Multi-electrode submerged arc welding method

Abstract

A multi-electrode submerged arc welding method enables, in multi-electrode submerged arc welding using five or six electrodes, a deep penetration and a large amount of deposit metal to be obtained by supplying a large current, and enables stable arc to be generated by respective electrodes by suppressing magnetic interference. Welding defects can be prevented, beads with a good shape or appearance can be obtained, and the welding speed can be increased.

Claims

1. A multi-electrode submerged arc welding method of butt welding of a thick steel plate by using multiple electrodes, wherein a thickness of the thick steel plate is 15 to 45 mm and the multiple electrodes consists of five electrodes, the method comprising: disposing, in a welding advance direction, a first electrode at a selected position, a second electrode behind and adjacent to the first electrode, a third electrode behind and adjacent to the second electrode, a fourth electrode behind and adjacent to the third electrode, and a fifth electrode behind and adjacent to the fourth electrode; and supplying first to fifth welding currents to the first to fifth electrodes such that a current density J.sub.1 (A/mm.sup.2) of the first electrode, a current density J.sub.2 (A/mm.sup.2) of the second electrode, a current density J.sub.3 (A/mm.sup.2) of the third electrode, a current density J.sub.4 (A/mm.sup.2) of the fourth electrode, a current density J.sub.5 (A/mm.sup.2) of the fifth electrode satisfy Formulae (6) to (9), an arc voltage of the first electrode E.sub.1 (V) and an arc voltage of the second electrode E.sub.2 (V) satisfy Formula (13), and a difference between a maximum bead width and a minimum bead width in an obtained welding joint is 2.5 mm or less in a length of 1000 mm of a steady region of the obtained welding joint, wherein: a distance L.sub.12 (mm), a distance L.sub.23 (mm), a distance L.sub.34 (mm), and a distance L.sub.45 (mm) satisfy Formulae (1) to (4), the distance L.sub.12 being a distance between a position where a welding wire of the first electrode is in contact with a surface of the thick steel plate and a position where a welding wire of the second electrode is in contact with the surface of the thick steel plate, the distance L.sub.23 being a distance between the position where the welding wire of the second electrode is in contact with the surface of the thick steel plate and a position where a welding wire of the third electrode is in contact with the surface of the thick steel plate, the distance L.sub.34 being a distance between the position where the welding wire of the third electrode is in contact with the surface of the thick steel plate and a position where a welding wire of the fourth electrode is in contact with the surface of the thick steel plate, and the distance L.sub.45 being a distance between the position where the welding wire of the fourth electrode is in contact with the surface of the thick steel plate and a position where a welding wire of the fifth electrode is in contact with the surface of the thick steel plate; a total welding current of the five electrodes is 4000 to 6000 A; the welding wire of the first electrode is provided perpendicular or tilted at a backsweep angle with respect to the thick steel plate; and alternating currents are supplied as welding currents to the second to fifth electrodes,
12 mm≤L.sub.12≤25 mm  (1)
12 mm≤L.sub.23≤25 mm  (2)
12 mm≤L.sub.34≤25 mm  (3)
12 mm≤L.sub.45≤25 mm  (4)
0.40≤J.sub.2/J.sub.1≤0.95  (6)
0.60≤J.sub.3/J.sub.2≤0.95  (7)
0.60≤J.sub.4/J.sub.3≤1.00  (8)
0.60≤J.sub.5/J.sub.4≤1.00  (9)
0≤E.sub.2−E.sub.1≤2  (13).

2. A multi-electrode submerged arc welding method of butt welding of a thick steel plate by using multiple electrodes, wherein a thickness of the thick steel plate is 15 to 45 mm and the multiple electrodes consists of six electrodes, the method comprising: disposing, in a welding advance direction, a first electrode at a front, a second electrode behind and adjacent to the first electrode, a third electrode behind and adjacent to the second electrode, a fourth electrode behind and adjacent to the third electrode, a fifth electrode behind and adjacent to the fourth electrode, and a sixth electrode behind and adjacent to the fifth electrode; and supplying a first to sixth welding currents to the first to sixth electrodes such that a current density J.sub.1 (A/mm.sup.2) of the first electrode, a current density J.sub.2 (A/mm.sup.2) of the second electrode, a current density J.sub.3 (A/mm.sup.2) of the third electrode, a current density J.sub.4 (A/mm.sup.2) of the fourth electrode, a current density J.sub.5 (A/mm.sup.2) of the fifth electrode, and a current density J.sub.6 (A/mm.sup.2) of the sixth electrode satisfy Formulas (6) to (10), an arc voltage of the first electrode E.sub.1 (V) and an arc voltage of the second electrode E.sub.2 (V) satisfy the following Formula (13), and a difference between a maximum bead width and a minimum bead width in an obtained welding joint is 2.5 mm or less in a length of 1000 mm of a steady region of the obtained welding joint, wherein: a distance L.sub.12 (mm), a distance L.sub.23 (mm), a distance L.sub.34 (mm), a distance L.sub.45 (mm), and a distance L.sub.56 (mm) satisfy Formulas (1) to (5), the distance L.sub.12 being a distance between a position where a welding wire of the first electrode is in contact with a surface of the thick steel plate and a position where a welding wire of the second electrode is in contact with the surface of the thick steel plate, the distance L.sub.23 being a distance between the position where the welding wire of the second electrode is in contact with the surface of the thick steel plate and a position where a welding wire of the third electrode is in contact with the surface of the thick steel plate, the distance L.sub.34 being a distance between the position where the welding wire of the third electrode is in contact with the surface of the thick steel plate and a position where a welding wire of the fourth electrode is in contact with the surface of the thick steel plate, the distance L.sub.45 being a distance between the position where the welding wire of the fourth electrode is in contact with the surface of the thick steel plate and a position where a welding wire of the fifth electrode is in contact with the surface of the thick steel plate, and the distance L.sub.56 (mm) being a distance between the position where the welding wire of the fifth electrode is in contact with the surface of the thick steel plate and a position where a welding wire of the sixth electrode is in contact with the surface of the thick steel plate; a total welding current of the six electrodes is 4000 to 6000 A; the welding wire of the first electrode is provided perpendicular or tilted at a backsweep angle with respect to the thick steel plate; and an alternating current is supplied as a welding current to the second to sixth electrodes,
12 mm≤L.sub.12≤25 mm  (1)
12 mm≤L.sub.23≤25 mm  (2)
12 mm≤L.sub.34≤25 mm  (3)
12 mm≤L.sub.45≤25 mm  (4)
12 mm≤L.sub.56≤25 mm  (5)
0.40≤J.sub.2/J.sub.1≤0.95  (6)
0.60≤J.sub.3/J.sub.2≤0.95  (7)
0.60≤J.sub.4/J.sub.3≤1.00  (8)
0.60≤J.sub.5/J.sub.4≤1.00  (9)
0.60≤J.sub.6/J.sub.5≤1.00  (10)
0≤E.sub.2−E.sub.1≤2  (13).

3. The method according to claim 1, wherein a direct current is supplied to the first electrode in a reverse polarity.

4. The method according to claim 2, wherein a direct current is supplied to the first electrode in a reverse polarity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1(a) and 1(b) show a side view schematically illustrating an example in which a welding wire is tilted.

(2) FIG. 2 is a cross-sectional view of a weld zone schematically illustrating an example of a penetration constriction.

REFERENCE SIGNS LIST

(3) 1 thick steel plate 2 electrode tip 3 welding wire 4 weld metal

DETAILED DESCRIPTION

(4) A multi-electrode submerged arc welding method is a technique for performing submerged arc welding with five or six electrodes disposed in line in the welding advance direction. When using five electrodes, an electrode disposed at the front in the welding advance direction is a first electrode; electrodes disposed behind are sequentially a second electrode, a third electrode, and a fourth electrode; and an electrode at the end is a fifth electrode. When using six electrodes, another electrode is disposed behind the fifth electrode, and this electrode at the end is a sixth electrode.

(5) In the above manner, it is possible to obtain a faster welding speed than that in the conventional art by increasing the number of electrodes used.

(6) The total welding current to be supplied to the electrodes is higher than or equal to 4000 A to prevent insufficient penetration and an insufficient amount of deposit metal accompanying with an increase in the welding speed. In addition, the total welding current to be supplied to the electrodes is lower than or equal to 6000 A to prevent the shape of beads from being degraded as a result of supplying an excessive welding current. That is, when using five electrodes, the total welding current to be supplied to the first to fifth electrodes is 4000 to 6000 A; and when using six electrodes, the total welding current to be supplied to the first to sixth electrodes is 4000 to 6000 A.

(7) Although the five or six electrodes are disposed in line, if the interval between adjacent electrodes (interval between electrodes disposed adjacent to each other) is too small, the electrodes, particularly contact chips come in contact with each other, which lead to arc disturbance. Accordingly, various problems including generation of a welding defect and degradation of shape of beads are generated. On the other hand, if the interval between adjacent electrodes is too large, molten metal to be formed by the electrodes is separated, and welding becomes unstable.

(8) Accordingly, the first to sixth electrodes are disposed such that a distance L.sub.12 (mm), a distance L.sub.23 (mm), a distance L.sub.34 (mm), a distance L.sub.45 (mm), and a distance L.sub.56 (mm) satisfy Formulae (1) to (5), the distance L.sub.12 being a distance between a position where a welding wire of the first electrode is in contact with a surface of a thick steel plate and a position where a welding wire of the second electrode is in contact with the surface of the thick steel plate, the distance L.sub.23 being a distance between the position where the welding wire of the second electrode is in contact with the surface of the thick steel plate and a position where a welding wire of the third electrode is in contact with the surface of the thick steel plate, the distance L.sub.34 being a distance between the position where the welding wire of the third electrode is in contact with the surface of the thick steel plate and a position where a welding wire of the fourth electrode is in contact with the surface of the thick steel plate, the distance L.sub.45 being a distance between the position where the welding wire of the fourth electrode is in contact with the surface of the thick steel plate and a position where a welding wire of the fifth electrode is in contact with the surface of the thick steel plate, and the distance L.sub.56 being a distance between the position where the welding wire of the fifth electrode is in contact with the surface of the thick steel plate and a position where a welding wire of the sixth electrode is in contact with the surface of the thick steel plate.
12 mm≤L.sub.12≤25 mm  (1)
12 mm≤L.sub.23≤25 mm  (2)
12 mm≤L.sub.34≤25 mm  (3)
12 mm≤L.sub.45≤25 mm  (4)
12 mm≤L.sub.56≤25 mm  (5)

(9) That is, when using five electrodes, the electrodes are disposed to satisfy Formulae (1) to (4); and when using six electrodes, the electrodes are disposed to satisfy Formulae (1) to (5).

(10) The current densities of the welding currents to be supplied to the electrodes affect the arc pressures. Accordingly, to maintain the arc pressures to suitable values, current density ratios between adjacent electrodes are maintained to appropriate values. Specifically, the welding currents are supplied such that a current density J.sub.1 (A/mm.sup.2) of the first electrode, a current density J.sub.2 (A/mm.sup.2) of the second electrode, a current density J.sub.3 (A/mm.sup.2) of the third electrode, a current density J.sub.4 (A/mm.sup.2) of the fourth electrode, a current density J.sub.5 (A/mm.sup.2) of the fifth electrode, and a current density J.sub.6 (A/mm.sup.2) of the sixth electrode satisfy Formulae (6) to (10).
0.40≤J.sub.2/J.sub.1≤0.95  (6)
0.60≤J.sub.3/J.sub.2≤0.95  (7)
0.60≤J.sub.4/J.sub.3≤1.00  (8)
0.60≤J.sub.5/J.sub.4≤1.00  (9)
0.60≤J.sub.6/J.sub.5≤1.00  (10)

(11) That is, when using five electrodes, the welding currents are supplied to satisfy Formulae (6) to (9); and when using six electrodes, the welding currents are supplied to satisfy Formulae (6) to (10). Accordingly, the arc pressures can be maintained at suitable values and, as a result, it is possible to prevent a welding defect and form beads with a good shape. A current density J.sub.n(A/mm.sup.2) of an n-th electrode is a value calculated from Formula (11).
J.sub.n=I.sub.n÷[π×(D.sub.n.sup.2÷4)]  (11)

(12) I.sub.n: welding current (A) to be supplied to n-th electrode

(13) D.sub.n: diameter (mm) of a welding wire of n-th electrode

(14) n: integer from 1 to 6

(15) Although any of direct current and alternating current may be supplied as welding current to the first electrode among the five or six electrodes, from the view point of increasing the depth of penetration, direct current as welding current (direct current) is preferably supplied. In addition, it is preferable to supply the welding current by using the welding wire as a positive (+) electrode and the thick steel plate as a negative (−) electrode (so-called “reverse polarity”). By supplying direct current as welding current in reverse polarity, melting of the thick steel plate is promoted, thereby further increasing the effect of increasing the depth of penetration.

(16) Together with an increase in the depth of penetration by the first electrode, the large welding current strengthens the backward flow of molten metal, and the arc at the second electrode and subsequent ones are likely to be disturbed. However, by supplying the welding current to satisfy Formulae (6) to (10), the arcs can be stabilized.

(17) Furthermore, at the second electrode and subsequent ones, alternating current as welding current is supplied, thereby preventing magnetic interference. Thus, it is possible to prevent a welding defect and form beads with a good shape. In the alternating current, “current” means “effective current”, and “voltage” means “effective voltage”.

(18) In addition, if the arc voltage of the second electrode is lower than the arc voltage of the first electrode, the arc voltage of the second electrode becomes unstable. However, if the arc voltage of the second electrode is too high, penetration formed by the first electrode and penetration formed by the second electrode and the subsequent electrodes are separated from each other, forming a constriction near the center of penetration. To avoid this phenomenon, an arc voltage E.sub.1 (V) of the first electrode and an arc voltage E.sub.2 (V) of the second electrode are preferably set to satisfy Formula (12), more preferably, Formula (13).
0≤E.sub.2−E.sub.1≤3  (12)
0≤E.sub.2−E.sub.1≤2  (13)

(19) As illustrated in FIGS. 1(a) and 1(b), the welding wire of the first electrode is preferably provided to be perpendicular or tilted at a backsweep angle α with respect to the thick steel plate. This is because deep penetration can be obtained thereby. The backsweep angle α of the first electrode is preferably 0 to 15°.

(20) The welding wires of the second electrode and the subsequent electrodes may be provided perpendicular or tilted with respect to the thick steel plate.

(21) FIG. 1(a) is an example in which a welding wire 3 is tilted so that the end thereof may be located behind an electrode tip 2 in the advance direction, and an angle α formed by a line perpendicular to a thick steel plate 1 and the welding wire 3 is called a backsweep angle. FIG. 1(b) is an example in which the welding wire 3 is tilted so that the end thereof may be located in front of the electrode tip 2 in the advance direction, and an angle β formed by a line perpendicular to the thick steel plate 1 and the welding wire 3 is called an upsweep angle. Each arrow W in FIGS. 1(a) and 1(b) represents the welding advance direction.

(22) Even if a thick steel plate with a thickness of 15 mm or more is used, it is possible to prevent a welding defect and obtain beads with a good shape. In addition, welding can be performed at a high welding speed. However, since the groove width is too large with a thick steel plate with a thickness exceeding 45 mm, molten metal does not spread sufficiently and the groove is likely to remain. Therefore, our methods are applicable to a thick steel plate with a thickness of 15 to 45 mm.

(23) Our methods are highly effective when applied to a thick steel plate with a thickness of 30 mm or more with which how to increase the welding speed is particularly a problem.

(24) The wire diameter (diameter of wire) of the welding wire to be used for each electrode is preferably 3.2 to 4.8 mm. In addition, as a welding flux, any of a fused flux and a agglomerated flux can be used.

(25) Furthermore, our welding method is suitably used for butt welding of a flat thick steel plate in addition to seam welding of a large-dimeter steel pipe such as a UOE pipe or a spiral pipe in which edges of a thick steel plate, having been formed into an open pipe, are brought into contact with each other. In addition, our methods are applicable to grooves of various shapes, and also applicable to one-side welding and both-side welding.

EXAMPLE

(26) One-side Y grooves were formed on thick steel plates with thicknesses of 19.1 mm and 38.1 mm, and multi-electrode submerged arc welding (1 pass) was performed to form welding joints. The thicknesses and groove shapes of the respective thick steel plates are shown in Table 1. Welding conditions are shown in Tables 2, 3, and 4. As for the electrodes, an electrode disposed at the front in the advance direction is a first electrode; electrodes disposed behind are sequentially a second electrode, a third electrode, a fourth electrode, a fifth electrode, and a sixth electrode. Note that types of welding current to be supplied to the first electrode are shown in Table 4, and alternating currents as welding currents were supplied to the electrodes other than the first electrode. Each “welding heat input” in Table 2 was obtained by dividing the total of “welding current× arc voltage” obtained for respective electrodes by “welding speed”.

(27) TABLE-US-00001 TABLE 1 Groove No. Thickness (mm) Angle (°) Depth (mm) 1 38.1 70 15.0 2 38.1 70 15.0 3 38.1 70 15.0 4 38.1 70 13.5 5 38.1 70 15.0 6 38.1 70 15.0 7 38.1 70 15.0 8 38.1 70 15.0 9 38.1 70 15.0 10 38.1 70 15.0 11 38.1 70 15.0 12 38.1 70 15.0 13 38.1 70 15.0 14 38.1 70 15.0 15 38.1 70 15.0 16 38.1 70 15.0 17 19.1 90 7.0 18 19.1 90 7.0 19 19.1 90 7.0 20 19.1 90 6.0 21 19.1 90 7.0 22 19.1 90 7.0 23 19.1 90 7.0 24 19.1 90 7.0 25 38.1 70 15.0

(28) TABLE-US-00002 TABLE 2 First Second Third Fourth Fifth Sixth Welding Welding Electrode Electrode Electrode Electrode Electrode Electrode Speed Heat I E D I E D I E D I E D I E D I E D (mm/ Input No. (A) (V) (mm) (A) (V) (mm) (A) (V) (mm) (A) (V) (mm) (A) (V) (mm) (A) (V) (mm) min) (kJ/mm) 1 1350 32 4.0 1100 34 4.0 900 38 4.0 700 40 4.0 500 42 4.0 — — — 1200 8.19 2 1350 32 4.0 1100 34 4.0 900 38 4.0 700 40 4.0 500 42 4.0 — — — 1200 8.19 3 1350 32 4.0 1100 34 4.0 900 38 4.0 700 40 4.0 — — — — — — 1200 7.14 4 1350 32 4.0 1100 34 4.0 900 38 4.0 700 40 4.0 — — — — — — 1200 7.14 5 1350 32 4.0 1280 34 4.0 1210 38 4.0 730 40 4.0 500 42 4.0 — — — 1200 9.15 6 1350 32 3.2 850 34 4.0 750 38 4.8 700 40 4.8 650 42 4.8 — — — 1200 7.80 7 1350 32 4.0 1290 34 4.0 900 38 4.0 700 40 4.0 500 42 4.0 — — — 1200 8.51 8 1350 32 3.2 800 34 4.0 750 38 4.0 700 40 4.0 650 42 4.0 — — — 1200 7.71 9 1350 32 4.0 1100 34 4.0 1060 38 4.0 700 40 4.0 500 42 4.0 — — — 1200 8.49 10 1350 32 4.0 1100 34 4.0 940 38 4.8 700 40 4.8 500 42 4.8 — — — 1200 8.27 11 1450 32 4.0 1200 34 4.0 1000 38 4.0 850 40 4.0 770 42 4.0 720 42 4.0 1450 9.18 12 1450 32 4.0 1200 34 4.0 1000 38 4.0 880 40 4.0 800 42 4.0 750 42 4.0 1450 9.33 13 1350 32 4.0 1100 34 4.0 900 38 4.0 700 40 4.0 500 42 4.0 — — — 1200 8.19 14 1350 32 4.0 1100 34 4.0 900 38 4.0 700 40 4.0 500 42 4.0 — — — 1200 8.19 15 1350 32 4.0 1100 34 4.0 900 38 4.0 700 40 4.0 500 42 4.0 — — — 1200 8.19 16 1250 35 3.2 1000 37 3.2 950 38 4.0 760 39 4.0 640 40 4.0 — — — 1200 8.60 17 1400 34 4.0 1150 36 4.0 950 37 4.0 750 38 4.0 550 39 4.0 — — — 2300 4.54 18 1280 33 3.2 1150 37 4.0 850 40 4.0 700 42 4.0 550 44 4.0 — — — 2300 4.50 19 1400 34 4.0 1340 36 4.0 950 37 4.0 750 38 4.0 550 39 4.0 — — — 2300 4.72 20 1400 34 4.0 1150 36 4.0 950 37 4.0 750 38 4.0 — — — — — — 2300 3.98 21 1250 34 4.0 950 36 4.0 750 38 4.0 550 40 4.0 550 42 4.0 — — — 2300 3.87 22 1200 34 4.0 900 36 4.0 750 38 4.0 550 40 4.0 560 42 4.0 — — — 2300 3.84 23 1350 32 4.0 1100 34 4.0 900 38 4.0 700 40 4.0 500 42 4.0 — — — 2300 4.27 24 1350 33 4.0 1250 36 4.0 1000 39 4.0 900 41 4.8 600 43 4.8 — — — 2300 4.99 25 1350 32 4.0 1100 36 4.0 900 38 4.0 700 40 4.0 500 42 4.0 — — — 1200 8.30 I Welding current E Arc voltage D Wire diameter

(29) TABLE-US-00003 TABLE 3 Distance between Thick Steel Distance between Positions Electrode Angle (°) *1 Plate and Electrode Tip (mm) where Welding Wire are First Second Third Fourth Fifth Sixth First Second Third Fourth Fifth Sixth in Contact with Surface of Elec- Elec- Elec- Elec- Elec- Elec- Elec- Elec- Elec- Elec- Elec- Elec- Thick Steel Plate (mm) No. trode trode trode trode trode trode trode trode trode trode trode trode L.sub.12 L.sub.23 L.sub.34 L.sub.45 L.sub.56 1 −10 0 +12 +24 +40 — 23 25 30 32 32 — 20 20 20 20 — 2 −10 0 +12 +24 +40 — 23 25 30 32 32 — 20 20 20 20 — 3 −10 0 +12 +24 — — 23 25 30 32 — — 20 20 20 — — 4 −10 0 +12 +24 — — 23 25 30 32 — — 20 20 20 — — 5 −10 0 +12 +24 +40 — 23 25 30 32 32 — 20 20 20 20 — 6 −10 0 +12 +24 +40 — 23 25 30 32 32 — 20 20 20 20 — 7 −10 0 +12 +24 +40 — 23 25 30 32 32 — 20 20 20 20 — 8 −10 0 +12 +24 +40 — 23 25 30 32 32 — 20 20 20 20 — 9 −10 0 +12 +24 +40 — 23 25 30 32 32 — 20 20 20 20 — 10 −10 0 +12 +24 +40 — 23 25 30 32 32 — 20 20 20 20 — 11 −12 −5 +5 +15 +30 +45 23 25 30 32 32 32 22 22 22 22 22 12 −12 −5 +5 +15 +30 +45 23 25 30 32 32 32 22 22 22 22 22 13 −10 0 +12 +24 +40 — 23 25 30 32 32 — 25 25 25 25 — 14 −10 0 +12 +24 +40 — 23 25 30 32 32 — 12 12 12 12 — 15 −10 0 +12 +24 +40 — 23 25 30 32 32 — 27 27 27 27 — 16 0 +10 +20 +30 +40 — 27 30 33 33 33 — 18 18 22 22 — 17 −12 −3 +5 +16 +26 — 25 27 32 34 34 — 22 22 24 24 — 18 −5 +5 +15 +25 +35 — 27 29 35 35 35 — 22 22 18 18 — 19 −12 −3 +5 +16 +26 — 25 27 32 34 34 — 22 22 24 24 — 20 −12 −3 +5 +16 — — 25 27 32 34 — — 22 22 24 — — 21 −12 −3 +8 +25 +45 — 32 34 39 41 41 — 21 21 23 25 — 22 −12 −3 +8 +25 +45 — 32 34 39 41 41 — 21 21 23 25 — 23 −12 −3 +8 +25 +45 — 32 34 39 41 41 — 21 21 23 25 — 24 −10 0 +8 +16 +25 — 27 29 34 36 36 — 20 18 15 12 — 25 −10 0 +12 +24 +40 — 23 25 30 32 32 — 20 20 20 20 — *1 + Upsweep angle − Backsweep angle 0 Perpendicular

(30) TABLE-US-00004 TABLE 4 Welding Current Current Supplied No. Total (A) J.sub.2/J.sub.1 J.sub.3/J.sub.2 J.sub.4/J.sub.3 J.sub.5/J.sub.4 J.sub.6/J.sub.5 to First Electrode 1 4550 0.81 0.82 0.78 0.71 — AC 2 4550 0.81 0.82 0.78 0.71 — DC 3 4050 0.81 0.82 0.78 — — AC 4 4050 0.81 0.82 0.78 — — AC 5 5070 0.95 0.95 0.60 0.68 — AC 6 4300 0.40 0.61 0.93 0.93 — AC 7 4740 0.96 0.70 0.78 0.71 — AC 8 4250 0.38 0.94 0.93 0.93 — AC 9 4710 0.81 0.96 0.66 0.71 — AC 10 4590 0.81 0.59 0.74 0.71 — AC 11 5990 0.83 0.83 0.85 0.91 0.94 AC 12 6080 0.83 0.83 0.88 0.91 0.94 AC 13 4550 0.81 0.82 0.78 0.71 — AC 14 4550 0.81 0.82 0.78 0.71 — AC 15 4550 0.81 0.82 0.78 0.71 — AC 16 4600 0.80 0.61 0.80 0.84 — DC 17 4800 0.82 0.83 0.79 0.73 — AC 18 4530 0.58 0.74 0.82 0.79 — DC 19 4990 0.96 0.71 0.79 0.73 — DC 20 4250 0.82 0.83 0.79 — — DC 21 4000 0.76 0.79 0.73 0.91 — DC 22 3960 0.75 0.83 0.73 1.02 — DC 23 4550 0.81 0.82 0.78 0.71 — AC 24 5100 0.93 0.80 0.63 0.67 — DC 25 4550 0.81 0.82 0.78 0.71 — AC

(31) A steady region of each of the obtained welding joints was visually observed to determine whether there was a welding defect. In addition, the bead width was measured in a length of 1000 mm of the steady region of the welding joint, and a difference between a maximum and a minimum was obtained.

(32) Next, three cross-sectional macroscopic observation samples were taken from the steady region of each of the welding joints. Then, a distance from the surface of the thick steel plate to the end of the penetration was measured, and an average of the values at the three cross sections was evaluated as the penetration depth. Similarly, the cross-sectional macroscopic observation samples were observed to determine whether there was a penetration constriction. The penetration constriction here means, as illustrated in FIG. 2, a penetration width M.sub.CENTER near the center of penetration being smaller than a penetration width M.sub.BOTTOM on the side of penetration end.

(33) The results are shown in Table 5. In Table 5, *3 represents a welding joint for which the difference between the maximum bead width and the minimum bead width exceeds 2.5 mm, from which it is found that welding was unstable. In addition, *2 in Table 5 represents a welding joint for which penetration was shallow, which is, the penetration depth being less than 0.65 times as deep as the thickness of the plate.

(34) TABLE-US-00005 TABLE 5 Difference between Welding Maximum Bead Pene- Defect Width and tration and Bead Minimum Bead Depth No. Appearance Width (mm) (mm) Note 1 No Abnormality 1.1 26.2 Example 2 No Abnormality 1.8 27.6 Example 3 Underfill   4.6 *3 26.3 Comparative Example 4 No Abnormality 1.9   24.6 *2 Comparative Example 5 No Abnormality 2.2 26.8 Example 6 No Abnormality 1.9 28.7 Example 7 No Abnormality   2.6 *3 27.1 Comparative Example 8 No Abnormality   3.2 *3 28.2 Comparative Example 9 No Abnormality   3.4 *3 26.0 Comparative Example 10 No Abnormality   3.6 *3 25.9 Comparative Example 11 No Abnormality 2.2 25.7 Example 12 High Excess   3.0 *3 26.0 Comparative Weld Metal Example Height 13 No Abnormality 2.3 26.6 Example 14 No Abnormality 1.7 27.4 Example 15 No Abnormality   2.9 *3 25.6 Comparative Example 16 No Abnormality 2.4 28.0 Example 17 No Abnormality 2.0 13.8 Example 18 No Abnormality 2.3 15.1 Example 19 High Excess   4.7 *3 14.2 Comparative Weld Metal Example Height 20 No Abnormality 1.7   12.3 *2 Comparative Example 21 No Abnormality 1.4 14.6 Example 22 No Abnormality   2.9 *3   12.2 *2 Comparative Example 23 No Abnormality 2.1 13.6 Example 24 No Abnormality 2.2 13.6 Example 25 No Abnormality 1.9 26.0 Example *2: Penetration depth is less than 0.65 times as deep as thickness of plate *3: Difference between maximum bead width and minimum bead width exceeds 2.5 mm

(35) Table 5 shows that in each of the Examples to which our method was applied (Nos. 1, 2, 5, 6, 11, 13, 14, 16, 17, 18, 21, 23, 24, and 25), deep penetration and high deposition rate were obtained, thereby obtaining beads with a good shape or appearance while preventing a welding defect at a high welding speed of 1200 mm/minute or higher.

(36) In each of the Examples other than Nos. 18 and 25, the arc voltage E.sub.1 (V) of the first electrode and the arc voltage E.sub.2 (V) of the second electrode satisfy Formula (12) and, accordingly, a penetration constriction was not formed.

(37) In each of Examples of Nos. 6 and 16, a welding joint of a thick steel plate with a thickness of 38.1 mm was formed by using a small-diameter (3.2 mm) welding wire for the first electrode. In Example of No. 18, a welding joint of a thick steel plate with a thickness of 19.1 mm was formed by using a small-diameter (3.2 mm) welding wire for the first electrode. In these Examples, the current density was increased by using a small-diameter welding wire and, as a result, deeper penetration than in the other Examples using a plate with the same thickness was obtained.

(38) Nos. 2 and 16 are examples in each of which direct current as welding current was supplied to the first electrode to form a welding joint of a thick steel plate with a thickness of 38.1 mm, and Nos. 18, 21, and 24 are examples in each of which direct current as welding current was supplied to the first electrode to form a welding joint of a thick steel plate with a thickness of 19.1 mm. In these Examples, by supplying the direct-current welding current to the first electrode, penetration was deeper than that in the other Examples using a plate with the same thickness.

(39) On the other hand, in each of Comparative Examples of Nos. 3, 4, and 20, a welding joint was formed by using four electrodes. Thus, welding was performed at the same welding speed as that in the Examples, which resulted in underfill and insufficient penetration.

(40) In each of Nos. 7, 8, 9, 10, and 19, the current densities of the welding currents did not satisfy Formulae (6) to (10) and, thus, the arc was disturbed, and the bead width largely varied.

(41) In No. 12, the total welding current exceeded 6000 A and, thus, the bead width largely varied.

(42) In No. 22, the total welding current was less than 4000 A, which resulted in insufficient penetration. In addition, the current densities did not satisfy Formula (9) and, thus, the bead width largely varied.