304H Stainless Steel: Properties and Key Applications
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Table Of Content
Table Of Content
304H stainless steel is a high-temperature variant of the widely used 304 stainless steel, classified as an austenitic stainless steel. This grade is primarily alloyed with chromium (18-20%) and nickel (8-10.5%), with a carbon content that is higher than standard 304, typically around 0.04% to 0.10%. The increased carbon content enhances strength at elevated temperatures, making 304H particularly suitable for applications in environments where high temperatures are a concern.
Comprehensive Overview
304H stainless steel exhibits excellent corrosion resistance, high strength, and good weldability. Its austenitic structure provides superior toughness and ductility, making it ideal for applications requiring significant mechanical stress. The primary advantages of 304H include its ability to withstand high temperatures (up to 870°C or 1600°F) and its resistance to oxidation and scaling. However, its higher carbon content can lead to reduced corrosion resistance in certain environments compared to lower carbon grades.
In terms of market position, 304H is commonly used in the petrochemical, oil and gas, and power generation industries, where high-temperature applications are prevalent. Historically, it has been significant in the development of heat exchangers, boilers, and pressure vessels.
| Pros | Cons |
|---|---|
| Excellent high-temperature strength | Reduced corrosion resistance compared to lower carbon grades |
| Good weldability | Susceptible to sensitization if not properly heat-treated |
| High resistance to oxidation | Higher cost due to alloying elements |
Alternative Names, Standards, and Equivalents
| Standard Organization | Designation/Grade | Country/Region of Origin | Notes/Remarks |
|---|---|---|---|
| UNS | S30409 | USA | Closest equivalent to AISI 304L with higher carbon content |
| AISI/SAE | 304H | USA | Used for high-temperature applications |
| ASTM | A240/A240M | USA | Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip |
| EN | 1.4948 | Europe | Equivalent grade in European standards |
| JIS | SUS304H | Japan | Japanese Industrial Standard equivalent |
The differences between 304H and its equivalents, such as 304L, primarily lie in the carbon content, which affects their performance in high-temperature environments. While 304L is preferred for its lower carbon content and better corrosion resistance, 304H is chosen for applications requiring higher strength at elevated temperatures.
Key Properties
Chemical Composition
| Element (Symbol and Name) | Percentage Range (%) |
|---|---|
| C (Carbon) | 0.04 - 0.10 |
| Cr (Chromium) | 18.0 - 20.0 |
| Ni (Nickel) | 8.0 - 10.5 |
| Mn (Manganese) | 2.0 max |
| Si (Silicon) | 1.0 max |
| P (Phosphorus) | 0.045 max |
| S (Sulfur) | 0.030 max |
The primary role of chromium in 304H is to enhance corrosion resistance, while nickel contributes to the steel's toughness and ductility. The controlled carbon content improves high-temperature strength, making it suitable for demanding applications.
Mechanical Properties
| Property | Condition/Temper | Test Temperature | Typical Value/Range (Metric) | Typical Value/Range (Imperial) | Reference Standard for Test Method |
|---|---|---|---|---|---|
| Tensile Strength | Annealed | Room Temp | 520 - 750 MPa | 75 - 109 ksi | ASTM E8 |
| Yield Strength (0.2% offset) | Annealed | Room Temp | 205 - 310 MPa | 30 - 45 ksi | ASTM E8 |
| Elongation | Annealed | Room Temp | 40% min | 40% min | ASTM E8 |
| Hardness (Rockwell B) | Annealed | Room Temp | 70 - 90 HRB | 70 - 90 HRB | ASTM E18 |
| Impact Strength | Charpy (20°C) | 20°C | 40 J min | 29.5 ft-lbf | ASTM E23 |
The combination of high tensile and yield strength, along with good ductility, makes 304H suitable for applications that experience mechanical loading and require structural integrity.
Physical Properties
| Property | Condition/Temperature | Value (Metric) | Value (Imperial) |
|---|---|---|---|
| Density | Room Temp | 7.93 g/cm³ | 0.286 lb/in³ |
| Melting Point/Range | - | 1400 - 1450 °C | 2552 - 2642 °F |
| Thermal Conductivity | Room Temp | 16.2 W/m·K | 112 BTU·in/h·ft²·°F |
| Specific Heat Capacity | Room Temp | 500 J/kg·K | 0.12 BTU/lb·°F |
| Electrical Resistivity | Room Temp | 0.72 µΩ·m | 0.72 µΩ·in |
| Coefficient of Thermal Expansion | Room Temp | 16.0 x 10⁻⁶/K | 8.9 x 10⁻⁶/°F |
The density and melting point of 304H indicate its robustness, while its thermal conductivity and specific heat capacity are critical for applications involving heat transfer, such as heat exchangers.
Corrosion Resistance
| Corrosive Agent | Concentration (%) | Temperature (°C/°F) | Resistance Rating | Notes |
|---|---|---|---|---|
| Chlorides | 3-5% | 20-60°C / 68-140°F | Fair | Risk of pitting |
| Sulfuric Acid | 10% | 20-40°C / 68-104°F | Poor | Not recommended |
| Acetic Acid | 10% | 20-60°C / 68-140°F | Good | Moderate resistance |
| Sea Water | - | Ambient | Good | Suitable for marine applications |
304H exhibits good resistance to a variety of corrosive environments, particularly in atmospheric conditions and diluted acids. However, it is susceptible to pitting corrosion in chloride environments and should be avoided in concentrated sulfuric acid applications. Compared to 316 stainless steel, which contains molybdenum for enhanced pitting resistance, 304H may not perform as well in highly corrosive environments.
Heat Resistance
| Property/Limit | Temperature (°C) | Temperature (°F) | Remarks |
|---|---|---|---|
| Max Continuous Service Temp | 870°C | 1600°F | - |
| Max Intermittent Service Temp | 925°C | 1700°F | - |
| Scaling Temperature | 800°C | 1472°F | - |
| Creep Strength considerations | 600°C | 1112°F | Begins to decrease |
304H maintains its strength and oxidation resistance at elevated temperatures, making it suitable for applications in heat exchangers and pressure vessels. However, prolonged exposure to temperatures above 870°C can lead to oxidation and scaling, necessitating careful design considerations.
Fabrication Properties
Weldability
| Welding Process | Recommended Filler Metal (AWS Classification) | Typical Shielding Gas/Flux | Notes |
|---|---|---|---|
| TIG | ER308L | Argon | Good results with proper technique |
| MIG | ER308L | Argon/CO2 mix | Requires preheat for thicker sections |
| SMAW | E308L | - | Suitable for thicker sections |
304H is generally considered to have good weldability, but preheating and post-weld heat treatment are recommended to minimize the risk of cracking and sensitization. Proper filler metal selection is crucial for maintaining corrosion resistance.
Machinability
| Machining Parameter | 304H | AISI 1212 | Notes/Tips |
|---|---|---|---|
| Relative Machinability Index | 60% | 100% | 304H is less machinable than 1212 |
| Typical Cutting Speed (Turning) | 30-50 m/min | 60-80 m/min | Use sharp tools and proper coolant |
304H has moderate machinability, and while it can be machined effectively, it requires careful attention to tooling and cutting speeds to avoid work hardening.
Formability
304H exhibits good formability, allowing for cold and hot forming processes. However, due to its work hardening characteristics, careful control of bending radii and forming speeds is necessary to avoid cracking.
Heat Treatment
| Treatment Process | Temperature Range (°C/°F) | Typical Soaking Time | Cooling Method | Primary Purpose / Expected Result |
|---|---|---|---|---|
| Solution Annealing | 1010 - 1120 °C / 1850 - 2050 °F | 30 min | Air or Water | Dissolve carbides, improve ductility |
| Stress Relief | 600 - 800 °C / 1112 - 1472 °F | 1-2 hours | Air | Reduce residual stresses |
Heat treatment processes such as solution annealing are critical for optimizing the microstructure of 304H, enhancing its mechanical properties and corrosion resistance.
Typical Applications and End Uses
| Industry/Sector | Specific Application Example | Key Steel Properties Utilized in this Application | Reason for Selection |
|---|---|---|---|
| Oil and Gas | Heat exchangers | High-temperature strength, corrosion resistance | Required for harsh environments |
| Power Generation | Boiler tubes | High strength, oxidation resistance | Essential for efficiency |
| Chemical Processing | Pressure vessels | Corrosion resistance, weldability | Safety and reliability |
| Food Processing | Equipment and piping | Corrosion resistance, ease of cleaning | Hygiene standards |
- 304H is often selected for heat exchangers due to its ability to withstand high temperatures and resist oxidation.
- In the oil and gas industry, it is used in pressure vessels where high strength and corrosion resistance are critical.
- The food processing sector utilizes 304H for its hygienic properties and ease of cleaning.
Important Considerations, Selection Criteria, and Further Insights
| Feature/Property | 304H | 316 | 321 | Brief Pro/Con or Trade-off Note |
|---|---|---|---|---|
| Key Mechanical Property | High strength | Better corrosion resistance | Good high-temp stability | 304H is stronger but less resistant |
| Key Corrosion Aspect | Fair in chlorides | Excellent in chlorides | Good in high-temp applications | 316 is preferred for marine environments |
| Weldability | Good | Excellent | Good | 316 may require special filler for high-temp |
| Machinability | Moderate | Moderate | Moderate | All require care to avoid work hardening |
| Approx. Relative Cost | Moderate | Higher | Moderate | 304H is cost-effective for high-temp applications |
| Typical Availability | Common | Common | Less common | 304H is widely available in various forms |
When selecting 304H, considerations include its cost-effectiveness for high-temperature applications, availability, and the specific mechanical and corrosion properties required for the intended use. While it offers significant advantages in strength and heat resistance, its susceptibility to pitting in chloride environments may necessitate careful evaluation against alternatives like 316 stainless steel for specific applications.