Precipitation evolution and creep strength modelling of 25Cr20NiNbN austenitic steel

Abstract

25Cr-20Ni-Nb-N is a high strength and oxidation-resistant austenitic stainless steel intended for Ultra-Supercritical (USC) power plants. In this work, the precipitation evolution, and creep strength at 650 and 750°C for up to 100 000 h are predicted. Six precipitates are considered in the thermokinetic calculation by MatCalc: M23C6, η (Cr3Ni2SiN), σ, G, Z, Nb(C,N). For the creep strength prediction, three hardening mechanisms are taken into account: dislocation, precipitation, and solid solution hardening. Both matrix composition and precipitation evolution, calculated with MatCalc, are used for modelling the precipitation and solid solution hardening. It is found that the dislocation hardening, followed by precipitation hardening gives the largest contribution to the creep strength. The most important precipitates strengthening phases are found to be Z-Phase and Nb(C,N), which are nucleated at the dislocations. The model for the creep rate can represent how the creep exponent is raised with increasing applied stress and reduced temperature.