Stainless Steel 304 flanges are among the most widely used piping components in industrial, commercial, and structural applications. Known for their excellent corrosion resistance, good mechanical strength, and reliable fabrication characteristics, SS 304 flanges are commonly selected for general-purpose piping systems operating under moderate temperature and pressure conditions.
This blog provides a technically accurate overview of Stainless Steel 304 flange specifications, aligned strictly with established ASTM, ASME, and international standards.
1. What is Stainless Steel 304?
Stainless Steel 304 is an austenitic chromium-nickel stainless steel and the most widely used stainless steel grade globally.
It is designated as:
UNS S30400
AISI Type 304
EN 1.4301 (European designation)
Its typical composition of 18–20% chromium and 8–10.5% nickel provides excellent resistance to atmospheric corrosion and many mildly aggressive chemical environments.
2. Chemical Composition of SS 304 (UNS S30400)
According to ASTM material specifications, the chemical composition limits are:
| Element | Composition (%) |
|---|---|
| Chromium (Cr) | 18.0 – 20.0 |
| Nickel (Ni) | 8.0 – 10.5 |
| Carbon (C) | ≤ 0.08 |
| Manganese (Mn) | ≤ 2.00 |
| Silicon (Si) | ≤ 1.00 |
| Phosphorus (P) | ≤ 0.045 |
| Sulfur (S) | ≤ 0.030 |
| Nitrogen (N) | ≤ 0.10 |
| Iron (Fe) | Balance |
Chromium enhances corrosion resistance through the formation of a passive oxide layer, while nickel improves toughness and ductility.
3. Standards for Stainless Steel 304 Flanges
Stainless Steel 304 flanges are manufactured under internationally recognized standards covering material and dimensions.
Material Standards
ASTM A182 Grade F304 – Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves
ASTM A240 – Stainless Steel Plate (when flanges are plate-cut)
ASME SA182 F304 – ASME equivalent of ASTM A182
Under ASTM A182, SS 304 flanges are designated as:
ASTM A182 F304 (UNS S30400)
These standards define chemical composition, mechanical properties, heat treatment, and testing requirements.
Dimensional Standards
Flange dimensions and pressure ratings are governed by:
ASME B16.5 – Pipe Flanges and Flanged Fittings (NPS ½" to 24")
ASME B16.47 – Large Diameter Steel Flanges (NPS 26" to 60")
EN 1092-1 – European flange standard
These standards specify:
Pressure-temperature ratings
Facing types
Tolerances
Bolt hole dimensions
Marking requirements
4. Mechanical Properties (Annealed Condition)
As per ASTM A182 for Grade F304:
| Property | Minimum Value |
|---|---|
| Tensile Strength | 515 MPa |
| Yield Strength (0.2% offset) | 205 MPa |
| Elongation | 30% (minimum for forgings, per ASTM A182) |
| Hardness | ≤ 201 HB (typical limit for annealed material) |
Mechanical properties may vary slightly depending on product form and standard.
5. Heat Treatment
ASTM A182 F304 flanges are supplied in the solution-annealed condition.
Key points:
Cannot be hardened by heat treatment
Strength can increase through cold working
Rapid cooling after solution annealing maintains corrosion resistance
Proper heat treatment ensures uniform austenitic microstructure and optimal performance.
6. Pressure Class Ratings
Under ASME B16.5, Stainless Steel 304 flanges are available in pressure classes:
Class 150
Class 300
Class 400
Class 600
Class 900
Class 1500
Class 2500
Pressure-temperature ratings are defined by ASME B16.5 based on material group and operating temperature.
7. Types of Stainless Steel 304 Flanges
Common types include:
Weld Neck Flange (WN)
Slip-On Flange (SO)
Blind Flange (BL)
Socket Weld Flange (SW)
Threaded Flange (TH)
Lap Joint Flange (LJ)
Selection depends on pressure requirements, piping design, and service conditions.
8. Corrosion Resistance
Stainless Steel 304 flanges provide:
Excellent resistance to atmospheric corrosion
Good resistance to many organic and inorganic chemicals
Oxidation resistance up to approximately 870°C (intermittent service)
However, they may be susceptible to:
Chloride-induced pitting
Crevice corrosion
Stress corrosion cracking in high-temperature chloride environments
For more aggressive chloride exposure, Grade 316 is generally recommended.
9. Applications of Stainless Steel 304 Flanges
Due to their versatility, SS 304 flanges are widely used in:
Water treatment plants
Food processing systems
Chemical processing (mild environments)
Oil and gas (non-severe corrosive service)
HVAC systems
Structural piping
Pharmaceutical industries
Their excellent weldability makes them suitable for fabricated piping assemblies.
10. 304 vs 304L Flanges
The primary difference between 304 and 304L is carbon content:
304: Maximum 0.08% carbon
304L: Maximum 0.03% carbon
Lower carbon in 304L reduces the risk of carbide precipitation during welding, making it more suitable for heavy welded applications.
Under ASTM A182, the low-carbon version is designated as:
ASTM A182 F304L
Key Advantages of Stainless Steel 304 Flanges
Good corrosion resistance in general environments
Excellent weldability and fabrication characteristics
Wide availability in standard pressure classes
Cost-effective compared to higher alloyed stainless steels
Reliable performance in moderate temperature and pressure service
Conclusion
Stainless Steel 304 flanges (ASTM A182 F304 / UNS S30400), manufactured in accordance with ASTM and ASME standards such as ASTM A182 and ASME B16.5, provide a dependable solution for general-purpose piping systems. With 18–20% chromium and 8–10.5% nickel content, they offer balanced corrosion resistance, mechanical strength, and fabrication flexibility.