EFFECT OF HOMOGENIZATION HEAT-TREATMENT ON THE MICROSTRUCTURE AND HEAT-AFFECTED ZONE MICROFISSURING IN WELDED CAST ALLOY-718

The effect of homogenization temperature on microfissuring in the heat -affected zones of electron-beam welded cast INCONEL 718 has been stud ied. The material was homogenized at various temperatures in the range of 1037 degrees C to 1163 degrees C and air-cooled The homogenized ma terial was then electron-beam welded by the bead-on-plate welding tech nique. The microstructures and microfissuring in the heat-affected zon e (HAZ) were evaluated by analytical scanning electron microscopy (SEM ). The grain boundary segregation of various elements was evaluated by secondary ion mass spectroscopy (SIMS). It was observed that the tota l crack length (TCL) of microfissures first decreases with homogenizat ion temperature and then increases, with a minimum occurring in the sp ecimen heat treated at 1163 degrees C. This trend coincides with the v ariation in segregation of B at grain boundaries with homogenization t emperature and has been explained by equilibrium and nonequilibrium se gregation of B to grain boundaries during the homogenization heat trea tment. No other element was observed to segregate at the grain boundar ies. The variation in volume fraction of phases like delta-Ni3Nb, MC c arbide, and Laves phases does not follow the same trend as that observ ed for TCL and B segregation at the grain boundaries. Therefore, micro fissuring in HAZ of welded cast INCONEL 718 is attributed to the segre gation of B at the grain boundaries.