4340 Steel Alloy: Composition, Properties, and Industrial Applications

A Complete Guide for Engineers, Manufacturers, and Material Specialists

The keyword 4340 steel alloy is widely searched by professionals across aerospace, automotive, oil & gas, and manufacturing industries—thanks to its remarkable combination of strength, toughness, and versatility. As a high-strength low-alloy (HSLA) steel, 4340 has become the go-to material for demanding structural and mechanical applications where both fatigue resistance and wear performance are critical.

In this detailed SEO-focused article, we explore everything you need to know about 4340 steel alloy—from its chemical composition to heat treatment, mechanical properties, and real-world uses. For premium-quality alloy steel products tailored to international specifications, sasaalloy is your trusted global supplier.

What Is 4340 Steel Alloy?

4340 steel is a nickel-chromium-molybdenum alloy steel known for its:

• High tensile strength

• Excellent toughness

• Good fatigue and impact resistance

• Deep hardenability through heat treatment

It belongs to the AISI (American Iron and Steel Institute) 4000-series, which is designated for alloy steels with enhanced performance over carbon steels.

4340 can be supplied in various conditions such as annealed, normalized, quenched and tempered, or vacuum arc remelted (VAR) for aerospace-grade purity.

Chemical Composition of 4340 Steel

The typical composition of 4340 steel is:

• Carbon (C): 0.38–0.43%

• Manganese (Mn): 0.60–0.80%

• Silicon (Si): 0.15–0.30%

• Nickel (Ni): 1.65–2.00%

• Chromium (Cr): 0.70–0.90%

• Molybdenum (Mo): 0.20–0.30%

• Phosphorus (P): ≤ 0.035%

• Sulfur (S): ≤ 0.040%

These alloying elements significantly enhance the steel’s strength, toughness, and heat-treatability compared to plain carbon steel.

Mechanical Properties of 4340 Steel

The mechanical performance of 4340 steel can vary depending on the heat treatment applied, but it typically offers:

• Tensile Strength: up to 980–1,200 MPa (or more with heat treatment)

• Yield Strength: 800–1,100 MPa

• Elongation: 10–20%

• Hardness (HRC): 24–50 (heat treatment dependent)

• Charpy Impact (at −40°C): high values, making it suitable for cold environments

This combination makes 4340 a top choice for high-stress, high-wear components.

Heat Treatment and Hardening

4340 steel alloy can undergo a variety of heat treatment processes to enhance specific properties:

• Annealing: Softens the material for machining

• Normalizing: Refines grain structure for improved toughness

• Quenching and Tempering: Increases strength and hardness

• Austempering: Optimizes strength-to-toughness ratio

• Cryogenic Treatment: Improves dimensional stability

4340 responds well to surface hardening techniques like nitriding or induction hardening for added wear resistance without compromising core toughness.

sasaalloy offers 4340 steel in multiple delivery conditions depending on customer requirements.

Applications of 4340 Steel Alloy

4340 is engineered for critical structural parts that experience high stresses and require superior strength, toughness, and fatigue resistance.

Aerospace Industry:

• Landing gear components

• Crankshafts

• Gear shafts and main structural supports

• High-performance fasteners

Automotive and Racing:

• Axles and drive shafts

• Transmission gears

• Engine parts under high load

• Suspension components

Oil & Gas:

• Drill collars

• Frac pump shafts

• Couplings for downhole equipment

• Wellhead equipment

Heavy Machinery:

• Press rams

• Tooling dies

• Industrial rollers

• High-load pins and bolts

Thanks to its excellent hardenability and shock resistance, 4340 is suitable for both dynamic and static high-stress applications.

Advantages of 4340 Steel Alloy

• High Strength-to-Weight Ratio: Critical for aerospace and automotive sectors

• Good Machinability: Especially in annealed condition

• Excellent Toughness: Performs well in extreme impact and fatigue scenarios

• Deep Hardenability: Ideal for large cross-sections requiring uniform hardness

• Corrosion Resistance (Moderate): Can be improved with coatings or surface treatments

For extreme environments, 4340 offers better reliability than basic carbon steels or lower-grade alloy steels.

4340 vs 4140 Steel: What’s the Difference?

While both are alloy steels, 4340 contains significantly more nickel, which enhances its toughness and deep hardenability. In contrast, 4140 is more commonly used for medium-duty applications where less toughness is acceptable.

sasaalloy supplies both 4140 and 4340 alloy steels and can help you choose the right material for your application.

Welding and Fabrication Considerations

4340 is weldable, but due to its high hardenability, preheating and post-weld heat treatment are often necessary to avoid cracking and ensure toughness. For critical applications, controlled welding procedures and filler materials should be selected to maintain structural integrity.

Surface Treatments and Coatings

To enhance corrosion resistance and wear life, 4340 can be finished with:

• Black oxide coating

• Zinc or cadmium plating

• Nitriding for surface hardness

• Phosphate coating for anti-friction

sasaalloy offers custom finishing services for 4340 steel components to match specific performance or aesthetic requirements.

Certifications and Standards

4340 steel is produced under various global standards, including:

• AISI 4340 (USA)

• AMS 6414 (aerospace grade)

• ASTM A29/A29M

• EN 34CrNiMo6 (Europe)

• JIS SNCM439 (Japan)

sasaalloy supplies certified 4340 steel that complies with industry-specific standards and testing requirements.

Why Choose sasaalloy for 4340 Steel Alloy?

• Wide inventory of 4340 bars, blocks, forged rings, and precision-machined parts

• Supply in various conditions: annealed, Q&T, hardened, rough turned

• In-house cutting, heat treating, and machining capabilities

• Full traceability and MTC (EN10204 3.1 or 3.2)

• Global export experience and fast delivery

sasaalloy serves aerospace OEMs, oilfield equipment manufacturers, defense contractors, and automotive suppliers worldwide with consistent quality and expert support.

Conclusion

4340 steel alloy stands as one of the most reliable and versatile high-strength materials available today. Its unique balance of strength, toughness, and deep hardenability make it ideal for mission-critical applications across multiple sectors. Whether you need forged bars for aerospace shafts or quenched and tempered stock for drilling equipment, 4340 delivers unmatched performance under pressure.

To ensure material integrity and precision delivery, partner with sasaalloy—your global source for advanced alloy steel solutions.