8620 Steel: Properties and Key Applications Overview

8620 steel is a medium-carbon alloy steel that is widely used in various engineering applications due to its excellent mechanical properties and versatility. Classified as a low-alloy steel, it primarily contains chromium and molybdenum as its alloying elements, which significantly enhance its strength, toughness, and hardenability. The typical chemical composition of 8620 steel includes approximately 0.18-0.23% carbon, 0.70-0.90% manganese, 0.15-0.25% chromium, and 0.10-0.20% molybdenum.

Comprehensive Overview

8620 steel is known for its good balance of strength, ductility, and toughness, making it suitable for applications that require high wear resistance and the ability to withstand impact loads. The alloying elements, particularly chromium and molybdenum, contribute to its hardenability, allowing it to achieve high hardness levels through heat treatment processes.

Advantages:
- High Strength and Toughness: 8620 steel exhibits excellent tensile strength and impact resistance, making it ideal for heavy-duty applications.
- Good Machinability: It can be easily machined in its annealed state, which is beneficial for manufacturing complex parts.
- Versatile Heat Treatment: The steel can be heat-treated to achieve desired hardness and strength levels, enhancing its performance in various applications.

Limitations:
- Corrosion Resistance: Compared to stainless steels, 8620 has lower corrosion resistance, which may limit its use in highly corrosive environments.
- Weldability Issues: While it can be welded, preheating and post-weld heat treatment are often necessary to avoid cracking.

Historically, 8620 steel has been used in the automotive and aerospace industries for components such as gears, shafts, and crankshafts, where high strength and durability are critical. Its market position remains strong due to its balance of performance and cost-effectiveness.

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	A829	USA	Standard specification for alloy steel
EN	1.6523	Europe	Similar properties, minor compositional differences
JIS	SCr420	Japan	Equivalent with slight variations in alloying elements

The differences between these grades can affect their performance in specific applications. For example, while 1.6523 may offer slightly better hardenability, G86200 is often preferred for its availability and cost.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.18 - 0.23
Mn (Manganese)	0.70 - 0.90
Cr (Chromium)	0.15 - 0.25
Mo (Molybdenum)	0.10 - 0.20
Si (Silicon)	0.15 - 0.40
P (Phosphorus)	≤ 0.035
S (Sulfur)	≤ 0.040

The primary alloying elements in 8620 steel play crucial roles:
- Carbon (C): Enhances hardness and strength through heat treatment.
- Chromium (Cr): Improves hardenability and corrosion resistance.
- Molybdenum (Mo): Increases strength at elevated temperatures and enhances toughness.

Mechanical Properties

Property	Condition/Temper	Typical Value/Range (Metric)	Typical Value/Range (Imperial)	Reference Standard for Test Method
Tensile Strength	Annealed	620 - 850 MPa	90 - 123 ksi	ASTM E8
Yield Strength (0.2% offset)	Annealed	350 - 550 MPa	51 - 80 ksi	ASTM E8
Elongation	Annealed	20 - 30%	20 - 30%	ASTM E8
Hardness (Rockwell C)	Quenched & Tempered	28 - 34 HRC	28 - 34 HRC	ASTM E18
Impact Strength (Charpy)	-40°C	27 J	20 ft-lbf	ASTM E23

The combination of these mechanical properties makes 8620 steel suitable for applications requiring high strength and toughness, such as in gears and shafts that experience dynamic loading.

Physical Properties

Property	Condition/Temperature	Value (Metric)	Value (Imperial)
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	31.2 BTU·in/h·ft²·°F
Specific Heat Capacity	Room Temperature	0.46 kJ/kg·K	0.11 BTU/lb·°F
Coefficient of Thermal Expansion	Room Temperature	11.5 x 10⁻⁶/K	6.4 x 10⁻⁶/°F

Key physical properties such as density and thermal conductivity are significant for applications involving heat treatment and thermal processing. The relatively high melting point allows for effective processing at elevated temperatures.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C)	Resistance Rating	Notes
Chlorides	Varies	Ambient	Fair	Risk of pitting
Sulfuric Acid	Low	Ambient	Poor	Not recommended
Sodium Hydroxide	Low	Ambient	Fair	Susceptible to stress corrosion cracking

8620 steel exhibits moderate resistance to corrosion, particularly in atmospheric conditions. However, it is susceptible to pitting in chloride environments and should be avoided in acidic or highly alkaline conditions. Compared to stainless steels like 304 or 316, 8620's corrosion resistance is significantly lower, making it less suitable for marine or chemical processing applications.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	400 °C	752 °F	Above this, properties degrade
Max Intermittent Service Temp	500 °C	932 °F	Short-term exposure only
Scaling Temperature	600 °C	1112 °F	Risk of oxidation at high temps

At elevated temperatures, 8620 steel maintains its strength but may experience oxidation and scaling. It is crucial to consider these factors when designing components for high-temperature applications.

Fabrication Properties

Weldability

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

8620 steel can be welded using common processes like MIG and TIG. However, preheating is often necessary to prevent cracking, especially in thicker sections. Post-weld heat treatment can also help relieve stresses and improve toughness.

Machinability

Machining Parameter	8620 Steel	AISI 1212	Notes/Tips
Relative Machinability Index	60	100	Good machinability in annealed state
Typical Cutting Speed	30 m/min	50 m/min	Adjust for tool wear

8620 steel offers good machinability, especially when in the annealed condition. It is important to use appropriate cutting tools and speeds to optimize performance and tool life.

Formability

8620 steel can be cold and hot formed, but care must be taken to avoid work hardening. The minimum bend radius should be considered during forming operations to prevent cracking.

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	Softening, improving ductility
Quenching	820 - 860 °C / 1508 - 1580 °F	30 minutes	Oil or water	Hardening
Tempering	400 - 600 °C / 752 - 1112 °F	1 hour	Air	Reducing brittleness, improving toughness

Heat treatment processes significantly alter the microstructure of 8620 steel, enhancing its hardness and strength. The transformation from austenite to martensite during quenching is critical for achieving desired mechanical properties.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection
Automotive	Gears	High strength, toughness	Durability under load
Aerospace	Shafts	Good machinability, heat resistance	Precision components
Oil & Gas	Drill bits	Wear resistance, impact strength	Performance in harsh environments

Other applications include:
* - Hydraulic cylinders
* - Crankshafts
* - Fasteners

8620 steel is chosen for these applications due to its excellent combination of strength, toughness, and machinability, making it suitable for components that experience dynamic loads and require high durability.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	8620 Steel	AISI 4140	AISI 4340	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High strength, good toughness	Higher strength	Higher toughness	8620 is more cost-effective
Key Corrosion Aspect	Fair	Poor	Fair	8620 is better for moderate environments
Weldability	Moderate	Good	Moderate	8620 requires pre/post-heat treatment
Machinability	Good	Moderate	Poor	8620 is easier to machine than 4340
Formability	Good	Fair	Poor	8620 can be formed more easily
Approx. Relative Cost	Moderate	Higher	Higher	8620 is often more economical
Typical Availability	High	Moderate	Moderate	8620 is widely available

When selecting 8620 steel, considerations include its cost-effectiveness, availability, and suitability for specific applications. While it may not have the corrosion resistance of stainless steels, its mechanical properties make it a reliable choice for many engineering applications. Additionally, its performance in various heat treatment processes allows for customization to meet specific requirements.

In summary, 8620 steel is a versatile and widely used alloy that offers a balance of strength, toughness, and machinability, making it suitable for a variety of demanding applications across multiple industries.