D6AC Steel
D6AC is a Ni-Cr-Mo-V ultra-high-strength alloy steel (0.42–0.48% C, 0.90–1.20% Ni, 0.90–1.20% Cr, 0.90–1.10% Mo, 0.05–0.10% V) originally developed for solid-fuel rocket motor cases, where the combination of ultra-high tensile strength and good fracture toughness at light weight is critical. When heat treated to the standard condition, D6AC achieves 220,000–240,000 psi tensile strength with fracture toughness (K₁₂) values of 60–80 ksi√in, providing an excellent combination for damage-tolerant design in thin-wall pressure vessels and structural frames.
D6AC is produced exclusively by vacuum arc remelting (VAR) per AMS 6431 to ensure the high cleanliness required for fracture-critical applications. It can be welded by electron beam, TIG, or MIG processes in the annealed condition, then heat treated after welding — a significant advantage for fabricated rocket motor cases and structural shells that would be impractical to forge as single pieces.
Product Forms & Size Range of D6AC Steel
D6AC bar, forging stock, sheet, plate, strip, and tubing are all available per AMS 6431 as a single multi-form specification. Round bar is available in diameters from 0.500 inch to 12.000 inches; plate in thicknesses from 0.040 inch to 4.000 inches; tubing in a range of outside diameters and walls. All forms are VAR processed for maximum cleanliness.
HEAT TREATMENT OF D6AC STEEL
D6AC is austenitized at 1650°F (899°C), oil or air quenched, and double tempered at 1000°F (538°C) for two hours minimum each cycle. The double temper at 1000°F is critical — it provides the optimum combination of 220 ksi strength and maximum fracture toughness for the rocket motor case application. Lower tempering temperatures are not used because they produce excessive sensitivity to hydrogen embrittlement in thin-wall welded structures.
300M Steel
Silicon-vanadium modified 4340 for landing gear at 280 ksi minimum — the most specified ultra-high-strength steel for commercial and military aircraft landing gear.
H11 Tool Steel
Chromium hot-work tool steel at 280+ ksi with exceptional toughness — used for highly stressed fasteners, dies, and structural components.
Certified Material for Critical Service
D6AC steel is supplied to AMS 6431 (bars, forgings, sheet, strip, plate, and tubing — all VAR processed). Material certifications document full VAR chemistry, mechanical properties, and fracture toughness values. Non-destructive inspection per AMS 2631 (ultrasonic) is typically required for all aerospace product forms.