Note: Descriptions are shown in the official language in which they were submitted.
~0~7748
The present invention relates to an improvement in the method for
longitudinally seam-welding a pipe-blank for welded steel pipe, from the
inside along a groove or cleft by the metal arc welding process, involving
using direct electric current supplied to a plurality of consumable electrodes
arranged in tandem through at least one current supplying cable as the
welding current.
This application is a division of application Serial No. 274,579
filed March 23, 1977.
Conventionally, in loDgitudinally seam-welding, for manufacturing
a welded steel pipe, the groove or cleft of an 0-shaped plate (hereinafter
called the "pipe-blank"), which is formed into a cylindrical shape by a
forming process, e.g. the U-0 process (abbreviation of the U-ing/0-ing pro-
cess) or the bending roll process J takes place, it is the usual practice
of welding to employ an inside welding machine equipped with a welding
torch attached to the free end of a boom, the boom having a length at least
equal to that of the pipe-blank to be welded, the fixed end of the boom
being fixed to a carriage. The boom is inserted into the pipe-blank in ad-
vance. The pipe-blank iB then longitudinally seam-welded from the inside
along the groove or cleft with a consumable electrode. Such consumable
electrode is fed through the welding torch while the boom is moved by the
carriage, together with the current supply cable serving to supply welding
current to the consumable electrode in the withdrawal direction from the
pipe-blank, i.e., in the welding direction. An inside welding machine may
be e~uipped with two welding torches, one leading and the other trailing,
and two consumable electrodes. However, an inside welding machine may have
one welding torch and one consumable electrode, or it may be provided with
more than two welding torches and more than two ccnsumable electrodes.
Another conventional method of welding a pipe-blank involves
adopting a welding process which includes using direct electric current as
7~
iOQ7748
the welding current with a consumable electrode as the anode, namely, the
reverse-polarity GMA welding process (GMA welding process is the abbrevi-
ation of the gas metal arc welding process which involves carrying out weld-
ing while shielding a molten metal and a welding arc produced in the space
between the base metal and the consumable electrode from open air with
shielding gases, e.g. an inert gas and a carbon dioxide gas). In such
process, the welding arc from the consumable electrode is deflected toward
the upstream side of the welding direction, i.e., in the opposite direction
to that of welding, and takes the form as if it were drawn in by the molten
metal. When the welding arc takes such form as if it is drawn in by the
molten metal as mentioned above, the plasma jet stream produced at the tip
of the consumable electrode is also deflected toward the molten metal and
acts on the molten metal as a dynamic pressure. This pushes the molten
metal away toward the upstream side of the welding direction, i.e. in the
opposite direction to that of welding. As a result, the space below the
consumable electrode becomes substantially dry without molten metal, thus
impairing the affinity between molten droplets from the consumable electrode
and the base metal at the groove or cleft of the pipe-blank. Welding de-
fects, e.g. undercut of bead, humping bead and lack of fusion of base metal
thus tend to occur easily. In addition, frequent occurence of boiling and
spattering in the molten metal tends to result in a deteriorated appearance
of the weld bead. When the welding arc is deflected as described above,
furthermore, the tip of the consumable electrode is melted only on one side.
Under such circumstances, the transfer mode of molten droplets from the con-
sumable electrode cannot be a desirable spray transfer, but takes an unde-
sirable mixed form of globular transfer and short-circuit transfer. As a
result, coarse spatters are splashed with a crac~ling short-circuiting noise
and aré deposited on the weld bead surface, thus leading to a deteriorated
appearance of the weld bead~ Moreover, splashed spatters are deposited on
1~7748
the opening st the tip of the shielding nozzle of the welding torch to dis-
turb the gas shield and entangle the air. In this case, it may practically
be impossible to carry out satisfactory welding.
The above-mentioned deflection of the welding arc toward the up-
stream side of the welding direction, i.e., in the opposite direction to
that of welding and the resulting irregular weld bead and welding defects
are not limited only in the case of the conventional GMA welding process,
but occur also in the case of the submerged-arc welding process using
direct electric current as the welding current. In both cases, it has been
difficult to obtain a sound weld substantially free from welding defects.
The irregular weld bead snd welding defects e.g. undercut of
bead, humping bead, lack of fusion of base metal and spattering, observed
in longitudinally seam-welding a pipe-blank for welded steel pipe from the
inside along a groove or cleft by the conventional welding process as men-
tioned above, is believed to be attributable to the deflection of a plasma
jet stream toward a lten metal caused by the deflection of a welding arc.
The deflection of the welding arc is brought about by a line of magnetic
force produced by the direct electric current for welding flowing through
a current supply cable introduced into the pipe-blank. This will be
described more specifically hereafter.
An object of a broad aspect of the present invention is therefore
to provide an improvement in the method for longitudinal seam-welding a
pipe-blank for welded steel pipe from the inside along a groove or cleft by the
metal arc welding process by using direct electric current as the welding current
supplied to a plurality of consumable electrodes in tandem, through at least
one current supply cable as the welding current, thereby to provide a sound
weld substantially free from welding defects.
An object of another aspect of the present invention is, in such
a method, substantially to prevent magnetization of the pipe-blank and the
resultiDg magnetic arc blow of the welding arc by causing lines of magne-
-- 3 --
1097748
tic force produced by the direct electric current for welding flowing
through the current supply cables introduced into the pipe-blank to cancel
each other, and also substantially to prevent splattering of molten droplets
from the straight polarity consumable electrodes.
In accordance with a broad aspect of the present invention, an
improvement is provided in a welding method which comprises applying the
metal arc welding process comprising using direct electric current supplied
to a plurality of consumable electrodes in tandem through at least one cur-
rent supply cable as the welding current, and longitudinally seam-welding
a pipe-blank for welded steel pipe from the inside along a pipe-blank for
welded steel pipe from the inside along a groove or cleft with the respec-
tive consumable electrodes fed through a plurality of welding torches in
tandem, while moving a boom through the pipe-blank relative thereto in the
same direction as that of welding, the boom being equipped with the welding
torches at the free end thereof and having a length at least equal to that
of the pipe-blank, the improvement comprising: using an even number of,
i e.at least two, consumable electrodes; connecting half of at least two
- even numbered consumable electrodes in the reverse-polarity manner, i.e., to
be positive in polarity and u~ing them as leading electrodes; connecting
20 the remaining half of the consumable electrodcs ln the straight-polarity
manner, i.e., to be ncgative in polarity and using them as trailing elec-
trodes; applying the GMA welding process on the siide of the reverse-polarity
consumable electrodes which are the leading electrodes; and applying the
submerged-arc welding process on the side o the straight-polarity consuma-
ble electrodes which are the trailing electrodes; ,hereby substantially pre-
venting magnetization of the pipe-blank and the resulting magnetic arc blow
of the welding arc by causing lines of magDetic force produced by the direct
electric c~rrent for welding flowing through the current supply cables in-
trod~ced into the pipe-blank to cancel each other, and also substantially
S preventing spattering of molten droplets from the straight-polarity con-
-- 4 --
10~7748
sumable electrodes.
By one variant, the boom is inserted in advance into the pipe-
blank together with at least one current supply cable; and the pipe-blank
is longitudinally seam-welded while withdrawing the boom, together with at
least one current supply cable, from the pipe-blank relative thereto in
the same direction as that of welding.
By another variant, the pipe-blank is longitudinally seam-welded
while inserting the boom into the pipe-blank relative thereto, together
with at least one current supply cable , in the same direction as that of
welding.
By another aspect of this invention, a method is provided for
longitudinally seam-welding a pipe-blank from the inside thereof applying
the metal arc welding process including: supplying DC current to an even
number of consumable electrodes in tandem through current supply cables, the
- current supply cables extending alongside each other longitudinally within
the pipe-blank; and longitudinally seam-welding the pipe-blank for welded
steel pipe from the inside along a groove or cleft by means of the respective
consumable electrodes fed through a respective even number of welding torches i
tandem, while moving a boom carrying the welding torches through the pipe-
blank relative to the pipe-blank in the same direction as that of welding,
the boom having a free end whibh carries the welding torches and having a
length at least equal to that of the pipe-blank, the method comprising the
steps of: c~nnecting one half of the even number of consumable electrodes
leading in the welding direction to direct electr~c current so as to be
positive in polarity, and connecting the other half of the even number of
consumable electrodes trailing in the welding direction to direct electric
current so as to be negative in polarity, whereby the direction, longltudinal-
ly of the pipe-blank, of the direct electric current flow to the one half
of the electrodes is opposite to-to the d~rection of the direct electric
lQ~7748
current flow to ebe other half of the electrodes so that the magne~ic fields
produced by the current flows cancel one another to prevent magnetization
of the pipe-blank; and wherein the metal arc welding is effected by means
of the trailing other half, in negative polarity, of the electrodes, by
producing a welding arc within a flux accumulation between the pipe-blank
A and the half of electrodes.
By yet another aspect of this invention, a method is provided for
longitudinally seam-wélding a pipe-blank from the inside thereof applying
the metal arc welding process including: supplying DC current as the weld-
ing current to an even number of consumable electrodes in tandem through10
current supply cables, the current supply cables extending alongside each
other longitudinally with the pipe-blank; and longitudinally seam-welding
the pipe-blank for welded steel pipe from the inside along a groove by means
of the respective consumable electrodes fed through a respective even numr
ber of welding torches in tandem, while moving a boom carrying the welding
torches through the pipe-blank relative to the pipe-blank in the same direc-
tion as that of welding, the boom having a free end which carries the weld-
ing torches and having a length at least equal to that of the pipe-blan~,
the method comprising the steps of: connecting one half of the even number
of consumable electrodes leading in the welding direction to direct electric
2Q
current 80 as to be positive in polarity, and connecting the other half of
the even number of consumable electrodes trailing in the welding direction
to direct electric current so as to be negative dn polarity, whereby the
direction, longitudinally of the pipe-blank, of the direct electric current
flow to the one half of the electrodes is opposite to the direction of the
direct electric current flow to the other half of the electrodes so that
the magnetic fields produced by the current flows cancel one another to pre~
vent magnetization of the pipe-blank; and wherein the metal arc welding is
effected by means of the leading one half, in positive polarity, of the
electrodes, by producing a molten metal and a welding arc in an initial
lQ"7748
leaclln
A space between the pipe-blank and the~one half of electrodes, and shielding
the molten metal and the arc with a shielding gas.
In the accompanying drawings,
Figure 1 is a schematic side view illustration of a conventional
method according to the prior art for longitudinally seamrwelding a pipe-
blank for welded steel pipe from the inside along a groove or cleft, and
an apparatus for the implementation thereof;
Figure 2 is a partially enlarged schematic drawing illustrating
a form of welding in the conventional reverse-polarity GMA welding process
of the prior art involving using direct electric current as the welding cur-
rent with a consumable electrode as the anode;
Figure 3 is a partial cutaway schematic side~view illustrating a
form of welding in longitudinally seam-welding a pipe-blank for welded
steel pipe from the inside along a groove by the conventional revefse-polari-
ty GMA welding process of the prior art;
Figure 4A and 4B are vector diagrams illustrating the relation be-
tween the direction of the electric current flowing through a welding arc,
the magnetizing direction at the groove of a pipe-blank for welded steel
pipe, and the direction of the force acting on the welding arc at the groove
in the conventional reverse-polarity GMA welding process of the prior art;
Figure S is a schematic side view illustrating an embodiment of
the method of one aspect of the present invention;
F~gure 6 is a photograph showing the state of a weld bead ob-
tained by the conventional reverse-polarity GMA welding process of the
prior art which is outside the scope of aspects of the prese~,invention,
and in which the leading and trailing consumable electrodes are connected
reversely in polarity with respect to each other; and
Figure 7 is a photograph showing the state of a weld bead obtained
by the method of an aspect of the present invention.
:lQ97748
As seen in schematic form in Figure 1, in longitudinally seam-
welding for manufacturing a welded steel pipe, a groove or cleft of an O-
shaped plate (namely a pipe-blank), which is formed into a cylindrical shape
by a forming process, e.g. the U-0 process or the bending roll process, it
is the usual practice of welding to employ an inside welding machine
equipped with a welding torch 1 attached to the free end of boom 2, the
boom 2 having a length at least equal to that of the pipe-blank 3 to be
welded. The fixed end of the boom 2 is fexed to a carriage 4. The boom 2
is inserted into the pipe-blank 3 in advance. The pipe-blank 3 is then
longitudinally seam-welded from the inside along the groove with a consuma-
ble electrode 5. Consumable electrode 5 is fed through the welding torch
1, while the boom 2 is moved by the carriage 4, together with a current sup-
ply cable 9 which serves to supply welding current to the consumable elec-
trode 5. The current supply cable 9 is moved in the withdrawal direction
from the pipe-blank 3, i.e., in the welding direction, as indicated by the
arrow in the drawing. As shown, the inside welding machine may be equipped
with two welding torches 1, 1, one leading, and the other trailing, and
two consumable electrodes 5, 5, one leading, and the other trailing. How-
ever, the inside welding machine may have one welding torch and one consuma-
ble electrode, or it may be provided with more than two welding torches andmore than two consumable electrodes.
Pigure 2 shows, in partially enlarged schematic form, a conven-
tional method of welding a pipe-blank, when adopting a welding process in-
volving using direct electric current as the welding current with a con-
sumable electrode as the anode, e.g , the reverse-polarity GMA welding pro-
cess. The welding 6 from the consumable electrode 5 is deflected toward
the ~pstream side of the welding direction indicated by the arrow in the
drawing, i.e., in the opposite direction to that of the welding and takes
the form as if it were drawn in by the molten metal 7. ~len the welding
-- 8
10"7748
arc 6 takes that form as if it is drawn in by the molten metal 7 as men-:
tioned above, the plasma ~et stream produced at the tip of the consumable
electrode 5 is also deflected toward the molten metal 7 and acts on the
molten metal 7 as a dynamic pressure. This pushes the molten metal 7
away toward the upstream side of the welding direction, i.e., in the oppo-
site direction to that of the welding. As a result, the space below the con-
sumable electrode 5 becomes substantially dry without any molten metal,
thus impairing the affinity between molten droplets from the consumable
electrode 5 and the base metal, at the groove of the pipe-blank 3. Welding
defects, e.g. undercut of bead, humping bead and lack of fustion of the
base metal thus tend to occur easily. In addition, frequent occurrence of
boiling and spattering in the weld bead surface, tends to result in a de-
teriorated appearance of the weld bead. When the welding arc 6 is deflected
as described above, the tip of the consumable electrode 5 is melted only on
one side as shown in Figure 2, Under such circumstances the transfer mode
of molten droplets from the consumable electrode 5 cannot be a desirable
spray transfer, but takes an undesirable mixed form of globular transfer
and short circuit transfer. As a result, of course, spatters are splashed
and are deposited on the weld bead surface, thus leading to a deteriorated
appearance of the weld bead. Moreover, as indicated before, splashed spat-
ters are deposited on the opening at the tip of the shielding nozzle (not
shown) of the welding torch to disturb the gas shield and entangle the air.
In this case, it may be practically impossible to carry out welding.
In longitudinally seam-welding a pipe-blank for welded steel pipe
from the inside along a groove or cleft by the conventional reverse polarity
GMA weld~ng process comprising using direct electric current as the welding
current with a consumable electrode as the anode, direct current for weld-
ing flows, as shown in the partial cutaway schematic side view given in Fig-
ure 3, in the opposite direction to that of welding, indicated by an arrow
in the drawing, through a current supply cable 9 introduced into the pipe-
_ g _
10"7748
blank 3. Therefore, a line of magnetic force 10 which is clockwise asviewed from the right-hand side of the drawing, is produced by the direct
electric current, and a strong magnetic field is formed around the current
supply cable 9. As a result, the pipe-blank 3 is strongly magnetized in
the same clockwise direction 11 as that of the line of magnetic force 10 by
the magnet~c field, thus causing leakage of a strong line of magnetic force
from the groove of the pipe-blank 3, and a strong magnetic field is formed
at the groove. When welding a groove or cleft where such a strong magnetic
field is formed, the welding arc from a consumable electrode 5, which is a
flow of charged corpuscle, is deflected by the strong magnetic field at the
groove. This phenomenon is called the magnetic arc blow of the welding arc.
The relation between the direction of the electric current flow-
ing through a welding arc, the magnetizing direction at the groove or cleft
of a pipe-blank and the direction of the force acting on the welding arc at
the groove or cleft is illustrated in the vector diagrams of Figures 4A and
4B. In these drawings, A is the direction of the electric current flowing
in the welding arc 6 through the consumable electrode 5; B is the magne-
tizing direction at the groove of the pipe-blank 3; C is the welding direc-
tion; and F is the direction of the force acting on the welding arc 6. As
shown in Figures 4A and 4B, the directiGn F of the force acting on the weld-
ing arc 6 is the same as that of the direct electric current for weldingflowing through the current supply cable 9 (refer to Figure 3) and is opposite to
the welding direction C. As described hereinabove with reference to Figure
2, therefore, the welding arc 6 is deflected toward the upstream side of
the welding direction C, i.e., in a direction opposite to that of welding
C, thus resulting in an irregular weld bead and welding defects, e.g. un-
- dercut of bead, humping bead, lack of fusion of base metal and spattering.
The above mentioned magnetic arc blow of the welding arc is wide-
ly observed when applying the arc welding process using direct electric
current as the welding current.
-- 10--
lOg7748
A procedure has been disclosed in parent application Serial No.
274,579 filed March 23, 1977 for the purpose of substantially preventing
the aforementioned magnetization of the pipe-blank and the resulting magne-
tic arc blow of the welding arc. That application proposed a procedure for
longitudinally seam-welding a pipe-blank for welded steel pipe from the in-
sidc along a groove or cleft which comprised: applying the metal arc weld-
ing process comprising using direct electric current supplied to a plurality
of consumable electrodes in tandem through a current supply cable as the
welding current; using at least two even-numbered consumable electrodes;
connecting half of the at least two even-nu~bered consumable electrodes in
the reverse-polarity, i.e., to be positive in polarity; connecting the re-
maining half of the at least two even-numbered consumable electrodes in the
straight-polarity manner, i.e., to be negative in polarity; and longitudin-
ally seam-welding the pipe-blank from the inside along the groove or cleft
with the respèctive consumable electrodes fed through a plurality of weld-
ing torches in tandem, while moving a boom through the pipe-bIank relative
thereto in the same direction as that of welding, the bo~m being equipped
with the welding torches at the free end thereof and having a length at
least equal to that of the pipe-blank; thereby substantially preventing
magnetization of the pipe-blank and the resulting magnetic arc blow of the
welding arc by causing lines of magnetic force produced by the direct elec-
tric current for welding flowing through the cable introduced into the pipe-
blank to cancel each other.
The metal arc welding process referred to above is a welding pro-
cess well known to a person having ordinary s~ill in the art, which com-
prises, with the use of heat of a welding arc produced between a base metal
and a consumable electrode, melting the consumable electrode from the tip
thereof and conducting welding with molten droplets from the consumable
electrode.
~0~77~8
According to the above-mentioned method, it is certain that a
sound weld substantially free from welding defects can be obtained by the
substantial prevention of the occurrence of the magnetization of a pipe-
blank and the resulting magnetic arc blow of the welding arc in longitudinal-
ly seam-welding the pipe-blank from the inside along the groove or cleft.
In carrying out this method, however, although no problem is posed
on the side of the consumable electrode connected in the reverse-polarity
manner, i.e., to be positive in polarity, on the side of the consumable
electrode connected in the straight-polarity manner, i.e., to be negative
in polarity, the strong pushing-up force intrinsic to a straight-polarity
welding arc blows off molten droplets from the straight-polarity consumable
electrode to abbve the tip of the consumable electrode in the form of spat-
ters, which are deposited on the weld bead surface and the-surroundings,
thus somet~mes deteriorating the appearance o the weld bead. This has some-
times caused problems in manufacturing welded steel pipes for pipe-line
under particularly strict requirement for a satisfactory weld bead surface
quality. Furthermore, in carrying out a welding continuously for a long
time, the 6patters are deposited on the opening of the shielding nozzle,
disturb the smooth flow of shield gas and impair the stability of welding
2Q arc. In extreme cases, entangle air even caused coarse bubbles in molten
metal.
Further studies have therefore been made in order to solve such
difficulties as mentioned above. As a result, it has been found that the
appearance of a weld bead can be dramatically improved, in the above-men-
tioned welding method in which leading and trailing consumable electrodes
are-connected reversely in polarity with respect to each other, by using
the reversc-polarity consumable electrode as the leading electrode, apply-
ing the GMA welding process on the side of the reverse-polarity consumable
electrode, using the straight-polarity consumable electrode which has posed
- 12 -
1097748
a problem in GMA welding process as the trailing electrode, and applying
the submerged-arc welding process on the side of the straight-polarity con-
sumable electrode.
The submerged-arc welding process referred to above is a welding
process, well known to a person having ordinary skill in the art, which
comprises accumulating a granular flux on a base metal in advance and carry-
out the metal arc welding in this accumulation of flux.
Aspects of the present invetnion have been made based on the
aforementioned fiDdings. More specifically, in the method of an aspect of
the present invention, the conventional metal arc welding process is
applied, involving using direct electric current supplied to a plurality
of consuma~le electrodes in tandem through current supply cables as the
welding current. Furthermore, at least two even-numbered consumable elec-
trodes are used, half of the consumable electrodes being connected in the
reverse-polarity manner, i.e., to be positive in polarity, and the remain-
ing half being connected in the straight-polarity manner, i.e., to be nega-
tive in polarity. As 6hown in the schematic side view of Figure 5, a boom
Z, having a length at least equal to that of a pipe-blank to be welded 3,
i~ equipped with a leading welding torch la and a trailing torch lb in tan-
dem at the free end thereof. The fixed end thereof is fixed to a carriage
~not shown), and is inserted in advance into the pipe-blank 3 together with
current supply cables 9a and 9b. A leading consumable clectrode 5a fed
through the leading welding torch la is connected vla current supply cable
~a in the reverse-polarity manner, l.e., to be positive in polarity, and a
trailing consumable electrode 5b fed through the trailing welding torch lb
is connected via current supply cable 9b in the straight-polarity manner,
i.e., to be negative in polarity. Pipe-blank 3 is longitudinally seamr
welded from the inside along the groove or cleft in the welding direction
lQ~7748
` aæ indicated by an arrow 14 in the drawing, by applying the GMA welding
process on the side of the leading consumable electrode 5a, i.e., the re-
verse-polarity consumable electrode 5a, and by applying the submerged-arc
welding process on the site of the trailing consumable electrode 5b, i.e.,
the straight-polarity consumable electrode 5b, while withdrawing the boom
2, together with the cables 9a and 9b, from the -pipe-blank 3 in the weld-
ing direction as indicated by the arrow 14.
In the aforementioned method of an aspect of the present inven-
tlon, as shown in Figure 5, the direct electric current flowing through
the current supply cable 9a flows in the direction as indicated by arrow
12 from right to left toward the consumable electrode 5a in the drawing.
A clockwise line of magnetic force 13 as viewed from the right-hand side of
the drawing is therefore produced by the direct electric current. On the
other hand, the dircct electric current flowing through the current supply
cable 9b flows in the direction as indicated by an arrow 15 from left to
right from the consumable electrode 5b. A counter-c~ockwise line of magne-
tic force 16, ~ust reverse in the direction to the line of magnetic force
13, a~ viewed from the right-hand side of the drawing, is therefore pro-
duced by the direct electric current. Consequently, because of the differ-
ence in polarity between thc leading consumable electrode Sa and the trail-
ing consumable electrode 5b, the direction of the line of magnetic force 13
is opposite to that of the line of magnetic force 16. Therefore, the lines
of magnetic force 13 and 16 cancel each other, and as a result, the pipe-
blank 3 is hardly magnetized. It is thus possible substant~ally completely
to prevent occurrence of the magnetic arc blow of the welding arc and the
resulting irregular weld bead and welding defects, e.g. undercut of bead,
humping bead, lac~ of fusion of base metal and spattering.
AB mentioned above, in the method of an aspect of the present in-
vention, the submerged-arc welding process is applied on the side of the
.
- 14 -
748
trailing consumable electrode 5b, i.e., the straight-polarity coDsumable
electrode 5b conn~cted to be negative in polarity. Therefore, all molten
droplets splashed on the gide of the straight-polarity consumable electrode
by the strong pushing-up forcc intrinsic to a straight-polarity welding arc,
are absorbed in a molten slag of flux and are not deposited on the surface
of the weld bead as spatters, thus leading to a sound weld substantially
free from welding defects. Furthermore, since the leading consumable elec-
trode 5a is connected in the reverse-polarity manner, i.e., to be positive
in an aspect of the present invention, better results can be obtained, be-
cause of the easy availability of a deeper fusion penetration required on
the side of the leading consumable electrode.
The magnetization of a pipe-blank can be substantially prevented
also by another embodiment of the method of an aspect of the present in-
vention. This embodiment comprises, also with reference to Figure 5, con-
necting contrary to the teachings of the first embodiment, the consumable
electrode 5a in the straight-polarity manner, i.e., to be negative in
polarity, and the consumable electrode 5b, in the reverse-polarity manner,
i.e., to be positive in polarity. The pipe-blank 3 is longitudinally seamr
welded from the inside along the groove or cleft in the direction opposite
to that indicated by the arrow 14, by applying the GMA welding process on
the side of the consumable electrode 5b, i.e., the reverse-polarity consuma-
ble electrode 5b and applying the submerged-arc welding process on the side
of the consumable electrode Sa, i.e., the straight-polarity consumable elec-
trode 5a, while inserting the boom 2, together with the current supply
cables 9a and 9b, into the pipe-blank 3 from the right-hand side of the
drawing by the carr~age (not shown in the drawing), thereby substantially
preventing magnetic arc blow of the welding arc and the resulting irregular
weld bead and weld defects. In this case the consumable electrode 5b is
the leading electrode, and the consumable electrode 5a is the trailing
electrode.
- 15 --
1097748
The method of aspects of the present invention is now described
in detail by way of the following example.
EXA~PLE
. .
In longitudinally seam-welting two pipe-blanks for welded steel
pipe from the inside along a groove or cleft, having a wall thickness of 1
inch, an outside diameter of 48 inches and a length of 12 meters by the
tandem-headed metal arc welding process which includes using direct elec-
tric current supplied to two leading snd trailing consumable electrodes
through current supply cableg as the welding current, the welding method of
the method of one aspect of the present invention described above with re-
ference to Figure 5 was applied to a pipe-blank. That method comprises,
also with reference to Figure 5, inserting the boom 2 in advance into the
pipe-blank 3 together with the current supply cables 9a and 9b. The boom
2 is eguipped with the leading welding torch la and the trailing welding
torch lb in tandem at the free end thereof and is fixed to a carriage (not
shown) at the fixed end thereof. - The leading consumable electrode 5a, fed
through the leading welding torch la, is connected in the reverse-polarity
manner, i.e., to be positive in polarity via the current supply cable 9a.
The trailing consumable electrode 5b, fed through the trailing welding
tbrch lb, is connected in the straight-polarity manner, i.e., to be nega-
tive in polarity via the current supply cable 9b. The pipe-blank 3 is
longitudinally seam-welded from the inside along the groove or cleft with
the leading and trailing consumable electrodes Sa and 5b in the welding di-
rection as indicated by the arrow 14 in the drawing, by applying the GMA
welding process on the side of the leading consumable electrode 5a, i.e,,
the reverse-polarity consumahle electrode 5a and applying the submerged-
arc welding process on the side of the trailing consumable electrode 5b,
i.e., the straight-polarity consumable electrode 5b, while withdrawing the
boom 2, together with the current supply cables 9a and 9b, from the pipe-
blank by the carriage in the same direction as that of welding iDdicated
- 16 -
10~77918
by the arrow 14.
On the other hand, the conventional reverse-polarity GMA welding
process, which is outside the scope of the present invèntion, was applied
to the other pipe-blank for comparison purposes, such method including con-
necting both of the leading consumable electrode and the trailing consumable
electrode reversely in polarity with respect to each other, and applying the
GMA welding process on both sides of the leading and trailing consumable
electrodes.
The state of the weld beads obtained as a result is shown in photo-
graphs of Figures 6 and 7.
In the weld beat obtained by the ~MA welding process which is out-
side the scope of the present invention, and in which both of the leading
consumable electrode and the trailing electrode are connected reversely in
polarity with respect to cach other, as shown in the photograph of Figure
6, not only is the appearance poor, but the X-ray test also revealed such
welding defects as lack of fu~ion of base metal and undercut of bead. This
is attributable to the strong pushing-up force intrinsic to a straight-
polarity weldlng arc blows off molten droplets from the straight-polarity
consumable electrode to above the tip of the consumable electrode in.the
form of spatters, which are deposited on the weld bead surface and the
surroundings.
The weld bead obtained by the welding method of an aspect of the
present invention, in contrast, has the appearance as shown in the photo-
graph of Figure 7, with no significant trace of welding defects observed
even by X-ray test and by an ultrasonic flaw detection test, thus permitting
ascertainment that a very sound weld can be obtained by the welding method
of an aspect of the present invention.
Similarly good results have also been obtained by the welding
method of another aspect of the present invention, which comprises, also
with reference to ~igure 5, connecting, contrary to the foregoing, the con-
- 17 -
10~7~748
sumable electrode 5a in the straight-polarity manner, i.e~, to be negative
in polarity, and the consumable electrode 5b in the reverse-polarity manner,
i.e., to be positive in polarity. The pipe-blank-3 is longitudinally
seamrweldet from the inside along the groove or cleft with the consumable
electrodes 5a and 5b in a direction opposite to that indicated by the arrow
14, by applying the GMA welding process on the side of the consumable elec-
trode 5b, i.e., the reverse-polarity consumable electrode 5b and applying
the submerged-arc welding process on the side of the consumable electrode 5a,
i.e., the straight-polarity consumable electrode 5a, while inserting the boom
2, together with the current supply cables 9a and 9b, into the pipe-blank
3 from the right-hand side of tbe drawing by the carriage (not shown in the
drawing). In this case, the consumable electrode 5b is the leading
electrode, and the consumable electrode 5a, the trailing electrode.
The above description of the method of aspects of the present in-
vention has covered only the case where a pipe-blank is fixed while moving
a boom equipped with welding torches in tandem. The essential requirement
in aspects of the present invention is, however, that the welding torches
move in the welding direction. Therefore, the method of other aspects of
the present invention covers also cases in which the pipe-blank is moved
while the boom equipped with the welding torches is fixed. In other words,
in the method of aspects of the present inventionk it suffices to move the
boom equipped with the welding torches through the pipe-blank relative
thereto in the same direction as that of welding.
According to the method of aspects of the present nvention, as
described above in detail, it is possible substantially completely to pre-
vent not only the occurrence of the magnetization of the pipe-blank caused
by direct electric current flowing through the current supply cables intro-
duced into the pipe-blank and the resulting magnetic arc blow of the welding
arc but also substantially completely to prevent the occurrence of spatter-
- 18--
lQ~7748
ing on the side of the straight-polarity consumable electrode caused by
the strong pushing-up force intrinsic to the straight-polarity welding arc.
Therefore, it is possible to obtain a sound weld bead, thus providing in-
dustrially useful effects.
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