Behavior of spectral emissivity of metal in oxidation process

Abstract

The emissivity change caused by oxidation might produce a large error in temperature measurement by a pyrometer in a steel making process, such as continuous annealing line of cold rolled steel. We measured in detail the change of spectral emissivity of metal samples during an oxidation process. It was confirmed that there were two types of emissivity changes caused by the oxidation. One type was the oscillation, and the other was monotonous increase. In cold rolled steel samples, the former appeared when the Si and Mn concentrations were low, and the latter appeared when the Si and Mn concentrations were high. These phenomena can be explained by the fact that surface silicon oxide layer prevented further oxidation. Regarding the relationship between oxide thickness and emissivity change, emissivity oscillated as the result of the interference at the thin oxide layer on the sample surface. Because emissivity changes in a very complex way during an oxidation process, it is difficult to compensate the change of emissivity. In order to solve this problem we employed a multiwavelength pyrometer. This method was effective when emissivity was less than 0.8 and the oscillation did not occur. We manufactured a pyrometer using four wavelengths, and the accuracy of temperature measurement was ±10 °C with the emissivity range from 0.4 to 0.8 in a laboratory experiment. Another method was proposed which could be applied to the oscillation.