Reference
Heat treat stock allowance
How much material to leave for the finishing pass after hardening. Get this number wrong in either direction and it costs real money — too little scraps the block, too much doubles the finishing operation.
| Part / grade type | Largest dimension | Allowance per side | Why |
|---|---|---|---|
| Small, simple, air hardening (A2, D2, H13) | Under 100 mm | 0.10 - 0.20 mm | Single light finishing pass |
| Medium block, air hardening | 100 - 300 mm | 0.20 - 0.40 mm | Normal hard milling allowance |
| Large plate, air hardening | Over 300 mm | 0.40 - 0.80 mm | Flatness dominates; plan a grinding pass |
| Oil hardening (O1, D3, 52100) | Any | 0.30 - 1.00 mm | Quench gradient moves the part 3-4x more than air |
| Thin or asymmetric sections | Any | Add 50 - 100% | Uneven section = uneven transformation = warp |
| Long slender parts (L:D over 6:1) | Any | Add 50 - 100% | Consider vertical hanging in the furnace |
| Pre-hardened (P20, 4140PH, 17-4) | Any | None | No heat treat step — finish to size directly |
Rule of thumb: air hardening grades move in the region of a quarter of what oil hardening grades move. Where dimensional stability matters, choosingA2 over O1 is frequently cheaper overall than paying to machine out the distortion of an oil quench — even though A2 costs more per kilo.
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The drawing note that makes quotes comparable
Four lines. They remove most of the ambiguity that makes hardened steel quotes vary by multiples between shops.
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.Adapt the hardness and the allowance from the table above. The fourth line is the one almost everybody leaves off, and it is the one that causes parts to fail inspection while being geometrically fine.
Common questions
What happens if I leave too little?
A low spot does not clean up, and the block is scrap after all the soft machining hours already in it. This is the expensive failure, which is why shops pad the allowance when the drawing is silent.
What happens if I leave too much?
You are now hard milling or grinding a millimetre of 60 HRC steel off every surface. That is slow, burns CBN inserts, and puts heat into the part. It is the failure that quietly doubles the finishing cost.
Why do air hardening grades move so much less?
An oil quench pulls heat out fast, so the surface transforms to martensite while the core is still hot and plastic. The core then transforms later against an already-hard shell, and the part fights itself. Air cooling is gentle: the gradient is shallow and the transformation is closer to simultaneous through the section.
Do I need stress relief as well?
On anything large or asymmetric, yes. Rough machine, stress relieve at 600-675 °C for about an hour per 25 mm of section, then semi-finish. That lets the part move in a controlled way before hardening, rather than in the quench where it does damage.
What about the datums?
This is the step most drawings forget. The reference faces move in the furnace too. Leave more stock on them than anywhere else, and say on the drawing that they are to be re-established by grinding before finish machining — otherwise the shop picks its own datums and every dimension is measured from the wrong place.
Free to reuse underCC BY 4.0with a link back. Longer treatment of the mechanism is inheat treat distortion.