EN24 Steel: Properties and Key Applications

EN24 Steel, also known as 4340 steel, is a high-strength alloy steel classified as a medium-carbon alloy steel. It primarily consists of iron, carbon, and several alloying elements, including nickel, chromium, and molybdenum. These elements significantly enhance the steel's mechanical properties, making it suitable for demanding applications in various industries.

Comprehensive Overview

EN24 steel is renowned for its excellent toughness, high fatigue strength, and good machinability, making it a preferred choice in applications requiring high strength and durability. The primary alloying elements—nickel, chromium, and molybdenum—contribute to its hardenability and overall performance. Nickel enhances toughness, chromium improves corrosion resistance and hardenability, while molybdenum increases strength and stability at elevated temperatures.

Advantages:
- High Strength: EN24 exhibits impressive tensile and yield strength, making it ideal for heavy-duty applications.
- Good Toughness: The alloy's toughness allows it to withstand impact loads without fracturing.
- Versatile Machinability: It can be machined effectively, allowing for intricate designs and components.

Limitations:
- Cost: EN24 is more expensive than lower-grade steels due to its alloying elements.
- Weldability: While it can be welded, special precautions are necessary to avoid cracking.
- Heat Treatment Sensitivity: The steel requires precise heat treatment to achieve desired properties, which can complicate processing.

Historically, EN24 has been widely used in the aerospace, automotive, and engineering sectors, where high-performance materials are critical. Its market position remains strong due to its reliability and versatility in various applications.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	G43400	USA	Closest equivalent to EN24
AISI/SAE	4340	USA	Commonly used designation
ASTM	A829	USA	Standard specification for alloy steel
EN	24	Europe	European standard designation
DIN	1.6582	Germany	Similar properties, minor compositional differences
JIS	SNCM439	Japan	Comparable, but with different alloying ratios
ISO	42CrMo4	International	Equivalent with slight variations

The differences between these equivalent grades can affect selection based on specific application requirements. For instance, while G43400 and 4340 are often interchangeable, variations in heat treatment processes can lead to differences in mechanical properties.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.38 - 0.43
Si (Silicon)	0.15 - 0.40
Mn (Manganese)	0.60 - 0.90
Cr (Chromium)	0.90 - 1.20
Mo (Molybdenum)	0.15 - 0.25
Ni (Nickel)	1.65 - 2.00
P (Phosphorus)	≤ 0.035
S (Sulfur)	≤ 0.035

The key alloying elements in EN24 steel play crucial roles:
- Carbon (C): Increases hardness and strength through heat treatment.
- Nickel (Ni): Enhances toughness and resistance to impact.
- Chromium (Cr): Improves hardenability and corrosion resistance.
- Molybdenum (Mo): Increases strength at high temperatures and enhances hardenability.

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 - 1100 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 (HRC)	Quenched & Tempered	Room Temp	28 - 34 HRC	28 - 34 HRC	ASTM E18
Impact Strength (Charpy)	Quenched & Tempered	-20°C	30 - 40 J	22 - 30 ft-lbf	ASTM E23

The combination of these mechanical properties makes EN24 steel particularly suitable for applications involving high mechanical loading, such as gears, shafts, and heavy machinery components. Its high yield strength and toughness enable it to endure 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	-	460 J/kg·K	0.11 BTU/lb·°F
Electrical Resistivity	-	0.0000017 Ω·m	0.0000017 Ω·in
Coefficient of Thermal Expansion	20 - 100 °C	12 x 10⁻⁶ /K	6.67 x 10⁻⁶ /°F

The density and melting point of EN24 steel indicate its suitability for high-temperature applications. Its thermal conductivity is moderate, making it effective for heat dissipation in mechanical components. The coefficient of thermal expansion is relatively low, which helps maintain dimensional stability under temperature fluctuations.

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 Solutions	5-10	20-40 °C (68-104 °F)	Fair	Risk of stress corrosion

EN24 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, such as AISI 304 or 316, EN24's corrosion resistance is significantly lower, making it less suitable for applications in corrosive environments.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	300	572	Suitable for high-temperature applications
Max Intermittent Service Temp	400	752	Short-term exposure only
Scaling Temperature	600	1112	Risk of oxidation beyond this limit
Creep Strength considerations begin around	400	752	Performance may degrade at high temps

EN24 steel maintains its strength and toughness at elevated temperatures, making it suitable for applications involving heat. However, care must be taken to avoid prolonged exposure to temperatures above 300 °C (572 °F) to prevent degradation of mechanical properties.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
MIG	ER80S-Ni	Argon + 2-5% CO2	Preheat recommended
TIG	ER80S-Ni	Argon	Requires post-weld heat treatment
Stick	E7018	-	Preheat and post-weld treatment recommended

EN24 steel can be welded using various processes, but it requires careful control of preheat and post-weld heat treatment to avoid cracking. The use of nickel-containing filler metals is recommended to enhance toughness in the weld area.

Machinability

Machining Parameter	EN24 Steel	AISI 1212	Notes/Tips
Relative Machinability Index	60%	100%	EN24 is less machinable than 1212
Typical Cutting Speed (Turning)	30-50 m/min	80-120 m/min	Use carbide tools for best results

EN24 steel has moderate machinability, requiring slower cutting speeds compared to free-machining steels like AISI 1212. Carbide tools are recommended for effective machining.

Formability

EN24 steel exhibits good formability, particularly when hot worked. Cold forming is also possible, but care must be taken to avoid excessive 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	600 - 650 / 1112 - 1202	1 - 2 hours	Air	Softening, improving machinability
Quenching	830 - 860 / 1526 - 1580	30 minutes	Oil	Hardening, increasing strength
Tempering	500 - 650 / 932 - 1202	1 - 2 hours	Air	Reducing brittleness, enhancing toughness

The heat treatment processes for EN24 steel significantly influence its microstructure and properties. 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)
Aerospace	Aircraft landing gear	High strength, toughness	Critical for safety and performance
Automotive	Drive shafts	Fatigue resistance, machinability	Essential for durability
Oil & Gas	Drill bits	Hardness, wear resistance	Required for extreme conditions
Heavy Machinery	Gears and shafts	High tensile strength, impact resistance	Necessary for heavy loads

Other applications of EN24 steel include:
- Structural components in machinery
- High-stress fasteners
- Tooling and dies

The selection of EN24 for these applications is primarily due to its high strength and toughness, which are critical for performance and reliability in demanding environments.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	EN24 Steel	AISI 4140	AISI 4340	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High strength	Moderate strength	High strength	EN24 and 4340 are similar, but EN24 has better toughness
Key Corrosion Aspect	Fair	Fair	Fair	All three grades are susceptible to corrosion
Weldability	Moderate	Good	Moderate	4140 is easier to weld than EN24
Machinability	Moderate	Good	Moderate	4140 has better machinability than EN24
Formability	Good	Fair	Good	EN24 and 4340 are better for forming than 4140
Approx. Relative Cost	Higher	Moderate	Higher	EN24 is more expensive due to alloying elements
Typical Availability	Moderate	High	Moderate	4140 is more commonly available

When selecting EN24 steel, considerations include its cost-effectiveness, availability, and specific application requirements. While it is more expensive than some alternatives, its superior mechanical properties often justify the investment for critical applications. Additionally, its moderate weldability and machinability require careful planning during fabrication to avoid potential issues.

In summary, EN24 steel is a versatile and high-performance material suitable for a wide range of applications, particularly where strength and toughness are paramount. Its unique properties, combined with careful processing and treatment, make it a preferred choice in demanding engineering environments.