Quantification of transformation plasticity effects in steel welds

It has long been known that fusion welding generates substantial levels of residual stress, and that these stresses are generally detrimental to the integrity and performance of the components that have been joined. Such stresses result from the highly localised application of heat, which in turn leads to localised thermal contraction strains that are incompatible with material further away from the weld. A conventional strategy for reducing weld residual stresses would involve subjecting the item of interest to a post-weld heat treatment (PWHT) procedure, whereby it would be heated to an elevated temperature for a specified duration. However, if components are large or thick-walled, a PWHT operation is often not possible once they are assembled. As a consequence, high levels of detrimental tensile residual stresses often reside in the vicinity of welds.In the past few years an exciting area of research has emerged, based on the possibility of exploiting the solid-state phase transformations that occur in steels in order to mitigate the residual stresses that arise during welding. These transformations, or changes in the arrangement of atoms, have associated strains which, depending on the transformation mechanism and temperature, can be engineered to compensate for the thermal contraction strains that arise as a weld cools. In this way the design of smart weld filler metals with carefully engineered transformation temperatures could lead to dramatic reductions in the residual stresses that arise in welds, thus inspiring the development of a new philosophy for welding, based on prevention rather than cure . However, there are still some significant obstacles to the potential of this low-transformation-temperature (LTT) technology being realised. Firstly, in order to optimise the transformation temperature of a steel, it is vital that the magnitude of the transformation strains can be predicted beforehand. Other important challenges include the ability to design steels that have optimised transformation temperatures while also meeting other important material property requirements such as being tough or resistant to corrosion.In this work, the aim is to quantify the extent to which two mechanisms of transformation plasticity (i.e. Greenwood-Johnson transformation plasticity and variant selection) contribute to transformation strains in steels during welding thermal cycles. Greenwood-Johnson transformation plasticity arises, during a phase transformation, when the growth of a hard or strong daughter phase induces plastic flow (deformation) in the softer parent phase. Meanwhile, variant selection occurs when the presence of mechanical stress during a transformation favours the formation of some crystal orientations over others, leading to a transformation strain that is dependent on direction within the material. In quantifying the contribution of each of these mechanisms, a framework will be established for the inclusion of both mechanisms for transformation plasticity in to finite element models for welding.In this work state-of-the-art diffraction techniques will be applied, using neutrons and high energy X-rays, to investigate some complex aspects of the behaviour of steels during a solid-state phase transformation. The results that are obtained with these techniques will be validated against measurements made by more conventional means, such as dilatometry. This research will assist in the development of new steels that have improved performance after welding, and it will also improve our ability to assess the remaining life and likely performance of existing welded steel structures.

Benefits to the Public: The wider public both within the U.K. and internationally will benefit from this research through opportunities for improvements in the performance of steel components that have been welded, wherever the useful life of such components or structures is affected by residual stresses. Steels are ubiquitous, and they are by far the most widely used class of materials in construction. An improved understanding of the development of transformation plasticity, transformation strains and hence residual stresses in steels would thus lead to improvements in component performance and, either through life extension or weight reductions, reduced consumption. This will translated to environmental benefits and, in cases where the failure of a component could harm or lead to the loss of human life, there will be clear benefits to our safety. Benefits to the United Kingdom: The estimation of residual stresses that exist in and around welds is of significant concern to the integrity of the United Kingdom's nuclear submarine fleet, and hence the national security of the United Kingdom. It is vital that these stresses can be estimated with confidence to ensure that submarines remain in service for their intended life time, and that they can carry out operations as necessary while providing the safest possible environment. Benefits to Specific Industry Sectors: Power Generation: An improvement of our ability to predict the residual stresses in power plant welds would lead to corresponding increases in confidence in our ability to conduct safety assessments and to plan for plant shut downs and maintenance. This will, in turn, lead to improvements in the reliability of electricity supply, and will translate to cost savings which may be passed on to the consumer. An improved understanding of the development of residual stresses in welds would also offer the potential for power plants to operate critical components at higher temperatures which, in the case of thermal power plants, would reduce the quantity of greenhouse gas emissions for each unit of energy that is generated. Oil and Gas: Fatigue failures at welded joints are of critical concern in the construction and maintenance of off-shore platforms. Residual stresses are highly detrimental to the fatigue performance of welds subject to cyclic loading. An improved understanding of transformation plasticity in welds, together with the concurrent development of low-transformation-temperature steels, would lead to the reduction of residual stresses in joints and corresponding improvements in the safety and performance of off-shore platforms. Automotive: Since fatigue cracks generally initiate at spot welds, there is a substantial incentive to reduce associated residual stresses. An improved understanding of transformation plasticity would assist in the development of steels that were less prone to developing yield level residual stresses. Communications and Engagement: I am a member of a European Network for Structural Integrity, which comprises approximately 40 academics, scientists and engineers from across Europe. I will be presenting the outcomes of the research proposed here to this committee, and discussing opportunities for the uptake of the research findings with some of the members with whom I collaborate. Within the U.K., I have strong links with Rolls-Royce Marine, Serco and British Energy. I will present an overview of the outcomes of this research to engineers and scientists from these companies, together with a strategy for the incorporation of key findings in to structural integrity assessments on welds. I will also create a web page on the Materials Engineering website at The Open University, describing the underlying principles and outcomes from the proposed work, so that key information is available on the internet. The results of this research will also be published in the journal Acta Materialia and at key international conferences.