439 Stainless Steel: Properties and Key Applications
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Table Of Content
Table Of Content
Comprehensive Overview
439 stainless steel is classified as a ferritic stainless steel, primarily characterized by its chromium content, which typically ranges from 16% to 18%. This steel grade is known for its excellent oxidation resistance and moderate corrosion resistance, making it suitable for various applications in mildly corrosive environments. The primary alloying elements in 439 stainless steel include chromium (Cr), which enhances corrosion resistance and provides strength, and nickel (Ni), which improves ductility and toughness. However, 439 has a lower nickel content compared to austenitic grades, which affects its overall toughness and formability.
The most significant characteristics of 439 stainless steel include its good weldability, moderate strength, and resistance to scaling at elevated temperatures. It also exhibits good ductility and is less prone to stress corrosion cracking than some austenitic grades.
Advantages and Limitations
Pros:
- Corrosion Resistance: Offers good resistance to oxidation and corrosion in various environments.
- Weldability: Suitable for welding processes without requiring extensive pre- or post-weld treatments.
- Cost-Effectiveness: Lower nickel content makes it more economical compared to austenitic stainless steels.
Cons:
- Limited Corrosion Resistance: Not suitable for highly corrosive environments, especially those containing chlorides.
- Lower Toughness: Compared to austenitic grades, it has reduced toughness at low temperatures.
Historically, 439 stainless steel has been used in automotive exhaust systems, heat exchangers, and various architectural applications due to its balance of properties and cost-effectiveness. Its market position is solid, particularly in industries where moderate corrosion resistance and good mechanical properties are required.
Alternative Names, Standards, and Equivalents
| Standard Organization | Designation/Grade | Country/Region of Origin | Notes/Remarks |
|---|---|---|---|
| UNS | S43900 | USA | Closest equivalent to EN 1.4510 |
| AISI/SAE | 439 | USA | Minor compositional differences to 444 |
| ASTM | A240 | USA | Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip |
| EN | 1.4510 | Europe | Equivalent to AISI 439, used in European standards |
| JIS | SUS439 | Japan | Similar properties to AISI 439, used in Japanese applications |
The differences between equivalent grades, such as 439 and 444, primarily lie in their nickel content and corrosion resistance. Grade 444 typically offers better resistance to pitting and crevice corrosion due to its higher molybdenum content, making it more suitable for harsher environments.
Key Properties
Chemical Composition
| Element (Symbol and Name) | Percentage Range (%) |
|---|---|
| Cr (Chromium) | 16.0 - 18.0 |
| Ni (Nickel) | 0.5 - 1.0 |
| Mo (Molybdenum) | 0.0 - 0.5 |
| C (Carbon) | 0.03 max |
| Si (Silicon) | 1.0 max |
| Mn (Manganese) | 1.0 max |
| P (Phosphorus) | 0.045 max |
| S (Sulfur) | 0.03 max |
The primary role of chromium in 439 stainless steel is to enhance its corrosion resistance and provide oxidation resistance at elevated temperatures. Nickel contributes to the steel's ductility and toughness, while molybdenum, although present in smaller amounts, can improve pitting resistance. Carbon and silicon play roles in the steel's strength and hardness.
Mechanical Properties
| Property | Condition/Temper | Test Temperature | Typical Value/Range (Metric - SI Units) | Typical Value/Range (Imperial Units) | Reference Standard for Test Method |
|---|---|---|---|---|---|
| Tensile Strength | Annealed | Room Temp | 450 - 550 MPa | 65 - 80 ksi | ASTM E8 |
| Yield Strength (0.2% offset) | Annealed | Room Temp | 200 - 300 MPa | 29 - 44 ksi | ASTM E8 |
| Elongation | Annealed | Room Temp | 20 - 30% | 20 - 30% | ASTM E8 |
| Hardness (Rockwell B) | Annealed | Room Temp | 70 - 90 HRB | 70 - 90 HRB | ASTM E18 |
| Impact Strength | Charpy V-notch | -20°C | 30 J | 22 ft-lbf | ASTM E23 |
The combination of these mechanical properties makes 439 stainless steel suitable for applications requiring moderate strength and ductility. Its yield strength and tensile strength allow it to withstand various mechanical loads, while its elongation indicates good formability.
Physical Properties
| Property | Condition/Temperature | Value (Metric - SI Units) | Value (Imperial Units) |
|---|---|---|---|
| Density | Room Temp | 7.7 g/cm³ | 0.278 lb/in³ |
| Melting Point | - | 1400 - 1450 °C | 2552 - 2642 °F |
| Thermal Conductivity | Room Temp | 25 W/m·K | 17.3 BTU·in/(hr·ft²·°F) |
| Specific Heat Capacity | Room Temp | 500 J/kg·K | 0.119 BTU/lb·°F |
| Electrical Resistivity | Room Temp | 0.73 µΩ·m | 0.00000073 Ω·m |
| Coefficient of Thermal Expansion | Room Temp | 11.5 x 10⁻⁶ /K | 6.36 x 10⁻⁶ /°F |
The density of 439 stainless steel makes it suitable for structural applications where weight is a consideration. Its thermal conductivity is adequate for applications involving heat transfer, while the specific heat capacity indicates its ability to absorb heat without significant temperature changes.
Corrosion Resistance
| Corrosive Agent | Concentration (%) | Temperature (°C/°F) | Resistance Rating | Notes |
|---|---|---|---|---|
| Chlorides | 0 - 3 | 20 - 60 / 68 - 140 | Fair | Risk of pitting |
| Acetic Acid | 0 - 10 | 20 - 60 / 68 - 140 | Good | Limited resistance |
| Sulfuric Acid | 0 - 5 | 20 - 60 / 68 - 140 | Poor | Not recommended |
| Atmospheric | - | - | Excellent | Good resistance |
439 stainless steel exhibits good resistance to atmospheric corrosion and mild organic acids. However, it is susceptible to pitting and crevice corrosion in chloride environments, particularly at elevated temperatures. Compared to austenitic grades like 304 and 316, 439 offers lower resistance to chlorides but is more cost-effective for applications where high corrosion resistance is not critical.
Heat Resistance
| Property/Limit | Temperature (°C) | Temperature (°F) | Remarks |
|---|---|---|---|
| Max Continuous Service Temp | 850 °C | 1562 °F | Suitable for high-temperature applications |
| Max Intermittent Service Temp | 900 °C | 1652 °F | Can withstand short-term exposure |
| Scaling Temperature | 1000 °C | 1832 °F | Risk of oxidation beyond this limit |
At elevated temperatures, 439 stainless steel maintains its strength and oxidation resistance, making it suitable for applications such as exhaust systems. However, prolonged exposure to temperatures above 850 °C can lead to scaling and degradation of mechanical properties.
Fabrication Properties
Weldability
| Welding Process | Recommended Filler Metal (AWS Classification) | Typical Shielding Gas/Flux | Notes |
|---|---|---|---|
| TIG | ER439 | Argon | Good results with minimal distortion |
| MIG | ER439 | Argon/CO2 | Requires careful control of heat input |
439 stainless steel is well-suited for welding, particularly with TIG and MIG processes. Preheating is generally not required, but post-weld heat treatment may be beneficial to relieve stresses and improve toughness. Potential defects include porosity and lack of fusion, which can be minimized with proper technique.
Machinability
| Machining Parameter | [439 Stainless Steel] | Benchmark Steel (AISI 1212) | Notes/Tips |
|---|---|---|---|
| Relative Machinability Index | 40% | 100% | Requires slower speeds and higher feed rates |
| Typical Cutting Speed (Turning) | 30 m/min | 60 m/min | Use carbide tools for best results |
Machinability of 439 stainless steel is moderate; it requires slower cutting speeds compared to more machinable steels like AISI 1212. Carbide tooling is recommended to achieve better surface finishes and tool life.
Formability
439 stainless steel can be cold and hot formed, but it exhibits work hardening, which may require additional force during forming operations. The minimum bend radius should be considered to avoid cracking.
Heat Treatment
| Treatment Process | Temperature Range (°C/°F) | Typical Soaking Time | Cooling Method | Primary Purpose / Expected Result |
|---|---|---|---|---|
| Annealing | 800 - 900 °C / 1472 - 1652 °F | 1 - 2 hours | Air | Improve ductility and reduce hardness |
Heat treatment processes like annealing can significantly enhance the ductility and toughness of 439 stainless steel. The microstructure transforms during these treatments, leading to improved mechanical properties.
Typical Applications and End Uses
| Industry/Sector | Specific Application Example | Key Steel Properties Utilized in this Application | Reason for Selection (Brief) |
|---|---|---|---|
| Automotive | Exhaust systems | Corrosion resistance, heat resistance | Cost-effective and durable |
| Architecture | Architectural cladding | Aesthetic appeal, moderate corrosion resistance | Good appearance and durability |
| Food Processing | Equipment components | Corrosion resistance, ease of cleaning | Meets hygiene standards |
Other applications include:
* - Kitchen equipment
* - Heat exchangers
* - Industrial equipment
In automotive exhaust systems, 439 stainless steel is chosen for its ability to withstand high temperatures and resist oxidation, providing a balance between performance and cost.
Important Considerations, Selection Criteria, and Further Insights
| Feature/Property | 439 Stainless Steel | AISI 304 Stainless Steel | AISI 316 Stainless Steel | Brief Pro/Con or Trade-off Note |
|---|---|---|---|---|
| Key Mechanical Property | Moderate Strength | High Strength | High Strength | 439 is more cost-effective |
| Key Corrosion Aspect | Fair in Chlorides | Good in Chlorides | Excellent in Chlorides | 439 is less resistant to pitting |
| Weldability | Good | Excellent | Excellent | 439 requires less pre-treatment |
| Machinability | Moderate | Good | Moderate | 439 is less machinable than 304 |
| Formability | Moderate | Good | Moderate | 439 work hardens more quickly |
| Approx. Relative Cost | Lower | Higher | Higher | Cost-effective for moderate use |
| Typical Availability | Common | Very Common | Common | 439 is widely available |
When selecting 439 stainless steel, considerations include its cost-effectiveness and availability, particularly in applications where high corrosion resistance is not critical. Its moderate strength and good weldability make it suitable for various engineering applications, while its susceptibility to pitting in chloride environments should be carefully evaluated.