One of the most effective ways to reduce the brittleness of boride layers is the formation of layers with a composite structure. To create a composite structure of the diffusion layer, it is proposed to form, in addition to borides, several additional phases that are located in the layer arbitrarily or in an orderly manner. For this purpose, the samples made of 38Cr2MoAl steel are strengthened by a complex chemical-thermal treatment, namely, boriding is carried out after cementation or nitrocementation. The analysis of microstructures shows that after various modes of surface hardening of steel, it can be argued that complex chemical-thermal treatment leads to the formation of a composite structure of steel with the grinding of borides in the layer, the dispersion of which increases with an increase in the temperature of diffusion saturation. The paper shows that the main mechanisms of boron diffusion are such mechanisms as reaction (the movement of atoms is carried out by moving the chemical reaction front); diffusion along dislocations, block boundaries, subgrains and grains, $etc$. reaction diffusion occurs by diffusing boron everywhere the boride layer to the main reaction front, which is located at the boundary of two phases: iron–boride Fe$_2$B and boride Fe$_2$B–boride FeB. First, the embryos are formed, and then the needles of boride Fe$_2$B. These needles grow until they close and form a continuous layer of Fe$_2$B borides. Individual needles are then formed on the surface of this boride layer, which also form a second continuous layer of FeB boride. The needle-like structure of the boride layer indicates rapid growth of boride zones in the direction perpendicular to the surface of the samples, while along the surface the development of these phases was insignificant. This formation was due to the peculiarities of the structure of these phases, due to which boron atoms move rapidly only in a certain direction. As found, the boride layer displaces the carbon that is in the layer hardened by cementation (nitrocementation) deep into the steel. Due to the preliminary hardening treatment by cementation (nitrocementation), boride needles with rounded edges are formed and then crushed to the depth of the diffusion layer, which is due to the presence of a large number of carbides (nitrides, carbonitrides) in the surface layer, which do not allow boride needles to grow in the direction perpendicular to the surface of the samples due to their high density. Grinding and rounding of boride needles has a positive effect on the further operation of the product, thereby making it possible to eliminate stress concentrators and local zones that can cause cracks in the presence of sharp needles and reduces surface brittleness due to the formation of a crushed composite structure.