JP2015123457A - BUTTING WELDING METHOD OF Ni STEEL PLATE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a butting welding method of an Ni steel plate, capable of forming a welding part having sufficient strength and low temperature toughness, and also excellent in welding efficiency.SOLUTION: An Ni-based alloy including Ni of 55% or more in a mass percentage and having an austenite structure is used for a welding material, and a pair of side plates 20 being Ni steel plates including Ni of 3%-15% in a mass percentage and having a ferrite structure are but-welded. A first wire W1 held by a first welding torch E1 and a second wire W2 held by a second welding torch E2 are arranged on both sides of a butting part constituted of the pair of side plates 20, and at least welding of a first layer is simultaneously performed from both sides of the butting part by DC bar minus polarity.

Description

  The present invention relates to a butt welding method for Ni steel plates.

  A low temperature tank for storing liquefied gas such as LNG (liquefied natural gas) and LPG (liquefied petroleum gas) includes an inner tank and an outer tank covering the outer periphery of the inner tank as an example. The inner tub has a circular bottom, a side erected along the periphery of the bottom, and a roof plate that covers the upper opening of the side. For example, the side portions are arranged side by side with a plurality of arc-shaped side plates extending in the circumferential direction of the tank, and are connected to each other by butt welding.

  The welded portion of each side plate is required to have sufficient strength and low temperature toughness. Therefore, Ni steel having a ferrite structure such as 9% Ni steel is employed for the side plate (base material). As the welding material, for example, a Ni-based alloy having an austenitic structure containing 55% or more of Ni by mass percent is employed. In order to prevent the occurrence of poor penetration and poor fusion as much as possible, each side plate is usually welded from both front and back surfaces with a positive polarity of a DC bar. As the welding method, a submerged arc welding method or a TIG welding method is employed.

  In the conventional welding method, after the first layer was welded at the butt portion from one surface of the pair of side plates, the back chipping process was performed, and then the first layer on the other surface was welded. According to this method, it is possible to remove defects at the bottom of the groove at the butted portion of the pair of side plates by the back chipping process, but the welding process (particularly the back chipping process) takes a long time.

  If the back chipping process can be omitted, the construction period of the welding process can be greatly shortened. Here, in Patent Document 1, welding is performed simultaneously from both sides of the groove, and by suppressing the undissolved residue and the occurrence of defects in the first layer of the welded portion, the back chipping process is omitted and the welding efficiency is improved. A method is disclosed.

JP-A-61-206564

  In the dissimilar joint made of Ni steel and Ni-base alloy as described above, the alloy component of the weld metal portion in the welded portion is diluted by the melted base material and becomes thinner than the alloy component of the welding material. This phenomenon is called “dilution”, and the ratio of this dilution is called “dilution rate”.

  In order to ensure the strength and low temperature toughness of the welded part, it is important to reduce the penetration of the base material into the welded metal part, that is, to suppress the dilution rate. However, the welding method described in Patent Document 1 is intended for a mild steel co-joint joint, and by simultaneously welding from both sides, the penetration of the base metal into the weld metal part increases, that is, the dilution rate increases. No countermeasures for doing are described.

  Therefore, an object of the present invention is to provide a butt welding method for Ni steel sheets, which can form a welded portion having sufficient strength and low temperature toughness and is excellent in welding efficiency.

  In order to solve the above problems, according to one embodiment of the present invention, a Ni-based alloy containing 55% or more Ni by mass and having an austenite structure is used as a welding material, and 3% or more and 15% or less by mass. A method of butt welding a pair of Ni steel sheets containing Ni and having a ferrite structure, wherein welding electrodes are arranged on both front and back sides of a butt portion constituted by the pair of Ni steel sheets, and welding of at least a first layer, It is simultaneously performed from both the front and back sides of the abutting portion with a negative polarity of the DC bar.

  Thus, by welding simultaneously from the front and back both sides of a butt | matching part, the melt | dissolution residue of a groove bottom part and generation | occurrence | production of a defect are suppressed, and a back chipping process can be skipped. Therefore, the work period of the welding process can be shortened.

  In addition, welding can be performed more quickly than in the case where welding is sequentially performed on each side. Thereby, the improvement of welding efficiency can be aimed at.

  Further, by setting the polarity of the welding electrode to a negative DC rod, it is possible to reduce the penetration of the base material into the weld metal portion, that is, to suppress the dilution rate, compared to the case of using a positive DC rod. Therefore, even when simultaneously welding from both the front and back sides of the Ni steel plate, it is possible to prevent a decrease in strength and low temperature toughness of the welded portion accompanying an increase in the dilution rate.

  The Ni content in the Ni steel sheet is preferably 3% or more and 15% or less in terms of mass percentage from the viewpoint of securing strength and low temperature toughness. Further, the Ni content in the Ni-based alloy is preferably 55% or more by mass percent from the viewpoint of ensuring the toughness of the weld metal part.

  Further, a welded portion having sufficient strength and low temperature toughness by using a welding material having a pair of Ni steel plates having sufficient strength by having a ferrite structure as a base material and having an austenite structure and having excellent low temperature toughness. Can be formed.

  Moreover, in another aspect of the present invention, the polarity of the welding electrode can be AC. As a result, in the same way as when the polarity of the welding electrode is a negative DC rod, compared to when the polarity of the welding electrode is a positive DC rod, the penetration of the base material into the weld metal part can be reduced, that is, the dilution rate can be suppressed. . Therefore, even when simultaneously welding from both the front and back sides of the Ni steel plate, it is possible to prevent a decrease in strength and low-temperature toughness of the welded portion accompanying an increase in dilution rate.

  In another aspect of the present invention, the welding can be performed in a lateral posture. As described above, when the welding is performed in a lateral orientation, it is possible to perform the welding under the condition that gravity acts in the same direction on both sides of the pair of Ni steel plates, and a uniform welded portion is formed on both sides of the Ni steel plates. Easy to form.

In another aspect of the present invention, the welding can be performed by a submerged arc welding method.
This welding method is particularly advantageous because the effect is great when welding is performed with a relatively large current as in the submerged arc welding method.

  In another aspect of the present invention, the pair of Ni steel plates may constitute a side portion of a low-temperature tank for storing liquefied gas.

  If a pair of Ni steel plates is used as a member constituting the side portion of such a low temperature tank, a welded portion having sufficient toughness can be formed even in an extremely low temperature environment such as being in contact with liquefied gas.

  As is apparent from the above description, according to the present invention, it is possible to provide a butt welding method for Ni steel sheets that can form a welded portion having sufficient strength and low temperature toughness and is excellent in welding efficiency.

It is sectional drawing which shows the structure of the low temperature tank which concerns on embodiment. It is a front view which shows the structure of the side plate of a low-temperature tank. It is a figure for demonstrating the welding method which concerns on embodiment. It is sectional drawing which shows the structure of the butt joint periphery of a side plate. It is a figure for demonstrating the welding method of a modification. It is a figure which shows the positional relationship of a 1st wire and a 2nd welding wire.

  Generally, in an inner tank of a low-temperature tank, a welded portion formed to connect a plurality of side plates constituting the side portion is a Ni steel plate for a low-temperature pressure vessel such as steel containing Ni of about 9% by mass, Inconel (Registered trademark), Hastelloy (registered trademark), etc., and a welding material made of a Ni-based alloy containing Ni of 55% or more by mass percentage. As a welding method for forming such a welded portion, for example, a submerged arc welding method is used. From the standpoint of preventing poor penetration and poor fusion, the polarity is usually a positive DC bar.

  When the base material used in a low temperature environment is connected by welding, such as the side plate constituting the side of the inner tank of the low temperature tank, it is required to form a welded portion having sufficient strength and low temperature toughness. It is done. Here, when the end portions of the pair of side plates are welded simultaneously from both sides of the side plate, dilution occurs because the penetration of the base material into the weld metal portion becomes larger than when the end portions are welded from one side of the side plate. The rate becomes high and it becomes difficult to form a weld having sufficient strength and low temperature toughness.

  This problem exists not only when welding the first layer but also when welding the second and subsequent layers.

  The same problem as described above also exists in common when a welding method other than the submerged arc welding method is used.

  The embodiment described below can solve the above-described problems.

<Embodiment>
Embodiments of the present invention will be described below with reference to the drawings.

  In FIG. 1, the structure of the low temperature tank 1 produced using the welding method of the Ni steel plate which concerns on this embodiment is shown.

  The low-temperature tank 1 includes a bottomed cylindrical inner tank 2 and an outer tank 3 arranged so as to cover the outer periphery and the upper part of the inner tank 2. A heat insulating material (not shown) is disposed between the inner tank 2 and the outer tank 3. The low temperature tank 1 is configured as a storage tank for storing a liquefied gas such as LNG, PNG, ethylene, or the like in the inner tank 2 at a high pressure.

  The inner tub 2 includes a disc-shaped bottom 2a, a large-diameter and cylindrical side 2b erected along the circumferential direction of the periphery of the bottom 2a, and an upper portion of the side 2b via a knuckle plate (not shown). And a dome-shaped roof plate 2c made of rolled steel and covering the opening. As an example, the bottom 2a and the roof plate 2c are configured by connecting a plurality of Ni steel plates having a rectangular main surface. As shown in FIG. 2, the side portion 2b has several arcuate side plates 20b arranged side by side in the circumferential direction of the bottom portion 2a in a state where the longitudinal direction is horizontal and the short side direction is vertical, respectively. Several to tens of sheets are stacked and arranged, and the side plates 20b are brought into contact with each other and connected by welding.

  The side plate 20 is made of a Ni steel plate having a ferrite structure (body-centered cubic lattice (bcc) structure), excellent in low-temperature toughness, and excellent tensile strength at normal and low temperatures. In this embodiment, a 9% Ni steel plate conforming to the JIS standard (JIS G 3127 SL 9N) is used for the side plate 20. As an example, the thickness of the side plate 20 is set in a range of 6 mm to 50 mm which is the thickness of the inner tank 2 of a general LNG tank, but the thickness of the side plate 20 is not limited to this. The side plate 20 is bent so as to follow the circumferential direction of the side portion 2b.

  Each side plate 20 constituting the side portion 2b increases in thickness as it is positioned below, and is adjusted to withstand the pressure of the liquefied gas stored in the inner tank 2. Moreover, a pair of vertical edge part 21 (butting | matching part) of the adjacent side plate 20 is connected, for example by standing covering arc welding or TIG welding. Moreover, a pair of horizontal edge part 22 (butting | matching part) of the adjacent side plate 20 is connected by the double-sided simultaneous horizontal direction submerged arc welding shown below, and has favorable toughness also in a low temperature environment (for example, -5 degrees C or less environment). A weld J is formed.

  The outer tub 3 includes a side portion 3a that covers the side portion 2b of the inner tub 2 from the outside, and a roof plate 3b that covers the roof plate 2c of the inner tub 2 from above via a knuckle plate (not shown). The outer tub 3 is configured by connecting a plurality of rolled steel plates by welding, for example.

  Next, the welding method of the side part 2b is demonstrated using FIGS.

  First, along the circumferential direction of the periphery of the bottom portion 2a, a plurality of side plates 20 positioned at the first stage of the side portion 2b are picked up by a crane and arranged, and the side plates 20 are fixed to each other by temporary welding and a jig. . Thereafter, the pair of vertical end portions 21 of the side plates 20 butted against each other are welded by, for example, vertical covering arc welding or TIG welding.

  After welding the vertical ends 21 of the side plates 20 positioned in the first stage and welding the side plates 20 to the bottom 2a, the side plates 20 positioned in the second stage are connected to the side plates 20 of the first stage. Place with a crane above. Then, the side plates 20 positioned in the second stage are erected and fixed to each other by temporary welding and a jig in a state in which the side plates 20 are positioned in the vertical direction. In this state, a pair of vertical end portions 21 of the side plates 20 that are abutted to each other along the horizontal direction are welded by, for example, vertical covering arc welding or TIG welding in the same manner as the side plates 20 positioned in the first stage. .

  Subsequently, the pair of horizontal end portions 22 of the side plates 20 positioned in the first and second stages that are in contact with each other are welded by the following double-sided simultaneous submerged arc welding method.

  Specifically, first, a pair of known flux supply devices (not shown) are prepared. As shown in FIG. 3, the upper surfaces of the flux belts B1 and B2 of the flux supply device are arranged in the vicinity of a pair of horizontal end portions 22 that face each other in the vertical direction from both sides of the side plates 20 arranged vertically. . A granular flux F is placed on the flux belts B1 and B2, and the flux F is disposed between and in the vicinity of the pair of horizontal ends 22.

  Subsequently, as an example, a welding apparatus having a first welding torch E1 and a welding apparatus having a second welding torch E2 (both apparatuses are not shown) are separately prepared. Further, as a welding material (consumable electrode) having a ferrite structure (face-centered cubic lattice (fcc) structure) and containing a larger amount of Ni than the side plate 20, for example, a Ni-based alloy containing 55% or more of Ni by mass percent The welding electrode (welding wires W1, W2) to be prepared is prepared. The welding wire W1 is held on the first welding torch E1 and the welding wire W2 is held on the second welding torch E2 so as to be fed out. The first welding torch E1 and the second welding torch E2 are arranged on both front and back sides of the pair of horizontal end portions 22 of the adjacent side plates 20, and the welding wire W1 from the first welding torch E1 and the welding wire from the second welding torch E2 to each other. W2 is inserted into the flux F between the pair of horizontal ends 22.

  In this state, each welding apparatus is operated so that the polarity of the DC bar is negative, and the first layer is welded simultaneously from both the front and back sides of the horizontal end portion 22 constituted by the pair of side plates 20.

  Thereafter, also for the second and subsequent layers, the welded portion J having the weld metal portion 30 as shown in FIG. 4 is formed by repeating the welding work by the same method as the welding of the first layer.

  In this embodiment, the first layer of the welded portion J and the second and subsequent layers sequentially stacked thereon are formed by this welding method. However, another welding method is applied to the second and subsequent layers of the welded portion J. May be formed.

  If the welding wires W1 and W2 are made negative in this way, the penetration of the base material Ni steel into the Ni-based alloy constituting the weld metal portion 30 is made smaller than when the welding wires W1 and W2 are made positive. To reduce the dilution rate. Therefore, even when welding is performed simultaneously from the front and back both sides of the pair of horizontal end portions 22, it is possible to prevent a decrease in strength and low temperature toughness due to an increase in dilution rate.

  Moreover, in this welding method, since a pair of horizontal edge part 22 is simultaneously welded from the both surface sides of each side plate 20, compared with the conventional welding method which welds a pair of horizontal edge part 22 from one side of each side plate 20 for each side, it welds. The construction period can be shortened. In addition, processing such as a back chipping process by gouging for suppressing poor penetration and defects at the bottom of the groove becomes unnecessary. Thereby, welding efficiency can be improved rather than the conventional welding method.

  As described above, based on the main welding method, when the circumferential joint is formed on each side plate 20 located in the first and second steps along the circumferential direction of the bottom portion 2a, the plurality of side plates 20 are arranged in a plurality of steps in the vertical direction. They are connected based on the welding method, and a vertical joint and a peripheral joint are formed and stacked. Thereby, the side part 2b of predetermined height can be obtained.

<Modification>
FIG. 5 shows a modification of the embodiment.

  In this modification, two welding devices are used, and welding of at least the first layer in the horizontal end portion 22 constituted by the pair of side plates 20 is performed with the polarity of the welding electrodes (welding wires W1, W2) of each welding device being an alternating current. Simultaneously from both the front and back surfaces of the pair of horizontal ends 22. In addition, the ratio (positive ratio) which each side plate 20 becomes positive with respect to the electric potential of welding wire W1, W2 can be 50%, for example.

  Even in the case of performing the double-sided simultaneous horizontal submerged arc welding method of such a modification, there is a timing at which the potentials of the welding wires W1 and W2 are negative with respect to the potentials of the side plates 20, and thus the same as in the above embodiment. The dilution rate can be reduced. Therefore, even when simultaneously welding from both the front and back sides of the Ni steel plate, it is possible to prevent a decrease in strength and low temperature toughness due to an increase in dilution rate.

<Other matters>
The present invention is not limited to the above embodiments and modifications, and the configuration can be changed, added, or deleted without departing from the spirit of the present invention.

  In the said embodiment and modification, although the submerged arc welding method was illustrated as a welding method, the welding method is not limited to a submerged arc welding method, The welding method using other arc welding may be sufficient.

  In the embodiment and the modification, the welding wires W1 and W2 which are welding electrodes are arranged one on each side of each side plate 20, but based on a so-called tandem welding method, a plurality of welding wires and a welding torch for holding the welding wires are provided. May be arranged on both sides of each side plate 20 and simultaneous welding may be performed.

  In the above embodiment, the base material side plate 20 is made of steel containing 9% Ni by mass, but this is an example of Ni steel defined as a standard for the base material used in the LNG tank. In addition, as a mass percentage of the amount of Ni contained in the base material, it is of course possible to take a numerical value other than 9%. The amount of Ni contained in the Ni steel plate is preferably in the range of 3% to 15% by mass percent. The amount of Ni contained in the welding wires W1 and W2 is 55% or more in mass percent, which is a numerical value determined as a standard for the base material and the molten material used in the LNG tank.

  3 to 5 illustrate the configuration in which the groove and the root surface are formed on the pair of horizontal end portions 22 from both sides of each side plate 20, the configuration of the pair of horizontal end portions 22 is not limited thereto. For example, a groove may be formed only on one side of each side plate 20.

  The “simultaneous welding” in the present invention is not limited to the case where welding is performed by orienting the tips of the welding wires W1 and W2 to the exact same position from both sides of each side plate 20, and each longitudinal length of each side plate 20 is determined. This includes the case where welding is performed by shifting the tips of the welding wires W1 and W2 within a certain range along the direction. For example, as shown in FIG. 6, the welding wire W1 held by the first welding torch E1 is made to precede the welding wire W2 held by the second welding torch E2 by a distance L along the longitudinal direction of each side plate 20. When welding is performed, the welding wire W2 is included in the “simultaneous welding” in the present invention if welding is performed with the tip of the welding wire W2 oriented in the molten pool 30a formed by welding the welding wire W1. In FIG. 6, for convenience, the horizontal end portion 22 and the weld metal portion 30 of the side plate 20 are shown in a straight line, and the flux F and the flux belts B1 and B2 are not shown.

  In the said embodiment and its modification, although the welding method of this invention was applied to the double-sided simultaneous horizontal welding of the horizontal edge part 22 comprised by a pair of side plate 20, the welding method of this invention is a pair of used in a low-temperature environment. When welding the butt | matching part comprised with Ni steel plate from front and back both surfaces, it is possible to apply, without limiting a welding direction. Therefore, for example, the welding method of the present invention may be applied to the connection of the so-called Ni steel plates for low-temperature pressure vessels, such as welding of the vertical ends 21 of the side portions 20 and each Ni steel plate constituting the bottom portion 2a. Or in the structure arrange | positioned inside the inner tank 2 of the low-temperature tank 1, you may apply the welding method of this invention to the welding method of the butt | matching part comprised by the structural members of the said structure.

  The “low temperature” in the present invention refers to a temperature range of −5 ° C. or lower. “Normal temperature” refers to a temperature range of 5 ° C. or more and 35 ° C. or less.

  In the present invention, the welding material is not limited to the welding wire, and other materials such as a welding rod may be used.

  The Ni steel sheet used in the present invention may contain elements such as Cr and Mn in addition to carbon and Ni.

  As mentioned above, this invention has the outstanding effect which can form the weld part which has sufficient intensity | strength and low temperature toughness, and can provide the butt welding method of the Ni steel plate excellent in welding efficiency. Therefore, it is beneficial to apply widely as a welding method that can exhibit the significance of this effect.

E1, E2 Welding torch J Welded part W1, W2 Welding wire 1 Low temperature tank 2 Inner tank 2a Bottom plate 2b, 3a Side part 2c, 3b Roof plate 3 Outer tank 20 Side plate (base material)
21 Vertical end portion 22 Horizontal end portion 30 Weld metal portion 30a Molten pool

Claims (5)

A Ni-based alloy containing 55% or more Ni in mass percent and having an austenite structure is used as a welding material, and a pair of Ni steel plates containing 3% or more and 15% or less Ni in mass percent and having a ferrite structure are butt-matched A method of welding,
The welding electrodes are arranged on both front and back sides of the butt portion constituted by the pair of Ni steel plates, and at least the first layer is welded simultaneously from both the front and back surfaces of the butt portion with a negative polarity of a DC rod, Ni Butt welding method for steel plates. A Ni-based alloy containing 55% or more Ni in mass percent and having an austenite structure is used as a welding material, and a pair of Ni steel plates containing 3% or more and 15% or less Ni in mass percent and having a ferrite structure are butt-matched A method of welding,
Welding electrodes are arranged on both front and back sides of the butt portion constituted by the pair of Ni steel plates, and welding of at least the first layer is simultaneously performed from both the front and back surfaces of the butt portion using the polarity of the welding electrode as an alternating current. A butt welding method for Ni steel sheets.   The butt welding method for Ni steel sheets according to claim 1 or 2, wherein the welding is performed in a lateral orientation.   The butt welding method for Ni steel sheets according to any one of claims 1 to 3, wherein the welding is submerged arc welding.   The pair of Ni steel plates is a butt welding method for Ni steel plates according to any one of claims 1 to 4, which constitutes a side portion of a low-temperature tank for storing liquefied gas.

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