1.2311 (P20) plastic mould steel: properties, hardness, equivalents

1.2311 (40CrMnMo7) is the pre-hardened plastic mould steel sold at 28–34 HRC and machined in that condition — no heat treatment, no distortion. Composition, hardness, polishing and P20 equivalents.

Last updated 12 October 2026

What is 1.2311?

1.2311 (40CrMnMo7) is the steel most plastic moulds are built from, and the thing to understand about it is not its analysis but how it is sold: already hardened. A block arrives quenched and tempered to 28–34 HRC, the cavity is cut into it at that hardness, and the mould goes into the press without ever seeing a heat-treatment furnace. Nothing moves, nothing cracks, nothing needs re-grinding to size — which on a 600 kg mould base is worth far more than the extra hardness a through-hardened tool could offer.

The analysis exists to serve that trick. At 0.40 % carbon the steel reaches 28–34 HRC after tempering at around 600 °C, which is soft enough to machine with ordinary carbide tooling and hard enough to resist the clamping, injection and ejection loads of a few hundred thousand cycles. The 1.5 % manganese and 2 % chromium give it enough hardenability to come out of the quench uniformly at 250 mm section without nickel; above that the core falls away, which is exactly where 1.2738 with its 1 % nickel takes over.

The third member of the family is the one toolrooms actually buy most of. 1.2312 is the same steel with sulphur added: it machines about 20 % faster and costs the same, but the sulphide stringers that do the lubricating also ruin a polished surface and make welding unreliable. The division of labour is settled practice — 1.2312 for mould bases and bolsters, 1.2311 for cavities that will be polished, textured or welded, and 1.2738 for anything large.

AISI P20 is the North American counterpart; it is leaner on manganese, so a thick P20 block reads lower in the core than a 1.2311 block of the same size. For mould bases they are interchangeable, but the hardness certificate is what should be compared, not the designation.

1.2311 chemical composition

EN ISO 4957:2018, plastic mould steels (40CrMnMo7). AISI P20 to ASTM A681 is leaner on manganese: C 0.28–0.40, Mn 0.60–1.00, Cr 1.40–2.00, Mo 0.30–0.55. The high manganese of 1.2311 substitutes for part of the molybdenum and is what gives a 400 mm block a usable core hardness without nickel.

1.2311 mechanical properties

EN ISO 4957 specifies the chemistry and the delivery hardness range; the tensile figures are the normal correlation for a quenched and tempered 0.40 % C low-alloy steel at that hardness, not standardised values. The only number on the certificate that matters in practice is the **delivery hardness**, and it is measured on the surface — on a block over 250 mm the core will read lower.

1.2311 equivalent grades

Identical = same composition and mechanical limits within rounding. Near = one limit differs (e.g. carbon max or impact temperature). Functional = interchangeable for most uses after an engineering check. Full cross-reference for 1.2311 →

Old and superseded designations

Product forms and tolerance standards

Product forms: blocks and plate, pre-hardened and tempered (+QT), flat bar, pre-hardened, round bar, pre-hardened, machined mould bases and bolsters.

Dimensional tolerances: pre-hardened blocks: supplier tolerances, normally +3/+6 mm per face for finish machining; ground or milled all over on request; plate thickness: EN 10029 class B where supplied as plate; flatness and parallelism on machined mould bases: supplier specification, typically 0.05 mm/m.

Key property values

1.2311 delivery hardness

Supplied pre-hardened and tempered to 280–325 HB, that is 29–34 HRC. Typical guaranteed bands are 300 ± 20 HB. The hardness is measured on the surface; a block above 250 mm thick reads 260–300 HB at the core and the drop is normal, not a defect. Specify the hardness at the depth that matters if the cavity is cut deep into a thick plate.

Why it is not hardened further

The mould steel compromise: at 30 HRC a cavity can be milled, drilled, tapped and EDM-cut in the delivered state and will hold its dimensions for the life of the mould. Re-hardening to 42–44 HRC is possible (oil quench from 840–870 °C, temper at 550 °C) and occasionally done for small inserts, but it means distortion, re-grinding and the risk of cracking a machined cavity — so it is almost never done to a finished mould. Where a harder surface is needed, it is added locally by nitriding or flame hardening, not by re-quenching the block.

Surface hardening

Gas or plasma nitriding at 480–520 °C gives 600–700 HV to a depth of 0.2–0.4 mm with no measurable distortion and no loss of core hardness — the tempering temperature of the delivered block is above the nitriding temperature, which is why it works. Used on slides, guides, gate areas and ejector holes. Flame or induction hardening reaches 50–54 HRC locally. Hard chrome plating, PVD CrN and nickel plating are all applied on 1.2311 cavities for wear and release.

Polishing and texturing

1.2311 polishes to a mirror finish (Ra < 0.05 µm) and takes photo-etched textures cleanly — this is the single reason to pay for it instead of the free-machining 1.2312. Standard practice is to stone, then paper down to 1200 grit, then diamond paste. Remelted versions (Impax Supreme and equivalents) are bought where the moulding is optically critical, because ordinary continuously cast material carries enough inclusions to show pinholes at high polish.

Section size and hardenability

Uniform 28–34 HRC up to about 250 mm section; from 250 to 400 mm the core typically falls to 26–31 HRC, and beyond 400 mm it is below the useful band. The remedy is nickel: 1.2738 holds the same hardness to 600 mm and beyond. Do not solve it by specifying a higher delivery hardness — a 38 HRC block is noticeably harder to machine and gains nothing where the load is clamping, not wear.

Physical properties

Density 7.85 g/cm³, E = 205 GPa, thermal conductivity ≈ 34 W/m·K at 20 °C — better than any of the high-alloy tool steels, which matters directly for cycle time. Expansion 12.3 × 10⁻⁶/K (20–200 °C). Maximum sensible service temperature about 500 °C; in a mould the limit is set by the polymer, not the steel.

Weldability

Weldable, and mould repair welding is routine — this is the main advantage over the sulphur-bearing 1.2312. Preheat to 200–300 °C, TIG or micro-TIG with a matching filler (or a slightly softer filler where the weld will be polished), keep interpass temperature above 200 °C, cool slowly and temper at 500–550 °C, which is below the original tempering temperature so the block keeps its hardness. Laser cladding is used for small cavity corrections because the heat-affected zone stays under 0.5 mm and no post-weld temper is needed. A weld that will be polished must be made with matching filler, or it will show as a different-coloured patch after polishing.

Machining, forming and heat treatment

Machined in the delivered pre-hardened condition at 30 HRC: machinability ≈ 55–60 % relative to 1 % C free-cutting steel, entirely routine with coated carbide at moderate speeds. Deep cavities are milled dry or with air blast; tapping at 30 HRC needs quality taps and correct hole sizes. Drilling for conformal cooling lines is the usual difficulty on large blocks — use a through-coolant drill. EDM is standard for ribs and sharp corners; the white layer should be stoned off a surface that will be polished. If a cavity is to be stress-relieved after heavy rough machining, do it at 500–550 °C, never higher, or the delivered hardness is lost.

Typical applications

• Plastic injection mould cavities, cores and inserts
• Mould bases, bolsters and support plates (more often 1.2312)
• Blow-moulding and extrusion-blow tooling
• Die casting holder blocks, frames and bolsters for zinc
• Rubber and compression moulds
• Hydroforming and foam tooling
• Jigs, fixtures and machine parts needing 30 HRC without heat treatment

Frequently asked questions

Related grades

Verified against EN ISO 4957:2018; EN ISO 4957:2018, plastic mould steels; ASTM A681-08 (P20 comparison). Last checked: October 2026.