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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5

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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15

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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16

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

Documents

Orbital-Welding Facts En