SUS431 Stainless Steel
SUS431 is a martensitic stainless steel defined by Japanese Industrial Standards (JIS), renowned for its high strength, excellent corrosion resistance, and good mechanical properties after heat treatment. It is widely used in applications requiring a balance of strength, toughness, and resistance to corrosion, particularly in automotive, aerospace, and industrial sectors.
Basic Information
Definition: SUS431 is a chromium-nickel martensitic stainless steel, where “SUS” denotes “Stainless Steel” in JIS terminology. It is designed to achieve high strength through heat treatment (quenching and tempering) while maintaining better corrosion resistance than lower-alloy martensitic grades like SUS410 or SUS420.
Corresponding Standards: Conforms to JIS G4303 (Stainless steel bars) and JIS G4311 (Hot-rolled stainless steel plates, sheets, and strips). Its international equivalents include:
European: X17CrNi16-2 (EN 10088-1)
American: 431 (ASTM A276 / AISI 431)
Chinese: 1Cr17Ni2 (GB/T 1220)
Key Characteristics: High tensile strength (up to 1100 MPa after heat treatment), good ductility, moderate corrosion resistance, and weldability with proper procedures.
Chemical Composition
The composition of SUS431 is carefully balanced to promote martensitic transformation during heat treatment, ensuring strength while retaining chromium for corrosion resistance:
Element Content Range Role in the Steel
Carbon (C) 0.11–0.17% Promotes martensite formation during quenching, enhancing strength; controlled to avoid brittleness.
Chromium (Cr) 15.0–17.0% Primary element for corrosion resistance, forming a passive chromium oxide layer.
Nickel (Ni) 1.5–2.5% Improves toughness, ductility, and corrosion resistance; stabilizes austenite to aid martensitic transformation.
Manganese (Mn) ≤1.0% Enhances hardenability and strength; limits sulfur-induced brittleness.
Silicon (Si) ≤1.0% Acts as a deoxidizer; improves high-temperature oxidation resistance.
Phosphorus (P) ≤0.04% Minimized to prevent grain boundary brittleness.
Sulfur (S) ≤0.03% Controlled to avoid hot cracking during welding or forming.
Iron (Fe) Balance Base metal providing structural integrity.
Physical Properties
Density: ~7.75 g/cm³ (slightly lower than austenitic stainless steels like SUS304).
Melting Point: ~1430–1480°C.
Thermal Conductivity: ~25 W/(m·K) at room temperature (lower than carbon steels but higher than some martensitic grades).
Coefficient of Thermal Expansion: ~10.5×10⁻⁶/°C (20–100°C), ensuring dimensional stability in temperature cycles.
Elastic Modulus: ~200 GPa, providing rigidity in structural applications.
Magnetic Properties: Ferromagnetic (magnetic) in the heat-treated condition, unlike austenitic stainless steels.
Mechanical Properties
SUS431’s mechanical properties are highly dependent on heat treatment, with quenching and tempering unlocking its full strength potential:
Property Annealed State Quenched & Tempered (Typical)
Yield Strength (Rp0.2) ~350 MPa ≥800 MPa (varies with tempering temp)
Tensile Strength (Rm) 600–800 MPa 1000–1200 MPa
Elongation (A) ≥20% ≥12–15%
Hardness ~200 HB 30–38 HRC (after tempering at 200–300°C)
Impact Toughness (Charpy) ~60 J/cm² ~40–50 J/cm² (varies with tempering)
Heat Treatment Effect: Quenching from 950–1050°C (followed by oil or air cooling) forms hard martensite. Tempering at 200–300°C retains high strength, while higher tempering (500–600°C) improves toughness but reduces strength.
Ductility-Strength Tradeoff: Higher tempering temperatures increase elongation and toughness but decrease yield and tensile strength.
Heat Treatment
SUS431 requires heat treatment to achieve its optimal properties, as it is relatively soft in the annealed state:
Annealing: Heating to 650–750°C, holding, then slow cooling to soften the steel, improve machinability, and relieve internal stress.
Quenching: Heating to 950–1050°C (to form austenite), followed by rapid cooling (oil or air) to transform austenite into hard, brittle martensite.
Tempering: Reheating quenched steel to 200–600°C, then cooling to reduce brittleness, adjust strength-toughness balance, and relieve residual stress.
Note: Avoid tempering at 400–500°C to prevent “temper embrittlement,” which reduces toughness.
Processing Performance
Weldability: Moderate—welding requires preheating (200–300°C) and post-weld annealing to prevent cracking and restore corrosion resistance. Martensitic formation during cooling can cause hardness and brittleness in the heat-affected zone (HAZ).
Formability: Fair in the annealed state, as the soft microstructure allows bending, stamping, or rolling. Poor formability in the hardened state due to high strength and brittleness.
Machinability: Good in the annealed state (HB ~200) with high-speed steel or carbide tools. Hardened SUS431 (HRC ≥30) is more challenging, requiring sharp tools and slower speeds to avoid wear.
Surface Treatment: Polishes well to a smooth finish; can be passivated (with nitric acid) to enhance the chromium oxide layer and improve corrosion resistance.
Corrosion Resistance
SUS431 offers better corrosion resistance than lower-chromium martensitic steels (e.g., SUS410) but is less resistant than austenitic grades (e.g., SUS304):
Atmospheric Resistance: Resists rust in dry or moderately humid environments; performs well in urban or industrial atmospheres.
Water Resistance: Tolerates fresh water but may corrode in saltwater or brackish environments (inferior to SUS316).
Chemical Resistance: Resists mild acids (e.g., dilute acetic acid) and alkalis but is susceptible to pitting in chloride-rich environments (e.g., coastal areas, deicing salts).
Passivation: Critical for maximizing corrosion resistance—removes free iron from the surface to stabilize the chromium oxide layer.
Application Fields
SUS431 is valued for its high strength and moderate corrosion resistance, making it suitable for demanding applications:
Automotive Industry:
Exhaust system components (manifolds, brackets) requiring heat and corrosion resistance.
Fasteners, shafts, and gears in high-stress parts (e.g., transmission systems).
Decorative trim and structural parts needing a balance of strength and appearance.
Aerospace & Defense:
Aircraft components (landing gear parts, brackets) requiring high strength-to-weight ratios.
Missile and weapon system parts resistant to wear and moderate corrosion.
Industrial Machinery:
Valves, pumps, and shafts for fluid handling in non-aggressive environments.
Cutting tools, dies, and molds requiring hardness and wear resistance.
Medical Devices:
Surgical instruments (scalpels, forceps) due to good strength, sterilizability, and moderate corrosion resistance.
General Engineering:
High-strength fasteners, bolts, and nuts for outdoor or industrial structures.
Watch cases, jewelry, and decorative hardware (polished finish).
Advantages and Limitations
Advantages
High Strength: Among the strongest stainless steels when heat-treated (tensile strength up to 1200 MPa).
Toughness: Balances strength and ductility after proper tempering, avoiding brittleness.
Cost-Effective: More affordable than austenitic stainless steels (e.g., SUS304) while offering better strength than low-alloy steels.
Magnetic: Useful in applications requiring magnetic properties (e.g., sensors, magnetic clamps).
Limitations
Corrosion Resistance: Inferior to austenitic grades; not suitable for saltwater or highly corrosive environments.
Welding Challenges: Requires pre- and post-weld heat treatment to prevent cracking, increasing production complexity.
Formability Limits: Cannot be cold-formed in hardened states, restricting design flexibility.
Temperature Sensitivity: Loses strength above 300°C; long-term exposure to high temperatures may degrade the passive layer.
Comparison with Similar Steels
Steel Grade Type Tensile Strength (Quenched/Tempered) Corrosion Resistance Key Advantage
SUS431 Martensitic 1000–1200 MPa Moderate High strength + better corrosion than lower martensitic grades.
SUS410 Martensitic 600–800 MPa Low Lower cost, simpler heat treatment.
SUS304 Austenitic 515–725 MPa (annealed) Excellent Superior corrosion resistance, non-magnetic.
SUS420J2 Martensitic 1200–1400 MPa Moderate Higher hardness but lower toughness than SUS431.
In summary, SUS431 is a versatile martensitic stainless steel that bridges the gap between strength and corrosion resistance. Its ability to be heat-treated for high strength, combined with moderate formability and weldability, makes it a go-to material for applications demanding durability in non-extreme corrosive environments.