JP2013046932A - Shielding gas for mag welding, method for mag welding, and weld structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a shielding gas for metal active gas (MAG) welding, a method for MAG welding and a weld structure, in which quality in welding a narrow bevel is improved.SOLUTION: The shielding gas for MAG welding welds a high chromium steel containing 8-13 wt.% Cr using a solid wire containing 8-13 wt.% Cr in a one pass per layer manner to weld a narrow bevel in which a ratio of a gap W1 of the bevel between base metals to a thickness H1 of the pair of base metals is 0.4 or less and an angle θ1 of the bevel is 10° or less. The shielding gas for MAG welding is formed of a three-component mixture gas composed of: 5-17 vol% carbon dioxide gas; 30-80 vol% helium gas; and the balance argon gas.

Description

  The present invention relates to a shielding gas for MAG welding of high Cr steel, a MAG welding method, and a welded structure.

In recent years, power generation equipment such as power generation turbines and boilers tend to be used under high temperature and high pressure in order to improve thermal efficiency. For this reason, high Cr steel (for example, 9Cr steel, 12Cr steel) excellent in strength at high temperatures has been developed as a constituent material of power generation equipment.
And because welding is useful for the production of power generation equipment, welding technology for high Cr steel has been developed. For example, a technique for improving the arc stability during welding by incorporating a rare earth metal into a welding material for mag-welding high Cr steel is disclosed (see Patent Document 1).

JP 2001-219292 A

However, the above-mentioned welding materials are not always easy to manufacture with a suitable rare earth metal content, and are more expensive than general high Cr steel mag welding materials. It is difficult to secure.
An object of this invention is to provide the shielding gas for MAG welding, the MAG welding method, and the welded structure which aimed at the quality improvement in narrow groove welding.

  The shielding gas for mag welding according to one aspect of the present invention is a high-Cr steel containing 8 wt% or more and 13 wt% or less of Cr, and a solid wire containing 8 wt% or more and 13 wt% or less of Cr. The ratio (W1 / H) of the thickness H1 of the pair of base materials to the gap interval W1 between these base materials (W1 / H) is 0.4 or less, and the groove angle θ1 is 10 °. Shielding gas for MAG welding for welding the following narrow gaps, which is 5% to 17% by volume carbon dioxide, 30% to 80% by volume helium gas, and the balance is argon gas. It consists of 3 kinds of mixed gas.

  ADVANTAGE OF THE INVENTION According to this invention, the shielding gas for MAG welding, the MAG welding method, and the welded structure which aimed at the quality improvement by narrow groove welding can be provided.

It is sectional drawing showing an example of the shape of a narrow groove. It is sectional drawing showing an example of the shape of a general groove. It is sectional drawing showing the welding state in a narrow groove | channel. It is a photograph showing the cross section of the test body of Example 3 (welding with ternary shielding gas of He—Ar—CO ₂ ) of the present invention. Is a photograph showing a cross section of the test body (welding binary shielding gas Ar-CO ₂₎ Comparative Example 1 of the present invention.

Hereinafter, embodiments of the present invention will be described.
The present inventors examined a shielding gas for mag welding of high Cr steel. As a result, by using a ternary shielding gas of He—Ar—CO ₂ , it becomes possible to perform MAG welding with excellent arc stability, weld metal wettability, and penetration of the open tip even in a narrow groove. I found out.

  Mag welding is a type of arc welding. In arc welding, an arc discharge is generated between a base material and an electrode (welding wire), and the base material and the welding wire are melted and joined by the high temperature of the arc. In MAG welding, an arc generated from an electrode (welding wire) is covered with a shielding gas in which inert gas and carbon dioxide gas are mixed. As a result, the arc is stabilized and the atmosphere is prevented from entering the molten metal.

  Narrow groove welding means, for example, arc welding of a gap (groove) having a small angle facing or contacting the end of a thick plate at a small interval compared to the plate thickness. Here, in particular, it is considered to perform multi-layer welding of the cleaves with one pass for each layer. Multi-layer welding (multi-layer welding) is welding in which a plurality of layers of weld beads are stacked.

FIG. 1 is a cross-sectional view showing an example of the shape of a narrow groove. The end surfaces of the base materials 11 and 12 such as thick plates have a gap (gap) 13 and are arranged so that their lower ends are in contact with each other. The distance W1 (for example, 20 mm or less) between the base materials 11 and 12 at the bottom of the gap 13 is smaller than the thickness H1 (for example, 50 mm) of the base materials 11 and 12 (W1 <H1).
Further, the angle θ1 formed by the wall surface of the gap 13 (end surfaces of the base materials 11 and 12) is small, for example, 10 ° or less.

  FIG. 2 is a cross-sectional view illustrating an example of the shape of a general groove. End surfaces of the base materials 21 and 22 such as thick plates are arranged so as to face each other with a gap (gap) 23. In this example, the angle formed by the wall surface of the gap 23 (end surfaces of the base materials 21 and 22) changes in two stages, θ21 and θ22.

Here, in FIG. 1, the ratio between the thickness H1 of the base materials 11 and 12 and the interval W1 (W1 / H1) is 0.4 or less and the angle θ1 is 10 ° or less is called a narrow groove.
In addition to this, the thickness H1 and the interval W1 may satisfy the following conditions.
When H1 ≦ 200mm, W1 ≦ 20mm
When H1> 200mm, W1 ≦ 30mm

  As shown in FIG. 2, when the angle θ1 is not constant (for example, when the angle θ1 changes in a plurality of stages), the substantially maximum angle is set as the angle θ1. In FIG. 1, the lower ends of the base materials 11 and 12 are in contact with each other. However, even if the lower ends of the base materials 11 and 12 are not in contact with each other, there is no problem as a narrow groove.

  FIG. 3 is a cross-sectional view showing a welding state in a narrow groove. End surfaces of the base materials 31 and 32 such as thick plates are disposed with a gap (gap) 33. A backing plate 34 is disposed below the base materials 31 and 32. The weld beads 41 to 44 are disposed in the gap 33. Since one weld bead 41 to 44 is arranged per layer (in the vertical direction on the paper surface), this welding is a three-layer welding with one pass for each layer.

  If the width W31 of the beads 41 to 44 is sufficiently large with respect to the interval W30 between the base materials 31 and 32, it is considered that the penetration depth of welding in the base materials 31 and 32 is sufficient.

  High Cr steel is an alloy material containing iron as a main component and containing a relatively high concentration (8 to 13% by mass) of Cr. Cr is an important element for improving the corrosion resistance, oxidation resistance and creep strength of a metal material. Less than 8% by weight of Cr has little effect on corrosion resistance. Further, when 13 wt% or more of Cr is contained, δ ferrite is crystallized and the strength and brittleness are lowered. Therefore, high Cr steels used at high temperatures and high pressures such as power generation equipment (power generation boilers, turbine parts) generally contain 8 to 13% by weight of Cr.

  In addition, as high Cr steel, more desirable Cr content is 8.5 to 11% by weight. The shield gas for mag welding according to the present embodiment, which will be described later, can be preferably applied to mag welding with high Cr steel containing 8.5 to 11% by weight of Cr.

  A wire (welding wire) containing 8 to 13% by weight (or 8.5 to 11% by weight) of Cr, which is a common metal (same type of metal), is used for welding high Cr steel (common metal welding). ).

  However, compared with low carbon steel, etc., welding of high Cr steel is not always easy, and narrow groove welding (especially, multi-layer (overlay) welding in a narrow groove) is difficult. Let us consider narrow-groove welding of high Cr steel using a general shielding gas for mag welding (a mixed gas containing 20% by volume of carbon dioxide in argon gas). In this case, the slag of high Cr steel is hard and it is difficult to remove the slag at the open tip, so the slag does not float during the next pass welding, and the slag is likely to be caught at the open tip. Further, since the bead wettability is poor, it tends to be a convex bead, and welding defects such as poor fusion are likely to occur.

The shielding gas for mag welding according to the present embodiment is composed of three mixed gases of 5 volume% or more and 17 volume% or less of carbon dioxide gas, 30 volume% or more and 80 volume% or less of helium gas, and the balance is argon gas.
The reasons for limiting the components of shield gas for MAG welding are described below.

  In the arc stability of MAG welding, it is important that an oxidizing gas such as oxygen or carbon dioxide is included in the shielding gas. In order to stabilize the arc in MAG welding, it is important that the arc spot of the arc is stably formed. However, if an oxide is present on the cathode side, the cathode spot is more easily generated. When the shield gas is composed only of inert gas such as argon gas or helium gas, there is no oxidizing gas in the cathode side atmosphere, so oxide formation is poor on and around the molten pool surface, and the cathode spot is It is difficult to stabilize. As a result, arc instability behavior such as excessive arc spreading or wobbling occurs. In particular, in a narrow groove, the arc tends to become unstable due to an excessive spread of the arc and an arc rising to the groove wall. As the arc becomes unstable, welding defects and defects such as poor fusion at the open end, bead shape failure, and spattering will occur.

  When the shielding gas contains carbon dioxide, the oxide that becomes the source of the cathode spot is generated by the oxidizing power of the carbon dioxide gas, and the cathode spot is stabilized. By containing carbon dioxide, the arc itself contracts due to the thermal pinch effect, and the rigidity and directivity of the arc are improved. Therefore, since the shielding gas contains carbon dioxide, the stability of the arc is improved, the generation of spatter is reduced, and the open tip can be effectively melted.

  If the carbon dioxide concentration is less than 5% by volume, the arc stability is not sufficient, and if it exceeds 17% by volume, spatter increases, which is not preferable. That is, if the carbon dioxide content is too high, the arc becomes too tight and the detachment of the droplets from the welding wire is hindered, making the arc unstable and increasing spatter. The shield gas according to the present embodiment has a lower carbon dioxide content than a general MAG welding shield gas (a mixed gas containing 20% by volume of carbon dioxide in argon gas). For this reason, the oxidizing power of the shielding gas is slightly weakened, and bead oxidation and generation of slag on the bead surface are reduced.

  If oxygen is used as the oxidizing gas, its oxidizing power is excessive, and the appearance of welding and the penetration into the base metal are not always good. For this reason, oxygen is not employed as the oxidizing gas of the shield gas for mag welding according to the present embodiment.

  Since helium gas has a higher potential gradient than argon gas, the arc voltage during welding increases, and heat generation by the arc increases. As a result, since the heat input to the base material increases, the melting of the base material is accelerated, the amount of melting of the base material increases, and the penetration becomes deep. In the case of a narrow groove, the open tip can be sufficiently melted, sufficient penetration is obtained, and poor fusion is reduced. In addition, the increase in the amount of the base metal melted promotes the generation of metal vapor effective in improving the arc stability from the molten pool, so the arc stability is also slightly improved. In addition, the heat generated by the arc increases, so that the base metal is heated by heat conduction, and the gas pressure during arcing decreases due to the low helium gas density, improving the wettability of the weld metal (bead). To do.

  When the concentration of helium gas is less than 30% by volume, the increase in penetration and the improvement of wettability are not sufficient. On the other hand, if the concentration of helium gas exceeds 80% by volume, arcing is difficult to occur at the start of welding (deterioration of arc start performance) and shielding properties from the atmosphere due to the low density of helium gas. Deteriorates (atmosphere easily enters molten metal).

  As described above, the shielding gas for the narrow gap welding of high Cr steel is 5 to 17 volume% carbon dioxide gas, 30 to 80 volume% helium gas, and the balance is argon gas 3 A seed gas mixture is suitable.

Narrow groove mag welding of high Cr steel can be performed using these three mixed gases.
At this time, it is preferable to perform pulsed MAG welding under welding conditions in which the peak current is 350 to 500 A, the base current is 40 to 100 A, and the pulse frequency is 100 to 400 Hz. Sputtering and fume can be reduced by using pulsed mag welding.

  The peak current secures the electromagnetic pinch force and contributes to the detachment of the droplets from the welding wire. When the peak current is less than 350 A, the electromagnetic pinch force is weak, so that it is difficult to separate from the welding wire until the droplet becomes a large mass. As a result, there is a possibility that a large amount of spatters and fumes are generated because the transition from one pulse to one droplet is lost. On the other hand, when the peak current exceeds 500 A, the arc force that pushes up the droplets becomes too strong, and regular droplet detachment from the welding wire becomes difficult, resulting in one-pulse multiple droplet transfer.

  The base current continues the arc and contributes to the stable shaping of the droplets. If the base current is less than 40 A, arc breaks and short circuits are likely to occur. On the other hand, when the base current exceeds 100 A, the arc force that contributes to the transfer of the droplet increases, and the droplet fluctuates, making it difficult to stably shape the droplet.

  The pulse frequency affects the droplet size per pulse and the synchronization rate between the pulse and droplet transfer. If the pulse frequency is less than 100 Hz, the droplets per pulse become too large, and a short circuit is likely to occur between the droplets and the molten pool. Further, when the pulse frequency exceeds 400 Hz, the droplet transfer deviates from one pulse / one droplet transfer and does not synchronize with the pulse.

  Pulse welding conditions affect the droplet transfer mode, and in turn, spatter and fume generation and weld defects. In other words, if the average current is too small, the penetration of the open tip becomes unsatisfactory in narrow groove welding, resulting in poor fusion. On the other hand, if the average current is too large, the cooling rate of the weld pool is slowed down and hot cracking occurs.

  As described above, the peak current is 350 to 500 A, the base current is 40 to 100 A, and the pulse frequency is 100 to 400 Hz as pulse mag welding conditions for high Cr steel.

Here, by using the three kinds of mixed gases, it becomes easy to set the welding posture to all postures in the mag welding of high Cr steel. The total posture is a general term for all of the downward posture, the horizontal posture, the vertical posture, and the upward posture.
In general, in a horizontal posture, a vertical posture, an upward posture, and particularly in an upward posture, the molten pool hangs down due to gravity, wettability deteriorates, and the bead shape tends to be convex. In multi-layer welding, welding on a convex bead induces welding defects such as poor fusion. Therefore, by using the three types of mixed gas, it is possible to perform welding without impairing the wettability of the weld metal even in all-position welding.

  As described above, when high-Cr steel containing 8 to 13% by weight of Cr is subjected to narrow gap welding with one layer and one pass using a solid wire containing 8 to 13% by weight of Cr, It is preferable to use a three-mixed gas composed of 17% by volume carbon dioxide gas, 30% by volume to 80% by volume helium gas, and the balance argon gas as the shielding gas. Even in narrow gaps, it is possible to perform welding with excellent arc stability, weld metal wettability, and penetration at the open end.

  An example is described about the shielding gas suitable for the mag welding of the high Cr steel which concerns on this embodiment.

[Example 1]
In order to confirm the characteristics and effects of the shielding gas according to this embodiment, confirmation tests of various characteristics were performed.
Using a 9Cr steel plate, mag welding was performed in a narrow groove. At this time, a mixed gas of He gas, CO ₂ gas and Ar gas was used as the shielding gas, and the composition (volume%) was changed. Arc stability, degree of oxidation, slag generation, wettability, spatter generation, and penetration depth were evaluated.
The welding conditions in this test are as follows.

<Welding conditions>
・ Welding method: narrow groove mag welding 1pass / layer
-Welding base material: A182 F91 equivalent (ASTM)
-Welding wire: AWS A5.28 ER90S-B9 equivalent, φ1.2
・ Peak current: 400-500A
・ Base current: 50-70A
・ Pulse frequency: 100-200Hz
・ Welding voltage: 28-32V

In order to clarify the characteristics obtained by the mag welding with the shielding gas according to the present embodiment, as a comparative example, a mixed gas containing 20% by volume of CO ₂ gas in an Ar gas base conventionally used as a shielding gas for mag welding, and Welding was performed using a binary mixed gas of Ar—He.

  Each test item was evaluated as follows, and was divided into four stages: extremely good (“◎”), good (“○”), somehow good (“△”), and bad (“×”).

(1) Arc stability The arc was visually observed during welding and judged from the temporal change state of the arc.
(2) Degree of oxidation The appearance of the bead after welding was visually observed and judged from the degree of bead discoloration.
(3) Slag generation amount The appearance of the bead after welding was visually observed and judged from the area occupied by the slag with respect to the bead.
(4) Wettability The appearance of the bead after welding was visually observed and judged from the shape of the bead in the groove. If the bead is concave, the wettability of the bead in the groove is good, and if the bead is convex (convex bead), the wettability of the bead in the groove is not good.
(5) Spatter generation amount The appearance of the bead after welding was visually observed and judged from the area occupied by spatter with respect to the bead.
(6) Penetration depth Based on the ratio of the width W31 of the beads 41 to 44 (W31 / W30) to the interval W30 between the base materials 31 and 32 in FIG. 3, the penetration depth was evaluated in four stages. The width W31 is the horizontal width of the boundary between the beads 41 and 42.
The above results are shown in Table 1.

The He—Ar—CO ₂ ternary shield gas according to the present embodiment has improved arc stability because carbon dioxide gas is added to the ternary shield gas of Ar—He. Since the Ar-He binary shield gas does not contain an oxidizing gas, the degree of oxidation and the amount of slag generation are good, but the cathode spot becomes unstable, the arc is not stable, and there is a significant amount of sputtering.

The arc stability was insufficient with about 3% carbon dioxide, and the arc stability was good with more than 5% carbon dioxide. Further, the ternary shielding gas of He—Ar—CO _{2 according to} the present embodiment has a carbon dioxide gas content of 15% by volume or less, compared with a shielding gas of 80% by volume Ar-20% by volume CO _2. The amount of oxygen was small, and the degree of oxidation, slag generation, and spatter generation were reduced. When the amount of spatter generated is large, not only the quality is poor, but also the contact tip, shield gas nozzle, and the like are consumed heavily, and the replacement frequency increases, which is not preferable in production.

Furthermore, He because of the inert gas, reduced effect on the mechanical properties of the weld metal, the tensile strength of the weld metal is equivalent to that applied by 80 volume% Ar-20 volume% CO ₂ gas, present It can be said that the toughness is generally improved by the He—Ar—CO ₂ ternary shielding gas according to the embodiment because the amount of oxygen is small.

FIGS. 4 and 5 show test specimens welded in Example 3 (He—Ar—CO ₂ ternary shielding gas) of the present invention and Comparative Example 1 (Ar—CO ₂ binary shielding gas), respectively. It is a photograph showing a cross section. In each of Example 3 and Comparative Example 1, three layers of beads B01 to B03 and B11 to B13 are formed by one layer and one pass.

  As shown in FIG. 4, in Example 3, the wettability is good (the shape of the bead B03 is concave upward), the slag is not found, and the penetration depth is also good. From FIG. 5, in Comparative Example 1, the wettability is poor (bead B13 shape is convex upward), welding defects S1 and S2 involving slag are generated at the open tip, and the penetration of the groove is good. I can't say that. If the bead shape is convex as shown in FIG. 5 (if the wettability is not good), a dent is formed in the outer peripheral portion A1 of the beat, which causes a welding failure such as slag entrainment. On the other hand, when the bead shape is concave as shown in FIG. 4 (when wettability is good), it is difficult to form a dent in the outer peripheral portion A0 of the beat, and welding defects such as slag entrainment are unlikely to occur.

[Example 2]
Next, an example of welding construction of a structure typified by a power generation boiler and a turbine will be described using the He—Ar—CO ₂ ternary shield gas according to the present embodiment.
In recent years, power generation equipment such as power generation turbines and boilers tend to be used under high temperature and high pressure in order to improve thermal efficiency. For this reason, high Cr steel (for example, 9Cr steel, 12Cr steel) excellent in strength at high temperatures has been developed as a constituent material of power generation equipment. Typical examples of components of power generation equipment include turbine piping, valves, and turbine nozzles that serve as passages for high-temperature and high-pressure steam generated in a boiler.

Conventionally, these high Cr steel welded structures are often constructed by coated arc welding or TIG welding with good welding workability and quality, including during factory production and on-site installation. Compared with MAG welding, welding efficiency is inferior. Therefore, by using the He—Ar—CO ₂ ternary shielding gas according to this embodiment, it becomes possible to construct these high Cr steel structures by mag welding with good welding workability and quality. , Manufacturing cost can be reduced. In addition, the manufacturing cost can be further reduced by using a narrow groove design for the groove.

As a quantitative evaluation of manufacturing cost reduction, we present an example of all-position welding of turbine piping. As the welding process, covered arc welding, automatic TIG welding, and automatic mag welding using He—Ar—CO ₂ ternary shielding gas according to the present embodiment were selected, and comparison was made among these three processes. The size of the turbine pipe was 500 A used for the main steam reed pipe and the wall thickness was 50 t. Table 2 shows the parameter values used for calculating the welding time.

The narrow groove and the general groove in this embodiment are shown in FIGS. 1 and 2, respectively, with an angle θ1 = 1 to 6 °, θ21 = 60 to 90 °, θ22 = 10 to 30 °, an interval W1 = 4 to 12 mm, W2 = 2 to 6 mm and thickness H1 = H2 = 50 mm. The groove cross-sectional areas of the narrow groove (FIG. 1) and the general groove (FIG. 2) are about 500 mm ² and about 1500 mm ² , respectively.

For TIG welding, a narrow groove widely used in all-position automatic TIG pipe welding was selected. In addition, narrow gaps similar to TIG welding were selected for MAG welding (conventionally, automatic welding in all positions with high Cr steel narrow gaps is difficult due to poor wettability and penetration at the open end. However, by using the He—Ar—CO ₂ ternary shielding gas according to the present embodiment, narrow gap welding is possible). The groove cross-sectional area of the coated arc welding was three times the narrow groove cross-sectional area. For welding amount, welding efficiency and arc time rate, general values were selected for each welding process in the main welding process.

Table 3 shows the welding time calculated using the parameters in Table 2. The weld line length used in calculating the required welding amount mm ³ was about 800 mm.

As shown in Table 3, by applying automatic mag welding using He—Ar—CO ₂ ternary shielding gas according to the present embodiment, it is about one-third of clad arc welding and about four minutes of TIG welding. It is possible to perform welding in one time.

(Other embodiments)
Embodiments of the present invention are not limited to the above-described embodiments, and can be expanded and modified. The expanded and modified embodiments are also included in the technical scope of the present invention.

11, 12 Base material 13 Gap 31, 32 Base material 33 Gap 34 Backing plate 41-44 Weld bead

Claims (6)

A high-Cr steel containing 8% by weight or more and 13% by weight or less of Cr, and a solid wire containing 8% by weight or more and 13% by weight or less of Cr, in one pass and one pass, A shield for mag welding for welding a narrow groove having a ratio (W1 / H) of the gap H1 between the thickness H1 and the base metal of 0.4 or less and an angle θ1 of the groove of 10 ° or less. Gas,
A shielding gas for mag welding characterized by consisting of a mixed gas of 5 volume% or more and 17 volume% or less of carbon dioxide gas, 30 volume% or more and 80 volume% or less of helium gas, and the balance argon gas. 5% by volume or more and 17% by volume or less of carbon dioxide gas, 30% by volume or more and 80% by volume or less of helium gas, the shielding gas composed of three kinds of mixed gas of argon gas, and 8% by weight or more and 13% by weight or less The ratio of the thickness H1 of a pair of high Cr steels containing 8 wt% or more and 13 wt% or less of Cr and the gap interval W1 between the base metals using a solid wire containing Cr A magnet welding method characterized by welding a narrow groove having (W1 / H) of 0.4 or less and an angle θ1 of the groove of 10 ° or less. The mag welding method according to claim 2, wherein multilayer overlay welding is performed in one pass and one pass. 4. The mag welding method according to claim 2, wherein pulse mag welding is performed under welding conditions of a peak current of 350 A or more and 500 A or less, a base current of 40 A or more and 100 A or less, and a pulse frequency of 100 Hz or more and 400 Hz or less. The mag welding method according to any one of claims 2 to 4, wherein the welding posture is an all posture.   A welding structure produced using the shielding gas for mag welding according to claim 1 or the mag welding method according to any one of claims 2 to 5.

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