SUS431 is a martensitic stainless steel known for its high strength, good corrosion resistance, and moderate toughness. It is widely used in applications requiring a balance of mechanical performance and resistance to corrosion, particularly in environments where strength is a primary requirement. Below is a detailed overview:
1. Chemical Composition
The key chemical components (weight percentage) of SUS431 are as follows:
Chromium (Cr): 15.00% – 17.00% (critical for corrosion resistance, forming a passive oxide film)
Nickel (Ni): 1.25% – 2.50% (enhances toughness and corrosion resistance, distinguishing it from lower-nickel martensitic grades)
Carbon (C): 0.15% – 0.25% (enables martensitic transformation during heat treatment, contributing to high strength)
Silicon (Si): ≤1.00%
Manganese (Mn): ≤1.00%
Phosphorus (P): ≤0.040%
Sulfur (S): ≤0.030%
Iron (Fe): Balanced
2. Mechanical Properties
SUS431’s properties are highly dependent on heat treatment (e.g., quenching and tempering). Typical values after optimal heat treatment are:
Tensile strength: ≥1000 MPa (can reach 1200 – 1400 MPa with specific heat treatment)
Yield strength: ≥800 MPa
Elongation (in 50mm): ≥10%
Hardness: ≥32 HRC (Rockwell C) after quenching and tempering (can exceed 40 HRC with higher carbon content)
3. Physical Properties
Density: Approximately 7.75 g/cm³
Melting point: Around 1425 – 1510°C
Thermal conductivity: ~25 W/(m·K) (similar to other martensitic stainless steels)
Coefficient of thermal expansion: ~10.3 × 10⁻⁶/°C (lower than austenitic grades like 304)
Electrical resistivity: ~70 μΩ·cm
Magnetic: As a martensitic steel, it is ferromagnetic.
4. Key Characteristics
High Strength: Through quenching and tempering, it achieves much higher tensile and yield strengths than ferritic or austenitic stainless steels (e.g., SUS304), making it suitable for load-bearing components.
Good Corrosion Resistance: The combination of chromium and nickel provides better corrosion resistance than lower-alloy martensitic steels (e.g., SUS410) and carbon steel. It resists atmospheric corrosion, mild acids, and organic compounds but is less resistant than austenitic grades (e.g., SUS304) in harsh environments.
Heat Treatability: Its martensitic structure allows for heat treatment (quenching + tempering) to adjust strength and toughness, balancing hardness and ductility as needed.
Moderate Machinability: Machinability is better than high-carbon martensitic steels but inferior to free-cutting grades like SUS430F. It requires sharp tools and proper cooling to avoid work hardening.
5. Application Fields
Aerospace and Defense: Components like aircraft fasteners, landing gear parts, and missile structural elements (due to high strength and corrosion resistance).
Automotive Industry: High-performance parts such as valve stems, exhaust system components, and drive shafts (resisting high temperatures and mechanical stress).
Oil and Gas Industry: Valve parts, pump shafts, and drill components (for moderate corrosion resistance and strength in non-severe downhole environments).
Medical Devices: Surgical instruments and orthopedic implants (where strength, corrosion resistance, and biocompatibility are required, though 316L is more common for critical implants).
General Engineering: High-strength bolts, gears, and shafts in machinery where corrosion resistance surpasses that of carbon steel.
6. Heat Treatment
Heat treatment is critical to optimizing SUS431’s properties:
Quenching: Heat to 980 – 1050°C, hold, then water or oil quench to form a hard martensitic structure.
Tempering: Reheat to 200 – 650°C (depending on desired properties):
Low tempering (200 – 300°C): Maximizes hardness and strength (but lower toughness).
High tempering (500 – 650°C): Improves toughness and ductility (with slight reduction in strength).
7. Limitations
Corrosion Resistance: Inferior to austenitic grades (e.g., 304, 316) in high-chloride or strong acid environments (prone to pitting corrosion in seawater).
Weldability: Welding is challenging due to hardening in the heat-affected zone (HAZ), which may cause cracking. Pre-heating (200 – 300°C) and post-weld tempering are recommended.
Temperature Limits: Not suitable for continuous use above 600°C, as strength decreases and oxidation resistance diminishes.
In summary, SUS431 is a high-strength martensitic stainless steel valued for its balance of mechanical performance and corrosion resistance. It is ideal for applications requiring both strength and moderate corrosion resistance, serving as a cost-effective alternative to more expensive high-nickel alloys in non-severe environments.
