8640 Steel: Properties and Key Applications
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Table Of Content
Table Of Content
8640 steel is a medium-carbon alloy steel that is primarily classified as a low-alloy steel. It is known for its excellent toughness, strength, and wear resistance, making it suitable for a variety of demanding applications. The primary alloying elements in 8640 steel include nickel, chromium, and molybdenum, which enhance its mechanical properties and overall performance.
Comprehensive Overview
8640 steel is characterized by its balanced composition, which typically includes approximately 0.40% carbon, 0.70% manganese, 0.50% chromium, 0.25% molybdenum, and 1.50% nickel. This combination of elements contributes to its high tensile strength and good ductility, allowing it to withstand significant stress and deformation without failure. The presence of nickel and chromium improves the steel's hardenability, while molybdenum enhances its resistance to wear and fatigue.
Advantages of 8640 Steel:
- High Strength and Toughness: 8640 steel exhibits excellent mechanical properties, making it ideal for applications requiring high strength and impact resistance.
- Good Hardening Capability: The alloying elements allow for effective heat treatment, resulting in improved hardness and wear resistance.
- Versatility: It can be used in various applications, including automotive, aerospace, and heavy machinery.
Limitations of 8640 Steel:
- Weldability Issues: Due to its alloying elements, 8640 can be challenging to weld without proper preheating and post-weld heat treatment.
- Cost: The alloying elements can make 8640 steel more expensive compared to lower-grade steels.
Historically, 8640 steel has been used in critical applications such as gears, shafts, and other components that require high strength and durability. Its market position is strong, particularly in industries that demand reliable performance under extreme conditions.
Alternative Names, Standards, and Equivalents
| Standard Organization | Designation/Grade | Country/Region of Origin | Notes/Remarks |
|---|---|---|---|
| UNS | G86400 | USA | Closest equivalent to AISI 8640 |
| AISI/SAE | 8640 | USA | Commonly used designation |
| ASTM | A829 | USA | Standard specification for alloy steel |
| EN | 1.6511 | Europe | Equivalent grade in European standards |
| JIS | SNCM439 | Japan | Similar properties but with minor compositional differences |
The table above highlights various designations for 8640 steel across different standards. Notably, while SNCM439 is often considered an equivalent, it may have slight variations in composition that could affect performance in specific applications, particularly in terms of hardenability and toughness.
Key Properties
Chemical Composition
| Element (Symbol and Name) | Percentage Range (%) |
|---|---|
| C (Carbon) | 0.38 - 0.43 |
| Mn (Manganese) | 0.60 - 0.90 |
| Cr (Chromium) | 0.40 - 0.60 |
| Mo (Molybdenum) | 0.15 - 0.25 |
| Ni (Nickel) | 1.30 - 1.70 |
The key alloying elements in 8640 steel play significant roles:
- Nickel (Ni): Enhances toughness and impact strength, particularly at low temperatures.
- Chromium (Cr): Improves hardenability and resistance to wear and corrosion.
- Molybdenum (Mo): Increases strength at elevated temperatures and enhances resistance to softening.
Mechanical Properties
| Property | Condition/Temper | Test Temperature | Typical Value/Range (Metric) | Typical Value/Range (Imperial) | Reference Standard for Test Method |
|---|---|---|---|---|---|
| Tensile Strength | Quenched & Tempered | Room Temp | 850 - 1000 MPa | 123 - 145 ksi | ASTM E8 |
| Yield Strength (0.2% offset) | Quenched & Tempered | Room Temp | 650 - 850 MPa | 94 - 123 ksi | ASTM E8 |
| Elongation | Quenched & Tempered | Room Temp | 15 - 20% | 15 - 20% | ASTM E8 |
| Hardness (Rockwell C) | Quenched & Tempered | Room Temp | 28 - 34 HRC | 28 - 34 HRC | ASTM E18 |
| Impact Strength | Quenched & Tempered | -20°C (-4°F) | 30 - 50 J | 22 - 37 ft-lbf | ASTM E23 |
The mechanical properties of 8640 steel make it suitable for applications that require high strength and toughness, such as in automotive and aerospace components. Its ability to maintain strength under stress and resist deformation is critical for structural integrity in demanding environments.
Physical Properties
| Property | Condition/Temperature | Value (Metric) | Value (Imperial) |
|---|---|---|---|
| Density | - | 7.85 g/cm³ | 0.284 lb/in³ |
| Melting Point | - | 1425 - 1540 °C | 2600 - 2800 °F |
| Thermal Conductivity | 20°C | 45 W/m·K | 31 BTU·in/h·ft²·°F |
| Specific Heat Capacity | 20°C | 0.49 kJ/kg·K | 0.12 BTU/lb·°F |
The density and melting point of 8640 steel indicate its robustness, while its thermal conductivity and specific heat capacity are essential for applications involving thermal management.
Corrosion Resistance
| Corrosive Agent | Concentration (%) | Temperature (°C/°F) | Resistance Rating | Notes |
|---|---|---|---|---|
| Saltwater | 3.5% | 25°C (77°F) | Fair | Risk of pitting |
| Sulfuric Acid | 10% | 20°C (68°F) | Poor | Not recommended |
| Chlorides | 1% | 30°C (86°F) | Fair | Susceptible to stress corrosion cracking |
8640 steel exhibits moderate corrosion resistance, particularly in saline environments where pitting can occur. It is not recommended for use in acidic environments, as it can suffer from significant degradation. Compared to grades like 4140 and 4340, which have better corrosion resistance due to higher chromium content, 8640 may require protective coatings or treatments in corrosive settings.
Heat Resistance
| Property/Limit | Temperature (°C) | Temperature (°F) | Remarks |
|---|---|---|---|
| Max Continuous Service Temp | 400°C | 752°F | Suitable for moderate heat |
| Max Intermittent Service Temp | 500°C | 932°F | Short-term exposure only |
| Scaling Temperature | 600°C | 1112°F | Risk of oxidation above this temp |
At elevated temperatures, 8640 steel maintains its strength but may begin to oxidize if not properly protected. Its performance in high-temperature applications is adequate, but care must be taken to avoid prolonged exposure to extreme conditions.
Fabrication Properties
Weldability
| Welding Process | Recommended Filler Metal (AWS Classification) | Typical Shielding Gas/Flux | Notes |
|---|---|---|---|
| MIG | ER80S-Ni1 | Argon + CO2 | Preheat recommended |
| TIG | ER80S-Ni1 | Argon | Post-weld heat treatment advised |
Welding 8640 steel requires careful consideration of preheating and post-weld heat treatment to avoid cracking and ensure the integrity of the weld. The recommended filler metals enhance the properties of the weld and maintain compatibility with the base material.
Machinability
| Machining Parameter | 8640 Steel | AISI 1212 | Notes/Tips |
|---|---|---|---|
| Relative Machinability Index | 60 | 100 | Moderate machinability |
| Typical Cutting Speed (Turning) | 30 m/min | 50 m/min | Use carbide tools for best results |
Machining 8640 steel can be challenging due to its hardness, requiring appropriate tooling and cutting speeds to achieve optimal results. It is advisable to use carbide tools and maintain proper coolant flow to enhance tool life.
Formability
8640 steel exhibits moderate formability, suitable for cold and hot working processes. However, it is subject to work hardening, which can limit its ability to be formed without proper techniques. Bend radii should be carefully calculated to avoid cracking during forming operations.
Heat Treatment
| Treatment Process | Temperature Range (°C/°F) | Typical Soaking Time | Cooling Method | Primary Purpose / Expected Result |
|---|---|---|---|---|
| Annealing | 700 - 800 °C / 1292 - 1472 °F | 1 - 2 hours | Air or furnace | Softening, improved ductility |
| Quenching | 850 - 900 °C / 1562 - 1652 °F | 30 minutes | Oil or water | Hardening, increased strength |
| Tempering | 400 - 600 °C / 752 - 1112 °F | 1 hour | Air | Reducing brittleness, improving toughness |
The heat treatment processes significantly affect the microstructure of 8640 steel, transforming it from a softer, more ductile state to a harder, more brittle state through quenching, followed by tempering to achieve a balance of hardness and toughness.
Typical Applications and End Uses
| Industry/Sector | Specific Application Example | Key Steel Properties Utilized in this Application | Reason for Selection (Brief) |
|---|---|---|---|
| Automotive | Gears | High strength, toughness | Required for load-bearing components |
| Aerospace | Aircraft components | High strength-to-weight ratio | Critical for performance and safety |
| Heavy Machinery | Shafts | Wear resistance, fatigue strength | Essential for durability under stress |
Other applications include:
- Oil and gas drilling equipment
- Military vehicle components
- Tooling and dies
The selection of 8640 steel in these applications is primarily due to its high strength, toughness, and ability to withstand harsh operating conditions.
Important Considerations, Selection Criteria, and Further Insights
| Feature/Property | 8640 Steel | AISI 4140 | AISI 4340 | Brief Pro/Con or Trade-off Note |
|---|---|---|---|---|
| Key Mechanical Property | High strength | Higher toughness | Better fatigue resistance | 4140 is more ductile, 4340 offers superior toughness |
| Key Corrosion Aspect | Moderate | Fair | Good | 4140 and 4340 have better corrosion resistance |
| Weldability | Moderate | Good | Fair | 4140 is easier to weld, 4340 requires more care |
| Machinability | Moderate | Good | Fair | 4140 is easier to machine |
| Approx. Relative Cost | Moderate | Moderate | Higher | 4340 tends to be more expensive |
| Typical Availability | Common | Common | Less common | 4340 may have limited availability |
When selecting 8640 steel, considerations include its mechanical properties, cost-effectiveness, and availability. While it offers a good balance of strength and toughness, alternatives like 4140 and 4340 may be more suitable for specific applications, particularly where higher toughness or corrosion resistance is required. Additionally, the welding and machining characteristics should be evaluated based on the intended fabrication processes to ensure optimal performance in the final application.