISI 4340 per ASTM A322" — same specificity for the alloy steel bar used in large shafts. "AISI D2 per ASTM A681" — the tool steel specification with its associated heat treatment parameters. "6061-T6 per AMS-QQ-A-200/8" — aluminum structural plate to the aerospace specification. Unacceptable examples: "alloy steel" — dozens of grades qualify as alloy steel, and each has different heat treatment parameters. "Tool steel" — same problem on a larger scale. "4140 or equivalent" — this is ambiguous and invites substitution without the buyer's approval. "Hardened steel" — describes the condition after processing, not the starting material. The grade specification matters for heat treatment because the grade determines all of the cycle parameters: austenitizing temperature (1,475–1,525 °F for 4340, 1,550–1,600 °F for 4140 — different by 75 °F, enough to produce different results), quench medium selection (oil for 4340 in most sections, faster quench for 1045 in thin sections), tempering temperature curve (4340 runs 3–5 HRC higher than 4140 at equivalent temper — enough to matter for hardness targets), and achievable hardness range (tool steels reach 60+ HRC; carbon steels max out around 55 HRC). The heat treater cannot select the correct cycle without knowing the specific grade. When existing stock of a different grade than specified is to be substituted, the substitution must be approved in writing before processing begins (ASM Handbook, Vol. 4A, ASM International, 2013; ASTM A29; ASTM A681).

How should the heat treatment process be specified?​

The process must be named using the industry-standard term that corresponds to the intended metallurgical outcome. Common processes and their standard designations: "Anneal per [specification]" for full annealing — used when maximum softness and machinability are required. "Spheroidize anneal per [specification]" for the extended sub-critical cycle used on tool steels and high-alloy grades. "Normalize per [specification]" for the supercritical-soak, air-cool cycle that refines grain structure. "Stress relieve per [specification]" for the sub-critical cycle that reduces residual stress without microstructural change. "Quench and temper per [specification]" for through-hardening by austenitize, quench, and temper — the standard through-hardening cycle for alloy steel components. "Induction harden per [specification]" for surface hardening on specific features. "PWHT per ASME Section VIII Div 1 Table UCS-56" for code-required post-weld heat treatment of pressure vessels. "Solution treat and age per AMS 2770" for aluminum components requiring T6 temper. The process name is paired with either a specific procedure reference (AMS, ASM, internal procedure) or a hardness target that defines the intended outcome. Do not specify multiple processes ambiguously — "anneal and heat treat" does not tell the heat treater whether the anneal and the heat treatment are two separate operations or the same operation (they can't be; annealing is itself a heat treatment process). Specify each operation in its proper sequence with the intended outcome for each. For parts requiring multiple thermal operations (anneal before machining, then quench-and-temper after machining), specify both operations on the drawing with clear sequence indication — "1. Anneal to 179–197 HB before machining. 2. Quench and temper to 28–34 HRC after finish machining" — rather than collapsing them into a single ambiguous note (ASM Handbook, Vol. 4A, ASM International, 2013; Machinery's Handbook, 31st ed., Industrial Press, 2020).

How should hardness range and test location be specified?​

Hardness must be specified as a range (both minimum and maximum), in a specific scale, at a specific location on the part. Acceptable examples: "28–34 HRC at finished outside diameter surface" — specific scale, specific range, specific location. "321–401 HB core, measured at mid-length cross-section after finish grinding" — specific scale, specific range, specific location defined by a sketch reference. "58–62 HRC tread surface, measured at four equally-spaced positions around circumference" — specific scale, specific range, specific location defined by geometry plus measurement frequency. "50 HRC minimum at 0.100 inch effective case depth" — specifies both the case depth and the minimum hardness at that depth. Unacceptable examples: "hardened" — no quantitative specification. "approximately 30 HRC" — no upper limit, no tolerance. "32 HRC nominal" — single value without tolerance is not actionable; the heat treater cannot produce exactly 32 HRC every time, and some range must be specified. "surface hardness 58 Rc" — single value, no tolerance, ambiguous scale (HRC and Rc are both used, but the specification should be consistent). The hardness range must reflect what the process can actually produce on the specified material in the specified section size. A 4140 drawing calling out "45–50 HRC at the core of a 5-inch section" is not meetable — 4140 hardenability is inadequate for 45 HRC at that section depth; either the grade must be changed to 4340, or the hardness range must be reduced, or the location must be redefined (surface rather than core). The drawing designer should check achievability against the steel's hardenability data (Jominy curves, hardenability bands) before releasing the specification (ASM Handbook, Vol. 4A, ASM International, 2013; ASTM E18; ASTM E10; SAE J406).

What additional constraints should be called out to avoid common errors?​

Several constraints should be added explicitly to avoid the most frequent heat treatment errors. Minimum tempering temperature: for alloy steels (4140, 4340, 8640) in impact-loaded or cyclic service, specify "minimum tempering temperature 600 °F" (or 400 °F for high-hardness applications where the lower band is required) to keep the heat treater out of the 450–570 °F temper embrittlement range. A hardness callout alone does not prevent embrittlement — "32–36 HRC" can be achieved by tempering at 530 °F, which is in the embrittlement zone. Specifying the minimum temper temperature prevents this error. Maximum austenitizing temperature: for high-carbon or highly alloyed steels where grain coarsening is a concern, specify "austenitize at 1,550 °F maximum" or the equivalent. Quench medium: when the quench medium matters for distortion or cracking risk (alloy steels in complex shapes that must be oil-quenched, not water), specify "oil quench" or "polymer quench at 15% concentration." For 4140 and 4340 in most industrial work, oil quench is the default and need not be called out. For 1045 or other low-hardenability grades requiring water quench, the water quench specification is important. Case depth tolerance for induction-hardened parts: specify both the minimum effective case depth and the acceptable tolerance: "0.100–0.150 inch effective case depth, measured per SAE J423." Test method and instrument: specify "per ASTM E18" for Rockwell, "per ASTM E10" for Brinell, so there is no ambiguity about test procedure. These additional specifications do not burden the heat treater with unnecessary constraints — they prevent errors that occur when the defaults vary between heat treaters (ASM Handbook, Vol. 4A, ASM International, 2013; ASTM E18; ASTM E10; SAE J423).

How should induction hardening be specified?​

Induction hardening specifications add complexity because multiple parameters must be controlled simultaneously: surface hardness, case depth, core hardness, and hardened pattern geometry. A complete induction hardening specification includes: the hardened location (specific feature on the part — "crane wheel tread surface," "shaft bearing journal 2.500 inch diameter," "gear tooth flank and root"); the pattern width or coverage area (how far the hardened zone extends beyond the functional surface); the minimum surface hardness in HRC ("52 HRC minimum at surface"); the minimum effective case depth ("0.100 inch minimum effective case depth at 50 HRC"); the core hardness (typically the pre-induction condition — "core hardness as per prior heat treatment, typically 28–34 HRC on 4140 Q&T substrate"); any critical dimensions or tolerances that must not change during hardening (in practice, induction hardening produces minimal distortion, but specifying this is still useful for mission-critical parts); and the measurement method for case depth verification — typically destructive sectioning with microhardness traverse per SAE J423, performed on a witness coupon heat-treated with the production lot or on a first-article basis. The common specification error for induction hardening is calling out surface hardness alone without case depth — a 55 HRC surface reading could be achieved with a 0.020-inch case (too thin for most service applications) or a 0.250-inch case (correct), and the drawing does not distinguish between these outcomes. Always specify both surface hardness and case depth, with their tolerances (ASM Handbook, Vol. 4C, ASM International, 2014; SAE J423; AMS 2759/10).

What documentation and verification should the specification require?​

The specification should state what documentation the customer expects with the shipment. Typical requirements: "Heat treatment certification including furnace chart and hardness verification report" — the minimum for most industrial work. "ASME Section VIII PWHT documentation per applicable code paragraphs" — for code-regulated pressure vessel weldments. "Material traceability from mill heat number through heat treatment lot" — for high-reliability or regulated applications. "Hardness test report with minimum five readings across the specified surface, results in individual values (not pass/fail only)" — for applications where trend data matters. "Certificate of conformance to AMS 2759/1" — for aerospace heat treatment of carbon and low-alloy steel. "First-article inspection report for the first production part with case depth microhardness traverse" — for induction-hardened components where case geometry is mission-critical. Stating the documentation requirement on the drawing (or in the associated quality plan) ensures the heat treater provides it as part of the contracted work, rather than as a discretionary addition that the customer might or might not receive. UTEC Industrial applies a consistent documentation standard — cycle chart, hardness verification, cycle parameters, equipment identification — to all heat treatment work, so the customer receives complete documentation as standard practice regardless of whether the drawing specifies it (ASM Handbook, Vol. 4A, ASM International, 2013; ASME Section VIII Div 1, UW-40; AMS 2759).

What are the most common specification errors and how are they caught?​

Frequent specification errors fall into five categories. Category 1, missing parameters: grade not specified, hardness not ranged, test location ambiguous, documentation requirements not stated. Category 2, incompatible combinations: a hardness specified on a grade that cannot achieve it at the specified section size; a case depth specified without accompanying surface hardness; a through-hardness specified at a core depth the grade cannot reach. Category 3, embrittlement-zone risk: a hardness range that can be met by tempering in the 450–570 °F embrittlement range on alloy steel, with no minimum tempering temperature specified. Category 4, contradictory specifications: "anneal and heat treat" without sequence clarity; "harden to 55 HRC, then stress relieve at 1,100 °F" (the stress relief above the tempering temperature reduces hardness below the target); a hardness spec that requires Q&T combined with a distortion tolerance that requires minimum-quench-stress processing. Category 5, industry-standard-mismatch: using aerospace AMS references for industrial work (unnecessarily strict, expensive, and often adds lead time for Nadcap compliance); using industrial ASTM references for aerospace work (may not meet the aerospace quality requirement even if the metallurgical parameters are the same). The heat treater's order-entry review process catches most of these errors before processing begins — but catches them as clarification requests that delay the job. Higher-quality drawings go through processing without clarification and deliver faster. UTEC Industrial's intake process reviews every drawing for these patterns at order entry, and clarifications are requested before the cycle is scheduled — preventing re-work at the cost of a brief delay rather than absorbing the cost of re-processing a non-conforming part (ASM Handbook, Vol. 4A, ASM International, 2013; Machinery's Handbook, 31st ed., Industrial Press, 2020).

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References​

  • ASM International. (2013). ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes. ASM International.
  • ASM International. (2014). ASM Handbook, Volume 4C: Induction Heating and Heat Treatment. ASM International.
  • Machinery's Handbook (31st ed.). (2020). Industrial Press.
  • AMS 2759: Heat Treatment of Steel Parts, General Requirements. SAE Aerospace.
  • AMS 2770: Heat Treatment of Wrought Aluminum Alloy Parts. SAE Aerospace.
  • ASME Boiler and Pressure Vessel Code, Section VIII Division 1 (current edition). American Society of Mechanical Engineers.
  • ASTM A29: Standard Specification for General Requirements for Steel Bars, Carbon and Alloy, Hot-Wrought. ASTM International.
  • ASTM A681: Standard Specification for Tool Steels Alloy. ASTM International.
  • ASTM E18: Standard Test Methods for Rockwell Hardness of Metallic Materials. ASTM International.
  • ASTM E10: Standard Test Method for Brinell Hardness of Metallic Materials. ASTM International.
  • SAE J406: Methods of Determining Hardenability of Steels. SAE International.
  • SAE J423: Methods of Measuring Case Depth. SAE International.

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