4350 Steel: Properties and Key Applications

4350 Steel is classified as a medium-carbon alloy steel, primarily known for its excellent hardenability and strength. The primary alloying elements in 4350 steel include chromium (Cr), nickel (Ni), and molybdenum (Mo), which significantly enhance its mechanical properties and resistance to wear. This steel grade is often used in applications requiring high strength and toughness, making it suitable for various engineering and manufacturing processes.

Comprehensive Overview

4350 steel is recognized for its unique combination of strength, toughness, and wear resistance, making it an ideal choice for components subjected to high stress and fatigue. The alloying elements play a crucial role in defining its characteristics:

• Chromium enhances hardenability and corrosion resistance.
• Nickel improves toughness and impact strength.
• Molybdenum contributes to high-temperature strength and stability.

The main advantages of 4350 steel include its ability to withstand high loads and its excellent fatigue resistance, which is critical in applications such as gears, shafts, and heavy machinery components. However, it also has limitations, such as lower weldability compared to other steel grades and a tendency to be more expensive due to its alloying elements. Historically, 4350 steel has been significant in industries requiring reliable performance under demanding conditions, maintaining a strong market position due to its desirable properties.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	G43500	USA	Closest equivalent to AISI 4340
AISI/SAE	4350	USA	Similar to 4340 but with slight variations in composition
ASTM	A829	USA	Standard specification for alloy steel
EN	1.7225	Europe	Equivalent to AISI 4340 with minor differences
JIS	SNCM439	Japan	Similar properties but with different alloying elements
ISO	35CrMo4	International	Comparable in terms of mechanical properties

The subtle differences between these grades can affect performance in specific applications. For instance, while G43500 and 1.7225 are often considered equivalent, the slight variations in nickel and molybdenum content can influence hardenability and toughness.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.38 - 0.43
Cr (Chromium)	0.70 - 0.90
Ni (Nickel)	1.65 - 2.00
Mo (Molybdenum)	0.15 - 0.25
Mn (Manganese)	0.60 - 0.90
Si (Silicon)	0.15 - 0.40
P (Phosphorus)	≤ 0.035
S (Sulfur)	≤ 0.040

The key alloying elements in 4350 steel contribute significantly to its performance. For example, the presence of chromium enhances hardenability, allowing for deeper penetration during heat treatment. Nickel improves the toughness of the steel, making it less brittle, while molybdenum enhances its strength at elevated temperatures.

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	980 - 1,100 MPa	142 - 160 ksi	ASTM E8
Yield Strength (0.2% offset)	Quenched & Tempered	Room Temp	850 - 950 MPa	123 - 138 ksi	ASTM E8
Elongation	Quenched & Tempered	Room Temp	12 - 15%	12 - 15%	ASTM E8
Reduction of Area	Quenched & Tempered	Room Temp	50 - 60%	50 - 60%	ASTM E8
Hardness (Rockwell C)	Quenched & Tempered	Room Temp	28 - 34 HRC	28 - 34 HRC	ASTM E18
Impact Strength (Charpy)	Quenched & Tempered	-20°C (-4°F)	30 - 50 J	22 - 37 ft-lbf	ASTM E23

The combination of these mechanical properties makes 4350 steel suitable for applications requiring high strength and toughness, such as in the automotive and aerospace industries. Its ability to withstand significant mechanical loading while maintaining structural integrity is a key factor in its selection for critical components.

Physical Properties

Property	Condition/Temperature	Value (Metric)	Value (Imperial)
Density	Room Temp	7.85 g/cm³	0.284 lb/in³
Melting Point	-	1,400 - 1,540 °C	2,552 - 2,804 °F
Thermal Conductivity	Room Temp	45 W/m·K	31 BTU·in/h·ft²·°F
Specific Heat Capacity	Room Temp	460 J/kg·K	0.11 BTU/lb·°F
Electrical Resistivity	Room Temp	0.0000017 Ω·m	0.0000017 Ω·in
Coefficient of Thermal Expansion	Room Temp	11.5 x 10⁻⁶ /K	6.36 x 10⁻⁶ /°F

The density and melting point of 4350 steel indicate its robustness, while the thermal conductivity and specific heat capacity suggest its suitability for applications involving thermal cycling. The electrical resistivity is relatively low, making it a good conductor of electricity, which can be advantageous in certain applications.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Atmospheric	-	-	Fair	Susceptible to rust
Chlorides	3-5	20-60 °C (68-140 °F)	Poor	Risk of pitting
Acids	10-20	20-40 °C (68-104 °F)	Poor	Not recommended
Alkaline	5-10	20-60 °C (68-140 °F)	Fair	Moderate resistance

4350 steel exhibits moderate corrosion resistance, particularly in atmospheric conditions. However, it is susceptible to pitting in chloride environments and is not recommended for use in acidic conditions. Compared to grades like 4140 and 4340, which have better corrosion resistance due to higher chromium content, 4350 may require protective coatings or treatments in corrosive environments.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	400 °C	752 °F	Suitable for high-temperature applications
Max Intermittent Service Temp	500 °C	932 °F	Short-term exposure only
Scaling Temperature	600 °C	1,112 °F	Risk of oxidation beyond this temp
Creep Strength considerations begin around	450 °C	842 °F	Important for long-term applications

At elevated temperatures, 4350 steel maintains its strength but may experience oxidation if not properly protected. Its performance in high-temperature applications makes it suitable for components in engines and turbines, where thermal stability is crucial.

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	Requires post-weld heat treatment
Stick	E80S-Ni	-	Suitable for thicker sections

Weldability of 4350 steel is moderate; preheating is often necessary to avoid cracking. Post-weld heat treatment can help relieve stresses and improve toughness in the weld zone.

Machinability

Machining Parameter	4350 Steel	AISI 1212	Notes/Tips
Relative Machinability Index	60%	100%	4350 is more challenging to machine
Typical Cutting Speed (Turning)	20-30 m/min	40-50 m/min	Use carbide tools for best results

Machinability of 4350 steel is lower than that of free-machining steels like AISI 1212. Optimal cutting speeds and tooling are essential to achieve desired surface finishes and tolerances.

Formability

4350 steel exhibits moderate formability. Cold forming is feasible, but care must be taken to avoid work hardening. Hot forming can be performed at elevated temperatures, allowing for better shaping without compromising material integrity.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	600 - 700 °C / 1,112 - 1,292 °F	1-2 hours	Air	Softening, improved ductility
Quenching	800 - 850 °C / 1,472 - 1,562 °F	30 minutes	Oil/Water	Hardening, increased strength
Tempering	400 - 600 °C / 752 - 1,112 °F	1 hour	Air	Reducing brittleness, enhancing toughness

Heat treatment processes significantly affect the microstructure and properties of 4350 steel. Quenching increases hardness, while tempering helps reduce brittleness, making the steel more suitable for dynamic applications.

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
Oil & Gas	Drill bits	Wear resistance, toughness	Essential for harsh environments
Heavy Machinery	Shafts	Fatigue resistance, strength	Necessary for durability under load

Other applications include:

• • Structural components in heavy machinery
• • High-stress fasteners
• • Tooling and dies

The selection of 4350 steel in these applications is driven by its ability to withstand high loads and resist wear, making it a preferred choice in demanding environments.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	4350 Steel	AISI 4140	AISI 4340	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High strength	Moderate strength	High strength	4340 offers better toughness
Key Corrosion Aspect	Fair resistance	Good resistance	Fair resistance	4140 has better corrosion resistance
Weldability	Moderate	Good	Moderate	4140 is easier to weld
Machinability	Moderate	Good	Moderate	4140 is easier to machine
Formability	Moderate	Good	Moderate	4140 offers better formability
Approx. Relative Cost	Higher	Moderate	Higher	4140 is often more cost-effective
Typical Availability	Moderate	High	High	4140 is widely available

When selecting 4350 steel, considerations include its cost-effectiveness, availability, and specific mechanical properties required for the application. While it offers excellent performance, alternatives like AISI 4140 may provide better corrosion resistance and machinability, making them suitable for different applications.

In conclusion, 4350 steel is a versatile medium-carbon alloy steel that excels in applications requiring high strength and toughness. Its unique properties, combined with careful consideration of fabrication techniques and environmental factors, make it a valuable material in various industries.