JPH0716753A - Method for plasma arc welding - Google Patents

Abstract

PURPOSE:To provide the weld zone of full penetration without generating the sag of the molten pool and the projecting bead mainly at the part to be welded where the welding position is successively changed. CONSTITUTION:In a welding method where the plasma is the heat source, the capacity of the plasma arc is periodically changed by periodically changing the flow rate of the plasma gas. It is preferable that the period to change the flow rate of the plasma gas is relatively low below 10Hz. Concerning the flow rate of the plasma gas, it is preferable that the peak flow rate is >=1 litre/min. in the case of the plasma key hole welding, while the peak flow rate is <=3 litre/min. in the case of the non-key hole welding. As for the mechanism to fluctuate the flow rate of the plasma gas, e.g. a needle valve for setting the base gas, a needle valve for setting the peak gas, and an ON-OFF solenoid valve are provided in parallel between a gas adjuster and a plasma arc welding machine, or between the gas adjuster and the welding torch. This welding method is mainly suitable for the case of the plasma arc welding of the part to be welded where the welding position is successively changed.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a plasma welding method,
The present invention mainly relates to a welding method suitable for performing plasma welding on a welding application site such as a medium- to large-diameter fixed pipe or a tank in which the welding posture is successively changed.

[0002]

2. Description of the Related Art Plasma welding is a welding method using a high energy beam as a heat source, and keyhole welding using plasma as a heat source is adopted from the viewpoint of improving efficiency.

Keyhole welding is the most characteristic backside welding method of plasma welding. A plasma arc with a relatively large current is narrowed down and locally heats the base material, and the plasma gas ejected by restraint strongly pushes down the molten metal, resulting in a small hole (keyhole). As the welding progresses, the tip of the keyhole advances while melting the base metal, but since the molten metal moves backward along the molten pool and blocks the rear of the keyhole, the keyhole is a welding method that maintains a nearly constant shape. is there
(See Figure 7).

Conventionally, in the case of plasma keyhole welding,
As seen in JP-A-60-27473, a pulsed current was used to control the plasma arc.

However, in this method, since the current is controlled, the rise and fall times of each pulse are instantaneous, so that the molten pool is easily disturbed, and when the current is high, the molten width is larger than the penetration depth. However, there is a problem in that it is less effective in preventing the burn-through of the molten pool and controlling it in the case of, for example, all-position welding.

That is, in full circumference welding of medium or large diameter horizontal fixed pipes, tanks, etc., all-position welding from downward to vertical to upward is performed from the first layer to the final layer. When keyhole welding was applied, there were problems such as drooling of the molten pool and convex beads, and shallow penetration.

The present invention solves the above-mentioned problems of the prior art, and mainly at a welding target portion where the welding posture changes successively, a deep-penetration weld portion is formed without causing drooling of the molten pool or convex beads. The purpose is to provide a plasma welding method that is obtained.

[0008]

As a means for solving the above-mentioned problems, the present invention, in a welding method using plasma as a heat source, periodically changes the plasma gas flow rate to periodically change the plasma arc capacity. The main point is the plasma welding method, which is characterized in that

The present invention will be described in more detail below.

[Action]

In the conventional plasma welding, the plasma gas flow rate is constant, but in the present invention, the ability of the plasma arc is periodically changed by periodically changing the plasma gas flow rate. . As the plasma gas flow rate increases, the gas stream turned into plasma is jetted at a high speed, and the nozzle and the thermal pinch force strongly push down the molten metal. Therefore, by periodically changing the plasma gas flow, it is possible to periodically change the capability of the plasma arc, that is, the penetration amount of the molten metal.

The cycle for changing the flow rate of the plasma gas is not particularly limited, but it is currently difficult to set the cycle to 10 Hz or less, and at 10 Hz or more, the molten pool is disturbed and a sound welding bead is obtained. Hateful. Therefore, the cycle is 10 Hz or less, preferably 3 Hz or less.

As the peak flow rate of the plasma gas,
In the case of keyhole welding, the plasma jet strength is insufficient at 1 liter / min or less, and it is difficult to perform stable keyhole welding. In the case of non-keyhole welding for any thick plate, if the peak flow rate is set to 3 liters / min or more, the splash of molten metal becomes remarkable and porosity occurs in the weld metal. Easy,
As a result of being severely cut or gouging,
It is difficult to stably obtain a good weld bead. Therefore, the peak flow rate of the plasma gas is preferably 1 liter / min or more in the case of keyhole welding and 3 liter / min or less in the case of non-keyhole welding.

The flow rate of the plasma gas at the time of the base is preferably 3 liters / min or less in consideration of blowout of molten metal, prevention of burn-through, and arc stability during welding.

The mechanism for pulsating the flow rate of the plasma gas can be appropriately configured, and for example, the mechanism having the configuration shown in FIG. 8 can be cited.

That is, normally, the plasma gas passes through a gas regulator from a gas cylinder and then directly enters the plasma welding machine. Then, the welding machine is connected to the welding torch together with the cooling hose and the welding cable, and plasma welding is performed.

The illustrated mechanism used to practice the invention is
It is a mechanism in which a base gas setting needle valve, a peak gas setting needle valve, and an ON-OFF solenoid valve are provided in parallel between the gas regulator and the plasma welding machine or between the gas regulator and the welding torch. Thus, the peak gas setting needle valve is closed at the base time, and the peak gas setting needle valve is opened at the peak time to open the peak gas setting needle valve, thereby changing the plasma gas flow rate.

The pulse period and pulse height can be freely changed by a solenoid valve. Also, the signal to the solenoid valve can be sent from a separately provided preset circuit, or as shown in the figure, the arc voltage or arc current during welding can be detected,
You may interlock with it. By pulsating the plasma gas flow rate in this manner, a deep-penetration weld bead can be obtained in all welding positions without increasing the bead width so much.

It goes without saying that other conditions for plasma welding are not particularly limited. For example, argon is used as the plasma gas. As the shield gas, argon or helium, or a mixed gas of argon containing hydrogen or CO ₂ is used. Care should be taken not to use hydrogen if cracking due to hydrogen is a problem. Welding current,
The voltage, speed, etc. may be appropriately determined. A filler material such as filler wire may be used.

Further, it can be applied to welding of various shapes and thicknesses of plate materials such as thin plates or thick plates, pipes or tanks, and keyhole welding or non-keyhole welding can be performed alone or in combination. If the first layer welding is sufficient, it is preferable to use keyhole welding. In the case of multi-layer welding, it is desirable that the first layer be keyhole welded and the second and subsequent layers be non-keyhole welded.

Next, examples of the present invention will be described.

[0021]

[Example 1]

In this example, a pipe having a relatively small outer diameter of 216 mm is welded in all positions. The groove is I groove (Fig. 1)
And 1-pass welding was performed under the welding conditions shown in Table 1. As shown in FIG. 2, the peak flow rate of the plasma gas is 2.4 liters / min, the base flow rate is 0.6 liters / min, the plasma arc is strengthened at the peak time, and the keyhole is formed, and the solidification rate at the base time is set. Can be increased to promote bead formation. The period is 1 Hz. Applicable steel material is JIS G3452 200A (outer diameter 216 mm, plate thickness 6 m
m).

[0023]

[Table 1]

[0024]

Example 2

In this example, an I-groove (FIG. 3) having a plate thickness of 12 mm is welded by one pass. Table 2 shows the welding conditions. As shown in Fig. 4, a strong plasma arc is obtained by increasing the peak plasma gas flow rate to 4.2 l / min with respect to the base flow rate of 0.4 l / min, and at a low cycle of 1 Hz. Bead formation can be facilitated. The applicable steel material is JIS SM490 (plate thickness 12 mm).

[0026]

[Table 2]

[0027]

Example 3

This example is an example in which keyhole welding and non-keyhole welding are selectively used by controlling plasma gas. Table 3 shows the welding conditions. Tube diameter 30 inches (76.
For the 2 cm) tube, the groove is Y-groove (Fig. 5), the plasma gas peak flow rate Pmax is increased on the relatively high current side in the first pass, and keyhole welding is performed. Controlled to a low level, non-keyhole welding was used for multi-pass welding. Applicable steel material is API 5L-X65
(Tube diameter 30 inches, plate thickness 19 mm).

[0029]

[Table 3]

In each of the examples, stable welding was possible, the bead shape was good, and no welding defects were observed.

[0031]

As described in detail above, according to the present invention,
Since the plasma gas flow rate is changed cyclically, it can be applied to the welding of base materials having various shapes and plate thicknesses, and in particular, the welding target portion where the welding position changes sequentially does not cause drooping or convex beads of the molten pool, Moreover, a deep-penetration welded portion can be obtained, and high efficiency can be achieved.

[Brief description of drawings]

FIG. 1 is a sectional view showing a joint shape according to a first embodiment.

FIG. 2 is a diagram showing changes in the plasma gas flow rate in the first embodiment.

FIG. 3 is a sectional view showing a joint shape according to a second embodiment.

FIG. 4 is a diagram showing changes in the plasma gas flow rate in the second embodiment.

FIG. 5 is a cross-sectional view showing a joint shape and a formation method in Example 3.

FIG. 6 is a diagram showing changes in the plasma gas flow rate in the third embodiment.

FIG. 7 is a diagram illustrating keyhole welding.

FIG. 8 is a diagram illustrating an example of a mechanism for pulsating the plasma gas flow rate.

Claims (4)

[Claims] 1. In a welding method using plasma as a heat source,
A plasma welding method characterized in that the capacity of the plasma arc is periodically changed by periodically changing the plasma gas flow rate. 2. The period for changing the plasma gas flow rate is 10H.
The method of claim 1 having a relatively low period of z or less. 3. The plasma gas flow rate according to claim 1 or 2, wherein the peak flow rate is 1 liter / min or more in the case of plasma keyhole welding, and the peak flow rate is 3 liters / min or less in the case of non-keyhole welding. The method described. 4. A base gas setting needle valve, a peak gas setting needle valve and an ON-OF between a gas regulator and a plasma welder or between a gas regulator and a welding torch.
The method according to claim 1, 2 or 3, wherein the F electromagnetic valve is provided in parallel to change the plasma gas flow rate.