Steam Oxidation and Surface Hardness of Power Plant Valve Materials Under the Ultra Super Critical Steam Condition

• Published in: Metals and materials international 2008-12, Vol.14 (6), p.721
• Main Authors: Ki O Song, Tong Yul Cho, Jae Hong Yoon, Chan Gyu Lee, Kee Sam Shin, Seon Ho Lee, Kee Won Urm, Jong Wook Lee, In Soo Kim
• Format: Article
• Language: Korean
• Subjects: 17 wt.% Cr-steel; Incoloy 901; oxidation; ultra-supercritical power plants
• ISSN: 1598-9623; 2005-4149

Current supercritical steam power plants operate at 3,600 psi and 1,000°F. If the steam temperature is raised from 1,000°F (538℃) to 1,150°F (621℃), the efficiency increases by 2 %. Therefore, study on the high temperature corrosion of power plant materials under ultra-supercritical conditions (USC) is necessary to protect the plant from corrosion. In this study, valve materials of 17 % Cr martensitic steels (17Cr steel), Incoloy 901 (I901) and their surface nitrided specimens were exposed to USC of 621℃ and 3600 psi (255 kg/㎠) steam for 200℃, 400℃, and 800 h. The oxidation of both 17Cr steel and I901 under the USC for 800 h is very small due to the formation of a protective thin oxide layer formation on the surface. The USC oxidation of both nitrided specimens were increased due to the decomposition and formation of active nitrogen from the non protective nitrides such as Fe4N, Fe2-3N, and CrN. The oxidation of nitrided 17Cr steel (n17Cr steel) is about two times higher compared to nitrided I901 (nI901). The surface hardness is improved by more than two times near the surface by nitriding, and the degradation of hardness by USC oxidation is rapid for n17Cr steel, but slow for nI901.