Emissivity of Grade 91 Ferritic Steel: Additional Measurements on Role of Surface Conditions and Oxidation Faten N. Al Zubaidi, a,b Kyle L. Walton, c * Robert V. Tompson, b,c Tushar K. Ghosh, c,d and Sudarshan K. Loyalka b,c a University of Technology, Department of Electromechanical Engineering, P.O. Box 19006, Baghdad, Iraq b University of Missouri, Mechanical and Aerospace Engineering, Columbia, Missouri 65211 c University of Missouri, Nuclear Science & Engineering Institute and Particulate Systems Research Center, Columbia, Missouri 65211 d University of Missouri, Chemical Engineering, Columbia, Missouri 65211 Received May 13, 2020 Accepted for Publication August 6, 2020 Abstract — Measurements and data are reported for the total hemispherical emissivity of Grade 91 steel [ASTM International (ASTM) A387 Grade 91] for the temperature range of 400 K to 1048 K using ASTM standard C835-06. The surfaces studied included (1) an electric discharge machining (EDM) cut, (2) Grade 91 steel sandblasted with 320-grit-sized alumina beads, (3) EDM-cut Grade 91 steel oxidized in air, and (4) Grade 91 steel sandblasted with 320-grit-sized alumina beads and oxidized in air. The EDM-cut Grade 91 steel was oxidized at 873 K and 1023 K for 1, 3, and 5 h. The 320-grit sandblasted Grade 91 steel was oxidized at 1023 K for 5 h. Air oxidations were carried out in a three-zone furnace at fixed airflow. It appears that under some conditions, Grade 91 steel can reach the emissivities required for passive heat removal during an air ingress accident. Keywords — Total hemispherical emissivity, Grade 91 steel, very-high-temperature reactors. Note — Some figures may be in color only in the electronic version. I. INTRODUCTION Ferritic 9Cr-1Mo steels were developed for use as struc- tural materials at elevated temperatures for power generation industries, particularly for steam generators. 1 Grade 91 steel is a candidate material for the very-high-temperature reactor (VHTR) reactor pressure vessel (RPV). VHTRs are intended to passively reject heat from temperature excursions through electromagnetic radiation. In radiant heat transfer, emissivity is a material property that is the ratio of the radiance of a material to the radiance of an ideal blackbody and varies from 0 to 1. Studies have determined that VHTR structural materials, mainly for the RPV, need a minimum emissivity of 0.80 to 0.85 to ensure adequate radiant heat transfer. 2,3 One accident scenario of great interest is the ingress of air following the loss of He coolant in a VHTR. The presence of oxygen and elevated temperatures can result in oxidation of VHTR structural materials. It is important to determine emissivity as it depends on the surface condition of the material. Spectral directional and total hemispherical emissiv- ities of Grade 91 steel have been reported in the literature. Data previously reported by Azmeh et al 4 focused on total hemispherical emissivity of Grade 91 steel for short- term oxidation in air at 1073 K. These oxidations were limited to 15 min since oxidations were done with an emissometer by heating the sample under vacuum and venting air into the chamber. The study also included roughening and light coating of graphite powder (NBG- 18) on the emissivity of Grade 91 steel. Cao et al. 5 measured the spectral normal emissivity of Grade 91 (T91) steel from wavelengths of 2 to 9 μm during in situ *E-mail: klwd32@mail.missouri.edu NUCLEAR TECHNOLOGY © 2020 American Nuclear Society DOI: https://doi.org/10.1080/00295450.2020.1808394 1