4340 Air Melt Steel
SAE 4340 is a nickel-chromium-molybdenum alloy steel (1.65–2.00% Ni, 0.70–0.90% Cr, 0.20–0.30% Mo, 0.37–0.44% C) and is among the most widely used high-strength structural steels in aerospace history. The air-melted grade governed by AMS 6415 and AMS 6484 develops ultimate tensile strengths of 200,000–260,000 psi in the quenched and tempered condition, combined with good fracture toughness and excellent fatigue resistance — a combination that has made it the benchmark structural steel for aircraft frames, landing gear, and missile cases for over five decades.
The air-melt designation distinguishes it from vacuum-processed grades (4340 VAR, governed by AMS 6414). Air melt is appropriate for the majority of aerospace structural applications; VAR processing is required only where maximum cleanliness and fatigue life in premium grade forgings are mandatory. Both grades share the same nominal composition; only the melt practice and resulting inclusion content differ.
Product Forms & Size Range of 4340 Air Melt Steel
4340 Air Melt round bar and forging stock per AMS 6415 is available in diameters from 0.500 inch to 16.000 inches. Bar is supplied annealed for machining, or normalized and tempered per specific strength requirements. Large-diameter bar and billet for forging is a primary product form, supporting the extensive use of 4340 in aerospace structural forgings.
Mechanical tubing in 4340 Air Melt per AMS 6415 is available in outside diameters and wall thicknesses per customer specification. Tubing is used for actuator bodies, hydraulic cylinders, and structural hollow sections in high-load aerospace applications. Cold-drawn tubing in smaller sizes is available for precision machined components.
HEAT TREATMENT OF 4340 AIR MELT STEEL
Austenitize at 1500–1550°F (816–843°C), oil quench, and double temper at 325–600°F (163–316°C) for strengths of 220,000–260,000 psi, or at 700–1000°F (371–538°C) for 200,000–220,000 psi with improved fracture toughness. The double temper cycle is mandatory for critical aerospace applications to eliminate retained austenite and fully relieve martensite transformation stresses. Normalizing at 1600°F (871°C) prior to hardening produces a uniform grain structure that improves both mechanical property consistency and fatigue resistance.
300M Steel
Silicon-vanadium modified 4340 with 280 ksi minimum tensile strength and superior fracture toughness — the premium landing gear structural steel.
SAE 4335 Steel
Higher-Ni Cr-Mo alloy for large-section forgings where 4340 hardenability is insufficient to develop full strength through the cross-section.
Certified Material for Critical Service
4340 Air Melt steel is supplied to AMS 6415 (standard bar and forging) and AMS 6484 (bars and forgings, alternate condition). Both specifications require certified mill test reports documenting chemistry and mechanical properties per heat and lot. Magnetic particle inspection and ultrasonic testing are typically required for forged components per applicable NAS or AMS NDT specifications.