From a mechanical testing perspective all welds (both main
seam welds and repairs) produced exceptional results in all
positions. Weld metal strength was adequately overmatched to
the base material achieving excellent yield and tensile strength
values. Weld and heat affected zone (HAZ) toughness was
characterised using both Charpy V-notch and crack tip opening
displacement (CTOD) methods. Charpy testing carried out at -10°C
utilising full size specimens achieved values of over 100 J in the
weld and 180 J in HAZ of semi-mechanised welds and over 130 J
in both the weld metal and HAZ of repair welds. CTOD testing
using B x 2B specimens at -10°C test temperatures delivered
values of 0.78 - 1.07 mm for weld metal and 1.26 - 1.33 mm for
the HAZ. Examples of CTOD fracture faces are shown in Figure 6.
The excellent toughness values in the HAZ indicate that the heat
inputs were low enough to avoid deleterious grain coarsening
in this region. The risk with such an approach when using high
CEV material can be the consequences of high hardenability,
but maximum HAZ hardness values of 287 Hv10 for the main
seam welds and 285 Hv10 for the repair welds HAZ are within
specification limits. Reported weld metal hardness values of
237 Hv10 and 219 Hv10 for the main seam welds and repair welds
respectively means there is no concern of high weld metal
hardness being encountered. This is deemed to be a consequence
of several factors associated with the set-up including the close
control of pre-heat and interpass using the induction heating
equipment and its configuration, along with consistent heat input
delivery and accurate individual weld bead placement.
Conclusions
This initial review of the GSFCAW process and particular set-up
delivered promising results, which
demonstrate it is suitable for this
application. WPS documents have been
developed from the qualification tests
that meet the requirements of the
National Grid specifications covering
welding onto pressurised pipelines. It
allows the delivery of consistent and
controlled heat inputs across all the
welding positions required ensuring
control over mechanical properties and
weld quality.
The benefits to welders undertaking
this type of work are also significant.
Less total welding time means less
exposure to fume and arc radiation as
well as the physical elements of welding
onsite, which by the very nature of the
application often takes place in trenches
and means workspaces can be cramped
and confined. Repeated operations
such as electrode changing have been
removed and the reduction in required
grinding activities has clear health and
safety benefits.
The travel speeds achieved using
the GSFCAW process can be up to
twice those recorded on similar sized
fittings welded using SMAW, as shown
in Figure 7. As a best estimate across
the various positions, an arc time saving
of approximately 20% was realised
during the work. When these gains
are combined with the reduction in
ancillary operations, such as changing
electrodes and slag removal, the
overall productivity gains of the system
are clear. Procedures developed during
the project support its implementation
for this type of welding and further
development in assessing the capability
of the system to be utilised for
circumferential fillet welds is now
being considered.
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