Specifying heat treatment correctly means defining the end result you need, not dictating the exact process a heat treater should follow. A good specification calls out the material, the desired final condition (usually a hardness range), and the applicable standard, then leaves the heat treater room to achieve those results. Getting this right on your engineering drawing or purchase order prevents costly rework and ensures every part meets its design intent.
Specify the End Condition, Not the Process
The most common mistake in heat treatment specifications is spelling out every step of the thermal cycle instead of stating what the finished part needs to look like. The Precision Machined Products Association recommends noting the end condition desired rather than the process. This way both the heat treater and the part user can check to the same standard. Spelling out the complete process will not guarantee the required end condition, because furnace characteristics, part geometry, and lot size all affect how a given recipe performs in practice.
That said, you still need to reference the process type. The goal is to pair a process name with a measurable outcome. NASA’s process specification for steel alloys illustrates this well with drawing notes like:
- Carbon and low alloy steels: “QUENCH AND TEMPER TO 160–180 KSI PER NASA/JSC PRC-2001”
- Martensitic stainless steels: “QUENCH AND TEMPER AT 450°F PER NASA/JSC PRC-2001”
- Austenitic stainless steels: “ANNEAL PER NASA/JSC PRC-2001” or “AGE HARDEN TO CONDITION H1025 PER NASA/JSC PRC-2001”
Each note names the process, states the target property or condition, and references a governing specification. That three-part formula works for nearly any application.
Essential Elements of a Heat Treatment Callout
Whether you’re writing a drawing note or a line item on a purchase order, your specification should cover these elements:
- Material designation: List the alloy grade in the heat treat condition in which it will be procured. Include the procurement specification alongside the material callout so the heat treater knows the starting point.
- Process type: Name the specific treatment or combination of treatments: normalizing, annealing, quench and temper, carburizing, nitriding, solution treat and age, etc.
- Final condition or temper: This is a hardness range, a tensile strength range, or a named condition (like H1025 for precipitation-hardened stainless). It gives the heat treater a measurable target.
- Governing specification: Reference the industry, company, or agency standard that defines acceptable practices, such as an AMS spec, a company process spec, or ASTM A1040 for harmonized steel grade compositions.
If your part requires surface hardening, you also need to specify case depth and surface hardness separately from core hardness. More on that below.
How to Write Hardness Requirements
Hardness is the most common measurable property called out in a heat treatment specification, and there are specific rules for how to express it. Per ASTM E18, Rockwell hardness values should never be written as a bare number. You must include the scale symbol so anyone reading the drawing knows which indenter and force were used. The correct format places the hardness number first, followed by “HR” and the scale letter.
For example: 64 HRC means a Rockwell C hardness of 64. If you’re using the B scale with a tungsten carbide ball, it would read 72 HRBW. Superficial scales follow the same pattern: 81 HR30N.
When setting your hardness range, keep in mind the inherent variability of the testing equipment itself. For Rockwell C scale readings below 35 HRC, testing machines are allowed a repeatability spread of up to 2.0 HR units and a maximum error of plus or minus 1.0 HR unit. At 35 HRC and above, repeatability tightens to 1.5 units. Above 60 HRC, the allowed repeatability is just 1.0 unit with plus or minus 0.5 error. Specifying a range narrower than these machine tolerances sets your heat treater up for failure, so build your acceptance window accordingly. A range of 58–62 HRC is reasonable. A range of 60–61 HRC is not.
Specifying Case Depth for Surface Hardening
If your part is carburized, carbonitrided, or nitrided, you need to specify how deep the hardened layer should extend. There are two ways to express this, and confusing them causes real problems.
Total case depth is the full distance that carbon or nitrogen has diffused inward from the surface. Under a microscope, it’s measured as the distance from the surface to the point where you can no longer distinguish the outer case from the inner core. Effective case depth is something more specific and more useful: the distance from the surface to a defined hardness level. For most applications, effective case depth is measured to the point where hardness drops to 50 HRC (sometimes 52 HRC).
Effective case depth is the better callout for most engineering applications because it ties directly to mechanical performance. A part with 0.040 inches of effective case depth at 50 HRC tells the heat treater exactly what to achieve and gives quality inspectors a clear pass/fail criterion. If you specify total case depth instead, you’re relying on a metallographer’s visual judgment, which introduces more subjectivity. Use effective case depth when you can, and always state the reference hardness.
Thermal Cycle Parameters Worth Specifying
In most cases, referencing a process specification and a final hardness range is enough. The heat treater picks the soak times, temperatures, and cooling media that work for your part geometry and their equipment. But some applications, particularly aerospace and defense work, require tighter control over the thermal cycle itself.
The key parameters in a thermal cycle are the austenitizing (or solution treating) temperature, the soak time at temperature, the cooling method, and for tempered parts, the tempering temperature and its soak time. To illustrate how much these can vary: in one study on a medium-carbon alloy steel, parts were austenitized at 860°C, soaked for 28 minutes (calculated based on carbon content, part shape, and thickness), then oil quenched. Tempering was then performed at temperatures ranging from 250°C to 650°C, with soak times from 120 to 240 minutes, followed by air cooling. Higher tempering temperatures and longer soak times produced lower hardness but greater toughness.
If you need to control these parameters directly, specify them as ranges rather than single values. For example, “temper at 900°F to 950°F for a minimum of 2 hours” gives the heat treater enough room to account for furnace calibration and load size while keeping the process within your engineering intent. Always specify the cooling medium (oil, water, polymer, air, furnace cool) when it matters to the outcome, because different quench media produce dramatically different cooling rates and therefore different final properties.
What to Require in Certification Documents
Your specification should state what documentation you expect back from the heat treater. At minimum, a Certificate of Conformance should confirm that the parts were processed per the referenced specification, list the actual hardness readings, and identify the specific furnace load or batch. For higher-criticality parts, you may want to require time-and-temperature data from the furnace recorder, showing the actual thermal profile the parts experienced.
For case-hardened parts, the certification should include effective case depth measurements from a sample or test coupon. If your specification calls out specific mechanical properties like tensile strength, the cert should include test results from specimens processed alongside the production parts.
Requiring a diagram of sensor locations and calibration records for the furnace’s temperature instruments is standard practice in regulated industries. Calibration records should note the correction factor for each sensor, defined as the greatest deviation observed during calibration. This gives you traceability if a property dispute arises later.
Putting It All Together on a Drawing
A complete heat treatment callout on an engineering drawing typically looks like a note in the title block area or the notes section. It combines all the elements discussed above into a concise statement. For a through-hardened steel shaft, the note might read: “HEAT TREAT: QUENCH AND TEMPER TO 28–32 HRC PER AMS 2759.” For a carburized gear, it might read: “CARBURIZE AND HARDEN PER AMS 2759/7. SURFACE HARDNESS 58–62 HRC. EFFECTIVE CASE DEPTH 0.030–0.045 IN. AT 50 HRC. CORE HARDNESS 25–40 HRC.”
Notice that both examples reference a standard, give hardness as a range with proper notation, and for the surface-hardened part, define both case depth and core hardness. The material callout elsewhere on the drawing tells the heat treater what alloy they’re working with and its starting condition. Together, these elements give the heat treater everything needed to plan the process and give your quality team everything needed to verify it.