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Transcript of Orbital-Welding Facts En
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Orbital welding facts
V02 2010
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Original edition, Polysoude Nantes Frankreich SAS
Photos, plans and drawings are used as help to understanding and are thus not contractual.
All rights reserved. No total or partial reproduction of this work can made, under any format or by any
means, electronic or mechanical, including photocopy, recording or computer techniques, without thewritten authorization of the publisher.
Published by Fronius International GmbH
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C O N T E N T S
1. Preace 5
2. What is orbital welding? 5
3. Recapitulation o the TIG (GTAW) process 5
4. Reasons to select orbital welding 11
5. Industries which apply the orbital TIG
welding process successully 12
6. Specifcities o the orbital weld process 14
7. Hardware Components o Orbital Welding Equipment 15
8. Programmable system controllers 16
9. Orbital welding heads 18
10. Wire eeder units 21
11. Functionalities o the orbital welding equipment 21
12. Weld cycle programming 28
13. Real time data recording 31
14. Tube to Tube welding 32
15. Tube to Tube or Pipe to Pipe orbital welding with fller wire 38
16. Orbital tube to tube sheet welding 43
17. Conclusion 50
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1. Preace
Among industrial joining processes, orbital
TIG welding has meanwhile become a well-
established method, although a considera-
ble lack o inormation about the variouspossibilities o this challenging technique
still remains in public. Aerospace industry,
aviation, high speed trains, nuclear indus-
try, pharmaceutical industry, ood industry,
tiny microelectronic devices - just to name a
ew o the most exciting applications - rely
on orbital welding, but the equipment to
ensure our daily supply with electric current,
oil and gas also depends on orbital welding
techniques.
In this booklet basic inormation is provided
about the orbital weld process and the related
equipment: technical approach, advantages,
common and special applications, but also
restrictions and limits. To give an idea how
reality looks like, the text is illustrated bynumerous application examples.
Tables and designs shall help engineers and
welding experts, as well as project managers,
to get quick answers whether orbital welding
could oer solutions corresponding to
their needs. To get specifc answers to your
questions, consult the customer service
team.
2. What is orbital welding?
Whenever high quality results are required,
orbital welding is the frst choice or the joi-
ning o tubes. The welding torch - in most
cases, the TIG-welding (Tungsten Inert Gas)
process is used - travels around the tubes tobe joined, guided by a mechanical system.
The name orbital welding comes rom the cir-
cular movement o the welding tool around
the workpiece.
Generally, orbital welding technique covers
two main felds o application:
z Tube to tube / pipe to pipe joining.z Tube to tube sheet welding.
In the frst group, all kinds o tube joining
are included: butt welding and welding o
anges, bends, T-fttings and valves, i.e. the
entire tubing and piping requirements.
The second group concerns the manuac-
turing o boilers and heat-exchangers and
comprises the dierent welding tasks related
to tube to tube sheet welding operations.
3. Recapitulation o the TIG (GTAW) process
An electric arc is maintained between the
non-consumable tungsten electrode and the
workpiece. The electrode supports the heat
o the arc; the metal o the workpiece melts
and orms the weld puddle.
The molten metal o the workpiece and the
electrode must be protected against oxy-
gen in the atmosphere; an inert gas such as
argon serves as shielding gas.
I the addition o fller metal becomes
necessary, fller wire can be ed to the weld
puddle, where it melts due to the energy
delivered by the electric arc.
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3.2. Types o weld currents
Two kinds o current are applied in the TIG
welding technique:
X Direct Current (DC) is most requently
used to weld nearly all types o materials.
X Alternating Current (AC) is preerred to
weld aluminium and aluminium alloys.
I DC is used, the electrode is connected
as cathode to the negative terminal o the
power source; this confguration is namedDCEN or Direct Current Electrode Negative. In
this case, the electrons o the electric arc ow
rom the electrode with negative polarity to
the workpiece with positive polarity. Up to
70 % o the released energy is considered to
heat up the workpiece, which means an e-
ciency o 0.7 (useul energy/released energy).
The confguration DCEP or Direct Current
Electrode Positive is not used in the TIG
process except o some very special appli-
cations in aluminium welding. In this mode
however, most o the heat is transmitted to
the tungsten electrode, so already at low
weld current intensities, very large electrode
diameters, compared to TIG DCEN, become
necessary to carry o the heat.
In the AC mode, the electrode is switched
periodically between positive and negativepolarity. During the time o positive polarity
the tungsten electrode acts as the anode,
due to the cleaning eect produced, the
oxide layer on the surace o the workpiece
will be destroyed. During the time o nega-
tive polarity the tungsten electrode acts as
cathode, the heat necessary to melt the alu-
minium is applied to the workpiece; in this
phase the electrode can then cool down.
3.1. Advantages/Inconveniences o the TIG (GTAW) process
1 - Nearly all metals can be joined.
2 - Dierent kinds o steel, stainless steel
included, can be welded as well as reractoryor wear-resistant nickel alloys, aluminium,
copper, gold, magnesium, tantalum, tita-
nium, zirconium, and their alloys; even brass
and bronze can be welded in certain cases; i
fller wire is applied, workpieces consisting o
dissimilar alloys or batches can also be joined
together.
3 - All welding positions are possible.
4 - The process is very stable and reliable;
the occurrence o weld deects can be redu-ced to less than 1 %.
5 - No slag or umes are developed during
welding.
6 - The aecting weld parameters can beadjusted in a wide range and mostly inde-
pendent one o each other.
7 - TIG welding can be carried out with or
without fller wire.
8 - The arc voltage, which is directly rela-
ted to the arc length, and the weld current
intensity oer a wide range o variations
and can be controlled automatically.
3.1.1. Advantages
3.1.2. Inconveniences
1 - Compared to other arc welding proc-
esses, the deposition rate o the TIG process
is relatively low.
2 - Time-intensive and costly development is
necessary to determine the weld procedures
and the exact values o weld parameters
which are necessary to control the process.
3 - The welding equipment is sophisticated;
it requires much more capital investment
cost than gear or manual welding.
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3.3. Tungsten electrodes
3.3.1. Types o electrodes
Tungsten is a highly reractory metal with a
melting point o 3,410 C. It withstands the
heat o the electric arc and keeps its hardnesseven i it becomes red hot. In the past, tho-
riated tungsten electrodes have been widely
used or TIG welding, but as thorium is a low-
level radioactive element, special grinding
equipment is required to ensure a sae dispo-
sal o the grinding particles.
Today, dierent alloyed tungsten electrodes
are preerred, e.g. Ceriated or Lanthanated
types, which are ree o any radioactivity. In
addition, their perormance is comparable to
that o thoriated tungsten electrodes.
3.4. Filler metals
1 - The welding seam must be reinorced.
2 - I carbon steel or mild steel have to be
welded.
3 - In case o a preparation o the tube ends,
or example a J or V preparation.
4 - To prevent metallurgical ailure i the
tubes to be welded are made o dissimilarmetals or alloys.
A well-known example is the welded connec-
tion between carbon steel and Stainless
Steel 316, where a fller wire made o Stainless
Steel 309 or nickel base alloy Inconel 82 is
added.
5 - I the alloys change their composition or
structure during welding.
Alloying elements can evaporate during the
weld process or orm a new compound. For
example, chromium carbide is developed i
chromium combines with carbon. The resul-
ting lack o metallic chromium can cause anunwanted loss o corrosion resistance at the
heat aected zone.
The application o fller wire may become necessary under the ollowing conditions:
3.3.2. The Electrode grinder
To get the precise end preparation and su-
fcient repeat accuracy which is necessary to
maintain a stable arc and a consistent levelo weld penetration, a special electrode
grinder should be used.
The design o the grinder must ensure that
the grind marks on the tapered part run in
correct alignment with the grain structure
o the electrode: lengthwise. This guaranties
better ignition and improved arc stability.
Correct: lengthwise grinding marks
Incorrect: circumerential grinding marks
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3.5. Gases
3.5.1. Welding gases
Argon is commonly used as shielding gas
in the TIG process. It provides good arc stri-
king characteristics and excellent arc stabi-lity even at low amperages, the energy o
the arc is confned to a narrow area. Argon
is also compatible with all types o base
materials.
Shielding gas or standard TIG welding
purposes should have a purity o 4.5, i.e. a
purity level o 99.995 %. Metals which are
classifed as delicate to weld or example;
titanium, tantalum, zirconium and their alloys
require a purity o at least 4.8, which means apurity level o 99.998 %.
To increase the weld energy, 2 % to 5 %
hydrogen can be added to the argon. Besides
a higher energy input o 10 % to 20 % resul-
ting in a better penetration and aster wel-
ding speeds, argon / hydrogen mixtures have
reducing properties helping to protect the
molten metal against the inuence o oxygen
rom the surrounding atmosphere. However,
mild and carbon steels absorb hydrogenwith the possible result o porosity and cold
cracking, so the use o hydrogen containing
gas mixtures is not recommended; or the
welding o aluminium and titanium they are
strictly orbidden.
The weld energy can also be increased by
argon/helium mixtures with helium contents
o 20 %, 50 % or 70 % or even pure helium.
Helium has no detrimental eects on tita-
nium, so it is used especially to weld the
pure metal or titanium containing alloys.
Mixtures o argon, helium and nitrogen
are used to weld Duplex and Super Duplex
steels.
Unlike argon, helium is a good heat conduc-
tor. The arc voltage under helium is muchhigher than under argon, so the energy
content o the arc is strongly increased. The
arc column is wider and allows deeper pene-
tration. Helium is applied or the welding
o metals with high heat conductivity like
copper, aluminium and light metal alloys.
As helium is a lightweight gas, compared to
argon its ow rate or identical gas coverage
must be increased two to three times.
The ollowing table indicates the qualifca-tion o dierent welding gases and mixtu-
res according to the base materials to be
joined:
Ar Ar + H2
Ar + H Ar + N2
He Ar Argon
Mild steel / Carbon steel *** ** ** * ** N2 Nitrogen
Austenitic steel *** ** ** ** ** H2 HydrogenDuplex / Super duplex steel ** ** ** *** ** He Helium
Copper *** x *** ** *** *** Recommended
Aluminium *** x *** * *** ** Possible
Titanium *** x *** x *** * Not to be used
x Prohibited
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3.5.2. Backing gases
Most applications o orbital welding require
an outstanding quality to the inside o the
root, as this is the part o the weld which
will be in direct contact with the transpor-
ted medium. To avoid any risk o oxidation,beore, during and ater the welding opera-
tion the hot metal at the inside o the tube
must be prevented rom coming in contact
with oxygen in the atmosphere. Depending
on the material to be welded, reducing com-
ponents like N2
or H2
are added to the bac-
king gas. The most typical backing gases andmixtures applicable or the dierent base
metals are:
Ar N2
Ar + H2
ou N2
+ H2 Ar Argon
Mild steel / Carbon steel *** *** * N2 Nitrogen
Austenitic steel *** *** *** H2 Hydrogen
Duplex / Super duplex steel ** *** ** *** Recommended
Copper *** ** ** ** Possible
Aluminium *** * x * Not to be used
Titanium *** x x x Prohibited
3.6. Weld energy
3.6.1. The Inuence o heat input
The heat input cannot be measured, but onlycalculated; its quantity is used e.g. to com-
pare dierent weld procedures or a given
weld process. The heat input inuences the
cooling rate and the HAZ (Heat Aected
Zone) o the weld. A lower heat input allows
us to obtain aster cooling rates and a smaller
HAZ. With ast cooling rates, microstructure
modifcations o the base metal like grain
growth or precipitations can be minimised,
avoiding the loss o too much mechanical
strength or corrosion-resistance. For many
materials, e.g. sophisticated heat-treated and
stainless steels, the heat input is limited by
the specifcations o the manuacturer.
In manual welding, to obtain a particular heat
input, the welder must keep the arc length
continuously at a specifed level, by that the
arc voltage remains constant at the desiredweld current intensity. But additionally, as the
heat input is inuenced signifcantly by the
travel speed, the manual welder must fnish
the weld within a fxed period o time. Only
well-trained welding sta with excellent skills
is able to meet these requirements.
In automatic Gas Tungsten Arc Welding, the
process parameters arc voltage and weld cur-
rent intensity, as well as travel speed and wire
eed rates are controlled and kept constantby the microelectronic devices unctioning
within the power source, so the demand to
respect a specifed heat input does not cause
any problems.
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3.6.2. Formula to calculate heat input
The energy per unit length o the weld (Heat
Input) HI released by the electric arc during
welding is calculated using the ollowing
equation:
HI = 60 x U x I / S
HI = heat input [J / mm or J / in]
U = arc voltage [V]
I = current [A]
S = travel speed [mm / min or in / min]
Using the above cited equation or heat
input calculation, the characteristics o the
applied weld process are not taken into
account. A weld process dependent e-
ciency coecient "r" allows us to calculatea more comparable heat input values or di-
erent weld processes:
HI = 60 x U x I x r / S
In publications, the coecient "r" or TIG
(GTAW) welding, is expected to be in the
range o 0.6 to 0.8, i.e. 60 % to 80 % o the
energy released by the electric arc heats up
the workpiece while 20 % to 40 % escape
by radiation, heating up o the torch, the
shielding gas etc.
Ih
Ib
Iaverage
Tb Th
I (A)
T (ms)
Ih
Pulse current
Th
Pulse time
Ib
Background current
Tb
Background time
Expert inormation:
To calculate the average weld current Iaverage
when using pulsed current or orbital wel-
ding applications, the ollowing ormula has
to be applied:
Iaverage
= (Ih
x Th
+ Ib
x Tb) / (T
b+ T
h)
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4. Reasons to select orbital welding
The decision or the use o mechanised or automatic orbital TIG welding can be taken or di-
erent reasons: economic, technical, organisational, and others may be more or less important
or even become the decisive actor. The orbital welding process oers a large range o benefts
which qualifes it or industrial applications. The major advantages are:
4.1. Increased productivity compared to manual welding
Compared to manual TIG welding, the
mechanised or automatic process leads to
enhanced productivity. Repetitive work in
the shop or complicated assembly jobs on
site - orbital welding equipment guarantees
that approved weld sequences are reliably
repeated, hence time-consuming repair work
will be reduced to a minimum.
4.2. Consistent excellent weld quality
Generally, the weld quality obtained by
mechanised equipment is superior to that
o manual welding. Once an adequate weld
program has been developed, the weld
cycle can be repeated as oten as necessary,
without deviations and virtually without
weld deects.
4.3. Required skill levels o the operators
Certifed welders are dicult to fnd and well
remunerated. However, ater appropriate
training, skilled mechanics are able to ope-
rate orbital welding equipment perectly and
get excellent results. By using this equipment
expenditure on personnel can be reduced.
4.4. Environment
Orbital welding can be executed even under
harsh environmental conditions. Restricted
space or access, lack o visibility, presenceo radiation; once the weld head is positio-
ned properly, the weld can be accomplished
without problems rom a sae distance; oten
supported by a video transmission.
4.5. Traceability Quality Control
Modern orbital welding equipment is desig-
ned or real-time monitoring o the aecting
weld parameters; a complete weld protocol
can be generated and stored or output as a
printed document. Sophisticated data acqui-
sition systems operate in the background,
i they are connected directly to a superior
quality management system; automatic data
transer takes place without any interrup-
tions to the weld procedure.
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5. Industries which apply the orbital TIG welding process
successully
5.1. Aircrat industry
In the aircrat industry, which was the frst
one to recognize the importance o orbi-
tal welding or their purposes, more than
1,500 welds are necessary to complete the
high pressure system o one single plane.
Manual welding o the small, thin-walled
tubes is extremely dicult; fnally the requi-
red consistent joint quality cannot be gua-
ranteed. The only solution is to establish
welding procedures using orbital equipment.
In this way, the parameter values are reliably
controlled by the equipment and the fnal
welds meet the same quality level as the qua-
lifed test welds.
5.2. Food, diary and beverage industries
The ood, diary and beverage industries
need tube and pipe systems meeting deli-
cate hygienic requirements. Full penetration
o the welded joints is necessary; any pit,
pore, crevice, crack or undercut can become
a dead spot where the medium is trapped
and pathogenic bacteria growth, (Listeria
etc.), can occur. Smooth suraces everywhere
inside the tubes enable successul cleaning
and complete sterilisation o the system. The
requested surace quality can only be ensu-
red i orbital TIG equipment is used to weld
these critical joints. Thereore, most stan-
dards and specifcations oblige nowadays
the manuacturers o hygienic installations to
apply this process.
5.3. Pharmaceutical and biotechnology industries
Plants in pharmaceutical industries must be
equipped with pipe systems or the trans-
port and the treatment o the product and
or the sae supply o clean steam and injec-
tion water. For injection water and its deri-
vatives that are intended or injection intothe human body, the purity requirements
are particularly high. Any traces o corrosion
are absolutely orbidden, the corrosion resis-
tance o these welds must not be undermi-
ned, especially not by partial overheating
o the base material. Joints made by orbital
welding qualiy or extended corrosion resis-
tance. Additionally, to avoid any subsequentoxidation or corrosion, their smooth surace
can be passivated.
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5.4. Fabrication o semi-conductor devices
For the abrication o semi-conductor
devices, electro-polished stainless steel tubes
are installed as process gas lines, mostly
with an OD o 6.3 mm and a wall thicknesso 0.9 mm. The ultra-pure process gas must
pass the tubes without picking up moisture,
oxygen, particles or other contaminants. The
acceptance criteria or these installations
are very stringent: uniorm welds with small
weld beads to minimize the weld surace in
the tubes, ull penetration on the ID, absence
o discoloration, etc. Only experienced ope-rators working with reliable orbital welding
equipment are able to perorm this task,
oten even under adverse conditions on site.
5.5. Chemical industries
A considerable part o plant equipment
or chemical industries are manuacturedand installed by means o orbital welding.
Chemical apparatuses are comprise o tubes,
heat exchangers and converters which are
made o corrosion-resistant or reractory
metals or alloys o titanium, zirconium, nickel,
chrome etc.; not to orget the whole range o
dierent stainless steel types. As the service
lie o the installations depend directly on
the quality level o the welded joints, strict
control and traceability o the weld processare required by customers, inspection bodies
and standards authorities. For the assembly
o one heat exchanger, hundreds or even
several thousand aultless welds have to be
carried out, so here orbital welding becomes
a must to ensure the expected results.
5.6. Power generation
For the saety o power stations the whole
range o orbital joining techniques are
applied: tubes with small diameters or sen-
sing and control purposes must be connec-
ted, heat exchangers and other components
are manuactured using orbital tube to tube
sheet welding, and thick-walled tubes or
operation under high pressure and tempe-
rature must be assembled on site. The wel-
ding procedures and the weld quality are
generally under constant surveillance o the
respective authorities and external organisa-
tions, the required complete documentation
and traceability is ensured by the provision o
orbital equipment with online data acquisi-
tion systems.
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6. Specifcities o the orbital weld process
6.1. Typical Welding Positions
The denominations or pipe welding are specifed by the ASME code, section IX, and the European
Standards EN 287 / EN ISO 6947, both reer to the position o the tube to be welded.
6.2. Pulsed current
The essential characteristic o successul
orbital welding is the necessity to control the
bath o molten metal during the whole weld
cycle, taking into account the continuously
changing situation in the process. An orbital
weld o the PF / PG or 5G (fxed tube) type orexample must meet at each moment the ol-
lowing conditions:
1 - Alteration o the weld position and hence
o the inuence o the orce o gravity.
2 - Alteration o the thermal state o the
workpiece.
The most eective measure to keep the
control o all weld positions during the orbi-
tal weld cycle is to use a pulsed weld current.
Basically, a pulsed weld current toggles
between two dierent levels o intensity:
X During a time period Th
the weld current
remains at a high level Ih; here the volume o
the weld puddle increases to its maximum.
X During a time period Tb
the weld current
remains at a lower level Ib, allowing the weld
puddle to cool down and to decrease its
volume to a minimum, which mitigates the
awkward eects o the orce o gravity.
Ih
Ib
Tb
Th
Pulsed current is advantageous or a major
part o orbital welding applications, makingthe determination o welding parameters
easier and aster. However, i thick-walled
tubes o signifcant diameters with wall-thic-
kness over 10 mm and tube diameters above
114 mm are to be welded, the level o the low
current intensity may approach that one o
the high intensity, which results almost in an
un-pulsed current.
(1) (2)
AWS 1GISO PA
AWS 2GISO PC
AWS 5GISO PG
(1)/ PF
(2)
AWS 6GISO H-LO45
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6.3. Programming o sectors
In many cases, the only use o a pulsed weld
current is not sucient to obtain acceptable
orbital weld results. The parameters must be
adapted with regard to the actual require-ments o the weld. The course covered during
the weld cycle is hence divided into dierent
zones, which are called sectors. The weld
parameters are modifed i the border o one
sector to the next is crossed.
To explain the sector layout, a circle o 360
as symbol o the cross-section o the tubes to
be welded is divided into our sectors, each
covering 90. The frst sector begins at the
starting point D o the orbital weld, in thiscase at the 10.30 position, and ends at the
01.30 position.
Each sector corresponds to a specifc wel-
ding position:
z sector S1 rom 0 to 90 at position;z sector S2 de 90 180 vertically downposition;
z sector S3 de 180 270 overheadposition;z sector S4 de 270 360 vertically upposition.
S4
S3
S2
S1
0360
270 180
90
D
Depending on the weld position and the
thermal conditions o the workpiece, which
is heated up perpetually by the energy input
o the electric arc, the parameter values are
modifed at the beginning o each sector.
In the orbital weld practice, most oten the
sectors are not divided as regularly as shownin the example. The number o sectors
can also vary due to the dierent welding
applications.
7. Hardware Components o Orbital Welding Equipment
Independently o the welding tasks to be car-
ried out, orbital welding equipment is gene-
rally composed o the ollowing components:z A programmable system controller anda remote control unit, (distinct or as
integrated as part o the Welding Head).
z The Welding Head.z A wire eeder unit, i required by theapplication.
In any case, the perormance o the equi-
pment depends on the design o the aore
mentioned components.
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8.2. Portable power source FPA 2020
The smallest power source with a weight
o less than 30 kg delivers weld currents up
to 200 Amperes; it is operated on a 230 Volt
single phase supply. The programming and
parameter development is carried out via
an intuitive graphic user interace and a ull
unction remote control unit.
The man-machine-interace allows a com-
ortable management o weld cycles,
programs and weld parameters, sector-
programming is supported as well. The gas
solenoid valve or the purging gas can be
switched rom the remote control (on / o).
8. Programmable system controllers
8.1. General
X One Power Inverter to supply the welding
current. Today, state o the art sources are o
the inverter type.
X Programmable control unit which is
generally based on an integrated PLC.
X Cooling circuit or the torch and
the welding and clamping tools.
X AVR-system (actual value recording)
recording each welding sequence.
The power sources or orbital wel-
ding can be divided in 2 catego-
ries with specifc felds o application.
A power source or orbital applications is composed o several subassemblies with specifc
unctions each:
Orbital power source FPA 2020
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The FPA 2020 is equipped to handle up to our
axes o control, i.e. our devices can be pro-
grammed and controlled: the shielding gas
ow, the weld current intensities and pulse
rates, the travel speed o the welding head,
and wire eeding operations. A closed loopCooling System is present to operate water-
cooled orbital Weld Heads and Welding Tools
and is integrated as part o the machine.
Furthermore the FPA 2020 allows us to fnd
matching weld programs, (i the user speci-
fes basic inormation about size and mate-
rial o the tubes to be joined), using a touch
screen. The system consults its in-built data-
base to fnd similar applications or suggestsweld parameters determined by progressive
calculations.
Orbital system controller FPA 2030 with power source
8.3. System controller FPA 2030 with power source
Medium-sized power sources or orbital wel-
ding are too heavy to be carried; they aremounted on a carriage to keep them mobile.
These power sources are or connection to
three-phase 400 Volt outlets or eature a
multi-voltage input, they generate welding
currents up to 500 Amperes. For the dialog
with the operator, the power sources are
equipped with a convenient man-machine-
interace and a ull unction remote control
unit.
Medium-sized power sources are designed to
handle up to six axes, which can be program-med and controlled. Usually these axes are
attributed to the shielding gas ow, the weld
current intensities and pulse rates, the tra-
vel speed o the welding head, the wire ee-
ding operations, and Arc Voltage Control &
Oscillation. The purging gas can be switched
on / o rom the remote control as well.
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Open welding heads were conceived as a tool
or orbital TIG welding with or without fller
wire. The diameters o the tubes to be welded
cover a range rom 8 mm up to 275 mm (ANSI5/16" to 11").
Open welding heads o the U-type are
equipped with a TIG-torch with gas diuser.
Sucient gas protection is achieved only at a
zone around the torch which is covered by the
shielding gas streaming out o the gas lens.
During the welding process, the arc can be
watched and controlled directly by the ope-
rator. The asymmetrical design o the open
heads allows welding to be carried out at avery short distance to a wall or a bend.
The positioning o the welding torch can be
carried out manually or by means o motorized
slides (Arc Voltage Control and oscillation).
9.1.2. Open welding heads o the U type
Open welding head MU
9. Orbital welding heads
9.1. Tube to tube welding heads
Closed chamber welding heads are especially
designed or autogenous welding o tubes
without fller wire; their dierent sizes cover
a range o diameters between 1.6 mm and
168 mm (ANSI 1/16" to 6"). Besides austenitic
stainless steel, metals susceptible to oxidation
like titanium or zirconium and their alloys can
be welded with excellent results. Depending
on the application, one or two pairs o clam-
ping shells or TCIs (Tube Clamping Inserts) areneeded to fx the closed chamber head on to
the tubes to be welded.Closed chamber welding head MW
9.1.1. Closed chamber welding heads
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9.1.3. Carriage-type welding heads
Open orbital welding heads o the carriage
type travel around the tubes or pipes on
appropriate rails or tracks, which can be
mounted on any tube OD rom 114 mm
(3 1/2") upwards. The wall thickness o the
tubes and pipes concerned always requires
multi-pass welding, the robust design o the
carriage weld heads enable them to carry the
necessary equipment such as a heavy duty dri-
ving motor, a torch with an AVC and oscillation
device and a wire eeder bearing spools with a
weight o up to 5 kg. Additionally, video came-
ras can be mounted, allowing the operator to
watch and saeguard the weld process.
Due to the application, these welding heads
can be equipped with TIG torch with gas lens,
assuring the protection o the zone covered by
the shielding gas.
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9.2. Tube to tube sheet welding heads
9.2.1. Enclosed orbital tube to tube sheet welding heads without fller wire
Enclosed welding heads are designed or
TIG welding (GTAW) o tube to tube sheet
applications, i they can be accomplishedwithout fller wire. With these weld heads,
ush or slightly protruding tubes with a mini-
mum internal diameter o 9.5 mm (3/8") can
be welded, the maximum diameter being
33.7 mm (1 1/3").
The weld is carried out in an inert atmosphere
inside a welding gas chamber, providing very
good protection against oxidation.
For clamping, a mandrel is inserted intothe tube to be welded and expanded
mechanically.
By means o a weld lance which is mounted at
the ront o the weld head, internal bore wel-
ding can be carried out at tube I.D. between
10 mm and 33.7 mm (13/32" and 1 1/3").
9.2.2. Open tube to tube sheet welding heads with or without fller wireOpen orbital tube to tube sheet weld heads
which can be used with fller wire cover the
whole range o applications rom tubes with
an I.D. o 10 mm (13/32") up to tubes with a
maximum O.D. o 60 mm. The TIG torch tra-
vels around the tubes, which can be protru-
ding, ush or recessed.
The welding heads are equipped with a TIG-
torch with gas diuser. A sucient gas pro-
tection is achieved only at the zone around
the torch which is covered by the shielding
gas streaming out o the gas lens. I oxygen
sensitive materials need to be welded, the
gas protection can be improved by installinga gas chamber.
The welding heads can be equipped with
an integrated wire eeder unit A pneuma-
tic clamping device can be used to hold the
weld head in working position on the tube
plate, enabling several welding heads to be
operated by just one person. Welding lances
allow the operator to carry out internal bore
welding with gapless joints behind a tube
sheet or a double tube sheet.
Tube to tube sheet welding head TS 34
Tube to tube sheet welding
head TS 2000
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10. Wire eeder units
Generally, a wire eeder unit can be integrated
into the orbital welding head or specifed as an
external wire eeder unit. The choice o the ee-
ding unit depends on the availability o the fller
wire, which must be available on suitable spo-
ols; urthermore on the conditions o use, the
constraints o the application and the requested
mobility o the equipment.
11. Functionalities o the orbital welding equipment
11.1. Gas management
There are two possibilities when controlling
the gas management o an orbital weldinginstallation:
1 - A manually adjustable pressure reducer
with ow meter, installed at the gas supply,
(cylinder or network), an electric valve which
can be opened and closed by the control unit
o the power source .
2 - An adjustable pressure reducer is instal-
led at the gas supply (cylinder or network),
an electronic device inside the power source
controls the gas ow rate. Power sources ororbital welding are equipped to control up
to three gases: two welding gases and one
additional gas, e.g. backing gas. The so-called
Bi-Gas unction o a power source allows the
unit to change the type o welding gas when
the electric arc is initiated, which is especially
advantageous i helium is used as shielding
gas. To avoid requently occurring problems
caused by ignition diculties under helium,
the ignition is initially carried out under argon
and, ater the arc has become stable, the wel-
ding gas supply is switched to helium.Depending on the standard o the particular
orbital welding equipment, the welding gas
ow is continuously monitored. In case o an
interruption o the welding gas supply, the
ignition o the arc is blocked. I during welding
the gas ow rate drops below a actory-adjus-
ted value, the weld cycle will be aborted auto-
matically. By this measure, severe damage o
the workpiece and equipment is avoided.
Integrated wire eed unit
on a TS 2000 welding head
Integrated wire eed unit
on a MUIV welding head
External wire eed unit
KD-4000
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1
2
11.2. Current
11.2.2. Welding current
11.2.1. Arc ignition
Current control unctions
The standard method o striking an arc is to
apply high voltage surges. The column o
shielding gas between the electrode and
the workpiece becomes ionised and takes
on conducting properties. As a consequence
an arc is struck and the weld current begins
to ow. This ignition method is the com-mon standard or all types o orbital welding
equipment.
This ignition technique is limited by the
cable length between the power source and
the welding head, which depending on the
kind o application, must not exceed 30 m to
25 m. I the welding head is equipped with an
AVC device, a so-called Touch & Retract can
be carried out instead. The torch is moved
towards the workpiece until the tungsten
electrode touches its surace. Smoothly ate-
rwards it is drawn back (lited). The potential
to initialise the weld current is applied in thesame moment. Once the arc is struck the torch
can be moved to the programmed arc length.
Any tungsten inclusion in the weld seam is
reliably excluded.
IB
IP
tB tP
IS
tS tUp tDs tE
IE
The welding current is one o the aecting para-
meters o the TIG process; thereore its intensi-ties must be controlled accurately by the power
sources. A precision o 1 Amp is guaranteed
i the welding current intensity rests below
100 Amps, or intensities exceeding 100 Amps a
precision o 1 % is ensured. To meet the requi-
rements o the dierent applications, dissimi-
lar current types are supplied by the power
sources:
X Un-pulsed current (1): no variation o the
current intensity
X Pulsation (2): this current is commonly
used or standard orbital TIG welding;the maximum requency o pulsations,
who makes sense, is 5 Hz (with wire).
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Downslope
To avoid a crater occurring at the end o the
weld, the welding current cannot be inter-
rupted instantaneously. During a downslope,
the weld current intensities are decreasedlinearly to values between 30 A and 4 A,
aterwards the current is shut o. The higher
intensities are adapted to tubes with a more
signifcant wall thickness. Downslope unction
11.3. Torch rotation
During welding the torch must rotate with
the desired linear travel speed around
the tube or pipe. Standard orbital welding
applications require a linear travel speed range
between 50 mm/min and 200 mm/min.
Torch rotation control unctions
vWtA
tDsT
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In most cases the travel speed remains un-
pulsed, but it can also become pulsed and
synchronized to the weld current pulsations.
It is possible to program dierent speeds
during base and pulse current. Usually, as in
the case o step pulsed welding, rotation stops(V = 0 mm/min) during the high current level,
whereas during the base current period the
torch moves orward.
The achieved speed precision is 1 % o the pro-
grammed value. Welding equipment can be
operated using impulse emitters or tachome-
ter encoders on request.
These pulses are also processed by the
control system o the power source to iden-
tiy the actual position o the torch relative
to the start point, which means that the pro-
gramming o a weld cycle can be carried outusing angular degrees instead o time spans.
Intuitive programming is possible because
one tour o the torch always covers 360 per
pass, independently o the linear welding
speed and the tube or pipe diameter.
11.4. Wire eeding
Power sources or orbital welding are equip-ped to control dierent types o wire eeder
units; the attainable wire speeds range rom
0 to 8,000 mm/min, a precision o about 1 %
is attained.
Standard unctions o wire eeding which are
managed by all power sources are the control
o the wire start and stop as well as a pulsed
eeding rates. The wire eeding pulses can be
synchronised to the pulses o the weld cur-
rent; the wire speed is kept at a high levelwhen the weld current is at its high level, and
is decreased during low level current. The
independence between wire speed and weld
current oered by the TIG process allows the
reversal o synchronisation; the wire is ed at
a high speed when the current intensity is
low; the wire arrives at a small weld puddle
and melts with resistance. The mechanical
stability o the wire can be used to push the
bath o molten metal to get a convex root
pass surace at the inside o the workpiece.At the end o welding, a wire retract unction
allows the reversal o the eeding direction.
The wire end is drawn back a ew millimetres,
avoiding the ormation o a terminal wire ball
or, even worse, the wire resting stuck in the
weldment.
Expert inormation:
1 - Common diameters o wire or welding
purposes range between 0.6 mm and 1.2 m;
the best choice or standard orbital weldingis a proper wire with 0.8 mm diameter.
2 - The melting rate o the wire depends
not only on the precision o the wire eed
speed, but also on the precision o the
wire itsel: a variation o 0.02 mm at a wire
with a diameter o 0.8 mm represents a
dierence o already 5 % o added metal.
Wire eeding control unctions
vDBvDP
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11.5.1. Theoretical approach
During welding, it is important to keep the arc length constant; but there are no simple
methods to measure it. In any case, i the welding conditions do not change, each particulararc length corresponds to a related arc voltage. This phenomenon is used to control the dis-
tance between the electrode and the workpiece during welding.
The characteristic o arc voltage at dierent
arc lengths and welding current intensities
are shown in the graph below:
U2-h
I-b I-h
U (V)
I (A)Imini
U1-hU2-b
U1-b
2 mm
1 mm
At an arc length o 1 mm, the arc voltage measu-
red between the electrode and the workpiece at
dierent welding current intensities is characteri-
sed by the black line.
The red line shows the result o the same mea-surement at an arc length o 2 mm.
X Rule n 1: at the same weld current (I-b
) an
increase o the arc length provokes a higher
arc voltage (increasing rom U1-b
to U2-b
).
X Rule n 2: i the arc length is maintained
(weld current intensity exceeds Imini
) and the
weld current increases (rom I-b
to I-h
), the arc
voltage also increases (rom U1-b
to U1-h
).
X Rule n 3: i a dierent type o shielding
gas is used (with other weld parameters
remaining unchanged), the arc length will
change: i the shielding gas changes e.g. rom
argon to an argon-hydrogen mixture, the arc
becomes signifcantly shorter.
X Rule n 4: i the geometry o the elec-
trode diers (taper angle, tip diameter), the
arc length at a given weld current changes
or, at a constant arc length, the arc voltagechanges.
X Rule n 5: i a pulsed weld current is
applied, the arc voltage pulsations are not
proportional.
I1-h
U1-b
U1-h
I
U
I1-b
T
T
Each change o the weld current intensity
provokes a peak o the arc voltage which is
commonly known as overshoot.
11.5. AVC (Arc Voltage Control)
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11.5.3. Programmable distance between electrode and workpiece
Besides the AVC control, the torch position
can be determined by the programmed
distance between electrode and workpiece
unction. Here, starting rom a reerence
value, the torch is moved by a motorized
slide over the selected distance in mm to thedesired height.
The mentioned unction is oten used to get
the electrode in position e.g. with tube to
tube sheet applications or, i special welding
tools are used, to ollow the complex sur-
ace geometry o a workpiece in piggyback
position.
11.6. Oscillation
I a weld preparation is applied to the tube
ends, the groove to be flled becomes rela-
tively wide, especially in the case o an
increased wall thickness. Dierent to thestringer bead technique, where several
passes are required to complete one layer,
the groove can be covered completely by
one layer i the torch is moving perpendicu-
larly rom one side to the other between the
sidewalls o the weld prep. This movement isgenerated by a motorised slide and control-
led by the oscillation system.
11.5.2. AVC eatures
As or most orbital welding applications, a pulsed current is applied; the rules 1 and 2 must be
taken into account, making specifc adjustments necessary to get a stable arc length.
X Restriction o the voltage measurement tothe period o the low or o the high welding
current. During the period without measu-
rement the AVC slide is temporarily blocked,
the electrode position does not change. The
adjustment is simple, only one parameter
value is requested to get a stable arc length
X Extended arc voltage measurement
during the period o the low and o the high
welding current. This type o AVC control can
be used i thermal pulsing (pulse requency