JP2009233691A - Plasma welding process and outer gas for use in plasma welding process - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform welding of stainless steel product having a thickness of ≥8 mm so as to stably form a satisfactory back bead by a plasma welding process. <P>SOLUTION: The plasma welding process uses a plasma welding torch in which an insert chip 2 is provided on the circumference of a tungsten electrode 1. A shield cap 3 is provided on the circumference of the insert chip 2. The front end part of the tungsten electrode is located on the inner side of the front end part of the insert chip. A center gas is made to flow into a gap between the tungsten electrode and the insert chip, and an outer gas is made to flow into a gap between the insert chip and the shield cap. When a stainless steel is welded by plasma welding, an inert gas is used for the center gas, and a mixed gas comprising 0.5-2 vol.% by volume of carbon dioxide with the balance consisting of an inert gas is used for the outer gas. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a plasma welding method and an outer gas used in the plasma welding method, and is capable of forming a stable back bead even with a thick material to be welded.

The plasma welding method is classified as a non-consumable electrode type welding method together with the TIG welding method. However, since the thermal concentration is superior to the TIG welding method, the bead width is narrow and welding can be performed at high speed. It can be welded with little distortion.
Moreover, the plasma welding method can perform keyhole welding which is a single-sided back wave welding method using a plasma arc with high energy density.

In keyhole welding, a plasma arc pushes away molten metal and penetrates a base material to form a keyhole. As the welding progresses, this keyhole forms a weld pool by forming a molten pool by moving the molten metal back along the wall surface.
For this reason, the plate | board thickness which can perform the one-pass single-sided welding of I type groove | channel is about 0.6 to 6 mm with a mild steel plate, and is about 0.1 to 8 mm with a stainless steel plate.

FIG. 1 schematically shows an example of a welding torch used in such a plasma welding method.
Reference numeral 1 in FIG. 1 denotes a tungsten electrode. The tungsten electrode 1 is a rod-shaped electrode made of tungsten containing a small amount of a rare earth element oxide such as tungsten or lanthanum oxide.

  This tungsten electrode 1 is surrounded by an insert tip 2. This insert tip 2 is a pipe-like one, and is provided coaxially with a gap with respect to the tungsten electrode 1. Moreover, although not shown in figure, a cooling water circulates through the inside and the insert chip | tip 2 is cooled.

  The insert tip 2 is further surrounded by a shield cap 3. The shield cap 3 has a pipe shape, is spaced from the insert tip 2 and is coaxial.

A center gas made of an inert gas such as argon or helium flows in the gap between the tungsten electrode 1 and the insert tip 2, and hydrogen is added to the inert gas such as argon or helium in the gap between the insert tip 2 and the shield cap 3. The outer gas which consists of mixed gas which added 3-7 vol% is comprised so that it may flow.
The center gas functions as a plasma gas, and the outer gas functions as a shield gas.

  Further, the current from the pilot arc power source 4 is applied to the tungsten electrode 1 and the insert tip 2 to ignite the preliminary plasma, and then the current from the main arc power source 5 is applied to the tungsten electrode 1 and the workpiece 6. The plasma arc is configured to flow from the tungsten electrode 1 to the workpiece 6.

Furthermore, the tip end portion of the tungsten electrode 1 is arranged at a position inside the tip end portion of the insert tip 2 and is in a state where it does not protrude outward from the tip end portion of the insert tip 2.
As a result, the tungsten electrode 1 is encased in a center gas made of an inert gas and is not exposed to an oxidizing gas, and is not oxidized or consumed during welding, and no spatter is generated. High quality welding for a long time is possible, and the running cost can be reduced.
For this reason, the plasma welding method is widely used mainly in welding construction for manufacturing pressure vessels, piping and joints.

However, in the conventional plasma welding method, it is difficult to stably form a back bead in welding of stainless steel and carbon steel having a thickness of 8 mm or more, and due to the influence of gravity, the weight of the molten metal is reduced. There is a problem that the shape of the back bead is not stable because it cannot be tolerated. Therefore, welding is performed by applying a backing metal to the back side of the welded portion.
In addition, since the back bead is not stable, the finish of the front bead is affected, and there is an inconvenience such as reworking.
JP 2003-31414 A JP 2006-26644 A JP 2004-298963 A

  Therefore, an object of the present invention is to weld a steel material having a thickness of 8 mm or more by a plasma welding method so that a good back bead is stably formed.

To solve this problem,
According to the first aspect of the present invention, an insert tip is arranged around the tungsten electrode, a shield cap is arranged around the insert tip, and the tip of the tungsten electrode is located inside the tip of the insert tip. Outer gas used when plasma welding is performed using a plasma welding torch in which a center gas composed of an inert gas flows in the gap between the electrode and the insert tip, and an outer gas flows in the gap between the insert tip and the shield cap Because
This outer gas is an outer gas for plasma welding characterized in that it is a mixed gas of carbon dioxide gas 0.5-2 vol% and the balance inert gas.

According to a second aspect of the present invention, an insert tip is disposed around the tungsten electrode, a shield cap is disposed around the insert tip, and the tip of the tungsten electrode is located inside the tip of the insert tip. Using a plasma welding torch in which a center gas flows in the gap between the electrode and the insert tip, and an outer gas flows in the gap between the insert tip and the shield cap,
When performing plasma welding, a plasma welding method is characterized in that an inert gas is used as the center gas, and a mixed gas of carbon dioxide gas of 0.5 to 2 vol% and the remaining inert gas is used as the outer gas.

  The invention according to claim 3 is the plasma welding method according to claim 2, wherein a backing metal is not applied to the material to be welded during plasma welding.

  According to this invention, plasma welding is performed by flowing a center gas composed of an inert gas through a welding torch and flowing a mixed gas in which carbon dioxide gas is mixed in an inert gas in an amount of 0.5 to 2 vol% as an outer gas. Penetration is obtained and the back bead can be stabilized. For this reason, since it is possible to weld a steel material having a plate thickness of 8 mm to 12 mm without applying a backing metal, no cost is required for the production and installation of the backing metal (made of copper). In addition, good welding can be easily performed even on an object to be welded such as a pipe or a container which cannot use a backing metal.

  In the present invention, for example, when plasma welding is performed using the plasma welding torch shown in FIG. 1, a center gas composed of an inert gas such as argon or helium or a mixed gas thereof is provided in the gap between the tungsten electrode 1 and the insert tip 2. In the gap between the insert tip 2 and the shield cap 3, carbon dioxide gas is 0.5 to 2 vol%, preferably 0.6 to 2 vol%, and inert gas 98 to 99 such as argon, helium or a mixed gas of argon and helium. An outer gas composed of a mixed gas of 0.5 vol%, preferably 98 to 99.4 vol% is flowed.

In the present invention, such a combination of the center gas and the outer gas can provide deep penetration and an effect of stabilizing the back bead. When the carbon dioxide gas concentration in the outer gas is less than 0.5 vol% and exceeds 2 vol%, the width of the back bead is not uniform, the bead meanders and unevenness occurs, and the bead becomes unstable.
In addition, even when a mixed gas of inert gas and oxygen is used as the outer gas, or when a mixed gas of inert gas and hydrogen is used, the width of the back bead is uneven, and the meandering and unevenness of the bead And the bead becomes unstable.

The point of using an inert gas as the center gas in plasma welding is known, but the point of using a mixed gas of carbon dioxide gas 0.5-2 vol% and inert gas 98-99.5 vol% as the outer gas is known. Absent.
In TIG welding, it has been proposed to use such a mixed gas as a shielding gas. However, since the welding principle is different and the structure of the welding torch is also different, the effects are also different.

In TIG welding, a mixed gas in which carbon dioxide gas of 0.5 vol% or less is mixed with argon as a shielding gas may be used. In this case, the function of the carbon dioxide gas is to make the convection in the molten pool inward convection and deeply penetrate To do.
On the other hand, in the present invention, the function of carbon dioxide gas when the mixed gas is used as the outer gas has not been fully elucidated, but the surface tension of the molten pool is reduced, thereby reducing the viscosity of the molten metal in the entire molten pool. It is assumed that the keyhole is smoothly formed and the back bead is favorably affected.

  The flow rate of the center gas varies depending on the welding conditions, the type of the material to be welded, etc., but it is usually preferably about 0.1 to 5 liters / minute. Also, the flow rate of the outer gas varies depending on the welding conditions, the type of the material to be welded, etc., but it is usually preferably about 5 to 20 liters / minute.

A direct current is used as the welding current, but a pulse current is preferred. As the pulse current, the current waveform is a rectangular wave, the pulse frequency is 20 to 100 Hz, the base current is 30 to 80 A, the peak current is 30 to 200 A, and the ratio of the peak period to the base period (pulse width) is 1:05 to 1: Although it is desirable to set it as 5, it is not limited to this range.
When the peak current is increased, the spread of the generated plasma arc is narrowed, and a keyhole is easily generated, which is suitable for welding a steel material having a large plate thickness.

The preferred range of the welding speed varies depending on the type and thickness of the material to be welded, but is usually about 3 to 10 cm / min.
The welding posture may be downward, upward, or standing. In the upward and standing postures, lowering the frequency of the pulse current is advantageous for preventing molten metal from dripping and for forming a back bead.
The inclination angle of the torch is desirably about 0 to 30 degrees.

The inner diameter of the tip of the insert tip 2 of the welding torch is important because it affects the spread of the plasma arc to be generated, and it is appropriate to set it to 5 mm or less, preferably about 2 mm.
In addition, the material to be welded is not particularly limited, but since a normal steel material is used and the keyhole is well formed, a steel material having a large plate thickness, such as stainless steel or carbon steel having a thickness of 8 to 12 mm, is used. preferable.

  In the welding of the present invention, it is not always necessary to apply a backing metal to the back side of the material to be welded during welding. This is because the back bead is formed stably. For this reason, good welding can be performed even on a material to be welded to which a backing metal such as a pipe or a container cannot be applied.

Hereinafter, in order to confirm the effect of the present invention, a characteristic confirmation test was performed according to the following test examples.
(Test Example 1)
Under the following welding conditions, plasma welding was performed with a bead-on-plate using a stainless steel plate thickness of 8 mm, and the stability of the back bead was examined.

<Welding conditions>
Welding method: Plasma welding (non-consumable electrode welding)
Welding base material: SUS304, plate thickness 8mm
Welding method: Plasma welding method (downward posture)
Electrode: Tungsten with 2% lanthanum oxide φ4.8mm
Center nozzle base metal distance: 3.5mm
Torch tilt angle: Advance angle 20 degrees Welding current: Peak current = 120A Base current = 50A
Welding speed: 6 cm / min
Pulse width: 50%
Pulse frequency: 50Hz
Nozzle inner diameter: 2mm
Back money: None

The combinations of the center gas and the outer gas used are as follows. All are volume ratios.
1) Center gas: 100% Ar Outer gas: Mixed gas of 97-99.5% Ar and 0.5-3% CO _{2 For} comparison, 2) Center gas: 100% Ar Outer gas: 100% Ar
3) Center gas: 100% Ar Outer gas: Mixed gas of 99-99.5% Ar and 0.5-1% O ₂ 4) Center gas: Mixed gas of 100% Ar and 7% H ₂ Outer gas The mixed gas flow rate of 93% Ar and 7% H _{2 was set} to 1.6 liter / min for the center gas and 10 liter / min for the outer gas.

The results are shown in FIG. In FIG. 2, the photograph which image | photographed the external appearance of a front bead and a back bead is shown, and the pass / fail is judged from the external appearance.
○: Pass: The width of the back bead is uniform, and there is no meandering or unevenness and it is stable.
X: Fail: The width of the back bead is not uniform, and there is meandering and unevenness and it is unstable.
From the results of FIG. 2, it can be seen that a stable back bead can be formed by using a mixed gas of argon 98 to 99.5% and carbon dioxide gas 0.5 to 2% as the outer gas.

(Test Example 2)
Under the following welding conditions, plasma welding was performed with a bead on plate using a stainless steel plate thickness of 12 mm, and the stability of the back bead was examined. As a result, it was confirmed that a stable back bead was formed even with a stainless steel plate having a thickness of 12 mm.

<Welding conditions>
Welding method: Plasma welding (non-consumable electrode welding)
Welding base material: SUS304, plate thickness 12mm
Welding method: Plasma welding method (downward posture)
Electrode: Tungsten with 2% lanthanum oxide φ4.8mm
Center nozzle base metal distance: 5mm
Torch tilt angle: Advance angle 10 degrees Welding current: Peak current = 150A Base current = 100A
Welding speed: 6 cm / min
Pulse width: 20%
Pulse frequency: 20Hz
Nozzle inner diameter: 2mm
Back money: None

Center gas: 100% Ar
Outer gas: Ar-1% CO ₂

Center gas flow rate: 1.7 liters / minute Outer gas flow rate: 15 liters / minute

It is a schematic block diagram which shows the torch for plasma welding in this invention. It is a photograph which shows the result of a test example.

Explanation of symbols

1 ... Tungsten electrode, 2 ... Insert tip, 3 ... Shield cap

Claims (3)

An insert tip is placed around the tungsten electrode, a shield cap is placed around the insert tip, and the tip of the tungsten electrode is located on the inner side of the tip of the insert tip, in the gap between the tungsten electrode and the insert tip. An outer gas used when performing plasma welding, using a plasma welding torch in which a center gas composed of an inert gas is flowed and an outer gas is allowed to flow in the gap between the insert tip and the shield cap,
An outer gas for plasma welding, wherein the outer gas is a mixed gas of carbon dioxide gas of 0.5 to 2 vol% and the remaining inert gas. An insert tip is placed around the tungsten electrode, a shield cap is placed around the insert tip, and the tip of the tungsten electrode is located on the inner side of the tip of the insert tip, in the gap between the tungsten electrode and the insert tip. Using a plasma welding torch that flows the center gas and flows the outer gas through the gap between the insert tip and the shield cap,
When performing plasma welding, a plasma welding method is characterized in that an inert gas is used as a center gas, and a mixed gas of carbon dioxide gas of 0.5 to 2 vol% and the remaining inert gas is used as an outer gas.   3. The plasma welding method according to claim 2, wherein a backing metal is not applied to the material to be welded during plasma welding.

Images