Tempered steels are unalloyed and alloyed machine manufacturing steels whose chemical compounds are suitable for hardening especially due to carbon content and display high firmness against a certain tensile strength after the tempering process. In tempered steels, high strength and ductility is desired as well as hardenability properties. To be able to obtain adequate level of hardness, tempered steels contain relatively more carbon. Since hardness depth is the most important criterion for thick cross-section parts, these parts are manufactured from alloyed tempered steels. Tempering process is described as the combination of hardening and then damascening processes in which the steel part will gain high firmness properties. Tempered steels are widely used for manufacturing of various machinery and engine parts, forged parts, including a great variety of bolts, nuts and studs, crankshafts, axles, controller and drive parts, piston arms, several types of axles, teeth, etc. due to their superior mechanical properties at the end of the tempering process. Thus, tempered steels are the most manufactured and used type of steel after construction and unalloyed steels.