4340 Steel properties
4340 is a nickel-chromium-molybdenum alloy steel prized for its exceptional strength and toughness, even in large sections. It is used for the most demanding shafts, gears, and aerospace parts. Values below are for the quenched-and-tempered condition.
Mechanical properties
| Property | Value |
|---|---|
| Yield strength | 150 ksi (1035 MPa) |
| Ultimate tensile strength | 170 ksi (1170 MPa) |
| Elongation | 13% |
| Hardness | 363 HB (Brinell) |
| Modulus of elasticity | 29,700 ksi (205 GPa) |
Physical properties
| Property | Value |
|---|---|
| Density | 0.284 lb/in³ (7850 kg/m³) |
| Melting point | about 1427 °C |
Thermal properties
| Property | Value |
|---|---|
| Thermal conductivity | 44.5 W/m·K |
| Thermal expansion | 12.3 µm/m·°C |
| Specific heat | 475 J/kg·K |
Values are typical for the grade and condition shown. Actual properties vary with temper, heat treatment, and product form, so use these for comparison and preliminary design and confirm against material certificates for final work.
Compare with other materials
See the Material Properties Comparison, Yield Strength, and Density charts.
Typical uses
4340 is chosen for heavily loaded shafts, landing gear, gears, crankshafts, and aerospace structural parts. Its nickel content gives outstanding toughness and deep hardenability, so very large sections can be hardened fully while resisting impact and fatigue.
How it compares
4340 is among the strongest and toughest of the common alloy steels, outperforming 4140 in toughness and hardenability thanks to added nickel. It is more expensive and harder to machine and weld, so it is reserved for critical, highly stressed components.
FAQ
What makes 4340 steel special?
Its nickel-chromium-molybdenum alloying gives very high strength with excellent toughness and deep hardenability, even in large sections.
What is 4340 used for?
Critical high-stress parts: aircraft landing gear, heavily loaded shafts, gears, and crankshafts where strength and toughness are both essential.
Is 4340 stronger than 4140?
In the quenched-and-tempered condition 4340 reaches higher strength and is notably tougher and more deeply hardenable, owing to its nickel content.