P20 vs P20-Ni: picking a mold steel for thick-section molds

The default plastic mold steel, and its upgrade

P20 is the pre-hardened steel most Karachi mold shops reach for first when quoting an injection or die-casting mold, work directly connected to the Karachi Tools, Dies and Moulds Centre's mission to support Pakistan's plastics and automotive tooling sectors. But P20 has a limit worth knowing before quoting a large mold: hardenability through thick sections.

What's in standard P20

P20 runs 0.28 to 0.40% carbon, 1.40 to 2.00% chromium, and 0.30 to 0.55% molybdenum, with manganese and silicon rounding out the balance. That's enough alloying to harden well in moderate section thicknesses, reaching around 300 Brinell (roughly 30 HRC) in the pre-hardened condition supplied, with tensile strength in the 965 to 1030 MPa range and reasonable ductility, 20% elongation at break. It's supplied pre-hardened specifically so mold shops can machine complex geometry without needing to heat treat the finished mold themselves, one less process step for a shop that isn't set up for controlled hardening. It's referenced under ASTM A681, UNS T51620, or DIN 1.2330, and tempering after machining, if needed, runs 482 to 593°C for a final hardness between 37 and 28 HRC.

Why P20 alone isn't enough for big molds

In a large mold with a thick core, the center of that section may not harden as uniformly as the surface, leaving a softer core than the mold designer intended, a real problem when the whole point of the mold steel is consistent hardness across the cavity. That's exactly the gap P20-Ni, also known as DIN 1.2738, is built to close.

What the nickel addition does

P20-Ni adds 0.90 to 1.20% nickel to a composition otherwise close to standard P20. Nickel improves hardenability through thickness, meaning a thick mold section hardens more uniformly from surface to core than standard P20 would achieve. It's specifically positioned for large plastic molds under high core stress, die-casting dies, and hydroforming tools, up to roughly 400mm sections in some published references. Physically it runs 7.80 g/cm³ density, 205 GPa elastic modulus, and 29.0 W/mK thermal conductivity, with a typical strength around 1080 N/mm². Most P20-Ni is supplied already pre-toughened and needs no further heat treatment unless the application has special requirements, but when hardening is needed the process runs 840 to 870°C followed by an oil or hot-bath quench, landing around 52 HRC, with soft annealing available at 710 to 740°C for jobs that need to be machined further afterward.

When to reach for 2083 instead

Neither P20 nor P20-Ni is the right call when the mold needs corrosion resistance and a mirror-polish finish, medical device molds and optical component molds, for instance. 2083, a high-chromium stainless mold steel, is the grade for that narrower, more technical segment, relevant to Karachi mold shops serving Sialkot's medical device and instrument exporters where the finished plastic parts often need a surface finish and material purity standard mold steels can't reliably deliver.

Quoting the right grade

Ask about section thickness before defaulting to standard P20. A mold under roughly 150 to 200mm in its thickest section is usually fine with P20. Beyond that, or wherever core hardness uniformity genuinely matters to the customer, quote P20-Ni and explain why, the price difference is easier to justify once a customer understands it's solving an actual hardening problem, not just an upsell.

Working on a mold and unsure which grade fits the section thickness? Send us the drawing and we'll help you spec it.