Tool steels: machine soft and harden, or buy hard and cut it
Tool steels are specified by people who need wear resistance, and machined by people who have to deal with what wear resistance does to a cutting edge. The central decision on every tool steel part is the sequence: machine soft and harden afterwards, or buy hardened material and cut it hard.
That decision drives cost more than the grade does.
The grades you will meet
| Grade | Type | Working hardness | Distinguishing property |
|---|---|---|---|
| A2 | Air hardening cold work | 57–62 HRC | Low distortion, good all-rounder |
| D2 | High carbon high chrome cold work | 58–62 HRC | Excellent wear resistance, poor toughness |
| O1 | Oil hardening cold work | 57–62 HRC | Cheap, easy to machine soft, distorts more |
| H13 | Hot work | 44–52 HRC | Thermal fatigue resistance, holds up hot |
| S7 | Shock resisting | 54–58 HRC | Impact tolerance |
| M2 | High speed steel | 62–65 HRC | Red hardness, cutting tools |
| P20 | Mould steel | 28–34 HRC | Supplied pre-hardened, machinable as-is |
D2 is the one buyers over-specify. Its wear resistance comes from large chromium carbides, which also make it brittle and abrasive to machine. If the part sees impact, A2 or S7 is usually the better answer.
H13 is the mould and die workhorse. Its lower working hardness makes it far more machinable than the cold work grades, and it is often specified for parts that never get hot, simply because it is easy to buy.
P20 deserves attention because it ships pre-hardened at 28–34 HRC and is designed to be machined in that condition. For mould bases and plates it removes the heat treatment step entirely.
Soft machining, then harden
Advantages. Fast cutting — annealed tool steel runs 80–150 m/min with coated carbide. Cheap tooling. Complex geometry is straightforward.
The cost. Heat treatment moves the part. How much depends on the grade and the section:
| Grade | Typical dimensional change through hardening |
|---|---|
| A2 | ±0.05 % — low, which is its selling point |
| D2 | −0.06 % to +0.10 %, varies with section |
| O1 | +0.10 % to +0.20 %, and distortion-prone |
| H13 | ±0.05 % to 0.10 % |
| M2 | +0.05 % to +0.15 % |
On a 300 mm die plate, 0.1 % is 0.3 mm. That has to be left as grind stock on every critical feature, with a finishing operation after heat treatment to bring it back.
There is also risk. Quench cracking, decarburisation and distortion beyond allowance are real failure modes, and they happen after most of the machining cost has already been spent. Distortion and stress relief covers the sequence that reduces it.
Hard machining
Advantages. No distortion after the fact. One operation. Dimensions are what you measured. No risk of losing a part at the last step.
The cost. CBN tooling, lower speeds, higher hourly rates, and a hard ceiling: above roughly 65 HRC — M2 territory — turning and milling largely give way to grinding and EDM. Hard turning or grinding has the thresholds.
How to choose
Hard machine when the part is small to medium, the tolerances are tight, the features are few, distortion would be unrecoverable, or the material is available pre-hardened at the right hardness.
Soft machine and harden when the part is large, the geometry is complex, the quantity is high enough that cycle time dominates, or a finishing operation after heat treatment is already in the plan.
Hybrid, which is what most real parts do: rough soft, harden, finish hard or grind. It costs an extra setup and removes most of the risk from both routes.
Machining data, annealed condition
Coated carbide, flood coolant:
- Milling: 80–140 m/min, 0.08–0.20 mm/tooth
- Turning: 100–180 m/min, 0.15–0.35 mm/rev
- Drilling: 20–35 m/min
D2 sits at the low end of every range because the carbides are abrasive even when the matrix is soft. Expect noticeably shorter tool life on D2 than on A2 at the same hardness — the hardness number does not capture it.
What to write on the drawing
Material: AISI A2 tool steel, supplied annealed. Harden and temper to 58–60 HRC after rough machining. Finish grind features marked G after heat treatment. Hardness verified on the finished part, three locations.
That specifies the grade, the supply condition, the target hardness, the sequence and the acceptance test. “D2, hardened” leaves every one of those open, and each open question widens the quote — why the same drawing comes back at three different prices.
Also worth stating: whether the heat treatment must be done by a named or approved source, and whether you need the heat treat certificate with the parts.
Background: tool steel.
Send a tool steel drawing and we will come back with the sequence as well as the price.