The demand for new and improved materials for the energy sector is rising as more efficient power plants needs to be built to meet the challenges of tomorrow. However, at the moment more expensive Ni-based alloys will be used to meet the requirements for the next generation of power plants, Advanced Ultra-Supercritical (A-USC). Therefore, a more economical Heat Resistant Material(HRM) candidate has to be developed, which will fulfill the requirements of exhibiting a minimum creep rupture strength of 100MPa at 700°C for 100 000h. One such candidate is austenitic stainless steels.

In this study, the microstructural stability of long-term aged and creep tested austenitic stainless steels is investigated. Nine austenitic stainless steels with different chemical compositions have been creep tested until failure at temperatures ranging from 600°C to 800°C for a duration of up to 30 years. Microstructural characterization of undeformed (aged) and deformed (crept) regions of the creep tested specimens was carried out using Light Optical Microscopy (LOM) and Field Emission Scanning Electron Microscope (FEG- SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD) techniques. In addition, prediction and quantification of stable phases were simulated with Thermo-Calc and TC-PRISMA.

The experimental observations were focused on the influence of several parameters such as alloying elements, temperature and deformation on the microstructural developments in aged and crept regions. The results clearly showed that the high Ni containing alloys are very stable at high temperatures. However, low Ni alloys can undergo a martensitic transformation at high temperature due to an unstable austenitic structure caused by Cr depletion.

In addition, a clear correlation between the test temperature and the Cr2N precipitation has been noted. Above 700°C, Cr2N is the dominant precipitate and below 700°C, sigma phase is the most prominently observed phase. It is also noted that the precipitation of phases and the volume fraction of these phases increase with deformation, as new sites for nucleation form.

Moreover, dynamic recrystallization (DRX) have also been observed in some crept specimens. The comparison between experimental results from EBSD phase mapping and thermodynam- ical simulations, reveals that the Thermo-Calc gives sufficient indication on the precipitation of possible stable phases at equilibrium. However, phases such as Cr2N and G-phase/η- nitrides(M6C) are not predicted. Furthermore, estimations of the volume fractions are not correctly predicted, as these are either over- or underestimated.