Because this material has a somewhat higher carbon content than structural steel, it is harder and the yield strength is higher. It is used in many types of machinery, such as road construction machines, machine tools and production machines. etc.
This often concerns the moving parts of machines, such as gears, shafts, levers and couplings. Material properties which are particularly desirable for such parts can be obtained by heat treatment.
An example of this is:
Hardening and tempering of steel
In a case where the material from which machine parts are made needs to be extra hard (for instance, to improve wear-resistance) this can be realised by means of heat treatment hardening, followed by tempering. The product to be hardened is heated to the temperature at which the crystal structure of iron changes from spatially-centred cubic (ferritic) to plane-centred cubic (austenitic). Depending on the carbon content, the temperature at which the conversion of ferrite to austenite begins is about 800 °C. The carbon dissolves in the austenite. After being kept at the right temperature for long enough, the material is quickly cooled by immersion in water or oil.
The cooling speed is such that the material cannot resume the 'normal' ferritic structure, so the austenitic structure, complete with the dissolved carbon, remains. This structure, called martensite, is relatively hard and brittle.
In order to improve the ratio between the increased hardness and the decreased toughness, the material is then kept at a specific, elevated temperature, below the temperature at which it was hardened, for a period of time . This procedure is called 'tempering'.