AISI P20 (1.2311): Best for mold sections under 200 mm, high-gloss optical surfaces, weld-repair molds, and cost-sensitive tooling. | P20+Ni (1.2738): Required for sections over 250 mm to ensure uniform through-hardness; same polishability as standard P20. | P20+S (1.2312): Choose for maximum machining economy (25% faster cutting speeds), but avoid for optical surfaces or glass-filled/abrasive polymers.
What are P20-series plastic mold steels?
The P20 family of steels occupies a well-defined position in the tooling world: pre-hardened, medium-alloy Cr–Mn–Mo steels delivered to the mold shop already at working hardness (typically 28–36 HRC / 280–320 HB). No further heat treatment is required before machining cavities or mold frames — a decisive advantage over grades like H13 or D2 that require a full quench-and-temper cycle after rough machining.
This pre-hardened delivery condition is the core reason P20-series steels dominate plastic injection mold construction worldwide. The mold maker receives forged blocks or seamless rolled rings already at correct hardness, machines them directly to final dimensions, and puts the tooling straight into production.
Three main variants share this base composition, each adding or modifying one specific element to address a limitation of the standard grade:
All three grades share the same base composition (C–Cr–Mn–Mo) and the same pre-hardened delivery condition. The nickel in P20+Ni and the sulfur in P20+S are targeted modifications for specific use cases, not general upgrades or downgrades. Selecting the correct grade — not always the most expensive — is how engineers avoid overpaying or underperforming.
Many procurement engineers default to P20+Ni for all applications to be safe, adding unnecessary material cost for thin-section molds where it provides zero metallurgical benefit. Conversely, specifying standard P20 for a 500 mm thick mold base leads to soft, inconsistent core properties. This guide gives you the data to decide correctly for every application.
Chemical composition — where the three grades actually differ
The table below shows nominal chemical composition ranges per AISI/DIN standards. Highlighted cells show where each variant diverges from the standard P20 baseline.
| Element | AISI P20 · 1.2311 | P20+Ni · 1.2738 | P20+S · 1.2312 | Function in Steel |
|---|---|---|---|---|
| C (carbon) | 0.35–0.45% | 0.35–0.45% | 0.35–0.45% | Primary hardness former |
| Cr (chromium) | 1.70–2.00% | 1.80–2.10% | 1.70–2.00% | Hardenability, wear resistance |
| Mn (manganese) | 1.30–1.60% | 1.30–1.60% | 1.30–1.60% | Hardenability, deoxidation |
| Mo (molybdenum) | 0.30–0.45% | 0.15–0.25% | 0.30–0.45% | Deep hardenability, temper resistance |
| Ni (nickel) | ≤ 0.25% trace | 0.85–1.15% ▲ | ≤ 0.25% trace | Thick-section hardenability |
| S (sulfur) | ≤ 0.030% controlled | ≤ 0.030% controlled | 0.05–0.10% ▲ | MnS inclusions → chip-breaking |
| Si (silicon) | 0.20–0.40% | 0.20–0.40% | 0.20–0.40% | Deoxidation, hardness support |
| P (phosphorus) | ≤ 0.030% | ≤ 0.030% | ≤ 0.030% | Controlled for toughness |
Why nickel transforms thick-section performance in P20+Ni (1.2738)
Nickel in P20+Ni does not increase peak surface hardness — the 280–320 HB specification is identical to standard P20. Nickel's role is to slow the bainite/pearlite transformation kinetics during quenching, allowing austenite more time to transform to martensite even at the slow cooling rates found in the cores of large cross-sections. This property — called hardenability — is measured by the Jominy end-quench test. P20+Ni consistently maintains hardness uniformity through sections exceeding 400 mm; standard P20 drops off significantly beyond 200–250 mm.
Why sulfur improves machinability in P20+S (1.2312) — and what it costs
Sulfur reacts with manganese in the steel matrix to form manganese sulfide (MnS) inclusions. These elongated particles act as internal chip-breakers: they interrupt the chip, reducing cutting force, operating temperature, and tool wear per unit of material removed. Cutting speeds can typically be increased 15–25% on P20+S for the same tool life.
The same MnS inclusions that improve machinability are stress concentrators that reduce transverse impact toughness and fatigue life. P20+S (1.2312) must not be specified for molds processing glass-filled or abrasive polymers at high injection pressures, for components under cyclic loading, or for any application requiring optical-grade surface polish. It is a specialist grade, not a general substitute for P20.
Mechanical properties in pre-hardened delivery condition
All three grades are supplied pre-hardened (quenched and tempered) by the forging manufacturer. The values below are representative for 280–320 HB delivery condition.
Hardness at delivery is nearly identical across all three grades. Never base grade selection on hardness specification alone — you will always reach the wrong answer. The differentiating properties are section hardenability (P20+Ni advantage) and machinability index (P20+S advantage). Jiangsu Liangyi supplies forged P20 blocks and rings in all three grades, pre-hardened to 280–320 HB, with EN 10204 3.1 material test certificates.
Section hardenability: the most critical selection factor
Section hardenability determines whether the target hardness (e.g., 300 HB) is achieved uniformly through the full cross-section of a forged block, or only near the surface. For large mold bases this is not theoretical — a 400 mm thick platen measuring 310 HB at the surface but only 240 HB at the core has inconsistent structural stiffness and unpredictable cavity tolerances when machined at depth.
Practical rule: Specify P20+Ni (1.2738) whenever the cross-section of the forged block or ring exceeds 250 mm. Below this threshold, standard P20 (1.2311) achieves essentially the same through-hardness at significantly lower material cost. The nickel premium in P20+Ni is not justified for thin-section components.
Machinability comparison: where P20+S earns its place
Machinability encompasses achievable cutting speed for a given tool life, chip formation, surface finish at depth, tool wear rate, and heat generation. The three grades differ significantly on this axis.
Recommended CNC cutting parameters — external turning, pre-hardened 300 HB
| Parameter | P20 · 1.2311 | P20+Ni · 1.2738 | P20+S · 1.2312 |
|---|---|---|---|
| Roughing cutting speed | 100–140 m/min | 90–120 m/min | 130–180 m/min |
| Finishing cutting speed | 140–200 m/min | 120–170 m/min | 180–240 m/min |
| Roughing feed rate | 0.25–0.40 mm/rev | 0.20–0.35 mm/rev | 0.30–0.50 mm/rev |
| Roughing depth of cut | 2–5 mm | 2–4 mm | 3–6 mm |
| Recommended tooling | PVD carbide P25–P35 | PVD carbide P20–P30 | Uncoated or TiN P30–P40 |
| Coolant | Flood (sulfurized oil) | Flood (sulfurized oil) | Flood or MQL |
| Chip form | Long, curling | Long, tough | Short, self-breaking ✓ |
P20+Ni machines approximately 10% slower than standard P20 because the nickel addition increases toughness (resisting chip shear), and the Mo content is slightly lower in 1.2738 than in 1.2311. Budget for longer cycle times or reduce cutting speeds by 10–15% when switching from P20 to P20+Ni on an existing program.
Polishability and achievable surface finish
For optical or high-gloss plastic parts — lenses, display bezels, cosmetic packaging — the mold cavity finish is directly transferred to the molded part. Polishability determines the achievable Ra value and whether micro-pits or orange-peel texture appear at fine grit levels.
P20+S shows noticeably inferior polishability. The MnS inclusions that make it faster to machine create micro-pits and orange-peel texture when the surface is lapped and polished to fine grades. P20+S must not be specified for optical or SPI-A1/A2 surface requirements.
Standard P20 and P20+Ni polish to equivalent quality — both achieve Ra < 0.025 µm with correct polishing procedures using progressively finer diamond abrasives. For SPI-A1 or A2 finish, select P20 or P20+Ni based on section size only. P20+S is reserved for textured, EDM-finished, or non-optical surfaces.
Heat treatment parameters
All three grades are typically supplied pre-hardened by the forging manufacturer. If re-hardening is required after welding or design change, apply the following parameters.
Elevated sulfur content significantly increases hot cracking susceptibility during welding. Preheat to minimum 200 °C is mandatory before welding; use low-hydrogen electrodes (H4 or H2 class); post-weld temper immediately at 500–550 °C for a minimum of 2 hours. For molds requiring significant cavity weld repair, P20 (1.2311) or P20+Ni (1.2738) are strongly preferred over P20+S.
Application selection guide
The following scenarios map real-world mold-making applications to the correct P20-series grade.
Large injection mold bases — section >300 mm
P20+Ni (1.2738) is required. Standard P20 will not achieve uniform through-hardness. Nickel ensures consistent core-to-surface properties in thick platens and large mold frames — the primary reason this grade exists. See available sizes and specifications for custom P20+Ni open die forgings.
General-purpose cavity inserts — section <200 mm
Standard P20 (1.2311) is fully adequate and the most cost-effective choice. Cross-section does not push the limits of P20 hardenability; P20+Ni adds 15–25% material cost with no metallurgical benefit at this scale.
Complex CNC mold frames — machining economy priority
P20+S (1.2312) is appropriate where the primary driver is reducing CNC cycle time on complex multi-axis toolpaths, and surface finish requirements are not optical-grade and polymer abrasivity is low.
Optical lens molds and high-gloss part tooling
P20 or P20+Ni, depending on section size. Both achieve equivalent polishability (Ra < 0.025 µm, VDI 6). Select P20+Ni only if section exceeds 250 mm. P20+S must never be specified for SPI-A surface requirements.
Die casting — zinc and low-temperature alloys
P20 or P20+Ni depending on section size. P20-series is appropriate for zinc and tin-bismuth die casting. For aluminum die casting, H13 (1.2344) is preferred due to its superior hot hardness and thermal fatigue resistance.
Prototype molds and short-run tooling
P20+S offers the fastest machining cycle time for prototype tools where long-term durability and optical finish are secondary to speed-to-first-part. Typical mold life: 100,000–300,000 shots with unfilled polymers.
Grade decision matrix
Use this matrix to confirm your selection. Each row represents a key performance criterion; indicators show how well each grade satisfies the requirement.
| Criterion | P20 · 1.2311 | P20+Ni · 1.2738 | P20+S · 1.2312 |
|---|---|---|---|
| Thin-section hardenability (<200 mm) | Excellent | Excellent | Excellent |
| Thick-section hardenability (>300 mm) | Not suitable | Excellent | Not suitable |
| Machinability / CNC cycle time | Adequate | Adequate (10% slower) | Excellent (+25%) |
| Polishability — optical / SPI-A finish | Excellent | Excellent | Not suitable |
| Weldability / mold repair | Excellent | Excellent | Requires care |
| Impact toughness under cyclic load | Excellent | Excellent | Reduced (MnS) |
| Glass-filled / abrasive polymer runs | Excellent | Excellent | Not recommended |
| Relative material cost | Lowest | +15–25% vs P20 | Low (≈ P20) |
| EDM / electrical discharge machining | Excellent | Excellent | Adequate |
| Prototype / short-run tooling economy | Adequate | Adequate | Best choice |
Summary verdict by grade
- Cavity inserts — sections under 200 mm
- High-gloss or optical surface finish
- Molds requiring weld repair
- Cost-sensitive production tooling
- General-purpose mold frames
- Mold bases >250 mm thick
- Platens and frames over 400 mm
- Long production runs (>1 M shots)
- Optical / high-gloss in large blocks
- Core-to-surface uniformity required
- Maximum CNC machining economy
- Prototype and short-run molds
- Textured or EDM-finished surfaces
- Unfilled / lightly filled polymers only
- Sections under 200 mm
Frequently asked questions
Request a Quote for AISI P20 Forged Parts
Jiangsu Liangyi manufactures custom AISI P20, P20+Ni (1.2738), and P20+S (1.2312) open die forgings and seamless rolled rings — 30 kg to 30,000 kg single piece — from our ISO 9001 certified factory in Jiangyin, Jiangsu Province, China. EN 10204 3.1 material test certificates supplied as standard. Response within 24 hours.