Thesis (M.Sc.)(Chemistry)--University of Pretoria, 2004. / Summaries in English and Afrikaans. Includes bibliographical references.

The objective of this investigation was to determine the influence of work roll cooling on roll thermal behavior. The investigation has been conducted utilizing a previously developed computer model [1]. Three industrial operations were evaluated and features of an optimum configuration have been identified. The magnitude of the thermal zone near the roll surface within which the cyclic temperature variation is confined has been evaluated for different roll cooling conditions. A two dimensional thermal model of a work roll has been developed to predict the thermal field inside the roll, based on which the thermal crown of the roll was calculated under an assumption of axi-symmetric deformation. The heat transfer equation was solved by the numerical finite difference method, the Alternating Direction Implicit (ADI) method. This method makes it possible to solve the problem on a personal computer making it suitable for industrial application due to the minimum requirements of computer storage and time. Also because the thermal model simulates operating conditions including the cooling configuration it is of value in studying a variety of industrial rolling conditions. The influence of roll spray arrangement, roll gap heat transfer and mill pacing on roll thermal crown has been investigated. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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Fusion welding processes cause residual stress due to the uneven heat distribution produced by the moving welding torch. These residual stresses are characterised by a large tensile component in the welding direction. Due to the self-equilibrated nature of the residual stress, compressive ones are present in the far field next to the weld seam, which can cause different kind of distortion such as bending or buckling. Welding residual stress can be responsible of premature failure of the components, such as stress crack corrosion, buckling, and reduction of fatigue life. Localised rolling is a stress engineering technique that can be used to reduce the residual stress and distortion caused by welding. It induces plastic strain in the rolling direction, counteracting the plastic strain produced during welding. In this thesis three techniques were investigated, pre-weld rolling, post-weld rolling, and in situ rolling. These techniques have been seldom studied in the past, particularly pre-weld rolling; consequently the mechanisms are poorly understood. Finite element models allow stress and strain development during both welding and rolling processes to be better understood, providing an improved understanding of the mechanisms involved and aiding process development. A literature survey was done to find the state of the art of the computational welding mechanics simulations, stress management, and the residual stress measurement techniques, as well as the knowledge gaps such as, the thermal losses through the backing-bar in the thermal simulation, the frictional interaction in the rolling process, and the material properties of the steel used in the models. In the literature not many models that investigate the management of welding residual stress were found. After this, the general considerations and assumptions for the welding thermal mechanical models presented in this thesis were discussed. The effect of different backing-bar conditions, as well as different material properties where investigated. Both influenced the residual stress profile to varying degrees. In particular, temperature dependent heat loss to the backing-bar was necessary to capture the improved heat loss near the weld. The distortion predicted by the model was investigated to determine whether it was due to bending or buckling phenomena. Lastly, the temperature distribution and residual stress predictions were validated against thermocouple and neutron diffraction measurements conducted by Coules et al. [1–3]. Pre-weld rolling was the first of the three rolling methods considered, in which rolling is applied to the plates before performing GMA butt-welds. The principle behind this technique consisted in inducing tensile residual stress in the weld region before welding; therefore, it is similar to mechanically tensioning the weld, which can significantly reduce the residual stress and distortion. However, there was no significant change in the tensile residual stresses. On the other hand, it was possible to achieve a small reduction in the distortion, when the plates were rolled on the opposite surface to the weld; rolling in this way induced distortion in the opposite direction to the distortion induced by welding, reducing the magnitude of the latter. These results were compared with experiments conducted by Coules et al. [1,4]. A subsequent investigation combined pre-weld rolling with post-weld heating. With this additional process the residual stress and distortion were significantly reduced, and flatter residual stress profile was achieved. The post-weld rolling and in situ rolling techniques were discussed afterwards. In the post-weld rolling models, rolling was applied after the weldment was cooled to room temperature. In in situ rolling the roller was applied on top of the weld bead at some distance behind the torch, while it was still hot. The principle behind these techniques consisted in applying positive plastic strain to the weld bead region by a roller, counteracting the negative plastic strains produced in the welding process. Two roller profiles were investigated, namely, grooved, and double flat rollers. The post-weld rolling on top of the weld bead models, which used the grooved roller, showed good agreement against experimental results, producing a large reduction of the residual stress and distortion. Some discrepancies were present when the weld toes were rolled with the dual flat roller. The former roller was more efficient for reducing residual stress and distortion. The influence of different friction coefficients (between the roller and weldment, and between the backing-bar and the weldment), were investigated. It showed significant dependency on the residual stress distribution when high rolling loads were used. The frictional interaction constrained the contact area inducing more compressive stress in the core of the weld bead; therefore it produced more tensile residual stress in the surface of the weldment. Additionally, the influence of rolling parameters on the through-thickness residual stress variation was investigated. Low loads only influence the residual stress near the surface, while high loads affected the material through the entire thickness. When the dual flat roller was used to roll next to the weld bead, significant compressive residual stress was induce in the weld bead; however, the residual stress reduction was very sensitive to the contact of the roller to the weld toes; therefore, when rolling a weld bead that varies in shape along the weld, the residual stress reduction is not uniform and varies along the length. On the other hand, the in situ rolling did not produced significant residual stress or distortion reduction in all the cases analysed. The rolling occurred when the material was still hot and the residual stress was subsequently formed as the material cooled to room temperature. Numerical modelling was a very useful tool for understanding the development of stress and plastic strain during the welding and rolling processes.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Three aspects of ring rolling are investigated in this thesis, single stage closed-pass rolling, multi-stage rolling and radial-axial rolling. In Chapter Two, which deals with single stage closed-pass rolling, the rolling of a range of symmetrical and non-symmetrical profiles is investigated., The effects of feed rate, lubrication, small variations in the geometry of the roll and blank shape on profile filling are examined experimentally. The occurrence of defects and surface cracking and the stability of rolling when producing non-symmetrical profiles are examined. Rolling force and torque are predicted theoretically and comparisons with experimental results are made. In Chapter Three the production of a bicycle rim is investigated, following an enquiry from a cycle manufacturer. A multi-stage ring rolling operation is employed. The problems of profile filling, cracking and surface finish are solved. 'Means of providing stability to the rolling process and diameter control are developed and tested. The circumference is controlled to within 1 mm with radial thickness variations within 1% of the specified thickness. Various aluminium alloys and a magnesium alloy are used as test materiaLs. A theory for the dimensional limitations in relation to profile folding in the final stage is given. In Chapter Four a newly installed experimental axial rolling system is described. The system was designed and manufactured in the Department and it was provided with control and measuring equipment. Stability of the rolling process is examined theoretically and experimentally for axial and radial-axial ring rolling operations and the control system is examined. Rolling force and torque and power dissipation are predicted theoretically and compared with experimental results.

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