Polysoude Field Reports
Gapless Welding of Cooling Spirals Using
the Hot Wire TIG Process
A welding process used to ensure high quality joints in
the field of Tank and Equipment manufacturing
The Principle of Hot Wire TIG Welding – Resistance Heating of Filler Wire
Considerably Increases the Deposition Rate
With conventional TIG welding
applications, the heat required to
melt the Filler Wire, (app. 30 %
of the energy delivered by the
arc), is taken from the molten
pool. Any increase in the
deposition rate is limited by the
time necessary to heat up and
melt the filler wire.
The technique used to facilitate
Hot Wire TIG welding is based
on Cold Wire TIG equipment.
The filler wire is moved by a
feeding mechanism from the
wire spool to the contact
nozzle. The wire between the
contact nozzle and the molten
pool is warmed up by
resistance heating.
A second Power Source is
necessary to maintain this
resistance heating.
This Secondary Current circuit
is closed by the permanent
contact between the wire and
the molten pool.
The preheating temperature of
the wire can be varied within a
wide range by controlling the
intensity of the Hot Wire
Current.
The energy finally needed to
melt the Filler Wire, which is
taken from the molten pool, is
decreased by the preheating of
the wire. Thus, a considerably
higher volume of filler metal
can be added at higher
welding speeds while reducing
the energy input per unit
length of the weld at the same
time.
The crucial advantage of the TIG welding process, compared to processes using a melting
electrode, is the independence between the energy delivered by the arc and the quantity of the
added filler metal. The application of the Hot Wire technique allows us to exploit this advantage
still further, especially at higher deposition rates. The benefits of independent control of energy
input and added filler metal quantity are clearly to be seen at the beginning and end of the weld,
where the size of the weld pool can be modified specifically. This is an indispensable
precondition to producing welds corresponding to the highest quality requirements.
Polysoude Field Reports
1
Welding of Cooling Spirals for tank and equipment manufacturing
To maintain the desired chemical reactions at the interior of dedicated tanks, many of them must be
temperature controlled from the exterior. Therefore Cooling Spirals are welded on the outside of
their walls. A medium ensuring heat transmission flows continually through these Cooling Spirals.
If not excluded by temperature conditions, water is often considered to be the best choice. In many
regions it is available at a fair price and its heat transmission characteristics are outstanding. But, due
to its provenance, this water can be enriched with suspended or dissolved substances so lowering its
long-term heat transfer characteristics or causing corrosion of metallic surfaces.
During design and manufacturing, any elements of the construction susceptible to allow corrosion
must be avoided or eliminated. Furthermore, steam is used more and more as medium for heat
transmission. For these reasons, the welding of the Cooling Spirals must be executed free of notches
and undercut.
For tightly welded constructions (fillet joints without root fusion), MIG/MAG (GMAW) welding
delivers optimal performance concerning economic and industrial requirements. The limits of the
process are reached if welds without any undercut are demanded. Even with the application of pulse
technique and cored wires the obtained results do not meet the specified requirements.
In the past, the only possibility to overcome this problem was to execute manual TIG welding of the
root pass. The low deposition rate of the process and the necessary weld seam preparation with a
constant gap is time-consuming and cost-intensive.
Therefore, a solution had to be found to ensure reliable and reproducible root fusion combined with
economic production preconditions.
The Hot Wire TIG (GTAW) welding process, with its mostly independent control of heat energy
input and quantity of added filler wire, it is apparent that it is the process of choice. If the Hot Wire
technique is applied, the expected flexibility can be achieved even with high deposit rates.
The realisation of two pilot applications has finally shown the suitability of the process in practical
use.
Polysoude Field Reports
2
Application of the Hot Wire TIG Process for
Gapless Welding of Cooling Spirals
The application of the Hot Wire TIG process allows welding of Cooling Spirals without joint
preparation directly on the wall of the vessel.
Before final welding, the Cooling Spirals are positioned on the outside, reducing the gap width as
much as possible, then they are tacked by TIG welding. At a wall thickness of the Cooling Spirals of
3 mm a partially appearing gap of maximal 0.8 mm can be compensated, if it is not situated at the
beginning of the weld.
For Cooling Spirals with a wall thickness of 2.5 mm or more usually joint preparations with a
chamfer or reduced wall thickness in the weld area are selected.
Gapless
To get root passes free of notches and undercut, it is imperative that backing gas protection be used.
Depending on the required throat thickness and according to the valid specifications, the welding of a
final capping pass can be applied.
Polysoude Field Reports
3
Multi-Process Welding Equipment well established for
Tank and Equipment Manufacturing
Besides the choice of the best suited welding method, PC-controlled multi-process welding
equipment of the PC-Series of Power Sources allows precise realisation and monitoring of
the selected parameters. Combined with standard peripheral units, PC Power Sources form an
integrated modular system ready to solve mechanised welding problems.
Programmed welding parameters are applied directly to control the movements of electromechanical components by the Power Source. The modular design of the system allows a
configuration of the Power Source which makes it possible to adapt the installation perfectly
to each particular welding task.
Wire Feeding, Gas Flow, AVC and Torch Oscillation are piloted by modules which can be
integrated directly into the control of the Power Source. Electro-mechanical components for
mechanised welding like Column & Booms, Rotators, and Positioners etc. are controlled and
synchronised by a multi-axes plug-in board of the Power Source.
To weld the Cooling Spirals without interruption, the movement of the Rotator or Positioner
and the movement of the Column & Boom are synchronised.
Depending on the type of Column & Boom used, the following configurations are possible:
Colum and boom
(The motion of the column and boom on its proper
carriage is synchronised with the rotation of the
workpiece)
Motorised Cross-Slide of the Seam
Tracking System
Configuration Type 1:
For the longitudinal displacement, the Column & Boom is moved on the ground using its
proper carriage. The fine positioning of the TIG-Torch is ensured by a Seam Tracking
System with Tactile Sensor.
Polysoude Field Reports
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Boom
(The motion of the boom is synchronised with the
rotation of the workpiece)
Motorised Cross-Slide of the Seam
Tracking System
Configuration Type 2:
The Boom of the Column & Boom equipment is moved synchronously to the rotation of the
workpiece. The fine positioning of the TIG-Torch is ensured by a Seam Tracking System with
Tactile Sensor.
A Seam Tracking System with Tactile Sensor and Slide is installed on the Boom; this
controls the positioning in relation to the weld preparation of the Cooling Spiral. The system
is separated and works independently from the power source, it is characterised by a very
robust design.
Equipment:
Motorised Cross-Slides for AVC and Torch
Oscillation, (mounted on an adjustable
interface), Wire Feeder Unit, and sockets
allowing the changing of the Torch and the
connection of the Seam Tracking System
with Tactile Sensor.
Polysoude Field Reports
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