Heat Treatment of Sintered Functionally Graded Ti(C,N)-WC-Co Cermets in Nitrogen Atmospheres

摘要

Gradient structures developed at the surface of Ti(C, N)-WC-(W,Ti)(C, N)-Co cermets have been modified by graphite additions to the powder mixtures and by sintering in inert and reactive atmospheres. The combination of high C and high N activities during sintering avoids the formation of η phase and the presence of large WC grains near the surface of the specimens. The cermets free of η phase present a discontinuous outer layer consisting of a mixture of nitrogen-rich (Ti,W)(N,C) carbonitrides and WC grains in a matrix of Co. HIPing at a high N_2 pressure of the previously sintered specimens is observed to produce significant changes in the gradient zones. In the samples highly decarburised during sintering, HIPing in N2 is observe to remove the η phase up to a depth of about 1 mm from the surface. Non-decarburised materials present after HIPing a continuous layer mainly consisting of a nitrogen-rich (Ti,N)(N,C) carbonitride. Nevertheless, the HIP treatment is also found to enhance the growth of large WC grains near the surface of the specimens. Both low C and high C alloys sintered in N_2 present these microstructural defects, being larger for low C alloy. Dissolution and reprecipitation phenomena are likely causing the observed coarsening of the WC phase, since these grains, though smaller, are already present after sintering. In the high C alloy sintered in Ar, surface WC grains are generated during the HIPing process and present a much lower size. In this case, WC grains need to be nucleated by decomposition of the continuous η phase surface layer obtained after sintering, a process that limits the final WC grain size.