4140 and 4340 are both medium-carbon alloy steels. Both heat-treat to high strength. Both are workhorses in shafts, axles, gears, and structural components. The difference is 1.8% nickel in 4340. That nickel buys you deeper hardening, better toughness at low temperatures, and 30% more cost. For high-cycle fatigue and aerospace applications, it's the right choice. For general mechanical work, 4140 does the job.
| Element | 4140 (%) | 4340 (%) | Role |
|---|---|---|---|
| Carbon | 0.38–0.43 | 0.38–0.43 | Hardenability, strength after heat-treat |
| Chromium | 0.80–1.10 | 0.70–0.90 | Hardenability, tempering resistance |
| Molybdenum | 0.15–0.25 | 0.20–0.30 | Reduces temper embrittlement, hardenability |
| Manganese | 0.75–1.00 | 0.60–0.80 | Hardenability, deoxidation |
| Nickel | — | 1.65–2.00 | Deeper hardening, low-temp toughness |
| Silicon | 0.15–0.30 | 0.15–0.30 | Deoxidation |
| Phosphorus (max) | 0.035 | 0.025 | Impurity — 4340 tighter spec |
| Sulfur (max) | 0.040 | 0.025 | Impurity — 4340 tighter spec |
The nickel addition in 4340 has two effects:
Both alloys develop their rated properties through quench-and-temper heat treatment. The "condition" in the spec determines tensile strength:
| Condition | 4140 UTS | 4140 Yield | 4340 UTS | 4340 Yield |
|---|---|---|---|---|
| Annealed | 655 MPa | 415 MPa | 745 MPa | 475 MPa |
| Normalized | 1020 MPa | 655 MPa | 1280 MPa | 860 MPa |
| Q&T at 540 °C (30 HRC) | 1100 MPa | 980 MPa | 1175 MPa | 1050 MPa |
| Q&T at 425 °C (38 HRC) | 1240 MPa | 1100 MPa | 1310 MPa | 1170 MPa |
| Q&T at 315 °C (45 HRC) | 1620 MPa | 1420 MPa | 1760 MPa | 1590 MPa |
| Q&T at 205 °C (50 HRC) | 1900 MPa | 1620 MPa | 2040 MPa | 1860 MPa |
In any given temper, 4340 is 5–10% stronger than 4140 in both UTS and yield — mostly due to the nickel contribution.
4140 hardens ~25 mm deep in oil quench. 4340 reaches 50 mm+ deep. For large shafts that need uniform properties core-to-surface, 4340 is required.
Landing gear, hydraulic actuators, rotor masts — 4340 is spec'd for low-temperature toughness (-55 °C service) and fatigue performance. AMS 6415 is the common aerospace 4340 spec.
Crankshafts, drive axles, connecting rods in high-performance engines. 4340's reduced phosphorus/sulfur and nickel toughness give better fatigue life under repeated loads.
Arctic pipelines, off-road equipment for northern climates. 4140's ductile-brittle transition is around -20 °C; 4340's is below -40 °C.
Grade 8 fasteners are typically 4140. Higher-grade aerospace fasteners (GR 9, NAS specs) use 4340.
For 80% of mechanical applications, 4140 is the better buy:
If your part doesn't need aerospace certifications, through-harden huge cross-sections, or service at arctic temperatures — 4140 is almost certainly the right answer.
In the annealed state, both machine reasonably well:
In prehardened condition (28–32 HRC), both are harder but workable:
Above 40 HRC, machining becomes impractical — switch to grinding or EDM for final features.
For precision parts: rough-machine in annealed state (oversize by 0.3–0.5 mm per surface), send out for Q&T heat treat, finish-grind or finish-mill to final dimensions. This yields the best combination of dimensional accuracy and mechanical properties.
Raw bar-stock pricing (approximate 2024 US averages):
| Grade & Condition | $/kg (Ø50 mm round) |
|---|---|
| 4140 annealed | $3.20 |
| 4140 Pre-hard (28–32 HRC) | $3.80 |
| 4340 annealed | $4.10 |
| 4340 Q&T (32–35 HRC) | $4.80 |
| 4340 aerospace (AMS 6415) | $7.50 (full traceability adds cost) |
Stocked sizes at fobproto:
If your spec says "4340" and the application is general-purpose mechanical work (not aerospace, not thick section), consider asking your design engineer if 4140 would work. It's often an unnecessary over-spec that traces back to a decades-old drawing.
Email [email protected] with your drawing. For heat-treated parts, include target HRC and critical-feature tolerances — we coordinate the Q&T cycle and finish machining.
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