Heat Treatment Optimisation of Electron Beam Welded Reactor Pressure by Vessel Steel

Abstract

Reactor pressure vessel (RPV) integrity is a key limiting factor for the lifetimes of nuclear power plants, as replacing them during its operational lifetime is impracticable. SA508 Grade 3 steel is currently widely used as a RPV material for light water reactors, and conventional joining techniques (e.g., multi-pass arc welding) are employed for RPV construction. However, with nuclear new build looking at plant lifetimes in excess of 60 years, the need for improved RPV material and joining techniques has been highlighted. In response to these requirements, SA508 Grade 4N steel and electron beam welding (EBW) were identified as potential upgrades. EB welds are autogenous, meaning no filler metal is used, and hence, in theory, it should be possible to homogenise microstructure and properties across the weld given a suitable post weld heat treatment (PWHT). This thesis investigates the development of novel PWHTs for EB welded Grade 4N steel, with the aim of fully homogenising the microstructures and properties across the weld.It was found that microstructure and hardness was successfully ho- mogenised across the parent and weld in all heat treatments involving re-austenitisation at 860C, but compositional microsegregation per- sisted in the fusion zone (FZ). Although homogenisation at 1200C eliminated this segregation, the weld area remained visible optically after etching, owing to the presence of coarse compositional banding in the forged parent material (PM). Furthermore, homogenisation at 1200C led to the formation of unusual prior austenite grain structures following subsequent re-austenitisation at 860C, which may not be desirable. Nevertheless, a full re-austenitisation was found to clearly homogenise the microstructures and hardness values across the weld, and this augurs well for the future application of EB welding to nuclear pressure vessels. To aid in the further understanding of grain morphology response to heat treatment, the austenite grain growth behaviour in the FZ and PM were studied and compared with other. It was found that on the whole, prior austenite grain (PAG) size increased with austenitisation temperature and hold time. However, a two-stage grain growth behaviour was observed in the parent material consisting of moderate grain growth (up to 950C) followed rapidly increasing PAG size (at 1000C). A single regime was observed in the fusion zone characterised by steadily increasing grain size. Small differences in Al and N content between the parent material and fusion zone led to a 70°C difference in the temperature at which AlN was predicted to dissolve: 1010C in the parent and 940C in the fusion zone. Furthermore, the NanoSIMS maps show the nitride population in the parent material is drastically larger than in the fusion zone. The differences in composition, pinning particle population, austenite grain growth behaviour and ultimately grain size may lead to difficulties in successfully applying a truly homogenising PWHT to electron beam welded SA508 Grade 4N steel. The continuous cooling transformation (CCT) behaviour of the FZ and PM regions were also studied to determine whether there was any fundamental difference in their heat treatment response. Using a dilatometer, transformation start temperatures could be identified for martensite and bainite, and it was found that the transformation start temperatures, both Bs and Ms, for all cooling rate cases were similar and that there was no significant difference in the CCT behaviour between the PM and FZ regions. This is promising for achieving full uniformity across the material with a PWHT. Many aspects of homogenisation through PWHT look promising, but further investigation into the effect of the resulting large and unusual prior austenite grain structures are warranted. Additionally, an understanding of the effect of microsegregation on irradiation effects (e.g., irradiation hardening) would help determine whether the extra effort to carry out homogenising PWHTs is necessary from a safety and RPV integrity standpoint.

Date of Award	10 Sept 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	John Francis (Supervisor) & Ed Pickering (Supervisor)