Evolution of secondary phases in austenitic stainless steels during long-term exposures at 600, 650 and 800 °C

Three austenitic steels (18Cr–8Ni, 18Cr–10Ni, 21Cr–30Ni), used for long-term applications at temperatures between 600 and 800 °C were investigated. In the investigation, metallography, transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy, and scanning electron microscopy were used. In additional to the experimental measurements, thermodynamic predictions were done using the ThermoCalc software and the non-commercial database STEEL16F. Various combinations of M23C6, sigma, and MC phases were identified in the austenite matrix of these steels. It was confirmed experimentally that extra large particles (up to 10 μm) observed in the 21Cr–30Ni steel are M23C6, even though this carbide was not predicted as the equilibrium carbide at service temperature (800 °C). The analytical-experimental approach, combining thermodynamic predictions and experimental measurements, was found to be reliable for the characterization of austenitic steels.