Overview
4340 is the premium aerospace alloy steel — the workhorse for landing gear forgings, high-strength fasteners, drive shafts, and gun barrels. The chemistry adds ~1.8% nickel to the 4140 Cr-Mo base, which buys three improvements:
- Better toughness — Charpy impact at equivalent strength is significantly higher than 4140. The nickel suppresses cleavage fracture.
- Deeper hardenability — through-hardens to ~150 mm in oil quench versus ~75 mm for 4140. Critical for large forgings.
- Better cryogenic toughness — usable down to ~-60°C versus ~-40°C for 4140.
The cost is significant — 4340 runs ~2× the price of 4140 due to nickel content. Specify 4340 only when its toughness, hardenability, or cryogenic properties are needed.
The selection logic:
| Need | Choose |
|---|---|
| Light through-hardening (<\15 mm) | 4140 |
| Heavy through-hardening (>75 mm) | 4340 |
| Aerospace landing gear | 4340 |
| Industrial shafts (cost-sensitive) | 4140 |
| Cryogenic structural | 4340 |
| Maximum strength fasteners (>1500 MPa UTS) | 4340 |
| Carburizing alternative | 8620 (lower-cost carburizing alloy) |
| Welded structure | 4130 (better weldability) |
Heat treatment — the design space
Same temperature-temper relationship as 4140, but through-hardens much deeper due to nickel:
| Temper | UTS | Yield | Elong. | Hardness | Use |
|---|---|---|---|---|---|
| Annealed | 110 ksi | 70 ksi | 22% | 220 HB | Machining stock |
| Normalized | 185 ksi | 125 ksi | 12% | 363 HB | Pre-Q&T |
| Q&T 1200°F (650°C) | 150 ksi | 130 ksi | 18% | 32 HRC | High toughness |
| Q&T 1100°F (600°C) | 180 ksi | 160 ksi | 16% | 38 HRC | Landing gear standard |
| Q&T 1000°F (540°C) | 220 ksi | 200 ksi | 14% | 47 HRC | High strength |
| Q&T 800°F (425°C) | 250 ksi | 225 ksi | 12% | 52 HRC | Very high strength |
| Q&T 400°F (205°C) | 290 ksi | 265 ksi | 8% | 55 HRC | Maximum strength |
Through-hardening capability is the headline 4340 advantage:
| Section | Recommended Q&T alloy |
|---|---|
| <\15 mm | 4140 (cost-effective) |
| 25–75 mm | 4140 oil quench |
| 75–150 mm | 4340 oil quench |
| 150–250 mm | 4340 (with quench control) |
| >250 mm | 4340 modified (vacuum-melted, AMS 6414) |
Aerospace heat treatment specifications
For aerospace-grade 4340, the AMS specifications control more than the temper temperature:
- AMS 6414 — premium aircraft quality 4340 modified, vacuum- arc remelted (VAR) for inclusion control. The landing gear standard.
- AMS 6415 — aircraft quality 4340, air-melted. Used for less-critical aerospace applications.
- AMS 6359 — 4340 sheet and strip.
- AMS-S-5000 — 4340 forgings with controlled grain flow.
VAR-melted 4340 has dramatically better fatigue behavior than air-melted material due to reduced inclusion content. Critical for landing gear and other high-cycle fatigue service.
Machining notes
Annealed 4340 machines reasonably — slightly harder than 4140 in any equivalent condition:
- Coated carbide (TiAlN preferred); HSS for low-volume
- Speed: 60–150 SFM annealed; 40–100 SFM Q&T
- Feed: 0.005–0.020 in/rev
- Cutting fluid recommended
- Tool life ~50–60% of A36
For Q&T 4340 above 40 HRC, machining gets specialized:
- CBN or ceramic tooling
- Light depths of cut
- Very low feeds
- Most production above 45 HRC uses grinding instead of machining
The aerospace machining sequence for landing gear:
- Forge to near-net shape (controlled grain flow per AMS-S-5000)
- Anneal the forging
- Rough machine to leave grinding stock (~2–5 mm per surface)
- Heat treat to target strength + temper
- Stress relieve if needed
- Finish grind to final tolerance
- Shot peen fatigue-critical surfaces
- Hard chrome plate wear surfaces
- Cadmium plate / chrome-free corrosion protection
- Paint outer surfaces
Welding considerations
4340 is weldable but requires more care than 4140:
- Preheat to 250–350°C before welding sections above 12 mm
- Low-hydrogen electrodes (E11018-M, E12018-M) mandatory
- Slow cooling under insulation
- PWHT at 600–650°C for any structural welds
- For Q&T 4340 assemblies, PWHT essentially mandatory — weld zone is over-quenched and brittle without
In aerospace practice, welded 4340 structure is typically avoided — landing gear and high-stress fittings are forged or single-piece machined parts. Welding introduces a potential weak link and inspection burden that aerospace designers prefer to eliminate.
For welded Q&T 4340 assemblies (rare):
- Machine components in annealed condition
- Weld assembly with preheat and low-hydrogen practice
- Heat treat complete assembly to Q&T
- Finish machine to final tolerances
Corrosion considerations
Same baseline as 4140 — none. The nickel content doesn’t provide stainless behavior. 4340 rusts in any moist environment.
Standard aerospace corrosion protection for 4340:
- Cadmium plate (legacy) — sacrificial coating, hydrogen embrittlement risk requires post-plate bake-out at 200°C / 4 hours
- Zinc-nickel plating (modern replacement for cadmium) — chrome-free alternatives gaining adoption per ROHS pressure
- Hard chrome on wear surfaces (cylinder rods)
- Chromate conversion as paint primer
- Polyurethane topcoat
- Sealant in faying surfaces
For marine aircraft (naval) service, additional measures:
- Corrosion-resistant primer (epoxy) over chromate
- Saltwater drainage design
- Periodic inspection and re-coating
- Sealant maintenance in fastener holes
Hydrogen embrittlement — the key 4340 service hazard
4340 in high-strength temper (>1200 MPa UTS / >35 HRC) is highly susceptible to hydrogen embrittlement from:
- Pickling acids during surface preparation
- Electroplating (cadmium, zinc, nickel)
- Cathodic protection systems
- Hydrogen-containing service environments
- Welding without low-hydrogen practice
Mandatory mitigations:
- Post-plate bake-out at 200°C for 4 hours within 4 hours of plating completion
- Re-bake at 200°C for 23 hours within 23 hours of any cathodic-protected service
- Low-hydrogen welding practice — moisture-controlled electrode storage, dry shielding gas
- Limit maximum hardness for chloride-exposed service (typically <\15 HRC for marine aerospace)
The aerospace failure mode is delayed brittle fracture — hydrogen-loaded high-strength steel cracks under static load hours or days after the loading begins. Multiple historical aerospace failures traced to insufficient post-plating bake-out.
Applications by industry
- Aerospace landing gear — main landing gear cylinder forgings, trunnion forgings, drag braces. The canonical 4340 use. Always vacuum-arc remelted (AMS 6414) for fatigue critical service.
- Aerospace structural fittings — engine mounts, wing-fuselage attachment, high-stress brackets. Premium aircraft quality.
- Heavy-duty fasteners — aerospace bolts, high-strength industrial bolts (ASTM A490, A354 BD).
- Defense — gun barrels (M1 Abrams main gun, naval gun barrels, artillery tubes), ammunition cases, military vehicle structural.
- Heavy automotive / racing — race engine crankshafts and connecting rods, heavy truck drive shafts and axles.
- Oil and gas — drill collars, downhole tool components, high- pressure piping.
- Marine propulsion — large vessel propeller shafts, gearbox shafts, rudder posts.
- Industrial machinery — heavy-duty shafts, rolling mill rolls, hydraulic press components.
- Power generation — turbine shafts (some grades), generator rotor components.
Failure modes worth designing around
Hydrogen embrittlement is the dominant 4340 in-service failure mode in high-strength conditions. Aerospace QA programs include mandatory bake-out, hardness restrictions for chloride service, and periodic inspection. Field failures occur when bake-out is skipped or insufficient.
Stress corrosion cracking in chloride environments at high strength. Aerospace specifications restrict maximum hardness for chloride-exposed components — landing gear in naval aircraft service typically tempered to ~38 HRC max rather than 50+ HRC.
Atmospheric corrosion without coating — same as A36. Standard aerospace protective stack mandatory.
Decarburization during heat treatment without protective atmosphere — reduces surface fatigue strength on shafts and rods. Critical for landing gear service. Use protective atmosphere (endothermic gas) or salt bath; alternatively, leave grinding stock and remove decarb after heat treatment.
Quench cracking in thick sections during water quench — 4340’s high hardenability creates significant crack risk. Standard aerospace practice: oil quench only (water quench rarely used). For very large landing gear forgings, polymer quench (controlled cooling rate) is sometimes specified.
Temper embrittlement at 350–550°C during slow cooling from higher temper temperatures. Always temper above 600°C or below 350°C, and cool rapidly through the embrittlement range. Critical for impact-toughness-sensitive aerospace service.
Fatigue at machined fillets, threads, and surface defects — 4340 in fatigue service is sensitive to surface finish. Mandatory mitigations:
- Polish fatigue-critical surfaces to 16 Ra or better
- Generously radius all stress concentrations
- Shot peen for compressive surface residual stress
- Cold-expand fastener holes (Fatigue Technology Split-Sleeve)
- VAR-melted material for high-cycle service
Welded structural service without proper preheat + PWHT — HAZ cracking and reduced strength. Aerospace standard practice: avoid welded 4340 structure for primary load paths; use forgings or machined parts. For maintenance repair of welded 4340, follow rigorous procedure with preheat, low-hydrogen practice, and PWHT.
Specifying 4340 when 4140 is adequate is a common designer error — the 2× cost premium is justified only by toughness, deep through-hardening, or cryogenic requirements. For sections under 75 mm requiring through-hardening, 4140 is the cost- effective choice.
Specifying air-melted 4340 for fatigue-critical aerospace applications — air-melted 4340 has higher inclusion content and significantly worse fatigue behavior than vacuum-arc remelted (VAR) AMS 6414. For landing gear and other high-cycle fatigue service, VAR-melted material is essentially mandatory.