Heat Resistant Steel: Properties and Key Applications
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Heat resistant steel is a specialized category of steel designed to maintain its mechanical properties at elevated temperatures. These steels are primarily classified as alloy steels, often containing significant amounts of chromium, nickel, and molybdenum, which enhance their resistance to oxidation and creep deformation. The primary alloying elements in heat resistant steel include:
- Chromium (Cr): Improves oxidation resistance and enhances high-temperature strength.
- Nickel (Ni): Increases toughness and ductility at elevated temperatures.
- Molybdenum (Mo): Enhances strength and resistance to softening at high temperatures.
Characteristics and Properties
Heat resistant steels are characterized by their ability to withstand high temperatures while maintaining structural integrity. Key properties include:
- High-temperature strength: Retains strength and hardness at elevated temperatures.
- Oxidation resistance: Forms a protective oxide layer that prevents further degradation.
- Creep resistance: Ability to resist deformation under prolonged exposure to high temperatures and stress.
Advantages and Limitations
| Pros | Cons |
|---|---|
| Excellent high-temperature strength | Higher cost compared to standard steels |
| Good oxidation resistance | Limited availability in some grades |
| Suitable for extreme environments | May require special welding techniques |
Heat resistant steels are commonly used in industries such as power generation, aerospace, and petrochemical processing. Their historical significance lies in their development for applications requiring durability and reliability in harsh conditions.
Alternative Names, Standards, and Equivalents
| Standard Organization | Designation/Grade | Country/Region of Origin | Notes/Remarks |
|---|---|---|---|
| UNS | S31000 | USA | Austenitic stainless steel, good oxidation resistance |
| AISI | 310 | USA | Similar to UNS S31000, often used interchangeably |
| ASTM | A213 | USA | Standard specification for seamless ferritic and austenitic alloy steel tubes |
| EN | 1.4845 | Europe | Equivalent to AISI 310, minor compositional differences |
| JIS | SUS310S | Japan | Similar to AISI 310, lower carbon content for improved weldability |
The differences between these grades can affect performance, particularly in terms of weldability and oxidation resistance. For example, while UNS S31000 and AISI 310 are often used interchangeably, the specific heat treatment and processing can lead to variations in mechanical properties.
Key Properties
Chemical Composition
| Element (Symbol and Name) | Percentage Range (%) |
|---|---|
| Carbon (C) | 0.08 - 0.15 |
| Chromium (Cr) | 19.0 - 22.0 |
| Nickel (Ni) | 9.0 - 12.0 |
| Molybdenum (Mo) | 0.0 - 0.5 |
| Silicon (Si) | 0.0 - 1.0 |
| Manganese (Mn) | 0.0 - 2.0 |
| Phosphorus (P) | ≤ 0.045 |
| Sulfur (S) | ≤ 0.030 |
Chromium is crucial for oxidation resistance, while nickel enhances toughness. Molybdenum contributes to high-temperature strength, making these elements vital for the performance of heat resistant steels.
Mechanical Properties
Room Temperature Properties
| Property | Condition/Temper | Typical Value/Range (Metric) | Typical Value/Range (Imperial) | Reference Standard for Test Method |
|---|---|---|---|---|
| Tensile Strength | Annealed | 515 - 690 MPa | 75 - 100 ksi | ASTM E8 |
| Yield Strength (0.2% offset) | Annealed | 205 - 310 MPa | 30 - 45 ksi | ASTM E8 |
| Elongation | Annealed | 40 - 50% | 40 - 50% | ASTM E8 |
| Hardness (Rockwell B) | Annealed | 70 - 90 HRB | 70 - 90 HRB | ASTM E18 |
Elevated Temperature Properties
| Property | Condition/Temper | Test Temperature | Typical Value/Range (Metric) | Typical Value/Range (Imperial) | Reference Standard for Test Method |
|---|---|---|---|---|---|
| Creep Strength | 1000°C | 1000°C | 100 - 150 MPa | 14.5 - 21.8 ksi | ASTM E139 |
| Hardness | Quenched & Tempered | 600°C | 150 - 200 HB | 150 - 200 HB | ASTM E10 |
The combination of high tensile strength and elongation makes heat resistant steel suitable for applications that require both strength and ductility under mechanical loading, particularly in high-temperature environments.
Physical Properties
| Property | Condition/Temperature | Value (Metric) | Value (Imperial) |
|---|---|---|---|
| Density | Room Temperature | 7.9 g/cm³ | 0.284 lb/in³ |
| Melting Point | - | 1400 - 1450 °C | 2552 - 2642 °F |
| Thermal Conductivity | Room Temperature | 16 W/m·K | 92 BTU·in/(hr·ft²·°F) |
| Specific Heat Capacity | Room Temperature | 500 J/kg·K | 0.12 BTU/lb·°F |
| Electrical Resistivity | Room Temperature | 0.72 µΩ·m | 0.0000013 Ω·in |
The density and melting point are critical for applications involving high thermal loads, while thermal conductivity affects heat dissipation in components exposed to extreme temperatures.
Corrosion Resistance
| Corrosive Agent | Concentration (%) | Temperature (°C/°F) | Resistance Rating | Notes |
|---|---|---|---|---|
| Sulfuric Acid | 10% | 25°C/77°F | Fair | Risk of pitting |
| Chlorides | 3% | 60°C/140°F | Good | Susceptible to stress corrosion cracking |
| Atmospheric | - | - | Excellent | Forms protective oxide layer |
Heat resistant steel exhibits good resistance to various corrosive environments, particularly in high-temperature applications. However, it can be susceptible to pitting and stress corrosion cracking in chloride environments. Compared to other grades, such as AISI 316, heat resistant steel may offer superior high-temperature performance but might lag in certain acidic environments.
Heat Resistance
| Property/Limit | Temperature (°C) | Temperature (°F) | Remarks |
|---|---|---|---|
| Max Continuous Service Temp | 1150°C | 2100°F | Suitable for prolonged exposure |
| Max Intermittent Service Temp | 1200°C | 2192°F | Short-term exposure |
| Scaling Temperature | 1000°C | 1832°F | Begins to lose oxidation resistance |
| Creep Strength Considerations | 800°C | 1472°F | Critical for design |
Heat resistant steel performs well at elevated temperatures, maintaining mechanical integrity and oxidation resistance. However, scaling can occur at temperatures above 1000°C, necessitating careful consideration in design and application.
Fabrication Properties
Weldability
| Welding Process | Recommended Filler Metal (AWS Classification) | Typical Shielding Gas/Flux | Notes |
|---|---|---|---|
| TIG | ER310 | Argon | Good for thin sections |
| MIG | ER310 | Argon/CO2 | Suitable for thicker sections |
| Stick | E310 | - | Requires preheat |
Heat resistant steel can be welded using various methods, but preheating is often necessary to avoid cracking. Post-weld heat treatment may also be required to relieve stresses.
Machinability
| Machining Parameter | Heat Resistant Steel | AISI 1212 | Notes/Tips |
|---|---|---|---|
| Relative Machinability Index | 50 | 100 | Requires slower speeds |
| Typical Cutting Speed (Turning) | 20 m/min | 40 m/min | Use carbide tools |
Machinability is lower compared to standard steels, necessitating specific tooling and cutting speeds to achieve optimal results.
Formability
Heat resistant steel can be formed through both cold and hot processes. Cold forming may lead to work hardening, while hot forming allows for more complex shapes without significant risk of cracking.
Heat Treatment
| Treatment Process | Temperature Range (°C/°F) | Typical Soaking Time | Cooling Method | Primary Purpose / Expected Result |
|---|---|---|---|---|
| Annealing | 1000 - 1150°C / 1832 - 2102°F | 1 - 2 hours | Air cool | Reduce hardness, improve ductility |
| Quenching | 900 - 1000°C / 1652 - 1832°F | 30 minutes | Water/oil | Increase hardness |
| Tempering | 600 - 700°C / 1112 - 1292°F | 1 hour | Air cool | Reduce brittleness |
Heat treatment processes significantly affect the microstructure and properties of heat resistant steel, enhancing its performance in high-temperature applications.
Typical Applications and End Uses
| Industry/Sector | Specific Application Example | Key Steel Properties Utilized in this Application | Reason for Selection (Brief) |
|---|---|---|---|
| Power Generation | Boiler tubes | High-temperature strength, oxidation resistance | Essential for durability under extreme conditions |
| Aerospace | Engine components | Creep resistance, toughness | Critical for safety and performance |
| Petrochemical | Reactor vessels | Corrosion resistance, high-temperature strength | Necessary for reliability in harsh environments |
Other applications include:
-
- Heat exchangers
-
- Industrial furnaces
-
- Gas turbines
Heat resistant steel is chosen for these applications due to its ability to withstand extreme temperatures and corrosive environments, ensuring longevity and reliability.
Important Considerations, Selection Criteria, and Further Insights
| Feature/Property | Heat Resistant Steel | AISI 316 | AISI 304 | Brief Pro/Con or Trade-off Note |
|---|---|---|---|---|
| Key Mechanical Property | High-temperature strength | Good corrosion resistance | Good formability | Heat resistant steel excels in high-temp applications |
| Key Corrosion Aspect | Moderate in acids | Excellent in chlorides | Good in atmospheric | 316 offers better corrosion resistance in certain environments |
| Weldability | Requires preheat | Good | Good | Heat resistant steel may need special techniques |
| Machinability | Moderate | Good | Good | Heat resistant steel requires slower speeds |
| Approx. Relative Cost | Higher | Moderate | Lower | Cost reflects performance capabilities |
| Typical Availability | Limited | Widely available | Widely available | Availability can affect project timelines |
When selecting heat resistant steel, considerations include cost-effectiveness, availability, and specific performance requirements. Its unique properties make it suitable for niche applications where standard steels may fail, providing a critical advantage in demanding environments.