AMS 6415
AMS 6415 is the SAE Aerospace Material Specification for SAE 4340 air-melted nickel-chromium-molybdenum alloy steel in the bar, forging, and mechanical tubing product forms. It is the most widely specified and broadly used high-strength alloy steel specification in the aerospace supply chain. The 4340 composition (1.65–2.00% Ni, 0.70–0.90% Cr, 0.20–0.30% Mo, 0.37–0.44% C) develops tensile strengths of 200,000–260,000 psi in the quenched and tempered condition with retained fracture toughness and fatigue resistance that has made it the benchmark structural alloy steel for aircraft primary structure for over fifty years.
The “air melt” designation distinguishes AMS 6415 from the vacuum-arc-remelted (VAR) grade governed by AMS 6414. Air melt is the standard for the majority of structural applications; VAR is reserved for the most demanding fracture-critical applications where maximum cleanliness and fatigue life are required. Both share the same nominal 4340 composition and mechanical property requirements; only melt practice and resulting inclusion level differ.
AMS 6415 material heat treated to 260,000 psi (the maximum standard strength condition) achieves minimum 260,000 psi UTS, 217,000 psi yield, 8% elongation, and 35% reduction of area. Fracture toughness (K₁₂) at this strength is typically 50–60 ksi√in for air-melt material. The double temper at 325–400°F is standard for landing gear and primary structural applications to ensure complete retained austenite transformation. For strengths of 200,000–220,000 psi, fracture toughness increases to 70–90 ksi√in with significantly improved damage tolerance.
- Aircraft landing gear main struts and drag links
- Wing carry-through box and fuselage primary structure forgings
- Helicopter airframe primary structural members
- Missile airframe primary structural forgings
- Naval ordnance gun barrel forgings and housing bodies
- Aerospace actuator bodies and cylinder forgings
AMS 6484 — 4340 Air Melt Bar (Alt)
Alternate 4340 air melt bar specification for different heat treatment conditions and procurement requirements.
AMS 6257 — 300M Bar
Silicon-vanadium modified 4340 at 280 ksi minimum — the premium landing gear structural steel where 4340 fracture toughness is insufficient.
Certified Material for Critical Service
AMS 6415 certifications include chemical composition per heat, mechanical properties per lot (tensile, yield, elongation, reduction of area, Charpy impact, and hardness), melt practice documentation (air melt), grain size, and ultrasonic inspection per AMS 2631 for fracture-critical forging applications. Magnetic particle inspection per AMS 2641 or ASTM E1444 is typically required for forged components. Full heat-lot traceability is maintained through all processing steps per AS9100.