Steels have been used since the Iron Age and their importance in the development of industry has been enormous. Steels are the most important alloys utilized as structural material. They are straightforwardly related to engineering. The microstructure of most steels is well known by now as well as the effects of the heat treatments in changing their mechanical properties [1]. For instance, the hardness of the AISI 5150 steel could vary from ∼ 20 to 60 HRC depending on its heat treatment [1]. The differences in mechanical properties of a given steel are the result of different microstructures formed during cooling. This statement generally means that the highest hardness in the iron–carbon systems is obtained due to a diffusionless transformation called martensite formation and the lowest hardness is obtained due to a diffusion transformation, which causes the ferrite and/or pearlite formation by a eutectoid reaction. Both martensite obtained during rapid cooling and ferrite-pearlite obtained during slow cooling or near the equilibrium, come from austenite [1]. Therefore, both the steel microstructure and the steel mechanical properties are related to steel thermal history