AISI/SAE 4340 is a low-alloy steel built around a nickel-chromium-molybdenum combination that gives it one of the best strength-to-toughness balances of any commonly machined steel grade. The nickel content improves core toughness and impact resistance even in large sections, while chromium and molybdenum contribute hardenability and elevated-temperature strength. Carbon content sits in a similar range to other Cr-Mo alloy steels (roughly 0.38–0.43%), but the added nickel is what sets 4340 apart, letting it achieve high tensile strength without becoming brittle.

Because it hardens deeply and uniformly through thick cross-sections, 4340 is a common choice where a part must survive heavy cyclic loading or shock without failing — aircraft landing gear components, crankshafts, high-strength bolts, gears, axles, and other critical structural and drivetrain parts. It is typically supplied and machined in an annealed or normalized-and-tempered condition, then finish heat-treated afterward for components that need maximum strength, or fully heat-treated before finish machining when tight dimensional control on the hardened part is required. Its combination of strength and fracture toughness is why 4340 shows up disproportionately often in aerospace and heavy-equipment specifications compared with lower-alloy steels.

4340 is noticeably tougher to machine than plain carbon or even standard Cr-Mo alloy steels, and the nickel content is the main reason why. Nickel raises the material's toughness and ductility, which means chips shear less cleanly and cutting forces run higher for a given depth of cut — the material resists deformation rather than fracturing cleanly at the shear zone. Combined with the chromium-molybdenum carbides also present in the alloy, this produces faster abrasive wear on the flank and a greater tendency toward built-up edge if cutting speeds or geometries aren't well matched to the material.

Heat generation at the cutting edge is a bigger factor here than on lower-alloy grades, so tool grades with strong hot hardness and thermal stability tend to outperform general-purpose grades. Chip control also needs more attention: 4340 can produce long, stringy chips in its softer conditions, so chipbreaker geometry matters more than it would on a free-machining steel. In hardened or high-strength conditions, expect shorter chip life and more pronounced notch wear at the depth-of-cut line.

Rigid machine setups, adequate coolant delivery to manage heat, and carbide grades selected specifically for tough alloy steels rather than mild steel are the biggest levers for controlling tool wear and holding tolerance on 4340. Reducing cutting speed slightly in exchange for a more wear-resistant grade is often the better trade-off than pushing speed and burning through inserts.