Overview
SAE/AISI 4340 Steel is a premium nickel-chromium-molybdenum alloy steel renowned for its exceptional combination of ultra-high strength, toughness, fatigue resistance, impact resistance, and hardenability.
Compared with SAE 4140, 4340 contains additional nickel, significantly improving toughness, ductility, and deep-section hardenability. It is widely used in aerospace, defense, oil & gas, heavy machinery, motorsports, and critical structural applications where failure is not an option.
SAE 4340 is commonly used in:
- Aircraft Landing Gear
- Aerospace Components
- Heavy-Duty Shafts
- Crankshafts
- Gears & Pinions
- Oil & Gas Equipment
- Military Components
- High-Performance Racing Parts
Because of its outstanding mechanical performance, SAE 4340 is often considered one of the highest-performance engineering alloy steels available.
Material Specification
| Property | Value |
|---|---|
| Material Designation | SAE/AISI 4340 |
| Standard | ASTM A29 / SAE J404 |
| UNS Number | G43400 |
| Material Family | Nickel-Chromium-Molybdenum Alloy Steel |
| Density | 7.85 g/cm³ |
| Magnetic Property | Magnetic |
| Standard Forms | Bar, Plate, Forgings, Tube, Shafting |
Chemical Composition
| Element | Content (%) |
|---|---|
| Iron (Fe) | Balance |
| Carbon (C) | 0.38–0.43 |
| Nickel (Ni) | 1.65–2.00 |
| Chromium (Cr) | 0.70–0.90 |
| Molybdenum (Mo) | 0.20–0.30 |
| Manganese (Mn) | 0.60–0.80 |
| Silicon (Si) | 0.15–0.30 |
| Phosphorus (P) | ≤0.035 |
| Sulfur (S) | ≤0.040 |
The nickel content significantly enhances toughness, fatigue resistance, and deep hardening capability.
Mechanical Properties
Annealed Condition
| Property | Value |
|---|---|
| Tensile Strength | 745–860 MPa |
| Yield Strength | 470–620 MPa |
| Elongation | 20–25% |
| Hardness | 200–250 HB |
| Elastic Modulus | 205 GPa |
Quenched & Tempered Condition
| Property | Value |
|---|---|
| Tensile Strength | 1080–1450 MPa |
| Yield Strength | 740–1250 MPa |
| Elongation | 10–18% |
| Hardness | 32–50 HRC |
Ultra-High Strength Condition
| Property | Value |
|---|---|
| Tensile Strength | Up to 1900 MPa |
| Yield Strength | Up to 1600 MPa |
| Hardness | Up to 55–60 HRC |
Physical Properties
| Property | Value |
|---|---|
| Density | 7.85 g/cm³ |
| Melting Point | 1425–1540°C |
| Thermal Conductivity | 44 W/m·K |
| Electrical Resistivity | 0.22 μΩ·m |
| Thermal Expansion Coefficient | 12.3 × 10⁻⁶/K |
| Specific Heat Capacity | 475 J/kg·K |
Corrosion Resistance
| Environment | Performance |
|---|---|
| Indoor Environment | Good |
| Dry Atmosphere | Good |
| Outdoor Environment | Moderate |
| Humid Environment | Moderate |
| Marine Environment | Poor |
| Chemical Environment | Poor |
Although better than carbon steels, SAE 4340 is not considered corrosion resistant.
Recommended surface treatments:
- QPQ / Salt Bath Nitriding
- Gas Nitriding
- Black Oxide
- Electroless Nickel Plating
- Hard Chrome Plating
- Zinc Plating
- Phosphate Coating
Weldability
| Welding Process | Rating |
|---|---|
| TIG Welding | Moderate |
| MIG Welding | Moderate |
| Arc Welding | Moderate |
| Resistance Welding | Fair |
| Spot Welding | Fair |
Due to its high alloy content and hardenability:
- Preheating Required
- Controlled Cooling Recommended
- Post-Weld Heat Treatment Recommended
For critical aerospace components, welding is generally minimized.
Machinability
| Process | Rating |
|---|---|
| CNC Turning | Good |
| CNC Milling | Good |
| Drilling | Good |
| Grinding | Excellent |
| Threading | Good |
| Broaching | Good |
Machinability Comparison
| Material | Machinability |
|---|---|
| 1018 | Excellent |
| 1045 | Good–Excellent |
| 4140 | Good |
| 4340 | Good |
| 8620 | Good |
| D2 Tool Steel | Fair |
4340 is typically machined in the annealed or pre-hardened condition.
Heat Treatment
Annealing
| Parameter | Value |
|---|---|
| Temperature | 815–870°C |
| Cooling Method | Furnace Cool |
Normalizing
| Parameter | Value |
|---|---|
| Temperature | 845–900°C |
| Cooling Method | Air Cool |
Quenching
| Parameter | Value |
|---|---|
| Temperature | 830–860°C |
| Cooling Medium | Oil |
Tempering
| Parameter | Value |
|---|---|
| Temperature | 200–650°C |
| Purpose | Strength & Toughness Adjustment |
Nitriding
| Property | Value |
|---|---|
| Surface Hardness | 60–72 HRC |
| Wear Resistance | Outstanding |
Induction Hardening
| Property | Value |
|---|---|
| Surface Hardness | 55–62 HRC |
| Hardened Depth | 1–10 mm |
International Equivalent Grades
| Standard | Equivalent Grade |
|---|---|
| SAE/AISI (USA) | 4340 |
| ASTM (USA) | ASTM A29 Grade 4340 |
| UNS | G43400 |
| GB/T (China) | 40CrNiMoA |
| EN (Europe) | 34CrNiMo6 |
| DIN (Germany) | 1.6582 / 34CrNiMo6 |
| JIS (Japan) | SNCM439 |
| ISO | 34CrNiMo6 |
Available Forms
| Product Type | Availability |
|---|---|
| Round Bar | Yes |
| Flat Bar | Yes |
| Alloy Plate | Yes |
| Forgings | Yes |
| Seamless Tube | Yes |
| Mechanical Tube | Yes |
| Ground Shafting | Yes |
| Aerospace Grade Bar | Yes |
Typical Applications
Aerospace Industry
- Landing Gear Components
- Aircraft Structural Parts
- Engine Mounts
- Aerospace Fasteners
Oil & Gas Industry
- Drill Collars
- Downhole Tools
- High-Pressure Components
- Wellhead Equipment
Power Transmission Industry
- Heavy-Duty Shafts
- Splined Shafts
- Couplings
- Transmission Components
Gear Manufacturing
- High-Load Gears
- Pinions
- Worm Gears
- Gear Shafts
Defense Industry
- Armored Vehicle Components
- Weapon System Components
- Military Hardware
Motorsport Industry
- Racing Axles
- Crankshafts
- Suspension Components
- Drivetrain Parts
Manufacturing Characteristics
| Manufacturing Process | Rating |
|---|---|
| CNC Turning | Excellent |
| CNC Milling | Excellent |
| Precision Grinding | Outstanding |
| Heat Treatment | Outstanding |
| Nitriding | Outstanding |
| Induction Hardening | Outstanding |
| Forging | Excellent |
| Hard Chrome Plating | Excellent |
| QPQ Treatment | Excellent |
Advantages
Ultra-High Strength
One of the strongest engineering steels available.
Exceptional Toughness
Outstanding resistance to shock and impact.
Superior Fatigue Resistance
Ideal for cyclic loading applications.
Excellent Hardenability
Can achieve deep and uniform hardness.
Outstanding Wear Resistance
Especially after nitriding.
Aerospace Proven Material
Widely used in mission-critical applications.
Limitations
Higher Material Cost
More expensive than 4140 and carbon steels.
More Difficult Welding
Requires controlled procedures.
Limited Corrosion Resistance
Needs protective coatings.
Higher Machining Costs
Especially after heat treatment.
Comparison with Other Alloy Steels
| Property | 4340 | 4140 | 4130 | 8620 | 40CrNiMoA |
|---|---|---|---|---|---|
| Nickel Content | High | None | None | High | High |
| Strength | Exceptional | Very High | High | High | |
| Toughness | Outstanding | Excellent | Excellent | Excellent | |
| Hardenability | Outstanding | Excellent | Good | Excellent | |
| Fatigue Resistance | Outstanding | Excellent | Excellent | Good |
Comparison with Engineering Materials
| Material | Yield Strength | Hardness Potential | Fatigue Resistance |
|---|---|---|---|
| SAE 4340 | 740–1600 MPa | 60 HRC | Outstanding |
| SAE 4140 | 655–1200 MPa | 58 HRC | Excellent |
| SAE 4130 | 650–900 MPa | 45 HRC | Excellent |
| 17-4PH Stainless | 860–1170 MPa | 44 HRC | Excellent |
| Maraging Steel 300 | 1700–2000 MPa | 52 HRC | Exceptional |
| Titanium Ti-6Al-4V | 880–1100 MPa | Moderate | Excellent |
Available Surface Finishes
Raw Finishes
- Annealed Finish
- Normalized Finish
- Quenched & Tempered Finish
- Aerospace Grade Finish
Protective Finishes
- QPQ Treatment
- Gas Nitriding
- Black Oxide
- Electroless Nickel Plating
- Hard Chrome Plating
- Zinc Plating
Decorative Finishes
- Powder Coating
- Spray Painting
- Electrophoretic Coating (E-Coating)
Frequently Asked Questions (FAQ)
What is SAE 4340 steel?
SAE 4340 is a nickel-chromium-molybdenum alloy steel known for exceptional strength, toughness, fatigue resistance, and hardenability.
What is SAE 4340 used for?
Typical applications include:
- Aircraft Landing Gear
- High-Load Shafts
- Crankshafts
- Gears
- Oil & Gas Equipment
- Defense Components
What is the difference between SAE 4340 and SAE 4140?
| Property | SAE 4340 | SAE 4140 |
|---|---|---|
| Nickel Content | Yes | No |
| Toughness | Higher | Lower |
| Strength | Higher | Lower |
| Hardenability | Better | Excellent |
What is the difference between SAE 4340 and 40CrNiMoA?
SAE 4340 and 40CrNiMoA are generally considered equivalent grades and are interchangeable in most engineering applications.
Can SAE 4340 be nitrided?
Yes. Nitriding can achieve surface hardness of 60–72 HRC with excellent wear resistance.
Is SAE 4340 suitable for aerospace applications?
Yes. SAE 4340 is widely used in aircraft landing gear, structural components, and high-strength aerospace hardware.
Can SAE 4340 be welded?
Yes, but preheating and post-weld heat treatment are recommended.
What hardness can SAE 4340 achieve?
- Quenched & Tempered: 32–50 HRC
- Induction Hardened: 55–62 HRC
- Nitrided Surface: 60–72 HRC
Why is SAE 4340 widely used in critical engineering applications?
Because it combines:
- Ultra-High Strength
- Exceptional Toughness
- Outstanding Fatigue Resistance
- Excellent Hardenability
- Proven Reliability
making it ideal for mission-critical components.
Does GCNOV provide custom manufacturing services for SAE 4340?
Yes. GCNOV provides:
- CNC Turning
- CNC Milling
- Aerospace Component Manufacturing
- Shaft & Gear Machining
- Nitriding
- QPQ Treatment
- Heat Treatment
- Precision Grinding
- Hard Chrome Plating
- Prototype Development
- Low-Volume Production
- Mass Production