HR3C (25Cr-20Ni-Nb-N) is a key material used in heat exchangers in supercritical power plants. Its creep properties and microstructural evolution has been extensively studied at or below 650 °C. The precipitation evolution in HR3C steel after creep rupture at elevated temperatures of 700 °C and 750 °C with a stress range of 70~180 MPa is characterized in this paper. The threshold strength at 700 °C and 750 °C were determined by extrapolation method to be σ105700= 57.1 MPa and σ105750=37.5 MPa, respectively. A corresponding microstructure investigation indicated that the main precipitates precipitated during creep exposure are Z-phase (NbCrN), M23C6, and σ phase. The dense Z-phase precipitated dispersively in the austenite matrix along dislocation lines, and remained stable (both size and fraction) during long-term creep exposure. M23C6 preferentially precipitated at grain boundaries, and coarsening was observed in all creep specimens with some continuous precipitation of granular M23C6 in the matrix. The brittle σ phase formed during a relatively long-term creep, whose size and fraction increased significantly at high temperature. Moreover, the σ phases, grown and connected to form a large “island” at triple junctions of grain boundaries, appear to serve as nucleation sites for high stress concentration and creep cavities, weakening the grain boundary strength and increasing the sensitivity to intergranular fracture.
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.
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