AMS 6431
AMS 6431 is the SAE Aerospace Material Specification for D6AC nickel-chromium-molybdenum-vanadium ultra-high-strength alloy steel, covering bar, sheet, plate, strip, forgings, and mechanical tubing in a single specification. All product forms covered by AMS 6431 are produced by vacuum arc remelting (VAR) as a mandatory requirement — reflecting the fracture-critical nature of D6AC applications in rocket motor cases and aircraft structural shells where non-metallic inclusions are not acceptable as fatigue crack initiation sites. D6AC achieves 220,000–240,000 psi tensile strength with fracture toughness (K₁₂) of 60–80 ksi√in at double-temper 1000°F condition.
A unique characteristic of AMS 6431 D6AC is its weldability in the annealed condition: components can be gas tungsten arc welded (GTAW) in the annealed state, then heat treated after welding to achieve full strength in both the parent metal and the weld zone. This allows economical fabrication of large-diameter rocket motor cases from rolled and welded cylinder sections rather than requiring monolithic forgings — a key reason D6AC was chosen for the Minuteman and Peacekeeper missile motor cases.
AMS 6431 requires minimum 220,000 psi UTS at the double-temper 1000°F condition, with minimum 11% elongation and 40% reduction of area. Fracture toughness minimum of 60 ksi√in (K₁₂) is commonly specified by procurement drawings. Charpy V-notch at the 220 ksi condition is typically 35–50 ft-lb. The double-temper at 1000°F is critical — tempering at lower temperatures produces the 240 ksi condition but significantly reduced fracture toughness, while the 1000°F double temper provides the optimal combination for the rocket motor case pressure vessel application.
- Minuteman, Trident, Peacekeeper ICBM solid rocket motor cases
- Aircraft structural primary frames and fuselage barrel sections
- Aerospace pressure vessels and accumulator shells
- Aircraft ordnance body forgings requiring weldability
- Hypersonic vehicle structural skins and frames
AMS 6257 — 300M Bar
300M Si-V modified 4340 at 280 ksi minimum — the standard landing gear steel for commercial and military aircraft worldwide.
AMS 6487 — H11 Bar/Forging
Chromium hot-work tool steel at 280+ ksi with outstanding toughness for aerospace fasteners and structural forgings.
Certified Material for Critical Service
AMS 6431 certifications include chemical composition per VAR heat, melt practice documentation (VAR required), mechanical properties per lot (tensile, yield, elongation, reduction of area, Charpy), fracture toughness where specified, dimensional inspection, and non-destructive inspection per AMS 2631 (UT) and applicable magnetic particle or dye penetrant inspection standards. Welding procedure qualifications must be documented per the applicable military or aerospace welding specification. Full heat-lot traceability is maintained.