AMS 6257
AMS 6257 is the SAE Aerospace Material Specification for 300M alloy steel in the bar and forging product forms — the standard-grade (air-melted) version of this ultra-high-strength landing gear steel. 300M is a silicon (1.45–1.80%) and vanadium (0.05–0.10%) modified derivative of 4340 that achieves a minimum ultimate tensile strength of 280,000 psi with retained fracture toughness (K₁₂ typically 50–70 ksi√in) that significantly exceeds plain 4340 at comparable strength levels. The higher silicon content enables tempering at 600°F minimum — well above the tempered martensite embrittlement range — providing critical resistance to hydrogen-assisted delayed fracture in landing gear components processed with cadmium plating.
Bar and forging stock to AMS 6257 is supplied in the annealed condition for rough machining and forging, then heat treated after completion of all material-removal operations to minimize heat treat distortion. AMS 6257 is the standard specification for commercial and military aircraft landing gear forgings worldwide, used on programs from the Boeing 737 family to the F-15 and F-18 fighters.
AMS 6257 requires a minimum UTS of 280,000 psi with minimum 8% elongation and 30% reduction of area in the standard heat treatment condition. The minimum 600°F temper temperature specified in AMS 6257 is a critical safety requirement — it ensures the steel passes the hydrogen embrittlement resistance test per ASTM F519 Method 1a, which simulates the hydrogen charging experienced during electroplating operations. Fracture toughness testing per ASTM E399 is typically required by landing gear procurement drawings to verify compliance with damage-tolerant design allowables.
- Commercial and military aircraft landing gear main fittings and struts
- Landing gear drag struts, side struts, and torque arms
- Hydraulic actuator cylinders for landing gear retraction
- Arresting hook assemblies for carrier-based aircraft
- Helicopter skid landing gear components
Certified Material for Critical Service
AMS 6257 certifications include chemical composition per heat (with explicit silicon and vanadium content), melt practice (air melt), mechanical properties per lot at minimum 280,000 psi strength level, fracture toughness where specified, hydrogen embrittlement resistance testing per ASTM F519, and non-destructive inspection per AMS 2631 (UT) and AMS 2641 (MT). All certifications must include the specific tempering temperature used, which must be 600°F minimum per AMS 6257 requirements.