Method for Tungsten Grain Size Analysis in Coiled Lamp Filaments

A method of tungsten grain size analysis in coiled lamp filaments has been developed. It is shown to be a valuable tool in design and optimization of heat treatment schedules, predictions of sag rates, failure analyses as well as standardization and benchmarking of non-sag tungsten grain structure in coiled lamp filaments. The procedure consists of two measurements on a polished and etched cross-section of a lamp coil: a number of observed “grains” and a number of examined turns. The resulting figure of merit, a grain-per-turn ratio, provides a useful relative measure of grain size (grain aspect ratio) in recrystallized non-sag tungsten lamp coils. The method permits rapid measurement, does not require any special instrumentation, and is performed directly from a stage of a standard light microscope.