4140 is a chromium-molybdenum alloy steel. The chromium content provides good hardness penetration, and the molybdenum content ensures uniform hardness and high strength. 4140 chrome-molybdenum steel can be oil hardened to a relatively high level of hardness. The desirable properties of the 4140 include superior toughness, good ductility, and good wear resistance in the quenched and tempered conditions.

The 4140 cold finished annealed chromium-molybdenum alloy steel can be heated using various methods to yield a wide range of properties, hence it is often used as stock for forging as it has self-scaling properties. 4140 is capable of resisting creep in temperatures up to 538°C (1000°F) and maintaining its properties even after long exposure at comparatively high working temperatures.

Similar to grade AISI 4130 but with a slightly higher carbon content. The higher carbon content of AISI 4140 gives greater strength and heat treatment capabilities in comparison to AISI 4130.