Experimental research on the influence of surface conditions on the total hemispherical emissivity of iron-based alloys

Abstract

The influences of surface conditions, including surface roughness and oxidation state, on the total hemispherical emissivity were experimentally investigated using an improved steady-state calorimetric technique for iron-based alloy samples. The investigated samples were prepared using rough polishing grits and high-temperature oxidization. The surface micrograph, roughness and composition of the oxide layer were analyzed using scanning electron microscope (SEM) and auger electron spectroscope (AES) to quantitatively describe the surface characteristics. Results of two iron-based alloy samples with different surface roughnesses showed that the measured total hemispherical emissivity increased with the roughness. However, the effect of surface roughness on total hemispherical emissivity gradually became smaller at high temperatures above 1200K for the investigated roughness conditions of iron-based alloys. Another six iron-based alloy samples were pre-oxidized in a high-temperature air muffle furnace with three oxidized at 400°C and the other three oxidized at 600°C for various times. The results showed that the emissivities of samples oxidized at 400°C for various times were similar to each other and the un-oxidized substrate, while the emissivities of samples oxidized at 600°C increased with oxidation time. This distinction arose from the different oxide layer thickness as the oxidation time increased at 600°C, while the thickness was almost constant at 400°C. The characteristics of the oxide layer grown in different oxidation temperatures were different as well. For the same oxidation time, the emissivities of samples oxidized at the higher oxidation temperature were larger than the values of samples oxidized at lower oxidation temperature. At high temperature (600°C) oxidation process of iron-based alloys, the oxide layer might be compact and its density was large which would strength the capacity of radiation heat transfer of iron-based alloy, comparing with the oxide layer grown in low temperature (400°C) oxidation process at the same oxidation time.