4320 Steel: Properties and Key Applications

4320 steel is classified as a medium-carbon alloy steel, primarily known for its balance of strength, toughness, and wear resistance. This steel grade is characterized by its alloying elements, which typically include chromium (Cr), nickel (Ni), and molybdenum (Mo). These elements enhance the steel's hardenability and overall mechanical properties, making it suitable for various engineering applications.

Comprehensive Overview

4320 steel is often utilized in applications requiring high strength and toughness, such as in the manufacturing of gears, shafts, and other components subjected to dynamic loads. Its chemical composition typically includes around 0.20-0.25% carbon, 0.70-0.90% manganese, 0.15-0.25% molybdenum, and 0.40-0.60% nickel, with chromium content ranging from 0.70-0.90%. The presence of these alloying elements contributes to its excellent hardenability, allowing it to achieve high strength levels through heat treatment processes.

The primary advantages of 4320 steel include its good machinability, weldability, and the ability to be heat treated to achieve desired mechanical properties. However, it also has limitations, such as susceptibility to stress corrosion cracking in certain environments and a tendency to be less resistant to corrosion compared to stainless steels. Historically, 4320 steel has been significant in the automotive and aerospace industries, where its mechanical properties are critical for performance and safety.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	G43200	USA	Closest equivalent to AISI 4320
AISI/SAE	4320	USA	Commonly used designation
ASTM	A29/A29M	USA	General specification for alloy steels
EN	1.6523	Europe	Equivalent grade in European standards
DIN	34CrNiMo6	Germany	Minor compositional differences
JIS	SNCM420	Japan	Similar properties, but different heat treatment recommendations

The differences between these equivalent grades can affect selection based on specific application requirements. For instance, while G43200 and 1.6523 may have similar mechanical properties, their performance in specific environments or during particular manufacturing processes may vary.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.20 - 0.25
Mn (Manganese)	0.70 - 0.90
Cr (Chromium)	0.70 - 0.90
Ni (Nickel)	0.40 - 0.60
Mo (Molybdenum)	0.15 - 0.25
Si (Silicon)	0.15 - 0.40
P (Phosphorus)	≤ 0.035
S (Sulfur)	≤ 0.040

The key alloying elements in 4320 steel play significant roles:
- Nickel enhances toughness and impact strength, particularly at low temperatures.
- Chromium improves hardenability and corrosion resistance.
- Molybdenum contributes to strength at elevated temperatures and enhances the steel's overall toughness.

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	12 - 18%	12 - 18%	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 4320 steel make it suitable for applications involving dynamic loading and structural integrity requirements. Its high tensile and yield strengths, combined with good ductility, allow it to withstand significant stress without failure.

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	31.2 BTU·in/(hr·ft²·°F)
Specific Heat Capacity	20°C	460 J/kg·K	0.11 BTU/lb·°F
Electrical Resistivity	20°C	0.00065 Ω·m	0.00038 Ω·in

The density and melting point of 4320 steel indicate its robustness, while its thermal conductivity and specific heat capacity are essential for applications involving thermal management. The electrical resistivity is relevant for applications where electrical conductivity is a consideration.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C)	Resistance Rating	Notes
Chlorides	3-5%	20-60°C	Fair	Risk of pitting corrosion
Sulfuric Acid	10%	25°C	Poor	Not recommended
Sodium Hydroxide	50%	25°C	Good	Limited resistance

4320 steel exhibits moderate corrosion resistance, particularly in environments with chlorides and alkaline solutions. It is susceptible to pitting corrosion in chloride-rich environments and should be used with caution in acidic conditions. Compared to stainless steels like 304 or 316, 4320 steel's corrosion resistance is significantly lower, making it less suitable for applications in highly corrosive environments.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	400°C	752°F	Suitable for moderate temperatures
Max Intermittent Service Temp	500°C	932°F	Short-term exposure only
Scaling Temperature	600°C	1112°F	Risk of oxidation beyond this temp

At elevated temperatures, 4320 steel maintains its strength but may experience oxidation. Its performance in high-temperature applications is limited, and care should be taken to avoid prolonged exposure to temperatures above 400°C.

Fabrication Properties

Weldability

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

4320 steel is generally considered weldable, but preheating is recommended to minimize the risk of cracking. Post-weld heat treatment can help relieve residual stresses and improve toughness.

Machinability

Machining Parameter	4320 Steel	AISI 1212	Notes/Tips
Relative Machinability Index	70%	100%	4320 is less machinable than 1212
Typical Cutting Speed (Turning)	30-50 m/min	60-80 m/min	Use carbide tools for best results

Machinability of 4320 steel is moderate. Optimal cutting speeds and tooling can enhance performance, but challenges may arise due to its toughness.

Formability

4320 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)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	700 - 800	1 - 2 hours	Air	Softening, improved machinability
Quenching	850 - 900	30 minutes	Oil or Water	Hardening, increased strength
Tempering	400 - 600	1 hour	Air	Toughness improvement

Heat treatment processes significantly alter the microstructure of 4320 steel, enhancing its mechanical properties. Quenching increases hardness, while tempering reduces brittleness, resulting in a balanced material suitable for various applications.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection
Automotive	Gears	High tensile strength, toughness	Durability under load
Aerospace	Shafts	Lightweight, high strength	Performance and safety
Oil & Gas	Valve components	Corrosion resistance, toughness	Reliability in harsh environments

• Other applications include:
• Heavy machinery components
• Tooling and dies
• Structural components in construction

4320 steel is chosen for applications requiring a combination of strength, toughness, and wear resistance, making it ideal for components subjected to dynamic loads.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	4320 Steel	AISI 4140	AISI 4340	Brief Pro/Con or Trade-off Note
Key Mechanical Property	Moderate strength	High strength	Very high strength	4340 offers superior strength but is less ductile
Key Corrosion Aspect	Fair	Fair	Good	4340 has better corrosion resistance
Weldability	Good	Fair	Poor	4320 is easier to weld than 4340
Machinability	Moderate	Moderate	Poor	4140 is more machinable than 4340
Approx. Relative Cost	Moderate	Moderate	Higher	Cost varies with alloying elements
Typical Availability	Common	Common	Less common	4320 is widely available in various forms

When selecting 4320 steel, considerations include its mechanical properties, weldability, and cost-effectiveness. It is a versatile material suitable for various applications, but its corrosion resistance may limit its use in certain environments. Understanding the trade-offs between 4320 and alternative grades like AISI 4140 and AISI 4340 can help engineers make informed decisions based on specific project requirements.