HP 9-4-20 Steel
HP 9-4-20 (High-Performance 9% Ni, 4% Co, 0.20% C) is a cobalt-nickel ultra-high-strength alloy steel developed by the U.S. Air Force for large weldable aerospace structural components. With 8.5–9.5% Ni, 4.0–4.5% Co, 0.17–0.23% C, and additions of Cr and Mo, it achieves 200,000–220,000 psi tensile strength while retaining outstanding weldability without preheat in the annealed condition — a combination that distinguishes it from other ultra-high-strength steels and makes it suitable for large fabricated structures.
The high nickel content provides excellent fracture toughness (K₁₂ typically 90–120 ksi√in) and near-zero ductile-to-brittle transition temperature, allowing use in cold environments where conventional high-strength steels would be unacceptably brittle. HP 9-4-20 is a lower-strength variant of the HP 9-4 family, providing improved weldability over the 0.30% carbon HP 9-4-30 grade at a modest reduction in strength.
Product Forms & Size Range of HP 9-4-20 Steel
HP 9-4-20 bar and plate per AMS 6425 is available in round bar diameters from 0.750 inch to 12.000 inches and plate thicknesses from 0.250 inch to 5.000 inches. All product forms are produced by vacuum arc remelting (VAR) for the cleanliness required in aerospace fracture-critical applications. Bar is supplied in the annealed condition for machining and welding.
HEAT TREATMENT OF HP 9-4-20 STEEL
HP 9-4-20 is austenitized at 1500–1525°F (816–829°C), oil or air quenched, and tempered at 400–500°F (204–260°C) for two hours to achieve 200,000–220,000 psi. Post-weld heat treatment after fabrication welding is performed at 1000°F (538°C) to relieve welding stresses and restore fracture toughness in the weld heat-affected zone. The excellent post-weld response to heat treatment is one of the key advantages of the HP 9-4 alloy family over other ultra-high-strength steels.
HP 9-4-30 Steel
Higher-carbon variant achieving 220–250 ksi — greater strength than HP 9-4-20 with retained fracture toughness and weldability.
300M Steel
Si-V modified 4340 at 280 ksi — the standard ultra-high-strength steel for aircraft landing gear worldwide.
Certified Material for Critical Service
HP 9-4-20 steel is supplied to AMS 6425 (bars and plate, VAR). Certified mill test reports document composition including cobalt and nickel, mechanical properties, and fracture toughness values. Welding procedures must be qualified per applicable military or aerospace welding specifications.