JPH0819871A - Plasma key hole welding method - Google Patents

Abstract

PURPOSE:To excellently weld a key hole with a penetration bead protruded regardless of the groove state of the butt end part of a tube to be welded extending almost horizontally. CONSTITUTION:This plasma key hole welding method is adopted to weld all over the circumference of butt end parts of steel tubes 9 by jetting a pulse like plasma arc and plasma gas corresponding to a direct current pulse like plasma current from a torch 1 while revolving the torch 1 of the plasma key hole welding equipment along the outer circumference of the butt end parts of the steel tubes 9. Then, a pre-heating process to partially melt the outer circumferential side of the butt end parts of the steel tubes 9 by jetting a soft plasma whose flow rate of plasma gas is decreased lower than the normal value from the torch 1 and a welding process to execute key-hole-welding onto the partially molten butt end parts of the steel tubes 9 by jetting a pulse like plasma arc and plasma gas whose jet interval is increased by the supply of the direct current pulse like current decreased lower than the normal value only for the time of applying continuously a peak current value are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plasma keyhole welding method for welding a butt end of a pipe to be welded such as a steel pipe over the entire circumference.

[0002]

2. Description of the Related Art As a method of welding a butt end of a pipe to be welded over the entire circumference, a method using a plasma keyhole welding device (see Japanese Patent Laid-Open No. 5-111774) is generally known. This plasma keyhole welding device ejects a plasma gas in a jet shape together with a plasma arc with a high energy density that is thermally and electromagnetically converged from a nozzle of a torch that circulates along the outer periphery of the butt end of the pipe to be welded. In the above, a keyhole that melts and penetrates the butt end is sequentially formed in the orbiting direction of the torch.
At the butt end behind the torch, the keyhole is sequentially closed and the front bead and the back bead are sequentially formed by the cooling and solidification of the molten metal, thus performing the entire circumference welding.

In the plasma keyhole welding method using such a plasma keyhole welding device, steel pipes having a wall thickness up to about 9 mm can be butt welded without groove processing, and fixed steel pipes can be welded. It is also widely used for butt welding of steel pipes such as gas pipes, since the butt end can be welded over the entire circumference.

By the way, in the plasma keyhole welding method, the metal melted along with the formation of the keyhole is cooled and solidified while flowing under the action of gravity to form a backside bead. When butt-welding a steel pipe or the like that extends substantially horizontally as described above, the back bead is recessed from the inner peripheral surface of the steel pipe at the left and right side portions and the lower portion of the steel pipe, and a welding defect with insufficient strength is likely to occur. Therefore, a technique for preventing the occurrence of such a depression of the back bead has been proposed in Japanese Patent Laid-Open No. 5-111774.

Generally, the plasma arc ejected from the torch of the plasma keyhole welding apparatus described above is
The energy of one pulse is ejected in a pulsed manner in accordance with the supplied DC pulsed plasma current (see Japanese Patent Laid-Open No. 60-9578). . If the peak current value or the duration of the peak current changes, the formation of keyholes, the formation of blowholes, the formation of front beads and back bead, etc. also change accordingly.

In view of this, Japanese Patent Laid-Open Publication No. 60-9578 discloses a method in which each plasma current to be supplied is reduced in a slope shape or a step shape from a peak current value to a base current value so that a good front bead or backside wave can be obtained. There has been proposed a plasma keyhole welding method intended to form beads and prevent blowholes.

[0007]

By the way, FIG. 5 (a)
In the plasma keyhole welding method of welding the abutting ends of the pipes a, a to be welded as shown in Fig. 3, it is difficult to always keep the groove state constant because the groove state of the abutting ends is not known from the outside. is there. In this regard, the above-mentioned Japanese Patent Laid-Open No. 5-111
The prior arts described in Japanese Patent No. 774 and Japanese Patent Laid-Open No. 60-9578 are based on the assumption that the groove state at the butt ends of the pipes a, a to be welded is constant, so the groove state is not constant. In this case, the formation of the keyhole KH tends to be unstable, and it is impossible to completely prevent the occurrence of the depression b in the back bead (see FIGS. 5B and 5C).

Therefore, the present invention provides a plasma keyhole welding method which enables good keyhole welding in which the back bead has a convex shape, regardless of the groove state of the butt ends of the pipes to be welded extending substantially horizontally. The purpose is to provide.

[0009]

To this end, according to the present invention, a torch of a plasma keyhole welding apparatus is circulated along the outer periphery of a butt end of a pipe to be welded while the torch is turned into a DC pulsed plasma current. In the plasma keyhole welding method, in which a pulsed plasma arc and plasma gas are jetted according to the requirements to weld the butt end of the pipe to be welded over the entire circumference, a soft plasma with a plasma gas flow rate lower than the normal value is jetted from the torch. Preheating process to partially melt the outer peripheral side of the butt end of the pipe to be welded, and the pulsed plasma with the injection interval increased by the supply of the DC pulsed plasma current that is reduced from the normal value only for the duration of the peak current value. A welding step of keyhole welding the butt ends of the pipes to be welded, in which arc and plasma gas are ejected from the torch. It is the means.

[0010]

In the plasma keyhole welding method according to the present invention employing such means, first, in the preheating step, the torch is operated at the butt end of the pipe to be welded while injecting the soft plasma having the plasma gas flow rate reduced from the normal value. By circling along the outer circumference, the outer peripheral side of the butt end of the pipe to be welded is preheated and partially melted, so that the groove condition becomes constant.

Subsequently, in the welding process, the torch is pulsed while the plasma arc and the plasma gas having the injection intervals increased by the supply of the direct-current pulsed plasma current reduced only from the normal value only for the duration of the peak current value are pulsed. By wrapping around the outer periphery of the butt end of the welded pipe, the butt end of the welded pipe is keyhole welded over the entire circumference. At that time, since the pulse-shaped plasma arc and plasma gas ejected from the torch have an increased injection interval, the back bead is sufficiently cooled during that time, and the back bead is welded to both the left and right sides and the lower side of the welded pipe. The corrugated bead is formed in a state of protruding from the inner peripheral surface of the pipe to be welded.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be specifically described below with reference to the accompanying drawings. First, the overall structure of the plasma keyhole welding apparatus used in the plasma keyhole welding method of one embodiment is well known in the related art as described in the above-mentioned JP-A-5-111774 and JP-A-60-9578. Therefore, detailed description based on the drawings will be omitted.

Here, as shown in FIG. 2, the torch 1 mounted on the head of the plasma keyhole welding apparatus and rotating around the butt end of the pipe to be welded is covered with a cylindrical ceramic cap 2. Nozzle 4 at the tip of the torch body 3
Is attached by screwing, and the tip 5a is attached to the center of the nozzle 4.
A rod-shaped tungsten cathode 5 having a conical point is arranged.

The ceramic cap 2 and the torch body 3
A shield gas passage 6 through which an inert gas such as argon gas for shielding flows is formed in an annular shape, and in the torch body 3 and the nozzle 4, there are argon and hydrogen gas around the tungsten cathode 5. A gas passage 7 through which the mixed gas flows is formed. The tip of the gas passage 7 is tapered, and the plasma arc ejection hole 8 is continuous to the tip.

Next, a plasma keyhole welding method of an embodiment using the plasma keyhole welding apparatus having the torch 1 as described above will be explained. This plasma keyhole welding method, as shown in FIGS.
The torch 1 of the plasma keyhole welding apparatus is a butt end of the steel pipes 9 and 9 and is made by welding the butt ends of the steel pipes 9 and 9 installed substantially horizontally as welded pipes over the entire circumference. 1 (a) and 1 (b) along the outer circumference thereof are set to rotate clockwise.

The torch 1 set in this way is supplied with a direct-current pulsed plasma current to the tungsten cathode 5, so that the plasma is directed from the tip 5a of the tungsten cathode 5 toward the butt end of the steel pipes 9, 9. Ark P
A is ejected in a pulse shape from the plasma arc ejection hole 8 and a mixed gas of argon and hydrogen flows through the gas passage 7 to eject the plasma gas PG in a jet shape from the plasma arc ejection hole 8 (see FIG. 2). ).

Here, in welding the butt ends of the steel pipes 9, 9 all around, first, in the preheating step, as shown in FIG.
As shown in FIG. 3, a DC pulsed plasma current is supplied to the tungsten cathode 5 in a state where the flow rate of the plasma gas flowing through the gas passage 7 is about 1/2 to 1/3 of the normal value, and the plasma arc thus obtained. PA and plasma gas P
G, that is, soft plasma with reduced keyhole penetrating force is jetted from the torch 1 in a pulse shape, and the torch 1 is circulated along the outer periphery of the butt ends of the steel pipes 9 as shown in FIG. 1 (a). Let

By doing so, the butt ends of the steel pipes 9, 9 are preheated and partially melted on the outer peripheral side over the entire circumference, as shown by the diagonal lines in FIG. 1 (a), and the groove condition is shown in FIG. 4 (a). It is stabilized as shown in.

In the subsequent welding step, as shown in FIG. 3 (b), a normal peak current value preset according to the conditions such as the wall thickness of the butt ends of the steel pipes 9 is set, and the continuous peak current value is maintained. A direct current pulsed plasma current reduced to 40% to 60% of the normal value only for the time is supplied to the tungsten cathode 5, and the plasma arc PA and the plasma gas PG whose injection intervals are greatly increased according to the plasma current are supplied to the torch 1. The torch 1 is rotated along the outer circumference of the butt ends of the steel pipes 9 and 9 as shown in FIG.

As a result, at the butt ends of the steel pipes 9, 9, keyholes KH are sequentially formed as shown in FIG. 4 (a), and keyhole welding is performed over the entire circumference. that time,
Pulsed plasma arc PA ejected from torch 1
Since the injection intervals of the plasma gas PG and the plasma gas PG are greatly increased, the back bead 10 that closes the keyhole KH during that time is sufficiently cooled. Therefore, as shown in FIG. 1 (b), the back bead 10 is formed so as to project from the inner peripheral surface of the abutting end of the steel pipes 9 and 9 even on the left and right sides and the lower side of the steel pipes 9 and 9. (See FIG. 4 (c)).

[0021]

As described above, in the present invention, in the preheating step, the torch orbits along the outer periphery of the butt end of the pipe to be welded while injecting the soft plasma having the plasma gas flow rate reduced from the normal value. Then, the outer peripheral side of the butt end of the pipe to be welded is preheated and partially melted, so that the groove condition becomes constant.

In the subsequent welding process, the torch is welded while the plasma arc and the plasma gas having the injection intervals increased by the supply of the direct-current pulsed plasma current reduced only from the normal value only for the duration of the peak current value are pulsed. By wrapping around the outer circumference of the butt end of the pipe, the butt end of the pipe to be welded is keyhole welded over the entire circumference. At that time, since the pulse-shaped plasma arc and plasma gas ejected from the torch have an increased injection interval, the back bead is sufficiently cooled during that time, and the back bead is welded to both the left and right sides and the lower side of the welded pipe. The corrugated bead is formed in a convex shape protruding from the inner peripheral surface of the pipe to be welded.

Therefore, according to the present invention, regardless of the groove state of the butt ends of the pipes to be welded extending substantially horizontally, it is possible to perform good keyhole welding in which the back bead has a convex shape.

[Brief description of drawings]

FIG. 1 is a sectional view of a butt end portion of a steel pipe showing an operation of one embodiment of the present invention, (a) is a sectional view in a preheating step,
(B) is sectional drawing in a welding process.

FIG. 2 is a sectional view of a torch of a plasma keyhole welding apparatus used in one embodiment.

3A and 3B are graphs showing changes in a plasma current amount and a plasma gas flow rate in one example, FIG. 3A is a graph in a preheating process, and FIG. 3B is a graph in a welding process.

FIG. 4 is a partially enlarged sectional view of a butt end portion of a steel pipe in one embodiment, (a) is a partially enlarged sectional view in a preheating step, (b) is a partially enlarged sectional view at the time of forming a keyhole,
(C) is a partial expanded sectional view at the time of forming a back bead.

FIG. 5 is a partially enlarged sectional view of a butt end portion of a steel pipe in a conventional example, (a) is a partially enlarged sectional view before welding, and (b) is a sectional view.
FIG. 4A is a partially enlarged cross-sectional view when forming the keyhole, and FIG.

[Explanation of symbols]

 1 Torch 2 Ceramic Cap 3 Torch Main Body 4 Nozzle 5 Tungsten Cathode 5a Tip 6 Shield Gas Passage 7 Gas Passage 8 Plasma Arc Ejection Hole 8a Choke Hole 8b Tapered Hole 9 Steel Pipe 10 Back Wave Bead KH Keyhole

Claims (1)

[Claims] 1. A torch of a plasma keyhole welding apparatus is rotated along the outer circumference of a butt end of a pipe to be welded, and a pulsed plasma arc and plasma gas are ejected from the torch according to a DC pulsed plasma current. In the plasma keyhole welding method in which the butt end of the pipe to be welded is welded over the entire circumference, a soft plasma with a plasma gas flow rate lower than the normal value is jetted from the torch and the outer peripheral side of the butt end of the pipe to be welded. Partial melting by ejecting a pulsed plasma arc and plasma gas from a torch in which the injection interval is increased by the preheating process of partial melting of the Plasma keyhole welding method comprising a welding step of keyhole welding the butt ends of the welded pipes .