Heat treat distortion: stress relief, stock allowance and datum strategy

Nobody is surprised that a part moves during heat treatment. What surprises people is how much of the final cost is determined by decisions made before it goes in the furnace.

Three of them: how much stock you leave, whether you stress relieve, and which surfaces you plan to re-locate from afterwards.

Where the movement comes from

Two mechanisms, and they are worth separating because the fixes differ.

Volume change from phase transformation. Austenite becomes martensite, and martensite occupies more volume. Every part grows. For a typical tool steel this is in the region of 0.1-0.2% linear, modified by the amount of retained austenite and by tempering. It is broadly uniform and broadly predictable — you can plan for it.

Distortion from thermal gradients and residual stress. The part heats and cools unevenly, different sections transform at different times, and whatever residual stress was locked in from rolling, forging or rough machining releases. This is not uniform and not predictable in detail. It is the one that warps a plate into a potato chip.

The second is what stress relief attacks.

Stress relief, and why rough machining is the trigger

A rolled or forged block arrives with internal stress. Rough machining removes material asymmetrically, which unbalances that stress, and the block moves — sometimes immediately, sometimes in the furnace.

The standard sequence for a through-hardened part:

1. Rough machine, leaving generous stock — typically 2-3 mm per side on a large block.
2. Stress relieve. Heat to 600-675 °C, hold roughly one hour per 25 mm of section, slow cool. The part moves here, in a controlled way, where it does no harm.
3. Semi-finish machine, cleaning up whatever moved and leaving the real heat treat allowance.
4. Harden and temper.
5. Finish machine or grind.

Skipping step 2 on a large or asymmetric part is the single most common cause of a block that comes back from the furnace unrecoverable.

For parts that are already hardened and being finish-ground, a low-temperature stress relief after rough grinding — typically 25-55 °C below the tempering temperature — takes out grinding stress without touching the hardness.

How much stock to leave

This is the number shops most want on your drawing, and the one most often missing.

Rough guidance for the post-heat-treat allowance:

Part type	Allowance per side
Small, simple, air hardening (A2, D2)	0.1-0.2 mm
Medium block, air hardening	0.2-0.4 mm
Large plate (>300 mm), air hardening	0.4-0.8 mm
Oil hardening (O1, S7 heavy section)	0.3-1.0 mm
Thin or asymmetric sections, any grade	Add 50-100%

Air hardening grades move far less than oil hardening ones — that is the central argument in O1 vs A2. Choosing an air hardening grade is often cheaper than paying to machine out the distortion of an oil quench.

Two failure modes to avoid:

Too little allowance. The part moves more than you left, a low spot does not clean up, and the block is scrap after all the work already in it.

Too much allowance. You are now hard milling or grinding 1 mm of 60 HRC steel off every surface. That is slow, expensive, and puts heat into the part. See machining after heat treatment.

Datum strategy: the part everyone forgets

Here is the failure that costs the most and gets discussed the least.

You machine a part with features dimensioned from two reference faces. It goes to heat treat. It comes back, and those reference faces have moved — they are no longer flat, no longer square to each other, and no longer where they were.

The shop now has to re-establish datums before it can finish anything. If you have not said how, they will pick. If they pick differently from how your tolerances are referenced, every dimension is measured from the wrong place and the part fails inspection while being geometrically fine.

What to do:

Designate the post-heat-treat datums explicitly. A note: “after heat treatment, re-establish datums A and B by grinding; 0.3 mm stock allowed on both.”

Leave more stock on the datum faces than on anything else. They have to clean up completely, because everything else references them.

Where possible, make the datums large, flat and on solid material. A datum on a thin flange will have moved more than one on a 40 mm thick section.

Consider a sacrificial datum. An extra feature — a machined pad, a reamed hole pair — that exists only to locate the part after heat treat and gets removed at the end. On complex tooling this is routine practice and worth the material.

What to write on the drawing

A short heat treatment note covering four things:

HEAT TREATMENT
1. Rough machine, stress relieve 650 °C, 1 hr / 25 mm section, slow cool.
2. Harden and temper to 58-60 HRC. Double temper.
3. Allowance after HT: 0.3 mm per side; 0.5 mm on datums A and B.
4. Re-establish datums A and B by grinding before finish machining.

Four lines. They remove most of the ambiguity that makes hardened steel quotes vary wildly, and they stop the failure modes that turn a 30-hour block into scrap.

Send that note with the drawing on the RFQ and quotes from different shops become directly comparable, because they are all pricing the same plan.