The key difference is that 1.2738 steel plate is a pre-hardened plastic mold steel, while standard tool steel grades like D2, O1, or A2 are typically delivered in an annealed or soft condition and require heat treatment after machining. 1.2738 comes ready to use at a hardness of 280–325 HB (roughly 29–34 HRC), which eliminates the need for post-machining heat treatment, reducing distortion risks and lead times. Standard tool steels, by contrast, are often supplied at 180–220 HB and must be hardened, quenched, and tempered to reach their working hardness of 58–62 HRC. This fundamental difference in delivery condition and intended application drives everything from machining strategy to final part performance.
Let’s break this down with hard data. The chemical composition of 1.2738 (also known as 40CrMnNiMo8-6-4) includes roughly 0.38–0.45% carbon, 1.8–2.2% chromium, 0.9–1.2% manganese, 0.4–0.6% nickel, 0.15–0.25% molybdenum, and trace silicon. Compare that to a standard high-carbon, high-chromium tool steel like D2, which has about 1.5% carbon and 12% chromium. The lower carbon content in 1.2738 means it can be pre-hardened without becoming brittle, while the chromium and nickel add through-hardening capability and toughness. The nickel content in 1.2738 is a key differentiator—it improves core toughness and polishability, which is critical for plastic injection mold cavities that need a mirror finish. Standard tool steels like O1 (oil-hardening) or A2 (air-hardening) have negligible nickel, focusing instead on wear resistance through higher carbide content.
Now, let’s talk about the heat treatment reality. With standard tool steels, you’re looking at a multi-step process: preheating to 650–700°C, austenitizing at 980–1050°C (depending on grade), quenching in oil, air, or salt bath, and then double tempering at 180–550°C to achieve the desired hardness. This process introduces dimensional changes—typically 0.1–0.3% shrinkage or growth—and risk of cracking, especially in complex geometries. For a large mold base, that’s a nightmare. With 1.2738 steel plate, you skip all that. It’s delivered pre-hardened to 280–325 HB, which is the sweet spot for plastic mold applications. You machine it, EDM it, polish it, and it’s ready. No furnace, no distortion, no tempering. That’s a huge time and cost saver, especially for shops that don’t have in-house heat treatment capabilities.
But there’s a trade-off. The hardness of 1.2738 (29–34 HRC) is significantly lower than the 58–62 HRC you get from hardened D2 or A2. That means 1.2738 is not suitable for applications requiring high wear resistance, like cutting tools, stamping dies, or shear blades. For plastic injection molding, though, the wear requirements are lower—the main concerns are corrosion from plastic gases, thermal fatigue, and polishability. 1.2738 handles those well, especially when nitrided or coated. For example, a gas-nitrided 1.2738 surface can reach 650–750 HV (about 58–60 HRC) with a case depth of 0.1–0.3 mm, giving it a wear-resistant surface while retaining a tough core. Standard tool steels can also be nitrided, but they often require pre-hardening first, which adds steps.
Let’s look at machinability. 1.2738 in the pre-hardened condition machines at about 70–80% of the speed of mild steel, but it’s still very manageable with carbide tooling. Standard tool steels in the annealed condition (180–220 HB) machine even easier, but the problem is you have to heat treat them after machining, which can cause distortion. For large plates, that distortion is often unacceptable. That’s why 1.2738 is the go-to for large plastic mold bases, like those used in automotive bumpers, TV frames, or appliance housings. A typical mold base for a car bumper might be 1.5 meters by 1 meter by 0.5 meters thick. Trying to heat treat that in a standard furnace is impractical—you’d need a specialized vacuum furnace and even then, the risk of quench cracking is high. With 1.2738, you buy the plate pre-hardened, cut it, machine the cavities, and assemble it.
Now, let’s compare some numbers in a table for clarity:
| Property | 1.2738 (Pre-hardened) | D2 (Standard Tool Steel) | A2 (Standard Tool Steel) |
|---|---|---|---|
| Delivery hardness | 280–325 HB (29–34 HRC) | 180–220 HB (annealed) | 180–220 HB (annealed) |
| Working hardness | 29–34 HRC (as-is); 58–60 HRC (nitrided) | 58–62 HRC (heat treated) | 57–62 HRC (heat treated) |
| Carbon content | 0.38–0.45% | 1.5% | 0.95–1.05% |
| Chromium content | 1.8–2.2% | 12% | 5% |
| Nickel content | 0.4–0.6% | Negligible | Negligible |
| Typical applications | Plastic injection molds, large mold bases | Cutting tools, stamping dies, shear blades | Forming dies, punches, gauges |
| Heat treatment needed | No (pre-hardened) | Yes | Yes |
| Dimensional stability | Excellent (no post-machining heat treat) | Moderate (risk of distortion) | Good (air-hardening, less distortion) |
| Wear resistance | Good (with nitriding) | Excellent (high carbide content) | Very good |
| Polishability | Excellent (mirror finish possible) | Good (but can have carbide pullout) | Good |
Another angle is the cost. 1.2738 steel plate is typically priced higher per kilogram than standard tool steel grades in the annealed condition—roughly 20–30% more. But when you factor in the cost of heat treatment, which can be $1–3 per pound for commercial vacuum heat treating, plus the risk of scrapping a part due to distortion, the total cost of ownership often favors 1.2738 for large molds. For small tools or high-wear applications, standard tool steels are still more economical because you can heat treat them in a small furnace and the material cost is lower.
Let’s talk about thermal conductivity, which matters for mold cooling. 1.2738 has a thermal conductivity of about 35–40 W/mK at room temperature, which is decent for a steel. Standard tool steels like D2 have lower conductivity, around 20–25 W/mK, due to their high alloy content. That means 1.2738 can dissipate heat faster during the injection molding cycle, reducing cycle times and improving part quality. For high-volume production, that’s a real advantage. Also, 1.2738 has a coefficient of thermal expansion of about 12.5 x 10^-6 /°C, which is similar to other tool steels, so thermal expansion mismatch isn’t an issue.
Now, let’s get into the nitty-gritty of polishability. For plastic molds that need a mirror finish (like for optical lenses or clear parts), the steel must be free of inclusions and have a uniform microstructure. 1.2738, with its nickel content, produces a fine, uniform tempered martensite structure that polishes to a high gloss. Standard tool steels like D2 have large, hard chromium carbides that can pull out during polishing, leaving pits. That’s why 1.2738 is preferred for high-gloss applications. The sulfur content in 1.2738 is typically low (0.005% max) to avoid sulfide inclusions that cause pitting. Some suppliers offer a “ESR” (electroslag remelted) version for even better polishability.
Let’s also consider weldability. 1.2738 is weldable using preheat and post-weld heat treatment, which is common for repairing mold cavities or adding inserts. Standard tool steels are notoriously difficult to weld due to their high carbon content—they require preheat to 300–500°C and slow cooling to avoid cracking. For 1.2738, preheat to 200–300°C and post-weld tempering at 500–550°C is sufficient. This makes it more forgiving in repair situations.
One more point: availability. 1.2738 steel plate is widely stocked in thicknesses from 20 mm to 600 mm, with widths up to 2 meters and lengths up to 6 meters. Standard tool steel plates are often stocked in smaller sizes because they’re intended for smaller parts. For large mold bases, 1.2738 is the default choice. If you’re sourcing 1.2738 steel plate, you’ll find it in pre-hardened condition from major mills like ThyssenKrupp, Bohler, and Daido. The material is typically supplied to ASTM A681 or DIN 1.2738 standard, with ultrasonic testing for internal soundness.
Let’s look at some real-world applications. A manufacturer of automotive interior trim parts uses 1.2738 for mold cavities that run glass-filled nylon. The mold runs 24/7, and the cavity surface is nitrided to 0.2 mm depth. After 500,000 cycles, the cavity shows minimal wear. The same manufacturer tried using A2 tool steel for a similar part, but the heat treatment distortion caused the cavity to be out of tolerance by 0.05 mm, requiring rework. The 1.2738 mold went straight into production without issues. In another case, a medical device molder uses 1.2738 for molds that produce clear polycarbonate parts. The mirror finish on the cavity is maintained for over 1 million cycles with periodic repolishing.
From a mechanical properties standpoint, 1.2738 has a tensile strength of about 900–1000 MPa in the pre-hardened condition, with elongation of 10–12% and impact toughness of 20–30 J (Charpy V-notch). Standard tool steels in the hardened condition have tensile strengths of 1800–2200 MPa but elongation of 1–2% and impact toughness of 5–10 J. So 1.2738 is much tougher, meaning it can withstand the cyclic stresses of injection molding without cracking. That toughness comes from the nickel and the lower carbon content, which prevents the formation of brittle martensite.
Another difference is in the manufacturing process. 1.2738 is often produced via the electric arc furnace (EAF) route, followed by vacuum degassing and argon stirring to reduce gas content. Standard tool steels may use the same process, but they often require additional steps like ESR or VAR (vacuum arc remelting) for high-performance applications. For 1.2738, the standard quality is sufficient for most plastic mold applications, but ESR grades are available for critical optical molds.
Let’s talk about the heat treatment of 1.2738 itself. Even though it’s pre-hardened, you can still heat treat it further if needed. For example, if you want a higher hardness for a specific application, you can austenitize at 840–870°C, quench in oil, and temper at 180–200°C to get 50–52 HRC. But this defeats the purpose of using pre-hardened steel—you’d be better off starting with a standard tool steel. The real value of 1.2738 is in the “as-delivered” condition.
In terms of international standards, 1.2738 is equivalent to AISI P20 modified (with nickel), or to the Japanese standard SKT4. The European standard is 40CrMnNiMo8-6-4. The “modified” version with nickel is sometimes called “P20+Ni” or “1.2738 HH” (higher hardness). Standard P20 (1.2311) has no nickel and is delivered at 280–325 HB, but it has lower toughness and polishability. So the nickel addition in 1.2738 is a real upgrade.
Now, let’s address the elephant in the room: why would you ever choose a standard tool steel over 1.2738? The answer is simple: when you need extreme wear resistance, high hardness, or the ability to cut other materials. For example, a stamping die for 0.5 mm thick stainless steel sheet needs a hardness of 60–62 HRC and high wear resistance. 1.2738 at 30 HRC would wear out in minutes. For that, you need D2, M2, or powder metallurgy tool steels. Similarly, a cutting tool for aluminum needs high hardness and edge retention. 1.2738 is not designed for that. But for plastic injection molding, blow molding, compression molding, and rubber molding, 1.2738 is the standard.
Let’s also consider the surface treatment options. 1.2738 can be nitrided, nitrocarburized, PVD coated, or chrome plated. Nitriding is the most common, giving a case depth of 0.1–0.3 mm and surface hardness of 650–750 HV. This improves wear resistance and corrosion resistance without affecting the core toughness. Standard tool steels can also be nitrided, but they often require pre-hardening to 50–55 HRC first, which adds cost. For 1.2738, you can nitride it directly in the pre-hardened condition, saving a step.
One more practical point: when you buy 1.2738 plate, you need to specify the thickness and hardness range. Some suppliers offer “HH” (high hardness) at 320–345 HB, or “H” (standard) at 280–320 HB. The HH grade is better for wear resistance but slightly less tough. For most applications, the standard grade is fine. The plate is typically supplied with a black oxide finish or mill scale, which you’ll need to remove before machining.
In terms of machining parameters, for 1.2738 at 300 HB, you’d use carbide inserts with a speed of 100–150 m/min, feed of 0.1–0.3 mm/rev, and depth of cut of 1–4 mm. For standard tool steels in the annealed condition, you can use higher speeds (150–200 m/min) but you’ll need to account for the heat treatment distortion later. For EDM, 1.2738 machines well with a surface finish of 0.4–0.8 Ra achievable. For grinding, use aluminum oxide or CBN wheels.
Let’s not forget about corrosion resistance. 1.2738 has moderate corrosion resistance due to the 2% chromium content. It can rust in humid environments, so it’s often stored with a rust preventive oil. For molds running PVC or other corrosive plastics, a stainless steel like 1.2083 (420 stainless) is better, but 1.2738 can be chrome plated or nitrided to improve corrosion resistance. Standard tool steels like D2 have higher chromium (12%) but still aren’t stainless—they can rust if not properly maintained.
Finally, let’s talk about the supply chain. 1.2738 is a commodity grade, so it’s widely available from multiple mills. The lead time for standard sizes is usually 2–4 weeks from stock, while custom sizes may take 6–8 weeks. Standard tool steel grades are also widely available, but the heat treatment adds another 1–2 weeks to the lead time. For urgent projects, 1.2738 is the faster option.