IMPACT OF ROUGHNESS LENGTH SCALE ON SPECTRAL EMISSIVITY DURING INTERCRITICAL ANNEALING OF ADVANCED HIGH STRENGTH STEELS

摘要

Uncertainty in the spectral emissivity of advanced high strength steels (AHSS) causes pyrometry errors and thermal excursions during intercritical annealing, leading to defective material properties. To understand this phenomenon, it is crucial to decouple the influence of evolving surface composition and topography on near-infrared spectral emissivity, and determine the pertinent roughness length scale. To this end, the spectral emissivity of DP780 and DP980 coupons heated according to industrial cycles and quenched an intermittent times was characterized over 0.25-25 μm. The radiative properties are compared to contact and optical profilometry measurements, in the context of several theories that relate surface topography to spectral emissivity. The results highlight the fact that average roughness parameters are inadequate to characterize spectral emissivity, and suggest that the radiative properties pertinent to pyrometry are sensitive to roughness scales smaller than the corresponding detection wavelengths.