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The benefits of high strength steels in terms of reduced material volume due to enhanced mechanical performance have been known for some time. Large diameter transmission linepipe steels of minimum 690MPa ('X1OO') yield strength have been developed throughout the previous decade, and have recently become commercially available. Before these steels are used in linepipe construction projects, fimdamental work regarding their ability to be field welded required undertaking. This thesis presents data arising from girth welding experiments involving a variety of X 100 linepipe steels, welding consurnables and welding processes. Target girth weld mechanical properties thought suitable for a strain-based MOO pipeline design were proposed at the outset of the research. Optimisation of pulsed gas metal arc welding waveforms for the single and tandem wire processes, alongside the establishment of the base material properties formed an early part of the research. An extensive programme of solid wire welding consumable evaluation was then undertaken for single, tandem and dual torch narrow gap welding processes. The majority of equipment and procedures used throughout the work were as close to current field practice as possible, to minimise the time required to transfer the technology to the field situation. Work then focussed on the optimised alloy levels and welding procedure requirements for the production of full girth welds, using a variety of industry pipeline welding standards and supplemental techniques to assess the joint integrity. It has been demonstrated that, subject to careful selection of welding consumable and fairly precise control of welding process variables and parameters, there are no major problems in obtaining weld metal strength levels of at least 120 MPa above the 690 MPa specified minimum yield strength (SMYS) of the parent pipe. This objective has been achieved in welds made usirig all three mechanised process variants examined. The desired target properties of strength and toughness were achieved with a variety of consumables and pipe materials of different composition. Tie-in and repair procedures were also developed during the course of the research, with particular attention focussed on the application of high strength rutile flux cored ýVires. These wires attained strength levels overmatching the pipe specified minimum yield strength (690MPa), but would not reach the guaranteed overmatch level of 81 OMPa. An examination of the thermocycles associated with four mechanised narrow gap welding techniques (single, tandem, dual and dual tandem) was undertaken. The experimental technique developed allowed the solidifying weld bead to be monitored, as well as the cumulative temperature cycles experienced by the underlying layers. Succesful determination of the cooling rates, times and transformation temperatures allowed a comparative evaluation of the four processes, using an optimum weld metal composition suitable for single wire welding of X100. This led to an understanding of the metallurgical history, and its consequent effect on the associated mechanical and microstructural properties. A similar series of experiments was undertaken to examine these effects using variations in preheat with a single wire process. In most cases considerable property variations were attained for'the same weld metal chemistry, joint geometry and arc energy, highlighting the sensitivity of the process and procedure in achieving the required properties. The high cooling rates determined from the thermocycle experiments explained the microstructural and mechanical properties attainable from lean alloying levels. A series of metal cored wires, based around the same alloy as for the thermocycle experiments, was consequently manufactured to examine small changes in weld metal chemistry. The individual wires involved changes in carbon, nickel, molybdenum and chromium to examine potential property variations arising from a highly controlled narrow gap welding procedure. The results again highlighted the sensitivity of the narrow gap welding technique in generating considerable property variation within the weld metal. Tolerance ranges for specific alloying additions to attain the proposed strength levels with a single and tandem wire process were derived from the data.

Creep failure of welds in high-temperature power plant steam piping systems is known to be a potential cause of plant failure. Creep behaviour of plain pipes with circumferential welds and cross-weld specimens have received fairly extensive attention. However, research into the creep behaviour of welded thick-walled branched steam pipes has received less attention. Consequently, this thesis addresses improving the understanding of the creep behaviour for this type of geometry. Numerical and analytical methods are used to assess the creep behaviour of typical power plant branched pipe geometries. The effects of various geometric and material parameters on the creep stress and creep life behaviour of the connections are studied. In particular, the effect of the differing creep properties associated with the various material regions of the weld are investigated. The importance of incorporation of weld properties in creep life assessments is thus assessed. Finite element steady-state and continuum damage mechanics creep analyses have been used to identify the relative creep strength of typical connections compared to plain pipes. The work identifies typical creep rupture locations within branched pipe welds and the associated damage accumulation at and around these positions. Various creep life assessment methods/procedures are used in practise: these are mainly the British Standard codes, British Energy's R5 procedure, steady state creep approaches and continuum damage mechanics approaches. The relative accuracy and conservatism of these distinct approaches are addressed for the application to typical branched pipes. The general formulation of steady-state creep stress is applied to the parametric study of weld materials in a typical multi-material welded branched pipe. An approximate interpolation technique for power-law creep is implemented to reduce the number of analyses needed to span a wide range of material parameters. The method is used to estimate the creep stresses and lives at several critical regions within the various material zones of the weld. The advantages of the technique are related to the small number of analyses required and the simple and compact way of presenting the results for weld design and life assessment purposes.