Microstructural investigation of creep behaviour in Grade 92 power plant steels

The aim of this research project was to obtain a deep and quantified understanding of the effects of chemical composition and heat treatment on the microstructures of Grade 92 parent metals, and in turn, their corresponding creep behaviour. Comprehensive microstructural characterisation on the virgin and creep-tested samples was carried out in this study to give valid and reliable data in order to correlate the microstructural changes with creep behaviour, which can provide steel manufacturers with useful information to optimise the manufacturing route for fabrication.

Predicted long term creep behaviour based on the numerical extrapolations of short term data has been shown to be overestimated due to series of microstructural degradations. In particular, creep cavitation, which is equally important compared to other microstructural degradation mechanisms during creep exposure, was investigated by using a wide range of complementary microscopy techniques including field emission gun scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM) on focused ion beam (FIB) prepared thin foil lift-out samples with energy dispersive X-ray spectroscopy (EDS), dual beam serial sectioning with post image 3D reconstruction on the cavities and particles, and combined in-lens SEM examination and EDS analysis. BN, MnS and Al2O3 inclusion particles were observed to be the preferential cavity nucleation sites. Within these three types of inclusion particles, BN was the most frequent particle observed to be associated with cavities. [Continues.]