SUS 304 stainless steel, commonly referred to as 18-8 stainless, is renowned for its versatility and widespread use. It offers a unique combination of excellent corrosion resistance, mechanical strength, and formability, making it a preferred choice for numerous applications across various 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 stainless steel alloy, characterized by its composition of 18-20% chromium and 8-10.5% nickel. This balanced composition is crucial for its signature properties, including high corrosion resistance and good strength. 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.
Why Choose SUS 304 Stainless Steel?
SUS 304 stainless steel is widely preferred due to its excellent balance of properties, including corrosion resistance, strength, and formability. These attributes make it an ideal choice for various applications across industries. Its versatility, combined with cost-effectiveness compared to higher-alloyed grades like 316, makes it a default selection for many stainless steel applications.
Composition and Microstructure
The composition of SUS 304 is precisely controlled to ensure optimal properties, such as high corrosion resistance, strength, and ductility. Key elements include chromium, nickel, carbon, manganese, silicon, phosphorus, and sulfur.
| Element | Composition (%) |
|---|---|
| Carbon (C) | 0.08 max |
| Chromium (Cr) | 18.0-20.0 |
| Nickel (Ni) | 8.0-10.5 |
| Manganese (Mn) | 2.0 max |
| Silicon (Si) | 1.0 max |
| Phosphorus (P) | 0.045 max |
| Sulfur (S) | 0.03 max |
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:
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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.
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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.
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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.
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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 stainless steel is utilized in a wide range of applications across industries due to its versatility and cost-effectiveness. Common uses include food processing equipment, architectural components, chemical processing equipment, vehicle parts, medical instruments, and household appliances.
- 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.
Common Questions About SUS 304 Stainless Steel
Q: What is the difference between SUS 304 and SUS 316 stainless steel?
A: SUS 316 contains molybdenum, which provides better corrosion resistance against chlorides compared to SUS 304.
Q: Is SUS 304 suitable for high-temperature applications?
A: Yes, SUS 304 maintains its strength and toughness at high temperatures, but for extreme conditions, variants like 304H may be more suitable.
Q: Can SUS 304 be used in marine environments?
A: While SUS 304 offers good corrosion resistance, it can be susceptible to pitting and crevice corrosion in chloride-rich environments. For marine applications, higher-alloyed grades might be preferable.
Conclusion
As a leading material in various industries, SUS 304 stainless steel offers unparalleled versatility and performance. Its unique blend of corrosion resistance, strength, and formability makes it an indispensable choice for engineers and designers seeking high-quality, long-lasting solutions. 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.
