When you’re sourcing a 1.2311 flat bar for tooling, mold bases, or precision machining, the first thing you need to know is that the term “precision” here isn’t just marketing fluff—it refers to specific dimensional tolerances, surface finish grades, and hardness consistency that are baked into the material’s standard and custom processing. 1.2311 is a pre-hardened tool steel, equivalent to AISI P20 + Ni, with a typical hardness range of 28–32 HRC. The precision specification for a flat bar of this grade is governed by the EN 10088-3 standard for stainless steels and the EN 10083-3 for quenched and tempered steels, but in practice, most suppliers reference the DIN 1.2311 material data sheet and the ISO 2768-m or ISO 2768-f tolerance classes for machined surfaces. For a standard flat bar, the width and thickness tolerances are usually ±0.5 mm for dimensions up to 100 mm, and ±1.0 mm for dimensions between 100 mm and 300 mm. Length tolerances are typically +5 mm / -0 mm for cut-to-length bars. Surface finish on a precision flat bar is often specified as Ra ≤ 1.6 µm (63 microinches) for ground surfaces, and Ra ≤ 3.2 µm (125 microinches) for mill-finished surfaces. These numbers are critical because they directly affect how much material you need to remove during final machining, and whether your mold cavity or die set will hit the required fit. If you’re ordering a precision 1.2311 flat bar, you should always request a certified mill test report (MTR) that includes the actual chemical composition (C: 0.35–0.45%, Si: 0.20–0.40%, Mn: 1.30–1.60%, Cr: 1.80–2.10%, Ni: 0.90–1.20%, Mo: 0.15–0.25%) and the hardness reading in HRC taken at three points along the bar. The precision 1.2311 flat bar from reputable suppliers often comes with a guaranteed flatness of 0.5 mm per 300 mm length, and a parallelism of 0.3 mm across the width. These specs are not just nice-to-haves; they determine whether your bar will warp during wire EDM or heat treatment, and whether you can hold a ±0.01 mm tolerance on your finished part.
Let’s break down the dimensional precision in more detail. For a 1.2311 flat bar, the standard thickness range is from 6 mm to 300 mm, and width from 25 mm to 800 mm. The precision specification for thickness is often split into two categories: “hot rolled” and “pre-ground.” Hot rolled bars have a thickness tolerance of ±0.5 mm for sizes under 50 mm, and ±1.0 mm for sizes 50–150 mm. Pre-ground bars, which are surface-ground on both sides, tighten that to ±0.1 mm for thickness up to 100 mm, and ±0.2 mm for 100–200 mm. Width tolerance for pre-ground bars is typically ±0.3 mm for widths up to 300 mm, and ±0.5 mm for 300–600 mm. Length tolerance is usually +5 mm / -0 mm for standard bars, but if you order a precision cut, you can get +1 mm / -0 mm. Surface roughness on pre-ground bars is typically Ra 0.8–1.6 µm, which is suitable for direct use in mold bases without additional surface preparation. Hardness consistency is another precision factor: the 28–32 HRC range is typical, but some suppliers offer a tighter band of 29–31 HRC for critical applications like injection molding cores or die casting inserts. The microstructure of a properly processed 1.2311 flat bar should be tempered martensite with fine carbides, and the grain size should be ASTM 7 or finer. This ensures uniform wear resistance and polishability. If you’re working with EDM, the bar’s electrical conductivity is also a consideration—1.2311 has a resistivity of approximately 0.55 µΩ·m, which is standard for tool steels. The bar’s density is 7.85 g/cm³, and its thermal expansion coefficient is 11.5 × 10⁻⁶ /°C (20–200°C). These physical properties are important for predicting dimensional changes during machining or heat treatment. For example, if you’re cutting a 200 mm long bar, a 50°C temperature rise during grinding will cause a 0.115 mm expansion—that’s enough to throw off your tolerance if you don’t account for it.
Now, let’s talk about the real-world implications of these precision specs. In a typical mold shop, a 1.2311 flat bar is used for cavity plates, core plates, and bolster plates. The precision specification directly affects the fit between the plate and the mold base frame. If your bar is out of flatness by more than 0.5 mm over 300 mm, you’ll have to shim or grind the plate, which adds cycle time and cost. If the hardness is inconsistent—say, 28 HRC on one end and 32 HRC on the other—you’ll get uneven wear in the cavity, leading to premature tool failure. That’s why many precision suppliers offer a “guaranteed flatness” of 0.3 mm per 300 mm for bars up to 50 mm thick, and 0.5 mm per 300 mm for thicker bars. They also offer a “guaranteed hardness” of 30 ± 1 HRC for a premium. The surface finish is equally critical: if you’re using the bar as a mirror-finish mold surface, you need a starting roughness of Ra ≤ 0.8 µm, which requires a pre-ground surface. If you’re using it for a structural component, Ra ≤ 3.2 µm is usually acceptable. The chemical composition also plays a role in precision: the nickel content (0.90–1.20%) improves through-hardening, so a 1.2311 bar can be heat-treated to a uniform hardness even in sections up to 400 mm. The chromium content (1.80–2.10%) provides corrosion resistance—not enough for stainless steel, but enough to resist rust during storage if the bar is oiled. The molybdenum (0.15–0.25%) improves toughness and prevents temper embrittlement. These compositional details are not just academic; they directly affect the bar’s machinability, weldability, and polishability. For instance, a bar with a sulfur content above 0.030% will machine easier but may have poorer polishability. That’s why precision suppliers often specify a “clean steel” grade with low sulfur (≤0.010%) and low phosphorus (≤0.020%) for high-polish applications.
Let’s look at some data from a typical mill test report for a precision 1.2311 flat bar. The table below shows the actual measured values from a 50 mm × 200 mm × 1000 mm bar that was pre-ground on both sides:
| Parameter | Specification | Measured Value |
|---|---|---|
| Thickness (mm) | 50.0 ± 0.1 | 50.02 |
| Width (mm) | 200.0 ± 0.3 | 200.15 |
| Length (mm) | 1000 +5 / -0 | 1003 |
| Flatness (mm/300mm) | ≤ 0.3 | 0.18 |
| Parallelism (mm) | ≤ 0.3 | 0.12 |
| Surface Roughness (Ra, µm) | ≤ 1.6 | 1.2 |
| Hardness (HRC) | 28–32 | 30.5 |
| Hardness Uniformity (ΔHRC) | ≤ 2 | 1.0 |
| Carbon (C, %) | 0.35–0.45 | 0.40 |
| Silicon (Si, %) | 0.20–0.40 | 0.32 |
| Manganese (Mn, %) | 1.30–1.60 | 1.45 |
| Chromium (Cr, %) | 1.80–2.10 | 1.95 |
| Nickel (Ni, %) | 0.90–1.20 | 1.05 |
| Molybdenum (Mo, %) | 0.15–0.25 | 0.20 |
This data is typical for a premium supplier. Notice that the measured flatness (0.18 mm/300mm) is well within the ≤0.3 mm spec, and the hardness uniformity (ΔHRC = 1.0) is tighter than the ≤2 spec. That’s the kind of precision you need if you’re making a mold that will run 500,000 cycles without rework. The surface roughness of Ra 1.2 µm is good for a ground surface—you can polish it down to Ra 0.05 µm with standard diamond paste. If you’re using the bar for a hot runner manifold or a slide guide, you might need a tighter thickness tolerance of ±0.05 mm, which is available from some suppliers who offer “precision ground” bars with a ganged grinding process. That level of precision is typically quoted separately and costs 20–30% more than standard pre-ground bars. The length tolerance of +5 mm / -0 mm is standard for cut-to-length bars, but if you need a bar that is exactly 1000 mm for a CNC fixture, you can request a “precision cut” at +1 mm / -0 mm for an additional charge. The width tolerance of ±0.3 mm is typical for pre-ground bars, but if you’re using the bar as a die insert with a press-fit into a pocket, you might need a width tolerance of ±0.05 mm, which is achievable with a surface grinder or a blanchard grinder. That’s a different product category—often called “precision ground flat stock” or “tool steel flat stock”—and it’s available in 1.2311 from specialty suppliers. The key takeaway is that precision is not a single number; it’s a set of specifications that you need to match to your application. For a general-purpose mold base, the standard tolerances are fine. For a high-precision injection mold with tight shutoffs, you need the tighter specs.
Another angle to consider is the heat treatment response of a precision 1.2311 flat bar. Because it’s pre-hardened, you don’t usually need to heat treat it after machining, but if you do—for example, if you’re using it for a die that needs higher hardness—you need to know the bar’s “as-delivered” structure. A precision bar should have a uniform tempered martensite structure with no retained austenite. That’s important because retained austenite can cause dimensional instability during machining or thermal cycling. The standard practice is to triple temper the bar at 580–620°C after quenching, which gives the 28–32 HRC range. If you need a higher hardness, you can re-heat treat the bar to 40–45 HRC, but you’ll need to account for a 0.1–0.2% dimensional change in the thickness and width. That’s why precision bars are often supplied with a “stock allowance” of 0.5–1.0 mm per side for final grinding after heat treatment. The thermal expansion coefficient of 11.5 × 10⁻⁶ /°C means that a 200 mm bar will expand by 0.23 mm when heated from 20°C to 120°C—that’s a significant shift if you’re holding a ±0.01 mm tolerance. So, if you’re machining the bar in a hot environment, you need to compensate for thermal expansion. The elastic modulus of 1.2311 is 210 GPa, which is typical for tool steels. The yield strength is approximately 750–850 MPa in the pre-hardened condition. That’s high enough for most mold applications, but if you’re designing a thin-walled cavity, you need to check the deflection under clamping force. For a 1.2311 flat bar used as a mold plate, the recommended minimum thickness is 20 mm for a 300 mm × 300 mm cavity area, to avoid deflection under 100 tons of clamping force. These are the kinds of engineering decisions that depend on the precision specification of the bar.
Finally, let’s talk about the supply chain and quality assurance for precision 1.2311 flat bars. The best suppliers source from mills that use vacuum degassing and argon shielding during casting, to minimize inclusions and gas porosity. The bar is then hot rolled, annealed, and quenched and tempered in a controlled atmosphere furnace. The precision grinding is done on a blanchard grinder or a surface grinder with a magnetic chuck, and the bar is checked for flatness with a feeler gauge and a straight edge. The hardness is verified with a Rockwell tester at three points: one at each end and one in the middle. The chemical composition is verified by optical emission spectroscopy (OES) or combustion analysis. The surface roughness is measured with a profilometer. All of this data is recorded on the MTR, which should be included with every shipment. If the supplier doesn’t provide an MTR, you’re taking a risk. The precision 1.2311 flat bar from Asia Tools, for example, comes with a full MTR and a guarantee of flatness, parallelism, and hardness uniformity. They also offer a “no-warp” guarantee for bars up to 100 mm thick, meaning that if the bar warps more than 0.5 mm over 300 mm during your first machining pass, they’ll replace it. That’s the kind of service that matters when you’re running a production job. The price for a precision 1.2311 flat bar varies by size and tolerance: a standard 50 mm × 200 mm × 1000 mm bar costs about $150–$200, while a precision ground bar with ±0.05 mm thickness tolerance costs $250–$350. The lead time is typically 1–2 weeks for standard bars, and 2–4 weeks for precision ground bars. If you need a bar that is 300 mm thick and 600 mm wide, you’re looking at a custom order with a 4–6 week lead time and a price of $800–$1,200. The weight of a 1.2311 flat bar is 7.85 kg per mm of thickness per square meter of area, so a 50 mm × 200 mm × 1000 mm bar weighs 78.5 kg. That’s important for shipping costs and handling. The bar is typically shipped with a protective oil coating and wrapped in kraft paper or plastic film. If you’re storing it for more than a few weeks, you should apply a rust inhibitor or keep it in a dry environment. The precision specification is not just about the numbers on the MTR; it’s about the entire process from melt to delivery. When you order a precision 1.2311 flat bar, you’re paying for that process, and you should get a product that meets your dimensional, hardness, and surface finish requirements without surprises.