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There are many potential advantages to welding structural ferritic steels with a power-beam process such as electron beam welding. These include low distortion, high welding speed, and computer control. The microstructure developed by a typical EB weld is poor from a mechanical properties perspective, however, and this limits the applications in which it is used. This work investigates the microstructures developed in an EB welded structural steel with the aim of further understanding and promoting a sound microstructure. A single steel composition was welded using a wide range of conditions. This resulted in the full gamut of possible microstructures being produced. These welds indicated that there may be a connection between the solidification microstructure and the room temperature microstructure in this steel. Further experiments were carried out that varied certain weld conditions and the link was confirmed. Two hypotheses were suggested to explain the observed microstructures and these were tested by replicating the EB weld microstructure with an A-TIG weld and then quenching to 'freeze' the partially developed microstructure. It was discovered that the poor microstructure in this particular steel was the result of Mn segregation, which caused lines of preferential nucleation through out the weld and thus, an aligned ferritic microstructure. The A1:O ratio in the weld metal is known to be a particular problem for EB welds and, in the final experimental chapter, an experiment was designed to test the hypothesis that excess soluble A1 in the steel prevents the nucleation and growth of acicular ferrite.

This thesis deals with the changes in chemical composition and microstructure that occur when a joint between two different steels is heat treated for prolonged periods of time at a temperature below that at which austenite forms. Such joints are common place in power plant where components with different purpose are joined by welding. The aim of the work was to characterise the changes that occur at the weld junction and to quantitatively model them such that the method has predictive power. Metallographic analysis on dissimilar steel welds and bonds, heat-treated over a range of times and temperatures showed the ease of decarburised and carburised zone formation and their associated microstructures. In the low-alloy steel, rapid carbide dissolution close the dissimilar interface leaves room for grain growth that forms a decarburised zone. In the associated carburised zone, perception at and carbide coarsening occurred on grain boundaries and intra-granularly due to the associated increase in carbon concentration. In vanadium-containing low-alloy steels, stable vanadium-alloy carbides were found to complicate carbon partitioning generating wider, shallower decarburised zones compared to the smaller, almost carbide-free ones in vanadium-free steels. A model, developed for estimating the decarburised zone width in ferritic transition joints considers the carbon concentration as a function of distance from the dissimilar interface resulting from chemical potential gradients in the joint. Written in FORTRAN 77, the model uses the Crank-Nicholson implicit finite difference solution of Fick's second law, and was found to accurately predict decarburised zone widths based on measurement taken from welds and bonds from a given series of tempering tests. The existence of substantial carbon chemical potential gradients in the transition joint causes carbides to dissolve in the low-alloy steel during heat-treatment, releasing carbon to diffuse across the dissimilar interface to form a corresponding carburised zone. Naturally, the temperature and time of the heat treatment influences carbon diffusion, but the particular alloy combination in the transition joint also affects the amount of carbon partitioning.

One of the aims of the study has been the evaluation of suitable wires for welding API-X80 and En8 grade steels, on the basis of experimental results. The experimental results (from Charpy, CTOD and tensile testing together with general microscopy) were obtained using commercially available flux and metal cored wires. Weld metal tensile properties similar to those of the parent metal were obtained, although the fracture properties were generally inferior. An exception to this was shown by the En8 steel welded with a high nickel content wire which exhibited toughness values greater than those recorded for the parent metal. However, the high nickel content makes the use of such a weld in certain corrosive environments problematic. The best toughness properties amongst the X80 welds were shown by the higher oxygen containing welds. For these X80 welds, mechanical test results correlated strongly with microstructure, which varied from coarse to fine ferrite. In order to improve the toughness properties, experimental wires were produced using a combination of computer modelling (weld prediction program) and statistical methods (Taguchi methods). These provided successful in that significant proportions of acicular ferrite were obtained. However, the toughness values did not show a corresponding improvement. This was believed to be due to the presence of inclusions, essential in order to nucleate acicular ferrite, but which have a direct effect on the fracture resistance of the weld metal. The presence of grain boundary ferrite morphologies, microphases and nitrogen may also have had an embrittling effect. It was suggested that reducing the oxygen levels and controlling the presence of other elements would have a beneficial effect with regard to toughness properties. Finally the problem of Fracture Path Deviation (FPD), where the crack path deviates out of weld metal during Charpy impact testing, was examined using autogenous and wire feed welds produced in BS 4360 50B steel. To examine the factors controlling FPD, both experimental work and finite element modelling of the Charpy specimen were undertaken. The results indicated the importance of weld width and orientation on the FPD phenomena. It was suggested that the toughness testing of narrow fusion zone laser welds (approximately 2 to 3 mm) should be undertaken using CTOD rather than Charpy testing.

The work described in this thesis details the investigation into flash butt weld failure at Corus, Port Talbot, where the process is used to join coils of steel, permitting cold rolling without interruption. The aim of this work was to examine the factors which constitute weld failure and to determine whether failure could be eliminated. It was shown that weld failure cannot be predicted with appreciable levels of confidence with the use of neural networks based on inputs such as steel composition and the thickness and width of the coils to be joined. 817 weld breaks were recorded during 2006, of which all but two were observed during cold rolling. Analysis of the samples provided indicate that in some case the term weld break was incorrectly used to describe what was in fact failure in the base material, away from the joint. Prior to this work there were no reported data on the residual stress levels generated after flash butt welding of sheet steel with a thickness of 2.5 mm. It is now clear that in addition to the expected tensile stress in the weld, there is also a compressive stress of up to 200 MPa in the base material which extends far beyond the heat-affected zone. This result may go someway to explain why failure in the base material is observed outside the heat-affected zone during cold rolling. Finally, a post-weld heat treatment for a high hardening boron-steel was examined. The heat treatment was intended to supersede previous attempts to reduce the post-weld cooling rate by applying a voltage across the weld to locally heat the region to a desired temperature.

Two types of coating materials were prepared by a sol-gel process and classified into two systems. In system I, four different types of the optically transparent organic-inorganic hybrid coating materials produced by The Welding Institute (TWI) using a patent method called Vitresyn, identical in terms of the precursors, but different in terms of their relative proportions, were examined. The precursors used for system I were tetraethoxysilane (TEOS), 3-(trimethoxysilyl)propyl methacrylate (MPTMA) and an aliphatic urethane acrylate. The coating materials were deposited on aluminium, brass and polycarbonate substrates by a flow coating method, and cured under a UV lamp for various times from 2 minutes to 40 minutes. In system II, MPTMA, Methyltrimethoxysilane (MTMS) and trimethoxysilypropyl acrylate (PATMS) were used as precursors. No urethane acrylate was used in this system. TEOS was used in system I as the primary inorganic precursor and urethane acrylate was used as the source of organic component. MPTMA was used as both a secondary inorganic source and a coupling agent between the organic and inorganic phases. To ascertain the interaction relationships between organic and inorganic phases in the hybrid materials, Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR) were used. It was found that a siloxane (Si­O-Si) network is a backbone of the hybrid materials and that the network increases with increasing inorganic content in the hybrid materials. These organic-inorganic hybrid materials were mainly cross-linked by T³, Q³ and Q⁴ species. It was also found that UV curing leads to accelerated condensation reaction through the opening of C=C bonds in MPTMA and urethane acrylate. Microstructures in the coatings and interfaces between the coatings and the substrates were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The coating thickness is dependent on the relative amount of organic resin added to the hybrid materials. The coating thickness increases as more organic resin (urethane acrylate) is used. Neither silica domains nor pores were observed in the coatings. It was concluded that the hybrid coating materials are a single featureless amorphous phase regardless of the organic/inorganic ratios. Scratch testing was used to study the scratch resistance of the hybrid materials and the interfacial strength between the coatings and the substrates. A micro-indentation adhesion test using focused ion beam (FIB) technologies was also used for investigating interfacial strength. The results showed that scratch resistance increases with an increase in the relative amount of organic resin added.

No description available.

No description available.

No description available.

No description available.