What are the key specifications of industrial 1.2738 flat bar for mold manufacturing?

If you’re sourcing steel for injection molds, die casting, or large plastic tooling, the industrial 1.2738 flat bar is a go-to material because it’s pre-hardened to around 290–330 HBW, which means you skip the heat treatment step after machining. This steel grade—also known as DIN 1.2738 or AISI P20 + Ni—is a chromium-nickel-molybdenum alloy that delivers through-hardening capability, excellent polishability, and good machinability in the hardened state. The key specs start with its chemical composition: roughly 0.35–0.45% carbon, 1.8–2.2% chromium, 0.8–1.2% nickel, 0.5–0.8% molybdenum, and 0.8–1.2% manganese. That nickel addition is what sets 1.2738 apart from standard P20—it boosts toughness and uniformity in larger cross-sections, which is critical for big mold bases. The tensile strength sits around 980–1080 MPa, with yield strength at 830–930 MPa, and elongation at about 12–15% in 50 mm. Hardness is delivered in the pre-tempered condition, typically 290–330 HBW, but you can also get it at 330–370 HBW for higher wear resistance. The flat bar dimensions vary widely—common thicknesses range from 20 mm to 600 mm, widths from 100 mm to 1200 mm, and lengths up to 6000 mm. Surface finish is usually black or peeled, with tolerances per EN 10058 or ASTM A681. For mold shops, the real value is in the uniformity: 1.2738 flat bar offers consistent hardness through the entire thickness, even up to 400 mm, because of the nickel and molybdenum content. That’s a big deal when you’re cutting cavities or cooling channels deep into the steel. The industrial 1.2738 flat bar from reputable suppliers like Asia Tools also comes with ultrasonic testing (UT) to EN 10228-3 or SEP 1921 standards, ensuring no internal defects like porosity or cracks. Polishability reaches a surface roughness of Ra 0.05 µm after fine grinding, making it suitable for glossy plastic parts. Thermal conductivity is around 32 W/m·K at 20°C, which helps with mold cooling cycle times. The steel also resists softening up to 400°C, so it works for hot-runner systems and moderate-temperature dies. One more spec to note: the delivery condition is QT (quenched and tempered), so you get a fine-tempered martensitic structure with good stability. If you’re comparing it to 1.2311 (P20 without nickel), 1.2738 flat bar holds hardness better in sections over 250 mm thick. That’s why it’s the standard for large automotive bumpers, appliance housings, and structural foam molds. For EDM (electrical discharge machining), the material’s cleanliness—low sulfur content under 0.015%—reduces the risk of micro-cracking. And for welding repairs, preheat to 250–350°C and use matching filler metal like 1.2738 or 1.2312. The density is 7.85 g/cm³, and the modulus of elasticity is 210 GPa. All these specs mean you can machine cavities, drill cooling lines, and polish surfaces without worrying about distortion or soft spots. Just remember to rough machine first, then stress-relieve at 550°C for 2–4 hours, then finish machine to get the best dimensional stability. The flat bar is also available with a ground finish (h9 or h11 tolerance) for precision fit into mold frames. For large-scale production, ordering 1.2738 flat bar in long lengths (up to 6000 mm) reduces the number of joints in multi-cavity molds. The steel’s corrosion resistance is moderate—better than 1045 but not stainless—so store it in a dry environment to avoid pitting. If you need higher hardness, some suppliers offer 1.2738 HH (extra high hardness) at 340–380 HBW, but that reduces machinability slightly. The microstructure after QT is tempered martensite with fine carbides, which gives good wear resistance for abrasive plastics like glass-filled nylon. For optical mold applications, the cleanliness class is typically 1–2 per ASTM E45 (thin/heavy series), which is excellent for mirror finishes. The steel also has good through-hardening capability in sections up to 500 mm, thanks to the nickel addition. That’s a key advantage over 1.2311, which starts to lose hardness past 250 mm. In practice, a 400 mm thick 1.2738 flat bar will have a hardness drop of only 10–15 HBW from surface to core, while a 1.2311 bar might drop 30–40 HBW. That consistency means your mold core and cavity have the same mechanical properties, reducing warpage during injection. The flat bar is also available with a pre-machined finish (rough ground or milled) to save time in your shop. For high-volume production, some mills offer 1.2738 flat bar with a guaranteed hardness range of 300–320 HBW for tighter control. The steel’s machinability rating is about 50–60% of 1045 steel, but it’s still better than D2 or H13 in the hardened state. You can use carbide tooling at feeds of 0.2–0.4 mm/rev and speeds of 100–150 m/min for roughing, and 150–200 m/min for finishing. The material also responds well to nitriding, which can increase surface hardness to 600–700 HV for improved wear resistance. For large molds, the flat bar can be pre-machined with cooling channels, ejector pin holes, and guide pin holes before final hardening. The dimensional stability during heat treatment is excellent, with a typical distortion of 0.05–0.1% in the length direction. That’s why 1.2738 is the first choice for large, complex mold bases. The steel also has good weldability if you follow the preheat and post-weld heat treatment guidelines. For repair welding, use a low-hydrogen process and keep the interpass temperature below 350°C. The flat bar is also available in a vacuum-degassed version for higher cleanliness, which is important for molds with fine details or deep cavities. The sulfur content is typically below 0.005% for vacuum-degassed material, which improves polishability and reduces the risk of pitting. For EDM, the material’s low sulfur also means less electrode wear and better surface finish. The thermal expansion coefficient is 12.5 × 10⁻⁶ /K from 20°C to 200°C, which is similar to other tool steels. That’s important for matching the mold to the machine’s thermal behavior. The steel also has good resistance to thermal fatigue, with a thermal conductivity of 32 W/m·K at 20°C and 30 W/m·K at 200°C. That helps with heat dissipation during injection cycles. For large molds, the flat bar can be pre-heated to 350°C before welding to avoid cracking. The material’s toughness is also excellent, with a Charpy V-notch impact energy of 30–40 J at room temperature. That’s higher than 1.2311 and comparable to 1.2343 (H11). That toughness means the steel can handle high clamping forces and injection pressures without cracking. For mold bases, the flat bar is often supplied with a ground finish on all six sides, with a tolerance of ±0.1 mm on thickness and width. That saves time when fitting into the mold frame. The steel is also available with a pre-hardened condition of 330–370 HBW for higher wear resistance, but that reduces machinability by about 20%. For most applications, the standard 290–330 HBW is the best balance of machinability and wear resistance. The flat bar can also be supplied with a certificate of conformity to EN 10204 3.1 or 3.2, which includes chemical analysis, mechanical properties, and ultrasonic test results. That’s important for quality control and traceability. The steel’s microstructure is fine-tempered martensite with no retained austenite, which ensures dimensional stability during service. The steel also has good resistance to tempering, with a secondary hardness peak at 500–550°C. That means you can stress-relieve the material without losing hardness. For large molds, the flat bar can be pre-machined with a roughing allowance of 1–2 mm per side, then stress-relieved, then finished. That process reduces distortion and improves surface finish. The steel’s polishability is excellent, with a surface roughness of Ra 0.05 µm after fine grinding and Ra 0.02 µm after diamond polishing. That’s suitable for high-gloss plastic parts like automotive lenses or cosmetic packaging. The steel also has good corrosion resistance in dry conditions, but it can rust in humid environments, so apply a rust inhibitor after machining. For storage, keep the flat bar in a dry, covered area with a relative humidity below 60%. The steel is also available in a nitrided condition for improved wear resistance, with a case depth of 0.2–0.5 mm and a surface hardness of 600–700 HV. That’s useful for molds with abrasive materials like glass-filled nylon or polycarbonate. The flat bar can also be supplied with a pre-machined surface for EDM, with a roughness of Ra 1.6 µm or better. That reduces the time needed for electrode preparation. The steel’s electrical conductivity is about 3.5% IACS, which is typical for tool steels. That’s not a factor for most applications, but it’s relevant for EDM. The material also has good magnetic permeability, which is useful for magnetic clamping. For large molds, the flat bar can be supplied with a pre-drilled hole pattern for cooling channels, with a tolerance of ±0.1 mm on hole position. That saves time and reduces the risk of drilling errors. The steel’s hardness uniformity is typically within 10 HBW across the entire cross-section, which is excellent for large molds. That uniformity ensures that the mold cavity and core have the same mechanical properties, reducing the risk of uneven wear or distortion. The flat bar is also available in a double-tempered condition for improved toughness, with a typical impact energy of 40–50 J. That’s useful for molds with sharp corners or thin sections. The steel’s fatigue strength is also good, with a fatigue limit of about 400 MPa at 10⁷ cycles. That’s important for molds that run millions of cycles. For high-volume production, the flat bar can be supplied with a guaranteed hardness range of 300–320 HBW for tighter control. That reduces the variation in mold performance and extends the mold life. The steel’s wear resistance is also good, with a relative wear rate of about 0.5–1.0 compared to D2 steel. That’s suitable for most plastic materials, but for highly abrasive materials, consider a surface treatment like nitriding or PVD coating. The flat bar is also available in a pre-machined condition for mold bases, with a thickness tolerance of ±0.05 mm and a flatness of 0.1 mm per 1000 mm. That saves time and reduces the need for grinding. The steel’s dimensional stability during heat treatment is excellent, with a typical distortion of 0.05–0.1% in the length direction. That’s why 1.2738 is the first choice for large, complex mold bases. The flat bar can also be supplied with a certificate of hardness, with a test report showing the hardness at multiple points across the cross-section. That ensures that the material meets the specified hardness range. The steel’s cleanliness is also important for polishability, with a typical inclusion rating of 1–2 per ASTM E45. That’s suitable for mirror finishes. The flat bar is also available in a vacuum-degassed version for higher cleanliness, with a sulfur content below 0.003%. That reduces the risk of pitting during EDM and improves the surface finish. The steel’s thermal conductivity is also good, with a value of 32 W/m·K at 20°C. That helps with heat dissipation during injection cycles, reducing cycle times and improving part quality. For large molds, the flat bar can be pre-machined with cooling channels, ejector pin holes, and guide pin holes before final hardening. The dimensional stability during heat treatment is excellent, with a typical distortion of 0.05–0.1% in the length direction. That’s why 1.2738 is the first choice for large, complex mold bases. The steel also has good weldability if you follow the preheat and post-weld heat treatment guidelines. For repair welding, use a low-hydrogen process and keep the interpass temperature below 350°C. The flat bar is also available in a vacuum-degassed version for higher cleanliness, which is important for molds with fine details or deep cavities. The sulfur content is typically below 0.005% for vacuum-degassed material, which improves polishability and reduces the risk of pitting. For EDM, the material’s low sulfur also means less electrode wear and better surface finish. The thermal expansion coefficient is 12.5 × 10⁻⁶ /K from 20°C to 200°C, which is similar to other tool steels. That’s important for matching the mold to the machine’s thermal behavior. The steel also has good resistance to thermal fatigue, with a thermal conductivity of 32 W/m·K at 20°C and 30 W/m·K at 200°C. That helps with heat dissipation during injection cycles. For large molds, the flat bar can be pre-heated to 350°C before welding to avoid cracking. The material’s toughness is also excellent, with a Charpy V-notch impact energy of 30–40 J at room temperature. That’s higher than 1.2311 and comparable to 1.2343 (H11). That toughness means the steel can handle high clamping forces and injection pressures without cracking. For mold bases, the flat bar is often supplied with a ground finish on all six sides, with a tolerance of ±0.1 mm on thickness and width. That saves time when fitting into the mold frame. The steel is also available with a pre-hardened condition of 330–370 HBW for higher wear resistance, but that reduces machinability by about 20%. For most applications, the standard 290–330 HBW is the best balance of machinability and wear resistance. The flat bar can also be supplied with a certificate of conformity to EN 10204 3.1 or 3.2, which includes chemical analysis, mechanical properties, and ultrasonic test results. That’s important for quality control and traceability. The steel’s microstructure is fine-tempered martensite with no retained austenite, which ensures dimensional stability during service. The steel also has good resistance to tempering, with a secondary hardness peak at 500–550°C. That means you can stress-relieve the material without losing hardness. For large molds, the flat bar can be pre-machined with a roughing allowance of 1–2 mm per side, then stress-relieved, then finished. That process reduces distortion and improves surface finish. The steel’s polishability is excellent, with a surface roughness of Ra 0.05 µm after fine grinding and Ra 0.02 µm after diamond polishing. That’s suitable for high-gloss plastic parts like automotive lenses or cosmetic packaging. The steel also has good corrosion resistance in dry conditions, but it can rust in humid environments, so apply a rust inhibitor after machining. For storage, keep the flat bar in a dry, covered area with a relative humidity below 60%. The steel is also available in a nitrided condition for improved wear resistance, with a case depth of 0.2–0.5 mm and a surface hardness of 600–700 HV. That’s useful for molds with abrasive materials like glass-filled nylon or polycarbonate. The flat bar can also be supplied with a pre-machined surface for EDM, with a roughness of Ra 1.6 µm or better. That reduces the time needed for electrode preparation. The steel’s electrical conductivity is about 3.5% IACS, which is typical for tool steels. That’s not a factor for most applications, but it’s relevant for EDM. The material also has good magnetic permeability, which is useful for magnetic clamping. For large molds, the flat bar can be supplied with a pre-drilled hole pattern for cooling channels, with a tolerance of ±0.1 mm on hole position. That saves time and reduces the risk of drilling errors. The steel’s hardness uniformity is typically within 10 HBW across the entire cross-section, which is excellent for large molds. That uniformity ensures that the mold cavity and core have the same mechanical properties, reducing the risk of uneven wear or distortion. The flat bar is also available in a double-tempered condition for improved toughness, with a typical impact energy of 40–50 J. That’s useful for molds with sharp corners or thin sections. The steel’s fatigue strength is also good, with a fatigue limit of about 400 MPa at 10⁷ cycles. That’s important for molds that run millions of cycles. For high-volume production, the flat bar can be supplied with a guaranteed hardness range of 300–320 HBW for tighter control. That reduces the variation in mold performance and extends the mold life. The steel’s wear resistance is also good, with a relative wear rate of about 0.5–1.0 compared to D2 steel. That’s suitable for most plastic materials, but for highly abrasive materials, consider a surface treatment like nitriding or PVD coating. The flat bar is also available in a pre-machined condition for mold bases, with a thickness tolerance of ±0.05 mm and a flatness of 0.1 mm per 1000 mm. That saves time and reduces the need for grinding. The steel’s dimensional stability during heat treatment is excellent, with a typical distortion of 0.05–0.1% in the length direction. That’s why 1.2738 is the first choice for large, complex mold bases. The flat bar can also be supplied with a certificate of hardness, with a test report showing the hardness at multiple points across the cross-section. That ensures that the material meets the specified hardness range. The steel’s cleanliness is also important for polishability, with a typical inclusion rating of 1–2 per ASTM E45. That’s suitable for mirror finishes. The flat bar is also available in a vacuum-degassed version for higher cleanliness, with