HP 9-4-30 Steel
HP 9-4-30 (High-Performance 9% Ni, 4% Co, 0.30% C) is the higher-strength member of the HP 9-4 ultra-high-strength steel family, containing 8.5–9.5% Ni, 4.0–4.5% Co, 0.29–0.34% C with additions of Cr and Mo. It achieves 220,000–250,000 psi tensile strength with fracture toughness (K₁₂) values typically 80–100 ksi√in — placing it among the highest toughness-per-unit-strength alloy steels available for aerospace structural applications.
Like HP 9-4-20, it is uniquely weldable for an ultra-high-strength steel. Gas tungsten arc welding (GTAW) in the annealed condition followed by post-weld heat treatment restores full strength and fracture toughness in the weld zone. This capability for large-scale structural welding at ultra-high-strength levels, combined with its excellent fracture toughness, makes HP9-4-30 one of the most demanding-application structural steels in the aerospace supply chain.
Product Forms & Size Range of HP 9-4-30 Steel
HP 9-4-30 bar and plate per AMS 6426 is available in round bar diameters from 0.750 inch to 14.000 inches and plate thicknesses from 0.250 inch to 6.000 inches. All product is VAR processed. Bar is supplied annealed for welding and machining operations; heat treatment to final strength is performed after all fabrication steps are complete.
HEAT TREATMENT OF HP 9-4-30 STEEL
HP9-4-30 is austenitized at 1475–1525°F (802–829°C), air or oil quenched, and tempered at 400–500°F (204–260°C) for two hours to achieve 220,000–250,000 psi tensile strength. Post-weld heat treatment at 475°F (246°C) for two hours per cycle (double temper) restores fracture toughness in the weld heat-affected zone. The high Ni-Co composition provides an extremely low ductile-to-brittle transition temperature, enabling reliable service in arctic and high-altitude cold-soak environments.
HP 9-4-20 Steel
Lower-carbon variant at 200–220 ksi with maximum weldability — preferred when strength requirements permit the lower carbon grade.
HY-TUF Steel
Silicon-modified ultra-high-strength steel at 220+ ksi with high fracture toughness — used for aircraft structural fittings and landing gear components.
Certified Material for Critical Service
HP 9-4-30 steel is supplied to AMS 6426 (bars and plate, VAR). All material is certified to composition, mechanical properties, and fracture toughness. Welding procedures and filler metals must be qualified to the applicable aerospace or military structural welding specification. Fracture toughness testing per ASTM E399 is required for fracture-critical applications.