TPT July 2013

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Bending, end forming and swaging
90 July 2013
Narrow-gap TIG welding of P92
TWI is managing a TSB collaborative
project named “Verified approaches to
life management and improved design
of high temperature steels for advanced
steam plants – VALID”, a three-year
research programme focused on the
application of advanced processes
for P92 pipe welding, which started in
March 2011.
The project is a collaboration between
the following organisations: TWI Ltd,
Polysoude SAS Air Liquide UK Ltd,
Centrica Energy plc, Doosan Power
Systems Ltd, E.ON New Build &
Technology Ltd, Metrode Products Ltd
and SSE plc. The project is managed by
TWI Ltd and is partly funded by the TSB
under the Technology Programme ref:
100816.
The research programme includes
the production of P92 pipe welds with
the narrow-gap TIG (NG TIG), TOPTIG,
variable-polarity submerged-arc (VP
SAW), flux-cored (FCAW) and electron
beam (EB) welding processes. The
soundness of the experimental welds
is verified via non-destructive testing
(NDT) and mechanical testing. Following
these, an extensive creep testing
programme is planned to determine
their high-temperature properties.
The preparation and NDT/mechanical
testing of the narrow-gap TIG has been
recently completed with successful
results. Before commencing the
welding activities, Polysoude reviewed
its database of welding procedure, to
determine the process parameters to be
applied when welding P92. The following
were selected: Narrow groove joint:
9mm wide with a 2° slope, accounting
for cross seam shrinkage, hence
enabling the maintenance of a constant
groove width after each welding pass,
and the use of pure
argon welding gas,
known for its universal
availability.
At the same time,
the various phases
of the heating cycle,
such as pre and post-
heating, as well as
the post weld heat
treatment (PWHT)
conditions, were
agreed by the project
partners, based on
the current practice
(Ref 1). A grade
92-matching filler
metal was supplied
by Metrode (trade
name: 9CrWV). The
heating cycle is a key
factor in obtaining
optimum results when
welding P92, in order
to ensure the required
high temperature properties.
Following this stage, Polysoude
prepared a preliminary test weld in order
to develop and then optimise all welding
variables, in particular the operational
weldability of the selected welding wire
and the cross seam shrinkage curve
obtained with such consumable.
A first P92 weld was then produced
on a spare pipe to confirm the welding
parameters and to make the necessary
adjustments. Considering that high
magnetisation had been observed for
this test weld (±15 gauss or 1,200A/m),
the pipe pieces to be used for the
final weld were de-magnetised prior
to welding. The final weld was then
carried out according to the developed
procedure and conforming to the
required heating cycle. The soundness
of this weld was confirmed through
radiographic testing (RT).
The total arc time was two hours and
eighteenminutes,whereasapproximately
six hours would be expected for a
conventional weld groove, which shows
the high productivity obtainable with the
narrow-gap hot wire TIG process.
Following RT, the test piece was heat
treated at 760°C ±10°C for 4 hours. Four
monitoring thermocouples were installed
at different locations on the inside and
outside surface of the welded pipe, in
order to verify that a uniform temperature
could be reached and maintained
throughout the pipe wall thickness.
Prior to delivery to TWI and the other
project partners, Polysoude carried out
the NDT and mechanical tests typically
required by international standards for
the qualification of welding procedures,
namely penetrant testing (PT), RT,
macroscopic examination, transverse
tensile testing, hardness survey, impact
testing, bend testing as well as weld
metal chemical analysis and all-weld-
metal tensile testing. No defects were
detected and the mechanical test results
were in compliance with the acceptance
criteria established for the project. This
allowed progression to creep testing
which is currently underway at Doosan
Power Systems’ and E.ON’s laboratories.
Polysoude – France
Email:
Website:
Polysoude carriage-type welding head with narrow gap torch
Narrow gap
TIG welding