Key Properties and Applications of 1.2312 Flat Bar in Tool Steel
If you are working with tool steels, you need to know that 1.2312 flat bar is a specific grade that offers a unique balance of machinability, wear resistance, and dimensional stability. It is a pre-hardened tool steel, often used directly in the as-delivered condition without the need for additional heat treatment. The key properties include a typical hardness range of 28 to 32 HRC, a tensile strength around 900 to 1100 N/mm², and a density of approximately 7.85 g/cm³. Its chemical composition is what sets it apart: it contains about 0.40% carbon, 1.50% manganese, 1.90% chromium, 0.20% molybdenum, and critically, 0.10% to 0.15% sulfur. That sulfur addition is deliberate—it improves machinability significantly, making it easier to cut, drill, and mill compared to standard 1.2311 or 1.2738 grades. This is not a steel for high-wear applications like stamping dies; it is designed for plastic mold bases, injection mold frames, and structural components where you need good polishability and moderate strength.
One of the most practical aspects of 1.2312 flat bar is its pre-hardened condition. You get it directly from the mill with a uniform hardness profile, which means you can machine it to final dimensions without worrying about distortion from heat treatment. The sulfur content, while improving machinability, does reduce impact toughness and weldability. So if you plan to weld this material, you need to preheat it to around 250 to 300°C and use a low-hydrogen filler metal. The flat bar form is typically available in thicknesses from 10 mm to 200 mm and widths up to 600 mm, with lengths up to 3000 mm. The surface finish is usually black or peeled, depending on the supplier. For plastic mold applications, the polished surface finish can reach a mirror-like quality, with a surface roughness down to Ra 0.05 µm after proper polishing. This makes it suitable for molds that produce transparent or high-gloss plastic parts, like automotive lenses or cosmetic packaging.
When you compare 1.2312 flat bar to other tool steels, the data is clear. For example, 1.2311 (which has lower sulfur) has a machinability rating of about 70% of a free-cutting steel, while 1.2312 jumps to about 85%. That 15% improvement translates directly to lower machining costs and longer tool life for your CNC operations. The thermal conductivity is around 35 W/m·K, which is moderate for tool steels. This means it dissipates heat reasonably well during injection molding cycles, reducing cycle times. The coefficient of thermal expansion is about 12.5 × 10⁻⁶ /K, which is standard for this class of steel. In terms of wear resistance, it is not as good as high-carbon high-chromium steels like D2 or 1.2379, but for plastic mold bases, that is rarely a concern. The key is that you get a material that is easy to machine, dimensionally stable, and cost-effective. Typical applications include support plates, clamping plates, ejector plates, and mold frames for injection molding, blow molding, and compression molding.
For a deeper look at the data, here is a table summarizing the key mechanical and physical properties of 1.2312 flat bar in its pre-hardened condition:
| Property | Value | Unit |
|---|---|---|
| Hardness (delivered) | 28-32 | HRC |
| Tensile Strength | 900-1100 | N/mm² |
| Yield Strength | 700-850 | N/mm² |
| Elongation at Break | 10-14 | % |
| Impact Toughness (Charpy V-notch) | 15-25 | J |
| Density | 7.85 | g/cm³ |
| Thermal Conductivity | 35 | W/m·K |
| Coefficient of Thermal Expansion | 12.5 × 10⁻⁶ | /K |
| Machinability (relative to free-cutting steel) | 85 | % |
Now, let us talk about real-world applications. In the injection molding industry, 1.2312 flat bar is the go-to material for the mold base. The mold base is the structural frame that holds the cavity and core inserts. It does not directly contact the plastic, so it does not need extreme wear resistance. What it needs is good machinability to create the necessary holes, pockets, and cooling channels. The sulfur content makes drilling and tapping easier, which is critical when you have dozens of water lines and ejector pin holes. For example, a typical mold base for a 200-ton injection molding machine might use a 1.2312 flat bar that is 400 mm × 500 mm × 100 mm. The material cost is about 20% lower than using a higher-grade steel like 1.2738, and the machining time is reduced by 15 to 20%. That is a significant saving in both time and money.
Another application is in the production of extrusion dies for plastic profiles. The flat bar is machined into the die body, which must withstand moderate pressures and temperatures up to 200°C. The pre-hardened condition ensures that the die maintains its shape over long production runs. The sulfur content does not cause any issues here because the die is not subject to impact loads. For blow molding, the flat bar is used for the mold frame and the pinch-off inserts. The good polishability of 1.2312 allows for a smooth surface finish on the mold cavity, which is essential for producing clear plastic bottles. The typical surface roughness after polishing is around Ra 0.1 µm, which is acceptable for most blow molding applications. If you need a better finish, you can use a higher-grade steel like 1.2083, but that will cost more and be harder to machine.
From a supply chain perspective, 1.2312 flat bar is widely available from European and Asian mills. The standard delivery condition is annealed to a hardness of 28 to 32 HRC. You can also get it in a pre-machined condition, with the surfaces ground to a tolerance of ±0.1 mm. This is useful if you are making a large number of identical mold bases. The material is typically certified to DIN 1.2312 or AISI P20+S, which is the equivalent American grade. The sulfur content is tightly controlled to be between 0.08% and 0.15%. If it goes above that, the machinability improves but the toughness drops too much. If it goes below, the machinability suffers. So the mills are careful to hit that sweet spot. For quality assurance, you should always request a mill certificate that shows the chemical composition and hardness test results. Most reputable suppliers will provide this at no extra cost.
When it comes to machining, there are some specific guidelines you should follow. For turning, use carbide inserts with a grade of P20 to P30, a cutting speed of 150 to 200 m/min, and a feed rate of 0.2 to 0.4 mm/rev. For milling, use a cutting speed of 120 to 180 m/min and a feed per tooth of 0.1 to 0.2 mm. For drilling, use high-speed steel or carbide drills with a point angle of 118 degrees and a cutting speed of 20 to 30 m/min. The sulfur content creates a built-up edge on the tool, so you need to use a coolant to flush the chips away. A water-soluble coolant at a concentration of 5 to 10% works well. The material is also suitable for wire EDM, but the sulfur can cause some issues with the wire breakage if the flushing is not adequate. For EDM, use a lower current and a higher flushing pressure to avoid problems. For grinding, use a soft-grade aluminum oxide wheel with a grit size of 46 to 60. The material is not prone to cracking during grinding, but you should still use a coolant to prevent thermal damage.
One of the less discussed aspects of 1.2312 flat bar is its resistance to stress corrosion cracking. In plastic molding, the mold is exposed to moisture and sometimes aggressive chemicals from the plastic. The chromium content of 1.9% provides some corrosion resistance, but it is not as good as stainless steel. If you are molding PVC or other materials that release hydrochloric acid, you should use a stainless steel grade like 1.2083 or 1.2316. For most other plastics, 1.2312 is fine. The material is also not recommended for use in food contact applications unless it is coated or plated, because the sulfur content can migrate into the food. For medical device molds, you should use a higher-grade steel that meets the biocompatibility requirements. But for general industrial molds, 1.2312 is a workhorse that gets the job done at a reasonable cost.
For a practical example, consider a mold for a plastic chair. The mold base is made from 1.2312 flat bar that is 800 mm × 600 mm × 150 mm. The machining time for the base is about 40 hours on a CNC mill. If you used a standard 1.2311, the machining time would be about 48 hours, an increase of 20%. The material cost for 1.2312 is about $2.50 per kg, while 1.2311 is about $2.30 per kg. So the material cost difference is small, but the machining cost saving is significant. At a shop rate of $100 per hour, the 1.2312 saves $800 in machining costs. That is a real-world benefit that you can calculate for your own projects. The flat bar is also easier to weld if you need to add features like cooling channels or support ribs. The preheat temperature of 250°C is not too high, and the post-weld stress relief is usually not required for simple welds. For complex welds, you should stress relieve at 550°C for 2 hours and then slow cool in the furnace.
Another important factor is the dimensional stability of 1.2312 flat bar during heat treatment. Since it is delivered pre-hardened, you do not need to heat treat it. But if you do need to increase the hardness for a specific application, you can harden it to 50 to 52 HRC by austenitizing at 850°C, quenching in oil, and tempering at 200°C. However, this will cause some distortion, and you will need to grind the surfaces to final dimensions. The material is not designed for through-hardening, so the core will be softer than the surface. For most mold base applications, the pre-hardened condition is sufficient. If you need higher hardness, you should use a different grade like 1.2343 or 1.2379. The flat bar is also available in a nitrided condition, which gives a surface hardness of 60 to 65 HRC. This is useful for molds that run abrasive materials like glass-filled nylon. The nitriding depth is typically 0.1 to 0.3 mm, and the process is done at 520°C for 10 to 20 hours.
From a cost perspective, 1.2312 flat bar is one of the most economical options for mold bases. The price per kg is typically 10 to 15% lower than 1.2738, and 20 to 30% lower than 1.2083. The machinability advantage further reduces the total cost of ownership. For a typical mold base weighing 500 kg, the material cost is about $1,250, and the machining cost is about $2,000. If you use a more expensive steel, the material cost might be $1,500, and the machining cost might be $2,400. So the total cost saving is about $650, or 20%. That is a significant number for a mold shop that makes hundreds of molds per year. The flat bar is also easy to source from stock, with lead times of 2 to 4 weeks for standard sizes. For non-standard sizes, the lead time is 6 to 8 weeks. Most suppliers will cut the flat bar to your required length and thickness, and some will even provide pre-machined surfaces.
If you are looking for a reliable supplier of 1.2312 flat bar, you need to check their quality control procedures. The mill certificate should show the chemical composition, hardness, and ultrasonic testing results for internal defects. The flat bar should be free of cracks, porosity, and inclusions. The surface finish should be consistent, with no scale or rust. The dimensional tolerances should be within ±0.5 mm for thickness and ±1.0 mm for width. The flatness should be within 0.3 mm per meter. If you are buying in bulk, you can request a test report from an independent laboratory. The material should be stored in a dry environment to prevent corrosion. If you are using it for a critical application, you can also request a hardness test on each bar to ensure uniformity. The typical hardness variation within a single bar is ±1 HRC, which is acceptable for most applications.
In terms of environmental impact, 1.2312 flat bar is recyclable. The steel can be melted down and reused to make new tool steel. The sulfur content does not affect the recycling process. The energy consumption for producing 1.2312 is about 5,000 kWh per ton, which is similar to other tool steels. The carbon footprint is about 2.5 tons of CO2 per ton of steel. If you are concerned about sustainability, you can ask your supplier for the environmental product declaration. Some mills now offer green steel with a lower carbon footprint, but the cost is higher. For most mold shops, the standard 1.2312 is a good balance of performance and cost. The material is also compatible with standard machining coolants and lubricants, which are biodegradable. The waste chips can be collected and recycled. The used mold bases can be sold as scrap steel, which recovers about 30% of the original material cost.