A new form of cast Inconel 718 has recently been receiving attention. This new form is known as Microcast-X (MX) investment cast alloy 718. Specimens were given an initial pre-weld homogenization heat treatment at 1200$\sp\circ$C and water quenched followed by solution heat treatments at 1050, 1100, and 1150$\sp\circ$C which were designed to maintain a fine-grained and "clean" microstructure while allowing sufficient time for equilibrium segregation of sulfur. Both air cooling and water quenching were used for each solution heat treatment to study the effect of cooling rate on both boron segregation levels and HAZ microfissuring susceptibility. Electron beam welding is used in commercial repair welding and joining practices due to the high quality of weld achievable using this technique and was used in this study. HAZ microfissuring measurements as well as microstructural observations and characterization were conducted using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), EnergyDispersive Spectroscopy (SEM/EDS and TEM/EDS), Secondary Ion Mass Spectrometry (SIMS), and Orientation Imaging Microscopy (OIM). (Abstract shortened by UMI.)