8620H Steel: Properties and Key Applications

8620H steel is a medium-carbon alloy steel that is primarily classified as a low-alloy steel. It is known for its excellent hardenability, strength, and toughness, making it suitable for a variety of engineering applications. The primary alloying elements in 8620H steel include chromium (Cr), molybdenum (Mo), and nickel (Ni), which enhance its mechanical properties and resistance to wear and fatigue.

Comprehensive Overview

8620H steel is characterized by its balanced composition, which typically includes around 0.18-0.23% carbon, 0.70-0.90% manganese, 0.40-0.60% chromium, 0.15-0.25% molybdenum, and 1.00-1.50% nickel. This combination of elements contributes to its high tensile strength, good ductility, and excellent toughness, making it a preferred choice for applications requiring high strength and durability.

Advantages:
- High Strength and Toughness: 8620H exhibits excellent mechanical properties, making it suitable for heavy-duty applications.
- Good Hardening Capability: The alloying elements provide good hardenability, allowing for effective heat treatment processes.
- Versatility: It can be used in various applications, including gears, shafts, and other components requiring high strength.

Limitations:
- Weldability Concerns: While it can be welded, special precautions must be taken to avoid cracking.
- Cost: The alloying elements can make it more expensive than lower-grade steels.
- Corrosion Resistance: It is not as corrosion-resistant as stainless steels, which may limit its use in certain environments.

Historically, 8620H has been utilized in the automotive and aerospace industries, where its properties are critical for performance and safety. Its market position is strong, particularly in sectors that demand high-performance materials.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	G86200	USA	Closest equivalent to AISI 8620
AISI/SAE	8620	USA	Commonly used designation
ASTM	A29/A29M	USA	General specification for alloy steels
EN	1.6523	Europe	Minor compositional differences
DIN	20CrMo	Germany	Similar properties, but different alloying elements
JIS	SCM420	Japan	Comparable, but with different mechanical properties

The differences between these equivalent grades can affect performance, particularly in terms of hardenability and toughness. For instance, while both AISI 8620 and UNS G86200 are similar, the latter may have stricter compositional limits that can influence its mechanical properties.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.18 - 0.23
Mn (Manganese)	0.70 - 0.90
Cr (Chromium)	0.40 - 0.60
Mo (Molybdenum)	0.15 - 0.25
Ni (Nickel)	1.00 - 1.50
Si (Silicon)	0.15 - 0.40
P (Phosphorus)	≤ 0.035
S (Sulfur)	≤ 0.040

The primary role of the key alloying elements in 8620H steel includes:
- Chromium: Enhances hardenability and corrosion resistance.
- Molybdenum: Improves strength at elevated temperatures and enhances toughness.
- Nickel: Increases toughness and improves the steel's ability to withstand impact.

Mechanical Properties

Property	Condition/Temper	Typical Value/Range (Metric - SI Units)	Typical Value/Range (Imperial Units)	Reference Standard for Test Method
Tensile Strength	Quenched & Tempered	850 - 1000 MPa	123 - 145 ksi	ASTM E8
Yield Strength (0.2% offset)	Quenched & Tempered	650 - 850 MPa	94 - 123 ksi	ASTM E8
Elongation	Quenched & Tempered	15 - 20%	15 - 20%	ASTM E8
Reduction of Area	Quenched & Tempered	50 - 60%	50 - 60%	ASTM E8
Hardness (Rockwell C)	Quenched & Tempered	28 - 34 HRC	28 - 34 HRC	ASTM E18
Impact Strength (Charpy)	Room Temperature	40 - 60 J	30 - 44 ft-lbf	ASTM E23

The combination of these mechanical properties makes 8620H steel particularly suitable for applications involving dynamic loading and structural integrity, such as in the manufacturing of gears and shafts. Its high tensile and yield strength, along with good ductility, allow it to perform well under stress.

Physical Properties

Property	Condition/Temperature	Value (Metric - SI Units)	Value (Imperial Units)
Density	Room Temperature	7.85 g/cm³	0.284 lb/in³
Melting Point	-	1425 - 1540 °C	2600 - 2800 °F
Thermal Conductivity	Room Temperature	45 W/m·K	31 BTU·in/(hr·ft²·°F)
Specific Heat Capacity	Room Temperature	0.46 kJ/kg·K	0.11 BTU/lb·°F
Electrical Resistivity	Room Temperature	0.00065 Ω·m	0.00038 Ω·in
Coefficient of Thermal Expansion	Room Temperature	11.5 x 10⁻⁶ /K	6.4 x 10⁻⁶ /°F

The practical significance of 8620H's physical properties includes:
- Density: Provides insight into weight considerations for structural applications.
- Thermal Conductivity: Important for applications where heat dissipation is critical.
- Melting Point: Indicates suitability for high-temperature applications.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Chlorides	Varies	Ambient	Fair	Risk of pitting
Sulfuric Acid	10%	25 °C / 77 °F	Poor	Not recommended
Sodium Hydroxide	5%	25 °C / 77 °F	Fair	Susceptible to SCC
Atmospheric	-	Ambient	Good	Moderate resistance

8620H steel exhibits moderate corrosion resistance, particularly in atmospheric conditions. However, it is susceptible to pitting in chloride environments and should not be used in acidic or highly alkaline conditions. Compared to stainless steels like 304 or 316, which offer excellent corrosion resistance, 8620H is less suitable for applications exposed to harsh environments.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	400 °C	752 °F	Suitable for prolonged exposure
Max Intermittent Service Temp	500 °C	932 °F	Short-term exposure
Scaling Temperature	600 °C	1112 °F	Risk of oxidation beyond this temp
Creep Strength considerations begin	300 °C	572 °F	Important for high-temperature applications

At elevated temperatures, 8620H steel maintains its strength and toughness, but oxidation can become a concern. It is suitable for applications where temperatures do not exceed its maximum service limits, making it ideal for components in engines and machinery.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
MIG	ER70S-6	Argon + CO2	Preheat recommended
TIG	ER80S-Ni	Argon	Post-weld heat treatment
Stick	E7018	-	Requires preheating

8620H steel can be welded using various methods, but preheating is often necessary to minimize the risk of cracking. Post-weld heat treatment is also recommended to relieve stresses and improve toughness.

Machinability

Machining Parameter	[8620H Steel]	AISI 1212	Notes/Tips
Relative Machinability Index	60%	100%	Moderate machinability
Typical Cutting Speed (Turning)	30-50 m/min	80-100 m/min	Use carbide tools for best results

Machinability of 8620H is moderate; it requires careful selection of tooling and cutting speeds. Carbide tools are recommended for optimal performance.

Formability

8620H steel exhibits good formability, allowing for both cold and hot forming processes. However, it is essential to consider work hardening effects during cold working, which can increase the material's strength but may also lead to cracking if not managed properly.

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	800 - 850 °C / 1472 - 1562 °F	30 minutes	Oil or Water	Hardening, increased strength
Tempering	400 - 600 °C / 752 - 1112 °F	1 hour	Air	Toughness improvement

During heat treatment, 8620H undergoes significant metallurgical transformations. Quenching increases hardness, while tempering reduces brittleness, resulting in a balanced combination of strength 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	Critical for performance
Aerospace	Shafts	Good hardenability, fatigue resistance	Safety and reliability
Machinery	Crankshafts	Excellent toughness, wear resistance	Durability under stress

Other applications include:
- Oil and gas industry components
- Heavy machinery parts
- Structural components in construction

8620H steel is chosen for these applications due to its ability to withstand high stress and fatigue, making it ideal for critical components.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	8620H Steel	AISI 4140	AISI 4340	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High strength	Moderate strength	High strength	8620H offers good toughness vs. 4140's higher hardness
Key Corrosion Aspect	Fair	Poor	Fair	8620H better for moderate environments
Weldability	Moderate	Good	Fair	8620H requires preheating
Machinability	Moderate	Good	Fair	8620H is less machinable than 4140
Formability	Good	Fair	Poor	8620H is easier to form
Approx. Relative Cost	Moderate	Moderate	Higher	8620H is cost-effective for its properties
Typical Availability	Good	Good	Moderate	8620H is widely available

When selecting 8620H steel, considerations include its cost-effectiveness, availability, and suitability for specific applications. Its moderate corrosion resistance and weldability make it a versatile choice, while its mechanical properties ensure reliability in demanding environments.

In conclusion, 8620H steel is a robust material that balances strength, toughness, and versatility, making it a preferred choice in various engineering applications. Its unique properties and performance characteristics should be carefully evaluated against project requirements to ensure optimal material selection.