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In this thesis, different modelling techniques, including physically based molecular dynamics simulations and empirical models using neural network architectures have been used to address particular problems in the understanding of microstructural development in iron-based systems. The two main areas of investigation are concerned with the prediction of composition and mechanical properties of steel welds and the effect of irradiation on the grain boundary microstructure of a-iron, both very important industrial issues. Microstructural evolution models in steel welds require weld metal composition as their starting point. Extensive analyses have been carried out concerned particularly with the prediction of weld metal chemistry, and also complex mechanical properties such as toughness, using neural network techniques and a database developed for one pass per side submerged arc welds typical of those used in the manufacture of linepipe. The neural network techniques used were based on a Bayesian framework, implemented using Markov chain Monte Carlo methods. The results showed a significant advantage in the use of neural network models for prediction of toughness compared with simpler regression analyses. In order to study the effects of irradiation on the structure of Fe-based systems, a molecular dynamics methodology was initially set up to study the equilibrium relaxed atomic configuration of symmetric tilt and twist grain boundaries in a-iron. These structures have been classified in terms of both the energy and width of the grain boundary region and the atomic arrangement has been also analysed for the tilt models in terms of structuraJ units. Radiation damage has then been studied near the relaxed structures of a symmetrical tilt and a symmetrical twist boundary in a-iron. Collision cascades have been initiated.inthe structure by imparting an initial energy of 1 keY to a single Fe atom, i.e. a primary knock-on atom (PKA). The subsequent interaction of the cascade with the grain boundary has been studied using molecular dynamics simulations. As a result of radiation, reordering is produced in the atomic structure of the boundaries, the damage being more pronounced in the twist model studied. Clusters of interstitial atoms are produced at the boundary. Changes in the properties of the interfaces after irradiation are discussed.

The microstructure of high-strength weld deposits has been investigated using thermodynamic analysis and phase transformation theory, backed by experimental confirmation. The microstructures of both the fusion and reheated zones of multirun alloy-steel weld deposits have been studied. The transformation mechanism of acicular ferrite has been established, and a theory for reaustenitisation in steel weld deposits has also been proposed. In addition to the studies on weld metals, some model alloys were fabricated and tested in order to confirm some of the predictions made by the new theory for reaustenitisation. All the weld metals studied in the investigation possess good hardenability. The primary microstructures of the fusion zone of these deposits consist mainly of acicular ferrite with very little allotriomorphic ferrite.

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This thesis describes a number of approaches to the problems of automatic defect detection in ultrasonic Time of Flight Diffraction (TOFD) and X-ray radioscopic images of butt welds in steel plate. A number of novel image segmentation techniques are developed, two of which feature the use of backpropagation artificial neural networks. Two new methods for defect detection in ultrasonic TOFD images are described - the first uses thresholding of individual one-dimensional A-scans, and the second uses a neural network to classify pixels using two dimensional local area statistics. In addition, three new methods for defect detection in radioscopic images are described - the first is based on the use of two conventional spatial filters, the second uses grey level morphology to replace the 'blurring' stage of conventional "blur and subtract' procedures, and the third uses a neural network to classify pixels using raw grey level data at the input layer. It is considered that all five methods which have been developed show novelty in their methodology, design and implementation, most specifically in that (1) no previous methods for automatic defect detection in TOFD images, (2) very few successful implementations of grey level data processing by neural networks, and (3) few examples of local area segmentation of 'real' textured images for automatic inspection have been reported in the literature. The methods developed were tested against data interpreted by skilled NDT inspectors. In the case of the ultrasonic TOFD image processing, both automatic methods performed exceptionally well, producing results comparable to that of a human inspector. In the case of the radioscopic image processing, the ANN method also produced results comparable to that achieved by a human inspector and also gave comparable or consistently better results than those obtained using a number of existing techniques.

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No description available.

This study was undertaken with the objective of determining the effect of gap distance on the weld bead geometry and the mechanical properties of the weldments. Low-alloyed and low carbon steel plates were welded under different conditions where each weldment had different gap distance and weld bead grooves. The influences of welding parameters namely, welding speed, current and voltage on the weld bead were examined in terms of weld bead penetration and heat affected zone and weld metal zone hardness variations.

The focus of this research was to quantify the post-fire mechanical properties of 5083-H116 and 6082-T6 aluminum alloys. Post-fire exposure is considered heating the material to a particular temperature then cooling the material back to room temperature. The research included evaluating parent materials as well as welded samples. Post-fire mechanical properties of parent materials were evaluated at temperatures ranging from ambient to 500oC with isothermal and transient heating. Changes in material properties were evaluated through static tensile tests and hardness testing on cooled samples. Using this data, an assessment was performed to investigate the relationship between hardness and mechanical properties. For the alloys evaluated, empirical relationships were found between Vickers hardness and post-fire strength. Testing was also performed on butt welded samples of 6082-T6 exposed isothermally to temperatures ranging from ambient to 500oC. Vickers hardness profiles were measured across a sample to quantify the hardness of the weld, heat affected zone, and parent material. This was performed at room temperature and following different heat exposures. Static tensile tests were used to evaluate the effect of reheating on the welded samples. Post-fire strength of welded samples was strongly affected by weld geometry. Parent material hardness varied with reheating while weld hardness remained constant. At select temperatures, this resulted in the weld having a higher Vickers hardness than the parent material. Despite this tensile failure always occurred within the weld. / Master of Science