Methodology for the emissivity estimation of radiation-type thermometers using the latent heat generated during the phase transformation of steels

Abstract

This paper proposes a practical procedure for the emissivity estimation of radiation-type thermometers during the hot rolling process of steels. The temperature of a workpiece is typically measured by radiation-type thermometers during the hot rolling process of metals since such thermometers have several advantages to measure the temperature of moving objects in real time. However, a critical disadvantage of radiation-type thermometers is that infrared radiation strongly depends on the surface emissivity of an object, but determining emissivity is very challenging. Therefore, in this study, a new methodology for determining the emissivity of radiation-type thermometers was developed for the hot rolling process of steels based on the heat generation or absorption phenomena of phase transformations during this process. It was found that the starting temperature of phase transformation can be detected easily using radiation-type thermometers as well as thermocouple. We can thus identify the reference point for comparing the measured temperature by all the thermometers using the starting temperature of phase transformation during process. More interestingly, the transformation starting temperature of steel has a strong linear relationship with the cooling rate. Accordingly, it is possible to consider the transformation starting temperature as a material property that varies with cooling rate. The proposed technique was well applied to the steel wire rod cooling process and the optimal emissivity values of a pyrometer and thermocamera were identified efficiently, which is useful for many steel manufacturing applications. Finally, the more accurate temperature was achieved using the radiation-type thermometers by correcting the emissivity.