300M — 4340 modified with V+Si for Rm 1930-2070 MPa. The standard landing gear steel for large commercial aircraft. Better fracture toughness than standard 4340 at the same ultra-high-strength level.
300M (UNS K44220 / AMS 6257) per AMS 6257. Silicon-vanadium modified 4340 — Si (1.45-1.80%) raises Rm without reducing toughness; V (0.05-0.10%) refines grain. Results in Rm 1930-2070 MPa at significantly better fracture toughness than 4340 at same strength level. THE standard material for large commercial aircraft main landing gear (Boeing 777, 787, A330, A380 main gear legs) and military aircraft landing gear globally.
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