Where are high-strength steels used in aircraft structures?

High-strength steels are used where concentrated loads require high strength in minimum volume. Applications: Landing gear - Main and nose gear (300M, HP9-4-30), requires very high strength (~280 ksi UTS), toughness, and fatigue resistance; Flap tracks - High stress, wear resistance needed; Fasteners - High-strength bolts (H-11, 4340); Engine mounts - High concentrated loads; and Actuator components - Pistons, cylinders. Properties: UTS 180-300 ksi depending on alloy and heat treatment, High hardness for wear resistance, Susceptible to hydrogen embrittlement (requires careful processing), and Requires corrosion protection (cadmium plating being replaced). Alloys: 4340 - General purpose, moderate strength; 300M - Landing gear standard, excellent properties; Aermet 100 - High toughness at high strength; HP9-4-30 - Alternative to 300M. Steel represents ~5-10% of airframe weight but is essential for highly loaded components. Fatigue and SCC are primary concerns.