AISI 4340 is a high-strength, heat-treatable low-alloy steel alloyed with nickel (Ni), chromium (Cr), and molybdenum (Mo). This balanced chemistry delivers exceptional toughness, deep hardenability, high fatigue resistance, and mechanical stability even in large cross-section components.
Among hardenable alloy steels, 4340 is regarded as a premium structural material, often selected when grades such as 4140 or 4130 reach their performance limits.
Chemical Composition
| Element | % | Metallurgical Role |
|---|---|---|
| Carbon (C) | 0.38 – 0.43 | Strength, hardness |
| Nickel (Ni) | 1.65 – 2.00 | Toughness, fatigue resistance |
| Chromium (Cr) | 0.70 – 0.90 | Hardenability, wear resistance |
| Molybdenum (Mo) | 0.20 – 0.30 | High-temperature strength |
| Manganese (Mn) | 0.60 – 0.80 | Strength, deoxidation |
| Silicon (Si) | 0.15 – 0.30 | Strength, deoxidation |
| Phosphorus (P) | ≤ 0.035 | Controlled impurity |
| Sulfur (S) | ≤ 0.040 | Machinability |
| Iron (Fe) | Balance | Base metal |
Microstructure & Metallurgy
• Annealed: Ferrite + pearlite (machinable)
• Normalized: Fine pearlite for uniform strength
• Quenched: Martensitic structure
• Tempered: Tempered martensite (optimal strength–toughness balance)
Nickel enhances impact resistance and fracture toughness, while chromium and molybdenum ensure deep hardening and resistance to thermal softening.
Mechanical Properties
| Property | Typical Range |
|---|---|
| Tensile Strength | 745 – 1,860 MPa |
| Yield Strength | 470 – 1,500 MPa |
| Hardness | 217 – 525 HB |
| Elongation | 10 – 25% |
| Impact Toughness | Excellent (sub-zero capable) |
| Fatigue Resistance | Very high |
| Density | ~7.85 g/cm³ |
| Modulus of Elasticity | ~205 GPa |
Key Characteristics
Exceptional Hardenability
• Uniform hardness in thick sections
• Superior deep-section performance vs 4140
Outstanding Toughness
• Nickel resists brittle fracture
• Reliable under shock and extreme loads
Superior Fatigue Strength
• Excellent for cyclic and torsional loading
• Ideal for rotating machinery
High Strength-to-Weight Ratio
• Enables weight reduction
• Critical for aerospace and high-speed systems
Heat Treatment & Refining
Annealing: 830–860°C – machinability and stress relief
Normalizing: 870–900°C – grain refinement
Quenching: Oil quench from 830–860°C
Tempering: 200–650°C – tailored strength vs toughness
Surface Treatments:
• Nitriding for high surface hardness
• Induction hardening for localized wear resistance
• Selective carburizing for special designs
Machinability & Welding
• Machinability: Good in annealed or normalized condition
• Weldability: Fair – requires 200–300°C preheat and post-weld stress relief
• Forming: Best performed hot
Available Forms
✔ Hot rolled, cold drawn & forged bars
✔ Plates and sheets
✔ Billets and blooms
✔ Forged shafts, rings, and blocks
✔ Wires and rods
✔ Pre-hardened or Q&T components
✔ Precision-machined parts
International Standards & Equivalents
| Standard | Equivalent |
|---|---|
| AISI / SAE | 4340 |
| ASTM | A29, A322 |
| DIN | 34CrNiMo6 |
| EN | 1.6582 |
| JIS | SNCM439 |
| BS | EN24 |
Applications
Automotive & Heavy Machinery: Gears, transmission shafts, crankshafts, axles, couplings
Aerospace: Landing gear components, structural parts, high-strength fasteners
Oil & Gas / Energy: Drilling tools, couplings, power transmission shafts
4340 vs 4140
| Property | 4140 | 4340 |
|---|---|---|
| Alloying | Cr–Mo | Ni–Cr–Mo |
| Toughness | Good | Excellent |
| Hardenability | High | Very High |
| Cost | Lower | Higher |
| Thick Section Performance | Moderate | Superior |
Advantages
✔ Exceptional toughness and fatigue life
✔ Deep and uniform hardening
✔ Excellent performance under extreme stress
✔ Long service life for critical components
Limitations
⚠ Higher cost than 4140
⚠ Requires controlled welding procedures
⚠ Not corrosion-resistant without protection
AISI 4340 stands out as a premium alloy steel for applications where strength, toughness, fatigue resistance, and reliability are non-negotiable—making it a trusted choice for aerospace, oil & gas, defense, and heavy engineering components.