Machine D2 and A2 in the annealed state (~200–250 HB) at 100–180 m/min with coated carbide, then heat treat to 58–62 HRC and finish only the critical features by hard milling or grinding — this routes the heavy stock removal to the cheap, fast soft-cutting condition. Machine P20 directly in the pre-hardened state (~28–34 HRC, ~300 HB) at 80–150 m/min because it ships ready-to-cut and skips heat treat distortion entirely, which is why it dominates plastic injection mold cavities.
Tool and die steels are not one machining problem — they are several, and the dominant variable is the heat-treat condition the workpiece arrives in. The same D2 bar is a routine 220 HB cut when annealed and a PCBN-or-grind job at 62 HRC after hardening. Choosing when to cut relative to when to harden is the single decision that controls cycle time, tool cost, and final accuracy on a die or mold component. This guide sits between the hardened steel machining guide, which covers >45 HRC finishing in general, and hard turning vs grinding, which covers the finishing-process choice. For the broader material map across all alloy families, see the material machining complete guide.
Quick Tool & Die Steel Machining Reference
| Problem / Goal | Primary Action | Expected Impact |
|---|---|---|
| Heavy stock removal on D2/A2 cavity | Rough in annealed state (~200–250 HB) before heat treat | 2–4× higher MRR vs cutting the same steel hardened above 58 HRC |
| Mold cavity needing minimal post-HT work | Use pre-hardened P20 (~30 HRC); machine direct, no heat treat | Eliminates heat-treat distortion correction; saves 1–2 process steps |
| Distortion after hardening D2 | Leave 0.1–0.3 mm finish stock; air-harden, then finish-mill/grind | Holds final tolerance to ±5–15 µm vs uncorrected ±50–100 µm drift |
| Short carbide life milling 58–62 HRC die steel | Switch to AlCrN coating, drop f_z to 0.02–0.05 mm/tooth | Tool life can increase 1.5–2.5× vs TiAlN in rigid setups above 52 HRC |
| Chipping milling annealed D2 (12% Cr) | Reduce ae to 30–50% D, use trochoidal path | Shifts wear from chipping to abrasive flank wear; more predictable life |
| H13 hot-work die at 44–50 HRC | Hard-mill direct with TiAlN/AlCrN at 60–90 m/min | Avoids a re-hardening cycle; acceptable for 3D cavity finishing |
Why Heat-Treat State Decides the Whole Strategy
The machinability of a tool steel changes by a factor of roughly 3–5× between its annealed and fully hardened conditions, which is why the soft-machine-then-harden sequence remains the default route for high-removal die and punch work. Annealed D2 at 200–250 HB cuts like a tough alloy steel; the same D2 at 60–62 HRC has bulk hardness approaching the room-temperature hardness of uncoated WC-Co carbide (≈1,600–1,800 HV) and demands PCBN or grinding for anything but light finishing.
Three properties drive the difference. First, bulk hardness: above roughly 45 HRC, abrasive flank wear accelerates sharply because the workpiece carbide phases — M₇C₃ chromium carbides in D2 at ~1,800 HV, M₂C/M₆C in high-speed-grade steels — are harder than the carbide tool's binder. Second, carbide volume fraction: D2 carries ~12% Cr and ~1.5% C, forming a dense network of hard carbides even when annealed, so it is more abrasive than P20 or H13 at equal hardness. Third, distortion on hardening: every steel moves dimensionally during the quench, so finishing before hardening risks losing the tolerance you cut.
The practical rule is to push heavy material removal into the cheapest cutting condition and reserve the expensive hardened-state cut for only the features that must hold final tolerance. Air-hardening grades (D2, A2) distort less than oil- or water-hardening grades, which is why they tolerate near-net soft machining followed by a small finish allowance after heat treat. A2 (5% Cr, ~1% C) is the lower-carbide cousin of D2 and is often chosen specifically because its more uniform dimensional response during hardening simplifies the soft-then-finish workflow.
The Core Decision: Soft-Machine + Heat Treat vs Hard-Mill Direct
✦ Soft-Machine then Harden Best For
- High stock removal (deep cavities, punch blanks) where MRR matters most
- Air-hardening grades D2 and A2 that move predictably in the quench
- Final hardness 58–62 HRC required across the whole part
- Shops with in-house or reliable outsourced heat treat capacity
- Parts where 0.1–0.3 mm finish stock can be left for post-HT cleanup
✦ Hard-Mill / Machine Direct Best For
- Pre-hardened P20 (~30 HRC) plastic mold cavities — no heat treat at all
- H13 hot-work tooling used at 44–50 HRC (re-hardening not required)
- Late design changes or rework on an already-hardened block
- Low part counts where a heat-treat cycle's lead time dominates schedule
- 3D cavity finishing where remachining a hardened block beats re-cutting soft
Soft machining then hardening is the lower-cost route whenever the part needs full hardness above 58 HRC and the geometry allows a small finish allowance, because roughing at 200–250 HB removes material 2–4× faster than cutting the same steel at 60 HRC. The penalty is the distortion that the hardening cycle introduces: a D2 block can move 0.05–0.2 mm depending on section thickness and quench severity, so the workflow must leave 0.1–0.3 mm of finish stock on critical surfaces to be cleaned up after heat treat by hard milling, grinding, or EDM.
Pre-hardened P20 is machined directly in the supplied 28–34 HRC condition because it is delivered pre-tempered specifically to skip the heat-treat-and-correct cycle, which is the reason it became the standard for plastic injection mold bases and cavities. Buying P20 pre-hardened trades a modest reduction in maximum hardness (it is rarely used above ~36 HRC) for the elimination of an entire distortion-correction process step — a worthwhile trade when the mold sees plastic, not metal-forming loads.
H13 occupies the middle ground. As a hot-work die steel it is frequently used at 44–52 HRC, and many H13 cavities are hard-milled directly at that hardness with coated carbide rather than soft-machined and re-hardened, because the working hardness is already within reach of TiAlN/AlCrN end mills.
Best Practice
Leave 0.1–0.3 mm of finish stock — not zero — on every surface that carries a final tolerance when soft-machining a part destined for hardening. The hardening quench moves the geometry, and the only way to recover the tolerance is to have material left to cut. Sizing the allowance to the section thickness (thicker sections move more) is more reliable than a blanket value.
Speeds and Feeds by Steel and Heat-Treat Condition
The table below gives turning/milling starting points by grade and condition. All values assume rigid setups, coated carbide except where PCBN is noted, and uniform flank wear as the dominant wear mode — treat them as calibration starting points, not guarantees, and adjust to the ISO 3685 wear criterion observed on your machine.
| Steel | Condition | Hardness | Tool | Speed (m/min) | Feed (mm/rev or mm/tooth) |
|---|---|---|---|---|---|
| D2 | Annealed | 200–250 HB | TiAlN carbide | 100–150 | 0.10–0.25 mm/rev |
| D2 | Hardened | 58–62 HRC | Low-CBN PCBN (turn) | 80–130 | 0.05–0.12 mm/rev |
| A2 | Annealed | 200–230 HB | TiAlN carbide | 120–180 | 0.10–0.25 mm/rev |
| A2 | Hardened | 57–60 HRC | PCBN / AlCrN carbide (mill) | 60–110 | 0.03–0.08 mm/tooth |
| P20 | Pre-hardened | 28–34 HRC | TiAlN carbide | 80–150 | 0.08–0.20 mm/tooth |
| H13 | Annealed | ~200 HB | TiAlN carbide | 110–170 | 0.10–0.25 mm/rev |
| H13 | Hardened | 44–52 HRC | TiAlN/AlCrN carbide (mill) | 60–90 | 0.04–0.10 mm/tooth |
Speeds drop with rising hardness because cutting temperature and abrasive wear both climb. For the >45 HRC finishing detail, cross-reference the hardened steel machining guide. For SFM/chip-load calculation from these starting points, see the steel machining parameters guide.
Pre-hardened P20 runs slower than annealed D2 despite D2's higher carbide content because P20's ~30 HRC bulk hardness raises cutting forces and interface temperature relative to a 220 HB anneal, illustrating that condition, not just alloy chemistry, sets the achievable speed. Annealed D2 is more abrasive per unit volume but its lower bulk hardness allows higher cutting speed; this is why the soft-machine route is so productive even on a high-carbide grade like D2.
TiAlN coating is preferred for soft-machining annealed tool steel because its 3,000–3,500 HV hardness and oxidation resistance to ~800°C handle the moderate temperatures of P-group steel cutting, while AlCrN is preferred once the workpiece exceeds ~52 HRC because its oxidation resistance to roughly 1,100°C provides thermal margin at the higher interface temperatures of hard milling. Per the ISO 513 classification of hard cutting materials, P-group carbide grades target long-chipping steels such as annealed tool steels, which is the application group these soft-machining recommendations fall into.
Distortion Control and the Finish-Stock Allowance
Avoid Finishing Before Heat Treat
Do not finish-machine a part to final tolerance before hardening it. The quench can move dimensions 0.05–0.2 mm depending on grade, section thickness, and quench severity — finishing first throws away the tolerance you cut. Leave 0.1–0.3 mm of stock on toleranced surfaces and remove it after hardening by hard milling, grinding, or EDM.
Distortion behaviour varies sharply by hardening mechanism. Air-hardening grades (D2, A2) typically distort less than oil- or water-hardening grades because slower, more uniform cooling reduces the thermal gradients that drive warpage, which is the main reason die makers favour them for parts that must hold tight geometry after heat treat. Even so, an air-hardened D2 block of unequal section can move 0.05–0.2 mm, so the finish allowance is non-negotiable on toleranced features.
A simple way to size the allowance: scale it to the largest section-thickness change in the part, because thicker sections cool slower and move more. A near-uniform block may need only 0.1 mm; a part with a 3:1 section ratio is safer at 0.25–0.3 mm. Symmetric stock removal — roughing both sides of a plate evenly before hardening — reduces residual-stress-driven bowing more effectively than removing all the stock from one face, a practice borrowed from precision die plate work.
The finishing method after hardening then follows the hard turning vs grinding decision: hard milling or PCBN turning for continuous features at Ra ≥ 0.4 µm, grinding for Ra ≤ 0.2 µm or where the white layer must stay thin. EDM is the third path for deep, fine cavity detail that neither milling nor grinding can reach.
Grade-Specific Notes: D2, H13, P20, A2
| Grade | Type | Carbide load | Soft-machine route | Typical use hardness |
|---|---|---|---|---|
| D2 | Cold-work, air-hardening | High (12% Cr, M₇C₃) | Strongly preferred | 58–62 HRC |
| A2 | Cold-work, air-hardening | Medium (5% Cr) | Preferred (low distortion) | 57–60 HRC |
| P20 | Mold steel, pre-hardened | Low–medium | Machined direct (pre-hard) | 28–34 HRC |
| H13 | Hot-work | Low–medium | Either route | 44–52 HRC |
D2 tool steel is the most abrasive of these four grades because its ~12% chromium and ~1.5% carbon form a dense network of M₇C₃ chromium carbides at ~1,800 HV, the hardest common carbide phase in tool steels, so even annealed D2 rewards reduced radial engagement (30–50% D) and trochoidal milling to keep the wear mode abrasive rather than chipping. After hardening to 58–62 HRC, D2 finishing is squarely a PCBN-turning or grinding job — see the hardened steel machining guide for the >45 HRC parameter detail.
H13 hot-work tool steel is commonly machined directly at its 44–52 HRC working hardness because re-hardening adds a distortion cycle without changing the in-service hardness the die actually needs, making direct hard milling with TiAlN/AlCrN carbide the practical default for H13 cavity and core work. A2 air-hardening tool steel is often selected over D2 when dimensional stability through heat treat matters more than maximum abrasion resistance, because its lower carbide volume gives more uniform quench response. Pre-hardened P20 is machined in its as-supplied 28–34 HRC condition for plastic injection molds because it ships pre-tempered to eliminate the heat-treat-and-correct cycle entirely — the defining reason it dominates mold-base and cavity production.
Cutting speeds for tool steels must be selected for the as-machined hardness condition, not the nominal grade — the same alloy spans a 3-to-1 machinability range from annealed to hardened.
— Machining Data HandbookSummary
Route heavy removal to the soft condition, reserve hard machining for final-tolerance features, and machine pre-hardened P20 direct.
The tool-and-die machining decision is driven by heat-treat state, not just alloy: rough D2 and A2 annealed (200–250 HB) at 100–180 m/min with TiAlN carbide, leave 0.1–0.3 mm finish stock, harden to 58–62 HRC, then finish critical features by hard milling, PCBN turning, or grinding. Machine pre-hardened P20 directly at ~30 HRC because it skips heat treat, and hard-mill H13 at its 44–52 HRC working hardness rather than adding a re-hardening cycle. Match coating to condition — TiAlN for soft P-group cutting, AlCrN above ~52 HRC, PCBN for continuous turning above 58 HRC — and size the finish allowance to section thickness because air-hardening grades still move 0.05–0.2 mm in the quench.
Sources
- ISO 513:2012 — Classification and application of hard cutting materials for metal removal (P/M/K application groups for steel cutting)
- ISO 3685:1993 — Tool-life testing with single-point turning tools (VB_B = 0.3 mm flank wear criterion)
- Sandvik Coromant — Tool steel and hardened steel machining guidelines (speed/feed by hardness condition)
- Kennametal — Hard Part Turning and PCBN grade selection
- Machinery's Handbook, 31st Edition (tool steel classifications and machinability data)
Should I machine tool steel before or after heat treatment?
Machine most of the material before heat treatment in the annealed state, where tool steels cut 2–4× faster, then leave 0.1–0.3 mm finish stock and clean up critical features after hardening. Exceptions are pre-hardened P20, machined directly at ~30 HRC, and H13 dies used at 44–52 HRC, which are often hard-milled direct.
Why is P20 sold pre-hardened instead of annealed?
P20 is supplied pre-hardened to 28–34 HRC so plastic injection molds can be machined directly without a heat-treat cycle, eliminating quench distortion correction. At this hardness it cuts with coated carbide at 80–150 m/min, and it is rarely used above ~36 HRC because plastic molding loads do not require it.
What carbide grade and coating should I use for hardened D2 at 60 HRC?
For continuous turning of D2 at 58–62 HRC, use low-CBN-content PCBN (45–70% CBN, TiC binder) at 80–130 m/min. For milling hardened die steel above 52 HRC, AlCrN-coated carbide is preferred because its oxidation resistance reaches roughly 1,100°C, versus about 800°C for TiAlN.
How much distortion should I expect when hardening D2 or A2?
Air-hardening D2 and A2 typically move 0.05–0.2 mm during hardening, depending on section thickness and quench severity, with thicker and more asymmetric sections moving more. This is why you leave 0.1–0.3 mm of finish stock on toleranced surfaces and remove it after heat treat by grinding, hard milling, or EDM.
Can I hard-mill H13 without re-hardening it?
Yes — H13 hot-work dies are commonly hard-milled directly at their 44–52 HRC working hardness with TiAlN or AlCrN coated carbide at 60–90 m/min and 0.04–0.10 mm/tooth. Re-hardening would add a distortion cycle without changing the in-service hardness the die actually needs.
