Second-year NSIRC PhD student Yao Ren has just returned from Australia, where she spent six weeks working with the Australian Nuclear Science and Technology Organisation (ANSTO).
Yao’s work at ANSTO, carried out in collaboration with Brunel University London and Australia’s Bragg Institute, saw her using a scientific instrument known as KOWARI – a uniquely capable residual stress diffractometer.
ANSTO, which has been in operation for over 60 years, conducts research and development in relation to nuclear science and technology. It aims to ‘deliver excellence in innovation, insight and discovery’, and is well known for the OPAL reactor. OPAL is one of a small number of reactors with the capacity for the commercial production of radioisotopes, and among the best research reactors in the world.
Yao spent her time with ANSTO participating in a large-scale investigation into the effectiveness of various post-weld heat treatment (PWHT) procedures on stress relaxation of girth welds. The research is directly related to work she is currently undertaking as part of her studies with NSIRC.
Welding residual stress is an important variable in the assessment of structural integrity, as it increases the risk of brittle fracture. PWHT of welds is usually employed to mitigate the residual stresses in welded joints, and to improve the mechanical properties of the weld and heat-affected zone by tempering the material’s microstructure.
Normal practice, for PWHT is to place a component in an enclosed furnace to heat it uniformly, preventing the generation of additional thermal stresses and residual stresses. However, it is not always possible to place an entire component in a furnace (for example circumferential welds in a pipeline), so sometimes local PWHT is the only option.
Compared to fully controlled, furnace-delivered PWHT of welded joints, there has been little research into the effectiveness of local PWHT for residual stress relief. Yao’s research aims to understand the effects of various parameters in local PWHT, such as heating rate, cooling rate, soak temperature, hold time, heated band width, soak band width and insulation conditions on stress relief. Measuring residual stresses after PWHT will reveal the effect of different PWHT methods (locally and globally) on stress reduction in pipe girth welds.
Yao’s investigation used welded pipes 1000mm in length, with a 355.6mm outside diameter and 19mm wall thickness. Four of these were sent to ANSTO for residual stress measurement by neutron diffraction. For large components, neutron diffraction such as that made possible by KOWARI is excellent at obtaining three-dimensional residual stress information non-destructively within the interior material, for small test volumes as well as thick specimens. Stress conditions in d0 samples – un-stressed lattice parameter specimens - were also investigated.
Residual stresses in pipe girth welds and stress-free samples were also measured, both before and after PWHT. In the first part of the experiment, two pipes and four d0s were measured in as-welded condition. In each pipe, up to 40 measurement points were distributed around the weld centre and the adjacent parent metal – not only on the pipe’s outer and inner surfaces, but also throughout the pipe wall. All points were measured in three dimensions – in hoop, axial and radial directions – in order to generate full maps of residual stress distribution. The second part of the experiment saw lattice spacing measured again at the same locations on all four pipes and d0 samples after PWHT had taken place.
Yao also conducted PWHT experiments on pipes between the two allocations of the neutron diffraction beam time. Three of the pipes were locally heated using different widths of heating band, and the fourth heated globally in a conventional furnace. Other parameters affecting PWHT – heating rate, cooling rate, soaking temperature, soaking time and insulation band width – were controlled to be identical for all the pipes, in order to investigate the influence of heating band width on welding residual stress relaxation. The effect of these parameters will be investigated in subsequent simulations.
Yao said: ‘I feel privileged to be one of the students from NSIRC to be given the opportunity to use the KOWARI Instrument. It is unique in that it provides information not obtainable by any other technique.’
As the work placement was located just 40 minutes’ travel from central Sydney, it also gave Yao an opportunity to visit the Australian city.
‘I had chance to explore Sydney to go and look around,’ she said. ‘I was also lucky enough to be taken by my colleague at ANSTO, Ania, to the nearby beach after work – it was a really enjoyable evening, and I really appreciated this opportunity being given to me by NSIRC.’
NSIRC is a state-of-the-art postgraduate engineering facility, which provides its students with a number of opportunities to travel for their research. These include work placements such as Yao’s, as well as conference attendance and opportunities to present on their publications.