Pull up a supplier's data sheet for D2 or H13 tool steel and you'll usually find two very different numbers hiding under "hardness"—mix them up and you can wreck a machining job or have a die failing weeks early. Tool steel hardness isn't one fixed value. On the shop floor you work with the soft, machinable annealed condition. The part needs the hardened, tempered working hardness to survive in service. The gap between them—sometimes 30+ points on the Rockwell C scale—is the whole point of the heat treatment process.
Below is a straight-to-the-data comparison of O1, A2, D2, H13, S7, and M2 tool steel, with annealed and quenched-and-tempered ranges side by side, plus a quick note on why each grade lands where it does. You can stop cross-referencing three PDFs and just pick the number you need.
Tool Steel Hardness Chart by Grade
Numbers tell the story faster than paragraphs ever could. Here's the reference table, pulled straight from mill certs and supplier spec sheets:
Grade | Annealed Hardness | Typical Working Hardness |
|---|---|---|
200–250 HB | 57–62 HRC | |
187–223 HB | 45–57 HRC | |
212–248 HB | High-50s HRC | |
202–248 HB | Low-to-mid 60s HRC | |
183–212 HB | 58–62 HRC | |
Max 255 HB | 58–62 HRC |
Notice the annealed column uses Brinell. That's not a typo. Annealed tool steel is soft enough that Rockwell C readings get unreliable, so suppliers use Brinell. Convert it in your head and you're looking at roughly 85–100 HRB across the board—soft, forgiving, machinable.
The working hardness column is where the real spread shows up. S7 tops out around 45–57 HRC because it's built for shock resistance. M2 climbs into the low-to-mid 60s because high-speed steel needs to hold an edge under heat. The rough rule is that hardness potential follows S7 < A2/O1 < D2 < M2.
O1 Tool Steel Hardness
O1 is the grade most machinists reach for when a job calls for oil hardening without a lot of drama. Cross-reference ASTM A681 (UNS T31501) and you'll land on the same numbers most suppliers ship with: 190–212 HBW annealed, dropping toward the lower end near 183 HB on some mill certs. That's a soft, forgiving band—easy on tooling, easy on cycle time.
Quench it and O1 jumps to 63–66 HRC as-quenched, which is too brittle for real work. Tempering brings it back down fast: 200°C nets roughly 60 HRC, 250°C settles near 58 HRC, 300°C drops to about 55 HRC. Most punches, blanking dies, and shear blades land in the 58–60 HRC sweet spot, with the full working range spanning 57–62 HRC depending on how much toughness you're trading for wear resistance. DIN 1.2510 and JIS SKS3 are the equivalents you'll see on European and Japanese spec sheets.
A2 Tool Steel Hardness
A2 earns its reputation as the "no drama" air-hardening steel. Shops love it because it skips the oil or water quench entirely, which means less warping and fewer surprises. Supplier sheets typically list annealed hardness at 197–241 HBW, though tighter purchase-order specs often call for ≤235 HBW, with some mills targeting a soft-annealed 215 HB for easy machining.
Quench it and A2 jumps straight to 63–65 HRC as-quenched—too brittle to trust in a die. Tempering pulls it back into the usable zone. Around 150°C you're still sitting at 61–62 HRC. Push tempering up to 350–400°C and it settles near 57–58 HRC.
Most shops split the difference. They aim for 58–60 HRC for a toughness/wear balance on punches and forming tools, or 60–62 HRC when wear resistance matters more than shock absorption. In BHN terms, that working range maps to roughly 601–658 BHN, useful if your gauge only reads Brinell.
D2 Tool Steel Hardness
High-chromium D2 sits in its own category—part cold-work die steel, part semi-stainless thanks to that 12% chromium content. Annealed stock usually runs ≤255 HB. You'll see specs quoted anywhere from 217–255 HB, and some machining-grade suppliers push that ceiling to 269 HB. Ask your vendor for "≤255 HB annealed" and you're covered.
Once hardened, D2 lands in the 58–62 HRC working range. Shops chasing a balance of toughness and wear resistance target 58–60 HRC, and tempering around 500–550°F (260–288°C) gets you there reliably. Need maximum wear resistance for a blanking die? Push to 60–62 HRC. Properly tempered D2 can reach 62–64 HRC, with 64–65 HRC reported as-quenched in some cases, though that's brittle territory for most tooling.
One catch: hardened D2 at 58–62 HRC eats standard carbide tooling. CBN or ceramic inserts are the practical choice if you're grinding or finishing after heat treat.
H13 Tool Steel Hardness
H13 tops out lower than D2's brutal 60+ HRC on purpose because hot-work dies need to survive thermal cycling without cracking. Annealed stock runs 192–235 HB, with several mill certs capping it at 235 HB max for machinability during drilling, milling, and cavity work. A typical anneal cycle holds the steel at 1575–1625°F for roughly an hour per inch of thickness, then furnace-cools slowly down to 1000°F before air cooling.
Once hardened, H13 settles into 44–54 HRC working hardness, though most suppliers narrow that to 45–52 HRC for practical die service. Some spec sheets list a minimum of 52 HRC after quenching. In Brinell terms, that's roughly 469–572 BHN. Hot-work tooling exposed to repeated thermal shock often runs closer to 40–48 HRC, trading peak hardness for the toughness that keeps dies from cracking under cycling.
S7 Tool Steel Hardness
Shock resistance is all S7 does, and the hardness numbers show that compromise. Mill-delivered stock comes soft annealed, ready for rough and semi-finish machining. You get a hardness of 187–229 HB. Most supplier specs cap it at 225 HB, with a few allowing 230 HB max. Either way, it machines easily.
Quench S7 and you get 59–61 HRC right out of the quench, but it's too brittle for a die that takes repeated impact. Tempering drops it into service range fast:
400–450°F (204–232°C): ~57–58 HRC
500–600°F (260–316°C): ~55–56 HRC
700–900°F (371–482°C): ~52–54 HRC
Most shops settle on 56–58 HRC as the sweet spot for shock-resistant tooling, though some suppliers cite a working range of 56–60 HRC. For reference: 54 HRC is about 543 HB, 56 HRC is 577 HB, and 58 HRC is 615 HB.
M2 Tool Steel Hardness
M2 is supplied soft for machining, typically at 212–248 HB. Annealed hardness limits vary: some specifications set a maximum of 235 HB or 248 HBW, while others allow 270–280 HB. Check the agreed delivery specification rather than treating these limits as interchangeable.
After hardening and tempering, M2 typically reaches 60–65 HRC, with some specifications citing 60–66 HRC. As-hardened readings can reach 65 HRC, while tempered examples may settle near 62 HRC. Tempering within 540–600°C can produce hardness across approximately 58–65 HRC, depending on the complete treatment cycle. The target must suit the cutting tool; this range is not a single requirement for all drills and cutters.
Annealing schedules may involve heating to 830–900°C, followed by slow furnace cooling to 538–650°C, then air cooling. These figures represent different procedures, so use one approved cycle rather than combining the range endpoints.
Annealed vs. Hardened Tool Steel: Key Differences
Every tool steel grade lives a double life, though suppliers don't always spell it out on a spec sheet. It shows up at your shop as soft, annealed bar stock. It leaves as a hardened part doing a completely different job. Confuse the two numbers and you'll either burn up end mills trying to machine hardened material, or ship a die that's still too soft to survive its first production run.
The industry pattern is consistent once you see it across enough grades. Steel arrives annealed, generally somewhere in the 200–260 HB range, soft enough to drill, mill, and turn without fighting the material. After heat treatment, that same steel settles into a working hardness around 58–62 HRC for most cold-work and general-purpose grades. D3 (1.2080) tool steel is a good example: soft-annealed to ≤230–250 HB and hardened up to 57–62 HRC for service.
Some grades push further. M2 high-speed steel, annealed around 202–248 HB, climbs to 60–66 HRC once hardened. That extra hardness is what lets it hold a cutting edge under heat.
The compact rule worth memorizing:
- Annealed = machinable state, roughly 190–255 HB
- Working = hardened service state, roughly 54–66 HRC
Keep those two bands separate in your head, and reading any tool steel spec sheet gets a lot faster.
Tool Steel Hardness Conversion: HRC to HB
A single number rarely tells the whole story, which is why supplier sheets often list both scales side by side. Rockwell C hardness works great once tool steel is hardened, but Brinell still shows up on certs and inspection reports. Knowing how to flip between the two saves you a phone call to the metallurgist.
Paste this conversion table above your workbench:
HRC | HB (approx.) |
|---|---|
50 | 475–481 |
52 | 500–512 |
54 | 543 |
56 | 577 |
58 | 615 |
60 | 654 |
62 | 688 |
64 | 722 |
65 | 739 |
Shops that don’t need lab-grade precision use a simpler shortcut – HB ≈ 10 × HRC – which holds up decently through the middle of the hardened range. Run the numbers: 30 HRC lands near 300 HB, 45 HRC near 450 HB, and 60 HRC near 654 HB, matching the table above almost exactly.
This conversion only makes sense for hardened, quenched-and-tempered material. Rockwell C readings get unreliable below roughly 20 HRC, so Brinell stays the default gauge for soft, annealed tool steel sitting in the 150–250 HB zone. Once you’re past that threshold and into working hardness territory – 58–62 HRC for D2, O1, and A2 – Rockwell C becomes the more practical, faster-to-read scale on the shop floor.
Choosing Tool Steel Hardness for Your Application
Machinability and wear resistance pull in opposite directions, and that trade-off is why two hardness numbers exist for every grade. The steels that cut fastest under a drill or endmill are rarely the ones that survive longest in a die.
Machinability benchmarks make the trade-off clear. O1 rates around 85–90% machinability, cutting almost as easily as low-carbon steel. H13 runs 70–75%, still forgiving in the mill. A2 drops to 60–65%, and D2 falls to 45–55%. The heavy carbide content that gives D2 its wear resistance is the same carbide that dulls your tooling faster.
Tool Steel Hardness Ranges for Application
Instead of memorizing every grade, think in bands:
20–35 HRC — soft or pre-hardened, easy machining (P20-type steels)
36–44 HRC — medium hardness, tougher cutting
45–52 HRC — balanced wear resistance and toughness (H13-type service)
53–60 HRC — very hard, high wear resistance (most cold-work dies)
61–66 HRC — extreme hardness for cutting edges (M2 at 64–66 HRC, T15 pushing 67–69 HRC)
Conventional tool steel tops out around 68 HRC on the whole scale. Past that, you're into ceramics and carbides, not heat-treated steel anymore.
For pure machining ease, O1 and pre-hardened P20 win. For wear service, D2 and M2/T15 dominate. A2 and H13 split the difference — reasonable to machine, still hard enough to earn their keep in the die.
Conclusion
Annealed hardness and working hardness serve different purposes. The delivery condition supports machining; subsequent hardening and tempering develop the properties needed in service. The right hardness depends on the grade and application.
Use the comparison above to narrow your choice: D2 for abrasive-wear resistance, O1 for oil-hardening cold-work tools, A2 for a balance of wear resistance and toughness, and H13 for hot-work applications.
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