Introduction
304 stainless steel is part of the 300-series austenitic alloys and is one of the most widely used stainless steels worldwide. It contains approximately 18–20% chromium and 8–10.5% nickel, which gives it excellent corrosion resistance, good strength, and high formability.
In the annealed condition, it is non-magnetic and maintains good toughness even at very low temperatures. Because of this balance of properties, 304 is widely used in industries such as construction, food processing, chemical equipment, automotive manufacturing, and medical devices.
1. Equivalent Grades of 304 Stainless Steel
304 stainless steel is standardized globally under different systems:
| Country/Standard | Grade |
|---|---|
| China (GB) | 06Cr19Ni9 / S30408 |
| USA (ASTM/AISI) | 304 / S30400 |
| ISO | X5CrNi18-10 |
| Europe (EN/DIN) | 1.4301 |
| Japan (JIS) | SUS 304 |
| Korea (KS) | STS 304 |
| France (NF) | Z6CN18-09 |
| Russia (GOST) | 08X18H10 |
| Sweden (SS) | 2332 |
| UK (BS) | 304S15 |
2. Chemical Composition (ASTM A959)
| Element | Content (%) | Function |
|---|---|---|
| C | ≤ 0.08 | Increases strength and hardness |
| Si | ≤ 1.00 | Improves oxidation resistance |
| Mn | ≤ 2.00 | Stabilizes austenite structure |
| P | ≤ 0.045 | Controlled to maintain weldability |
| S | ≤ 0.03 | Improves machinability |
| Cr | 18.0–20.0 | Provides corrosion resistance |
| Ni | 8.0–11.0 | Improves ductility and toughness |
Key Mechanism
- Chromium forms a passive oxide layer (Cr₂O₃) that protects against corrosion and can self-repair.
- Nickel stabilizes the austenitic structure, ensuring good ductility and non-magnetic behavior.
3. Mechanical Properties
304 stainless steel offers an excellent balance between strength and ductility.
| Property | Annealed | Cold Worked |
|---|---|---|
| Tensile Strength (MPa) | 515–750 | ~1000 |
| Yield Strength (MPa) | ≥205 | 350–700 |
| Elongation (%) | ≥40 | 20–35 |
| Hardness (HB) | 150–200 | ~250 |
| Elastic Modulus | ~193 GPa | — |
| Poisson’s Ratio | 0.29 | — |
Key Insights
- In the annealed state, it is ideal for forming and welding.
- After cold working, strength can nearly double.
- Maintains toughness even at −196°C, making it suitable for cryogenic applications (e.g., LNG tanks).
4. Physical Properties
| Property | Value | Notes |
|---|---|---|
| Density | 7.93 g/cm³ | Standard structural material |
| Melting Point | 1400–1450°C | Good high-temperature resistance |
| Thermal Conductivity | 16.2 W/m·K | Lower than carbon steel |
| Specific Heat | ~500 J/kg·K | Stable heat capacity |
| Thermal Expansion | 17.2 μm/m·K | Important for design |
| Electrical Resistivity | 0.72 μΩ·m | Higher than carbon steel |
| Magnetic Permeability | ~1.02 | Non-magnetic when annealed |
Note: Cold working may introduce slight magnetism.
5. Corrosion Resistance
The corrosion performance depends heavily on the environment:
| Environment | Performance | Notes |
|---|---|---|
| Atmosphere | Excellent | Resistant to oxidation |
| Fresh Water | Excellent | Minimal corrosion |
| Weak Acids | Good | Suitable for food processing |
| Mild Alkaline | Good | Stable passive film |
| Chlorides / Strong Acids | Limited | Risk of pitting corrosion |
Engineering Note
304 is not recommended for:
- Marine environments
- High-chloride exposure
In such cases, 316/316L is preferred.
6. Processing and Manufacturing Characteristics
| Category | Key Features | Risks / Challenges | Recommended Practices |
|---|---|---|---|
| Formability | Excellent ductility; suitable for deep drawing, bending, and stamping | Work hardening during cold forming | Use intermediate annealing (900–950°C); select 304L for deep drawing to reduce cracking |
| Welding | Compatible with TIG, MIG, SMAW, and spot welding; weld strength matches base metal | Risk of intergranular corrosion (sensitization) | Use 304L or apply solution annealing; ensure shielding gas purity; use fillers like ER308/ER308L |
| Machining | Moderate machinability (~70–80% of 303); good for precision parts | Severe work hardening; long chips; heat buildup | Use carbide/coated tools; apply high-pressure cooling; increase feed rate for chip control |
| Heat Treatment | Good response to solution annealing; stable structure | Sensitization at 425–870°C | Solution anneal at 1040–1120°C (water cooling); avoid prolonged exposure in critical temperature range |
| Hot Processing | Suitable for forging and hot rolling | Surface oxidation and uneven heating | Process at 1150–950°C; ensure uniform heating and rapid cooling |
| Surface Finishing | Wide range of finishes: 2B, BA, No.4, 8K | Surface contamination (iron particles) | Apply passivation or electropolishing; for hygienic use keep Ra ≤ 0.8 μm |
7. Common Derived Grades
| Grade | Key Composition | Core Features | Advantages | Limitations | Typical Applications |
|---|---|---|---|---|---|
| 304 | C ≤ 0.08% | Standard grade with balanced properties | Good corrosion resistance, formability, and cost-effectiveness | Risk of sensitization at high temperature (425–815°C) | General applications: kitchenware, piping, construction, light industrial parts |
| 304L | C ≤ 0.03% | Low-carbon version of 304 | Excellent weldability; strong resistance to intergranular corrosion | Slightly lower strength than 304 | Welded structures, tanks, pipelines, chemical equipment |
| 304H | C = 0.04–0.10% | High-carbon version for high temperature | Improved creep strength and high-temperature durability | Poor corrosion resistance after welding; sensitive to sensitization | Boilers, heat exchangers, high-temperature pressure systems |
| 304N | N = 0.10–0.16% | Nitrogen-strengthened grade | Higher strength while maintaining ductility and corrosion resistance | Higher cost; less common than 304/304L | Structural components, transport equipment, load-bearing parts |
| 304LN | C ≤ 0.03%, N = 0.10–0.16% | Low carbon + nitrogen strengthening | Combines weldability and high strength; good corrosion resistance | More expensive; requires controlled processing | Offshore structures, chemical plants, high-performance welded parts |
| 304LHN | C ≤ 0.03%, N > 0.15% | Advanced high-strength variant | Very high strength with good corrosion resistance and weldability | Limited availability; higher material cost | Nuclear, aerospace, deep-sea, and critical structural components |
Conclusion
304 stainless steel remains the default choice in many industries due to its excellent balance of:
- Corrosion resistance
- Mechanical strength
- Formability
- Hygiene performance
However, proper grade selection is critical. For example:
- Use 304L for welding
- Use 316/316L for chloride environments
Understanding these differences helps engineers and buyers make better decisions and avoid costly failures.
