300M Steel
300M is an ultra-high-strength alloy steel derived from SAE 4340 by the addition of silicon (1.45–1.80%) and vanadium (0.05–0.10%). These additions raise the minimum tensile strength to 280,000 psi while markedly improving fracture toughness (K₁₂ typically 50–70 ksi√in) and resistance to hydrogen embrittlement relative to unmodified 4340 at comparable strength. 300M has become the primary material for modern commercial and military aircraft landing gear main fittings, struts, and actuator cylinders worldwide.
The silicon addition to 300M serves a critical role: it retards carbide coagulation during tempering, allowing the steel to be tempered at higher temperatures than plain 4340 while maintaining higher ultimate tensile strength. This means 300M can be heat treated to 280 ksi with a tempering temperature high enough (600°F / 316°C) to minimize hydrogen-assisted delayed fracture — a critical safety advantage for landing gear components exposed to cadmium plating or other hydrogen-generating processes.
Product Forms & Size Range of 300M Steel
300M bar and forging stock per AMS 6257 is available in round bar diameters from 0.750 inch to 20.000 inches and as large-section billet for forging. Standard grade (AMS 6257) is air-melted; premium grade (AMS 6419) is vacuum arc remelted (VAR) for maximum cleanliness and fracture toughness in the most critical applications. Bar is supplied in the annealed condition for rough machining before heat treatment to final strength.
HEAT TREATMENT OF 300M STEEL
300M is austenitized at 1575°F (857°C), oil quenched, and double tempered at 600°F (316°C) minimum. The 600°F minimum temper temperature is specified in AMS 6257 to ensure the steel passes hydrogen embrittlement resistance testing per ASTM F519 — critical for landing gear components that will be cadmium plated or otherwise processed in hydrogen-generating environments. The double temper is mandatory to eliminate retained austenite and fully stabilize the microstructure. Final tensile strength is 280,000 psi minimum, 300,000 psi typical.
D6AC Steel
Ni-Cr-Mo-V ultra-high-strength structural steel achieving 220–240 ksi — used for missile cases and aircraft structural frames where 300M exceeds the required strength.
4340 Air Melt Steel
The parent alloy of 300M — Ni-Cr-Mo structural steel for aircraft primary structure at 200–260 ksi.
Certified Material for Critical Service
300M steel is supplied to AMS 6257 (air melt bar and forging) and AMS 6419 (VAR bar and forging, premium). Both specifications require minimum 280,000 psi UTS with fracture toughness and hydrogen embrittlement resistance testing. Full traceability and certified mill test reports are required for all landing gear applications per applicable FAA-approved data or military drawing requirements.