Titanium carbonitride coated cemented carbide and cutting inserts made from the same

1. Field of the Invention A process for forming a titanium carbonitride cemented carbide cutting insert, said cemented carbide is comprised of more than 70 percent by weight tungsten carbide and has a matrix binder comprising cobalt with a cobalt enriched stratified surface. The process comprises chemically vapor depositing a layer of titanium carbonitride on said cemented carbide substrate by heating gaseous reactants comprising titanium chloride, methane, hydrogen, and nitrogen with an optimum methane/nitrogen ratio, under suitable conditions to form a titanium carbonitride coated insert having an eta phase in the cemented carbide substrate adjacent said titanium carbonitride coating. The eta phase comprises chemical compounds consisting essentially of cobalt, tungsten and carbon, and the titanium carbonitride surface is contacted with a carburizing gas for a sufficient time and at a sufficient temperature to convert substantially all of said eta phase to elemental cobalt and tungsten carbide, and to diffuse tungsten into the carbonitride coating, such that the grain boundaries of the said carbonitride layer is strengthened by the presence of tungsten due to solid solution strengthening and/or grain boundary strengthening.