1.2738 (P20+Ni) plastic mould steel: properties, hardness by section, equivalents

1.2738 (40CrMnNiMo8-6-4) is the nickel-alloyed pre-hardened mould steel, P20+Ni: 28–34 HRC held uniformly in blocks to 600 mm and beyond. Hardness by section, HH variant, polishing and equivalents.

Last updated 12 October 2026

What is 1.2738?

1.2738 is 1.2311 with about 1 % nickel, and that single addition is bought for one reason: it keeps the hardness in the middle of a big block. A mould plate 600 mm thick cannot be cooled fast enough at its core for a nickel-free analysis to reach 30 HRC there; the nickel shifts the transformation curves far enough to the right that the core transforms to the same tempered structure as the surface. A 1.2311 block of that size reads 240–270 HB in the middle. A 1.2738 block reads 275–315 HB.

That is the whole engineering argument, and it sets the selection rule precisely. Below about 250 mm section, 1.2738 offers nothing over 1.2311 and costs 15–25 % more. Above 400 mm, it is the only one of the two that works. Large automotive bumper and dashboard moulds, appliance housings, crate and pallet tools and thick-walled blow moulds are where it belongs.

The nickel brings two secondary effects worth knowing. It raises the impact energy at the same hardness, so a deep cavity with sharp internal corners is less likely to crack in service — and in a mould base the dominant load is clamping and cycling, not wear, so toughness is the property that ends the tool. It also makes repair welding more exacting: a nickel-alloyed weld pool is more sensitive to hydrogen and the filler must match, or the repair shows up after polishing as a different shade of grey.

A note on the designation. 1.2738HH is not a different steel. It is the same analysis delivered at 38–42 HRC instead of 29–34 HRC, achieved by tempering lower. It gives a harder cavity surface and better texture retention for long production runs, in exchange for slower machining and much less latitude for later stress relief — and its tempering temperature is now close to the nitriding temperature, so the two cannot always be combined.

1.2738 chemical composition

EN ISO 4957:2018, plastic mould steels (40CrMnNiMo8-6-4). The analysis is 1.2311 with nickel added at 0.90–1.20 %; nothing else changes. AISI has no separate designation — the material is sold as P20+Ni or P20 modified, and the nickel content on a North American certificate may be anywhere from 0.5 to 1.2 %.

1.2738 mechanical properties

EN ISO 4957 fixes the chemistry and the delivery hardness band only; tensile and impact figures are the usual correlation for this class at that hardness, not standardised minima. The figure that should be on the order is **the hardness and where it is measured** — on a 700 mm block the difference between a surface reading and a mid-thickness reading is what distinguishes a well-made block from a badly quenched one.

1.2738 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 1.2738 →

Old and superseded designations

Product forms and tolerance standards

Product forms: large blocks, pre-hardened and tempered (+QT), heavy plate, pre-hardened, machined and ground mould plates, +QT ESR / remelted quality for optical and textured cavities.

Dimensional tolerances: pre-hardened blocks: supplier tolerances, normally +3/+8 mm per face on large sizes; plate thickness: EN 10029 class B; six-sided machined blocks: typically ±0.1 mm on size and 0.05 mm/m on parallelism, supplier specification.

Key property values

1.2738 hardness by section

Delivered 280–325 HB (29–34 HRC) and uniform out to roughly 400 mm. From 400 to 700 mm the core typically reads 275–315 HB — within a few points of the surface, which is the entire point of the grade. Compare that with 1.2311, where the same 700 mm block falls to 240–270 HB in the middle. Specify where the hardness is to be checked when ordering large blocks; a surface reading on a poorly quenched block tells you nothing about the cavity depth.

The HH variant

1.2738HH is the same steel tempered to 360–400 HB (38–42 HRC). It resists surface deformation and holds photo-etched textures longer on runs above a million cycles, and it is sold mainly as remelted material. Costs: machinability drops noticeably, drilling and tapping become slower, and because the tempering temperature is now around 500 °C, a later stress-relief or nitriding cycle has far less headroom. Use it where the cavity surface itself is the wearing part, not as a general upgrade.

Toughness and why it matters here

At delivery hardness, longitudinal impact energy is typically 25–40 J (KV, +20 °C), roughly a third higher than the nickel-free grade at the same hardness, and the transverse-to-longitudinal ratio is better. In a large mould the failure mode is cracking from a sharp internal corner, a cooling-line intersection or an ejector hole under repeated clamping — not wear. Toughness, not hardness, is what keeps a 600 mm plate in service.

Nitriding and surface treatment

Gas or plasma nitriding at 480–520 °C gives 650–750 HV to 0.2–0.4 mm with no distortion; the nickel makes no practical difference to the result. Standard on slides, guide bushes, gate inserts and ejector plates. Flame or induction hardening reaches 52–56 HRC locally. Hard chrome, nickel plating and PVD CrN are all used on cavities for release and wear. For the HH variant, check the tempering temperature against the nitriding temperature before committing — there may be only 20–30 °C of margin.

Polishing, texturing and cleanliness

Polishes to Ra < 0.05 µm and etches cleanly, but at high polish the limit is the inclusion content, not the steel's analysis. Ordinary continuously cast 1.2738 shows pinholes on a mirror cavity; remelted material (ESR) is what is specified for optical, high-gloss or fine-textured surfaces and is sold under names such as Impax Supreme and M238. For a textured interior panel mould the difference is visible on the moulded part, not just on the steel.

Physical properties

Density 7.85 g/cm³, E = 205 GPa, thermal conductivity ≈ 33 W/m·K at 20 °C. Expansion 12.4 × 10⁻⁶/K (20–200 °C), 13.3 × 10⁻⁶/K (20–400 °C). The conductivity is high for an alloy steel and is the reason mould cycle times are set by the cooling layout rather than by the steel.

Weldability

Weldable and routinely repair welded, but less forgiving than the nickel-free grade. Preheat to 250–325 °C and hold it, use a matching nickel-bearing filler — a mismatched filler polishes to a visibly different shade and etches differently, which ruins a textured cavity — keep interpass temperature above 250 °C, cool slowly under insulation and temper at 500–540 °C, below the original tempering temperature. Hydrogen control matters more here than in 1.2311: dry the filler and keep the joint clean. Micro-TIG and laser cladding are preferred for cavity corrections because the heat-affected zone stays shallow and no full-block temper is needed; on an HH block laser cladding is effectively the only option, because there is no room to temper afterwards.

Machining, forming and heat treatment

Machined in the delivered condition at 30 HRC, machinability ≈ 50–55 % relative to 1 % C free-cutting steel — slightly tougher-cutting than 1.2311 because of the nickel, and the chip is more adhesive. Coated carbide, moderate speeds, rigid set-ups. On blocks of this size the real work is deep-hole drilling for cooling lines: use through-coolant drills and expect to peck, and plan the drill pattern before the cavity is cut. Heavy rough machining on a large plate releases enough residual stress to move it — rough, stress relieve at 500–540 °C, then finish. EDM is standard; stone the white layer off any surface that will be polished. At 38–42 HRC in the HH condition, allow roughly 30 % more cycle time and use rigid tooling.

Typical applications

• Large plastic injection moulds — bumpers, dashboards, interior panels, appliance housings
• Crate, pallet and bin moulds, thick-walled technical mouldings
• Mould plates, bolsters and support blocks above 250 mm
• Blow-moulding tooling and extrusion-blow moulds
• Rubber and compression moulds in large sizes
• Die casting holder blocks and frames
• Heavy jigs, fixtures and press structures needing 30 HRC through section

Frequently asked questions

Related grades

Verified against EN ISO 4957:2018; EN ISO 4957:2018, plastic mould steels; GB/T 1299-2014 (3Cr2MnNiMo comparison). Last checked: October 2026.