Chromium carbide coatings produced on ductile cast iron QT600-3 by thermal reactive diffusion in fluoride salt bath: Growth behavior, microstructure evolution and kinetics

摘要

A series of chromium carbide coatings were formed on ductile cast iron QT600-3 by thermal reactive diffusion (TRD) processes in fluoride salt bath at 850-980 degrees C for 10 min similar to 40 h under an argon atmosphere. Coatings were analyzed by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX). The results demonstrate that the microstructure of chromium carbide coatings formed on ductile cast iron QT600-3 at 850 degrees C is related to the treatment time. At first 10 min, there is only a thin layer of Cr7C3 in the coating. At 1 h, the outermost layer of the coating is a thin Cr23C6 layer, and there is a thin M3C layer beneath the M7C3 layer. With increasing treatment time, the thickness of all layers in the coatings increases. For the sample treated for longer than 1 h, the microstructure of coating is Cr23C6, M7C3 and M3C from the outermost layer to the substrate. The microstructure of chromium carbide coatings formed on high C content steel substrates via TRD processes is independent with the composition of the base salt. The growth kinetics of chromium carbide coating follows a parabolic law with an activation energy of 129.906 kJ/mol. The evolution of hardness of coatings is consistent with the microstructure evolution of coatings.