A survey of hemispherical total emissivity of the refractory metals in practical use

The measured hemispherical total emissivity for type 304 stainless steel and molybdenum above 1000 °C showed a few times higher value compared with the data obtained from polished metals given in the literatures. This result indicated that the emissivity of the metals with protective layer including oxide scale must be carefully considered not only in measuring temperature with the radiometric method but also the designing various devices where radiation phenomena were critical. The measured hemispherical total emissivity above 1000 °C decreased due to the vaporization of the protective layer on the specimen surface during the measurements. Thus, the upper limit of the measuring temperature in this method is ruled by the stability of these surface products in a vacuum at high temperatures. Therefore, such pulse heating methods those enable the more rapid heating within nano- or picoseconds are strongly recommended for the measurements of the emissivity of the metals those having the protective layer.