Investigation of creep damage and cavitation mechanisms in P92 steels

In typically processed creep stress enhanced ferritic steels (CSEF) it has been observed that the long term performance and creep rupture strength is below that predicted from a simple extrapolation of the short term creep data. One of the primary microstructural degradation mechanisms responsible for this behaviour is the development of creep voids. Nucleation, growth and interlinkage of voids will also result in a significant loss of creep ductility. In this study, the cavities in creep tested P92 steel have been characterised. It has been found that in the standard P92 samples studied, the cavities are associated with large (typically 1-2 μm) ceramic particles. Three particle compositions were observed and more detailed TEM investigations showed that these particles were BN, MnS and γ-Al2O3. Statistical analysis of the ceramic particle types showed that in all samples studied a very high proportion of the cavities was associated with BN particles. The shape of both the cavity and associated particle were studied in 3D using a Focussed Ion Beam/ Field Emission Gun Scanning Electron Microscope (FIB-FEGSEM) ‘slice and view’ technique. This showed that both the cavity and associated particle had very irregular shapes. Analysis in the head-gauge transition area of a creep test bar, where the exposure stress is low, showed small cavities associated with the BN particles. This provides strong evidence that the cavities were preferentially nucleating on the hard, irregularly shaped BN particles.