Irradiation Sensitivity of Hot Isostatically Pressed Steels

Abstract

Hot isostatically pressed steels are fast becoming a viable alternative to their more traditional forged counterparts for the construction of heavy nuclear plant components. This thesis describes a study in which the viability of using powder HIP'ed SA508 grade 3 steel for the construction of nuclear reactor pressure vessels is further assessed, by comparing its irradiation hardening sensitivity with that of forged analogues. A sample of HIP'ed SA508 grade 3 and samples of forged analogues subjected to different heat treatments were characterised using a range of microscopy techniques, including light optical microscopy, scanning electron microscopy coupled with electron backscatter diffraction mapping, transmission electron microscopy and scanning transmission electron microscopy, in order to understand how their different thermo-mechanical processing routes had affected their underlying microstructures. Proton irradiation was used as a surrogate for in-reactor neutron irradiations to irradiate specimens to doses of 0.1dpa (T=300oC, E=2MeV), 0.4dpa (T=225oC, E=2.5MeV) and 1.1dpa (T=225oC, E=2MeV). Mechanical testing of the resulting specimens, in order to determine their irradiation hardening, was performed using a combination of size-effect fitted and pile-up corrected nano-indentation, along with in-situ in-SEM mesoscale tensile testing coupled with high-resolution digital image correlation strain mapping. Characterisation of irradiation induced microstructural evolution, in order to understand the inter-play between known irradiation hardening mechanisms and the underlying microstructures of the materials, was performed using scanning transmission electron microscopy coupled with energy dispersive x-ray spectroscopy and transmission kikuchi diffraction mapping. It is concluded that HIP'ing SA508 grade 3 steel results in a microstructure containing a higher proportion of diffusively transformed polygonal ferrite and less displacively transformed bainite and Widmanstatten ferrite, which in turn results in a coarser microstructure containing fewer boundaries and in particular fewer sub-grain boundaries to act as sinks for both mobile solute elements and irradiation induced defects. This increases the availability of mobile defects and solute elements in the matrix for the formation of solute-defect clusters, increasing irradiation hardening sensitivity by as much as 40% in the materials tested. In order to maximise resilience to irradiation hardening in low alloy RPV steels, thermo-mechanical processing routes which promote the formation of diffusively transformed ferritic microstructures should therefore be avoided. Recommendations for future research are made accordingly.

Date of Award	1 Jan 1824
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philipp Frankel (Supervisor) & Grace Burke (Supervisor)