The carbon steel key stock has an unpolished (mill) surface, meets American Society for Testing and Materials International ASTM A108 specifications, and has a standard tolerance. The carbon steel grade provides a good balance of toughness, strength, and machinability. It is magnetic.

Key stock is used to make a machinery part known as a machine key, which fits into a mating keyway to transmit torque between shafts and hubs or bushings. Rectangular and square key stock shapes are the most common, while step key stock is used for damaged or inconsistent keyways.