4340 Alloy Steel is a high-strength, low-alloy steel belonging to the AISI 43xx series, alloyed primarily with nickel, chromium, and molybdenum. This alloy combination provides exceptional toughness, deep hardenability, superior fatigue resistance, and outstanding impact strength, even in thick cross-sections.
4340 steel is widely used for critical, heavily loaded components where failure is not an option, including aerospace structures, power transmission systems, automotive drivetrains, oil & gas equipment, defense hardware, and heavy engineering machinery.
Chemical Composition
| Element | Percentage (%) | Function |
|---|---|---|
| Carbon (C) | 0.38 – 0.43 | Strength & hardness |
| Nickel (Ni) | 1.65 – 2.00 | Toughness & impact resistance |
| Chromium (Cr) | 0.70 – 0.90 | Hardenability & wear resistance |
| Molybdenum (Mo) | 0.20 – 0.30 | High-temperature strength & fatigue resistance |
| Manganese (Mn) | 0.60 – 0.80 | Toughness & deoxidation |
| Silicon (Si) | 0.15 – 0.35 | Strength & oxidation resistance |
| Phosphorus (P) | ≤ 0.035 | Controlled impurity |
| Sulfur (S) | ≤ 0.040 | Controlled impurity |
| Iron (Fe) | Balance | Base metal |
Microstructure & Metallurgy
• Annealed condition: Fine ferrite–pearlite structure
• Quenched condition: Martensitic structure with high hardness
• Tempered condition: Refined martensite balancing strength and toughness
Nickel improves low-temperature toughness and crack resistance, while molybdenum prevents temper embrittlement and enhances fatigue performance.
Mechanical Properties
| Property | Annealed | Quenched & Tempered |
|---|---|---|
| Tensile Strength | 650 – 850 MPa | 930 – 1,400 MPa |
| Yield Strength | 470 – 680 MPa | 800 – 1,200 MPa |
| Elongation | 18 – 25% | 10 – 18% |
| Hardness | 180 – 230 HB | 28 – 40 HRC |
| Impact Toughness | Very Good | Excellent |
Heat Treatment Capabilities
Annealing: Improves machinability and ductility (830–870°C)
Normalizing: Refines grain structure (850–900°C)
Quenching & Tempering: Develops high strength and fatigue resistance
Case Hardening: Optional carburizing or nitriding for hard surfaces
Stress Relieving: Enhances dimensional stability after fabrication
Machinability & Weldability
• Good machinability in annealed or normalized condition
• Fair weldability with precautions
• Preheating (200–300°C) and post-weld stress relief recommended
Available Forms
✔ Plates & sheets
✔ Bars (round, flat, square, hex)
✔ Forgings (rings, shafts, discs)
✔ Tubes & pipes (seamless / welded)
✔ Billets & blooms
✔ Precision-machined components
Standards & Equivalent Grades
| Standard | Equivalent Grade |
|---|---|
| AISI / SAE | 4340 |
| ASTM | A29, A322, A519 |
| EN | 34CrNiMo6 |
| DIN | 1.6582 |
| BS | 817M40 |
| JIS | SNCM439 |
Applications
Aerospace: Landing gear, structural fittings, engine mounts
Automotive & Motorsports: Crankshafts, axles, drive shafts, gears
Oil & Gas / Energy: Drill collars, pressure components, pump parts
Heavy Engineering: Power transmission shafts, large gears, couplings
Defense & Power: Armor components, missile parts, turbine shafts
Why Choose 4340 Alloy Steel
4340 alloy steel is selected for safety-critical applications where exceptional strength, toughness, and fatigue resistance are required. Its deep hardenability ensures uniform performance even in thick sections, making it a trusted material for aerospace, oil & gas, heavy machinery, and defense industries.