Influence of thermal environment on metallographic structure characteristics of the electric arc bead pattern

Electrical apparatuses are prone to arc, which generally causes a fire, even an explosion hazard, when a flammable gas mixture is present, especially during industrial processes. Terrible fire scenes are challenging for fire investigations. In this work, by performing a simultaneous thermal analysis test we simulated a fire environment and found that as the oxygen concentration decreased, the oxidation/exothermic peak temperature of 'cause' bead became higher, but the melting temperature was unaffected. Results indicated that the bead pattern underwent oxidation at approximately 831 degrees C, melting initiated at approximately 1060 degrees C, and the pattern then disappeared. The melted pattern grain changes were divided into three critical temperature stages: Approximately 600 degrees C, the onset temperature at which the melted pattern grains began to be equiaxed; approximately 831 degrees C, at which the grains were interspersed with oxygen-containing material; and 831-1060 degrees C, when the grains disappeared, which is a criterion for identifying electrical fires. However, the boundaries remained throughout the thermal environment process. Moreover, the bead pattern demonstrated three metallographic regions: Deep layer (Region I), the intermediate layer (Region II), and surface layer (Region III). Region I was the most thermally sensitive, in which equiaxed crystals first appeared. Region III was the thermal reaction lag zone, in which the typical branching crystals finally disappeared, and Region II was intermediate between Regions I and III. The results may help fire investigators determine the fire scene temperature stages and provide support for fire evidence extraction.