Characterization, quantification and growth modelling of pores in 9Cr steels
Surya Deo Yadav
Institute for Materials Science and Welding , TU Graz
http://iws.tugraz.at/
11:20 - 11:40 Thursday 23 October 2014 Rechbauerstrasse 12, HSII Creep failure of materials under service conditions strongly rely on the formation and growth of cavities, encouraging the characterisation, quantification and modelling of the cavitation. This work focuses on cavities evolution in a P91 steel pipe in three conditions: as received, and after creep at 650⁰C 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), X-ray computed tomography (CT) and a light optical microscope (LOM) have been employed for the investigations.
This study reveals two types of cavities: the pre-existing ones, 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 creep cavity nucleation. Furthermore, the number density and volume fraction of these small cavities are calculated from 2D measurements applying stereology and compared to 3D results obtained by FIB serial sectioning.
Nearest neighbours distances between pre-existing pores, number density and volume fraction are calculated from 2D measurements using stereology. It is found that the nearest neighbours distance, mean radius and volume fraction of pores are increasing, while number density is decreasing with creep exposure time. This suggests that pre-existing pores are growing as well as agglomerating with exposure time. The pores agglomeration is confirmed by CT measurements. The pore growth is studied applying a physical growth model, and experimental results are compared with numerical simulation. From this research it is deduced that damage occurs by agglomeration and growth of pre-existing cavities. The developed model can predict the growth of pores as a function of temperature and load at service.
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