Heat-Affected Zone (HAZ) microfissuring occurs during electron beam welding of cast Alloy 718. In order to understand this phenomenon and find a solution, a series of investigations have been carried out to examine the microstructure of the base material in various heat treated conditions (homogenized, solution treated and aged) afld the microstructure of HAZ and fusion zone and to evaluate the weldability of Alloy 718 heat Íeated to different conditions. The microstructures were examined using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectrometry (SIMS), Electron Back-Scatter Diffraction (EBSD) technique and optical image analyzer. The weldability was evaluated by measuring crack length around the HAZ and the sum of individual crack lengfhs on each weldment section (Total Crack Iængth) was used to represent the material's weldability in this study.

The microstructure of cast Alloy 718 was observed to consist of A₂B Laves phase, δ-Ni₃Nb phase, M(C,N) carbonitrides and MN nitrides, with occasional presence of γ" and γ' in the interdendritic areas. The nature of grain boundaries in the cast alloy was examined using EBSD technique and it was found that 93% of the grain boundaries were of the random type. It was found that on increasing the homogenization temperature from 1037 to 1163°C, the amount of δ-Ni₃Nb and Laves phases decreased and that of M(C,N) carbonitrides remained constant. Solution heat treatment at 927°C enhances the precipitation of both inter- and intragranular δ-Ni₃Nb phase. Ageing heat treatment increases the material's hardness through the precipitation of γ"+γ'.

The grain boundary segregation after homogenization heat treatments, at temperatures ranging from 950 to 1140°C, followed by either air cooling or water quenching, was analyzed using SIMS. Boron segregation was observed, the extent of which decreased initially (from 950 to 1066°C) and then increased (fiom 1066 to 1240°C) when air cooling was used. In the case of water quenching, the boron segregation decreased continuously with an increase in homogenization temperature (from 950 to 1163°C). The occurrence of both equilibrium and non-equilibrium segregation could account for the segregation of boron. Segregation of carbon, sulphur and phosphorus was also examined using SIMS. However, none of these elements was found to be present at grain boundaries.

The examination of HAZ and fusion zone microstructures revealed that the majority of the precipitates in EB fi¡sion are MC-type carbides, with small amounts of Laves phase and M₂₃C₆. In HAZ, constitutional liquations of both Laes phase and M(C,N) occurred and resulted in resolidified Laves phase and M(C,N).

The Total Crack Length (TCL) after homogenization heat treatment decreased initially (fiom 1037 to 1066°C) and then increased (from i066 to 1163°C). No correlation between the amount of secondary phases and the TCL was observed. However, the variation in TCL with homogenization temperatures had a similar trend as that of the variation in boron segregation with temperature after air cooling.

This suggests that boron segregation plays an important role in controlling the HAZ microfissuring. A mechanism to explain the effect of boron on HAZ microfissuring has been proposed. The solution heat treatment reduced the TCL as compared to the homogenization treatment, while ageing t¡reatment caused an increase in the value of TCL.