Filament condition during hot-wire chemical vapor deposition conditions of multi-walled carbon nanotubes is a major concern for a stable deposition process. We report on the novel application of electron backscatter diffraction to characterize the carburization of tungsten filaments. During the synthesis, the W-filaments transform to W2C and WC. W-carbide growth followed a parabolic behavior corresponding to the diffusion of C as the rate-determining step. The grain size of W, W2C, and WC increases with longer exposure time and increasing filament temperature. The grain size of the recrystallizing W-core and W2C phase grows from the perimeter inwardly and this phenomenon is enhanced at filament temperatures in excess of 1,400°C. Cracks appear at filament temperatures >1,600°C, accompanied by a reduction in the filament operational lifetime. The increase of the W2C and recrystallized W-core grain size from the perimeter inwardly is ascribed to a thermal gradient within the filament, which in turn influences the hardness measurements and crack formation.