Variations in spectral emissivity of Usibor® 1500P steel during a two-step austenitization and in two different heating atmospheres, argon and air, are measured in-situ using a near infrared spectrometer and ex-situ with a Fourier transform infrared reflectometer. Phase transformations of the Al-Si coating and surface oxidation lead to the changes in surface phase composition and surface roughness, which in turn influences the spectral emissivity. In the first step heating at 610°C, the abrupt increase of spectral emissivity in argon and air is due to the transformation of the Al-Si metallic surface into the Al7Fe2Si intermetallic surface and an increase in surface roughness. Through the second step heating at 900°C, the progressive decrease of spectral emissivity with soak time in argon is caused by the surface phase transformation from Al5Fe2 into AlFe with a decrease in surface roughness. In contrast, surface oxidization with an increase in surface roughness significantly increases the spectral emissivity after two minutes soaking in air.

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