Precipitation in 25Cr20NiNbN austenitic steel after ageing at 750°C

The heat-resistant austenitic steel 25Cr20NiNbN (HR3C) has been widely used in super heater and re-heater tubes in ultra-supercritical (USC) boilers. The precipitation processes in the steel are complicated because of its high alloy element contents. Precipitation in a 25Cr20NiNbN steel has been investigated after thermal aging at 750 degrees C for period from 500 to 3000 h. The results indicate that M23C6 carbides generally precipitate along grain boundaries and that the major intragranular precipitates are NbCrN nitrides and M23C6 carbides. More precipitates are observed in the specimens after long term aging treatment: NbCrN nitrides on grain boundaries, as well as Cr3Ni2SiC carbides, have been found in the sample after aged for 2000 h at 750 degrees C. The formation of sigma phase has also been confirmed in the current steel after long term aging based on the results of X-ray diffraction (XRD), SEM and chromogenic methods.