Experimental study of vanadium carbide and carbonitride coatings

Previous authors have established that prior nitriding or nitrocarburising will enhance the thickness of the vanadised coating formed on tool steels by the thermal diffusion (TD) process. However, whereas the single TD treatment produces a uniform surface layer of vanadium carbide, the combined treatments result in a complex vanadium carbonitride coating. Such a coating can be expected to exhibit a hardness that is lower overall and decreases away from the surface with increasing nitrogen to carbon ratio. An experimental study was undertaken to assess the relative merits of nitriding, nitrocarburising and carburising prior to TD vanadising on the steel AISI H13. The study demonstrates the specific advantages of carburising, previously untried as a pre-treatment. It produces the thickest coating (about twice as thick as that due to vanadising alone) and, perhaps more importantly, the coating is essentially vanadium carbide and exhibits a uniform high hardness across its entire span.