Steel emissivity at high temperatures

A new test method was developed at VTT for the determination of the emissivities of different types of steel and other metallic materials as a function of the temperature of the material. The method is simple and inexpensive, but its use is limited to materials which possess a high thermal conductivity and are physically and chemically inert in the temperature range of interest (no melting or other phase transitions, no charring, burning or other chemical reactions). Furthermore, reliable results are only obtained above a minimum temperature, the value of which varies depending on experimental conditions but seems to be in the range from 150 ºC to 200 ºC. Three different steel types were used in testing the new method: two stainless steels and one carbon steel. The emissivities of the two stainless steels were determined for the temperature range from approximately 200 ºC to approximately 600 ºC. The emissivity of the carbon steel was determined for the temperature range from approximately 150 ºC to approximately 550 ºC. Uncertainty of the emissivity values obtained with the new method was estimated to be approximately ±20 %. Repeatability of the results was well within the ±20 % limit. The results were compared with those obtained using a method developed earlier by SP. The SP method gives emissivity at a fixed temperature; earlier the fixed temperature was 100 ºC, but the method was now upgraded so that 200 ºC also became possible. Thus, emissivity values at 200 ºC could be used to compare the two methods. For the two stainless steels, there was good agreement between results obtained using the new method and results obtained using the existing method. For the carbon steel, the differences were much larger than would have been expected. The reasons for the differences have not been found yet and should be further investigated.