P20 is a low-carbon tool steel designed primarily for making plastic injection molds. It belongs to the AISI P-series of mold steels and arrives from the supplier already hardened to 30-34 HRC (Rockwell C), which means manufacturers can machine it directly into mold shapes without needing to heat treat it first. This “pre-hardened” delivery state is P20’s defining advantage: it saves time, reduces cost, and eliminates the risk of distortion that comes with hardening a finished mold.
Composition and How It Works
P20 contains chromium, molybdenum, and manganese as its main alloying elements, with a carbon content between 0.28% and 0.40%. The chromium (1.4-2.0%) provides moderate corrosion resistance and helps the steel harden uniformly through thick cross-sections. Molybdenum (0.30-0.55%) adds strength and toughness, while manganese (0.6-1.0%) improves machinability. This combination gives P20 something called good “through-hardening ability,” meaning even large mold blocks harden consistently from surface to core rather than staying soft in the middle.
Under international standards, P20 goes by several names. In the European DIN system it corresponds to 1.2311 (40CrMnMo7), and a free-machining variant with added sulfur is designated 1.2312 (40CrMnMoS8-6). The French equivalent is 40CMD8, and its UNS designation is T51620. If you see any of these on a material certificate, you’re looking at essentially the same family of steel.
Mechanical Properties
In its standard pre-hardened condition, P20 has a tensile strength of 965-1,030 MPa (roughly 140,000-150,000 psi) and a yield strength of 827-862 MPa (120,000-125,000 psi). Its hardness sits around 30-34 HRC. For context, that’s significantly harder than structural steel but softer than the 44-52 HRC range typical of hardened tool steels used in high-wear applications.
What makes this hardness range useful is the balance it strikes. At 30-34 HRC, P20 is hard enough to resist the pressures inside a plastic injection mold (molten plastic is injected at thousands of psi), yet soft enough to machine with conventional cutting tools. Harder steels require grinding or electrical discharge machining (EDM), which is slower and more expensive.
Where P20 Is Used
Plastic injection molds are P20’s primary home. Whenever a manufacturer needs a mold for producing plastic housings, containers, consumer goods, or automotive interior parts, P20 is often the default choice for small and medium-sized tooling. It’s used for mold bases, cavity blocks, backing plates, and core inserts.
Beyond plastics, P20 serves in zinc alloy die-casting molds, where the lower melting point of zinc (compared to aluminum) keeps operating temperatures within P20’s comfort zone. It also finds use in rubber molds and stamping dies for moderate production runs. In toolmaking shops, its machinability makes it a practical raw material for punches, milling cutters, and other precision cutting tools.
Surface Finish and Polishability
One of P20’s standout qualities is its ability to take a high-gloss polish. This matters enormously in plastic injection molding, because the mold surface directly transfers to the finished plastic part. If you’ve ever held a glossy plastic phone case or a clear container with a smooth interior, there’s a good chance the mold was made from P20 or a similar grade.
The steel’s uniform microstructure, free of large carbide clusters, allows polishers to achieve mirror-like finishes without pitting or orange-peel texture. When the highest optical clarity isn’t needed but a smooth, cosmetically acceptable surface is, P20 delivers reliably without the cost premium of specialty mirror-finish steels.
Machinability and Welding
P20 machines well relative to other tool steels. Its pre-hardened state is specifically calibrated so that standard carbide tooling can cut it efficiently. Shops can drill, mill, and turn P20 without the specialized setups that harder tool steels demand. The free-machining variant (P20+S, or 1.2312) contains a small amount of added sulfur (around 0.05%) that further improves chip breaking during cutting, at a slight cost to polishability.
Welding P20 requires care. The steel’s alloy content makes it susceptible to cracking if welded cold, so preheating to around 250°C (482°F) for two hours before welding is standard practice. This prevents thermal shock at the weld zone. Mold shops routinely weld P20 to repair worn surfaces, fill design changes, or correct machining errors, and with proper preheating and post-weld stress relief, repaired molds can return to full production service.
Heat Treatment
Most users never need to heat treat P20, since it arrives pre-hardened. That said, some applications call for adjusting its properties. Tempering at various temperatures between 400°C and 700°C can fine-tune the balance between hardness and toughness. Lower tempering temperatures preserve more hardness, while higher temperatures soften the steel but increase toughness and relieve internal stresses.
Stress relieving is common after heavy machining. Removing large amounts of material from a pre-hardened block can introduce residual stresses that cause the mold to warp slightly over time or during use. A controlled heating cycle relaxes those stresses without significantly changing the steel’s hardness.
P20 vs. H13: Choosing the Right Steel
P20 and H13 are both widely used in mold and die work, but they serve different temperature ranges. H13 is a hot-work tool steel loaded with chromium, molybdenum, and vanadium. It maintains its hardness and strength at high temperatures and resists the repeated heating and cooling cycles (thermal fatigue) that destroy lesser steels. That makes H13 the go-to choice for aluminum die-casting molds, hot forging dies, and extrusion tooling, all applications where the tool face regularly exceeds 500°C.
P20, by contrast, works best at lower temperatures. It’s tougher and easier to machine than H13, and it costs less. But it softens under prolonged high heat, which limits it to plastic molding (where mold temperatures rarely exceed 200°C) and low-melting-point die casting like zinc. The decision between the two usually comes down to operating temperature: if the mold or die will get hot, H13 is the safer bet. For plastic molds where surface finish and machinability matter more than heat resistance, P20 is the more practical and economical choice.
Limitations Worth Knowing
P20 is not a stainless steel. Its moderate chromium content provides some resistance to atmospheric corrosion, but molds stored in humid environments or used with corrosive plastics (like PVC, which releases hydrochloric acid during processing) can develop surface rust or pitting. Protective coatings, proper storage, and regular maintenance help extend mold life in these situations.
Its hardness ceiling is another constraint. At 30-34 HRC, P20 wears faster than hardened steels in the 50+ HRC range. For molds expected to produce millions of parts, or for molding glass-filled plastics that are highly abrasive, a through-hardened steel like H13 or a specialty wear-resistant grade will last longer. P20 is best suited for low-to-medium production volumes or for prototyping, where its easy machinability and lower material cost offset the need for occasional refurbishment.