title: Effect of Prior Plastic Strain on the High Temperature Creep Deformation and Damage Response of Type 316H Stainless Steel
Creators: name: Nicol, Johannes
Publisher: name: The Open University
Year: 2022

description: Creep damage in ductile alloys is associated with creep deformation, crack growth and starts with the nucleation and growth of cavities. Under sustained high temperature and stress conditions, growing cavities can start to coalesce leading to microcracking and ultimate failure of a component. This mechanism can limit the lifetime of power plant components operating at high temperature. Many engineering components enter service in a cold-worked or prestrained condition as a result of manufacturing processes such as bending, forging, welding etc. Such pre-conditioning alters the creep resistance of the material significantly. Its effect on the creep deformation properties of a structure during service, and creep damage response can be advantageous for some materials but disadvantageous for others. Hence it is crucial to understand the effects of prior plastic strain when assessing the lifetime and safety of power plant components, for example in the context of nuclear power generation. The research set out in this thesis aims to examine the effect of prior plastic strain on subsequent creep deformation behaviour and development of damage in AISI Type 316H austenitic stainless steel, a material widely used in the fleet of Advanced Gas Cooled reactors operated by EDF Energy in the UK. A novel cylindrical hourglass-shaped test specimen was designed for the research where a constant applied load provided a variation in uniaxial stress and associated creep strain rate along the hourglass gauge length. A further innovation in this PhD work involved exploiting the potential of 3D digital image correlation (3D-DIC) for measuring spatially resolved creep deformation along the hourglass gauge section over long duration creep tests at a high temperature of 550◦C. The scope of testing included load-controlled creep tests carried out on 5 samples where 0, 4, 8, 12 and 16% of prior tensile plastic strain was introduced at room temperature. The prestraining was carried out on cylindrical samples before the hourglass shape was machined, ensuring a uniform level of prior plastic strain was present along the gauge section prior to creep experiments. It was found that prior plastic strain increased the creep resistance of the as-received material. Increasing plastic strain decreased the creep strain rate and creep ductility. On the other hand, it resulted in an increase in time to failure. After creep failure at the maximum stress location, small-angle neutron scattering (SANS) was utilised to investigate changes in creep cavitational damage as a function of applied stress, level of creep strain and prior plastic strain at room temperature. Two sets of experiments were performed using the D11 instrument at the ILL reactor source (France) and the SANS2D instrument at the ISIS spallation source (UK). Very similar scattering results were obtained from the two instruments. Furthermore, SANS data from the instruments were analysed using two independent analysis routes; a maximum entropy method (MAXE) and a Monte Carlo algorithm (McSAS). Since SANS is an indirect method for measuring creep cavitation, the microstructure of the specimens was also investigated using qualitative scanning electron microscopy (SEM) in order to interpret and verify the SANS cavitation observations. The SANS investigations revealed a strong correlation between the volume fraction and number density of creep cavities with applied stress and creep strain. Furthermore, an increasing number density of small creep cavities as a function of prior plastic strain was observed and verified by qualitative SEM studies. This is new evidence that prior plastic strain, induced at room temperature, introduces specific cavitational damage in Type 316H stainless steel. The macroscopic damage calculation based on the stress modified ductility exhaustion model revealed that the majority of damage for the series of prestrained specimens is caused by plastic hole growth as a consequence of inducing prior plastic strain rather than due to creep related diffusion processes.