4140 and P20 pre-hardened: the workhorse grades nobody writes about
Search for hardened steel machining and you get D2, H13 and CBN inserts at 62 HRC. Walk through an actual tool room and most of the steel on the floor is 4140 or P20 at around 32 HRC.
These are the grades that quietly do the work, and they behave differently enough from the through-hardened family to deserve their own notes.
What they are
4140 (42CrMo4, 1.7225, SCM440) is a chromium-molybdenum low alloy steel. Bought “PH” or “pre-hard”, it arrives quenched and tempered at 28-32 HRC. Bought annealed, it can be through hardened to around 50 HRC, though few people bother.
P20 (1.2311, and the sulphurised variant 1.2312) is a mould steel — essentially a modified 4140 with a composition tuned for polishability and section uniformity. Supplied pre-hardened at 28-36 HRC. 718H and NAK80 are the higher-spec relatives.
Both are supplied stress relieved, so they machine without the internal stress that makes an annealed block move as you cut it.
What you get
| 4140 PH | P20 (1.2311) | |
|---|---|---|
| Supply hardness | 28-32 HRC | 28-36 HRC |
| Tensile strength | ~1000 MPa | ~1000-1100 MPa |
| Machinability (vs 1% C steel) | ~60-65% | ~65-70% |
| Polishability | Fair | Good |
| Weldability | Fair, pre-heat needed | Good with correct filler |
| Typical use | Shafts, holders, structural tooling, die shoes | Mould cavities, cores, mould bases |
The headline: no heat treat operation. You machine it, you ship it. No three-week queue, no distortion allowance, no second finishing pass, no risk of a cracked block after twenty hours of work. On lead time alone that is frequently decisive — see pre-hardened vs through-hardened.
How they machine
At 30-ish HRC these are firmly in carbide territory. No CBN, no dry cutting, no special toolpaths. But they are not mild steel either, and treating them like it produces short tool life and poor finishes.
Practical notes:
Run coated carbide, not uncoated. TiAlN or AlTiN. The cutting temperature is high enough that an uncoated edge softens.
Keep the feed up. The common mistake is dropping feed when the material feels hard. That rubs rather than cuts, work hardens the surface, and kills the edge. Maintain chip load and drop the speed instead if you need to.
Climb mill. Conventional milling in pre-hardened steel starts the cut at zero chip thickness, which means the edge rubs before it bites — exactly the condition that work hardens the surface for the next pass.
Rigidity still matters. At 32 HRC cutting forces are substantially higher than in mild steel. Long tool overhangs chatter, and chatter in pre-hardened steel chips edges rather than just sounding bad.
Expect roughly half the tool life you would get in 1045 at the same parameters. That is normal, not a problem with the tooling.
Where they stop being enough
The reason not everything is made of P20 is wear life. At 32 HRC:
- A blanking die edge will round over quickly against anything but soft material.
- A mould gate running 30% glass-filled nylon will erode in tens of thousands of shots.
- A sliding wear surface under load will gall.
The rule of thumb in mould work: unfilled resins and moderate volumes are P20 territory; filled resins or high volumes need a through-hardened cavity, or at minimum a hardened insert at the gate and any other high-wear feature.
For 4140, the equivalent line is load and abrasion. It is an excellent shaft and holder material. It is not a cutting edge.
Nitriding: the middle path
Both grades take nitriding well, which is the standard way to extend them without going to a through-hardened grade. A nitrided case on P20 reaches 60-65 HRC equivalent at a depth of 0.1-0.3 mm, over a tough 32 HRC core.
Three things to know:
- Nitride after all machining. The case is thin and hard; machining through it defeats the purpose and destroys tooling.
- Dimensional growth is small but real — typically a few microns to 0.02 mm. On a precision fit, account for it.
- It does not fix a design problem. A thin case over a soft core will collapse under point loading. Nitriding extends abrasion life, it does not add bulk strength.
On the drawing
State the supply condition: “P20, pre-hardened 30-34 HRC, no further heat treatment”. That single line stops a shop from quoting a heat treat cycle you did not want.
If the part has one high-wear feature and the rest is ordinary, say so — an insert or a nitrided zone is usually cheaper than moving the whole part to a harder grade.
Send the drawing with the volume and the material being processed, and the quote will tell you whether the grade you picked matches the life you need.