Abstract
Shot peening can be an effective solution for the prevention or retardation of scale formation, and subsequent exfoliation, upon exposure of the inner tube to steam in coal-fired power plants. In this study, specimens of T91 tubes were shot peened and then exposed to 1-bar steam for 100–1000 h at 650 °C, and were then analyzed using Vickers hardness test and microscopic techniques OM, SEM, TEM, etc. The analysis indicates that the oxide scales are typically Fe2O3 on the topmost layer, Fe3O4 below, and a FeCr2O4 spinel on the bottom in both shot peening treated and untreated specimens. However, the oxide scale thicknesses of shot peened specimens are thinner, indicating that shot peened specimens have better oxidation resistance. In addition, numerous defects, such as voids and micro-cracks, were found in the untreated specimens, which are believed to cause exfoliation of the uppermost Fe2O3 layers of the specimens exposed to steam for 800 and 1000 h. By contrast, the shot peened specimens maintained a dense contact oxide scale with fewer defects.
Highlights
Owing to their microstructural stability, high temperature strength, and oxidation resistance, 9Cr-1Mo ferritic-martensitic (F-M) steels are universally found in high temperature applications in fossil fuel power plants
Tan et al [6] reported that the oxide scale with a three-layer microstructure composed of Fe2O3, Fe3O4 and (Fe, Cr)3O4 formed on T91 steel in an (Ar + H2O) atmosphere at 600–700 ◦C
Steam was generated by heating distilled water to ~ 650 ◦C; it was blown across the sample surface for 100, 200, 400, 600, 800, and 1000 h. further details of the steam exposure are available in reference [10]
Summary
Owing to their microstructural stability, high temperature strength, and oxidation resistance, 9Cr-1Mo ferritic-martensitic (F-M) steels are universally found in high temperature applications in fossil fuel power plants. During exposure to high temperature steam environments, oxidation is an unavoidable problem for boiler tubes, such as found in superheaters and reheaters [1,2]. The oxidation behavior of F-M steels has been investigated by several groups [3,4,5], and are dependent on various factors, including temperature, atmosphere, and oxygen concentration. Tan et al [6] reported that the oxide scale with a three-layer microstructure composed of Fe2O3, Fe3O4 and (Fe, Cr)3O4 formed on T91 steel in an (Ar + H2O) atmosphere at 600–700 ◦C. The studies by Chen et al [5] indicated that the oxide scales of T91 steel exhibited higher thickness, higher interconnected porosity, and weak adhesion to metal matrix for high oxygen concentration exposure. According to Wagner’s theory of oxidation [7], the oxidation rate depends highly on the diffusion of the ions across the oxide scale during the steady state of oxidation
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