Effects of prior austenite grain size on impression creep and microstructure in simulated heat affected zones of boron modified P91 steels

Abstract

The induction of this paper was to simplify the exaggeration effects of prior austenite size (PAG) size on microstructure, impression creep, creep-rupture life, creep-damage, and precipitate size in the sub-heat affected zones (sub-HAZs) of boron modified P91 steels. Simulation of sub-HAZs caused the manipulation in PAG size and microstructure. This manipulation reduced precipitate sizes (~50%) in P91B samples that lead to a 20% reduction in the non-homogeneous distribution of size of precipitates. Evaluation of minimum creep-damage rate (MCR) suggested FG (type-IV) and B-CG (type-III) as weakest links having a critical PAG size of 17 μm. But, creep-performance of B-CG was still better than most of the P91 samples. While PAG hardening limit was observed in CF/B-CF that made them the strongest links. Activation energy (AE) calculation showed that an increase in PAG size for P91 samples motivated MCR, inviting PAG embrittlement. While vice versa observed in P91B samples, inviting PAG hardening. P91B samples exhibited maximum creep-performance by the shrinking span of the creep-damage and increasing creep-rupture life. Mutual dependency between creep-rupture life and creep-damage showed that their relation was independent of microstructural features, thermal history, amount of deformation, and boron modifications. Whereas, limited creep-damage was found for samples having normalized creep-rupture life ≥ 2. This paper also discussed various diffusion mechanisms for sub-HAZs by simplifying inconsistency between calculated and actual AE that suggested both interstitial and substitutional nature of boron. Small-grained samples (PAG ≤ 17 μm) had lower AE, while big-grained samples (PAG > 17 μm) had higher AE.