SUS440C is a high-carbon martensitic stainless steel and the hardest variant in the 440 series, prized for exceptional wear resistance, high hardness, and moderate corrosion resistance. It is engineered for applications demanding extreme durability under intense friction, abrasion, or high stress, such as precision tools, high-performance bearings, and critical industrial components. Below is a detailed overview of its properties, composition, and practical applications.
Chemical Composition
SUS440C’s superior hardness stems from its high carbon content, paired with chromium and trace alloying elements that balance wear resistance and corrosion performance. Its typical composition is:
Carbon (C): 0.95%–1.20% (highest in the 440 series, enabling maximum hardness through martensitic transformation during heat treatment).
Chromium (Cr): 16.0%–18.0% (forms a protective chromium oxide layer, though slightly compromised by high carbon compared to lower-carbon 440 variants).
Molybdenum (Mo): 0.60% max (enhances wear resistance and strengthens resistance to pitting in specific environments).
Manganese (Mn): ≤1.00% (aids in steel processing and reduces brittleness during manufacturing).
Silicon (Si): ≤1.00% (improves high-temperature strength and oxidation resistance).
Phosphorus (P): ≤0.040% (controlled to prevent embrittlement).
Sulfur (S): ≤0.030% (minimized to avoid reducing corrosion resistance and ductility).
Iron (Fe): Balance (serves as the matrix for alloying elements).
Physical Properties
Density: Approximately 7.75–7.80 g/cm³ (consistent with other 440 series stainless steels, as density is minimally affected by carbon content).
Melting point: Around 1450–1510°C (slightly lower than SUS440A/B due to higher carbon, which reduces thermal stability).
Thermal conductivity: About 22–24 W/(m·K) (marginally lower than SUS440A/B, indicating slower heat transfer).
Coefficient of linear expansion: Approximately 10.0×10⁻⁶/°C (moderate thermal expansion, minimizing dimensional changes during heat treatment).
Electrical resistivity: Around 0.65–0.75 μΩ·m (higher than carbon steel, making it a poor electrical conductor).
Mechanical Properties (After Heat Treatment)
Heat treatment is critical to achieving SUS440C’s full potential, transforming its microstructure into a hard, wear-resistant martensite phase. Key mechanical properties include:
Hardness: 60–63 HRC (Rockwell hardness) after optimal heat treatment—the highest in the 440 series, providing exceptional wear resistance.
Tensile strength: 2000–2300 MPa (superior to SUS440A/B, suitable for high-stress load-bearing components).
Yield strength: 1600–1900 MPa (high resistance to plastic deformation, ensuring structural integrity under pressure).
Elongation: 2%–4% (lower ductility than SUS440A/B due to higher hardness, typical of high-carbon martensitic steels).
Impact toughness: 6–10 J/cm² (moderate toughness, balancing hardness with limited resistance to chipping or fracture).
Heat Treatment Process
SUS440C requires precise heat treatment to maximize hardness while avoiding excessive brittleness:
Annealing: Heating to 800–900°C, holding for 1–2 hours, then slow cooling in the furnace. This softens the material (hardness ≤269 HB) for machining and relieves internal stress.
Quenching: Heating to 1010–1070°C (to dissolve chromium carbides into the austenite matrix), holding briefly, then rapid cooling in oil. This forms a hard martensite structure (hardness ≈63 HRC).
Tempering: Heating to 150–300°C for 1–2 hours to reduce brittleness while retaining peak hardness. Higher tempering temperatures (e.g., 400–500°C) lower hardness but slightly improve toughness for applications requiring minimal impact resistance.
Corrosion Resistance
SUS440C offers moderate-to-low corrosion resistance compared to other 440 series steels, as its high carbon content reduces the effectiveness of the chromium oxide layer:
Resists atmospheric corrosion, freshwater, and mild organic acids in dry or low-moisture environments.
Performs adequately in indoor or controlled industrial settings but is less resistant than SUS440A/B and far less than austenitic stainless steels (e.g., SUS304) in humid, salty, or acidic conditions.
Limitations: Highly susceptible to pitting and crevice corrosion in chloride-rich environments (e.g., seawater, deicing salts) and corrodes rapidly in strong acids or alkalis.
Processing Performance
Machinability: Very poor in the hardened state due to extreme hardness. Machining must be done in the annealed condition (≤269 HB) using carbide tools, high-speed steel with lubrication, or specialized cutting fluids.
Grindability: Excellent grindability, allowing precise shaping, sharpening, and finishing—critical for tools and bearings requiring tight tolerances.
Weldability: Severely limited weldability due to high carbon content, which increases the risk of cracking and embrittlement. Preheating (300–400°C) and post-weld tempering are mandatory to reduce residual stress, though welding is generally avoided when possible.
Formability: Very low ductility in the hardened state; cold working (e.g., bending, stamping) must be performed in the annealed condition before heat treatment.
Applications
SUS440C is ideal for applications prioritizing extreme hardness and wear resistance over maximum corrosion resistance:
High-performance cutting tools: Precision knives, surgical blades, and industrial cutting edges for hard materials (e.g., metal, ceramics, composites).
Bearings and rollers: High-speed, high-load bearings, roller elements, and shafts in machinery with intense friction (e.g., aerospace engines, industrial turbines).
Mechanical components: Gears, valve seats, and nozzles in heavy machinery where wear from friction or abrasion is severe.
Aerospace and defense: Critical components like gyroscopes, missile guidance parts, and aircraft engine bearings requiring dimensional stability under stress.
Mold and die making: Small, high-wear molds and dies for shaping hard plastics, metals, or composites.
Medical devices: Surgical instruments (e.g., scalpels, forceps) where sharpness retention and sterilizability are key.
Comparison with Other 440 Series Stainless Steels
The 440 series varies by carbon content, directly impacting hardness and corrosion resistance:
Alloy Carbon Content Hardness (HRC) Corrosion Resistance Key Applications
SUS440A 0.60%–0.75% 56–59 Highest among 440s General knives, standard bearings
SUS440B 0.75%–0.95% 58–61 Moderate Heavy-duty bearings, industrial tools
SUS440C 0.95%–1.20% 60–63 Lowest among 440s High-wear components, precision tools
Advantages and Limitations
Advantages
Highest hardness and wear resistance in the 440 series, ideal for extreme friction or abrasion.
Superior strength and dimensional stability under high stress.
Excellent grindability for precise manufacturing of sharp or complex components.
Limitations
Lowest corrosion resistance in the 440 series, unsuitable for humid, salty, or acidic environments.
Poor weldability and formability, requiring specialized processing steps.
Higher brittleness than SUS440A/B, increasing the risk of chipping under impact.
In summary, SUS440C is a high-performance martensitic stainless steel designed for applications where extreme hardness and wear resistance are critical, with corrosion resistance taking a secondary role. Its unique properties make it indispensable for precision tools, high-load bearings, and components subjected to intense friction or stress.