Influence of Welding Currents on Microstructure and Microhardness of Ni45 Alloy Reinforced with Spherical Tungsten Carbides (40 mass%) by Plasma Transferred Arc Welding

Abstract

The Ni45 alloy reinforced with spherical tungsten carbides powder was deposited on the surface of 304 L stainless steel by plasma transferred arc welding (PTAW). To determine the microstructure and microhardness of the coating with different currents, X-ray diffraction, an optical microscope, a scanning electron microscope, an electron microprobe and a hardness tester were all employed. The experimental results indicated that the microstructure of the coatings consisted mainly of different morphological strengthening phases and tungsten carbides distributed in γ-Ni matrix. Uniformly dispersed tungsten carbides were mostly in their original shape. Welding currents had strong effects on dilution with the base material and also on the formation of transition zones where the tungsten carbides were completely melted near the interface. In addition, lower currents resulted in transverse cracks along the fringe of the tungsten carbides. Concerning hardness it was found that higher currents contributed to lowering the hardness of the coating because of the intermixing of Fe in the hardfacing, the dissolution of the tungsten and the re-precipitation of the secondary carbides in the matrix. However, with higher currents, the hardness in the transition zone was greater than that with lower ones, which was the result of the predominant effect of melted tungsten carbides.