P20 Tool Steel vs 718H: The Complete Guide to Plastic Mold Steel Selection

If you build plastic injection molds for a living, you already know the golden rule: A high-quality mold starts with high-quality raw material. Before the first metal chip even flies off your CNC machine, the most critical decision has already been made during procurement.

If you buy a substandard block of steel or specify the wrong grade, the consequences are disastrous. The mold might wear out prematurely, fail to hold a polish, or worse-crack under injection pressure. And when that happens, the toolmaker eats the cost.

At the heart of the global tooling industry, two grades of plastic mold steel dominate the conversation: P20 tool steel and its upgraded, high-performance cousin, 718H steel.

For decades, tooling engineers and procurement managers have debated when to use the industry-standard P20 and when to justify the extra material cost for a forged block of 718H. Add in the confusion of European engineering drawings asking for a 1.2738 steel equivalent or 1.2311 steel, and buying the right block of steel becomes a massive headache.

In this comprehensive guide, Promisteel's metallurgical experts are taking you straight to the material science. We are going to strip away the marketing jargon and compare the chemistry, the machinability, the polishability, and the true cost-of-ownership of these two legendary mold steels.

By the end of this article, you will know exactly which steel block to order for your next tooling project.

Let's start with the undisputed workhorse of the tooling material supply chain. If you look around an average mold-making facility, probably 70% of the raw material sitting on the racks is some variation of P20 tool steel.

P20 (an AISI standard designation) is a low-carbon, chromium-molybdenum alloy steel. If your OEM clients are in Europe, they will almost universally specify this material on their blueprints as 1.2311 steel (DIN standard: 40CrMnMo7).

The biggest selling point of P20 for a machine shop is how we deliver it. Unlike cold-work steels (like D2) that you have to rough-machine in a soft state and then send out for risky heat treatment, Promisteel supplies P20 Pre-Hardened.

Typical Hardness: Delivered at 28 to 32 HRC (Hardness Rockwell C).
The Benefit: You buy the steel block from us, you strap it to your CNC milling machine, you cut the cavity, and you are done. There is no subsequent quenching, no risk of the steel warping in a furnace, and no need to calculate complex shrinking allowances.

For general-purpose plastic molds (injecting PP, PE, ABS) where your customer needs a standard commercial finish and a lifespan of maybe 300,000 to 500,000 shots, a block of P20 tool steel is the most cost-effective, reliable raw material on the market. It machines beautifully and maintains excellent dimensional stability.

So, if P20 steel blocks are so great, why does 718H steel even exist in our inventory?

The limitations of standard P20 become painfully obvious when you are commissioned to build massive molds, or when your customers demand flawless, mirror-like optical finishes on the final plastic parts.

718H steel was originally developed by the Swedish steel giant ASSAB to solve these exact limitations. The "H" simply stands for "High Hardness."

If a European automotive client sends you a drawing specifying a 1.2738 steel equivalent (DIN 40CrMnNiMo8-6-4), 718H is exactly the steel grade you need to source.

Like P20, we deliver 718H in the pre-hardened condition. However, it arrives at your dock hardened to a slightly higher specification of 33 to 37 HRC.

But the real magic isn't just the few extra points of hardness on the surface. The real reason you pay a premium for 718H happens deep inside the core of the massive steel block, thanks to one specific alloying element: Nickel.

To truly understand the difference between these two grades of plastic mold steel, you have to look at the chemistry of the ingot.

Here is a side-by-side comparison of the core elements:

Element	P20 Tool Steel (1.2311)	718H Steel (1.2738)	The Real-World Machining Impact
Carbon (C)	0.35% - 0.45%	0.35% - 0.45%	Identical. Both are medium-carbon steels.
Chromium (Cr)	1.70% - 2.00%	1.80% - 2.10%	Very similar. Provides basic hardenability and wear.
Molybdenum (Mo)	0.15% - 0.25%	0.15% - 0.25%	Similar. Helps maintain toughness.
Nickel (Ni)	None / Trace	0.90% - 1.20%	The Elephant in the Room. This is why 718H costs more.

Let's say you win a contract to build an injection mold for a car bumper. You need to order a forged steel block that is 1 meter thick and weighs 5 tons.

If you order standard P20 tool steel, the steel mill heats the massive forging and quenches it. The outside surface cools fast and hits 30 HRC. But the center of that massive block cools very slowly. Because P20 lacks deep hardenability, the core of that 1-meter block might only be 20 HRC. When your CNC machine cuts a deep cavity into that block, you will hit that soft core. The mold will wear out unevenly on the production line, and your client will reject the tooling.

If you order 718H steel (a true 1.2738 steel equivalent), the 1% Nickel fundamentally changes how the steel cools at the mill. Nickel promotes "Through-Hardenability." Even in a 1-meter thick forged block, the hardness will drop very minimally from the surface to the core. You can machine deep cavities with absolute confidence that the steel at the bottom of the mold is just as tough as the steel on top.

Chemistry dictates how the steel behaves under your cutting tools. Let's look at how these two materials compare when they hit your shop floor.

If your client is manufacturing clear plastic parts (like automotive headlight lenses, cosmetic jars, or medical syringes), the mold cavity must be polished to an optical mirror finish (SPI A-1 or A-2 standards).

P20 Tool Steel: It can be polished to a decent commercial finish. However, because its microstructure isn't perfectly uniform, if your mold polishers try to push it to a high-gloss mirror finish, they will often encounter "orange peel" or microscopic pitting.
718H Steel: This is where 718H dominates as a raw material. Thanks to its higher purity, vacuum degassing during smelting, and the addition of Nickel, its microstructure is incredibly uniform. It takes a brilliant, high-gloss polish without orange peeling. If your customer demands cosmetics, you must buy 718H.

Think about the plastic dashboard of a car-it looks like leather. That texture is created by chemically etching the steel mold cavity with acid.

To get a uniform "leather" pattern, the steel block must react to the acid evenly. If there are hard and soft spots in the steel (segregation), the acid bites unevenly, ruining the expensive mold.
Because of its refined microstructure, 718H etches far more predictably and uniformly than P20, making it the mandatory standard for textured automotive interiors.

1.2311 steel (P20) is slightly softer (around 30 HRC) and lacks Nickel, making it generally easier and faster to rough-machine. It is gentle on your carbide milling inserts.
718H Steel is harder (up to 37 HRC) and the Nickel makes the material "gummier" to cut. Your machinists will need to slow down the CNC feed rates slightly and expect a bit more wear on cutting tools.

The global steel trade is notorious for confusing nomenclature. If you are a procurement manager buying raw material, you must ensure you are comparing apples to apples.

USA: AISI P20
Germany / Europe: DIN 1.2311 steel (40CrMnMo7)
China: 3Cr2Mo
Note: Standard P20 does not contain Nickel.

Sweden: ASSAB 718H
USA: AISI P20+Ni
Germany / Europe: DIN 1.2738 steel (40CrMnNiMo8-6-4)
China: 3Cr2NiMo
Procurement Warning: If a supplier quotes you a suspiciously cheap price for "718H," but the Mill Test Certificate (MTC) shows 0% Nickel, they are selling you standard P20 steel and labeling it as 718H. Always verify the chemistry!

At Promisteel, we guarantee the chemical integrity of our materials. When you order a 1.2738 steel equivalent from us, we provide full SGS/TUV third-party inspection reports confirming the exact Nickel content.

Don't risk your expensive tooling on unverified steel blocks.

> Sourcing a true 1.2738 equivalent shouldn't be a gamble. Promisteel supplies 100% chemically verified and Ultrasonic Tested (UT) P20 and 718H forged blocks. Every shipment comes with an authentic Mill Test Certificate (MTC).

[Get a Secure Quote for Your Raw Material Today]

We have covered the metallurgy and the machining properties. Now let's talk about your raw material budget.

718H steel is noticeably more expensive than P20 tool steel. The exact price gap fluctuates with the global Nickel commodities market, but you can generally expect a 20% to 40% premium per ton for 718H.

How do you decide what to order? Use this simple Promisteel procurement matrix:

You are building small to medium-sized molds (under 400mm thick).
The final plastic part has a matte finish or requires only a standard commercial polish (e.g., internal brackets, hidden structural parts, cheap toys).
Your client is injecting non-abrasive, standard commodity plastics.
Your project bid is highly cost-sensitive and the material budget is tight.

You are machining massive, deep-cavity molds (e.g., automotive bumpers, large TV back covers) where uniform core hardness is critical.
The customer requires a flawless, high-gloss SPI A-1 mirror finish.
The mold requires complex, uniform photo-etched texturing.
The mold is expected to run for over 500,000 cycles and you want maximum insurance against premature cavity wear.

In the competitive landscape of mold manufacturing, there is no "one size fits all" raw material. Choosing between P20 tool steel and 718H steel requires a deep understanding of the mold's geometry, the aesthetic requirements of the final plastic part, and your material budget.

Whether you need the reliable economy of 1.2311 steel or the premium, high-polish capabilities of a true 1.2738 steel equivalent, sourcing from a reputable stockist is your first line of defense against tooling failure.

At Promisteel, we specialize in premium plastic mold steel. We stock both P20 and 718H in massive forged blocks and plates, ready to be saw-cut and milled to your exact custom dimensions. Every steel block we ship is rigorously ultrasonic-tested (UT) to SEP 1921 Class C/c standards, ensuring your CNC machines hit solid, flawless steel from the surface to the core.

Don't gamble on your next block of steel. Contact the Promisteel Technical Sales Team today for a competitive material quote, and let us supply the perfect foundation for your next injection molding project.