4340 Steel vs 300M Steel
AISI 4340 and 300M are low-alloy ultra-high-strength steels. 300M is a modified 4340 with added Si and V, heat-treatable to higher tensile strength with better fracture toughness at peak hardness.
4340 Steel
UNS G43400
Composition (wt %)
| Element | Min | Max |
|---|---|---|
| Fe | 94.0 | 98.0 |
| Ni | 1.65 | 2.0 |
| Cr | 0.7 | 0.9 |
| Mo | 0.2 | 0.3 |
| Mn | 0.6 | 0.8 |
| C | 0.38 | 0.43 |
300M Steel
UNS K44220
Composition (wt %)
| Element | Min | Max |
|---|---|---|
| Fe | 93.0 | 97.0 |
| Si | 1.45 | 1.8 |
| Ni | 1.65 | 2.0 |
| Cr | 0.7 | 0.95 |
| Mo | 0.3 | 0.45 |
| C | 0.4 | 0.46 |
| V | 0.05 | 0.1 |
Key differences
| Aspect | 4340 Steel | 300M Steel | Engineering impact |
|---|---|---|---|
| Silicon content | 0.15-0.30% Si | 1.45-1.80% Si | High Si in 300M raises the tempering resistance, allowing tempering at higher temperature without softening. |
| Vanadium | Not specified | 0.05-0.10% V | V forms fine carbides that pin grain boundaries and refine the microstructure for higher toughness. |
| Typical UTS (Q&T) | 1280-1450 MPa | 1900-2070 MPa | 300M is one of the highest-strength commercial steels - used when 4340 runs out of capacity for landing gear. |
| Processing sensitivity | Forgiving - wide HT window | Tight HT and cleanliness control required | 300M must be vacuum-melted and consolidated to reach spec. 4340 is more tolerant of conventional processing. |
When to choose 4340 Steel
- General aerospace shafts and gears
- Heavy truck and off-road driveline components
- Torsion bars and large fasteners
When to choose 300M Steel
- Aircraft landing gear (F-16, F-18, 737 historic)
- High-strength rocket motor casings
- Ultra-high-strength fasteners where 4340 is inadequate
Frequently asked questions
Can I substitute 300M for 4340 directly?
Only with revised heat treat and hydrogen-embrittlement controls. 300M is more susceptible to H-embrittlement and requires strict baking after plating.
Does 4340 come in a vacuum-melted grade?
Yes - AMS 6414 and 6415 cover VAR or VIM-VAR 4340 for aerospace. 300M is always vacuum-melted (AMS 6417/6419).
Which is tougher at 2000 MPa tensile strength?
300M. 4340 cannot reliably reach 2000 MPa with useful toughness. 300M was specifically developed to fill that gap.