440C Stainless Steel | AISI 440C 1.4125 | High-Carbon Martensitic Stainless Steel Supplier

Heat Treatment:

• Soft Annealing: Heat slowly to 850-900°C (1560-1650°F), hold until uniformly heated, then slow furnace cool at maximum 10-20°C per hour to about 600°C, followed by air cooling. Alternatively, sub-critical annealing at 735-785°C with slow furnace cool is also effective. Resulting hardness: ≤269 HB, optimized for extensive machining operations.

• Stress Relieving: Heat to 650-700°C, hold 1-2 hours, slow cool in furnace. Relieves machining stresses before final hardening to minimize distortion.

• Hardening (Austenitizing): Heat to 1010-1065°C (1850-1950°F), with optimal temperature typically 1030-1050°C. Soak time proportional to section size (typically 20-30 minutes at temperature). Protect against oxidation and decarburization using controlled atmosphere furnaces, salt baths, or stainless foil wrapping. Higher austenitizing temperatures (up to 1100°C) can increase hardness but also increase retained austenite.

• Quenching: Oil quench (40-80°C) for most sections; oil quenching is necessary for heavy sections. Air quench possible for smaller sections. Quench to 50-70°C, then temper immediately. Hardness after quenching: 60-65 HRC depending on section size and austenitizing temperature.

• Sub-Zero Treatment (Cryogenic): For maximum hardness and dimensional stability, cool immediately after quenching to -70°C to -80°C (dry ice or liquid nitrogen), hold for 3-4 hours, then temper normally. This treatment transforms retained austenite to martensite, increasing hardness by 1-3 HRC and improving wear resistance. Particularly recommended for precision bearings and gauges.

• Tempering: Temper immediately after quenching while tool is still warm. Double tempering recommended for critical applications. Hold minimum 1 hour per 25mm section thickness each temper, cool to room temperature between tempers.

Processing Guide:

• Forging: Heat slowly and uniformly to 1050-1150°C (1920-2100°F). Do not forge below 900°C (1650°F). After forging, slow cool in furnace or insulating media (lime, mica, dry ashes) to prevent cracking. Follow with full annealing to restore machinability.

• Machining: In annealed condition, relatively easily machined, approximately 40% machinability rating compared to mild steel. Chips are tough and stringy, so chip breakers are important. Carbide or ceramic tools recommended. After hardening, machining becomes difficult and is typically performed by grinding.

• Grinding: Due to high hardness and carbide content, grinding requires careful wheel selection. Use aluminum oxide or CBN wheels with proper cooling to avoid burning and surface damage.

• Welding: Not commonly welded due to air-hardening and cracking tendencies. If necessary, preheat at 250°C, use Grade 420 filler for high hardness welds, or 309/310 for softer welds with higher ductility. Follow welding with a full anneal or stress relief. Post-weld heat treatment recommended to restore corrosion resistance and relieve stresses.

• Polishing: Can be polished to a good surface finish; avoid overheating during polishing to maintain surface integrity.

• Surface Treatments: Passivation enhances corrosion resistance in hardened and tempered condition. PVD coatings (TiN, CrN) can further improve wear resistance and reduce friction for cutting tools and bearings.