A chromium–nickel austenitic stainless steel was plasma nitrided at 535, 585, 650, 735, and 785°C. After nitriding, the structure, thickness, and hardness of the surface layer, as well as the appearance and height of surface irregularities, were determined. The wear and corrosion resistance of the nitrided steels were tested and the percentage of magnetic phases present was determined. It has been established that nitriding markedly raises the wear resistance of the steel at pressures of 50, 200, and 400 MPa. The corrosion resistance of the steel in 0.05 M Na2SO4 at pH 3 decreases as a result of nitriding. Nitriding leads to the appearance of a multi phase structure in the surface layer, the proportion of the ferromagnetic a and γ’ phases being dependant on nitriding temperature. After nitriding at 785°C the proportion of these phases is minimal. At the same time, the steel has a high hardness and wear resistance and is susceptible to passivation.
Two cast irons, pearlitic-ferritic gray and ferritic ductile, were plasma and gas nitrocarburized at the same temperature and for the same processing time to produce a compound zone of about 10-14 lm thick. It was demonstrated that both processes caused changes in the surface roughness of the irons, and the most dramatic increase of roughness was observed after gas nitrocarburizing of the gray cast iron. It was shown that the primary reason that the results were not the same is the difference in the nitriding mechanism. Significant penetration of the surface voids and imperfections between the graphite particles and the metallic matrix by ammonia molecules led to the formation of a locally thicker compound zone and a bulging of the metallic matrix above the surface. This phenomenon did not occur in the plasma process and as a result the surface changes were much smaller than in the gas process.
Steels Cr17Ni2 and Cr18Ni9Ti were plasma nitrided and nitrided in ammonia at 585°C for 16h, and the layers obtained examined by X-ray diffraction and microhardness measurements. Corrosion properties were determined in 0.05M Na2SO4 at pH 3.0 and 5.0, with and without added chloride, using a potentiodynamic polarisation method. Nitriding resulted in a deterioration of corrosion resistance, especially in the acidic (pH 3.0) solutions, whereas in the near neutral (pH 5.0) chloride free solution the deterioration was slight and limitea to the transpassive region. Ammonia nitrided layers with ε +γ nitrides had lower corrosion resistances than plasma nitrided layers with γ nitride alone. This can be explained by the high porosity of the ε nitride layers and the resultant exposure of diffusion zones. The corrosion resistance of diffusion zones with CrN precipitates was markedly lower than that of outer nitride zones and of steel cores.
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