AMS 6411
AMS 6411 is the SAE Aerospace Material Specification for SAE 4330 Modified (commonly designated 4330V or 4330M), a nickel-chromium-molybdenum-vanadium alloy steel in the bar, forging, and mechanical tubing product forms. The composition (1.65–2.00% Ni, 0.70–0.90% Cr, 0.35–0.45% Mo, 0.28–0.34% C, 0.03–0.10% V) provides improved fracture toughness and fatigue crack growth resistance compared to unmodified Ni-Cr-Mo grades at equivalent tensile strength levels of 180,000–220,000 psi.
The vanadium addition is the defining characteristic of the AMS 6411 alloy: vanadium forms fine carbides that pin austenite grain boundaries during heat treatment, producing a finer martensite microstructure with substantially improved fracture toughness (K₁₂ typically 15–20% higher than 4340 at 200 ksi). This improvement has made AMS 6411 the primary specification for damage-tolerant aircraft landing gear structural forgings in programs governed by MIL-STD-1530 fracture mechanics requirements.
AMS 6411 material heat treated to 200,000 psi UTS develops minimum fracture toughness (K₁₂) values of 80–100 ksi√in, compared to 60–70 ksi√in for 4340 at the same strength. Double tempering is mandatory per AMS 6411 to ensure complete retained austenite transformation and full microstructural stabilization. VAR processing is available for the highest cleanliness requirements in the most fracture-critical landing gear applications.
- Aircraft landing gear main fitting forgings
- Helicopter rotor head and dynamic component forgings
- Aircraft structural primary frame forgings
- Arresting hooks and landing wire tensioner bodies
- Actuator piston rods and cylinder bodies
- Missile airframe primary structural forgings
Certified Material for Critical Service
AMS 6411 certifications include chemical composition per heat (with explicit reporting of vanadium content), mechanical properties including tensile strength, yield strength, elongation, reduction of area, and Charpy impact energy per lot, and fracture toughness (K₁₂) where specified by the procurement drawing. Melt practice (air melt vs. VAR) must be documented. Full traceability by heat and lot is maintained through all processing steps.