Deformation of Tungsten Carbide Tools when Cutting Inconel 718

Summary

The present research studies the reasons for the failure of tungsten carbide tools while cutting superalloys. Using recommended cutting conditions a plateau separates the crater from the cutting edge of the tool when cutting AISI 4340. This plateau is totally absent in all cutting of Inconel 718 even in short, two-minute tests. The crater intersects the cutting edge leaving only a thin wedge of carbide which either breaks off or deforms and wears very rapidly, due to increased local temperatures because the rake and flank sources of heat are so close together. If one uses Borazon (General Electric, cubic boron nitride) to cut Inconel 718, the bottom of the crater is at the flank face, and during wear of the tool a ‘self-sharpening’ cutting edge is maintained.

For continuous temperature measurement of the tool during cutting operations, a Vanzetti Infrared Thermal Monitor was used. The temperature measurements carried out indicate, at recommended speeds, a sharp rise of the temperature at the beginning of the cutting operation; then a steady state, very slow increase, as the cutting continues; and finally, just before tool failure, a very rapid increase in the temperature again. At extremely high cutting speeds the steady state temperature regime is absent.

From profilometer traces across the flank wear land, after dissolving off the adhering Inconel 718 from the tool, it is clear that the surface roughness remains small after cutting over 1000 ft at the recommended speed but that at higher speeds this roughness increases very rapidly with both the number of feet cut and the rate of cutting.

Hot hardness measurements of tools up to 2200°F indicate that hot hardness alone is not sufficient to guarantee cutting performance.

Chemical interaction, such as an adherent Inconel layer on the crater and flank, removal of carbide grain from the crater, and breaking up and rounding of carbide grains contribute to the tool failure.