SAE 4140 Steel
SAE 4140 is a chromium-molybdenum alloy steel (0.38–0.43% C, 0.80–1.10% Cr, 0.15–0.25% Mo) that offers considerably higher hardenability than SAE 4130. In the quenched and tempered condition it achieves ultimate tensile strengths up to 200,000 psi with retained ductility and impact toughness, making it one of the most widely used alloy steels for aerospace bolts, hydraulic actuator bodies, structural fittings, and precision machined components.
The Cr-Mo composition provides deep, consistent hardening in cross-sections up to approximately 3 inches. SAE 4140 machines efficiently in the annealed or normalized condition and responds predictably to heat treatment. Its hardness can be further enhanced by induction hardening for wear-critical surface applications, while the core retains adequate toughness for cyclic loading environments.
Product Forms & Size Range of SAE 4140 Steel
SAE 4140 round bar per AMS 6381 is available in diameters from 0.375 inch to 14.000 inches in the annealed or quenched and tempered condition. Cold-finished bar in diameters up to 3.500 inches is available for direct machining to close tolerances. Forging quality grades are produced with restricted chemistry and ultrasonic inspection per procurement requirements.
Seamless mechanical tubing per AMS 6390 is produced in outside diameters from 0.500 inch to 8.000 inches with a range of wall thicknesses. SAE 4140 tubing is used for hydraulic cylinder barrels, actuator bodies, and structural hollow sections in aircraft ground support equipment and missile launchers.
Sheet, strip, and plate per AMS 6382 are available in thicknesses from 0.020 inch to 4.000 inches. Plate is the predominant form for housings, brackets, and machined fittings. Normalizing prior to plate rolling ensures a uniform grain structure and consistent through-thickness properties.
HEAT TREATMENT OF SAE 4140 STEEL
Austenitize at 1525–1575°F (829–857°C), quench in oil (or polymer for thinner sections), and temper at 400–1100°F (204–593°C) to the required strength level. Tempering at 400°F produces approximately 200 ksi UTS; at 1000°F, approximately 130 ksi with significantly improved toughness. Stress relief at 1000°F after rough machining minimizes dimensional movement during finish cuts. For case-hardening applications, SAE 4140 can be nitrided or induction hardened after final heat treatment.
SAE 4130 Steel
Lower-carbon Cr-Mo alloy with superior weldability for airframe structures and engine mounts — the weldable structural counterpart to 4140.
SAE 4150 Steel
Higher-carbon Cr-Mo for maximum hardness in heavily loaded gears and shafts where 4140 strength levels are insufficient.
Certified Material for Critical Service
SAE 4140 steel is supplied to AMS 6381 (bars, forgings, mechanical tubing), AMS 6382 (sheet, strip, plate), AMS 6390 (seamless tubing), and MIL-S-5626 (bars, forgings). Mill certification documents composition and mechanical properties. Procurement to restricted hardenability (H-band) is available where tight property consistency is required.