One step up from 4140 — the nickel addition gives better toughness at high strength levels and improved hardenability in heavy sections. The standard alloy for aircraft landing gear, high-performance crankshafts, and critical rotating machinery. Holds fatigue life better than 4140 at 50+ HRC hardness levels.
Nickel content allows through-hardening to ~90 mm diameter vs ~50 mm for 4140.
At 50 HRC, 4340 retains notch impact strength that 4140 does not — critical in aerospace.
Specified in aircraft landing gear (S-N curves significantly better than 4140 above 150 ksi UTS).
| Element | Content |
|---|---|
| Iron | Balance |
| Nickel | 1.65–2.00% |
| Chromium | 0.70–0.90% |
| Molybdenum | 0.20–0.30% |
| Carbon | 0.38–0.43% |
| Manganese | 0.60–0.80% |
Composition per ASTM A29. Specific mill test reports (MTR) available on request for production orders.
4340 typically costs 30–50% more. Use 4140 unless cross-section exceeds 50 mm or toughness at 50+ HRC is required.
Oil quench from 845 °C; tempering curves steep — specify hardness target (e.g., 38–42 HRC) rather than tempering temperature alone.
After electroplating (cadmium, zinc), bake immediately at 190 °C for 24 hours to drive off absorbed hydrogen.
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