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EPSRC Reference: EP/H048294/2
Title: Quantification of transformation plasticity effects in steel welds
Principal Investigator: Francis, Dr J
Other Investigators:
Researcher Co-Investigators:
Project Partners:
Rolls-Royce Plc
Department: Mechanical Aerospace and Civil Eng
Organisation: University of Manchester, The
Scheme: First Grant - Revised 2009
Starts: 01 April 2011 Ends: 31 July 2012 Value (£): 58,238
EPSRC Research Topic Classifications:
Materials Characterisation
EPSRC Industrial Sector Classifications:
Aerospace, Defence and Marine
Related Grants:
Panel History:  
Summary on Grant Application Form
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.
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Organisation Website: http://www.man.ac.uk