New correlative microscopy approaches to understand the link between local microstructure and creep cavity initiation

Abstract

Reheat cracks are known to occur in the heat affected zone of AISI Type 316H austenitic stainless steel boiler header in AGRs as a consequence of creep cavity coalescence and interlinkage. During long-term in service thermal aging at elevated temperatures (490-530∘C), microstructual evolution happens, and creep cavities nucleate and grow under residual and service stresses. This thesis aims to better understand the phase evolution and creep cavitation during the long-term in-service aging, the role of local microstructure on creep cavity initiation, and cavity nucleation and early-stage growth.
An ex-service Type 316H stainless steel boiler header material was investigated by multiple characterisation techniques aiming to reveal potential precipitation and cavitation sequence, and underlying mechanisms. The continuous nucleation and growth of M₂₃C₆ carbide precipitates can cause local elemental depletion in austenite stabilisers, promoting austenite-ferrite phase transformation. The formation of ferrite phase at the grain boundaries may accelerate creep cavitation by adding more interfaces and modifying local surface energy, leading to significant creep cavitation at random oriented grain boundaries.
A correlative large length-scale microscopy approach was introduced in this study. This approach uses stitching and image correlation algorithm to characterise microstructure and creep cavitation in a correlative way, making statistical and quantitative analysis possible to understand correlations between local microstructural parameters and creep cavitation. The approach was successfully applied on a creep tested ex-service specimen. The creep cavitation was found to strongly correlate with inhomogeneous deformation and grain boundary misorientation. The creep cavities prefer to form at the random grain boundaries where significant precipitation and strain heterogeneity are present. And Σ3 CSL boundaries have the most cavitation resistance. While other microstructural parameters such as grain size and global Schmid factor exhibited weak or neglectable correlations with cavitation.
Two low stress interrupted creep tests were performed in this study. The same correlative microscopy approach was applied aiming to better understand cavity nucleation and early-stage growth. Stress triaxiality appeared to play a dentrimental role in cavitation resistance. The correlative microscopy approach was shown to be limited at finding correlations between microstructural parameters and early-stage cavitation due to too small number of creep cavities at the early stage of creep.
This thesis shows that grain boundary precipitates play a significant role in creep cavitation in 316H steels, and using a combination of advanced microscopies allows us to understand the underlying mechanisms with new details.

Date of Award	27 Sept 2022
Original language	English
Awarding Institution	University of Bristol
Sponsors	Edf Energy
Supervisor	Peter Flewitt (Supervisor) & Tomas L Martin (Supervisor)