The paper deals with the issues of hardening the surface layer of steel parts by ionic chemical-thermal treatment. A model for predicting the depth of the nitrided layer depending on the steel composition, surface roughness of the part and average temperature in the chamber has been obtained by analytical and calculation methods. The results showed that the steel content of phosphorus and sulfur, as well as chromium and nickel content have high correlation coefficients. Based on the data of regression analysis it should be noted that the greatest influence on the depth of the nitrided layer has the content of phosphorus in steel. The second most important element, the content of which in the steel composition leads to an increase in the depth of the nitrided layer, is titanium. Insignificant influence on the depth of the nitrided layer has the content of vanadium and molybdenum, which allows us to exclude them from the model for predicting the depth of the nitrided layer. It is noted that high accuracy of surface treatment worsens the conditions of nitrogen penetration into the metal structure, while the temperature in the working zone, expectedly, leads to an increase in the depth of the hardened layer.