Characterisation and quantification of cavities in 9Cr martensitic steel for power plants

Abstract

This work focuses on the characterisation of cavities evolution in a P91 steel pipe in three conditions: as received, and after creep at 650°C and 60 MPa for 7000 and 9000 h. A field emission gun scanning electron microscope (FEG-SEM) equipped with focused ion beam (FIB) gun, a conventional scanning electron microscope (SEM) and a light optical microscope (LOM) have been employed for the investigation. This study reveals two types of cavities: the pre-existing cavities, which are rare in this type of heat resistant steels, with a mean diameter of 2·56 μm and the cavities produced during creep with diameters smaller than 0·6 μm. Lath boundaries and precipitates are found to be preferential sites for cavity nucleation. Furthermore, the number density and volume fraction of these small cavities are calculated from 2D measurements and compared to 3D results obtained by FIB serial sectioning.

Keywords:

• Creep cavities
• FIB serial sectioning
• Stereology
• Precipitates
• Martensite
• P91

Acknowledgements

The experimental work was carried out under the project Hot-Pipes: 826434, ‘Entwicklung eines SHM-Systems für die Überwachung von Hochtemperatur-Rohrleitungen in Kraftwerken’ and the analysis under the project Z-Ultra: 309916, ‘Z phase strengthened steels for ultra-supercritical power plants’. The authors would like to thank the FFG Austrian Research Promotion Agency in Vienna and the European Commission for their financial support.