The current generation of advanced gas-cooled nuclear reactors (AGRs) in the United Kingdom are approaching the end of their intended operating periods. Whilst awaiting the commissioning of new plant, there is a desire to extend the operational lifetime of the AGR fleet to maintain electricity supply to the UK grid. One of the factors determining lifetime extension of the AGRs is creep damage in boiler plant components, some of which are constructed from AISI Type 316H steel. These components operate under stress at elevated temperatures between 490-550°C for periods of 70,000 hours or longer.

Creep damage is manifest through the initiation and evolution of creep cavities in these components and is strongly linked to secondary phases in the steel, which are also evolving over time due to thermal ageing \[1\]. This study explores the microstructural relationship between secondary phase precipitates and creep cavitation in an ex-service 316H boiler header specimen, as well as controlled uniaxial creep tests on similar material.

Various multiscale materials characterisation techniques can be adopted to explore the prevalence of creep cavities on different grain boundaries. Three-dimensional tomography using X-ray and serial focused ion beam sectioning are used in combination with scanning and transmission electron microscopy to understand the relationship between cavity shape and distribution, grain boundary orientation and secondary phase distribution \[2\]. The presence of M~23~C~6~ carbides and bcc phases at the boundaries are observed to dramatically affect cavity initiation \[3\].  

This study uses transmission electron microscopy imaging and diffraction to explore how cavity formation depends on both the secondary phases present and the crystal orientation of adjacent austenite grains at the boundary, and correlates the behaviour of individual cavities with larger-scale behaviour and distribution using 3-D tomography. The implication of these experimental data on both computational modelling of creep and lifetime predictions for components will be discussed.

 

\[1\] A.D. Warren, A. Martinez-Ubeda, I.J. Griffiths and P.E.J. Flewitt, The implications of fabrication and cast-to-cast variability on thermal ageing in the creep range for AISI Type 316H stainless steel components, Metallurgical and Materials Transactions A, 50 (2), pp. 987-996 (2019)

\[2\] S. He, A.D. Warren, D. Knowles, P.E.J. Flewitt and T.L. Martin, Comparing 2D and 3D techniques for the characterisation of creep cavities, submitted to Proceedings of ESIA15 and ISSI-2019 Joint Conference on Engineering and Structural Integrity Assessment (2019)

\[3\] A.D. Warren, I.J. Griffiths, R.L. Harniman, P.E.J. Flewitt and T.B. Scott, The role of ferrite in Type 316H austenitic stainless steels on the susceptibility to creep cavitation, Materials Science and Engineering A, 635, pp.59-69 (2015)


The Role of Grain Boundary Orientation and Secondary Phases in Creep Cavity Nucleation of a 316h Boiler Header

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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