Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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- This invention relates to welding under conditions
of high pressure~ for example welding pipelines on the
; seabed. Some of the welding processes which are used
in normal ambient conditions have been tried success-
fully for underwa-ter welding. Normally for quality
welds at any dep-th in order -to remove water from the
weld pool and arc, the welding process is surrounded
by an inert gas envelope, contained in a small
transparent enclosure. At depths below the air
diving range (164ft. 50m, or 6 bars pressure) the
welder or some part of the welder is usually
enclosed in the inert gas envelope with the materials
needed for the welding process and the joint to be l;
welded. As depth increases,the pressure on the
arc increases.
Flux-shielded arc welding p~ocesses and in particular
the manual metal arc process and the flux-cored
wire process, have been used with success down to
50m, which is equivalent -to a pressure of about 6 bars.
However, at depths beyond this changes occur in complex
slag, metal or gas reactions that take place in the
arc, leading to changes in the deposited weld metal
composition, that in turn affect the mechanical prop-
erties of the weld~ Arc stability however is
maintained
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Conventional metal inert-gas welding can also be used at
shallow depths but beyond 50 m arc stability and metal transfer
become erratic, with the result that large globules of weld metal
are thrown from the end of the electrode on the plate surrounding
the weld pool. In addition, whilst at atmospheric pressure there
is little fume evolution in metal inert-gas welding, at pressures
above 6 bars there is copious fume evolution and this is obviously
a severe problem in underwater welding.
These difficulties with metal inert-gas welding have led to
the use at depths of proces'ses utilising flux for stablisation and
metal modification, despite the advantages that metal inert-gas
welding provides in the form of a high deposition rate and a lack
of complex slag, metal or gas reactions.
In a method of welding according to the present invention,
at pressures of at least 7 bars, a solid bare wire'metal inert gas
process is used with the el'ectrode negative with respect to the
workpiece and with an el'ectrode wire of diameter not greater than
1.4mm; in the preferred method embodying the invention, the slope
of the power supply, as seen from the welding arc, is between 6 and
15V/lOOA. This slope is higher than the slope normally used in
MIG welding, which is about 3 to 4V/lOOA. We have found that the
uSe of such a higher slope improves the stability of the process
and allows increased penetration to be achieved in the weld. The
preferred shape is 7V/lOOA. The term "slope" is here used in the
generally accepted sense to mean a negative slope.
Above a pressure of 10 bars, the advantages of the invention
are even more apparent.
Preferably, the inert gas is predominantly argon or helium.
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An oxidising gas such as oxygen or carbondioxide may be added to
the inert gas. A suitable
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mixture is one that contains at least 95% of
argon or helium and up to 5% of oxygen or
carbon dioxide.
In this specifica-tion, the expression
"conventional metal inert gas welcling" is intended
to mean solid bare-wire metal inert-gas welding
and in such a welding process carried out at
normal atmospheric pressure, it is customary to
make the electrode positive. The reason for -this
is that in this form of welding, the majority of the
heat is generated at -the cathode, i.e. the negative
workpiece and this permits good penetration to be
achieved and an adequate transfer of metal from
the consumable electrode. If the electrode were
made negative ? little hea-t would be generated at the
workpiece and there would be little penetration; the
majority of heat generation would -take place at
the electrode end, causing the melting of too
much of the electrode with the result that the
arc would tend to run back to the copper guide tube
surrounding the electrode. m us when the electrode
is made negative at normal a-tmospheric pressure
there is a very high deposition rate on the
surface of the workpiece but the arc may have
poor stability and there is little penetration.
Metal inert-gas weIding with a negative
electrode has had little practical use, because of
the disadvantages set out above, although some
mitigation of these disadvantages can be achièved with
the use of an argon-rich gas containing some oxygen
or C02
It has also been proposed to use an ~C current
for metal inert-gas welding, the positive electrode
half-cycles stabilising th~-arc and the negative
electrode half-cycles heating the wire.
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m e conditions necessary to achieve a balance
between the amount of metal melted from -the electrode
wire and the amount of energy at the workpiece in
.a mètal inert-gas welding process vary with the
pressure ~der which the weld is carried out. Using
the c~nt~nal metal inert gas welding technique,
as the pressure in which the process is carried out
increases above atmospheric pressure, a condition
is produced which leads to the fume and stability
10 problems discussed above. However, with the same ...
increase in pressure, the al-teration of the above-
mentioned balance acts in favour of electrode-negative
working, so -that at the above-mentioned pressure of
7 bars it becomes preferable to use electrode-negative ~
15 working and at a pressure of 14 bars the use of .
electrode-negative working is highly advantageous.
As an example, s-table arcs with little spatter
and with a relative absence of fume can be produced
even at a pressure in excess of 32 bars ~equivalent ~ ¦
to 310m of depth). .
MIG welding generally requires a flat
characteristic. .One way of increasing the power
supply slope (as seen ~rom the arc) is by increasing
the length of the leads between the power supply and
the welding head. In this way, the use of a method
.. .. ... ~embodying ~he.invention permits locating the power .
source on the ship, the long leads on the ship to the
undersea welding si-te giving stability to the welding
process. Another way of increasing -the power supply
slope, as seen ~rom the arc, is to increase the
resistance between the power supply and the welding
head by reducing the diameter
of the cable connection or by reducing the number
. ~. ..: ... .... of.cable~.,;.wh.ere.~e~eral.cables.are used in parallel.
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The same result can also be achie~ed by increasing
the inductance between the power supply and the
welding head, because over the short period
of time for which the arc demands rapid changes
of voltage and current, the effect of the inductance
on the apparen-t power supply slope, as viewed ~rDm
the arc, will be the same as the effect of the
previously proposed increase in resis-tance. m e
inductance can be introduced by increasing the
10 number of parallel connecting cables or by
including an inductive component in the leads.
m e increased slope also assists the first
run in a welding operation, which is very difficult
in conditions of high pressure in that there is a
tendency for the weld to burn through. The
tolerance in the choice of root gap is also
improved and narrow-gap welding can be employed~
for example, welding with an included joint angle
of ~0, in any position. ~enerally speaking, this
20 was not possible in air, especially in an
overhead position, with one-millimeter diameter
wire, as it was~difficult to ensure adequate sidewall
fusion. Narrow-gap welding ~educes welding time.
A further advantage of the use of metal inert-gas
welding in underwater conditions is that as it employs
a continuous electrode it lends itsel~ to mechanisation.
m e limit of welding by divers has now been practically
reached in the sense that ~t the depths now envisaged
a diver requires a very long period of decompression.
Consequently to weld at greater depths mechanised
; p~cesses will be highly desirable.
It is not necessary to vary the weld metal
composition with pressure.
One example of apparatus embodying the invention
for undersea welding-will now be described with
reference to the accompanying drawings, in which:-
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Figure 1 is a perspective view of automa-tic
MIG welding apparatus performir~g a pipe-welding
operation, and
Figure 2 shows the apparatus located in an
enclosure on the seabed and electrically connected
to a power source on a surface ship.
In the drawings, 10 represents a pipe to be
welde~d formed with tracks 12 for guiding a trolley
14 provided with wheels 16. The trolley 14 carries
10 a reel 18 on which is wound consumable electrode wire
having a diameter of lmm or less. The end of the
electrode wire passed through a welding head 22
supported by the trolley 14. l'he trolley 14 houses the
driving systems for the wheels 16 and the reel 18.
15 ~lectrical supplies ~or these driving systems and
~or the welding arc are provided from a box 24 -through
a cable 26. The box 24 may include an oscillation
control system ~or the welding head together with
a local power source in which case the power source
20 has a slope o~ between 6 and 15V/lOOA. Alternatively,
as shown in Figure 2, the box 2L~ located with the
trolley 14 in an enclosure 28 on the seabed 30,
includes the oscillatlon control system and also
serves to couple the cable 26 to a further cable 32
25 and thence to a power source on the ship 34. In the
latter case, the slope of the power supply means
including the ship-borne power source and the
cable connecting this power source to the welding arc,
is between 6 and 15V/lOOA. Additionally, the connections
30 between the power source and the electrode wire
are such that the electrode wire is elec-trically
negative wi-th respect to thepipe 10 during the
welding operation.
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In one series of welding trials, carried out
at pressures of 7, 14 and 32 bars and in overhead,
vertical and flat posi-tions, -the following were
maintained constant throughout:
1. Base plate - BS 4360 Grade 50D
2. Pla-te Thickness - l9mm
3. Joint Type - One sided single V butt,60
included angle.
4. Root face - 1.6 + 0.6mm.
5. Root gap - 2.0 + 0 5mm
6. Process - MIG
7. Polarity - DC electrode negative
8. Consumable elec-trode wire - BS2901 part 1 A18
9. Consumable electrode wire diameter - O.9 mm
10. Power source - 500A solid s-tate
11. Open circui-t voltage - 45V
12. V/A slope - 7V/lOOA
13. Circuit lead length - 4m
14. Added circuit inductance - nil
15. Welding nozzle dia. - 12.5mm.
16. Contact tip/work distance - 10-15rnm
].7. Shielding gas flow rate - 10-15 l/min at working
pressure.
18. Shielding gas composition
7 bars - argon/2% oxygen
14 bars - argon/1% oxygen
32 bars - argon/0.5% oxygen
19. Interrun cleaning and grinding - nil.
20. Interpass -time - 5-lOmin.
An analysis of the base pla-te metal BS4360
Grade 50D gave the following percentages by weight
C 0.14; S 0.012; P 0.020; Si 0.35; Mn 1.43; Cu 0.02;
Nb 0.035; Al 0.021; 0 .0178; N .0093.
An analysis of the filler wire BS2901 give the
following results in percentages by weight: C 0.09;
S 0.029; P 0.022; Si 0.94; Mn 1.55; Cu 0.24; Nb e .0057
Al 0.007; 0 .0053; N .0073.
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For the overhead position 6 passes were made,
for the flat and vertical positions there were
7 passes. In each case for the root run the wire
feed speed was 5 metres/minute and the -travel
speed 200mm/minute: for the other passes tne
wire speed was 7.1 metres/minute and the travel
speeds were be-tween ~10 and 80mm/minute. The
oscillation width progressi~ely increased from the
lOmm in the second pass to 22mm ~or the seventh
pass for the flat and vertical positions, while
for the second to sixth passes in-the overhead - ;
position the oscillation widths were 7, 13, 13,
14 and lOmm respectively. The oscillation fre~uencies
were between 15. 4 and 21.4 oscillations per minute.
15 No oscillation was used for the roo-t run. The metal -
deposition rate for the root runs was 1.5kg/Hr and
for the other passes 2.05kg/Hr.
; The results of weld metal Charpy impact tests
-- - for the welds made were as follows, the value
20 in Joules in each case being the average of
the values for three welds.
_ Temperature ( =
Position Pressure O _ -10 -3-o
: 25__ _ _ (bars) JoulesJoules Joules
Overhead _ 118 56 50
4 ,59 57 31
~2 66 68 50
Vertical 7 100 78 55
ll 14 90 81 55
30 " 32 90 67 54
Flat 7 76 79 46
" 14 81 71 54
.l 32 80 50 46
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