Preferential distribution of chromium and nickel in the borided layer obtained on synthetic Fe-Cr-Ni alloys

Synthetic ferrous alloys containing chromium and/or nickel were prepared and borided at 1173 K with powders containing B4C, KBF4 and SiC for times varying from 20 to 60 h. The surface layers composed of borides of type (Fe, M)B and (Fe, M)2B were characterized by means of X-ray diffraction, microscopic observations, analysis with the microprobe and microhardness measurements. A quantitative study was carried out on the differentiated distribution of chromium and nickel in the phases constituting the borided layer. The chromium spreads from the matrix towards the borided layer, where it is concentrated in the phase richest in boron (Fe, M)B. Nickel behaves in the opposite manner. Alloys without carbon were used in order to avoid the formation of an area containing carbides and boron carbides, situated between the borided layer and the matrix, which could influence chromium distribution. The distribution phenomenon is in accordance with what was found in the study of Fe-M-B ternary systems (M=Cr, Ni). The influence of chromium and nickel on the thickness, the morphology and the microhardness of the borided layer are discussed.