Laser spot thermography for defect detection on mild steel at higher temperatures (30-600 °c)

This paper studies the performance of laser spot thermography (LST) in defect detection on a mild steel sample at different temperatures (30-600 °C). In laser spot thermography, a laser spot is used to scan over the surface of the sample and the thermal profile is monitored using a thermal camera. For checking the feasibility/capability of laser spot thermography in defect detection under higher temperatures, a three-dimensional numerical model is developed using a commercial FE software package. This FE model is used to understand the heat transfer phenomenon during defect detection at higher temperatures, even after oxidation temperature. The influence of the oxide layer formation (scale) at higher temperatures (above 500 °C) in defect detection is established using the validated model. The thermal properties and optical properties of this oxide layer are different from those of the base metal; this will lead to a drastic variation in the thermal profile after the oxidation temperature. An oxide layer at 600 °C is introduced in the improved numerical model as a thin resistive layer at the top surface with a thickness of 50 μm. The thermal properties of the layer are assigned as the scale properties. FE modeling results show better agreement with experimental results even at 600 °C. Thus, the applicability of LST in high temperatures is experimentally proved.