Characterising creep crack growth behaviour in austenitic steel weldments

The main aim of this work is to develop an improved understanding of creep crack growth behaviour in C(T) specimens extracted from as-welded austenitic steel weldments, with particular emphasis on developing an improved understanding of the crack driving force in these specimens resulting from the combination of residual stress and applied loads.
Power-plant components operate at high temperatures where failures by creep mechanisms are possible. Some components can contain crack like defects which could grow by creep and fatigue processes. These defects usually initiate and grow in the vicinity of welds. Significant work has been performed to characterise the creep crack growth (CCG) rate with the steady state creep fracture mechanics parameter C* in laboratory tests on fracture mechanics specimens. In most cases, the crack growth properties are obtained by testing high constraint (side-grooved) Compact Tension C(T) specimens. However, in order to obtain CCG rates in different regions of a weldment (including the heat affected zone (HAZ) and weld metal), it is necessary to extract C(T) specimens that contain material from more than one weldment zone. For example, C(T) specimens with the crack in the HAZ will also contain significant amounts of weld metal and parent material. In addition, C(T) specimens extracted from as-welded (non-stress relieved) austenitic steel weldments have been shown to contain levels of residual stress that are sufficient to influence the behaviour of the specimens during CCG testing.
Weldments are particularly problematic due to their complex inhomogeneous structures that consist of numerous regions of variable grain sizes and microstructures, with a gradient of material properties that can be described as the undisturbed parent material (PM), heat affected zone (HAZ) and the weld metal (WM) Weldments are the principal source of failure in high temperature components, caused by creep mechanisms that are generally caused by residual stresses and influenced by material embrittlement. Welding residual stresses can induce creep strain accumulation during post weld heat treatment (PWHT) or operation in high temperature plant, resulting in a phenomena known as stress relief or reheat cracking in the HAZ, which is a major industrial concern. However, models to predict weldment failure are limited and, of great concern, there is a general deficiency in material property data available for weldments and their individual constituents, especially under multiaxial stress states. The current techniques to estimate weldment properties, including weld material simulation indentation and punch tests only consider the properties of WM/HAZ/PM in isolation. Hence, the effects of microstructure discontinuity, local property gradients leading to interactive deformation constraint effects, and welding residual stress cannot be accounted for. In addition, indentation and punch tests generate complex loading states and require considerable interpretation to transform their results into equivalent uniaxial test data. Significant scope for innovation therefore exists in weldment characterisation.
In-situ, high-resolution digital image correlation (DIC) measurements on tensile weldment specimens will enable the elastic-plastic and creep deformation and failure properties of weldment constituents and their interactive/constraint effects to be established. The mechanical properties measured from the DIC and mechanical tests will provide the accurate data required to validate FE simulations of weldments deformation and fracture behaviour.

It cannot be overstated how important reducing CO2 emissions are in both electricity production for homes and industry but also in reducing road pollution by replacing petrol/diesel cars with electric cars in the next 20 years. These ambitions will require a large growth in electricity production from low carbon sources that are both reliable and secure and must include nuclear power in this energy mix. Such a future will empower the vision of a prosperous, secure nation with clean energy. To do this the UK needs more than 100 PhD level people per year to enter the nuclear industry. This CDT will impact this vision by producing 70, or more, both highly and broadly trained scientists and engineers, in nuclear power technologies, capable of leading the UK new build and decommissioning programmes for future decades. These students will have experience of international nuclear facilities e.g. ANSTO, ICN Pitesti, Oak Ridge, Mol, as well as a UK wide perspective that covers aspects of nuclear from its history, economics, policy, safety and regulation together with the technical understanding of reactor physics, thermal hydraulics, materials, fuel cycle, waste and decommissioning and new reactor designs. These individuals will have the skill set to lead the industry forward and make the UK competitive in a global new build market worth an estimated £1.2tn. Equally important is reducing the costs of future UK projects e.g. Wylfa, Sizewell C by 30%, to allow the industry and new build programme to grow, which will be worth £75bn domestically and employ tens of thousands per project.

We will deliver a series of bespoke training courses, including on-line e-learning courses, in Nuclear Fuel Cycle, Waste and Decommissioning; Policy and Regulation; Nuclear Safety Management; Materials for Reactor Systems, Innovation in Nuclear Technology; Reactor Operation and Design and Responsible Research. These courses can be used more widely than just the CDT educating students in other CDTs with a need for nuclear skills, other university courses related to nuclear energy and possibly for industry as continual professional development courses and will impact the proposed Level 8 Apprenticeship schemes the nuclear industry are pursuing to fill the high level skills gap.

The CDT will deliver world-class research in a broad field of nuclear disciplines and disseminate this work through outreach to the public and media, international conferences, published journal articles and conference proceedings. It will produce patents where appropriate and deliver impact through start-up companies, aided by Imperial Innovations, who have a track record of turning research ideas into real solutions. By working and listening to industry, and through the close relationships supervisory staff have with industrial counterparts, we can deliver projects that directly impact on the business of the sponsors and their research strategies. There is already a track record of this in the current CDT in both fission and fusion fields. For example there is a student (Richard Pearson) helping Tokamak Energy engage with new technologies as part of his PhD in the ICO CDT and as a result Tokamak Energy are offering the new CDT up to 5 studentships.

Another impact we expect is an increasing number of female students in the CDT who will impact the industry as future leaders to help the nuclear sector reach its target of 40% by 2030.
The last major impact of the CDT will be in its broadening scope from the previous CDT. The nuclear industry needs to embrace innovation in areas such as big data analytics and robotics to help it meet its cost reduction targets and the CDT will help the industry engage with these areas e.g. through the Bristol robotics hub or Big Data Institute at Imperial.

All this will be delivered at a remarkable value to both government and the industry with direct funding from industry matching the levels of investment from EPSRC.