SUS 304 stainless steel, also known as 18-8 stainless or simply "304 stainless", is the most widely used and versatile austenitic stainless steel. Its excellent corrosion resistance, strength, and formability make it an ideal material for countless applications across industries. In this definitive guide, we'll dive deep into the composition, properties, uses, and fabrication of SUS 304 to help you understand why it's such an indispensable alloy.

What is SUS 304 Stainless Steel?

SUS 304 is an austenitic chromium-nickel stainless steel that contains 18-20% chromium and 8-10.5% nickel. This balanced composition gives 304 its signature properties:

• High corrosion resistance
• Good strength and ductility
• Excellent weldability and formability
• Attractive, easy-to-clean surface finish

304 stainless is designated as "SUS304" in Japanese JIS standards and "304" or "S30400" in American AISI/SAE and UNS standards, respectively. It's often referred to as "18-8" due to its 18% chromium and 8% nickel content.

Composition and Microstructure

The specific composition of SUS 304 is carefully controlled to achieve optimal properties:

The high chromium content allows a thin, protective chromium oxide layer to form on the surface, providing the alloy's signature corrosion resistance. Nickel stabilizes the austenitic microstructure, enhancing strength, ductility, and toughness. The low carbon content minimizes the risk of chromium carbide precipitation during welding, which can lead to corrosion.

Microscopically, 304 consists of austenite (face-centered cubic crystal structure) with some ferrite. This microstructure remains stable across a wide temperature range and is responsible for the alloy's excellent formability.

Mechanical and Physical Properties

SUS 304 offers an exceptional combination of mechanical strength, ductility, and toughness:

• Tensile Strength: 515 MPa min
• Yield Strength (0.2% offset): 205 MPa min
• Elongation: 40% min
• Hardness: 88 HRB max

304 stainless also maintains its strength and toughness at both high and cryogenic temperatures, making it suitable for extreme environments. Other key physical properties include:

• Density: 8.0 g/cm³
• Melting Range: 1399-1454°C
• Thermal Conductivity: 16.2 W/m·K at 100°C
• Coefficient of Thermal Expansion: 17.2 x 10⁻⁶/K at 0-100°C

Corrosion Resistance

One of the primary reasons for specifying SUS 304 is its exceptional corrosion resistance. The alloy performs well in a variety of environments:

• Atmospheric corrosion
• Foodstuffs and beverages
• Organic chemicals and dyes
• Inorganic chemicals (except halides)
• Sterilizing solutions
• Most petroleum products

However, 304 can be susceptible to pitting and crevice corrosion in warm chloride environments and to stress corrosion cracking above 60°C. For highly corrosive conditions, higher alloyed grades like 316 or duplex stainless steels may be more suitable.

Fabrication and Processing

SUS 304 is readily fabricated using standard methods, including:

• Forming: 304 has excellent formability and can be readily deep drawn, bent, and stamped. Its high work hardening rate allows more severe deformation than carbon steels.

• Machining: Despite its tendency to work harden, 304 is machinable using appropriate tooling and techniques. Free-machining grades like 303 are available for improved machinability.

• Welding: 304 is considered one of the most weldable metals and can be joined using all common welding processes. Matching filler metals (308L) are typically used.

• Finishing: To maintain corrosion resistance after fabrication, 304 should be properly cleaned and passivated. A wide range of finishes, from dull to mirror-like, can be achieved.

Proper fabrication practices, like using dedicated stainless tooling and avoiding contamination, are essential to ensure optimal results.

Applications

SUS 304 finds use in an incredible array of applications across industries:

• Food and Beverage: Brewing, dairy, and food processing equipment
• Architecture: Decorative trim, handrails, and structural components
• Chemical Processing: Tanks, piping, and heat exchangers
• Transportation: Vehicle exhaust systems and structural members
• Medical: Surgical instruments, implants, and equipment
• Home and Commercial: Sinks, cookware, appliances, and hardware

304 is often the default choice for many stainless applications due to its versatility, availability, and cost-effectiveness compared to higher-alloyed grades.

Variants and Related Grades

Several common variants of 304 exist for specialized applications:

• 304L: Lower carbon version (0.03% max) for improved weldability
• 304H: Higher carbon version (0.04-0.10%) for high-temperature strength
• 304N: Nitrogen-strengthened grade for higher strength and wear resistance

304 also belongs to the broader family of 18-8 austenitic stainless steels, which includes grades like:

• 301: Higher strength version of 304 due to higher work hardening rate
• 302: Lower nickel and higher chromium version of 304 for certain applications
• 303: Free-machining version of 304 with added sulfur for improved machinability

For more demanding applications, 304 is often upgraded to 316 stainless, which offers even better corrosion resistance, particularly against chlorides, due to its molybdenum content.

Conclusion

SUS 304 stainless steel is a remarkably versatile material that has become a mainstay across industries worldwide. Its combination of corrosion resistance, strength, and fabricability make it the go-to choice for countless products and components. By understanding the properties and capabilities of 304, engineers and designers can harness its full potential to create high-performance, long-lasting solutions.

As a leading stainless steel supplier, Xinguangyuan offers a comprehensive range of 304 products, from sheet and plate to bar and pipe. Our expert team is ready to assist you in selecting the right material and finish for your application. Contact us today to learn more about how SUS 304 can benefit your next project.