Surface hardening in steels is a process in which a chemical composition is changed by thermo-chemical processes in a determined region and, accordingly, some micro-structure is changed. In order to obtain a harder layer than the inner region starting from the surface to a certain depth, it is mostly provided by diffusion of elements such as nitrogen and carbon. The process is particularly important in low and medium carbon steels in terms of increasing wear resistance, tensile strength and fatigue strength. The amount of elements used in cementation together with the duration of cementation is extremely important in terms of the harmonious change of structural differentiation. The effect of size and position of inclusions on the cracked structures which is affected from heat treatment is presented in the paper.