This is a common comparison between two important alloy steels.
| Property | AISI 4140 (Cr-Mo) | AISI 4340 (Ni-Cr-Mo) |
|---|---|---|
| Key Alloy Difference | 0.8-1.1% Cr, 0.15-0.25% Mo | 1.65-2.00% Ni, 0.7-0.9% Cr, 0.2-0.3% Mo |
| Core Strength/Toughness | Very good. | Superior. Nickel significantly enhances toughness at equivalent strength levels. |
| Hardenability | Very good (up to ~100mm effective). | Excellent (up to ~150mm+ effective). |
| Cost | Lower. | Higher, due to nickel content. |
| Typical Use | General high-strength components: shafts, gears. | Ultra-critical components: aircraft landing gear, high-performance racing crankshafts, where exceptional fracture toughness is required. |
Selection Rule: Use 4140 where cost-effectiveness and high strength are key. Choose 4340 where maximum fracture toughness and fatigue performance in very large sections or under extreme loads are paramount.