347 Stainless Steel: Properties and Key Applications
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Table Of Content
Table Of Content
347 stainless steel is an austenitic stainless steel grade known for its excellent corrosion resistance and high-temperature strength. It is primarily alloyed with chromium (Cr) and nickel (Ni), with the addition of niobium (Nb) to stabilize the structure against carbide precipitation during welding and high-temperature service. This stabilization is crucial for maintaining the steel's integrity in environments where it may be exposed to elevated temperatures.
Comprehensive Overview
347 stainless steel is classified as an austenitic stainless steel, which is characterized by its face-centered cubic (FCC) crystal structure. This structure provides excellent ductility and toughness, making it suitable for various applications. The primary alloying elements in 347 stainless steel include:
- Chromium (Cr): Typically 17-19%, which enhances corrosion resistance and contributes to the steel's overall strength.
- Nickel (Ni): Usually 9-12%, which improves ductility and toughness, especially at low temperatures.
- Niobium (Nb): Added to stabilize the steel against carbide precipitation, particularly during welding.
Key Characteristics
347 stainless steel exhibits several significant properties:
- Corrosion Resistance: Excellent resistance to oxidation and corrosion in a variety of environments, including acidic and high-temperature conditions.
- High-Temperature Strength: Retains strength and toughness at elevated temperatures, making it suitable for applications in heat exchangers and furnace components.
- Weldability: Good weldability due to the presence of niobium, which minimizes the risk of carbide precipitation during welding.
Advantages and Limitations
| Advantages (Pros) | Limitations (Cons) |
|---|---|
| Excellent corrosion resistance | Higher cost compared to carbon steels |
| Good high-temperature strength | Lower strength compared to some alloy steels |
| Good weldability | Susceptible to stress corrosion cracking in certain environments |
347 stainless steel is commonly used in industries such as chemical processing, aerospace, and power generation due to its unique combination of properties. Its historical significance lies in its development as a solution for applications requiring both corrosion resistance and high-temperature performance.
Alternative Names, Standards, and Equivalents
| Standard Organization | Designation/Grade | Country/Region of Origin | Notes/Remarks |
|---|---|---|---|
| UNS | S34700 | USA | Closest equivalent to AISI 347 |
| AISI/SAE | 347 | USA | Commonly used designation |
| ASTM | A240 | USA | Standard specification for stainless steel plates |
| EN | 1.4550 | Europe | Equivalent designation in European standards |
| DIN | X6CrNiNb18-10 | Germany | Minor compositional differences to be aware of |
| JIS | SUS347 | Japan | Japanese Industrial Standard equivalent |
The differences between equivalent grades can affect selection based on specific application requirements. For example, while both 347 and 321 stainless steels are stabilized grades, 321 contains titanium, which may be preferable in certain environments where titanium's properties are beneficial.
Key Properties
Chemical Composition
| Element (Symbol and Name) | Percentage Range (%) |
|---|---|
| Cr (Chromium) | 17.0 - 19.0 |
| Ni (Nickel) | 9.0 - 12.0 |
| Nb (Niobium) | 0.5 - 1.0 |
| C (Carbon) | ≤ 0.08 |
| Mn (Manganese) | ≤ 2.0 |
| Si (Silicon) | ≤ 1.0 |
| P (Phosphorus) | ≤ 0.045 |
| S (Sulfur) | ≤ 0.03 |
The primary role of the key alloying elements in 347 stainless steel includes:
- Chromium: Enhances corrosion resistance and forms a protective oxide layer.
- Nickel: Improves toughness and ductility, especially at low temperatures.
- Niobium: Stabilizes the steel against carbide precipitation, particularly during welding and high-temperature applications.
Mechanical Properties
| Property | Condition/Temper | Typical Value/Range (Metric - SI Units) | Typical Value/Range (Imperial Units) | Reference Standard for Test Method |
|---|---|---|---|---|
| Tensile Strength | Annealed | 520 - 750 MPa | 75 - 109 ksi | ASTM E8 |
| Yield Strength (0.2% offset) | Annealed | 205 - 310 MPa | 30 - 45 ksi | ASTM E8 |
| Elongation | Annealed | 40% | 40% | ASTM E8 |
| Reduction of Area | Annealed | 60% | 60% | ASTM E8 |
| Hardness (Rockwell B) | Annealed | 85 - 95 HRB | 85 - 95 HRB | ASTM E18 |
| Impact Strength (Charpy V-notch) | -20°C | 40 J | 29.5 ft-lbf | ASTM E23 |
The combination of these mechanical properties makes 347 stainless steel suitable for applications that require high strength and ductility, particularly under mechanical loading conditions. Its ability to maintain strength at elevated temperatures also makes it ideal for structural integrity in high-temperature environments.
Physical Properties
| Property | Condition/Temperature | Value (Metric - SI Units) | Value (Imperial Units) |
|---|---|---|---|
| Density | Room Temperature | 8.0 g/cm³ | 0.289 lb/in³ |
| Melting Point/Range | - | 1400 - 1450 °C | 2552 - 2642 °F |
| Thermal Conductivity | Room Temperature | 16.2 W/(m·K) | 112 BTU/(hr·ft²·°F) |
| Specific Heat Capacity | Room Temperature | 500 J/(kg·K) | 0.119 BTU/(lb·°F) |
| Electrical Resistivity | Room Temperature | 0.72 µΩ·m | 0.0000013 Ω·in |
| Coefficient of Thermal Expansion | 20 - 100 °C | 16.0 x 10⁻⁶ /K | 8.9 x 10⁻⁶ /°F |
| Magnetic Permeability | Room Temperature | Non-magnetic | Non-magnetic |
The practical significance of key physical properties includes:
- Density: Affects weight considerations in structural applications.
- Thermal Conductivity: Important for heat exchanger applications where efficient heat transfer is required.
- Coefficient of Thermal Expansion: Critical for applications involving temperature fluctuations, as it affects dimensional stability.
Corrosion Resistance
| Corrosive Agent | Concentration (%) | Temperature (°C/°F) | Resistance Rating | Notes |
|---|---|---|---|---|
| Chlorides | 3-10 | 20-60 / 68-140 | Good | Risk of pitting |
| Sulfuric Acid | 10-30 | 20-50 / 68-122 | Fair | Susceptible to SCC |
| Nitric Acid | 10-30 | 20-60 / 68-140 | Excellent | Resistant to localized attack |
| Sea Water | - | Ambient | Good | Suitable for marine applications |
| Acetic Acid | 5-20 | 20-60 / 68-140 | Fair | Risk of localized corrosion |
347 stainless steel exhibits excellent resistance to a variety of corrosive environments, particularly in acidic and high-temperature conditions. Its resistance to pitting and stress corrosion cracking (SCC) makes it suitable for applications in chemical processing and marine environments. Compared to other stainless steel grades, such as 316 and 321, 347 offers superior performance in high-temperature applications due to its niobium stabilization.
Heat Resistance
| Property/Limit | Temperature (°C) | Temperature (°F) | Remarks |
|---|---|---|---|
| Max Continuous Service Temp | 925 | 1700 | Suitable for continuous service |
| Max Intermittent Service Temp | 870 | 1600 | Can withstand short-term exposure |
| Scaling Temperature | 1000 | 1832 | Begins to lose strength above this temp |
| Creep Strength considerations begin | 600 | 1112 | Important for long-term applications |
347 stainless steel performs well at elevated temperatures, maintaining its strength and oxidation resistance. However, care must be taken to avoid prolonged exposure to temperatures above 925 °C (1700 °F), as this can lead to scaling and loss of mechanical properties. Its oxidation resistance is particularly beneficial in applications such as furnace components and heat exchangers.
Fabrication Properties
Weldability
| Welding Process | Recommended Filler Metal (AWS Classification) | Typical Shielding Gas/Flux | Notes |
|---|---|---|---|
| TIG | ER347 | Argon | Good for thin sections |
| MIG | ER347 | Argon + 2% O₂ | Suitable for thicker sections |
| SMAW | E347 | - | Requires preheat |
347 stainless steel is highly weldable, thanks to its niobium content, which minimizes the risk of carbide precipitation during welding. Preheating may be required for thicker sections to avoid cracking. Post-weld heat treatment can enhance the mechanical properties and relieve residual stresses.
Machinability
| Machining Parameter | [347 Stainless Steel] | Benchmark Steel (AISI 1212) | Notes/Tips |
|---|---|---|---|
| Relative Machinability Index | 40% | 100% | Requires slower cutting speeds |
| Typical Cutting Speed (Turning) | 30 m/min | 60 m/min | Use carbide tooling |
347 stainless steel has moderate machinability. It is recommended to use slower cutting speeds and appropriate tooling to achieve optimal results. The presence of niobium can lead to work hardening, so careful attention to cutting conditions is necessary.
Formability
347 stainless steel can be formed using both cold and hot working processes. Cold forming is feasible but may require additional force due to work hardening. Hot forming is preferred for complex shapes, as it reduces the risk of cracking and improves ductility.
Heat Treatment
| Treatment Process | Temperature Range (°C/°F) | Typical Soaking Time | Cooling Method | Primary Purpose / Expected Result |
|---|---|---|---|---|
| Annealing | 1010 - 1120 / 1850 - 2050 | 1-2 hours | Air or Water | Relieve stresses, improve ductility |
| Solution Treatment | 1050 - 1100 / 1920 - 2010 | 1 hour | Water | Dissolve carbides, enhance corrosion resistance |
During heat treatment, 347 stainless steel undergoes metallurgical transformations that enhance its microstructure and properties. Annealing relieves internal stresses and improves ductility, while solution treatment dissolves carbides, enhancing corrosion resistance.
Typical Applications and End Uses
| Industry/Sector | Specific Application Example | Key Steel Properties Utilized in this Application | Reason for Selection (Brief) |
|---|---|---|---|
| Chemical Processing | Heat exchangers | High-temperature strength, corrosion resistance | Suitable for aggressive environments |
| Aerospace | Exhaust systems | High-temperature strength, weldability | Required for high-performance applications |
| Power Generation | Boiler components | Corrosion resistance, high-temperature strength | Essential for durability and safety |
| Marine | Shipbuilding | Corrosion resistance, weldability | Ideal for marine environments |
Other applications include:
- Food processing equipment
- Pharmaceutical manufacturing
- Oil and gas pipelines
347 stainless steel is chosen for these applications due to its unique combination of high-temperature strength and excellent corrosion resistance, ensuring reliability and longevity in demanding environments.
Important Considerations, Selection Criteria, and Further Insights
| Feature/Property | 347 Stainless Steel | 316 Stainless Steel | 321 Stainless Steel | Brief Pro/Con or Trade-off Note |
|---|---|---|---|---|
| Key Mechanical Property | Moderate strength | High strength | High strength | 316 offers better corrosion resistance in chloride environments |
| Key Corrosion Aspect | Good in acidic environments | Excellent in chlorides | Good in high-temperature applications | 321 is better for high-temperature applications |
| Weldability | Good | Excellent | Good | 316 is preferred for critical welds |
| Machinability | Moderate | Good | Moderate | 316 is easier to machine due to lower work hardening |
| Formability | Good | Good | Good | All grades are suitable for forming |
| Approx. Relative Cost | Moderate | Higher | Moderate | 316 is generally more expensive |
| Typical Availability | Common | Very Common | Common | 316 is widely available due to its popularity |
When selecting 347 stainless steel, considerations include cost-effectiveness, availability, and specific application requirements. Its unique properties make it suitable for a range of applications, particularly where high-temperature performance and corrosion resistance are critical. Additionally, safety considerations in high-stress environments should guide material selection, ensuring that the chosen grade meets all operational demands.
In summary, 347 stainless steel is a versatile and reliable material, particularly valued in industries where performance under extreme conditions is paramount.