JP3189678B2 - Nozzle for plasma arc welding and plasma arc welding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nozzle suitable for plasma arc welding of a relatively thin metal material having a thickness of 10 mm or less, and a plasma arc welding method using the nozzle.

[0002]

2. Description of the Related Art The plasma arc welding method is generally applied to welding of stainless steel or high alloy steel having a thickness of about 2 to 10 mm. This plasma arc welding method is characterized in that backside welding from one side is possible and that a weld metal with good cleanliness can be obtained. For this reason, it has been conventionally used for pipe welding of high-grade stainless steel welded steel pipes.

As is well known, the plasma arc welding method described above is a welding method in which the main arc in TIG welding (Tungsten Inert Gas Weld) is contracted to enhance the penetration capability of the main arc.

That is, in the TIG welding method, a distance between a tungsten electrode (cathode) and a material to be welded (anode) is 100 to 100 mm.
In this method, an arc of about 300 A is ignited, and welding is performed while shielding the vicinity of the arc with an inert gas such as Ar. However, in this case, the arc penetration capability is relatively small.

On the other hand, in the plasma arc welding method, welding is performed as follows.

FIG. 1 is a longitudinal sectional view for explaining the plasma arc welding method. As shown in FIG.
A tungsten electrode 2 is arranged in a nozzle 1 for supplying G and an operating gas DG, and Ar
In this method, the working gas DG, which is an inert gas, is supplied at a high speed.

Therefore, in the plasma arc welding method, the diameter between the tungsten electrode 2 and the workpiece M is covered with a high-speed gas stream of the supplied working gas DG, and the diameter is reduced by the cooling effect of the high-speed gas stream. Arc A fires. As a result, the energy density of the arc A increases, so that the penetration capability of the arc A is larger in the plasma arc welding method than in the TIG welding method.

The operating gas DG is naturally distinguished from the shielding gas SG for shielding the molten metal and the arc A from the atmosphere. Most of the operating gas DG is in a plasma state, and is normally ejected from a nozzle hole 10b having a diameter d of about 1 to 4 mm.

Further, the nozzle body 10 has a cooling water passage 10c therein, and by supplying the cooling water W, the nozzle body 10 is prevented from being melted and damaged by the heat of the working gas DG converted into plasma. In some cases, the nozzle body 10 does not have the above-described cooling water passage 10c therein.

As is apparent from the above description, the plasma arc welding method has a large arc A penetration capability. For this reason, the plasma arc welding method can be applied to welding up to a maximum thickness of about 10 mm, while the TIG welding method can be applied only to welding having a maximum thickness of about 4 mm, and the welding speed is dramatically improved. There is an advantage of gaining.

However, in the plasma arc welding method, when the current is increased to further increase the penetration capability of the arc A and high-speed welding is performed, the voltage of the nozzle body 10 increases, and an abnormal discharge phenomenon called a series arc occurs. It has the disadvantage of occurring.

That is, as shown in FIG. 1 described above, unlike the normal arc A which is ejected from the center of the nozzle body 10, the gap between the tungsten electrode 2 and the nozzle body 10, and between the nozzle body 10 and the material M to be welded. An abnormal arc SA referred to as the series arc is generated during the period.

The limit current value at which the series arc SA begins to occur slightly varies depending on the material of the nozzle body 10, the flow rate of the working gas DG, the diameter d of the nozzle hole 10b, and the like.
Although not always constant, it always occurs as the current increases.

[Means to solve the problem]

When the series arc SA occurs, the current value of the normal arc A decreases, and a welding defect such as a non-penetrating bead occurs.

Various methods have been conventionally proposed to prevent this series arc (for example, JP-A-63-5882 and JP-A-5-8047).

[0016]

However, these conventional methods are insufficient in their effect, and in fact, it is desired to provide a method capable of more effectively preventing the generation of a series arc.

The present invention has been made in view of the above circumstances, and has as its object to provide a plasma arc welding nozzle capable of increasing a limit current value at which a series arc starts to be generated as much as possible, and a nozzle for the plasma arc welding. And a plasma arc welding method using the same.

[0018]

Means for Solving the Problems The inventors have conducted various experiments to solve the above problems, and as a result, have found the following, and have accomplished the present invention.

As described above, the series arc is a series arc of an arc between the tungsten electrode and the nozzle body and an arc between the nozzle body and the material to be welded. Therefore, if one of the two arcs is prevented, an electrically closed circuit will not be formed, so that the other arc will not be generated and the series arc itself can be prevented. Should be.

Therefore, the present inventors have paid attention to the arc between the tungsten electrode and the nozzle body.

That is, in order for an arc to be generated between the tungsten electrode and the nozzle body, it is necessary that all of the cathode point, the arc column, and the anode point be stably maintained.

Here, the cathode spot is a red-hot tungsten electrode, and is constantly and stably maintained in a state having electron emission capability during the plasma arc welding.

Although the arc column tends to be blown toward the nozzle hole exit end (downward) of the nozzle body by the supplied operating gas flow, there is no doubt that the arc is in an environment where the arc can be stably maintained.

Therefore, what matters here is the anode point (nozzle body). In order to stably maintain the anode spot, it is necessary that the cation layer formed by the impact of the inflow of electrons stably stays on the anode surface. In other words, the arc is neutral as a whole, and a cation flow is generated in a direction opposite to the electron flow, and the source of the cations is the anode point.

From this viewpoint, considering whether the inner wall surface of the nozzle hole of the nozzle body can be an anode point, it is estimated that the temperature of the inner wall surface of the nozzle hole is considerably lower because the nozzle body is usually water-cooled from the inside. Is done. In addition, the cation layer on the inner wall surface of the nozzle hole is relatively unlikely to be stably maintained because an operating gas such as Ar gas flows in the nozzle hole.

However, the fact that a series arc is generated when the current is increased means that when the current value exceeds a certain limit, the cation layer on the inner wall surface of the nozzle hole is stabilized and the anode point is stably maintained. It is thought to be like this.

From the above, as a result of various studies on means for preventing the inner wall surface of the nozzle hole from becoming the anode point,
The surface roughness of the inner wall surface of the nozzle hole is measured according to JIS-B601 (19).
It has been found that when the center line average roughness (Ra ₇₅ ) measured by the method specified in 94) is 2 μm or less, the generation limit current of the series arc can be increased.

It has also been found that butt welding or pipe making using this nozzle can increase the welding speed and increase productivity.

The gist of the present invention based on the above findings is the following (1) and (2) a plasma arc welding nozzle and a plasma arc welding method.

(1) A metal-made plasma arc welding nozzle in which an electrode is disposed at an axial center portion, wherein the surface roughness of the inner wall surface of the nozzle hole formed in the metal nozzle body is reduced. And a center line average roughness (Ra ₇₅ ) of 2 μm or less.

(2) A plasma arc welding method characterized by performing butt welding or pipe welding using the nozzle of (1).

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a nozzle according to the present invention will be described in detail with reference to FIG.

As shown in FIG. 1, the nozzle 1 of the present invention
For example, the inner wall surface 10a of the nozzle hole 10b formed in the axial center of the nozzle body 10 made of metal such as copper, silver, gold, platinum, and palladium is measured by a method specified in JIS-B601 (1994). Center line average roughness (Ra ₇₅ )
Is mirror-finished to a surface roughness of 2 μm or less.

As described above, the inner wall surface 10 of the nozzle hole 10b is
The reason why the series arc SA does not occur when the surface of a is mirror-finished is unknown in detail, but is presumed to be due to the following reason.

That is, the inner wall surface 10a of the nozzle hole 10b
Smoothing the surface roughness of the nozzle, the flow velocity near the surface of the working gas DG supplied to the nozzle hole 10b increases, the stable stagnation of the cation layer is suppressed, and the anode is not maintained stably. As a result, the welding current increases. It is presumed that the series arc SA no longer occurs.

Here, if the surface roughness of the inner wall surface 10a of the nozzle hole 10b exceeds 2 μm in the center line average roughness (Ra ₇₅ ), the generation limit current of the series arc SA cannot be increased. In other words, when the welding current is increased,
A series arc SA occurs. This is clear from the experimental results shown in FIG.

FIG. 2 shows the results of examining the limit current of the series arc SA when the surface roughness of the inner wall surface 10a of the nozzle hole 10b, that is, the center line average roughness (Ra ₇₅ ) is variously changed. .

As is clear from FIG. 2, when the nozzle body 10 having a surface roughness of the inner wall surface 10a of 3.5 μm or more with a center line average roughness (Ra ₇₅ ) exceeding 2 μm is used, the series arc SA The generation limit current is 160A. On the other hand, when the nozzle body 10 having a surface roughness of 2 μm or less in center line average roughness (Ra ₇₅ ) is used, the generation limit current of the series arc SA is 200 A, and welding with a large current is difficult. It turns out that it is possible.

The experiment was conducted on a SUS304 having a thickness of 3 mm.
The diameter d of the nozzle hole 10b is 2.4 m for a plate material made of
The operation was performed under the condition that a normal temperature Ar gas was supplied at 1 liter / min as an operating gas DG using a copper nozzle body 10 of m.

As a method of increasing the generation limit current of the series arc SA, a method of increasing the flow rate of the working gas DG or a method of increasing the nozzle hole 10 as in the related art has been used.
There is a method of increasing the diameter d of b.

However, the former method has a problem in the formation of a weld bead, and if the flow rate is too large, welding defects such as undercut occur frequently. Therefore, there is a limit to the increase in the flow rate, and the series arc SA generation limit current cannot be increased much.

In the latter method, if the diameter d of the nozzle hole 10b is too large, the flow rate of the working gas becomes slow, so that the diameter of the arc A is reduced, and the penetrating ability is reduced. For this reason, there is a limit in increasing the diameter d.

The nozzle of the present invention, as shown in FIG. 1, all the inner wall surface 1 0 a surface roughness of the nozzle holes 1 0 b bored in the nozzle body 10, the center line average roughness (Ra ₇₅ ) It is most preferable to finish the mirror surface of 2 μm or less. However, it is not always necessary to finish the entire surface to a mirror surface of 2 μm or less. For example, even if the surface where the series arc SA is generated is confirmed by conducting an experiment in advance and only the surface of that portion is finished to a mirror surface of 2 μm or less. Good. This occurrence site of series arc SA depending on the shape and size of the nozzle holes 1 0 b is from slightly different. In this case, the processing cost can be reduced as compared with the case where the entire surface is finished to a mirror surface of 2 μm or less, so that an increase in the manufacturing cost of the nozzle itself can be suppressed.

When the nozzle of the present invention is used to perform plasma arc welding along the contact line of mutually butted abutting plate members, or both metal strips continuously bent into an open pipe shape by a group of forming rolls. When pipe contact welding is performed by plasma arc welding of the contact portion of the edge, welding with a large current can be performed without concern about generation of a series arc. As a result, the welding speed can be increased, and not only the productivity is significantly improved, but also a sound welded joint can be obtained.

[0045]

Example: Butt welding of a SUS304 steel plate having a thickness of 2 mm, an outer diameter of 50.8 mm, a thickness of 2 mm and a thickness of 3 mm.
SUS304 pipe was welded.

At this time, the nozzle d is 2.4 mm and 3.2 mm as shown in FIG.
A nozzle of the present invention variously changed in the range of 0.7~2.0μm all inner wall surfaces 1 0 a surface roughness of the drilled nozzle holes 1 0 b at _{Ra 75} to, the surface roughness Ra It is 3.6 at ₇₅
Commercially available conventional nozzles in the range of 12.5 μm were used.

Further, a normal temperature Ar gas was used as an operating gas, and its flow rate was changed in two ways, 1.0 liter / min and 1.2 liter / min. Further, the welding current was varied in the range of 120 to 270A. Furthermore, the welding speed is 1 m / m for a 2 mm thick material.
in, and 0.8 m / min for a material having a thickness of 3 mm.

Incidentally, for comparison, the above-mentioned Japanese Patent Application Laid-Open No.
No. 47, which has a coating of 5 μm.
Welding was performed in the same manner as described above, using a nozzle which was a CBN coating of m.

Then, the occurrence of a series arc was visually observed, and the limit current value at the time of occurrence of the series arc was confirmed. Further, the occurrence of undercut in the obtained welded joint was examined, and a case where neither a series arc nor an undercut occurred was evaluated as “good”.

The results are shown in Table 1 together with the specifications of the nozzle body used and other welding conditions.

[0051]

[Table 1]

As is clear from the results shown in Table 1, in the plate butt welding, the flow rate of the operating gas was 1.0 liter / m.
In the case of "in", the generation limit current value of the series arc when a commercially available conventional nozzle was used was 140 to 160 A (see Nos. 1 to 8).

On the other hand, when the nozzle of the present invention was used, the maximum value of the generation limit current of the series arc was 220 A (see Nos. 9 to 16).

Further, the surface roughness of the inner wall surface of the nozzle hole is Ra.
_When a commercial conventional nozzle of ₇₅ and 5.8 μm is used and the flow rate of the working gas is increased to 1.2 liter / min, the maximum value of the series arc generation limit current is from 160 A to 180 A.
, But undercuts occurred and sound welded joints could not be obtained (Nos. 5, 6 and 18, 19)
reference). In the case of the CBN coating nozzle, the generation limit current value of the series arc only increased from 160 A to 180 A (see No. 17).

In pipe welding, the limiting current value of the series arc at a wall thickness of 2 mm was 160 A for a commercially available conventional nozzle having an inner wall surface of the nozzle hole having a surface roughness of Ra ₇₅ and 5.8 μm ( Nos. 21 and 22). On the other hand, the limit current value for series arc generation when the nozzle of the present invention was used was 220 A (see Nos. 23 and 24).

Further, the generation limit current value of the series arc in the case of a wall thickness of 3 mm is 220 A for a commercially available conventional nozzle whose surface roughness of the inner surface of the nozzle hole is Ra ₇₅ and 6.1 μm.
(See Nos. 25 and 26). On the other hand, when the nozzle of the present invention is used, the generation limit current value of the series arc is
270A (see Nos. 27 and 28).

[0057]

According to the present invention, welding with a large current becomes possible. As a result, the welding speed can be increased, and the productivity is improved. In addition, a sound welded joint can be obtained under high-speed welding.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view showing an example of a plasma arc welding nozzle.

FIG. 2 is a diagram showing a relationship between a generation limit current of a series arc and a surface roughness of an inner wall surface of a nozzle hole formed in a nozzle body.

[Explanation of symbols]

 1: nozzle, 2: tungsten electrode, 10: nozzle body, 10a: inner wall surface of nozzle hole, 10b: nozzle hole, 10c: cooling water passage, 10d: packing, A: arc, SA: series arc, M: welded Material, W: cooling water, SD: shielding gas, DG: operating gas.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) B23K 10/00 B23K 10/02 H05H 1/34

Claims (2)

(57) [Claims] 1. A metal plasma arc welding nozzle having an electrode disposed at an axial center thereof, wherein a surface roughness of an inner wall surface of a nozzle hole formed in a metal nozzle body has a surface roughness of: A nozzle for plasma arc welding, wherein the center line average roughness (Ra ₇₅ ) is 2 μm or less. 2. A plasma arc welding method for a metal material, comprising performing butt welding or pipe making welding using the nozzle according to claim 1.