4340

4340 refers to AISI 4340, a nickel-chromium-molybdenum alloy steel known for a combination of high strength, toughness, and hardenability. It is widely used in components that must withstand high loads and fatigue, such as shafts, gears, and other critical parts in aerospace and automotive applications.

Composition and variants

The alloy typically contains about 0.40% carbon, with significant alloying additions of nickel (around 1.65–2.00%), chromium (approximately 0.70–0.90%), and molybdenum (about 0.20–0.30%), along with smaller amounts of manganese and silicon. Variants exist, including vacuum-melted forms such as 4340M, which aim to improve cleanliness and uniformity for demanding applications.

Properties and heat treatment

4340 offers high hardenability and toughness, enabling high strength after appropriate heat treatment. It is commonly solution-annealed or austenitized and then oil-quenched, followed by tempering to achieve target properties. The material is well-suited to surface-hardening processes like carburizing or induction hardening, which produce a hard exterior while retaining a tough core. In the annealed state it is relatively machinable for a steel of its class, but machining quality decreases after hardening.

Applications

Due to its combination of strength and toughness, 4340 is used for gears, shafts, aircraft main/landing gear components, crankshafts, connecting rods, and other high-stress parts. It is frequently chosen for forgings and bars intended for case hardening or full hardening, and it serves in both aerospace and automotive sectors, as well as other heavy industries requiring reliable performance under cyclic loading.

Overview

SAE/AISI 4340 remains a standard reference alloy in discussions of high-strength alloy steels, balancing machinability, weldability (with proper precautions), and strong performance when heat-treated.