Influence of Heat on Non-Ferrous Objects: Metallurgical Analysis in Fire Scene Investigations

NFPA 921 is a widely accepted standard to perform fire and arson analysis: it states that heat damage on items and components must be recognized to perform a more careful analysis of fire spread and to better define area and point of origin. With reference to metallic materials involved in a fire scene, NFPA 921 only indicate two useful parameters for temperature estimation, i.e. melting temperature and surface oxidation. However, the melting point of metals and alloys may define a wide range of temperatures (for example iron-based alloys normally melt between 1500 and 1600°C); in addition surface oxidation colors can only inform in a qualitative way. To better estimate the temperature range experienced by various items, much more information can be inferred from in-depth metallurgical analyses. Intergranular oxidation, recrystallization, second phase precipitations and incipient melting were some of the metallurgical features easily observable: if the investigator knows the approximate temperature of these microstructure modifications, a better estimate can be done of the “effective fire temperature” that the material might have reached. To this aim some components that can be found in fire a scene (aluminium airshaft, brass tap and copper gas pipe) were exposed to simulated fire conditions of known time durations and peak temperatures. The collected items were then examined by metallurgical techniques, identifying reference temperatures. The obtained results can give useful information to understand the fire scene.