Alloying elements include chromium and molybdenum, and as a result these materials are often informally referred to as chromoly steel (common variant stylings include chrome-moly, cro-moly, CrMo, CRMO, CR-MOLY, and similar).
Examples of applications for 4130, 4140, and 4145 include structural tubing, bicycle frames, gas bottles for transportation of pressurized gases, firearm parts, clutch and flywheel components, and roll cages.
4150 stands out as being one of the steels accepted for use in M16 rifle and M4 carbine barrels by the United States military.
The core of the material retains its bulk properties, while the outer surface is significantly hardened to reduce wear and tear.
This makes this grade of steel an excellent material for uses such as gears, piston pins, and crankshafts.