The question that decides the quote: which state is the steel in?
The first thing I look at on a tool steel RFQ is not the alloy name — it's whether the drawing says annealed, prehardened, or fully hardened. I've seen quotes triple between the same D2 part machined soft versus machined hard. If the drawing is silent, ask. Machining D2 at HRC 28 annealed is a completely different job from grinding D2 at HRC 60.
D2 vs H13 vs prehardened 4140
| Material | Typical hardness | Best for | Machining reality |
|---|---|---|---|
| D2 (1.2379) | Annealed ~HB 240; hardened HRC 58-62 | Cutting dies, punches, wear parts | Machines OK annealed; hardened state is grind/EDM only |
| H13 (1.2344) | Annealed ~HB 200; hardened HRC 44-52 | Die casting dies, hot work, plastic molds | Tougher than D2 to cut; excellent polishability |
| Prehardened 4140/42CrMo4 | HRC 28-32 as supplied | Mold bases, fixtures, prototype tooling | The easy one — machines fast, stable, no post-heat-treat warpage |
Here's the practical rule: if the part is a mold base, a fixture, or prototype tooling that needs to be ready in weeks, prehardened 4140 is the material. It machines at a good clip, holds ±0.01 mm without drama, and there's no heat treatment step to warp it. If the part is a working die that needs real wear resistance, D2 or H13 hardened after machining is the honest path — and you must plan for finish grinding or EDM in the hardened state.
Machining strategy: what actually works
Tool steel in the annealed or prehardened state cuts well with carbide — but it's nowhere near aluminum. Expect cutting speeds roughly a quarter of what you'd run on 6061, and feed the tool deep enough to get under the work-hardened skin that forms on the previous pass. Small light cuts on tool steel are the classic beginner mistake: each pass work-hardens the surface for the next, and the tool dies from rubbing, not from cutting.
- Use coated carbide (TiAlN) and climb milling — conventional milling pushes the work-hardened chip ahead of the cutter
- Keep the tool engaged: radial engagement under 10% of diameter on tool steel creates chatter and rapid flank wear
- For deep cavities, variable-flute end mills reduce the harmonics that wreck finishes in D2
- Plan the heat treat step: rough + semi-finish soft, leave 0.3-0.5 mm stock, harden, then finish grind or EDM
- Wire EDM is your friend on hardened tool steel — it cuts hardened blanks cleanly with no grinding distortion
Heat treatment and warpage
Every tool steel part moves in heat treatment. D2 grows and can distort a surprising amount if the part is asymmetric; H13 is more stable but still moves. If the drawing needs tight features after hardening, the process must be: machine to within 0.3-0.5 mm, heat treat, then finish grind or wire EDM the critical faces. That double handling is where the cost goes — it's why a hardened D2 die component costs two to three times the same part left soft.
Getting the RFQ right
Three lines on the drawing save everyone money: the material state you expect to receive (annealed / prehardened / hardened), the final hardness with range (e.g. HRC 58-62), and which features are critical in the finished state. Then say whether you'll heat treat or the shop should. A complete tool steel RFQ gets a straight quote; an incomplete one gets a quote with assumptions — and assumptions are where the surprises hide.
Frequently Asked Questions
What hardness is tool steel supplied at?
Annealed tool steel ships around HB 200-240 (about HRC 20). Prehardened grades like 4140 ship at HRC 28-32. D2 hardens to HRC 58-62, H13 to HRC 44-52. Always specify the state on the drawing.
Can you machine D2 after hardening?
Not with conventional milling. Hardened D2 (HRC 58+) is ground or wire-EDM only. If you need machined features after hardening, leave stock and finish with grinding or EDM.
Which tool steel is cheapest to machine?
Prehardened 4140 by a wide margin — it's the only 'tool steel' you can cut fast without heat treat planning. D2 and H13 in annealed state are similar to each other; the cost jumps when finish work happens after hardening.
Do tool steel parts need special inspection?
No special equipment, but hardness verification (Rockwell) matters, and if you ground after hardening, ask for the same CMM report you'd get on any precision part — plus a hardness certificate.