JIS SUS440C Stainless Steel: High-Carbon Martensitic Grade for Extreme Hardness & Wear Resistance

JIS SUS440C, designated under the Japanese Industrial Standards (equivalent to AISI 440C), is a high-carbon martensitic stainless steel renowned for its exceptional hardness, wear resistance, and moderate corrosion resistance. With a carbon content of 0.95-1.20%, it achieves the highest hardness among the 440 series (up to 60 HRC after heat treatment), making it ideal for precision components, bearings, and cutting tools in aerospace, medical, and industrial sectors. This article explores its chemical composition, mechanical properties, heat treatment processes, and critical application considerations.

1. Chemical Composition (JIS G4303 Standard)

2. Mechanical Properties (After Heat Treatment)

• Hardness (HRC): 58-60 (after quenching and tempering at 150-370°C)
• Tensile Strength (σb): ≥ 1900 MPa (varies with tempering temperature)
• Yield Strength (σ0.2): ≥ 1700 MPa (high resistance to plastic deformation)
• Elongation (δ): ≥ 2% (limited ductility due to martensitic structure)
• Impact Toughness (CVN): ~10 J (low; not suitable for high-impact applications)
• Max Service Temperature: 425°C (retains hardness up to 300°C; tempering occurs above 400°C)

3. Heat Treatment Process

1. Annealing: Heat to 840-900°C, slow cool in furnace to ≤30°C/h to soften (≤269 HB) for machining. Avoid air cooling to prevent rehardening.
2. Hardening (Quenching): Austenitize at 1010-1070°C, oil or air quench to achieve maximum hardness (60 HRC). Critical cooling rate: >600°C/s to avoid retained austenite.
3. Tempering:
   • 150-200°C: Secondary hardening peak (~60 HRC); minimal toughness improvement.
   • 200-370°C: Optimal balance (58-60 HRC) for cutting tools; reduces brittleness.
   • 370-540°C: Sacrifices hardness (50-55 HRC) for improved toughness (CVN ~20 J).
4. Stress Relieving: 150-200°C for 1-2 hours to minimize distortion post-machining without reducing hardness.
5. Cryogenic Treatment: Optional -70°C to -196°C soak to transform retained austenite, increasing dimensional stability and wear resistance by ~10-15%.

4. Key Application Fields

5. Comparison with Related Grades

6. Machining & Fabrication Guidelines

• Machining: Perform in annealed state (<269 HB); use carbide tools (e.g., KC850) with sulfurized cutting oils. Avoid interrupted cuts to prevent work hardening.
• Welding: Not recommended due to air-hardening risk; if necessary, preheat to 200-300°C, use ER410 filler, and post-weld temper at 600°C. Stress relief is mandatory.
• Corrosion Limitations: Prone to pitting in chloride environments (e.g., seawater). Passivate with 20-40% nitric acid at 50-70°C for 30-120 minutes to enhance surface Cr₂O₃ film.
• Surface Finishing: Electropolishing (10-15 µm removal) improves corrosion resistance and cleanliness for medical/food applications. Avoid excessive grinding heat to prevent untempered martensite.
• Quality Control: Verify hardness via Rockwell C (HRC) testing per ASTM E18; check microstructure for retained austenite (<5% ideal) via metallographic analysis (ASTM E407).