SUS440B Stainless Steel
SUS440B is a martensitic stainless steel within the 440 series, valued for its higher hardness and wear resistance compared to SUS440A, while maintaining moderate corrosion resistance. It is engineered for applications demanding durability under high friction or wear conditions, such as heavy-duty tools, precision bearings, and industrial components. Below is a comprehensive overview of its properties, composition, and practical uses.
Chemical Composition
SUS440B’s performance is shaped by its increased carbon content (relative to SUS440A) and balanced alloying elements, which enhance hardness while retaining corrosion resistance. Its typical composition is as follows:
Carbon (C): 0.75%–0.95% (higher than SUS440A, enabling greater hardness through martensitic transformation during heat treatment).
Chromium (Cr): 16.0%–18.0% (forms a protective chromium oxide layer to resist corrosion, consistent with the 440 series).
Molybdenum (Mo): 0.60% max (improves wear resistance and strengthens corrosion resistance in specific environments).
Manganese (Mn): ≤1.00% (aids in steel processing and reduces brittleness during manufacturing).
Silicon (Si): ≤1.00% (enhances high-temperature strength and oxidation resistance).
Phosphorus (P): ≤0.040% (controlled to prevent embrittlement and maintain toughness).
Sulfur (S): ≤0.030% (minimized to avoid compromising 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 martensitic stainless steels, including SUS440A and SUS440C).
Melting point: Around 1470–1520°C (slightly lower than SUS440A due to higher carbon content, suitable for heat treatment processes).
Thermal conductivity: About 23–25 W/(m·K) (slightly lower than SUS440A, indicating slower heat transfer).
Coefficient of linear expansion: Approximately 10.1×10⁻⁶/°C (moderate thermal expansion, reducing dimensional changes during heat treatment).
Electrical resistivity: Around 0.62–0.72 μΩ·m (higher than carbon steel, making it a poor conductor of electricity).
### Mechanical Properties (After Heat Treatment)
Heat treatment (quenching and tempering) is critical to unlocking SUS440B’s full potential, transforming its microstructure into hard martensite. Key mechanical properties include:
Hardness: 58–61 HRC (Rockwell hardness) after optimal heat treatment—higher than SUS440A, providing superior wear resistance for heavy-duty applications.
Tensile strength: 1800–2100 MPa (greater than SUS440A, suitable for load-bearing components under high stress).
Yield strength: 1500–1700 MPa (high resistance to plastic deformation, ensuring structural integrity under pressure).
Elongation: 3%–5% (lower ductility than SUS440A due to higher hardness, typical of high-carbon martensitic steels).
Impact toughness: 8–12 J/cm² (moderate toughness, balancing hardness with resistance to chipping or fracture).
Heat Treatment Process
SUS440B requires precise heat treatment to achieve its desired balance of hardness and toughness:
Annealing: Heating to 800–900°C, holding for 1–2 hours, then slow cooling in the furnace. This softens the material (hardness ≤260 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 ≈62 HRC).
Tempering: Heating to 150–300°C for 1–2 hours to reduce brittleness while retaining high hardness. Tempering at higher temperatures (e.g., 400–500°C) lowers hardness but improves toughness for applications requiring greater impact resistance.
### Corrosion Resistance
SUS440B offers moderate corrosion resistance, slightly lower than SUS440A due to its higher carbon content (which can reduce 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 austenitic stainless steels (e.g., SUS304) in humid, salty, or acidic conditions.
Limitations: More susceptible to pitting corrosion than SUS440A 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 best performed in the annealed condition (≤260 HB) using carbide tools or high-speed steel with lubrication.
Grindability: Excellent grindability, allowing precise shaping and edge sharpening for tools and bearings through grinding processes.
Weldability: Limited weldability due to high carbon content, which increases the risk of cracking. Preheating (250–350°C) and post-weld tempering are necessary to reduce brittleness and residual stress.
Formability: Low ductility in the hardened state; cold working (e.g., bending, stamping) should be done in the annealed condition before heat treatment.
### Applications
SUS440B is widely used in applications requiring higher hardness and wear resistance than SUS440A, with moderate corrosion resistance:
Heavy-duty cutting tools: Industrial knives, shears, and blades for cutting tough materials (e.g., metal, composites).
Bearings and rollers: High-load precision bearings, roller elements, and shafts in machinery with high friction or wear.
Mechanical components: Gears, valves, and nozzles in industrial equipment where durability under stress is critical.
Aerospace and automotive: Engine components, fuel system parts, and instrument bearings requiring high strength.
Mold and die making: Small molds and dies for shaping materials, leveraging its wear resistance and dimensional stability.
Comparison with Other 440 Series Stainless Steels
The 440 series (SUS440A, SUS440B, SUS440C) varies primarily in carbon content, directly influencing hardness and corrosion resistance:
Alloy Carbon Content Hardness (HRC) Corrosion Resistance Key Applications
SUS440A 0.60%–0.75% 56–59 Highest among 440s Knives, general bearings, tools
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
Higher hardness and wear resistance than SUS440A, suitable for heavy-duty applications.
Better strength and toughness balance than many carbon steels.
Excellent grindability for precise manufacturing of sharp or complex components.
Limitations
Lower corrosion resistance than SUS440A and austenitic stainless steels (e.g., SUS304).
Poor weldability and formability in the hardened state, requiring pre- and post-processing steps.
Susceptible to corrosion in chloride-rich or highly acidic environments.
In summary, SUS440B is a robust martensitic stainless steel designed for applications prioritizing high hardness, wear resistance, and moderate corrosion performance. Its unique balance makes it a preferred choice for industrial tools, heavy-duty bearings, and components subjected to intense friction or stress.