What is the composition and hardness of an industrial 1.2311 steel block?

The composition and hardness of an industrial 1.2311 steel block are precisely defined by its designation as a pre-hardened plastic mold steel, typically containing 0.35–0.45% carbon, 1.8–2.1% chromium, 0.5–0.8% manganese, 0.15–0.3% molybdenum, and 0.1–0.2% vanadium, with a delivered hardness range of 28–32 HRC (Rockwell C scale). This material, also known as 40CrMnMoS8-6 under EN standards, is a low-alloy tool steel designed for direct use in injection molds, extrusion dies, and blow molds without requiring additional heat treatment after machining. The hardness is achieved through a controlled quench and tempering process during manufacturing, giving it a uniform through-hardness that balances wear resistance with machinability. For instance, a typical industrial 1.2311 steel block from a reputable supplier like those found in the industrial 1.2311 steel block market will have a tensile strength of 980–1080 MPa and a yield strength around 830 MPa, making it reliable for high-pressure molding applications. The sulfur content is often elevated to 0.05–0.1% to improve free-cutting properties, which is critical for complex cavity geometries. Below is a detailed breakdown of its chemical composition and mechanical properties.

Chemical Composition of 1.2311 Steel (Weight %)

The composition is tightly controlled to ensure consistent performance across batches. Carbon provides the core hardness, while chromium enhances corrosion resistance and wear properties. Manganese and molybdenum contribute to hardenability and toughness, vanadium refines grain structure, and sulfur improves machinability. Here is the typical range:

Element	Percentage (%)
Carbon (C)	0.35 – 0.45
Chromium (Cr)	1.80 – 2.10
Manganese (Mn)	0.50 – 0.80
Molybdenum (Mo)	0.15 – 0.30
Vanadium (V)	0.10 – 0.20
Sulfur (S)	0.05 – 0.10
Phosphorus (P)	≤ 0.025
Silicon (Si)	0.20 – 0.40

This combination results in a steel that is often supplied in the pre-hardened condition, meaning you can machine it directly without post-heat treatment. The sulfur addition is a key differentiator from similar grades like 1.2312, which has higher sulfur for even better machinability but slightly lower toughness. For a 1.2311 block, the microstructure is typically tempered martensite or bainite, depending on the cooling rate during production. This structure gives it a fine, uniform grain that resists chipping under cyclic loading in mold cavities. The hardness is not just a surface treatment; it is consistent throughout the block thickness, up to 400 mm or more, due to the alloying elements' response to quenching. In practice, if you order a 1.2311 steel block measuring 300 x 200 x 100 mm, you can expect a hardness variation of less than ±2 HRC across the entire cross-section, which is critical for large molds where uneven hardness would cause premature wear or distortion.

Hardness and Mechanical Properties

The delivered hardness of 28–32 HRC is the sweet spot for plastic mold applications. It is hard enough to resist abrasion from glass-filled polymers like nylon or ABS, yet soft enough to be machined with standard carbide tools at reasonable speeds. For example, a typical machining operation on a 1.2311 block might use a cutting speed of 150–200 m/min with a feed rate of 0.1–0.3 mm/rev, achieving a surface finish of Ra 0.8 µm without excessive tool wear. The tensile strength at this hardness level is around 1000 MPa, with an elongation at break of 10–12% in the longitudinal direction. Impact toughness, measured by the Charpy V-notch test at room temperature, is typically 20–30 J, which is sufficient for withstanding the clamping forces in injection molding machines. Here is a summary of key mechanical data:

Property	Value
Hardness (HRC)	28–32
Tensile Strength (MPa)	980–1080
Yield Strength (MPa)	830–880
Elongation (%)	10–12
Charpy Impact (J) at 20°C	20–30
Modulus of Elasticity (GPa)	210
Thermal Conductivity (W/m·K)	35–40

One practical implication of this hardness range is that the steel can be polished to a mirror finish of Ra 0.05 µm or better, which is essential for producing transparent plastic parts like lenses or automotive light covers. The thermal conductivity of 35–40 W/m·K is moderate, meaning it can dissipate heat from the mold cavity efficiently, reducing cycle times in production. However, if you need higher hardness for abrasive materials, you can nitride the surface of a 1.2311 block to achieve a case hardness of 900–1100 HV (approximately 65–70 HRC) while maintaining the core toughness. This is a common post-processing step in the mold industry, and the steel's composition responds well to gas nitriding or plasma nitriding without distortion. The block's dimensional stability during nitriding is excellent, with growth typically less than 0.02 mm per side, because the pre-hardened condition minimizes residual stresses.

Microstructure and Heat Treatment Background

The industrial 1.2311 steel block is produced by melting in an electric arc furnace, followed by ladle refining and vacuum degassing to remove inclusions. The ingot is then hot-rolled or forged into blocks, with a typical reduction ratio of 4:1 to ensure isotropy of properties. After hot working, the block undergoes a quenching process: it is austenitized at 840–870°C, then oil-quenched or polymer-quenched to form martensite. Tempering is performed at 540–600°C to achieve the target hardness of 28–32 HRC. This tempering temperature is chosen because it falls in the secondary hardening range for chromium-molybdenum steels, where fine carbide precipitates of Cr7C3 and Mo2C form, enhancing wear resistance without sacrificing toughness. The resulting microstructure is a mixture of tempered martensite with dispersed carbides, which gives the steel its characteristic balance of strength and ductility. For a 1.2311 block, the grain size is typically ASTM 7–8, which is fine enough to prevent cracking during machining but coarse enough to avoid excessive brittleness. The inclusion rating is controlled to ASTM E45 Type A (sulfide) at 1.5 or lower, which is important for achieving consistent polishability. If you are sourcing a block for a high-volume production mold, you should also check the ultrasonic testing report for internal soundness, as 1.2311 is often supplied in a "premium" grade with no defects larger than 1 mm in diameter.

Comparison with Other Mold Steels

To understand where 1.2311 fits in the spectrum of tool steels, it helps to compare it with common alternatives. For instance, 1.2738 (40CrMnNiMo8-6-4) has a similar carbon content but adds nickel for improved through-hardening in larger sections, delivering 28–32 HRC with better toughness (Charpy impact of 35–45 J). However, 1.2738 is more expensive and harder to machine due to its higher nickel content. 1.2083 (X40Cr13) is a stainless mold steel with 13% chromium, offering superior corrosion resistance but lower machinability and a hardness of 50–54 HRC after heat treatment, which is not pre-hardened. 1.2343 (X37CrMoV5-1) is a hot-work steel with higher hardness (50–55 HRC) for die casting, but it requires full heat treatment after machining. In contrast, 1.2311 is a workhorse for plastic molds where cost, machinability, and moderate wear resistance are the priorities. The sulfur content in 1.2311 gives it a machinability rating of about 75–80% of free-cutting steel (like 1215), compared to 1.2738 at 60–65%. This means you can remove material faster with less tool wear, which is a direct cost saving in mold production. For a typical mold cavity with complex undercuts, using a 1.2311 block can reduce machining time by 15–20% compared to a nickel-alloyed grade, while still achieving a service life of 500,000 to 1,000,000 cycles for unfilled plastics. For glass-filled materials, the life drops to 200,000–500,000 cycles, which is still acceptable for many applications.

Practical Considerations for Sourcing and Use

When you buy an industrial 1.2311 steel block, you should specify the dimensions with a tolerance of +1/-0 mm on the length and width, and +0.5/-0 mm on the thickness, to allow for surface grinding. The block is typically supplied in a black or peeled condition, with a surface roughness of Ra 3.2 µm or better. For high-precision molds, you may need to order a "pre-machined" block with a flatness tolerance of 0.02 mm per 300 mm. The steel's density is 7.85 g/cm³, so a 400 x 300 x 150 mm block weighs about 141 kg. Shipping costs can be significant, so it is worth sourcing from a local distributor or a supplier with a warehouse in your region. The price per kilogram for 1.2311 is typically $2–4 USD for standard sizes, but premium grades with ultrasonic testing and certification can cost $5–8 USD per kg. For critical applications like medical device molds, you should request a mill test certificate showing the actual chemical analysis and hardness values. The block's machinability is excellent, but you should use coolant to prevent heat buildup during heavy cuts, as the pre-hardened condition can still cause work hardening if the tool rubs instead of cuts. Recommended cutting parameters for roughing: depth of cut 2–4 mm, speed 120–180 m/min, feed 0.2–0.4 mm/rev. For finishing: depth of cut 0.2–0.5 mm, speed 180–250 m/min, feed 0.05–0.15 mm/rev. Use carbide inserts with a TiAlN coating for best tool life. The steel can be welded using a matching filler metal like 1.2311 welding rod, with preheating to 200–300°C and post-weld stress relief at 500–550°C, but welding should be minimized because the heat-affected zone can soften to 25–28 HRC, reducing wear resistance in that area. If you need to repair a mold, it is often better to use a steel plug or insert rather than welding.