Temperature measurements during the galvanneal process

Abstract

During the galvanneal process, zinc-coated steel sheet is rapidly heat-treated in order to develop desirable characteristics in the material. The surface changes rapidly from a layer of highly specular liquid zinc with a spectral emissivity of approximately 0.1 to a highly diffuse intermetallic layer with an emissivity as high as 0.8. The variability of the emissivity makes application of traditional non-contact temperature measurement unreliable. This study explored the use of a linear relationship between two spectral emissivities together with Planck's equation to simultaneously determine temperature and spectral emissivities. A laboratory simulation of the galvanneal process was performed in order to obtain spectral radiance and temperature data, from which emissivities were calculated. Results obtained indicate that this dual-wavelength radiation thermometry method can infer temperature to within +/− 50 K. Hence, a simple linear relationship between the emissivities is not adequate for accurate temperature measurement. The results suggest that development of a nonlinear equation relating the emissivity values, or an equation including temperature as a parameter, would reduce the temperature errors.