SUS440A Stainless Steel
SUS440A is a martensitic stainless steel belonging to the 440 series, known for its balanced combination of hardness, corrosion resistance, and wear resistance. It is widely used in applications requiring sharp edges, durability, and moderate corrosion resistance, such as cutting tools, bearings, and precision components. Below is a detailed overview of its properties, composition, and applications.
Chemical Composition
SUS440A’s performance is defined by its high chromium content (for corrosion resistance) and controlled carbon and molybdenum levels (for hardness and wear resistance). Its typical composition is as follows:
Carbon (C): 0.60%–0.75% (provides hardness through martensitic transformation during heat treatment).
Chromium (Cr): 16.0%–18.0% (the primary alloying element, forming a protective chromium oxide layer to resist corrosion).
Molybdenum (Mo): 0.60% max (enhances wear resistance and improves corrosion resistance in specific environments).
Manganese (Mn): ≤1.00% (aids in steel processing and reduces brittleness).
Silicon (Si): ≤1.00% (improves strength and oxidation resistance at high temperatures).
Phosphorus (P): ≤0.040% (controlled to avoid embrittlement).
Sulfur (S): ≤0.030% (minimized to prevent reduced corrosion resistance and toughness).
Iron (Fe): Balance (serves as the matrix for alloying elements).
Physical Properties
Density: Approximately 7.75–7.80 g/cm³ (similar to other martensitic stainless steels).
Melting point: Around 1480–1530°C (varies slightly with composition, suitable for heat treatment processes).
Thermal conductivity: About 24–26 W/(m·K) (lower than austenitic stainless steels like SUS304, indicating slower heat transfer).
Coefficient of linear expansion: Approximately 10.2×10⁻⁶/°C (moderate thermal expansion, reducing dimensional distortion during heat treatment).
Electrical resistivity: Around 0.60–0.70 μΩ·m (higher than carbon steel, making it less conductive).
Mechanical Properties (After Heat Treatment)
SUS440A’s mechanical properties are significantly enhanced through heat treatment (quenching and tempering), which transforms its microstructure into hard martensite. Key properties include:
Hardness: 56–59 HRC (Rockwell hardness) after optimal heat treatment, providing excellent wear resistance for cutting and sliding applications.
Tensile strength: 1700–2000 MPa (high strength, suitable for load-bearing components).
Yield strength: 1400–1600 MPa (high resistance to plastic deformation under stress).
Elongation: 4%–6% (low ductility due to high hardness, typical of martensitic stainless steels).
Impact toughness: 10–15 J/cm² (moderate toughness, balancing hardness and resistance to chipping).
### Heat Treatment Process
Heat treatment is critical to achieving SUS440A’s desired properties. The standard process includes:
Annealing: Heating to 800–900°C, holding for 1–2 hours, then slow cooling in the furnace. This softens the material (hardness ≤250 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 or air. This forms a hard martensite structure (hardness ≈60 HRC).
Tempering: Heating to 150–300°C for 1–2 hours to reduce brittleness while retaining high hardness. Higher tempering temperatures (e.g., 400–500°C) may lower hardness but improve toughness.
Corrosion Resistance
SUS440A offers moderate corrosion resistance, primarily due to its chromium content:
Resists atmospheric corrosion, freshwater, and mild organic acids, forming a passive chromium oxide layer that inhibits further oxidation.
Performs adequately in dry or low-moisture environments but is less resistant than austenitic stainless steels (e.g., SUS304) in humid, salty, or acidic conditions.
Limitations: Susceptible to pitting corrosion in chloride-rich environments (e.g., seawater) and may corrode in strong acids or alkalis.
Processing Performance
Machinability: Poor in the hardened state due to high hardness. Machining is typically done in the annealed condition (≤250 HB) using high-speed steel or carbide tools.
Grindability: Excellent grindability, allowing precise shaping and sharpening of edges (e.g., blades, tools) through grinding processes.
Weldability: Limited weldability due to high carbon content, which increases the risk of cracking. Preheating (200–300°C) and post-weld tempering are required to reduce brittleness.
Formability: Low ductility in the hardened state; cold working (e.g., bending, stamping) is best performed in the annealed condition before heat treatment.
Applications
SUS440A is widely used in applications requiring a balance of hardness, wear resistance, and moderate corrosion resistance:
Cutting tools: Knives, scissors, blades, and surgical tools, leveraging its ability to retain sharp edges.
Bearings and bushings: Precision bearings, roller elements, and shafts for low-friction, high-wear applications.
Mechanical components: Valves, gears, and nozzles in machinery where durability is critical.
Aerospace and automotive: Small precision parts, such as fuel system components and instrument bearings.
Hardware and consumer goods: Locks, springs, and watch components, benefiting from its aesthetic appeal and wear resistance.
Comparison with Other 440 Series Stainless Steels
The 440 series includes SUS440A, SUS440B, and SUS440C, differing primarily in carbon content, which affects hardness and corrosion resistance:
Alloy Carbon Content Hardness (HRC) Corrosion Resistance Key Applications
SUS440A 0.60%–0.75% 56–59 Highest among 440s Knives, bearings, general tools
SUS440B 0.75%–0.95% 58–61 Moderate Heavy-duty bearings, cutting tools
SUS440C 0.95%–1.20% 60–63 Lowest among 440s High-wear components, precision tools
Advantages and Limitations
Advantages
High hardness and wear resistance after heat treatment.
Better corrosion resistance than most carbon steels and low-alloy steels.
Excellent grindability for precise edge sharpening.
Limitations
Lower corrosion resistance compared to austenitic stainless steels (e.g., SUS304).
Poor weldability and formability in the hardened state.
Susceptible to corrosion in chloride-rich or highly acidic environments.
In summary, SUS440A is a reliable martensitic stainless steel ideal for applications prioritizing hardness, wear resistance, and moderate corrosion performance. Its balance of properties makes it a popular choice in toolmaking, precision engineering, and consumer goods.