Microstructural Observation of Ni-WC Surface Alloyed Carbon Steel

Abstract

EPS (expanded polystyrene spheres) templates with thickness of a 10 mm coating on a surface that is mainly consisted of Ni-WC particles, are prepared. Carbon steels containing 0.45C%wt are then cast in the templates using V-EPC process (vacuum expandable pattern casting), forming the surface alloyed steels. The microstructures are observed and analyzed using optical microscope, SEM and EDS. Macrostructural observation showed that the surface alloyed zone is dense and there is no obvious defects, such as gas bubbles, occluded gas holes and delimitation. Microstructural investigations indicate that the samples from top to bottom are obviously consisted of three different zones, i.e., the top alloyed zone, the interim transitional zone and the bottom matrix zone. Ni-WC particles are totally decomposed during the molten steel infiltration. The microstructures in the surface alloyed zone are consisted of small amounts of fine WC+W2C carbides, large amounts of M3C+M7C3 carbides and dendritic matrix. There is a fine pearlite strip at the top of the transitional layer. Small amounts of carbide particles within the matrix grains and net carbide precipitation in the grain boundaries at the transitional zone can be observed.