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Title: Welding of thin sheet steels in marine applications
Author: Beres, Miloslav
ISNI:       0000 0004 2692 0366
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2010
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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.
Supervisor: Dye, David Sponsor: European Community Marie Curie Intra European Fellowship ; EPSRC ; Great Britain Sasakawa Foundation
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral