Evolution of the Microstructure and Properties of Pre-Boronized Coatings During Pack-Cementation Chromizing

The effect of chromizing time on the microstructure and properties of B-Cr duplex-alloyed coating prepared by a two-step pack-cementation process was investigated. The phases, microstructure, and element distribution of three coatings obtained were characterized by X-ray diffraction (XRD), secondary electron imaging (SEI), backscattering electron imaging (BSEI), and energy dispersive spectroscopy (EDS), respectively. The results show that as the chromizing time increases, the net-like Fe2B and rod-like CrFeB phases in the coating gradually disappear, and finally completely transform into the block-like Cr2B and CrxCy (Cr7C3 and Cr23C6) phases. The growth kinetics analysis shows that interface reaction dominates the coating growth during the early stage of chromizing, while atomic diffusion gradually controls the coating growth at the later stage. The evolution mechanism of the B-Cr duplex-alloyed coating was also discussed.