Aircraft landing gear
Pistons, axles, struts, torque links. The deep hardenability + toughness combination is essentially required for these high-cycle, high-stress parts.
Ni-Cr-Mo alloy steel · UNS G43400
4340 Alloy Steel
High-performance nickel-chromium-molybdenum alloy steel. The 1.65–2.00% nickel addition over 4140 gives 4340 deeper hardenability, higher toughness, and excellent fatigue resistance. The aerospace landing-gear and high-stress shaft standard.
UTS (Q&T)
1280 MPa
Density
7.85 g/cc
Modulus
200 GPa
Mach.
50 /100
When to use
Pick 4340 when 4140 isn’t deep-hardening or tough enough.
4340 is essentially 4140 with extra nickel. That nickel does two things: deepens hardenability (you can through-harden much thicker sections than 4140) and substantially improves toughness at high hardness. The combination puts 4340 in landing gear, helicopter rotor shafts, large gears, and torsion bars where 4140 would either be brittle at high hardness or fail to harden through.
Cost: roughly 1.8× the price of 4140 by weight. Often justified for aerospace structural applications; rarely justified for general industrial work where 4140 is plenty strong. Vacuum-melted 4340 (4340M, AISI 300M) is a step further — premium-quality, used in critical aircraft components.
Strengths
Why aerospace specifies it.
- Deep hardenabilityThrough-hardens to 100 mm+ section size. 4140 starts to lose hardness in the core above ~30 mm.
- Toughness at high strengthCharpy V-notch roughly 15–30 J at ~50 HRC. 4140 at the same hardness is roughly half that.
- Excellent fatigueStandard for cyclically loaded aerospace shafts and landing gear pistons. Fatigue limit ~580 MPa.
- Wide Q&T rangeUseful at hardness 28 HRC (~890 MPa) up through 55 HRC (2000 MPa). One alloy covers a lot.
- Vacuum-melt gradesAMS 6414 vacuum-melted is premium quality with controlled inclusions — landing gear standard.
Trade-offs
What you give up.
- Will rust like any alloy steelPlate, paint, or oil. Bare 4340 corrodes in days in humidity.
- Slightly worse machinability than 4140Mach 50/100 vs 4140’s 65/100. The nickel toughness comes through as tougher chips.
- Cost premium over 4140Roughly 1.8× the price by weight. Stock cost matters more on volume parts.
- Hydrogen embrittlementSame risk as 4140 at high hardness. Bake-out after acid pickling or plating required at ≥1000 MPa tensile (≈31 HRC) per ASTM B850.
- Welding requires preheat + post-weld HTWeldable but always with controlled preheat (~250 °C) and post-weld stress relief. Not for casual welding.
Specs
Mechanical, physical & chemical data.
Mechanical values shown for Q&T to 38–42 HRC (UTS ~1280 MPa) — a common landing-gear and shaft condition. Higher hardness Q&T values follow.
Mechanical
Properties (Q&T 38-42 HRC)
| Ultimate tensile strength | 1280 MPa (185 ksi) |
|---|---|
| Yield strength (0.2% offset) | 1100 MPa (160 ksi) |
| Elongation at break | 13% |
| Reduction of area | 45% |
| Modulus of elasticity | 200 GPa (29.0 Msi) |
| Shear modulus | 76 GPa (11.0 Msi) |
| Hardness (Q&T) | 38–42 HRC |
| Charpy V-notch impactat 21 °C | 75 J |
| Endurance limit10⁷ cycles, R=-1 | 580 MPa |
Q&T to 50 HRC: UTS 1750 MPa, YS 1500 MPa, elongation 10%. Annealed: UTS 745 MPa, YS 470 MPa, elongation 22%.
Physical & thermal
At room temperature
| Density | 7.85 g/cc (0.284 lb/in³) |
|---|---|
| Melting range | 1416 – 1455 °C |
| Specific heat | 0.475 J/g·°C |
| Thermal conductivity | 44.5 W/m·K |
| CTE (20–100 °C) | 12.3 µm/m·°C |
| Electrical resistivity | 0.248 µΩ·m |
| Magnetic | Ferromagnetic |
| Critical temp (Ac1) | 725 °C |
Physical properties essentially identical to 4140 — the difference is metallurgical (hardenability) not thermal.
Composition
Chemistry, weight %
| Carbon (C) | 0.38 – 0.43 |
|---|---|
| Nickel (Ni) | 1.65 – 2.00 |
| Chromium (Cr) | 0.70 – 0.90 |
| Molybdenum (Mo) | 0.20 – 0.30 |
| Manganese (Mn) | 0.60 – 0.80 |
| Silicon (Si) | 0.15 – 0.35 |
| Phosphorus (P) | 0.035 max |
| Sulfur (S) | 0.04 max |
| Iron (Fe) | balance |
The 1.65–2.00% Ni is the defining difference vs 4140. Vacuum-melted (E-grade) variants tighten S and P limits significantly.
Specifications
Common purchase specs.
AMS 6414
Bar, forgings, tubing — vacuum (VAR) remelted (premium aerospace)
AMS 6415
Bar, normalized & tempered
AMS 6359
Sheet, plate, strip
ASTM A29
Hot-rolled bar — general purpose
ASTM A322
Alloy steel bar
MIL-S-5000
Steel — chrome-nickel-molybdenum
AISI 4340 / SAE 4340
Standard designation
Machining
Machining considerations.
Machine in annealed or normalized condition, heat-treat to final hardness, then finish-grind. The annealed-machine / Q&T / grind-finish workflow is standard for high-tolerance 4340 parts. Through-hardened 4340 above 45 HRC is grind-only territory.
Cutting speed
150–250 SFM with carbide on annealed/normalized. Drop to 120–160 SFM at 38–42 HRC Q&T.
Feed rate
Aggressive constant feed. Nickel-toughness chips are stringy — chip-breaker geometry helps.
Tooling
Coated carbide (TiAlN) standard. CBN inserts for finishing at hardness above ~50 HRC.
Coolant
Flood. Through-spindle high-pressure for production turning.
Annealed first when possible
Rough and semi-finish in annealed (~217 HB). Q&T after, then finish-grind to size.
Climb mill
Same rule as 4140 — conventional milling generates heat that work-hardens the surface.
Allow for HT growth
Q&T from annealed grows ~0.001-0.002″ per inch in random directions. Symmetric machining helps predict it.
Grind, don’t mill, post-HT
Above ~45 HRC, finish operations should be grinding. Carbide handles up to ~50 HRC; CBN handles harder.
Applications
Where 4340 earns its premium.
Helicopter rotor shafts
Main rotor shafts, swashplate components. Fatigue resistance at 1100 MPa yield is hard to beat.
Large gears & shafts
Heavy-duty drive shafts, marine propulsion, mining equipment. Sections too large to through-harden in 4140.
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