Note: Descriptions are shown in the official language in which they were submitted.
V~
The present invention relates to an improvement in the method
for longitudinally seam-welding a pipe-blank for welded steel pipe, from
the i~side along a groove or cleft by the metal arc welding process,
involving using direct electric current supplied to a plurality of con-
sumable electrodes arranged in tandem through at least one current
supplying cable as the welding current.
Conventionally, in longitudinally 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-O process (abbreviation of the U-ing/0-ing
process) or the bending roll process, takes place. It is the usual
welding practice 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 fixPd to a carriage. The boom is inserted into the pipe-
blank in advance. The pipe-blank is 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 current supply cable serving to supply
20 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 equipped 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 con-
sumable electrodes.
Another conventional method of welding a pipe-blank invovles
adopting a welding process which includes using direct electric current as
the welding current with a consumable electrode as the anode, namely, the
- 30 reverse-polarity G.M.A. welding process (G.M.A. welding process is the
_ _ _ _ _ . _ _ .. . . _ . . . . . . .
- : ,., ':. . : .` . : ~ , :
- : : . . .. .
:: . ... ., :: :, : :: : . -, : '
.. - - : . ~ :
1~8`~
abbreviation of the gas metal arc welding process which involves carrying
out welding 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. "~n 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 i9
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 defects, e.g., undercut of bead,
humping bead and lack of fusion of base metal thus tend to occur easily.
-~ In addition, frequent occurrence 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 circum-
stances, the transfer mode of molten droplets from the consumable electrode
- cannot be a desirable spray transfer, but takes an undesirable mixed form
of globular transfer and short-circuit transfer. As a result, coarse
spatters are splashed with a crackling short-circuiting noise and are
deposited on the weld bead surface, thus leading to a deteriorated appear- - ;
ance of the weld bead. i~oreover, splashed spatters are deposited on the
opening at the tip of the shielding nozzle of the welding torch to disturb
the gas shield and entangle the air. In this case, it may practially be
-- 2 --
-: . , , , , . . :
:: . - . . - . . . ::. : . . :: ::: , :
impossible to carry out satisfactory welding.
The above-mentioned deflection of the welding arc toward the
upstream side of the welding direction, i.e., in the opposite direction
to that of welding and the resulting irregular w~ld bead and welding de-
fects are not limited only in the case of the conventional G.M.A. 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.
An object of a broad aspect of the present invention is there-
fore 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 G.M.A. welding process by using direct electric current as the
welding current supplied to a plurality of consumable electrodes, through
at least one current supply cable as the welding current, thereby to pro-
vide a sound weld substantially free from welding defects.
An object of another aspect of the present invention is substan-
tially to prevent the occurrence of the magnetization of the pipe-blank
for welded steel pipe and the resulting magnetic arc blow of the welding
arc in longitudinally seam-welding the pipe-blank from the inside along a
groove or cleft by the G.M.A. welding process.
In accordance with a broad aspect of the present invention, an
improvement is provided in a welding method for longitudinally seam-welding
a pipe-blank from the inside thereof applying the G.M.A. welding process
comprising: supplying D.C. current as the welding current to an even
number of consumable electrodes in tandem through at least one current
- supply cable, the current supply cable extending alongside each other
; longitudinally within the pipe-blank; and longitudinally seam-welding the
pipe-blan~ for welded steel pipe from the inside along a groove or cleft
30 with respective consumable electrodes fed through a respective plurality
B - 3 - ~
~ : . ' ~ , : : ................. ' , '- :
:: , , ' , . ~ .
.. . . . ~ ..
1~38V~313
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
direction as that of welding, the boom having a free end which carries
the welding torches and having a length at least e~ual to that of the
pipe-blank, the method comprising the steps of: using at least two even-
numbered consumable electrodes; connecting one-half of the consumable
electrodes to the D.C. current so as to be positive in polarity; connec-
ting the other half of the consumable electrodes to the D.C. current so as
to be negative in polarity; thereby to enable lines of magnetic force
produced by the direct electric current for welding flowing through at
least one current supply cable introduced into the pipe-blank to cancel
each other, thereby substantially preventing magneti~ation of the pipe- ~-
blank.
By one variant, the boom is inserted in advance into the pipe-
blank together with the current supply cables; and the pipe-blank is
longitudinally seam-welded while withdrawing the boom, together with the
current supply cables, 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, together with the current supply cables, into
the pipe-blank relative thereto in the same direction as that of welding.
By a further variant, the method includes connecting one-half,
leading in the welding direction, of the even number of electrodes to the
D.C. current so as to be positive in polarity, and connecting the other
half, trailing in the welding direction, of the even number of electrodes
to the D.C. current so as to be negative in polarity.
In the accompanying drawings,
Figure 1 is a schematic side view illustrating a conventional
method according to the prlor art for longitudinally seam-welding a pipe-
blank for welded steel pipe from the inside along a groove or cleft, and
- 4 -
. ::: .: - : ~: :: .
: -- : : :
.: . . . . . ..
~8{)B~
an apparatus for ~he implementation thereof;
Figure 2 is a partially enlarged schematic drawing illustrating
a form of welding in the conventional reverse-polarity G.M.A. welding
process of the prior art involvi.ng using direct electric current as the
welding current with a consumable electrode as ~he 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 or cleft by the conventional
reverse-polarity G.M.A. welding process of the prior art;
Figures 4A and 4B are vector diagrams illustrating the relation
between the direction of the electric current flowing through a welding
arc, the magneti~ing direction at the groove or cleft of a pipe-blank for
welded steel pipe, and the direction of the force acting on the welding
arc at the groove or cleft in the conventional reverse-polarity G.M.A.
welding process of the prior art;
Figure 5 is a schematic side view illustrating an embodiment of
the method of one aspect of the present invention;
Figure 6 is a photograph showing the state of a weld bead
obtained by the conventional reverse-polarity G.M.A. welding process of
the prior art; and
Figure 7 is a photograph showing the state of a weld bead
obtained by the method of an aspect of the present invention.
As seen in schematic form in Figure 1, in longitudinally seam-
welding for manufacturing a welded steel pipe, a groove or cleft of an ;
0-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
macine 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 fixed to a carriage 4. The
,, .
~ _ 5 _
108~3
boom 2 is inserted into the pipe-blank 3 in advance. The pipe-blank 3 is
then longitudinally sea~-welded Erom the inside along the groove or cleft
with a consumable electrode 5. Consumable electrode 5 i5 fed through the
welding torch 1, while the boom 2 is moved by the carriage 4, together
with a current supply cable 9 wh:ich serves to supply welding current to
the consumable electrode 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. ~s shown, the inside welding
machine may be equipped with two welding torches 1, 1, one leading, and
the other trailing, and ~wo consumable electrodes 5, 5, one leading, and
the other trailing. However, the 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 consumable electrodes.
- Figure 2 shows, in partially enlarged schematic form, a conven~
tional method of welding a pipe-blank, when adopting a welding process
involving using direct electric current as the welding current with a con-
sumable electrode as the anode, namely, the reverse-polarity G.M.A. welding
process. The welding 6 from the consumable electrode 5 is deflected toward
the upstream 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. When the welding
arc 6 takes that form as if it is drawn in by the molten metal 7 as men-
tioned above, the plasma jet 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 opposite
direction to that of the welding. As a result, the space below the
consumable electrode 5 becomes substantially dry without any molten metal,
thus impairing the affinity between molten droplets from the consumable
- 30 electrode 5 and the base metal, at the groove of the pipe-blank 3. Welding
.
- 6 -
-- . . - - : , - . ~
.:.......... ~- .- .
. ~ . : - : ,
., , ,, . . ~ .
- . : :
.. . . .
1~8~8~3
defects, e.g., undercut of bead, humping beand and lack of fusion 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
deteriorated 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 dropl~ets from the consumable electrode 5
cannot be a desirable spray transEer, but takes an undesirable mixed form
of globular transfer and short circuit transfer. As a result, coarse
spatters are splashed and are deposited on the weld bead surface, thus
leading to a deteriorated appearance of the weld bead. Moreover, as indi-
cated before, splashed spatters 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.
The irregular weld bead and wèlding 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
mentioned above, is believed to be attributable to the deflection of a
plasma jet stream toward a molten metal caused by the deflection of a
welding arc, and also that 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.
More specifically, for instance in longitudinally seam-welding a
pipe-blank for welded steel pipe from the inside along a groove or cleft
by the conventional reverse polarity G.M.A. welding process comprising
using direct ~electric current as the welding current with a consuamble
electrode as the anode, direct current for welding flows, as shown in the
~ 7 ~ ,
-'~ ' ~ `' ' : ' ,. - ` : ` ' `
~L~38~8~1L;3
partial cutaway schematic side view glven in Figure 3, in the opposite
direction to that of welding, as indicated by an arrow in the drawing,
through a current supply cable 9 introduced into the pipe-blank 3. There-
fore, a line of magnetic force lO,which is clockwise as viewed from the
right-hand side of the dra~ing, 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 clock-
wise direction 11 as that of the line of magnetic force 10 by the magnetic
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 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
or cleft. 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 of a
pipe-blank and the direction of the force acting on the welding arc at the
groove 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 magnetizing
direction at the groove of the pipe-blank 3; C is the welding direction;
and F is the direction of the force acting on the welding arc 6. As shown
in Figures 4A and 4B, the direction F of the force acting on the welding
arc 6 is the same as that of the direct electric current for welding
flowing through the current supply cable 9 (refer to Figure 3) and is
opposite to the welding direction C. As described hereinabove with refer-
ence 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
- 8 -
: . , , : -.
-: : . :
. .
1~8()~-~3
defects, e.g., undercut of bead, humping bead, lack of fusion of base
metal and spattering.
In its broad aspects, the present invention, which has been based
on the aEorementioned finding, enables, in longitudinally seam-welding a
pipe-blank from welded steel pipe from the inside along a groove or cleft
by the conventional G.M.A. welding process which involves using direct
electric current supplied to a plurality of consumable electrodes in tan-
dem through at least one current supply cable as the welding current,
lines of magnetic force produced by the direct electric current for welding
flowing through at least one current supply cable introduced into the pipe-
blank to cancel each other, by using at least two even-numbered electrodes,
connecting one-half of the consumable electrodes in the reverse-polarity
manner, i.e., to be positive in polarity, and connecting the other half
in the straight polarity manner, i.e., to be negative in polarity, thereby
substantially preventing magnetization of the pipe-blank and the resulting
magnetic arc blow of the welding arc.
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 consumable elec-
trodes in tandem through current supply cables as the welding current. Asshown in the schematic side view of Figure 5, a boom 2, having a length at
least equal to that of a pipe-blank to be welded 3, is equipped with a
leading welding torch la and a trailing welding torch lb in tandem 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. ~ leading consumable electrode 5a, fed through
- the leading welding torch la, is connected via current supply cable 9a in
- the reverse-polarity manner, i.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
':
9 _ ' , ',
3V~
manner, i.e., to be negative in polarity. Pipe-blank 3 is long~tudinally
seam-welded from the inside along the groove or cleft in the welding
direction as indicated by an arrow 14 in the drawing, with the leading
consumable electrode 5a, and the trailing consumable electrode 5b, while
withdrawing the boom 2, together with the current supply cables 9a and 9b,
from the pipe-blank 3 by the carriage.
In the aforementioned method of an aspect of the present inven-
tion, as shown in Figure 5, the direct electric current flowing through
the current supply cable 9a flows in the direction as indicated by an
arrow 12 from right to left toward the consumable electrode 5a ln 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 direct electric current flowing through
- the supply cable 9b flows in the direction as indicated by an arrow 15
from left to right from the consumable electrode 5b. A counterclockwise
line of magnetic force 16, just reverse in direction to the line of
magnetic force 13, as viewed from the right-hand side of the drawing, is
therefore produced by the direct electric current. Consequently, because
of the di~ference in polarity between the leading consumable electrode Sa
and the trailing 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
substantially completely to prevent the occurrence of the magnetic arc
blow of the welding arc and the resulting irregular weld bead and weldi~ng
defects, e.g., undercut of bead, humping bead, lack of fusion of base
metal and spattering.
The magnetization of a pipe-blank can be substantially preve~ed
also by another embodiment of the method of an aspect of the present
invention. This embodiment comprises, also with reference to Figure 5,
.
-- 10 --
--- .- -
..
:: . -
.. ,~ : .
.. ~, .
:
. .
- ' . :
~3V8~3
connecting, contrary to the teachings of the first embodiment, the consum-
able 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 i5 longitu-
dinally seam-welded from the inside along the groove or cleft with the
consumable electrodes 5a and 5b in the direction opposite to that indica~ed
by arrow 14, while inserting the boom 2, together with the current supply
cables 9a and 9b, into the pipe-~)lank 3 from the right-hand side of the
drawing by the carriage (not shown in the drawing). In this case, it is
clear that the consumable electrode 5b is the leading electrode, and the
consumable electrode 5a, is the trailing electrode.
In any of the aforementioned embodiments of the method of aspects
of the present invention, it is possible substantially completely to pre-
vent the magnetization of a pipe-blank, and hence substantially completely
to prevent the occurrence of the magnetic arc blow of the welding arc and
the resulting irregular weld bead and welding defects, even by connecting ~-
the leading consumable electrode in the straight-polariy manner, i.e., to
be negative in polarity, and a trailing consumable electrode, in the
reverse-polarity manner, i.e., to be positive in polarity. However, by
connecting the leading consumable electrode in the reverse-polarity manner,
-:
i.e., to be positive in polarity, and the trailing consumable electrode,
in the straight-polarity manner, i.e., to be negative in polarity, better~
- results can be obtained, because of the easy availability of a deeper
fusion penetration required on the leading electrode side. -
The method of an aspect of the present invention is now described
in more detail by way of the following example.
- EXAMPLE ~.
In longitudinally seam-welding two pipe-blanks for welded steel
pipe from the inside along a groové or cleft, having a wall thickness of
;~ 30 1 inch, an outside diameter of 48.inches and a length of 12 meters by the
''.' :
. .
~~` tandem-headed G~M.A. welding process which includes using direct electric
current supplied to two leadîng and trailing consumable electrodes through
current supply cables as the welding current, the method of an aspect of
the present invention described above with reference to Figure S 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 equipped with the
leading welding torch la and the trailing welding torch lb in tandem at
the free end thereof The boom 2 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 thP 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 torch
lb, is connected in the straight-polarity manner, i.e., to be negative in
polarity via the current supply cable 9b. The pipe-blank 3 is longitu-
dinally seam-welded from the inside along the groove or cleft with the
leading and trailing consumable electrodes 5a and 5b in the welding
direction as indicated by the arrow 14 in the drawing, while withdrawing
the boom 2,together with the current supply cables 9a and 9b from the
pipe-blank by the carriage.
On the other hand, the conventional reverse-polarity G.M.A~
welding process, which is outside the scope of the present invention, was
applied to the other pipe-blank for comparison purposes, such conventional
method including the same steps as mentioned above, except for the step of
connecting both of the above-mentioned leading consumable electrode and
the trailing c~nsumable electrode reversely in the reverse-polarity manner,
i.e., to be positive in polarity.
The state of the weld beads obtained as a resuLt`is shown in
photographs of Figures 6 and 7.
In the weld bead obtained by the conventional reverse-polarity
B - 12 -
:
-: . . : , - . : .
.~. . . . ~ . -.
: ~ ' " ~ ~ . ' .,' .:
~'- : . ' . ' '
-- lV~
G.~.A. welding process, as shown in the photograph of Figure 6, not only
is the appearance poor, but the X-ray test also revealed such weldlng
defects as lack of fusion of base metal and undercut of bead. This i9
attributable to the occurrnece of boiling of the molten metal caused by
the magnetic arc blow of the welding arc towards the upstream side of the
welding direction, i.e., in the opposite direction to that of welding.
The weld bead obtained by the welding method of an aspect of
the present invention, in contrast, has the appearance as shown in the
phorograph of Figure 7, with no significant trace of welding defects
observed even by the 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 inveniton, (as will be described
in ~xample 2), which comprises, also with reference to Figure 5, connec-
ting, contrary to the foregoing, the consumable electrode 5a in the
straight-polariy manner, i.e., to be negative in polarity, and the consum-
able electrode 5b inthe reverse polarity manner, i.e., to be positive in ~'
polarity. The pipe-blank 3 is longitudinally seam-welded 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, 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 carraige (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
invention 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 the process of aspects of the present invention covers also
cases in which the pipe-blank is moved while the boom equipped with the
. ~ - 13 -
-{~--~
. - - ., ... . - . - - - .
: :-..... -. ., .. - .. , -
. - : - . . . :
.,: ..... ~ - :. :. , - .. .
- -.::: -
:.
~ welding torches is fixed. In other words, in the method of aspects of the
present invention, it suffices to move the boom equipped with the welding
torches through the pipe-blank in the welding direction~
According to the method of aspects of the present inveition, as
described above in detail, in longitudinally seam-welding a pipe-blank for
welded steel pipe Erom the inside along a groove or cleEt by the G.M.A.
welding process which includes using direct current supplied to a plurality
of consumable electrodes through at least one current supply cable as the
welding current, it is possible substantially completely to prevent the
occurrence of the magnetization of the pipe-blank caused by direct electric
. . _ . .
current flowing through at least one current supply cable introduced into
the pipe-blank and the resulting magne~ic arc blow of the welding arc.
- Therefore, it is possible to obtain a sound weld bead, thus providing
industrially useful effects.
': ;
- 14 -
. ~ . .
: ', -:
