SUS430F is a free-cutting ferritic stainless steel, belonging to the 430 series and modified with sulfur to enhance machinability. It combines the basic corrosion resistance of ferritic stainless steels with improved processing performance, making it widely used in mechanical parts requiring both corrosion resistance and easy machining. Here is a detailed introduction:
1. Chemical Composition
The key chemical components (weight percentage) of SUS430F are as follows:
Chromium (Cr): 16.00% – 18.00% (the main element for corrosion resistance, forming a passive oxide film on the surface)
Carbon (C): ≤0.12% (low carbon content helps maintain good corrosion resistance and ductility)
Silicon (Si): ≤1.00%
Manganese (Mn): ≤1.00%
Sulfur (S): 0.15% – 0.35% (added to improve machinability by reducing cutting resistance)
Phosphorus (P): ≤0.040%
Iron (Fe): Balanced (the base element)
2. Mechanical Properties
Tensile strength: ≥450 MPa
Yield strength: ≥205 MPa
Elongation (in 50mm): ≥22%
Hardness: ≤183 HB (Brinell) or ≤88 HRB (Rockwell B)
Compared to the standard SUS430 stainless steel (without sulfur), SUS430F has slightly lower strength and ductility due to the addition of sulfur, but its machinability is significantly improved.
3. Physical Properties
Density: Approximately 7.7 g/cm³ (similar to other ferritic stainless steels)
Melting point: Around 1425 – 1510°C
Thermal conductivity: About 26 W/(m·K) (better than austenitic stainless steels like 304)
Coefficient of thermal expansion: ~10.4 × 10⁻⁶/°C (lower than austenitic stainless steels, reducing thermal stress)
Electrical resistivity: ~60 μΩ·cm
4. Key Characteristics
Excellent Machinability: The addition of sulfur forms sulfide inclusions (e.g., MnS), which act as “chip breakers” during cutting, reducing tool wear and improving surface finish. This makes it suitable for mass production of complex-shaped parts via turning, milling, drilling, etc.
Good Corrosion Resistance: Thanks to its chromium content, it forms a dense chromium oxide film, providing resistance to atmospheric corrosion, mild acids, and organic compounds. However, its corrosion resistance is lower than austenitic stainless steels (e.g., SUS304) and is not suitable for harsh environments like strong acids or seawater (prone to pitting corrosion).
Magnetic: As a ferritic stainless steel, it is ferromagnetic, which is a notable feature distinguishing it from non-magnetic austenitic stainless steels.
Moderate Ductility and Weldability: It has certain ductility for bending and forming, but welding may reduce corrosion resistance in the heat-affected zone (needs post-weld annealing for recovery in critical applications).
5. Application Fields
Mechanical Parts: Fasteners, shafts, gears, valves, and precision components requiring complex machining.
Automotive Industry: Parts like fuel injectors, sensor housings, and small mechanical components (balancing cost, machinability, and corrosion resistance).
Household Appliances: Components of kitchen utensils, electric fans, and power tools (where magnetic properties are acceptable).
Industrial Equipment: Pumps, meters, and instrument parts that do not contact strong corrosive media.
6. Limitations
Corrosion Resistance Limitations: Not suitable for environments with high chloride ions (e.g., seawater) or strong acids (e.g., sulfuric acid, nitric acid).
Weldability Constraints: Welding may cause grain growth and reduce toughness; post-weld heat treatment is recommended for critical structures.
Temperature Resistance: Not ideal for high-temperature applications (over 400°C) as it may become brittle.
In summary, SUS430F is a cost-effective ferritic stainless steel optimized for machinability, suitable for non-corrosive or mildly corrosive environments where complex machining and mass production are required. It is often chosen as an alternative to carbon steel when better corrosion resistance is needed, or as a lower-cost option compared to austenitic stainless steels in less demanding scenarios.