Modification of carbon steel surface by the Tenifer® process of nitrocarburizing and post-oxidation

Abstract

The surface of the non-alloyed carbon steel was subjected to thermochemical modification by salt bath nitrocarburizing with or without post-oxidation based on the Tenifer procedure in order to improve mechanical properties, corrosion and wear resistance. Nitrocarburized layers were characterized by testing their basic properties (compound layer thickness, nitrocarburizing depth, surface hardness) according to current standards. Detailed estimation of the quality of the modified metal surface was performed by additional testing of chemical compositions by EDX and X-ray diffraction, microstructure, surface roughness and topography, and corrosion resistance. The corrosion resistance was investigated in a water solution of 5% NaCl using electrochemical polarization methods and electrochemical impedance spectroscopy. The corrosion properties of the treated and nontreated samples are compared among themselves and with the properties of austenitic stainless steel.The surface layer obtained after nitrocarburization treatment consists mainly of ε-Fe2–3N and γ′-Fe4N. The surface layer of the samples after post-oxidation consists also of ε-Fe2–3N and γ′-Fe4N, but contains additionally Fe3O4.The results obtained show that nitrocarburizing and post-oxidation treatments, based on the Tenifer procedure, result in better mechanical and corrosion properties of non-alloyed carbon steel. The corrosion properties of the thermochemically modified non-alloyed carbon steel were improved and approach the corrosion properties of austenitic stainless steel, known for its very good corrosion resistance.