Pre-hardened or through-hardened: choosing the supply condition for tool steel
There is a decision upstream of every hard machining question, and it gets made — often by accident — when someone picks the material line on the drawing. Do you buy steel already at working hardness, or do you buy it soft, machine it, and harden it afterwards?
The two routes produce different parts, different lead times and different prices.
Pre-hardened: hardness comes in the bar
Grades like P20 (1.2311), 4140 pre-hard and 718H arrive from the mill at 28-36 HRC, already quenched and tempered, already stress relieved. You machine them at that hardness and ship. No heat treat operation, no distortion, no second finishing pass.
At 30-35 HRC, conventional carbide tooling works. It is harder going than mild steel and tool life is shorter, but it is ordinary machining — no CBN, no special toolpath strategy, no dry cutting.
What you give up is hardness. 32 HRC is not 58 HRC. For an injection mould running glass- filled resin, or a blanking die cutting hardened strip, it will wear out early.
Section size matters too. Large pre-hardened blocks have a hardness gradient — the core is softer than the surface. On a 400 mm thick plate that gradient is real, and if you machine deep into the block you expose softer material.
Through-hardened: hardness comes after machining
D2, H13, A2, O1, S7 and the rest arrive annealed at around 200-250 HB. You rough machine them easily, then they go to heat treat, come back at 55-62 HRC, and get finished.
What you gain is wear life — the whole reason these grades exist.
What you pay for is the heat treat cycle and its consequences:
- Two to three weeks of lead time at a commercial heat treater, sometimes more.
- Distortion in the quench, which means leaving stock for a finishing pass and re-establishing datums afterwards. See heat treat distortion and stock allowance.
- A hard machining or grinding operation at the end, with the tooling and process cost that carries.
- Risk. A cracked part after heat treat means starting over on a block you have already put twenty hours into.
The decision table
| Pre-hardened | Through-hardened | |
|---|---|---|
| Typical hardness | 28-36 HRC | 55-62 HRC |
| Grades | P20, 4140PH, 718H, NAK80 | D2, H13, A2, O1, S7, M2 |
| Heat treat step | None | Required, 2-3 weeks |
| Distortion risk | None | Real, planned for |
| Machining | Conventional carbide | CBN or coated carbide, or grind |
| Typical wear life | Moderate | High |
| Best fit | Prototype moulds, short-run tooling, mould bases, holder plates | Production dies, punches, cutting edges, high-volume cavities |
How to actually choose
Start from the shot count or the stroke count. A mould that will run 20,000 shots of unfilled ABS is a P20 job. A mould that will run 2 million shots of 30% glass-filled nylon is a through-hardened H13 job with the gate area possibly in something harder still.
Then check the section. If the part is a thin plate or has thin walls, through-hardening means distortion risk you may not want. Pre-hardened avoids it.
Then check the schedule. If the tool is needed in four weeks, the three-week heat treat queue makes the decision for you. This is why so many prototype and bridge tools are P20 — not because it is the best material, but because it is the fastest path to a working tool.
Then check the geometry. Deep, narrow features in hardened steel get expensive to finish. If the design has a lot of those, pre-hardened may cost less overall even if the wear life is marginal.
The middle option people forget
Some parts want both: a pre-hardened base with a through-hardened insert where the wear happens. A P20 cavity block with a D2 or powder-metallurgy insert at the gate gives you the wear resistance exactly where it is needed and the easy machining everywhere else. It adds an assembly step and needs a good fit, but it is frequently the cheapest tool over its whole life.
If you are not sure which side of the line your part falls on, put the shot count and the schedule on the RFQ alongside the drawing. That pair of numbers usually answers it without any further discussion.