1 The chief cause of structures of nonmartensitic type forming in the surface layers of case-hardened components is the impaired hardenability of the surface zones of the case because of the depletion of carbon and alloying elements in the austenite as a result of the formation of carbides and their subsequent oxidation and graphitization. 2. The dark component in nitrided steel appears in consequence of the formation of nitride phase, of the dissociation of this phase when it is replaced by carbide phase, of oxidation and graphitization of the carbide phase. As a result of these processes, pores filled with graphite and with chromium, manganese, and silicon oxides form in the layer. 3. The processes of formation of defects in carburization and nitriding under conditions of simultaneous diffusion of several elements have in common the formation of a second phase with its subsequent substitution by a phase that is more resistant under the given conditions. 4. In nitrided steel nitrogen may be contained in the solid solution, in bound form (in nitrides), and in the free molecular state. Then the nitrogen of the solid solution together with carbon determines the degree of martensitic hardening, the nitrogen in the nitrides determines the degree of nitride hardening, and the molecular nitrogen determines the degree of defectiveness of the structure. 5. The new method of gas carburization and nitriding ensures a considerable improvement of the mechanical properties of components.