4340 Steel
A Ni-Cr-Mo alloy steel, for high hardenability and strength
Typical values for the temper in the title
4340 adds about 1.8% nickel to the 4140 recipe. Nickel raises both hardenability and toughness, so it reaches very high strength (tensile above 1000 MPa) even in thick parts, while keeping good toughness.
It is used for critical parts: heavy shafts, gears, high-strength bolts, landing gear and aerospace parts.
Mechanical and physical properties
| Property | Value | Note |
|---|---|---|
| Tensile strength Rm | 1080–1470 MPa | |
| Yield strength Rp0.2 | 970–1380 MPa | |
| Elongation A | 12% | |
| Hardness | 220–430 HB | |
| Modulus of elasticity E | 205 GPa | |
| Density ρ | 7.85 g/cm³ | |
| Machinability | 50% | relative rating in the site database |
Chemical composition (wt %)
| Element | Range | Role |
|---|---|---|
| C carbon | 0.38–0.43 | strength and hardness |
| Ni nickel | 1.65–2.00 | hardenability and toughness |
| Cr chromium | 0.70–0.90 | hardenability |
| Mo molybdenum | 0.20–0.30 | hardenability |
| Mn manganese | 0.60–0.80 | |
| Si silicon | 0.15–0.35 | |
| Fe iron | balance |
Ranges per SAE J404 and ASTM A322. P max 0.035, S max 0.040. Equivalents in other standards are close in composition, not identical.
Conditions and heat treatment
In alloy steels, chromium, molybdenum and nickel raise hardenability: thicker parts reach full hardness even at the center. Properties are set mainly by heat treatment.
| Condition | Properties | When to choose it |
|---|---|---|
| Annealed | soft, for machining | delivery condition for machining |
| Normalized | uniform structure, medium strength | base condition, common for tubing |
| Quenched and tempered (Q&T) | high strength and good toughness; strength is set by the tempering temperature | loaded parts |
| Tempering range to avoid | tempering around 250–400 °C lowers toughness | choose a temperature outside the range |
Applications
- Heavy shafts
- Highly loaded gears
- High-strength bolts
- Landing gear and aerospace parts
- Critical forgings
Fabrication
Heat treatment
Oil quenched and tempered. For very high-strength parts, electroplating (for example cadmium or zinc) requires baking afterwards, to prevent hydrogen embrittlement.
Welding
Difficult: high preheat and post-weld heat treatment are needed.
Machining
Annealed or normalized.
Compared with similar materials
| Material | Rm / Rp0.2 | Welding | When it is the better choice |
|---|---|---|---|
| 4140 | 655–1000 / 415–835 MPa | Medium — Preheat 150C | cheaper, for less thick parts · material page |
| 300M | 1930 / 1690 MPa | Very difficult | an improved version, for even higher strength · material page |
| 4330V | 1170 / 1070 MPa | Medium | with vanadium, high toughness · material page |
Common mistakes
Plating a high-strength part without baking
Hydrogen from plating may cause delayed fracture. Bake per the specification.
Tempering at 250–400 °C
Toughness drops.
Welding without a procedure
High risk of cracking.
Equivalents, standards and supply
| EN | 34CrNiMo6 / 36CrNiMo4 (close) |
|---|---|
| DIN | 1.6511 |
| JIS | SNCM439 |
| Standards | ASTM A322 / AMS 6414 / EN 10083-3 |
| Forms | rod · forging · plate |
| Producers | TimkenSteel · Böhler · Villares |
Learn more
Values are typical and for comparison only, and depend strongly on condition and diameter. For design, rely on the standard and on the mill certificate. Sources: SAE J403 and J404, ASTM A29, A322 and A519 (composition), producer data sheets, and the METAL HUB materials database.
Updated: October 31, 2026