8640 Steel: Properties and Key Applications

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.