What Are the Key Properties and Uses of 1.2738 Flat Bar in Tooling Applications?
Chemical Composition and Its Impact on Performance
The specific alloying elements in the 1.2738 flat bar dictate its behavior under thermal and mechanical stress. Let's break down the numbers:
| Element | Weight % (Typical) | Role in Tooling |
|---|---|---|
| Carbon (C) | 0.35–0.45 | Provides core hardness and wear resistance; higher carbon increases edge retention but reduces toughness. |
| Chromium (Cr) | 1.8–2.2 | Enhances hardenability and corrosion resistance; helps maintain hardness at elevated temperatures (up to 400°C). |
| Manganese (Mn) | 1.3–1.8 | Improves hot workability and reduces brittleness; also aids in deoxidization during melting. |
| Molybdenum (Mo) | 0.15–0.25 | Increases tempering resistance and prevents softening when the tool is cycled at 200–300°C. |
| Nickel (Ni) | 0.8–1.2 | Boosts toughness and through-hardening in thick sections; critical for large molds where center hardness must match surface hardness. |
What this means on the shop floor: if you're cutting a 600 mm thick block for an automotive bumper mold, the nickel content ensures that the center of the bar reaches the same hardness as the outer skin after heat treatment. Without nickel, you'd get a soft core that would wear out prematurely. The molybdenum content is the reason why this steel can be used in hot-runner systems where the mold temperature reaches 250°C without losing its temper. In contrast, a cheaper 1.2311 grade (which lacks nickel) would show a hardness drop of 3–5 HRC after 100,000 cycles in the same application.
Mechanical Properties in Real-World Tooling
The 1.2738 flat bar is supplied in the pre-hardened condition, meaning you don't need to send it to a heat treater before machining. The typical mechanical values are:
- Hardness: 28–32 HRC (can be specified up to 36 HRC on request).
- Ultimate Tensile Strength: 900–1100 MPa.
- Yield Strength (0.2% offset): 700–850 MPa.
- Elongation at Break: 12–16%.
- Impact Toughness (Charpy V-notch, transverse): 15–25 J at room temperature.
These numbers are not just theoretical. In a real case from a German mold maker, a 1.2738 flat bar used for a washing machine door mold ran 1.2 million cycles before any visible wear appeared on the cavity surface. The same mold made from a 1.2311 steel needed a rework after 400,000 cycles. The difference comes down to the combination of nickel and molybdenum, which reduces the rate of adhesive wear when the molten plastic flows over the steel surface at 220°C and 800 bar injection pressure.
Another critical property is the polishability. The 1.2738 flat bar can achieve a mirror finish of Ra 0.01–0.02 µm when using the correct sequence of diamond pastes (9 µm, 3 µm, 1 µm). This is essential for optical-grade parts like headlamp lenses or clear polycarbonate panels where any surface defect would be visible in the final product. The steel's low inclusion content (typically < 0.02% sulfur) prevents pitting during polishing, which is a common problem with free-machining grades like 1.2085.
Applications in Injection Molding and Die Casting
You'll find the 1.2738 flat bar in virtually every sector of plastic injection molding. Here are the most common use cases with specific data points:
Automotive Interior Trim Molds: For large parts like door panels or dashboards, the mold cavity can weigh 3–5 tons. The 1.2738 flat bar is chosen because it can be welded (using matching filler metal with 0.4% C, 2% Cr, 1% Ni) without preheating if the section is under 200 mm thick. Welding is common when adding cooling channels or repairing damaged areas. The weld zone will have a hardness of 28–32 HRC after a post-weld stress relief at 550°C for 2 hours, matching the base material.
Household Appliance Molds: For refrigerator liners or washing machine drums, the mold must withstand high clamping forces (2000–4000 tons) and repeated thermal cycling. The 1.2738 flat bar has a thermal conductivity of 32 W/m·K at 100°C, which is about 20% higher than a 1.2343 H13 steel. This means faster cooling cycles and shorter part cycle times. A typical mold for a refrigerator inner liner made from 1.2738 can reduce cycle time by 8–12 seconds per part compared to a standard P20 steel, which translates to a 10–15% increase in production throughput.
Die Casting Dies for Zinc and Aluminum: While not as common as H13 for aluminum die casting, the 1.2738 flat bar is used for low-pressure die casting of zinc alloys (melting point 390°C) and for prototype aluminum dies where the production run is under 50,000 parts. The steel's tempering resistance allows it to hold hardness up to 350°C, but above that, it will soften. For a zinc die casting die for a faucet handle, a 1.2738 flat bar core lasted 80,000 shots before the first signs of heat checking appeared, compared to 120,000 shots for a premium H13 die. The trade-off is cost: the 1.2738 flat bar is about 30–40% cheaper per kilogram than H13, making it viable for short-run or prototype work.
Machining and Fabrication Considerations
When you're working with the 1.2738 flat bar, you need to adjust your cutting parameters to account for its hardness. Here are the recommended feeds and speeds for common operations:
| Operation | Cutting Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Tool Material |
|---|---|---|---|---|
| Rough turning | 120–160 | 0.3–0.5 | 3–5 | Carbide (ISO P20–P30) |
| Finish turning | 180–220 | 0.1–0.2 | 0.3–0.5 | Carbide (ISO P10–P15) |
| Face milling | 100–140 | 0.15–0.25 per tooth | 2–4 | Carbide with AlTiN coating |
| Drilling (HSS-Co) | 15–20 | 0.08–0.12 | N/A | HSS with 5% cobalt |
| Drilling (Carbide) | 40–60 | 0.05–0.10 | N/A | Solid carbide with TiAlN coating |
One thing that catches many machinists off guard: the 1.2738 flat bar has a tendency to work-harden if you use a dull tool or if you take a light cut with a negative rake angle. This is because the nickel content increases the strain-hardening coefficient. Always use a sharp insert with a positive rake angle (7–11 degrees) and maintain a constant chip load. If you stop feeding and let the tool rub, you'll create a hardened layer that can be 0.1–0.2 mm deep and 5–10 HRC harder than the base material. That layer will then chip your tool on the next pass.
For EDM (electrical discharge machining), the 1.2738 flat bar behaves well. The typical material removal rate for roughing is 0.3–0.5 mm³/min/A with a copper electrode, and the surface finish after finishing can reach Ra 0.8 µm. The recast layer (white layer) is usually 0.02–0.05 mm thick, which can be removed by a subsequent polishing step. If you're using wire EDM, the cutting speed is about 30–40% slower than for mild steel due to the chromium content, which reduces electrical conductivity.
Heat Treatment and Stress Relieving
Even though the 1.2738 flat bar is pre-hardened, you still need to perform a stress-relieving treatment after rough machining. The standard procedure is:
- Heat the block to 550–600°C at a rate of 50°C/hour.
- Hold for 2 hours per 100 mm of thickness (e.g., a 300 mm thick block holds for 6 hours).
- Cool in still air to room temperature.
This reduces the residual stresses from machining by 50–70%, which prevents distortion during final machining or during the mold's first heat cycle. If you skip this step, you risk the mold cavity warping by 0.1–0.3 mm after the first injection cycle, which would scrap the part.
If you need to reharden the 1.2738 flat bar (for example, to increase wear resistance for a high-volume production run), the process is:
- Austenitizing: 850–870°C for 30 minutes per 25 mm of thickness.
- Quenching: Oil quench (or polymer quench for large sections) to avoid cracking.
- Tempering: 550–600°C for 2 hours per 25 mm of thickness, twice.
- Resulting Hardness: 38–42 HRC.
However, rehardening is rarely done in practice because the risk of distortion is high, and the cost of reheat treatment often exceeds the cost of buying a new block of higher-grade steel like 1.2344 (H13). The main advantage of the 1.2738 flat bar is that you don't need to heat treat it at all—you machine it, polish it, and put it into production.
Comparison with Alternative Tool Steels
To give you a clear picture of where the 1.2738 flat bar fits in the tool steel landscape, here's a comparison with three common alternatives:
| Property | 1.2738 (P20+Ni) | 1.2311 (P20) | 1.2343 (H11) | 1.2083 (420 Stainless) |
|---|---|---|---|---|
| Hardness (supplied) | 28–32 HRC | 28–32 HRC | 48–52 HRC (annealed) | 30–34 HRC |
| Maximum service temperature | 350°C | 300°C | 600°C | 400°C |
| Polishability (Ra achievable) | 0.01 µm | 0.02 µm | 0.05 µm | 0.01 µm |
| Corrosion resistance | Low | Low | Low | High |
| Weldability | Good (with preheat) | Good | Fair (requires preheat and PWHT) | Good |
| Through-hardness in 400 mm section | ±2 HRC | ±5 HRC | N/A (not used for thick sections) | ±3 HRC |
| Relative cost per kg | 1.0 (baseline) | 0.85 | 1.5 | 1.3 |
The key takeaway: if you need a mold for a large part with high polish requirements and the mold temperature stays below 350°C, the 1.2738 flat bar is the most cost-effective choice. If you need corrosion resistance (for PVC or flame-retardant plastics), go with 1.2083. If you need high-temperature strength for aluminum die casting, go with 1.2343. The 1.2738 flat bar sits in the sweet spot of "good enough" for 80% of injection molding applications.
Surface Treatments and Coatings
Many mold shops apply surface treatments to the 1.2738 flat bar to extend its service life. The most common are:
- Nitriding (gas or plasma): Creates a 0.1–0.3 mm thick case with a hardness of 900–1100 HV. This reduces adhesive wear and improves release for sticky plastics like polycarbonate or ABS. The treatment temperature is 520–540°C, which is below the tempering temperature of the steel, so the core hardness is unaffected. A typical nitrided 1.2738 flat bar mold for a PC headlamp lens can run 2–3 times longer than an untreated one before needing polishing.
- PVD coating (e.g., TiAlN, CrN): Applied at 400–500°C, these coatings are 2–5 µm thick and provide a hardness of 2000–3000 HV. They are used for molds that run abrasive plastics like glass-filled nylon (30% glass fiber). The coating reduces the wear rate by a factor of 5–10, but it requires a smooth base surface (Ra < 0.1 µm) to avoid flaking.
- Chrome plating: Used for molds that run PVC, which releases hydrochloric acid during processing. The chrome layer (0.05–0.1 mm thick) provides corrosion resistance and a low coefficient of friction. However, it can peel if the substrate is not properly prepared, so it's less common than nitriding.
One important note: if you plan to nitride or coat the 1.2738 flat bar, you must ensure that the steel has been stress-relieved after rough machining. Otherwise, the high temperature of the treatment can cause dimensional changes of 0.05–0.1 mm, which would ruin the mold fit.
Practical Tips for Sourcing and Handling
When you buy a 1.2738 flat bar, always check the supplier's certificate of analysis for the actual hardness and chemical composition. Some suppliers sell "equivalent" grades that may have lower nickel content (e.g., 0.5% instead of 1.0%), which will reduce the through-hardness in thick sections. The standard specification for the 1.2738 flat bar is DIN EN ISO 4957, and the material number is 1.2738. Reputable suppliers will also provide ultrasonic testing results to confirm that the bar is free of internal defects like porosity or cracks. For a 400 mm thick bar, the acceptable defect size is typically less than 2 mm diameter.
Storage is also important. The 1.2738 flat bar should be stored in a dry environment, preferably at 20–25°C and 40–60% relative humidity. If the bar is stored outside or in a humid shop, surface rust can form within a few days. Rust pits are difficult to remove from the polished surface and can cause defects in the mold. If you need to store the bar for more than a month, apply a rust-preventive oil (e.g., VCI oil) and wrap it in plastic.
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