Many metal structures are assembled from thin plate with welded supports for stiffness
to resist local loadings. However, welded joints, which require large heat input, may
incur significant distortion in the finished plate. Although the causes of distortion are
known, and have been the focus of number of studies, there is still a lack of
fundamental understanding of process and physical parameters in causing distortion.
The overall aim of this work is to identify the interaction of process and physical
parameters in causing distortion of welded ferritic thin steel plates. Experimental
measurements and the finite element method are used to identify the relationship
between distortion and the influence of pre-existing (residual) stresses in the plates.
Effect of onset of transformation temperature on distortion is examined. An improved
comprehension of the mechanisms causing distortion, and a readily useable model to
explore alternatives has significant potential in wide range of industries and thus is a
major driving force for continued research. The ability to predict with reasonable
certainty the geometry of distortion will enable users to evaluate alternative design and
production parameters.
The work is divided into eight chapters:
The first chapter gives an introduction and lists the objectives of the research. A
theoretical exploration of the problem in addition to a survey of relevant work with
regard to the welding of ferritic steels, weld microstructures, residual stresses, finite
element modelling (FEM) and an overview of experimental techniques including
transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron
backscattered diffraction (EBSD), neutron diffraction (ND) is given in chapter 2. In
chapter 3 experimental investigation results including both post-weld and in-situ
microstructure observation and residual stress distribution are presented and discussed
[1, 2]. To provide a qualitative insight into fundamental understanding of development
of residual stress, a finite element model that considers both the thermal and the
transformation strains caused by solid-state phase transformation was developed and is
presented in the chapter 4 [3]. A validated finite element model for computation of
residual stresses is presented in the chapter 5 [4]. Special emphasis was placed on the effect of transformation temperature on residual stress development in both the actual
weld and the model. Most of the modelling results were validated against experimental
measurements. Chapter 6 presents a sensitivity study on the effect of parameter
changes on distortion. An attempt was made to elucidate both the effect of
transformation start temperature and the initial distortion on the final distortion [5].
Chapter 7 details application of a model for variant selection [6], which is based on
work published in [7, 8] to actual welds. This work was performed to elucidate the
effect of texture on residual stress. Finally the last chapter draws together the major
conclusions of the thesis, and suggests future avenues of investigation to progress the
research discussed here.