27th IFHTSE CONGRESS & EUROPEAN CONFERENCE ON HEAT TREATMENT 2022

Gas nitriding processes with cyclical rotation of ammonia and ammonia/air atmospheres are studied for surface strengthening of Cr-Ni corrosion resistant steels. One-, two-, and three-staged processes were experimentally investigated differing by gas atmospheres at the final stage of the processes. Gas-cyclic nitriding provides an intensification of saturation of alloyed steels with nitrogen due to the local effects of thermo-cycling. Microstructure and phase composition of diffusion layers are examined. During the final stage in the ammonia/air mixture, iron oxides remain in the compound zone; and a sharp decrease of micro-hardness is observed at the border of the layer with the core. Completion of the cyclic process with the stage of saturation in ammonia forms a nitride zone of γ'-phase. It was shown that three-staged process increases the thickness of the diffusion layer, and makes the micro-hardness profile smooth. Results of wear and impact toughness tests are discussed in correspondence with the layers’ structure. After three-staged nitriding, the steels have the best wear resistance and keep satisfactory toughness due to the absence of ε-phase.