JP2022135634A - One-side welding method and flux-cored wire - Google Patents

Abstract

To provide a one-side welding method that can inhibit the oxidation of back beads without a back shield, can achieve a good back bead shape, and allows improved welding ability, and a flux-cored wire.SOLUTION: The present invention discloses a one-side welding method that welds one side of a weld material by gas metal arc welding and causes melting up to the back side without a back shield. The gas metal arc welding uses a flux-cored wire that contains a shield gas containing an inert gas of 90 vol.% or more, and a metal fluoride. As a polarity for use in the gas metal arc welding, at least a period of straight polarity is provided during the welding.SELECTED DRAWING: Figure 1

Description

The present invention relates to a welding method and flux cored wire for back shieldless, single sided welding, particularly to high Cr content steel.

Structures such as boilers and heat exchangers in thermal power plants are required to have properties such as heat resistance and pressure resistance, so generally high Cr-containing steel containing a relatively large amount of Cr is used. Among structures using such high-Cr-containing steel, especially when manufacturing small-diameter pipes by butt welding, it is difficult to access from the inside, so the construction method is arc welding from one side, that is, the outside surface of the pipe. is necessary. This construction method is generally called single-sided welding or single-sided welding.

For example, in Patent Document 1, a welding material in which the amount of chromium is increased by 2.50% by mass or more and 6.00% by mass or less than the base material having a chromium content of 18% by mass or more and 25% by mass or less is used. , discloses performing pipe butt welds by gas tungsten arc welding. The above Patent Document 1 describes the use of _N2 or Ar gas as the back shield gas.
Further, Patent Document 2 discloses a welding method in which chromium-containing steel having a Cr content of 9 to 19% by mass is melt-welded while performing back shielding using argon gas. Patent Literature 2 describes that the back shield is an essential measure for preventing deterioration of corrosion resistance due to oxygen and nitrogen in the atmosphere being taken into the welded portion.

In this way, when single-sided welding of high Cr-containing steel is performed, back shielding is performed by filling the back side of the member to be welded (hereinafter also referred to as the base metal) with shielding gas during welding of the first layer, and the back bead, In other words, it is common to prevent the Uranami from being oxidized.
As the shielding gas used during welding, in order to protect the front side bead and arc, a normal shielding gas for welding such as He gas, Ar gas, _CO2 gas, and mixed gas of these is generally used. used for purposes. Ar gas is generally used to protect the back bead.

If single-sided welding is performed without using a back shield and only by supplying shield gas from the welding surface, that is, from the base metal surface side, the back bead will oxidize, making it impossible to produce a sound welded joint. In addition, since the back shield has the effect of supporting the molten pool with its pressure and cooling effect, burn-through may occur when single-sided welding is performed without using the back shield.
On the other hand, if a back shield is used, a sound back bead without oxidation can be obtained, but this method requires a large amount of gas because the back side of the base material, for example, the inside of the pipe, is filled with the shield gas. In addition, it is necessary to prepare different shielding gases for the first layer and the remaining layer, which poses a problem of cost reduction.

Patent Document 3 discloses a flux-cored wire in which the contents of metal oxides and metal fluorides in the wire are controlled. The flux-cored wire described in Patent Document 3 is also applicable to first-layer TIG (Tungsten Inert Gas) welding, and first-layer semi-automatic TIG welding using this wire does not require a back shield, that is, " Back shieldless construction is possible.

Japanese Patent Application Publication No. 2018-529522 JP 2007-254894 A JP 2016-112575 A

By the way, as shown in the above Patent Documents 1 to 3, TIG (Tungsten Inert Gas) welding is generally used in the first layer in single-sided welding construction that requires a back shield or construction without a back shield. target.
However, since the TIG welding method cannot increase the welding speed and the welding efficiency is low, further improvement of the welding efficiency is expected. Even when the flux-cored wire described in Patent Document 3 is used, the TIG welding method is applied to the first layer, so further improvement of the welding ability is a problem.
Further, a welding method capable of suppressing oxidation of the back bead and obtaining a favorable back bead shape without using a back shield and thereby performing welding at low cost is further disclosed. Improvement is required.

The present invention has been made in view of such problems, and is capable of suppressing oxidation of the back bead without a back shield, obtaining a favorable back bead shape, and improving welding performance. It is an object of the present invention to provide a welding method for single-sided welding and a flux cored wire.

The present inventors have made intensive studies to suppress melt-through of molten metal. It has been found useful to use a flux-cored wire, or flux-cored wire, and to use a welding method in which the polarity used in gas metal arc welding has at least a period of positive polarity during welding. In addition, the present inventors have found that by using a flux cored wire containing a metal fluoride, even the backside bead can be covered with slag, and oxidation of the backside bead can be suppressed without a back shield. I found
The present invention has been made based on these findings.

The above object of the present invention is achieved by the following configuration [1] relating to a single-sided welding method.

[1] A welding method for single-sided welding in which one side of a member to be welded is welded by gas metal arc welding and the back surface is melted without a back shield,
The gas metal arc welding is
a shielding gas containing 90% by volume or more of an inert gas;
Using a flux cored wire containing a metal fluoride,
As the polarity used for the gas metal arc welding,
A welding method for single-sided welding, characterized in that there is at least a period of positive polarity during welding.

Further, preferred embodiments of the present invention relating to the welding method for single-sided welding relate to the following [2] to [7].
[2] The welding method for single-sided welding according to [1], wherein the welding is performed with the DC positive polarity.

[3] The single-sided welding method according to [1] or [2], wherein the metal fluoride is an alkaline earth metal fluoride.

[4] Among the flux components contained in the flux cored wire, the content of flux components excluding metals and metal fluorides is 0.5% by mass or less with respect to the total mass of the wire. , the welding method for single-sided welding according to any one of [1] to [3].

[5] The single-sided welding according to any one of [1] to [4], wherein the flux cored wire contains a flux component having a boiling point of 800° C. or higher and 1400° C. or lower. Method.

[6] The welding method for single-sided welding according to any one of [1] to [5], wherein the flux cored wire contains Mg.

[7] The flux cored wire is
Containing a flux of 4% by mass or more and 30% by mass or less with respect to the total mass of the wire,
The single-sided welding according to any one of [1] to [6], wherein the flux contains 2% by mass or more and 15% by mass or less of non-metallic flux with respect to the total mass of the wire. welding method.

The above object of the present invention relates to the following [8] related to flux cored wires.
[8] A flux-cored wire used in the single-sided welding method according to any one of [1] to [7], wherein the flux-cored wire contains a metal fluoride. .

ADVANTAGE OF THE INVENTION According to the present invention, a welding method for single-sided welding and a flux cored that can suppress oxidation of the back bead without a back shield, obtain a good shape of the back bead, and improve welding performance. Wires can be provided.

FIG. 1 shows test no. It is a drawing substitute photograph which shows the cross section of back bead shape of T6.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail. The present invention is not limited to the embodiments described below, and can be arbitrarily modified without departing from the gist of the present invention.

[1. Welding method for single-sided welding]
The welding method for single-sided welding according to the present embodiment is a method in which one side of a member to be welded is welded by gas metal arc welding and the back surface is melted without a back shield. A shielding gas containing an inert gas and a flux-cored wire containing a metal fluoride are used, and the polarity used in gas metal arc welding has at least a period of positive polarity during welding.
In the present embodiment, welding efficiency can be improved because single-sided welding is performed by gas metal arc welding instead of TIG welding. In addition, since there is no back shield, the cost of gas for the back shield can be reduced.

The welding method according to the present embodiment is preferably used for single-sided welding of high Cr-containing steel, specifically for the first layer in butt welding of pipes. As mentioned above, in butt welding pipes, it is necessary to weld only from one side, since it is difficult to access from the inside. In general, when targeting steel materials with a Cr content of 1.25% by mass or less, back shields are often not applied, but steel materials with a Cr content of 2.25% by mass or more are welded. If targeted, a back shield is applied. Therefore, like the welding method according to the present embodiment, a welding method that can apply gas metal arc welding and can obtain a good back bead shape without a back shield is extremely effective.

As the steel material to be welded, a steel material having a Cr content of 1.5% by mass or more and 28.5% by mass or less can be preferably used. The Cr content of the steel material to be welded is more preferably 2.0% by mass or more, more preferably 2.25% by mass or more, and particularly preferably 2.5% by mass or more. preferable.
On the other hand, the Cr content of the steel material to be welded is more preferably 26% by mass or less, even more preferably 20% by mass or less, and particularly preferably 12% by mass or less.

The shielding gas, polarity, wire type and essential components in the gas metal arc welding method according to the present embodiment will be described in detail below.

<Shield gas: content of inert gas is 90% by volume or more>
The shielding gas used in the gas metal arc welding method according to this embodiment contains 90% by volume or more of inert gas. As a representative shielding gas with a high inert gas content, for example, if 100% by volume Ar shielding gas is used, the effect of suppressing the arc pressure and the effect of suppressing oxygen supply to the surface of the molten pool can be obtained. and burn-through resistance can be improved.

The effect of using 100% by volume inert shielding gas on arc pressure is discussed in detail below.
As the content of dissociable gases, ie active gases such as _CO2 , in the shielding gas increases, energy is consumed in the dissociation of molecules, resulting in a concentrated arc. Such a phenomenon of arc concentration is generally referred to as "arc contraction". When the arc is constricted and the current density increases, the electromagnetic pinch force works and the pressure of the arc increases, so burn-through is likely to occur.
On the other hand, if a shielding gas containing 90% by volume or more of a monatomic rare gas, that is, an inert gas such as Ar or He is applied, the arc spreads, the current density decreases, and the arc pressure decreases. As a result, the occurrence of burn-through can be suppressed.

Further, the influence of the inert gas content on the amount of oxygen supplied to the surface of the molten pool will be described in detail below.
Chalcogen elements such as oxygen and sulfur have the effect of lowering the surface tension of steel. Therefore, by reducing the amount of oxygen in the shielding gas, the molten pool can be maintained with high surface tension, and burn-through can be suppressed.

From the effect of the inert gas as described above, if the inert gas content in the shielding gas is less than 90% by volume, the arc pressure increases and the effect of holding the molten pool decreases, resulting in burn-through. may occur.
Therefore, the shield gas contains 90% by volume or more of inert gas, preferably 95% by volume or more, more preferably 98% by volume or more, and even more preferably 100% by volume.

Ar is preferably used as the inert gas. As the gas other than the inert gas contained in the shielding gas, an oxidizing gas such as carbon dioxide (carbon dioxide; CO ₂ ) or oxygen gas (O ₂ ), N ₂ gas, H ₂ gas, or the like is selected. preferably the balance is unavoidable impurities. Even if the shielding gas is designed not to selectively contain these oxidizing gases, N ₂ gas, H ₂ gas, etc., these gases may be contained as unavoidable impurities in the shielding gas. As for the content of unavoidable impurities, the lower the possibility of hindering the effects of the present invention, the lower the content of unavoidable impurities. is more preferred.

<Polarity: having at least a period of positive polarity during welding>
In gas metal arc welding, if welding is performed with positive polarity, the arc generally tends to be unstable. can. Therefore, in order to suppress melt-through of molten metal, at least a predetermined period of positive polarity should be provided during welding.
As an example of the case of having a predetermined positive polarity period, the positive polarity and the reverse polarity are alternately replaced as in the case of using an AC power supply. , DC positive, ie, Direct Current Electrode Negative (DCEN).
In this specification, the term "positive polarity" (EN: Electrode Negative) means a state in which the welding rod is connected to the negative electrode and the base material is connected to the positive electrode.

<Wire: Flux cored wire>
The gas metal arc welding method according to the present embodiment is intended to suppress oxidation of the back bead without a back shield. In this embodiment, a flux cored wire (FCW) is selected as the wire, and is designed so that even the back bead can be covered with slag and oxidation of the back bead can be suppressed.

Metals and metal compounds added to the flux cored wire (hereinafter also referred to as FCW or wire) according to the present embodiment, chemical components contained, and the like will be described below.

(metal fluoride)
In order to coat the beads with slag using FCW, it is common to add oxides such as TiO ₂ , SiO ₂ and ZrO ₂ to the FCW flux as slag-forming agents. In this embodiment, by adding a metal fluoride as a slag-forming agent to the FCW flux, the back bead can be covered with slag having a uniform thickness, and oxygen supply to the surface of the molten pool can be suppressed. can be done. Therefore, in this embodiment, welding is performed using a flux cored wire containing a metal fluoride. Thereby, the holding force of the weld metal can be increased, and the burn-through resistance is improved.
By using a metal fluoride as a main component in the slag-forming agent, the back bead can be covered with slag having a more uniform thickness, and oxygen supply to the surface of the molten pool can be suppressed. That is, with respect to the total mass of the slag-forming agent, the metal fluoride is preferably more than 50% by mass, more preferably 80% by mass or more, and the slag-forming agent contains only metal fluorides and unavoidable impurities. It is more preferable to consist of

In the present embodiment, as described above, since it has a positive polarity period, the arc tends to be unstable, but by using an alkaline earth metal fluoride as the metal fluoride, stability in positive polarity can be improved.

The effect of alkaline earth metals on arc stability is described in detail below.
Alkaline earth metal oxides have a lower work function than steel and iron-based oxides. Therefore, when a wire containing an alkaline earth metal fluoride is used for welding with positive polarity, the oxygen adsorbed to the flux reacts with the alkaline earth metal fluoride at a high temperature, It changes to an oxide and acts as an electron emission source.
Therefore, if an alkaline earth metal fluoride is used as the metal fluoride contained in the wire, electron emission from the wire can be facilitated and welding stability can be further improved.

In addition, when a substance with a low work function forms a cathode spot, thermionic emission becomes active, and energy corresponding to the work function is taken away from the cathode. As a result, the wire is cooled, so even if the conditions of Joule heat, that is, "welding current x arc voltage" are the same, the wire melting rate changes and the relationship between heat input and wire melting amount improves. becomes possible. That is, it is possible to reduce the amount of deposited metal while making the heat input high enough to melt the groove, so even when welding is performed using pure Ar shielding gas with low energy density, sufficient Melting of the groove becomes possible.
Therefore, it is preferable to use a wire containing a fluoride of an alkaline earth metal whose oxide has a low work function. For example, it is preferable to use a wire containing a fluoride of Ca or Sr.

(Flux components excluding metals and metal fluorides: 0.5% by mass or less)
As described above, in the present embodiment, the flux contains a metal fluoride as a slag-forming agent to suppress the oxygen content on the surface of the molten metal and maintain a high surface tension. It is possible to improve melt-down properties.
Therefore, it is preferable not to actively add oxides to the wire, and the components other than metals and metal fluorides in the flux are preferably 0.5% by mass or less with respect to the total mass of the wire. 0.3% by mass or less is more preferable.
Components other than metals and metal fluorides in the flux include, in addition to impurities contained in the flux, metal oxides other than metal fluorides, metal sulfides, metal nitrides, etc., as non-metal fluxes described later. An organic substance etc. are mentioned.

(Flux component with a boiling point of 800°C or higher and 1400°C or lower)
In welding under a positive inert gas shield, the position of the cathode spot near the tip of the wire is predominantly determined by the position of the low work function material, rather than by the short distance from the base metal, which is the anode. be. Convection currents constantly occur in the droplet, and the low work function material on the droplet surface moves around on the droplet surface. As a result, the current path changes from moment to moment, the electromagnetic pinch force acting on the droplet becomes unstable, and droplet transfer becomes unstable. Therefore, it is preferable to apply a droplet transfer driving force different from the electromagnetic pinch force.
If the wire contains a substance that evaporates in a specified temperature range as a flux component, current flows through the wire during welding, and when the temperature of the wire itself rises due to Joule heating, the temperature rise causes the flux inside the wire. evaporates. Then, the vaporized substance supplies gas pressure toward the tip of the wire, which can promote detachment of the droplet at the tip of the wire. The term "flux component" as used herein refers to each chemical substance included in the wire sheath as flux.

In order to obtain the effect of promoting droplet detachment as described above, it is preferable to add to the wire a flux component that vaporizes in the temperature range between the solid-liquid interface near the tip of the wire and the contact tip. Specifically, the wire used in the present embodiment preferably contains a flux component with a boiling point of 800° C. or higher and 1400° C. or lower, and may contain a flux component with a boiling point of 900° C. or higher and 1350° C. or lower. More preferably, it contains a flux component having a boiling point of 1000° C. or higher and 1300° C. or lower.
Such flux components, that is, chemical substances, include Li with a boiling point of 1330°C, Mg with a boiling point of 1091°C, Zn with a boiling point of 907°C, and _AlF3 with a boiling point of 1260°C.

(Mg)
Among the flux components described above, it is particularly preferable to add Mg metal powder or Mg-containing metal powder. Mg is a strong deoxidizing element that contributes to reducing the oxygen content of the surface of the molten pool and has a very low yield to the weld metal after solidification. It is preferable that the wire used in the present embodiment contain Mg in the flux from the viewpoint of reducing the oxygen content of the surface of the molten pool and further improving burn-through resistance without adversely affecting the performance of the weld metal. . The Mg content in the flux is preferably 1.0% by mass or less, more preferably 0.5% by mass or less, relative to the total mass of the wire. It should be noted that the lower limit of the Mg content is not particularly specified because the effect is exhibited when the Mg content exceeds 0% with respect to the total mass of the wire.

(Flux content: 4% by mass or more and 30% by mass or less)
As will be described later, the welding method according to the present embodiment is suitable for welding high-Cr-containing steel containing a relatively large amount of Cr. In order to control the chemical composition of the weld metal to correspond to that of 9% Cr steel when the wire sheath is made of Cr-free steel, it is necessary to add a large amount of metallic Cr to the flux. can be used to adjust the flux rate.
That is, the required flux rate, that is, the flux content, changes according to the alloy content of the target base material. However, since a steel type containing Cr can be used as the wire sheath, the addition amount of the alloy component and the flux rate do not uniquely correspond.
Therefore, the content of the flux is not particularly limited, and can be designed in various ways according to the composition of the outer layer, the required alloy content of the base material, and the like.

When the flux content is 4% by mass or more, a fixed amount of flux can be easily supplied to the wire sheath during manufacture of the wire. Also, if the flux content is 30% by mass or less, it is possible to prevent the wire sheath from becoming too thin, and to suppress the occurrence of troubles such as breakage during the manufacturing process.
Therefore, the flux content is preferably 4% by mass or more, more preferably 8% by mass or more, relative to the total mass of the wire. The flux content is preferably 30% by mass or less, more preferably 25% by mass or less, relative to the total mass of the wire.

(Content of non-metallic flux: 2% by mass or more and 15% by mass or less)
The non-metallic flux contained in the flux mainly has an effect as a slag-forming agent that forms slag and protects the front and back beads. Therefore, in the present embodiment, the non-metallic flux almost functions as a slag-forming agent. When the nonmetallic flux is 2% by mass or more with respect to the total mass of the wire, the surface of the back bead can be completely covered with slag, and oxidation can be suppressed. On the other hand, when the nonmetallic flux is 15% by mass or less with respect to the total mass of the wire, uniform slag can be formed, and the shape of the back bead after removing the slag is improved.
Therefore, the non-metallic flux in the flux is preferably 2% by mass or more, more preferably 3.5% by mass or more, and preferably 5.5% by mass or more with respect to the total mass of the wire. More preferably, it is particularly preferably 6.0% by mass or more. In addition, the non-metallic flux in the flux is preferably 15% by mass or less, more preferably 7.5% by mass or less, and preferably 7.0% by mass or less with respect to the total mass of the wire. More preferred.
Examples of non-metallic fluxes include oxides and sulfides in addition to the above fluorides. Moreover, in this embodiment, the slag-forming agent refers to fluorides, oxides, and sulfides among nonmetallic fluxes.

The flux cored wire used in the gas metal arc welding method according to the present embodiment is not particularly limited in terms of components other than the above, and is particularly preferably a flux cored wire that is generally used in welding high Cr content steel. can be used for As such a flux cored wire, for example, a wire whose alloy composition is adjusted with a target of a weld metal composition similar to that of the wire composition specified in JIS Z 3317 can be used.

Hereinafter, the numerical ranges according to JIS Z 3317 will be specifically described as the component amounts of the welding wire that can be used in the present embodiment, together with the reasons for the limitations. Furthermore, unless otherwise specified, each of the above elements may be contained in the flux-cored wire in the form of a metal or in the form of a compound in the flux-cored wire. It may be contained in the flux-cored wire in both forms. Therefore, regardless of the form in which each of the above elements is contained in the flux-cored wire, the values are defined by conversion values converted into single elements. For example, when Si is taken as an example, the Si content refers to the sum of Si conversion values of metal Si and Si compounds. Metallic Si includes simple Si and Si alloy.

<C: 0.15% by mass or less (including 0% by mass)>
C is an element that can be precipitated as a carbide in the weld metal to ensure creep strength. Therefore, if it is desired to design the weld metal so as to increase the creep strength, C may be added to the raw material of the wire, if necessary. In addition, C is also an austenite-forming element. When the C content exceeds 0.15% by mass with respect to the total mass of the wire, the Ac1 transformation point of the weld metal is lowered, and austenite transformation occurs during post-weld heat treatment. Creep strength may decrease. Therefore, when C is added for the purpose of obtaining a desired creep strength, the C content with respect to the total mass of the wire is preferably 0.15% by mass or less.

<Si: 1.00% by mass or less (including 0% by mass)>
Si acts as a deoxidizing agent when the weld metal is melted, reduces the oxygen content in the weld metal, and contributes to the improvement of toughness. Si also has the effect of reducing the interfacial tension of the molten metal when the weld metal is melted, thereby reducing welding defects such as incomplete fusion and overlap. Therefore, if it is desired to design the weld metal to have high toughness or if it is desired to reduce weld defects, Si may be added to the raw material of the wire, if necessary. Since Si is a ferrite-forming element, if the Si content exceeds 1.00% by mass with respect to the total mass of the wire, ferrite will remain in the weld metal, possibly deteriorating toughness. Therefore, when adding Si for the purpose of obtaining desired toughness, the Si content with respect to the total mass of the wire is preferably 1.00% by mass or less.

<Mn: 2.50% by mass or less (including 0% by mass)>
Mn is an element that acts as a deoxidizing agent when the weld metal is melted and can ensure the strength and toughness of the weld metal. For this reason, when it is desired to design so as to ensure strength and toughness, Mn may be added to the raw material of the wire as necessary. Since Mn is an austenite-forming element, if the Mn content exceeds 2.50% by mass with respect to the total mass of the wire, the Ac1 transformation point of the weld metal may decrease and the creep strength may decrease. Therefore, when adding Mn for the purpose of ensuring strength and toughness while sufficiently ensuring the desired creep strength, the Mn content relative to the total weight of the wire is preferably 2.50% by mass or less. .

<P: 0.03% mass or less (including 0 mass%)>
<S: 0.03% mass or less (including 0 mass%)>
Since the higher the P and S contents in the weld metal, the lower the crack resistance, the lower the P content and the S content in the welding wire, the better. Specifically, the P content and S content in the welding wire are more preferably 0.03% by mass or less with respect to the total mass of the wire.

<Cu: 0.50% by mass or less (including 0% by mass)>
Cu is an austenite-forming element and has the effect of suppressing the formation of ferrite, which adversely affects the toughness of the weld metal. For this reason, when it is desired to design so as to ensure toughness, Cu may be added to the raw material of the wire as necessary. If the Cu content exceeds 0.50% by mass with respect to the total mass of the wire, the Ac1 transformation point of the weld metal is lowered and the creep strength is likely to be lowered. Therefore, when Cu is added for the purpose of ensuring sufficient creep strength, the Cu content relative to the total mass of the wire is preferably 0.50% by mass or less. In some cases, the surface of the welding wire is plated with Cu in order to improve the electrical conductivity and feedability of the welding wire. Therefore, regardless of the presence or absence of Cu plating, the Cu content in the entire welding wire is preferably 0.50% by mass or less.

<Ni: 1.00% by mass or less (including 0% by mass)>
Ni is an austenite-forming element and has the effect of suppressing the formation of ferrite, which adversely affects the toughness of the weld metal. For this reason, when it is desired to design so as to ensure toughness, Ni may be added to the raw material of the wire as necessary. When the Ni content exceeds 1.00% by mass with respect to the total mass of the wire, the Ac1 transformation point of the weld metal is lowered, and the creep strength is likely to be lowered. Therefore, when Ni is added for the purpose of ensuring sufficient toughness while ensuring desired creep strength, the Ni content relative to the total mass of the wire is preferably 1.00% by mass or less.

<Co: 2.00% by mass or less (including 0% by mass)>
Co is an austenite-forming element and has the effect of suppressing the formation of ferrite, which adversely affects the toughness of the weld metal. For this reason, when it is desired to design to ensure toughness, Co may be added to the raw material of the wire as necessary. If the Co content exceeds 2.00% by mass with respect to the total mass of the wire, the Ac1 transformation point of the weld metal is lowered and the creep strength is likely to be lowered. Therefore, when Co is added for the purpose of ensuring toughness while sufficiently ensuring the desired creep strength, the Co content relative to the total weight of the wire is preferably 2.00% by mass or less.

<Cr: 12.00% by mass or less (including 0% by mass)>
Cr has the effect of ensuring the oxidation resistance, corrosion resistance, strength, etc. of the weld metal. Therefore, when it is desired to design so as to ensure corrosion resistance, strength, etc., Cr may be added to the raw material of the wire as necessary. Since Cr is a ferrite-forming element, if the Cr content exceeds 12.00% by mass with respect to the total mass of the wire, there is a high possibility that ferrite will precipitate in the weld metal and the toughness will deteriorate. Therefore, when Cr is added for the purpose of ensuring oxidation resistance, corrosion resistance and strength while sufficiently ensuring the desired toughness, the Cr content relative to the total weight of the wire should be 12.00% by mass or less. is preferred. Moreover, in the welding using the wire according to the present embodiment, when a steel material within a suitable range is used as the base material, it is more preferable to make the Cr content in the wire correspond to the Cr content in the base material. Therefore, when matching with the base material, it is more preferable to set the Cr content to 1.5% by mass or more with respect to the total mass of the wire.

<Mo: 1.20% by mass or less (including 0% by mass)>
Mo is a solid-solution strengthening element in steel, and has the effect of forming a solid solution in the weld metal and improving the strength of the weld metal. For this reason, when it is desired to design so as to ensure strength, Mo may be added to the raw material of the wire as necessary. Since Mo is also a ferrite-forming element, if the Mo content exceeds 1.20% by mass with respect to the total mass of the wire, ferrite will precipitate in the weld metal, increasing the risk of deterioration in toughness. Therefore, when Mo is added for the purpose of improving the strength while sufficiently ensuring the desired toughness, the Mo content with respect to the total mass of the wire is preferably 1.20% by mass or less.

<V: 0.50% by mass or less (including 0% by mass)>
V is a precipitation-strengthening element in steel and precipitates in the weld metal as a carbonitride to have the effect of improving the strength of the weld metal. Therefore, when it is desired to design so as to ensure strength, V may be added to the raw material of the wire as necessary. If the V content exceeds 0.50% by mass with respect to the total mass of the wire, the strength of the weld metal becomes too strong and the toughness is likely to deteriorate. Therefore, when V is added for the purpose of improving the strength while sufficiently ensuring the desired toughness, the V content is preferably 0.50% by mass or less with respect to the total mass of the wire.

<Nb: 0.15% by mass or less (including 0% by mass)>
Nb is a precipitation-strengthening element in steel, and has the effect of increasing the strength of the weld metal by being precipitated as a carbonitride in the weld metal. Therefore, when designing to ensure strength, Nb may be added to the raw material of the wire as necessary. If the Nb content exceeds 0.15% by mass with respect to the total mass of the wire, the strength of the weld metal becomes too strong and the toughness is likely to deteriorate. Therefore, when Nb is added for the purpose of ensuring strength while ensuring desired toughness, the Nb content with respect to the total weight of the wire is preferably 0.15% by mass or less.

<W: 2.00% by mass or less (including 0% by mass)>
W is a solid-solution strengthening element in steel, and has the effect of forming a solid solution in the weld metal and improving the strength of the weld metal. Therefore, when designing to ensure strength, W may be added to the raw material of the wire as necessary. Since W is also a ferrite-forming element, if the W content exceeds 2.00% by mass relative to the total mass of the wire, ferrite will precipitate in the weld metal, increasing the risk of deterioration in toughness. Therefore, when W is added for the purpose of improving the strength while ensuring the desired toughness, the W content is preferably 2.00% by mass or less with respect to the total mass of the wire.

<N: 0.07% by mass or less (including 0% by mass)>
N exerts a solid-solution strengthening effect in steel, and combines with Nb and V to precipitate as nitrides, thereby contributing to the improvement of the creep strength of the weld metal. For this reason, when it is desired to design so as to ensure strength, N may be added to the raw material of the wire as necessary. Since N is a strong austenite forming element, if the N content exceeds 0.07% by mass with respect to the total mass of the wire, the Ac1 transformation point of the weld metal is lowered, and the creep strength is likely to be lowered. Therefore, when N is added for the purpose of securing a desired creep strength, the N content is preferably 0.07% by mass or less with respect to the total mass of the wire.

<Ti: 0.30% by mass or less (including 0% by mass)>
Ti is a ferrite-forming element and precipitates ferrite in the weld metal, which adversely affects toughness. However, if Ti is contained in the weld metal in an appropriate content, the toughness of the weld metal can be improved. . Therefore, if it is desired to design the weld metal so as to ensure toughness, Ti may be added to the raw material of the wire, if necessary. Like Nb and V, Ti is a strong carbide-forming element, and combines with C to form needle-like carbides that precipitate in the weld metal. Carbides in this form are highly likely to significantly impair the toughness of the weld metal. Therefore, when Ti is added for the purpose of ensuring desired toughness, the Ti content relative to the total mass of the wire is preferably 0.30% by mass or less.

<Alkaline earth metal: 5.00% by mass or less (including 0% by mass)>
In this embodiment, as described above, the alkaline earth metal is added in the form of a fluoride of the alkaline earth metal, so that the wire contains the alkaline earth metal. Alkaline earth metals include Ca, Sr, Ba and Ra, and it is particularly preferred that at least one of Sr and Ca is contained. When the content of alkaline earth metal in the wire indicates the range in which the alkaline earth metal fluoride exhibits the most effective arc stabilization effect, the total amount of alkaline earth metal with respect to the total mass of the wire is 0. % by mass is preferred. The total amount of alkaline earth metals is preferably 5.00% by mass or less, more preferably 4.00% by mass or less.

The wire that can be used in the gas-shielded arc welding method according to the present embodiment is preferably composed of the above elements and the balance of Fe and unavoidable impurities. As unavoidable impurities, for example, Sn, Na, Li, Sb, and As may be contained. In addition, when each element mentioned as the said unavoidable impurity is contained in a welding wire as an oxide, O will also be contained as an impurity.

Moreover, the gas metal arc welding method according to the present embodiment is not particularly limited with respect to conditions other than those described above. However, there are suitable ranges for groove angle, root gap, wire projection length and root face. Each welding condition is explained below.

(groove angle)
The groove angle is not particularly limited, but if the groove angle is 120° or less, welding can be performed with even better welding efficiency. Therefore, in the V-shaped groove, the groove angle is preferably 120° or less, more preferably 100° or less.
On the other hand, when the groove angle is 45° or more, the shape of the back bead can be made flat. Therefore, the groove angle is preferably 45° or more, more preferably 60° or more, still more preferably 70° or more, and particularly preferably 75° or more.

When a U-shaped groove is applied, the radius of curvature of the groove bottom is preferably 2 mm or more, more preferably 4 mm or more, and even more preferably 6 mm or more.
The gas metal arc welding method according to the present embodiment is intended for first layer welding, and the groove angle is arbitrary except for the first layer applicable part, and the groove cross-sectional area is narrowed by using a two-stage groove or the like. may

(root gap)
The root gap is also not particularly limited, but if the root gap is 1 mm or more, the shape of the back bead will be even better. Therefore, the root gap is preferably 1 mm or more, more preferably 2 mm or more.
On the other hand, when the root gap is 8 mm or less, drop resistance can be further improved. Therefore, the root gap is preferably 8 mm or less, more preferably 6 mm or less, and even more preferably 4 mm or less.
In addition, when applying an appropriate groove angle and root gap, the groove shape may be a R-shaped groove or a J-shaped groove.

(Wire projection length)
The wire projection length is not particularly limited, but if the wire projection length is 30 mm or less, the welding current and the wire feeding amount can be easily adjusted. Therefore, the wire projection length is preferably 30 mm or less, more preferably 20 mm or less, and even more preferably 15 mm or less.

(root face)
The root face is not particularly limited, but if the root face is 2 mm or less, the back bead can be made even more flat. Therefore, the root face is preferably 2 mm or less, more preferably 1 mm or less, and even more preferably 0.5 mm or less. Note that the root face may be 0 mm.

[2. Flux cored wire used for single-sided welding method]
The flux cored wire used in the welding method for single-sided welding according to the present embodiment is the above [1. Welding method for single-sided welding], which contains a metal fluoride.
The type and content of the flux component contained in the wire, the type of metal fluoride, and the preferred content of Mg and the like are described in [1. Welding method for single-sided welding].

EXAMPLES The present invention will be described in detail below with reference to Examples and Comparative Examples, but the present invention is not limited to these.

[1. Welding method for single-sided welding]
(1-1. Material to be welded and flux cored wire)
Two steel plates with a thickness of 12 mm were prepared as materials to be welded, and were processed into a V-shaped groove with a root interval of 2.4 mm and a groove angle of 90°. The gas metal arc welding method according to the present embodiment is suitable for high Cr content steel, but this example is intended to evaluate the necessity of a back shield and the welding efficiency of gas metal arc welding. For this purpose, an SM490A steel plate was used as the material to be welded.
As the flux cored wire, welding having the same composition as the filler metal composition in accordance with G62A-9C1MV described in JIS Z 3317 regarding gas shielded arc welding filler rods and solid wires for molybdenum steel and chromium molybdenum steel A wire was used in which the amount of alloy components added was adjusted so as to obtain a metal.

(1-2. Gas metal arc welding)
Using various flux cored wires shown in Table 1 below, MIG (Metal Inert Gas) welding, which is gas metal arc welding with pure Ar gas as the shielding gas, is applied to the groove of the material to be welded in a downward posture. Layer penetration welding was performed. The wire feeding speed was set at 355 cm/min, and the welding voltage was set at 17.5V. The polarity used in the gas metal arc welding was DCEN (Direct Current Electrode Negative) or DCEP (Direct Current Electrode Positive).

[2. evaluation]
As evaluation items, since the construction method does not require a back shield, the presence or absence of oxidation of the back bead was evaluated, and the shape of the back bead was evaluated to determine whether gas metal arc welding can be applied to the first layer welding.

(2-1. Oxidation state of back bead)
The oxidation state was evaluated by visually observing the presence or absence of oxidation of the back bead after welding.
As the evaluation criteria, "A" (excellent) was given when the back bead was entirely covered with slag and when the slag was peeled off, a bead with metallic luster could be observed. In addition, the slag coating state of the back bead is not uniform and the metallic luster is partially weakened "B" (slightly poor), and the back bead is black and oxidized almost entirely "C" (poor) and Furthermore, when the bead dripped down, it was considered that the evaluation of the oxidation state was not worth it, and the evaluation was not performed and was given as "-". Regarding the oxidation state of the back bead, the target evaluation result was set to "A".

(2-2. Back bead shape)
The back bead shape was evaluated by visually observing the back bead after welding.
As an evaluation criterion, the state of the back bead was a shape that protruded from the back side of the base material, that is, a positive shape, and had a uniform height over the entire line of the stationary welding part. . In addition, "B" (good) indicates a shape with a partial recess on the back side of the base material, that is, a negative shape, and "C" indicates that welding was impossible due to dripping of the weld metal. (bad). Regarding the shape of the back bead, the target of the evaluation result was set to "B" or higher.

Table 2 below shows the types of flux-cored wires used, welding conditions, and evaluation results. In this example, the oxidation state of the back bead was "A" and the shape of the back bead was "B" or higher, that is, those that achieved the target in all evaluation results were regarded as acceptable. , and the others were rejected.

As shown in Tables 1 and 2 above, test no. T1-T7 use a shielding gas containing 90% by volume or more of an inert gas, have a period of positive polarity (DCEN), and are welded using a flux-cored wire containing a metal fluoride. For example. Therefore, it was possible to suppress oxidation of the back bead without a back shield, and the shape of the back bead was excellent or good.
In particular, test no. In T4 to T6, the content of the non-metallic flux was set to a more preferable value, so that the shape of the back bead was even more excellent.

FIG. 1 shows test no. It is a drawing substitute photograph which shows the cross section of back bead shape of T6. The grooves of the two steel plates 1 processed into the V shape as described above were subjected to first-layer urinami welding in a downward posture to form a weld metal 2. As a result, test No. The back bead 3 of T6 did not burn through and protruded from the back side of the steel plate 1 over the entire line of the stationary welding portion, and an excellent back bead shape could be obtained.

Test No. which is a comparative example. In T8, since the inert gas content in the shielding gas was less than 90% by volume, the oxidation state of the back bead was "B" (slightly poor).
Test No. which is a comparative example. T9 used a shielding gas of 100% by volume CO ₂ , resulting in bead drooling.
Test No. which is a comparative example. In T10, the flux cored wire used did not contain a metal fluoride, and the welding was performed as a direct current rod plus (DCEP), so bead dripping occurred.
Test No. which is a comparative example. Since T11 was welded as a direct current rod plus (DCEP), bead dripping occurred.

In the above examples and comparative examples, there were no cases in which the back bead was oxidized black almost entirely and the evaluation result was "C" (defective).
In addition, wire No. Test No. W6 was used and various shielding gases were used. Good arc stability was obtained in all of T6 to T9, since the wires contained Mg.

Claims (8)

A welding method for single-sided welding in which one side of a member to be welded is welded by gas metal arc welding and the back surface is melted without a back shield,
The gas metal arc welding is
a shielding gas containing 90% by volume or more of an inert gas;
Using a flux cored wire containing a metal fluoride,
As the polarity used for the gas metal arc welding,
A welding method for single-sided welding, characterized in that there is at least a period of positive polarity during welding. 2. The welding method for single-sided welding according to claim 1, wherein welding is performed with the polarity being DC positive. The welding method for single-sided welding according to claim 1 or 2, wherein the metal fluoride is an alkaline earth metal fluoride. The content of flux components excluding metals and metal fluorides among the flux components contained in the flux-cored wire is 0.5% by mass or less with respect to the total mass of the wire. The welding method for single-sided welding according to any one of 1 to 3. The welding method for single-sided welding according to any one of claims 1 to 4, wherein the flux cored wire contains a flux component having a boiling point of 800°C or higher and 1400°C or lower. The welding method for single-sided welding according to any one of claims 1 to 5, wherein the flux cored wire contains Mg. The flux cored wire is
Containing a flux of 4% by mass or more and 30% by mass or less with respect to the total mass of the wire,
Welding of single-sided welding according to any one of claims 1 to 6, wherein the flux contains 2% by mass or more and 15% by mass or less of non-metallic flux with respect to the total mass of the wire. Method. A flux cored wire used in the single-sided welding method according to any one of claims 1 to 7, characterized by containing a metal fluoride.

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