Welding at pressures greater than atmospheric pressure

When metal inert gas (MIG) welding is carried out at normal atmospheric pressure, the electrode is made positive with respect to the workpiece because the use of a negative electrode gives little penetration. In undersea welding, as the pressure increases the arc stability and metal transfer in MIG welding become erratic and there is copious fume evolution. According to the present invention, MIG welding at pressures greater than 7 bars is carried out with the electrode negative with respect to the workpiece and with an electrode wire of diameter not greater than 1.4 mm; the slope of the power supply, as seen from the welding arc, is preferably between 6 and 15 V/100 A, which is higher than the 3 to 4 V/100 A used in positive-electrode MIG welding at normal atmospheric pressure.

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 successfully 
for underwater welding. Normally for quality welds at any depth 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 (164 ft. 50 m, 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 welded. As depth increases, the pressure on the arc 
increases. 
Flux-shielded arc welding processes and in particular the manual metal arc 
process and the flux-cored wire process, have been used with success down 
to 50 m, 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 properties of the weld. 
Arc stability however is maintained. 
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 processes utilising flux for stabilisation 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 
greater than 7 bars, a metal inert gas process is used with the electrode 
negative with respect to the workpiece and with an electrode wire of 
diameter not greater than 1.4 mm; in the preferred method embodying the 
invention, the slope of the power supply, as seen from the welding arc, is 
between 6 and 15 V/100 A. This slope is higher than the slope normally 
used in MIG welding, which is about 3 to 4 V/100 A. 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 
slope is 7 V/100 A. 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. An oxidising 
gas such as oxygen or carbon-dioxide may be added to the inert gas. A 
suitable mixture is one that contains at least 95% of argon or helium and 
up to 5% of oxygen or carbon dioxide. 
In this specification, the expression "conventional metal inert gas 
welding" 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 heat 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. 
Thus when the electrode is made negative at normal atmospheric 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 welding with a negative electrode has had little practical 
use, because of the disadvantages set out above, although some mitigation 
of these disadvantages can be achieved with the use of an argon-rich gas 
containing some oxygen or CO.sub.2. 
It has also been proposed to use an AC current for metal inert-gas welding, 
the positive electrode half-cycles stabilising the arc and the negative 
electrode half-cycles heating the wire. 
The 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 metal inert-gas welding process vary with the pressure under which 
the weld is carried out. Using the conventional 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 problems discussed above. However, with the same 
increase in pressure, the alteration 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 working 
and at a pressure of 14 bars the use of electrode-negative working is 
highly advantageous. As an example, stable 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 310 m of depth). 
MIG welding generally requires a flat characteristic. One way of increasing 
the power supply slope (as seen from 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 the invention permits locating the power 
source on the ship, the long leads on the ship to the undersea welding 
site giving stability to the welding process. Another way of increasing 
the power supply slope, as seen from 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 cables, 
where several cables are used in parallel. The same result can also be 
achieved 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 apparent power supply slope, as viewed from the arc, will be the 
same as the effect of the previously proposed increase in resistance. The 
inductance can be introduced by increasing the number of parallel 
connecting cables or by including an inductive component in the leads. 
The 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 30.degree., in any position. 
Generally speaking, this 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 reduces 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 itself to 
mechanisation. The limit of welding by divers has now been practically 
reached in the sense that at the depths now envisaged a diver requires a 
very long period of decompression. Consequently to weld at greater depths 
mechanised processes will be highly desirable. 
It is not necessary to vary the weld metal composition with pressure.

In the drawings, 10 represents a pipe to be welded formed with tracks 12 
for guiding a trolley 14 provided with wheels 16. The trolley 14 carries a 
reel 18 on which is wound consumable electrode wire having a diameter of 1 
mm or less. The end of the electrode wire passed through a welding head 22 
supported by the trolley 14. The trolley 14 houses the driving systems for 
the wheels 16 and the reel 18. Electrical supplies for these driving 
systems and for the welding arc are provided from a box 24 through a cable 
26. The box 24 may include an oscillation control system for the welding 
head together with a local power source in which case the power source has 
a slope of between 6 and 15 V/100 A. Alternatively, as shown in FIG. 2, 
the box 24 located with the trolley 14 in an enclosure 28 on the seabed 
30, includes the oscillation control system and also serves to couple the 
cable 26 to a further cable 32 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 15 V/100 A. Additionally, the connections 
between the power source and the electrode wire are such that the 
electrode wire is electrically negative with respect to the pipe 10 during 
the welding operation. 
In one series of welding trials, carried out at pressures of 7, 14 and 32 
bars and in overhead, vertical and flat positions, the following were 
maintained constant throughout: 
1. Base plate--BS 4360 Grade 50D 
2. Plate Thickness--19 mm 
3. Joint Type--One sided single V butt, 60.degree. included angle. 
4. Root face--1.6.+-.0.6 mm. 
5. Root gap--2.0.+-.1.0 0.5 mm 
6. Process--MIG 
7. Polarity--DC electrode negative 
8. Filler--BS 2901 part 1 A18 
9. Filler diameter--0.9 mm 
10. Power source--500 A solid state 
11. Open circuit voltage--45 V 
12. V/A slope--7 V/100 A 
13. Circuit lead length--4 m 
14. Added circuit inductance--nil 
15. Welding nozzle dia.--12.5 mm. 
16. Contact tip/work distance--10-15 mm 
17. 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--5-10 min. 
An analysis of the base plate 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; O 0.0178; N 0.0093. 
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&lt;0.005; Al 0.007; O 0.0053; N 0.0073. 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 meters/minute and 
the travel speed 200 mm/minute: for the other passes the wire speed was 
7.1 meters/minute and the travel speeds were between 110 and 80 mm/minute. 
The oscillation width progressively increased from the 10 mm in the second 
pass to 22 mm for 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 10 mm respectively. The 
oscillation frequencies were between 15.4 and 21.4 oscillations per 
minute. No oscillation was used for the root run. The metal deposition 
rate for the root runs was 1.5 kg/Hr and for the other passes 2.05 kg/Hr. 
The results of weld metal Charpy impact tests for the welds made were as 
follows, the value in Joules in each case being the average of the values 
for three welds. 
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Temperature .degree.C. 
Pressure 0 -10 -30 
Position (bars) Joules Joules 
Joules 
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Overhead 7 118 56 50 
" 14 59 57 31 
" 32 66 68 50 
Vertical 7 100 78 55 
" 14 90 81 55 
" 32 90 67 54 
Flat 7 76 79 46 
" 14 81 71 54 
" 32 80 50 46 
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