How to specify heat treatment on a drawing so it comes back right

A vague heat treat note causes more rework than almost any other drawing callout. “Harden” on its own does not tell anyone what you need. Here is how to write a callout that a heat treater can quote, run and certify.

The four parts of a good callout

1. Material and starting condition. For example “4140 steel, annealed”.
2. The result. Hardness range, case depth or temper.
3. Where to test. Surface, core, a specific face or a test coupon.
4. Any governing spec. Customer or industry specs, including certification needs.

Examples

Need	Example callout
Through hardened tool steel	D2, harden and temper to 58 to 62 HRC, test on face A
Carburized gear	8620, carburize, harden and temper, effective case depth 0.025 to 0.035 in at 50 HRC, surface 58 to 62 HRC
Induction hardened shaft	1045, induction harden journal areas only as shown, 50 to 55 HRC, case 0.060 to 0.100 in
Nitrided part	4140 prehardened, gas nitride, case depth and surface hardness per spec
Aluminum	6061, solution treat and artificially age to T6

Adjust values to your design and material spec. The examples show the structure, not recommendations for your part.

Hardness: always a range

Write a minimum and maximum. Heat treatment has natural variation, and inspection needs a band to accept parts against. Use the hardness scale that fits the material and case: Rockwell C for hardened steels, Rockwell B or Brinell for softer conditions.

Case depth: say how it is measured

“Case depth 0.030 in” can mean effective case depth to a hardness value or total case depth. Say which, and the hardness value that defines it.

Mark areas that must not be hardened

Threads, areas that will be machined later and features that must stay soft need masking or selective hardening. Show them on the drawing.

Think about sequence

• Machine before or after heat treatment? Hard parts are slower and costlier to machine.
• Leave grind stock on surfaces that will move or need a precise finish after hardening.
• Stress relieve before final machining on parts that must stay flat.

Certifications

If you need hardness certificates, pyrometry compliance or an accredited heat treater, put it on the purchase order and the drawing. Adding it after parts are run usually means running them again.

Common mistakes

• “Harden” with no hardness value
• Hardness with no test location
• No mention of masking for threads
• Unrealistic distortion limits for the process chosen
• Material grade missing or wrong