Study On High Chromium Alloying Layer Preparation By Mechanical Alloying On Carbon Steel Surface

During mechanical alloying (MA) process, the powder undergoes cold welding and the resulted refinement. Meanwhile, due to the repeated impacts and cold welding, a fraction of powder tends to adhere to the surface of balls and the inner wall of the container, so as to form a coating layer. Based on this principle, the present paper reports the studies on high content Cr coating layer prepared by ball milling. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) were used to investigate the microstructure and elemental distribution. Micro-hardness test, heat shock experiment, high temperature oxidation experiment and electrochemical corrosion technique were carried out to study the properties of as-prepared coating layers.A high-content Cr coating layer was obtained on the inner surface of tubular substrate buy using single component Cr powder and different rotating speeds (300r/min, 400 r/min, 500 r/min). The results showed that the rotating speed had an important effect on the thickness of coating layer. There was a diffusion layer at the interface. For the coating layers with 10wt%RE-Si-Fe and 20wt%Al, these two components could both efficiently improve the thickness of coating layer, thickness of diffusion layer and the deposition rate of Cr powder.The results of properties investigations showed that the micro-hardness improved significantly compared to the original substrate. Furthermore, the micro-hardness at the interface decreased gradiently. In addition, the oxidation resistance and erosion resistance were higher than that of substrate. At the speed of 500r/min, the coating layer had the best oxidation resistance and erosion resistance. With the addition of RE, the spalling resistance was improved greatly and the erosion resistance in 3.5wt%NaCl was also improved. Furthermore, the spalling resistance and oxidation resistance were both enhanced because of the addition of Al.Due to the intense impacts during ball milling, the inner surface of the substrate and Cr powder were both activated, which favored the combination between powder and substrate. The diffusion of Cr and Fe atoms at the interface led to a coating layer possessing a metallurgical bonding characteristic. A large mount of Cr powder deposited on the layer due to cold welding which made the Cr coating layer become much thicker.