Multilayer coated cemented carbide cutting insert

Claim 1

A cutting insert comprising: a body having a rake face and a flank face; a cutting edge at a junction of said rake face and said flank face; said body composed of: a cemented carbide substrate consisting essentially of tungsten carbide grains, solid solution carbide grains containing tungsten and an element selected from the group consisting of titanium, tantalum, niobium, zirconium and hafnium, alone or together, and 6.1 to 6.5 weight percent cobalt; said substrate having a hardness of 90.8 to 91.6 Rockwell A and a magnetic coercive force of 110 to 180 oerstads; a coating bonded to said substrate; said coating including a backing layer bonded to said substrate, having a thickness of at least 5μ, and selected from the group consisting of the carbides, carbonitrides of titanium, hafnium and zirconium, alone or in combination; an active layer composed of alternating sublayers including at least a plurality of alumina sublayers separated from each other by an intermediate sublayer selected from the group consisting of the nitrides of titanium, zirconium and hafnium, alone or in combination, and said first aluminua sublayer bonded to said second layer; a finish layer bonded to the outermost alumina layer; said finish layer selected from the group consisting of the carbonitrides and nitrides of titanium, hafnium and zirconium, alone or in combination; wherein said alumina in said sublayer has an average grain size of between about 0.15 to 0.5 μ.