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To the Issue of Accuracy of Determining the Temperature of Fire Exposure by Fire Traces

After the fire is extinguished, it is required to study a set of the parameters, the most important of which is the temperature of fire exposure. The material of building metal structures, including hazardous production facilities that are in the zone of high temperatures, is undergoing changes. According to them, the magnitude of the temperature effect is determined, that is, metal structures act as natural thermal witnesses. Currently, the temperature effect is assessed visually by the colors of thin oxide films (tint colors) on the metal structures surface. It is experimentally proven that such an assessment is subjective. Recommendations based on this method cannot be considered sufficiently reliable. The purpose of the conducted study is to determine the effect of temperature loading on the hardness and magnetic properties of the carbon building steels. During heating and subsequent cooling, two synergetically affecting each other processes occur simultaneously in steel: a change in the structure and properties of the base metal, as well as the appearance of oxide films on the surface. It is established that the characteristics of the metal — hardness and coercive force do not correlate with each other in all the temperature ranges of heating. However, each exposure temperature corresponds to only one pair of hardness and coercive force values. Based on the results of the conducted experiments, the method was developed for determining the temperature effect by a complex change in the hardness and coercive force of the steel structures. The proposed method will allow to improve the reliability of expert opinions on the possibility of further operation of the buildings and structures after a fire.