CA1094649A - Method for manufacturing spiral-seam-welded steel pipe by gas shielded arc welding - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method for manufacturing a spiral-seam-welded steel pipe comprises forming a cylindrical pipe-blank by bringing into butt contact side edges of a moving steel strip while spirally winding the moving steel strip and continuously effecting an inside welding and an outside welding of the pipe-blank thus formed along a spiral seam line thereof by a gas shielded arc welding method while continuing to move the steel strip : the inside welding and said outside welding are effected each by means of a plurality of solid wire consumable electrodes having a diameter of from 3 to 8 mm, each of which is fed at a prescribed position, in a mixed gas comprising an inert gas and CO2 gas and/or O2 gas and fed at a rate of from 7 to 400 ? per minute per consumable electrode under the following conditions: a DC welding current of from 600 to 2,000 amperes, a welding voltage of from 20 to 36 volts and a welding speed of from 500 to 1,500 mm per minute; the inside welding being effected at a position within a range between a point deviated by a center angle of 12° of the pipe-blank from the lowest point of said pipe-blank toward the downstream of the moving direction of said steel strip, and a point deviated by a center angle of 20° of the pipe-blank from said lowest point toward the upstream of the moving direction of the steel strip; and the outside welding being effected at a position within a range between a point deviated by a center angle of 30° of the pipe-blank from the highest point of the pipe-blank toward the upstream of the moving direction of the steel strip, and the highest point.

Description

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. 1 : .. l '~...... REFERENCE TO PATENTS, APPLICATIONS AND PUBLICATIONS
. P:E:RTINENT TO THE INVENTION
.~ . So far as we kno-~, the document pertinent to the ;, . present invention is the U.S. Patent No. 4,071,732 of .. January 31, 1978 corresponding to Japanese Patent Publica-., . tion No. 9,571J78 of April 6, 1978 (Japanese Patent :~ Application No. 135,559/74 of November 27~ 1974).

.. ' The contents of the prior art disclosed in the ;' aforementioned document will be decicribed later in the 1'. . 10 "Baokground of'the Invention'l.
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FIEl-D OF THE IN~TLNTION
The present invention relates to a method fox manufacturing a spiral~seam-welded steel pipe by gas shielded arc welding.

BACK(',ROUND OF THE IMVENTION
A spiral-seam-welded steel pipe is manufactured by spirally winding a steel strip as the material while ' moving said steel strip, forminy a cylindrical pipe-blan]c by bringing into butt contact side edges of said steel strip, and effecting an inside welding and an outside welding o said pipe-blank along a spiral line a-t the side edges in butt contact of said pipe-blank ~hus formed, i.e., a spiral seam line. In the manufacture of a spiral~
seam-welded steel pipe, therefore, it is not necessary to select a width of steel strip in accordance with the pipe diameter as is necessary in the manufacture of a straight-seam-welded steel pipe. It is possible, within certain ., . .
limits, to ch~nge the pipe diameter at will and to manu-facture a welded steel pip~n~ of a large diamter as desired, irrespective of the width of the steel strip ¦ ~sed.

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More speciflcally, in the manufacture of a spiral-searll-welded steel pipe (hereinafter referred to as the "spiral pipe")/ arrangement is {nade so that a point at which side edges of the steel strip wound spirally and cylindrically are first brought into butt contact (herein-after referred to ~s the "connecting point"), i.e., a point at which a cylirldrical pipe--blank is first formed from the steel strip, may be iocated near the lowest point of the pipe blan~. ~hile always moving the steèl strip~
the inside of said pipe-blank is welded along a spiral seam line at a prescribed position near said connecting point. Then, the outside of said pipe-blank is welded along said seam line at a prescribed position near thehighest point of said pipe-blank, i.e.~ the point opposite to said connecting point relative to the center axis of said pipe-blank (hereinafter referred to as the "opposite point").
In the manufacture of a spiral pipe, as described above, , a moving steel strip is welded at a prescribed position.
Inside welding and outside welding accordingly proceed in ~0 a direction opposite to the moving direction of the steel strip. Since the seam line to be welded is in a spiral for~, the inside welding and the outside welding naturally I rec,7uire downhill and uphill welding in the both cases.
I The uphill welding as used herein is an upward w~lding, !

25and the downhill welding, a downward welding.
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. ~ . ( As is clear ~rom the description given ahove, in .~J ~ Ic t}le manufacture of a spiral pipe, both the inside w~
and the outside welding Erom the connecting point to the opposite point in the moving directiQn of the steel strip are efEected by the downhill welding, whereas bo~ the inside welding and the outside welding from the opposite point to the connecting point in the moving direction of the steel strip are carried out by the uphill welding.
When welding is conducted along an inclined surface, in general, molten metal at the center solidifying last of a molten pool flows out toward the lower portions of the surface, and a centrally concave bead tends to be formed.
This tendency is more remarkable as the inclination of the surface to be welded is larger, and as the cooling rate of the molten pool is higher. In the uphill welding, -the above-mentioned cen-tral conoave of bead is gradually j filled up, and a bead with a seriously projecting top may sometimes be formed. In the downhill welding, on the contrary, the concave at the bead center is never made up, and a centrally concave bead is formed. None of the bead with a seriously projecting top and the I centrally concave bead ~s mentioned above can be a sound bea-l. When manufacturing a spiral pipe~ therefore, the occurrence of unsound beads such as those mentioned above caused by the downhill welding and the uphill welding is , ~.. . . . . .
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; one of the important problems.

A spiral pipe has conventionally been manufactured by the submerged-arc welaing method with a Vie~J to raisins the reliabili-ty of the resultant weld and -the efficiency of wel.ding. The submersed-arc weldi.ng method is widely adopted as a method suitable for the manufacture of spiral pipes in larcJe quantities at a high speed, because of the very high welding efficiency, as compared with the other welding methocls, brought about by a large heat input in welding.

. However~ in the conventional manufacture of a spiral pipe by the s.ubmerged-arc welding, both the inside . welding and the outside weldiny of a pipe-blank are effected ; by the downhill welding. In other words, the inside weld-.~ 15 i.ng is carried out at a sligntly downstream position from . the connecting point in the moving direction of -the steel s-trip, and the outside welding is conduc-ted a-t a slightly upstream position Erom the opposite point in the moving . direç-tlon of the steel strip. In the outside weldiny, in this practice, the molten pool formed by -the downhill , welding passes the opposite point whi.le still in the molten s-tate and goes down. The molten pool, which has ~ once flow~ out touard the lower portion, flows back to the ~ original place of the pool, thus giving beads of a normal ~ ~ . .

shape. In the inside welding, in contrast, it is very difficult to prevent formation of centrally concave unsound beads, because the molten pool formed by the down-hill welding is seriously affected by the large welding heat inpu-t and the slow cooling rate under the effect of a layer of flux coveriny the molten pool, Especially when a plurality of electrodes in tandem are used in an attempt to improve the weldi.ng speed, not only the welding heat irlpUt increases further, but also, the mol~en pool ~ecomes longerO
For example; a molten pool in the submerged~arc welding has a length of about 150 mm in the case of tw.o electrodes, and about 250 mm even with three electrodes. When employing a plurality of electrodes in tandem, ~herefore, centrally concave unsound beads tend to be formed more~
For the purpose of preventing formation of centrally concave unsound beads caused by the downhill welding particularly in he inside welding, as mentioned above~ a method for manufacturing a spiral pipe by the su~merged-arc welding has been proposed. A consumable electrQde for inside welding is fed at a prescribed position;
a consumable electrode for outside welding is ~ed at a prescribed position, ~ steel strip moves relative to the ~ 7--bm:~

electrodes; a connecting poin-t, i~e., a point where side ~dges of the moving steel strip wound spirallly and cylindrically are first brought into butt con~act to first form a cylindrical pipe-blank is defined along with an opposite point, e.g., a point which is opposite to the connecting point relative to the center axis of said pipe~blank. The pipe-blank is positioned so that the connecting point may be near the lowest point of the pipe blank, i.e., near the 6 00 o'clock position, and the opposite point may be near the highest point of the pipe-blank, i.e., near the 12:00 o'clock position~
In this proposed method, the inside welding of the pipe-blank is effected by means of the inside electrode at a position which is deviated by a center angle of the ; pipe-~lank from the connecting point toward the upstream direction of the moving steel strip, so as to permit uphill welding. The outside welding is applied, as has conventionally been done, by means of the outside electrode at a position which is deviated by a center angle o~ the pipe-blank rom the opposite point toward the upstream direction of the moving steel strip, so as to permit downhill welding.
According to this method, it is possible to prevent bm 'l~
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Lt~) to some extent the formation of centrally concave beads caused by the downhill welding during the inside welding.
In -this method, however, the distance of deviation from the connecting point to the inside welding position is under the restrictions in pipe-making technology and welding equipment as described later. It is therefore difficult to provide a necessary distance of deviation for this purpose.
(1) Restriction in pipe-makiny technology:
A too long distance of deviation from the connQcting point to the inside welding position causes welding to take place before close contact between side edge~ of the steel strip, thus resulting in a seriously defective weld.
~2j Restriction in welding equipment:
In the submerged-arc welding, which requlres installation of a flux feed device in front of the electrode, the aforementioned distance of deviation from the connecting point to the inside welding position, mentioned aboveO cannot always be sufficient to satisfy the re~uired distance~ Fox example, in the case of a pipe having a diameter of l,SOQ mm, the maXi~um distance that can be provided bm:~

between the connecting point and the inside welding position is only about 30 mm, and with a distance of this order/ it is impossible to cover the length of a molten pool of about 70 mm for a single electrode, about 150 mm for two electrodes and about 250 mm for three electrodes. Occurrence of centrally concave beads as mentioned above cannot therefore be completely preventedO

It has also been proposed to limit the welding heat input to a lower level in the submerged-arc welding, with a view to obtaining a sound weld by preventing centrally concave beads from occurring during the inside welding. A smaller heat input however not only leads ko a very low weldiny efficiency, but also makes it difficult to remedy welding defects in the case where the butt contact between side edges of the steel strip becomes unstable.
In the inside welding, another submerged arc welding method has been proposed, which comprises using a plurality of electrodes, providing a long distance - between the electrodes, and forming a molten pool for each electrode~ to obtain a sound weld by preventing centrally concave beads from occurring. In this method, bm:~

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however, it ls necessary to remove flux used by a leading electrode befoxe startiny welding by a trailing electrod~, this being very difficult in practice.
As described above, a spiral pipe is manufactured by bringing into bu-tt contact two root faces of tapered side edges of a steel strip, forming a pipe-blank having a spiral seam line, and continuously welding the pipe-blank from the inside and the outside ~hereof along the seam line. However, because the steel strip often has a cham~er or oth~r shape defect, a gap which may be pxoduced between opposite root faces of the steel strip entraps flux during submerged-arc welding. If inside welding is continued in such a state, a defective weld containing thus entrapped flux is produced. Expecially when inside welding is carried out at the position deviated from the connecting point toward the upstream of the moving direction or the steel strip, -the butt contact between the side edges of the steel strip tends to he more unstable, thus producing such weldiny defects 20 as flux entrapping more often, As mentioned above, the su~merged-arc welding method bmo~
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is suitable for manuacturing spiral pip2S in large quantities at a high speed, whereas it is impossible to complete]y prevent occurrence of welding defects caused by downhill and uphill ~elding practices. With this Eact in view, in the U.S. Patent No. 4,071,732 of January 31, 1978 corresponding to Japanese Patent Publication No. 9,571~78 of April 6, 1978 (Japanese Patent Application No. 135,559/74 of November 27, 197A), there is disclosed a method of high speed and large current gas shielded arc welding of steel using a mixture of inert gas and active gas as shielding gas, which com-prises the steps of: using at least one solid wire con-sumable electrode of low alloy steel material with a diameter substantially between 3.0 mm and 6.4 mm; feeding lS said solid electrode with a welding current of from 600 to 1500 amperes; supplying a shielding gas of from 50 to 200 ~min per electrode so as to shield an arc yeneratedj and adjusting an arc voltage within a range of substantially Erom 23 to 36 volts (hereinafter referred to as the "prior art").

The welding method based on the prior ar-t is said to be well suited for the weldin~i of the straight or ; helical (spiral) seam of an open pipe or a pipe-blank for the manufacture of a very low temperature line pipe.
~hile, i~ the submeryed-arc welding, degradation of the " ~. . , ~ , .
,, impact resistance is ineYitable at portions near the weld unle~s the advantage of high effi.ciency i.s sacrificed, it is possible, according to the prior art, to conduct a high speed weldi.ng almost without degradation of the low-temperature toucJhness near the weld. Thus, the welding method of the prior art may be considered to be mOre suitable for the manufacture of a spiral. pipe than the submerged-arc welding method. However, the prior art discloses only a method of high speed welding which permits prevention of the decrease in the touyhness at low-temper-atures of the portions of a workpiece near the weld.
Application of the weldiny method of the prior art as it is to the manufacture of a spiral pipe does not therefore enable to completely prevent occurrence of defective beads as mentioned above caused by the downhill and uphill weld--ing practices, especially the former, which is inevitable in the manufacture of a spiral pipe.

SUM~ARY OF THE INVENTION
An object of the present invention is therefore to provide a method for manufacturing a spiral-seam-welded steel pipe at a high welding efficiency and a high operat-ing speed.

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The principal obj ec-t of the pxesent invention is to provide a me-thod for manufacturing a spiral-seam-welded steel pipe, which permits prevention of occurrence of centrally concave beads caused by downhill and uphill welding practices, especially the former, at a high w~lding efficiency and a high operating speed.

BRIEF DESCRIPTION OF THE DP~WINGS

Fig. l(A) is a schematic plan view illustrating an embodiment of the con~entional manufacture of a spiral-seam-welded steel plpe;

Fig. l(B) is a schematic side view illustrating an embodiment of the inside welding position and the bm:J c, lO~L~

outside welding position of a pipe-blank for a spiral-seam~welded steel pipe in the conventional submerged-arc-welding;

Fig. 2(A) is a sectional view illustrating a normal s-tate of butt contact between side edges of a cylindrically wound steel strip;

FigO 2(B) is a sectional view illustrating an a~normal state of butt contact betwe~n side edges of a cylindrically wound steel strip;

Fig. 3 is a schematic side view illustrating ranges o the inside welding position and the outside bm~

welding position in the case where each one consumable electrode is used in each of said positions in the method of the present invention;

Fig. 4tA) is a schematic side view illustrating ranges of the lnside welding position and the outside welding position in the case where each two consumable electrodes in tandem are used in each of said positions in the method of the present invention;

Fig. 4(B) is a schematic side view illustrating another range of the outside welding position in the case where two consumable electrodes in tandem are used in the method of the present invention;

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Fig. 5(A) is a schematic plan view illustrating ranges of -the inside we1ding position and the outside welding position in the case where one consumable electrode is used in each of said positions in the method of the present invention;

Fig. 5(B) is a schematic plan view illustrating ranges o the inside welding position and the outside welding position in the case where each two consumable electrodes in tandem are used in each of said positions in the method of the present invention;

Fig 5tC) is a schematic plan view illustrating another range of the outside welding position in the case bm~
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where two consumable electrodes in tandem are used in the method of the present inven-tion;

Fig. 6(A) is a sectional vlew illustrating an example of a groove cut at butt contac-t faces of side edges of a steel s-trip in the conventional submerged-arc welding method;

Fig. 6(B) is a sectional view illustrating an example of a groove cut at butt contact faces of side edges of a steel strip in the conventional gas shielded arc welding method;

Fig. 7(A) is a sectional view of a weld obtained by effecting the inside welding and the outside welding o~ a pipe~blank along the groove shown in Fig. 6(A), each by the conventional submerged-arc welding method by means bm~

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of two consumable elec-troclcs in tandem; and Fig. 7(B) is a sectional view of a weld obtained by effecting the inside welding and the outside welding of a pipe-blanX along the groove shown in Fig. 6(B) by S the method of the present inverltion, each by means of two consumable electrodes in tandem.

DETAIIED DESCRIPTION OF PREFERRED E~ODI~ENTS
_ _ _ We have carried out in-tensive studies to solve the various problems as mentionecl above in the manufacture of a spiral-seam-welded steel pipe (herein referred to as "spiral pipe" as mentioned previously), while giving due regards to the following points:

(a) The available methods of welding capable of giving a sound weld without causing occurrence of centrally lS concave beads include gas shielded arc welding methods such as the MIG-arc welding method (the abbrevia-tion of "metal inert gas arc welding me-thod") and the C2 arc welding method(the abbreviation of "carbon dioxide gas arc welding method").

(b) In the submerged-arc welding method, which gives a 6~) la~ge welding heat inp~l-t, it is possi~le to carry out welding at a hiqh speed, whereas part of the heat energy applied to the portion to be welded is cor-sumed for melting flux, thus requiring additional S heat energy. In the submerged-arc welding method, furthermore, the large heat input and the weld covered by flux result in a lower cooling rate OL the mo]-ten pool. In the gas shielded arc welding me-thod, in contrast, not only no flux is used, but also the molten pool is always cooled by cold shielding gas.
It is thereore possible to conduct a high speed welding with a smal]er welding heat input than in the submerged-arc welding method. In addition, the high cooling ra-te of the molten pool minimi~es occur-rence of centr~lly concave beads caused hy the out-flawing of molten pool, even when effec-ting a down-hill welding as mentioned previously.

(c) The conventional gas shielded arc welding method comprises using a small-diameter solid wire with a diameter of up to 2.4 mm and welding a workpiece with a welding current of up to 500 amperes in an atmosphere of an inert gas such as argon or helium or in an atmosphere mainly comprising carbon dioxide gas. Since it is possible to reduce the welding ~ ` .

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- heat input to a low level ~mder such welding conditions, a bead without a cen-ter concave can be obtained, and it is possible to avoid occurrence of a defrctive weld caused by en-trappr~d flu:~, brcause no f]ux is employed.
Under the above-mentioned welding conditions, however, the welding speed cannot exceed 500 mm per minute, which is only two to three times the speed in the manual welding. Therefore, in the manufacture of a spiral pipe of which the productivity depends almost exclusively on the welding efficiency, the conven-tional gas shielded arc welding method cannot be a suitable welding method because of the too low productivity.

As a result of -these considerations, we have obtained the following knowledge: ¦

~1) In the gas shielded arc welding method, the welding efficlency may be improved by increasing the welcling current.

(2) However, a simply increased welding current alone gives rise to a strong pinch force and an intensive plasma jet, because of the use of DC current as the welding current. An arc generated is thus considèra-bly squeezed to become a so-called hard arc, which, .
while giving a deep fusion penetra:ion, leads to a .; - i' .~. ' ' ' '~ '' . ' ' .

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narrower bead width because the smaller divercJence o arc, with more frequent spatters, ancl results in the for~ation of hurDping beads with serious undula-tions.

(3) In order to avoid production of a s-tl-ong pinch force and an intensive plasma jet, therefore, it is neces-- sary to reduce the welding current density, in addition to the increase of the welding current.

(4) To reduce the welcling current density, it suffices to use a consumable electrode with a larger diameter.
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(5) It is certain that a good bead without a center concave can easily be obtained by the gas shielded arc welding method. However, for the comple-te elimination of a center concave of a bead, it is very important to select appropriate positions for the inside welding and the outside welding of a pipe-blank.

The presen-t invention was made on the basis of these items of knowledge (1) to (5) aboveO More speci-fically, the present invention covers a method for manu- i facturing a spiral pipe, which comprises, forming a cylindrical pipe-blank by bringing into butt contact side edges of a rnoving steel strip whi].e spi.rally winding said moving steel strip and continuously effec-ting an inside welding and an outside welding of said pi.pe-blank thus formed along a spiral seam line thereof by a gas shielded arc welding rnethod while continuing to move said steel strip, each by means of at least one consumable electrode fed at a prescribed position; said gas shielded axc welding method com~rising using large-diameter solid wire as said consumable electrode, supplying a large DC welding current to said consumable electrode, and using as the shielding gas a gaseous mixture rnainly comprising an inert gas; said method being characterized in that said inside welding and said outside we].ding are effected under the following conditions:
~, (l) consumable electrode: a solid wire having a diameter of from 3 to 8 mm;

(2) welding current : from 600 to 2,000 amperes;

(3) welding voltage : from 20 to 36 volts;

~4) composition of shielding gas: a gaseous mixture con-sistiny essenti-ally of an inert gas and at I.east vne of carbon dioxide gas of from 5 to 50 vol.% and oxygen gas of from l to lO vol.%;

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(5) feed amount of said shielcli.ng gas. rom 70 to ~00 litres per minutes per consulnable elec-trode;

(6) ranye of weldiny positions:

(a) Inside welding position: within a range between a point deviated by a center angle of 12 of said pipe-blank to~ard the downstream of the - rnoving direction of said steel strip from a connecting point set so as to be located near the lowest point of said pipe-blank, i.e., a point at which sicle edges of said steel strip wound spirally and cylindrically are first brought into butt contact, and a point deviatecl by a center angle of 20 of said pipe-blank '~ toward the ups-tream of the moving direction of said steel strip from said connecting point;

(b) Outside welding position: within a range between a point deviated by a center angle of 30 of said pipe-blank toward the upstream of the moving direction of sai.d steel strip from an opposite point set so as -to be located near the highest point of said pipe-blank, i.e., a point which is opposite to said connecting point relative to the center axis of said pi.pe-' . , .

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,, blank~ and said opposite point; and

(7) welding speed: rom 500 to l,500 mm per minute.

Now, the reasons of limitincJ the weldincJf conditions of a pipe-blank to those mentioned above in the method of the present invention are desc~ibed belo~

. Consumable electrode.
If a cored wire is used as the consumable electrode in the manufacture of a spiral pipe, contents of the cored wire may come off because of the long feed distance of the 1'3 cored wire, and this tends to cause spatters. Furthermore, the quantity of iron becomes short unless the feed rate is raised. It is therefore desirable -to use a solid wire as the consumable elec-trode. Since the use of a strip-shaped solid wire leads to a wider arc generated, a round solid wire is preferable.

In the conventional gas shielded arc welding method, as mentioned pre-~iously, a small-diameter solid wire con-sumable ele~ctrode with a fdiameter of up to 2.4 mm is employed. Supply of a large DC welding current to such a small-diameter consumable electrode raises the current density, which in turn causes a strong pinch force and an ' ~ - 25 -T . f intensiye plasma iet to occur. As a result, a deep f~sion penetration is obtained, whereas the smaller divergence of ~ arc leads ~n a smaller width of beads, and such defective ' AS
beads as humpiny beads tend to occur easily. ~ described later, therefore, in the present invention using a large welding current, it is desirable to reduce the welding current density by employing a large-diameter electrode.

However, with a diameter of ihe consumab]e electrode of over 8 mm, an arc generated becomes longer and unstable depending upon the extent of the welding current as des-cribed later, and this may cause such adverse effects as:
the fusion penetration becomes insu~ficient; the transfer mode of molten droplets from the consumable electrode does not become a spray transfer, but tends to be a globular lS transfer; a lack of fusion takes place; or cracks are produced in the weld. In addition, when the consumable electrode has a too large diameter, it is difficul-t to ~end the electrode and hence to achieve smooth feeding and other handling of the electrode. The diameter of the con-sumable electrode should therefore be up to 8 mm.

With a diameter of the consumable electrode of under 3 mm, on the o-ther hand, the pinch force and the plasma jet become too strong depending upon the extent of the welding current as described later, thus resulting in .
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defective beads. The diaJneter of the consLImable elect-rode should therefore be at least 3 mm.

2. Welding curren-t:
In the present invention, a large DC welding curren-t is used for raising the welding speed of the yas .
shielded arc welding. However, depending upon the extent .
of the diameter of the consumable electrode as rnentioned above, a too large wel.ding current beyond the appropriate range leads to a higher welding current densi-ty as in the case with a too small-diameter consurnable electrode, thus causing production of a strong pinch force and an inten-sive plasma jet. Consequently, a satisfactory bead cannot be obtained because of the splashing of a molten pool and the production of spa-tters.
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When the welding current is too small off the appropriate range in relation to the consumable el~ctrode diameter, on the other hand, as in the case with a too t large-diameter consumable electrode, an arc generated becomes longer and unstable with such adverse effects as an insufficient fusion pene-tration~ the transfer mode of : molten droplets from the consumable electrode tending to be a ylobular transfer not desirable, production of a lac]c of fusion, and occurrence of spatters~

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According to the results of an experiment carried by us, the reliltion between the consumable electrode diameter and the appropriate ranye of welding current is as shown in the following table:

Table Diame-ter of consumable Appropriate range of electrode welding current (r~) (ampere) _ ~ ___ 3.2 600 - 900 _ ,_ 4.0 650 - 1000 . ____ 4.8 700 - 1200 _ ~.~ 750 - 1500 _ _ .

8 850 - ~000 _ _ _, _ ~, A5 is evident from the table given c~bove, in order to obtain normally shaped beads free from welding defects at a high welding speed while using a consumable electrode with a diameter of from 3 ~o 8 mm, the welding current should be ~7ithin a range of from 600 to 2,000 amperes.

When using t~o consumable electrodes in tandem, it is necessary, for a leading electrode (first electrode), to ensure a sufficient fusion penetration and -to avoid production of an intensive plasma jet. Therefore, it is ,' ' ' ' ' '~'; '' '' ' ` ' .

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desirable that the leading electrode should have a slightly larger diameter and the welding current supplied to the leading electrode should be slightly ].arger. For a trailing electrode (second electrode), on the other hand, it is necessary to place the emphasis on the stabili-zation of the bead shape, achievemlent of a sound weld and smooth feeding of a consumable electrode It is therefore desirable that the trailing electrode should have a slightly smaller diarneter and the welding current supplied to the trailing electrode should be slightly small.er.

With these facts in view, it is recornmendable, when using two consumable electrodes in tandem, to set the diameter and the welding current, respectively for the leading electrode and the trailing elec-trode, within the following ranges:

(1) Leading electrode:

(a) Diameter . from 4 to 8 rnm, (b) Welding current : from 650 to 2,000 amperes ~2) Trailing electrode:

(a) Diameter : from 3 to 6.~ mm (b) ~elding current : from 600 to 1,500 amperes.

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~ hen emplo~in~ at least three consumable elect-rodes in -tandem, the diameter and the weldiny current for the third and tne subse~uent electrodes may be -~he sar~e as those given above for the trailing electrode.

3. Welding voltage:

The welding ~r-~}~t has an important effect on the length of a generated arc and the shape of beads.
For example, a low welding voltage leads to acute bead tops, whereas a high weldirlg voltage results in gently-sloping bead tops.

More specifically, with a welding voltage of ~nder 20 volts, a generated arc becomes too short in length and more sensitive to the feed speed o~ -the con-sumable electrode. This tends to cause arc interruptions and re~ults in acute bead tops as mentioned above. The welding voltage should therefore be at least 20 volts.

With a welding voltage of over 36 volts, on arc the other hand, a generated~becomes ~oo long and more susceptible of the effect of magnetism, which in turn makes the arc unstable and tends to give a too wide divergence of the arc. The welding voltage should therefore be up to 36 volts.

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~ 4. CompositiQn of shielding gas;
When effecting a gas shielded arc welding by the use of a large-diameter consurnable electrode and a large DC welding current, as in the present invention, exclusive use of an inert gas such as argon and helium as the shielding gas leads to a longer length of a generated arc under the effect of thermal expansion and hence to an unstable arc, thus often causing welding defects such as a lack of fusion. When employing a gaseous mixture obtained by adding and mixing carbon dioxide gas (hereinàfter referred to as "C02 gas") and/or oxygen gas (hereinafter referred to as "2 gas") in an appropriate amount to an inert gas, the arc becomes more stable, being enclosed by the shielding gas.

However, mixing of under 5 vol.% C02 gas and/or under 1 vol.% 2 gas with the inert gas cannot give a desired effect as mentioned above. It is therefore neces-sary to add and mix at least 5 vol.% C02 gas and/or at least l vol.% 2 gas to the inert gas.

When mixing over 50 vol.% C02 gas and/or over 10 vol.% 2 gas with an inert gas, occurrence of spatters becomes more serious and causes choking up of the shielding gas nozzle. The state of shield provided by the shielding gas thus worsens, causing defective e~ternal view of beads and weldiny de~ects. There~ore, the amount to be mixed with an inert gas should be up to 50 vol.~ for CO2 gas and/or up to 10 vol.% for 2 gas.

When usins two cons~nable electrodes in tandem the leading electrode (the first electrode) tends to cause such welding defects as a lack of fusion because of the too high welding current density. It is therefore desir~ble, for the leading electrode, to slightly increase the amount of CO2 gas and/or 2 gas to be mixed with the inert gas. For the trailing electrode, on the other hand, it is desirable to slightly decrease the amount of CO2 gas and/or 2 gas to be mixed with the inert gas, because of the necessity to prevent worsening of the external appearance of beads and occurrence of oxidation products.

For these reasons, when employing -two consumable electrodes in tandem, the amount of CO2 gas and/or 2 gas to be mixed with the inert gas, respectively for the leading electrode and the trailing electrode, should preferably be wi-thin the following ranges:

Il) For the leading electrode, from 15 to 50 vol.%
, .
C2 gas and~or from 3 to 10 vol.% 2 gas.

(2) For the trailing electrode, from 5 to 30, more - . .

~'3~

p~efexably~ 5 to 15 vol.~ C02 gas and~or from 1 to 5 vol.% 2 gas.

When employing at least three consumab]e elect-rodes in tandem, the amount of C02 gas and/or 2 gas to he mixed with the inert gas may be the same as in thc trailiny electrode mentioned above.

If a groove is cut along a seam line to be welded, the welding current density is higher than in the case where no such yroove is cut. In the case where a groove is cut, the amount of C02 yas and/or 2 yas to be mixed with the inert gas may be the same as in the case of the leading electrode as mentioned above, and in the case where no groove is cut, the amount of C02 gas and/or 2 yas to be mixed with the inert gas may be the same as in the case of the trailing electrode as mentioned above.

5. Feed amount of shielding gas:
A stable arc is generated when welding is e-ffected by the use of a large-diameter consumable electrode and a larye welding current, even if the welding voltage varies to some extent, by shielding the generated arc with a shieldiny gas having a composition as described above.

.
r `

~;3~

Ho~eve~, with a feed amo-mt of the shielding gas with the above~mentioned composition of under 70 litres per minute per consumable electrode, a plasma jet brea~s the state of shielding and air is entrained, -thus causing serious spattering. The feecl amount of said shielding gas should therefore be at least 70 li-tres per minu-te per consumable electrode. When using at least two consumable electrodes in tandem, it is recommended, for the second and the subsequent electrodes, to supply said shielding gas in an amount of at least 100 litres per minute, more preferably, at least 120 litres per minute, with a view to ensuring a satisfactory bead shape.

When said feed amount of shielding gas is over 400 litres per minute per consumable electrode, on the other hand, the molten pool is pushed by said shielding 4~. 1 gas, and this may not only cause centrally concave beads, but also is uneconomical. The feed amount of shielding gas should therefore be up to 400 litres per min~lte per consumable electrode.
, .
6. Ranges of inside we]ding position and outside welding position:
Satisfactory beads without central concave can be obtained to some extent with the use of the aforementioned - 3~ ~

. .
.

consumable electrode and shielding gas ar-d with ~he afore-mentioned DC welding current and ~7elding voltage. In order to ensure achievement of sound beads free from center concave without welding defects, however, it is further necessary to carry out an inside welding and an outside welding of a pipe-blank within the following ranges of welding posi-tions.

(1) In the case where an inside welding and an outside welding are respective]y carried out by means of a single consumable electrode, it is necessary to form satisfactory beads wi-thout center concave while prevent-ing outflow of the molten pool by a single welding.

In the present invention, the inside welding with a single cons~nable electrode is therefore effected at a position within a range as shown in Figs. 3 and 5~A~. In Figs. 3 and 5(A), 2 is a steel strip moving in-the arrow direction, 4 is a consumable electrode for inside welding supplied at a prescribed position, "a" is a connecting point, i.e., a point at which side edges of the spirally and cylindrically wound steel strip 2 are first brou~ht into butt contact to form a cylindrical pipe-blank, and "0" is the center axis of -the pipe-blank.
The connecting point "a", is set so as to be near the :

..

lowest point of the pipe~blank, i.e., near the point of 6:00 o'clock position. ~s shown in rigs. 3 and 5(~), the inside welding is carried out with the use of the consumable electrode 4, within a ran~Je be-tween d point "c" deviated by a center angle of 12~ (31) of the pipe~
blank toward the upstream of the moving direction of the steel strip 2 Irom the connecting point "a", an~ said connect-ing point "a", so as to permit uphil~ welding. In other words, the inside welding with a single consumable electrode is efEected within a range oi an arc "ac" of the pipe-blank, corresponding to a center angle l (12).

The outside welding with a single consum~ble electrode is then carried out at a position within a range as shown in Figs. 3 and 5(A). In Figs. 3 and 5(A), 5 is a consumable electrode for outside welding supplied at a prescribed position, and "b" is an opposite point, i.e., a point which is opposite to the connecting point "a" relative to the center axis "0" of the pipe-blank.
The opposite point "b" is set so as to be near the high-est point of the pipe-blank, i.e., near the point of 12:00 o'clock position. When the portion inside-welded as mentioned above reaches near the opposite point "b", the outside welding of said portion is carried out.
More specifically, the outside welding is effected with he use o the consumable electrode 5 at a position - 3~ -i4~ .

within a range between a poin-t "d" deYiat.ed by a center ansle of 12~ (all~ of -the pipe~blank toward the upstream of the moving direction of the steel strip 2 from the opposi-te point "b", and said opposi-te point "b", so as to perrnit downhill welding. In other words, the outside welding with a single consumable electrode is conducted within a range of an arc "bd" of the pi.pe-blank, corres- .
pondiny to a center anyle Gl~ (12).

(2) In the case where an inside welding and an outside welding are respectively carried out by means of two consumable electrodes in tandem, it is not neces-sary to pay so much attention to the bead shape in the inside weldiny and the outside welding with a leading electrode (first electrode) whereas it is necessary, in . 15 this case, to place the importance on the achievement of a required depth of fusion penetration and the quality of the resultant weld. In the inside welding and the :~ outside welding by means of a trailing electrode ~second electrode), on the other hand, it is not necessary to pay so much attention.to the fusion penetration, while due regards should be given to the achievement of satis-factory beads free from a center concave.

: Furthermore, in the inside welding and the outside welding each with two consumable electrodes in tandem, ~, ': ... . :

the dist~lce bet~een the consumable electro~les, if at least 100 mm, causes formation of molten poo]s in the same number as that of the consumable electrodes. Since the molten pools thus formed are iar smaller in si~e than a single molte-rl pool formed by multiple consumable electrodes, it is possible to further effectively pre-vent occurrence of a central concave of the bead otherwise caused by outflow of the molten pool. Wnlen employing two consumable electrodes in tandem, therefore, it is desirable to separate the two consumable electrodes, i.e., the leading elec-trode and the trailing electrode by at least 100 mrn. If the two consumable electrodes are separated by a distance equal to the distance of forward movement of -the pipe-blank when the pipe~blank has made one turn~ it is possible to hold the leading electrode and the trailing electrode by means of a single boom because there is no necessity of sprinkling the flu~ and removing the resulting slag.

In the present invention, the inside welding with two consumable electrodes in tandem is therefore carried out at a position within a range as shown in Figs.
4(A) and 5(B). In Figs. 4(A) and 5(B), 2 is a steel strip moving in the arrow direction, 6 is a leading consumable electrode for inside welding sup?lied at a prescribed position, 7 is a trailing consumable electrode for inside . . .

.

welding supplied at a prescribed posi.tion, "a" is -the connecting poi.nt as in Figs. 3 and 5(~), and "0;' is ,he center axis of the pipe-blank as in Fig. 3. The first inside welding with the leading electrode is conducted at a position near the connecting point "a", and the second inside welding with the trailing electrode is effected at a position near the connecti.ng point "al"
having advanced from the oriyinal position thereof in the forward movement direction of the pipe-blank by a lO distance equal`to t'ne distance of forward movement oE
the pipe-blank when the pipe-blank has made one turn.

More specifically, as shown in Figs. 4(A) and - 5~B), the first inside welding by means of the leading elec-trode is effected at a position within a range between a point "e" deviated by a center angle of 12 (42) of the pipe-blank toward the downsteam of the moving direction of the steel strip 2 from the connecting point "a"~ and said connecting point "a", with the use of the leading consumable electrode for inside welding 6, so as to permit downhi.ll welding. The second inside welding with the trailing electrode is carried o~lt, on the other hand, at a position within a range between a point "f" de~iated by a center angle of 20 (93) of the pipe-blank toward the u?s-tream of the mo~ing direction of the steel st}ip 2 from said advanced connecting point , "al"~ and said ad~anced connecting point "al", hy means of the trailing consumable electrode for inside welding 7, so as to permit uphill welcling. In other words, the first inside welding with the leading electrode is effected within a range of an arc "ae" of the pipe-b]ank, corresponding to a center angle 02 (12), whereas the second inside welding with the trailing electrode is effected within a range of an arc "alf" of the pipe-blank, corresponding to a center angle ~3 (20).

The outside welding with two consumable electrodes in tandem is then carried out at a position within a range as shown in Figs. 4~A) and 5(B). In Figs. 4(A) and 5(B), 8 is a leading consumable electrode for outside welding supplied at a prescribed position, 9 is a trailing consumable electrode for outside welding supplied at a prescribed position, and "b" is the opposite point as in Figs. 3 and 5(A). When the portion inside-welded as mentioned above reaches near the opposite point "b", the first outside welding of said portion is carried out by means of the leading electxode, and when said portion reaches near the opposite point "b1" having advanced from the original position thereof in the forward movement direction of the pipe-blank by a distance equal to the distance of forward movement of the pipe-blank during one turn thereof, the second outside welding of said _ 40 -. . .. . .

6~

portion is effected by rneans of the trailing elec~rode.

More specifically, as shown in Figs. 4~A) and 5(B), the first outside welding by the leading electrode is effected at a position within a range between a point "g" deviated by a center angle o~ 12 (~2'~ of the pipe-blank toward the upstream of the moving direction o~ the steel strip 2 from the opposite point "b'`, and said opposite point "b", with the use of the leading consuma-ble electrode for outside welding, so as to per~it down-hill welding The second outside welding by the trailing electrode is, on the other hand, conducL~ed at a position within a range between a point "h" deviated by a center angle of 20 (33') of the pipe-blank toward the upstream of thè moving direction of the steel s-trip 2 from said advanced opposite point "bl", and said advanced opposite point "bl", with the use of the trailing consumable electrode for outside welding 9, so as to permit also downhill welding. In other words, the first outside welding by the leading electrode is conducted within a range of an arc "bg" of the pipe-blank, corresponding to a center angle 42' (12~), and the second outside welding by the trailing electrode is carried out within a range of an arc "blh" oE the pipe-blank, corresponding to a center angle ~3~ ~20).

.
.: .
_ 41 -. . . ...
.

64~

When a desired weldiny position can be provided, in the outside welding with two consumable electrodes in tandem, by separating the leading electrode Erom the trailing electrode, not by a distance equal to the dis-tance of fo~ward movement of the pipe-blank during one turn thereof, but by at least 100 mm, -the outsi~e welding may be effected at a position within a range shown in Fiys. 4(B) and 5(C). In Figs. 4(B) and 5(C), as in Figs.
4(A) and 5(s), 2 is the steel strip moving in the arrow direction, "0" is the center axis of the pipe-blank, "a"
is the connecting point, "b" is the opposite point, 8 is the leading consumable electrode for outside welding supplied at a prescribed positio]l, and 9 is the -trailing consumable electrode for outside welding supplied at a prescribed position. As shown in Figs. 4(B) and 5(C), .*
the first outside welding by the leading electrode is effected at a position within a range between a point "i"
deviated by a center angle of 15 of the pipe-blank toward the upstream of the moving direction of the steel strip 2 Erom the opposite point "b", and a point "j"
deviated by a center angle of 30 of the pipe-blank toward ~he upstream of the moving direction of the steel strip 2 from the opposite point "b", with the use of the leading consumable electrode for outside welding 8, so as to permit downhill welding. The second ou-tside welding by the trailjny electrode is, on the other hand, carried out at a position within a range between a point "k"
deviated by a center angle of 12 of the pipe~blank toward the upstream of the moving direction of the steel strip 2 frorn the opposite point "b`', and said opposite point "b", with the use of the trailing consumable electrode for outside welding 9, so as to permit also ~) e 1 ~l, n ~
,~ downhill w~i~i~g. In other words, the firs-t outside we].ding hy the leading electrode is effected wi.tnin a range of an arc "ij" of the pipe-blank, corresponding to a center angle ~2" (15), and -the second outside weldi.ng by the trailiny elec-trode is effected within a range of an arc "bk", corresponding to a center anyle (12).

The above description has covered the case wi~h two consumable electrodes in tandem, with reference to the detailed explanation of the ranges of welding posi~
tions and the distance between consurrable electrodes in the p~esent invention, and this is also -the case with at least three cons~able electrodes in tandem.

7. ~elding speed:
In the present invention, with a welding speed of under 500 mm per minute, the excessive welding heat - ~3 -' - input c~uscs boili.ng of the molten pool eYen with appropriate diameter of the consumable electrode, weld-ing current, welding voltage and feed amount of shielding gas, and hence a sound weld cannot be obtained. There-fore, the welding speed should be at least 500 mm per minute~ With a welding speed of over 1,500 mm per minute, on the other hand, the effect supplying shiel.ding gas in a large quantity is l.ost, resulting in an unstable generated arc, and a satisfactory bead cannot be obtained.
The welding speed should therefore by up to 1,50G mm per minute.

8. After-shielding gas:
For the purpose of preventing o~idation of the weld as well as obtaining satisfactory beads free from center concave, in the present inventionr an after-shield-ing gas is employed as required. More specifically, an - af-ter-shielding device is attached on the upstream side in the welding direction of a consumable electrode in the case with a single consumable electrode and of the last trailing consumable electrode in the case with at least two consumable electrodes in tandem. An after~
shielding gas consisting essentially of argon gas is directed from sald af-ter-shlelding device onto a molten ~ 44 -.

, ~

.
. . : . .

pool to shield said molten pool until the completion of solidification thereof~ , 1, ~lowever, with a feed amount of the after-shielding gas of u~der 80 litres per mir.ute, a desired effect cannot be obtained. The feed amount of after-shielding gas should therefore be at least 80 litres per minute, and for ob-tain-ing satsifactorily shaped beads, preferably at least 120 litres per minute. Even when an after-shielding gas is supplied at a rate of over 400 litres per minute, on the other hand, no further improvement of the effect is observed, I
and the excess is only uneconomical. Therefore, the feed amount of after-shielding gas should preferably be up to 400 liters per minute.

Now, the method of the present invention is des-'~ 15 cribed in more detail by means of an example in comparison with a reference case.

F.X~MPLE
A pipe-blank of a spiral pipe with a diameter of ],200 mm0 was prepared from a steel strip having a thick-ness of 22 mm. The groove shape of the pipe-blank was -the same as that popularly adopted in the conventional gas shielded arc welding, as shown in Fig. 6(B). Then, an I
- ~ ,~ , . ..
, inside weldincJ and an outside welding of the pipe-blank thus obtained w~re carried out along the groove under the following weldiny conditions by the gas shi.elded arc weldiny of the present i.nvention:

(1) Type of consumabJ.e electrode: two consumable elec-trodes, one being leading electrode and the other being trailing electrode;

(2) Diameter of consumable electrode:
(a) Leading electrode . 4.8 ~n 0, (b) Trailiny electrode : 4.0 mm ~;

(3) Distance betwecn consumable electrodes: a distance ~, equal to the distance of Eorward movement of the pipe-blank when the pipe-blank has made one turn, (4) DC welding current:

(a) Leadiny electrode : 800 amperes, (b) Trailing electrode : 700 amperes;

(5) Welding voltage:

(a) Lec~ding electrode : 30 vol-ts, . . , j , .

(b) Trailing electrode : same as in the leading electrode;
(6) Welding speed: 750 mm per minute;
(7) Welding position:
(a) Inside welding position (expressed by a center angle of the pipe-blank relatlve to the connecting point) ~i) Leading electrode : 0 (ii) Trailing electrode: 2 toward the upstream of the moving direction of the steel strip (uphill welding);
(b) Outside welding position (expressed by a center angle of the pipe-blank relative to the opposite point):
(i) Leading electrode : 6.7 toward the upstream of the moving direction of the steel strip (downhill welding), (ii) Trailing electrode: same as in the leadin~
electrode, i.e., 6.7 toward the upstream of the moving direction of the steel ~trip from and opposite point having advanced from the original position thereof in the forward movement direction of said pipe-blank by a distance equal to the distance of forward movement of said pipe-blank when said pipe-~lank has made one turn ~downhill welding) as specified in paragrap~ (3) of this Example.
(8) Composition and feed amount o~ the shielding gas:
(a) Leading electrode O argon gas mixed with 30 ~ol.%
~02 gas at a rate of 70 litre~ per minute~

bm: ~

. .

!

(b~ Trailin~ electrode ; argon gas mixed with lO vol.~ CO2 yas at a rate of 120 litres per minute;

(9) Ater-shieldin~ gas: argon gas at a rate of 120 litres per minute.

As a resul-t of the aforementioned inside welding and outside welding, a weld as shown in the sectional view of Fig. 7(B) was obtained. As shown in Fig. 7 (B), 'i the weld obtained by the method of the present invention had a central concave of 0 mm, and the defect rate ex- I -pressed by the number of defects per welding length showed a value of 0.005 per meter.

,~ Then, for the purpose of comparison, another pipe-blank for a spiral pipe wi-th the same diameter was lS prepared from the same steel strip as in the above-mentioned example. The groove shape of the pipe-blank was -the same as that commonly adopted in the conventional submerged-arc welding, as shown in Fig. 6(A). Then, an inside weldiny and an outside welding of the pipe-blank thus obtained were carried out along the groove under the following conditions by -the conventional submerged-arc welding method: ¦
1, - 48 - i , ( '' ~1) Type of consumable electrode; two consu~able electrodes i.n tandem, one heiny leading electrode! and the other beiny trailing electrode;

(2) Diameter of consl~able electrode:
(a) Leading electrode : 4.8 mm ~, (b) Trailiny electrode : 4.0 mm 0;

(3) Distance between consumable electrodes : 10 mm;

(4) DC welding current::
(a) Leading electrode : 1,050 amperes, (b) Trailing electrode : 650 amperes;

Y~ (5) Weldiny voltage:
(a) Leadi.ng electrode : 28 volts, (b) Trailing electrode : 35 volts;

(6) Welding speed: 800 mm per minute;

(7) Welding position:
- (a) Inside welding position texpressed by a centex ~ngle of the pipe-blank relative to the connect-ing point):

",~
,.

(i) Leading electrode; 3 toward the upstream. of the moving direction of the steel strip (uphill welding), (ii) Trailing electrode: 1 toward the upstream of the moving direction of the steel strip (uphill welding);
(b) Outside welding position (expressed by a center angle relative to the opposite point):
(i) Leading electrode: 10 toward the upstream of the moving direction of the stee], strip (downhill welding), (ii) Trailing electrode: 8.5 toward the upstream of the moving direction of the steel strip (downhill welding);
~.
(8) Flux: melting-type flux As a result of the aforementioned inside welding and outside welding, a weld as shown in the sectional view of Fig. 7(A) was obtained. As shown in Fig. 7(A), the weld obtained by the conven-tional submerged-arc welding method had a central concave of 0.5 mm, and the ' defect rate showed a value of 0.01 per meter.

As is clear from the comparison mentioned above, the weld obtained by the method of the present invention ~f~

presents no central concave and contains very few welding defects as compared with thc weld obt:ained by the con-ventional submeryed-arc welding method.

According -to -the method of the present invention, as described above in detail, i-t is possible to prevent occurrence of beads with a defective shape caused by the downhill welding and the uphill welding, especiall.y the - former, which are inevitable in the manufacture of a spiral-seam-welded s-teel pipe, and to manufacture a spiral-seam-welded steel pipe free from welcling defects such as a central concave of bead and a lac~ of fusion at a high weld-ing efficiency and a high operating speed, thus providing industrially useful e-ffects.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method for manufacturing a spiral-seam-welded steel pipe, which comprises forming a cylindrical pipe-blank by bringing into butt contact side edges of a moving steel strip while spirally winding said moving steel strip and continuously effecting an inside welding and an outside welding of said pipe-blank thus formed along a spiral seam line thereof by a gas shielded arc welding method while continuing to move said steel strip; said gas shielded arc welding method comprising using a plurality of consumable electrodes in tandem, made of a large-diameter solid wire, fed at a prescribed position, said plurality of consumable electrodes in tandem comprising a leading consumable elect-rode and at least a trailing consumable electrode, supplying a large DC welding current to said plurality of consumable electrodes, and using as the shielding gas a gaseous mixture mainly comprising an inert gas to shield an arc generated and a molten pool;

the improvement characterized in that both said inside welding and said outside welding are carried out under the following conditions;

(1) each of said consumable electrodes being a solid wire having a diameter of from 3 to 8 mm;
(2) the welding current being from 600 to 2,000 amperes;
(3) the welding voltage being from 20 to 36 volts;
(4) the composition of said shielding gas being a gaseous mixture consisting essentially of an inert gas and at least one of carbon dioxide gas of from 5 to 50 vol.%
and oxygen gas of from 1 to 10 vol.%;
(5) the feed rate of said shielding gas being from 70 to 400 litres per minute per consumable electrode;

(6) welding positions:
(a) inside welding position:
(i) the welding position of said leading consumable electrode being between a point deviated by a center angle of 12° of said pipe-blank toward the downstream of the moving direction of said steel strip from a connecting point set so as to be located near the lowest point of said pipe-blank, i.e., a point at which side edges of said steel strip wound spirally and cylindrically are first brought into butt con-tact, and said connecting point;

(ii) the welding position of said trailing consum-able electrode being between a point deviated by a center angle of 20° of said pipe-blank toward the upstream of the moving direction of said steel strip from said connecting point, and said connect-ing point; and (iii) the distance between said leading consumable electrode and said trailing consumable electrode being at least 100 mm;

(b) outside welding position:

(i) the welding position of said leading consumable electrode being between a point deviated by a center angle of 30° of said pipe-blank toward the upstream of the moving direction of said steel strip from an opposite point set so as to be located near the highest point of said pipe-blank, i.e., a point which is opposite to said connecting point relative to the center axis of said pipe-blank, and said opposite point; and (ii) the welding position of said trailing con-sumable electrode being between a point deviated by a center angle of 20° of said pipe-blank toward the upstream of the moving direction of said steel strip from said opposite point, and said opposite point;
and (7) the welding speed being from 500 to 1,500 mm per minute.

2. The method as claimed in claim 1, wherein said inside welding and said outside welding are effected each with the use of two consumable electrodes in tandem com-prising a leading electrode and a trailing electrode, under the following conditions.
(1) consumable electrode:
(a) leading consumable electrode: a solid wire having a diameter of from 4 to 8 mm, (b) trailing consumable electrode: a solid wire having a diameter of from 3 to 6.4 mm;

(2) welding current:
(a) welding current supplied to the leading consumable electrode: from 650 to 2,000 amperes, (b) welding current supplied to the trailing consumable electrode: from 600 to 1,500 amperes:;

(3) composition of shielding gas:
(a) composition of shielding gas supplied to the leading comsumable electrode: a gaseous mixture consisting essentially of an inert gas and at least one of carbon dioxide gas of from 15 to 50 vol.% and oxygen gas of from 3 to 10 vol.%, (b) composition of shielding gas supplied to the trailing consumable electrode: a gaseous mixture consisting essentially of an inert gas and at least one of carbon dioxide gas of from 5 to 30 vol.% and oxygen gas of from 1 to 5 vol.%;

(4) feed amount of said shielding gas: from 100 to 400 litres per minute per consumable electrode;

(5) range of inside welding positions:
(a) welding position of-leading consumable electrode:
within a range between a point deviated by a center angle of 12° of said pipe-blank toward the downstream of the moving direction of said steel strip from said connecting point, and said connecting point, and (b) welding position of trailing consumable electrode:
within a range between a point deviated by a center angle of 20° or said pipe-blank toward the upstream of the moving direction of said steel strip from a connecting point having advanced from the original position thereof in the forward movement direction of said pipe-blank by a distance equal to the distance of forward movement of said pipe-blank when said pipe-blank has made one turn, and said advanced connecting point.

3. The method as claimed in claim 2, wherein said outside welding is effected at a welding position within the following range:

(1) welding position of leading consumable electrode: within a range between a point deviated by a center angle of 12° of said pipe-blank toward the upstream of the moving direction of said steel strip from said opposite point, and said opposite point; and (2) welding position of trailing consumable electrode: within a range between a point deviated by a center angle of 20° of said pipe-blank toward the upstream of said steel strip from an opposite point having advanced from the original position thereof in the forward movement direction of said pipe-blank by a distance equal to the distance of forward movement of said pipe-blank when said pipe-blank has made one turn, and said advanced opposite point.

4. The method as claimed in claim 2, wherein a distance of at least 100 mm is provided between said leading consumable electrode and said trailing consumable electrode, and said outside welding is effected at a position within the following range:

(1) welding position of leading consumable electrode: within a range between a point deviated by a center angle of 15° of the pipe-blank toward the upstream of the moving direction of said steel strip from said opposite point, and a point deviated by a center angle of 30° of said pipe-blank toward the up-stream of the moving direction of said steel strip from said opposite point;
and, (2) welding position of trailing consumable electrode: within a range between a point deviated by a center angle of 12° of said pipe-blank toward the upstream of the moving direction of said steel strip from said opposite point, and said opposite point.

5. The method as claimed in any of claims 1, 2 or 3, wherein an after-shielding means is attached on the upstream side in the welding direction of the last trailing consumable electrode; and an after-shielding gas consisting essentially of argon gas is directed from said after-shielding means onto a molten pool at a rate of from 80 to 400 litres per minute so as to shield said molten pool until the solidification of said molten pool is completed.

6. The method as claimed in claim 4 wherein an after-shielding means is attached on the upstream side in the welding direction of the last trailing consumable electrode; and an after-shielding gas consisting essentially of argon gas is directed from said after-shielding mean onto a molten pool at a rate of from 80 to 400 litres per minute so as to shield said molten pool until the solidification of said molten pool is completed.