MICROSTRUCTURAL CHANGES INDUCED BY GRINDING OF NI-BASE SUPERALLOY IN 738 LC AND THEIR RELATIONSHIP TO MACHINING PARAMETERS

Subsurface microstructures, w ich have been produced by grinding of th e nickel-base alloy IN 738 LC, were examined with TEM after preparatio n of suitable cross-sections. Two types of superficial zones were obse rved, namely a deformation induced type 1 and a thermally affected typ e 2 microstructure. The latter one consisted of a several microns thic k gamma'-depleted zone. It could be shown by estimating temperature gr adients and flash temperatures, that grinding with corundum wheels can cause local melting at contact spots, thus leading to the observed ga mma'-depleted zone at the surface. The thickness of this zone was a li near function of grinding energy for a given grinding wheel specificat ion. If the heat conductivity of the grinding wheel material is much h igher than the one of the workpiece material, as is the case for cubic boron nitride (CBN), flash temperatures are considerably reduced, so that local melting cannot occur. Since local melting can be regarded a s a prerequisite to the initiation of deep cracks, grinding with CBN w heels seems to be a promising alterative in manufacturing the root pro files of turbine blades.