O1 vs A2: what an oil quench really costs you in distortion
O1 is the grade most shops learn on. It is inexpensive, forgiving in the annealed state, and it hardens at a temperature a small furnace can reach. A2 costs more per kilo and needs a hotter cycle.
On a material quote, O1 wins every time. On a finished part, it frequently does not — and the reason is the quench.
The two hardening routes
O1 is oil hardening. Austenitise around 790-815 °C, then quench into oil. The oil pulls heat out fast, and fast is what it takes to get the transformation you want in a steel with O1’s modest alloy content.
A2 is air hardening. Austenitise around 940-980 °C, then let it cool in still or forced air. The 5% chromium and 1% molybdenum slow the transformation enough that air cooling is sufficient.
Both land in the same hardness neighbourhood — O1 at 57-62 HRC, A2 at 57-62 HRC. The difference is entirely in how violently the part gets there.
Why the quench medium drives distortion
Distortion in heat treatment comes from two sources: thermal gradients during cooling, and the volume change as austenite transforms to martensite.
An oil quench creates a steep gradient. The surface transforms while the core is still hot and plastic, then the core transforms later against a surface that has already hardened. The part fights itself, and it moves.
Air cooling is gentle by comparison. The gradient is shallow, the transformation is closer to simultaneous through the section, and the part comes out much nearer the shape it went in.
The practical numbers, for a typical die plate:
| O1 (oil) | A2 (air) | |
|---|---|---|
| Typical size change | 0.1-0.2% | 0.02-0.06% |
| Typical shape distortion on a 200 mm plate | 0.15-0.4 mm | 0.03-0.10 mm |
| Risk of cracking at section changes | Moderate | Low |
| Decarburisation risk | Higher (longer exposure, lower alloy) | Lower |
Those are broad figures — geometry, section thickness and furnace practice move them considerably — but the ratio holds. A2 moves in the region of a quarter of what O1 moves.
What that costs downstream
Distortion is not a defect in itself. It becomes a cost when you have to machine it out.
With O1, you leave enough stock to clean up 0.3 mm of movement. That means a real finishing operation after heat treat: hard milling or grinding across the whole part, with the tooling and time that implies. It also means the part has to be re-located, and if your datums moved, that is its own problem — see datum strategy after heat treat.
With A2, you might leave 0.08 mm and take a single light pass. On some parts you finish before heat treat and skip the second operation entirely.
So the honest comparison is not “O1 costs less per kilo”. It is:
**O1 total = cheaper material + easy soft machining + heat treat + full hard finishing operation
- higher scrap risk**
A2 total = dearer material + slightly harder soft machining + heat treat + light finishing pass
On a small, simple, low-precision part, O1 wins. On anything with tight tolerances, a large flat face, or an awkward section, A2 usually wins outright.
Where each one genuinely belongs
O1 is the right answer for:
- Simple shapes with generous tolerances — spacers, backing plates, simple form tools.
- Small sections where the quench gradient is mild anyway.
- Parts you are grinding all over regardless, so the distortion gets removed for free.
- Low-volume work where material cost dominates because labour is minimal.
A2 is the right answer for:
- Anything where the part is finished to size before heat treat.
- Large flat plates where flatness matters.
- Parts with varying section thickness, where an oil quench invites cracking.
- Gauges and fixtures that have to stay dimensionally stable.
The third option
If you are choosing between them because you want dimensional stability, D2 is also air hardening and adds substantial wear resistance — at the cost of toughness and machinability. The A2 vs D2 comparison covers that trade.
And if the part is going to see impact rather than abrasion, neither is the right grade; S7 is.
On the drawing
If you specify O1, state the stock allowance you expect after heat treat, or state that the shop should determine it. Leaving it unsaid means one shop quotes 0.1 mm and another quotes 0.4 mm, and their prices will not be comparable.
If the part is a candidate for either grade, say so on the RFQ — “O1 or A2 acceptable” lets the shop route it the cheaper way, and on tooling work that decision is frequently worth more than anything you will negotiate on the unit price.