Microstructural characterization of an ultrahigh carbon and boron tool steel processed by different routes

A newly developed ultra high carbon and boron tool steel containing 0.8wt.%B-1.3wt.%C-1.6wt.%Cr was processed to obtain a fine grain microstructure following two routes. In the first route, several thermomechanical treatments of the as-cast material, including extensive warm rolling, were used to refine the microstructure. A microstructure consisting of large and small borocarbides in a ferritic matrix with a grain size of about 2 mu m was obtained. In the second route, powder metallurgy techniques, including consolidation of rapidly solidified powders by extrusion and hot isostatic pressing (HIP), were used. The powders were produced by argon atomization and exhibit a dendritic microstructure, which remains unchanged after consolidation by HIP at temperatures up to 900 degrees C. The microstructure after consolidation by extrusion at 1050 degrees C is coarser and consists of spherical borocarbide particles, 2 mu m in size, in a fine-grained ferritic matrix. In addition, the shear forces developed during the extrusion process improve the bonding between the powder particles. In comparison to the microstructures obtained by thermomechanical processing, the powder metallurgy material possesses a better homogeneity in the size and shape of borocarbide particles and a finer microstructure.