Annealing
Definition
The heat-induced reorganisation of the crystal structure of steel at temperatures above approximately 720°C, resulting in reduced hardness and tensile strength; measurable post-fire and used to estimate whether a steel member reached annealing temperature during the fire.
- Material
- Steel
- Temperature threshold
- Above approx. 720°C
- Effect on metal
- Reduced hardness and tensile strength
- Forensic use
- Fire scene temperature estimation
Common questions
How does an examiner measure whether annealing occurred without a thermometer at the scene?+
Post-fire hardness testing of recovered steel, such as springs or wire, compares the metal's hardness against known unaffected reference values; a measurable drop indicates the piece reached annealing temperature, giving a minimum estimate of local fire severity.
Why is annealed steel useful evidence for locating a fire's point of origin?+
Because heat exposure and resulting hardness loss vary with distance from the seat of the fire, mapping which steel components show annealing across a scene helps corroborate other origin indicators like char depth and burn patterns.
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