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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.

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An improved non‐destructive replication (NDR) metallography method was developed and applied successfully for the precipitates study in 1Cr‐1Mo‐0.25V turbine rotor steel. The preparation of the replicas for electron microscopy was carried out with a combination of electropolishing, etching, and acetate film setting for extraction medium. The exposure of the precipitates was improved by electropolishing followed by chemical etching. The electropolishing after application of acetate film, instead of stripping, resulted in a quality improvement of the extraction replica. Three types of precipitates (M23C6, Mo2C, and VC) were identified by TEM analysis of thin foil. The types, sizes, and chemical compositions of the precipitates from the bulk SEM observation and TEM analyses of thin foil specimen and the NDR extraction replica matched very well, indicating NDR extracted all types of the precipitates with high efficiency. Copyright © 2012 John Wiley & Sons, Ltd.