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 ('XIOO') 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
