AISI 4140's balance of properties makes it a workhorse material in demanding sectors:
Automotive & Transportation: Crankshafts, axle shafts, connecting rods, kingpins, differential gears, and high-strength bolts.
Aerospace (under specific AMS specs): Landing gear components, engine mounts, and other high-stress airframe parts.
Oil & Gas: Drill collars, tool joints, valve bodies, and pump shafts requiring strength and fatigue resistance.
Manufacturing & Machinery: Main spindles, heavy-duty gears, bolts, couplings, hydraulic rods, press rams, and die holders.
Tooling: Not for cutting edges, but often used for jigs, fixtures, and mold bases due to its stability and strength.
The choice between hot-rolled (for subsequent forging/machining/HT) and cold-drawn (for precision ground shafts) depends on the final component design and manufacturing route.