1.4401, commonly designated as SUS316 in Japanese standards, is a molybdenum-alloyed austenitic stainless steel renowned for its excellent corrosion resistance, particularly in chloride-rich environments. It is a versatile material widely used across industries requiring durability in aggressive conditions, offering a balance of mechanical properties, weldability, and formability.
Basic Information
Definition: 1.4401 (SUS316) is an austenitic stainless steel containing chromium, nickel, and molybdenum, with a moderate carbon content (≤0.08%). The addition of molybdenum distinguishes it from 304 stainless steel, significantly enhancing its resistance to pitting and crevice corrosion. It is the standard-grade counterpart to the low-carbon 316L (1.4404).
Corresponding Standards: Conforms to international specifications such as EN 10088-1 (European), JIS G4303 (Japanese, as SUS316), and ASTM A240 (American). Designations include:
DIN: 1.4401
UNS: S31600
JIS: SUS316
Key Characteristics: Superior corrosion resistance compared to 304, good high-temperature strength, excellent formability, and weldability (with proper post-weld treatment for critical applications).
Chemical Composition
The composition of 1.4401 (SUS316) is tailored to optimize corrosion resistance and structural stability:
Element Content Range Role in the Alloy
Chromium (Cr) 16.0–18.0% Forms a protective oxide layer, providing general corrosion resistance.
Nickel (Ni) 10.0–14.0% Stabilizes the austenitic microstructure, enhancing toughness and ductility.
Molybdenum (Mo) 2.0–3.0% Critical for resistance to pitting, crevice corrosion, and chloride-induced stress corrosion cracking (SCC).
Carbon (C) ≤0.08% Provides strength but is controlled to minimize carbide precipitation during welding.
Iron (Fe) Balance Base metal providing structural integrity.
Silicon (Si) ≤1.0% Acts as a deoxidizer during production.
Manganese (Mn) ≤2.0% Aids in austenite stabilization and improves hot workability.
Phosphorus (P) ≤0.045% Limited to prevent brittleness and reduce susceptibility to pitting.
Sulfur (S) ≤0.03% Minimized to avoid hot cracking during welding.
Physical Properties
Density: ~8.0 g/cm³ (slightly higher than 304 due to molybdenum content).
Melting Point: 1375–1400°C.
Thermal Conductivity: ~16 W/(m·K) at room temperature.
Coefficient of Thermal Expansion: ~16×10⁻⁶/°C (20–100°C), typical for austenitic stainless steels.
Elastic Modulus: ~193 GPa at room temperature.
Magnetic Properties: Non-magnetic in the annealed state; may develop slight magnetism after cold working but remains predominantly non-magnetic.
Mechanical Properties
1.4401 (SUS316) exhibits balanced mechanical properties suitable for diverse applications:
Property Annealed Condition (Typical Values)
Tensile Strength (Rm) 500–700 MPa
Yield Strength (Rp0.2) ≥205 MPa
Elongation (A) ≥40%
Hardness ≤217 HB
Impact Toughness (Charpy V-Notch) ≥100 J (at -196°C)
High-Temperature Performance: Retains strength and oxidation resistance up to 800°C, making it suitable for high-heat applications like exhaust systems.
Cryogenic Performance: Maintains excellent toughness at extremely low temperatures (down to -270°C), ideal for LNG and cryogenic storage.
Corrosion Resistance
1.4401 (SUS316) offers superior corrosion resistance compared to standard austenitic grades like 304:
Pitting Resistance: High PREN (Pitting Resistance Equivalent Number) (~30–36), calculated as PREN = %Cr + 3.3×%Mo + 16×%N, indicating strong resistance to chloride-induced pitting.
Chloride Environments: Performs well in seawater, brines, and industrial chlorides, though 316L (1.4404) is preferred for critical applications due to its lower carbon content.
Chemical Resistance: Resists sulfuric acid, phosphoric acid, and organic acids; suitable for both oxidizing and reducing environments.
Welded Joints: May require post-weld annealing to prevent intergranular corrosion in the heat-affected zone (HAZ) due to its higher carbon content compared to 316L.
Heat Treatment
1.4401 (SUS316) is typically used in the annealed condition to optimize corrosion resistance and ductility:
Annealing Process:
Temperature: 1050–1150°C.
Cooling: Water quenching to retain a fully austenitic microstructure, critical for maximizing corrosion resistance.
Post-Weld Annealing: Recommended for welded components in corrosive environments to dissolve chromium carbides formed in the HAZ, restoring corrosion resistance.
Stress Relief: Cold-worked parts can be stress-relieved at 800–900°C (followed by air cooling) to reduce residual stresses without compromising properties.
Processing Performance
Weldability: Good using standard methods (GTAW/TIG, GMAW/MIG, SMAW). Post-weld annealing is advised for corrosive applications to prevent intergranular corrosion.
Formability: Excellent cold formability for bending, rolling, deep drawing, and spinning. Work hardening occurs during cold working, so intermediate annealing may be needed for complex shapes.
Machinability: Moderate due to work-hardening tendencies. Requires sharp tools, low cutting speeds, and ample coolant to minimize tool wear and avoid overheating.
Polishability: Polishes well to a smooth, reflective finish, suitable for decorative or sanitary applications.
Application Fields
1.4401 (SUS316) is widely used in corrosive and high-performance environments:
Marine Engineering:
Ship components, offshore platforms, seawater pipelines, and desalination equipment.
Chemical Processing:
Reactors, valves, pumps, and storage tanks for handling acids, chlorides, and organic chemicals.
Food & Beverage Industry:
Processing equipment, tanks, and conveyors (resists cleaning agents and prevents contamination).
Medical Equipment:
Instrumentation and non-implantable devices (ease of sterilization and corrosion resistance).
Energy Sector:
Offshore oil and gas equipment, heat exchangers, and pollution control systems.
Architectural & Decorative:
Outdoor fixtures, coastal buildings, and high-end decorative elements (resists atmospheric corrosion).
Advantages and Limitations
Advantages
Superior Corrosion Resistance: Outperforms 304 in chloride-rich and aggressive chemical environments.
Temperature Versatility: Performs well from cryogenic to high (800°C) temperatures.
Excellent Formability: Easily shaped into complex geometries via cold working.
Good Weldability: Compatible with standard welding techniques (with optional post-weld annealing).
Limitations
Higher Cost: More expensive than 304 due to molybdenum and nickel content.
Carbon-Related Risks: Higher carbon content than 316L increases risk of intergranular corrosion in welded joints without post-treatment.
Work Hardening: Requires careful processing to avoid excessive hardness during cold forming.
Comparison with 316L (1.4404)
Feature 1.4401 (SUS316) 1.4404 (316L)
Carbon Content ≤0.08% ≤0.03%
Welded Corrosion Resistance May require post-weld annealing No post-weld annealing needed
Strength Slightly higher yield strength Marginally lower strength
Cost Slightly lower than 316L Higher (due to stricter carbon control)
Ideal Use Case General corrosive environments Critical welded structures in harsh chloride environments
In summary, 1.4401 (SUS316) is a reliable austenitic stainless steel offering a balance of corrosion resistance, mechanical properties, and processability. It is the material of choice for applications requiring durability in chloride-rich and moderately aggressive environments, bridging performance and cost between 304 and 316L.