Microstructural identification of phase transformation mechanisms in the NiS system close to NiS composition, owing mainly to optical & electronic microscopy on a series of partially transformed state were carried out. The specimens contained different proportions of Sulphur & different iron contamination. It was shown that, depending on composition & temperature, transformation mechanisms are different. NiS type with the slowest transformation rate causes late breaks in HST, & under bad conditions might survive the test. It was found that transformed volume fraction of just this type of NiS decreases when increasing temperature above 280 deg C. Consequently, conditions of HST should be modified. The proposal is to limit its temperature to 280 deg C, which would enhance the safety of the toughened glass. Additionally, new HST conditions are proposed.
