Microstructures and properties of high Cr content coatings on inner surfaces of carbon steel tubular components prepared by a novel mechanical alloying method

In the present work, a novel mechanical alloying method was developed to prepare high Cr content coatings on the inner surface of carbon steel tubular components using a planetary ball mill. The microstructure and elemental and phase composition of mechanically alloyed coatings at different processing conditions were studied using SEM, XRD, and EDX. It showed that a proper increase in the applied milling time and the disc rotation speed favored the improvement in the thickness, surface smoothness, densification level, and microstructural homogeneity of the deposited coatings. With processing conditions optimized (rotation speed of 500 rpm and milling time of 10 h), a fully dense, ∼120 μm thick, high Cr content coating, consisting of metal Cr and Fe-20Cr solid solution alloy, was metallurgically bonded to the inner substrate. Comparative studies on the microhardness, corrosion resistance, and anti-oxidation capability of carbon steel substrates with and without coatings were performed. It was found that the maximum microhardness of the coating reached HV0.1667, showing a threefold improvement upon the substrate. The coated surfaces exhibited favorable resistance against corrosion and thermal oxidation as compared with the bare substrate. Based on two important action mechanisms (i.e., friction effect and impact effect) associated with a planetary ball mill, a reasonable mechanism was presented for the formation of mechanically alloyed coatings on inner surfaces of tubular components.