Comparison between use of TiN-coated and uncoated high-speed steel and fine-grained cemented carbide tools in cutting of resulphurized 0·08 wt-%C free-cutting steel

A free-cutting 0·08%C steel has been turned dry using uncoated fine–grained cemented carbide tools and both uncoated and TiN-coated (by chemical vapour deposition)T 42 high-speed steel tools. Temperature estimates from the high–speed steel tools show that coating reduces the maximum rake-face temperature by about 125 K over a range of cutting speeds, permitting higher cutting speeds to be used before the onset of cratering. The wear mechanisms and built-up-edge behaviour have been investigated in terms of the tool/chip interface temperature and the relative plasticity of MnS inclusions. Manganese sulphide seems to have a higher affinity for the TiN coating than for the uncoated tools, permitting easier interfacial shear, and reducing contact area and built-up-edge size. With coated tools a built-up cap of workpiece material forms over the cutting edge, protecting it from wear; as a result there is only a small initial improvement in surface finish, but since nose wear is reduced, surface finish is maintained over a longer cutting period than with uncoated tools.

MST/212