KR100376492B1 - Method For Flux Cored Arc Welding 9%Ni Steel - Google Patents

Abstract

The present invention relates to a method of flux cored arc welding a 9% Ni steel base material used as a material for an LNG storage tank, which can prevent welding cracking and prevent a decrease in strength of the weld metal compared to the base material. In addition, it aims to provide a flux cored arc welding method of 9% Ni steel, which can simultaneously improve the economics and workability of welding materials.

The present invention is a method for flux flux arc welding of 9% Ni steel as a base material,

Weld using a Hastelloy-based welding material for initial welding of 5-50% of the number of dedicated layers, including the root pass, and using Inconel-based welding material for welding other welds. The flux cored arc welding method of 9% Ni steel is made into the summary.

Description

Flux cored arc welding method of 9% Ni steel {Method For Flux Cored Arc Welding 9% Ni Steel}

The present invention relates to a method of flux cored arc welding (FCAW) of 9% Ni steel base materials used as materials for LNG storage tanks, and more particularly, to Inconel-based and Hastelloy-based welding materials. And a method for flux cored arc welding of 9% Ni steel base materials.

The 9% Ni steel is a ferrite-based material having excellent low temperature toughness and strength at cryogenic temperatures, such as a typical LNG storage tank, and has a low thermal expansion coefficient, excellent weldability, and stable supply in large quantities. It is increasing.

This 9% Ni steel is divided into quenched and tempered QT material and 2 times normalized and then treated with NNT material, which is treated with annealing to improve toughness by utilizing stable residual austenite as needed. In some cases, quenching-melamelizing-tempering (QLT) is used. The welding of LNG storage tanks using 9% Ni steel is mainly applied to SMAW (Shielded Metal Arc Welding), GTAW (Gas Tungsten Arc Welding) and SAW (Submerged Arc Welding) methods.

Except for some horizontal welding parts using SAW, most of them still apply only SMAW. Recently, the FCAW method is being considered to improve welding efficiency.

On the other hand, the welding material applied to such welding should be similar to the base material, the strength and thermal expansion coefficient of the weld metal, high toughness at cryogenic temperature and excellent welding workability.

The excellent low-temperature toughness of the 9% Ni steel is obtained not only by the added Ni composition but by appropriate heat treatment in the manufacturing process.

On the other hand, large structures such as LNG tanks cannot be heat treated after welding, so in order to secure excellent low temperature toughness in the welded state, Ni alloys that can obtain excellent strength and toughness in the welded state are mainly used.

Inconel-based alloys, which are Ni alloys, are most widely used as materials that satisfy these characteristics, but Hastelloy-based alloys having improved high temperature cracking properties by adding a large amount of Mo are also used.

In the case of constructing 9% Ni steel with Inconel-based welding material containing about 70% Ni as the FCAW welding material, problems such as weld cracking in the first layer or deterioration of the weld metal strength arise. This problem is known to be due to the effect of dilution of components between the base metal and the weld metal in welding between dissimilar materials. A similar problem occurs in SAW welding. As a countermeasure for SAW construction, the Inconel-based welding material is used after welding the root part one or more times by manual welding using a coated arc welding rod. The method of constructing by SAW welding using C is widely adopted. According to this construction method, it is effective for problems such as crack resistance of SAW welding part using Inconel-based welding material and decrease in strength of weld metal, but economical efficiency and productivity are reduced by using manual welding in combination with automatic welding work by SAW welding. Done.

This solves the problem of lowering productivity due to the mixing of automatic and manual welding, and the addition of Mo is effective for the problem of crack resistance and strength reduction of SAW welded parts using Inconel-based welding material. Therefore, the amount of Mo is large in SAW welding of 9% Ni steel. The method of performing all-layer and all-pass welding construction using the added Hastelloy-type welding material is employ | adopted widely. In this case, the welding cracks such as the root pass and the decrease in the strength of the weld metal, etc. are eliminated by this method, and all welding can be performed by automatic welding. Compared to the above, efficiency can be increased. However, Hastelloy-based welding materials are very expensive compared to Inconel-based welding materials, which is disadvantageous in terms of economy.

As described above, welding 9% Ni steel using a Ni alloy welding material is a kind of dissimilar material welding. Therefore, various precautions are required in welding construction, and a general problem is that welding cracks, particularly in the crater portion, are used. The high temperature cracks and low yield strength compared to the base metal and the increase of construction cost due to high price.

The present inventors have conducted research and experiments to solve the above-mentioned problems of the prior art, and based on the results, the present invention proposes the present invention, which prevents welding cracking and reduces the strength of the weld metal compared to the base metal. The purpose of this invention is to provide a flux cored arc welding method of 9% Ni steel, which can not only prevent the damage, but also improve the economics and workability of welding materials.

1 is a groove cross-sectional view of a transverse flux cored arc welding.

2 is a cross-sectional view showing the Hastelloy-based weld metal (black portion) and Inconel-based weld metal (colorless portion) distribution of the weld portion according to the present invention.

3 is a macro photograph of the welded portion by mixing of welding materials

4 is a macro photograph of the first layer of Inconel-based welding material in which cracking occurs

Explanation of symbols on the main parts of the drawings

1, 2. . . Base material 3. . . Initial weld

4 . . . Hastelloy welding materials 5. . . Welds other than the initial weld

6. . . Inconel Welding Material

The present invention is a method for flux flux arc welding of 9% Ni steel as a base material,

Weld using a Hastelloy-based welding material for initial welding of 5-50% of the number of dedicated layers, including the root pass, and using Inconel-based welding material for welding other welds. A flux cored arc welding method of 9% Ni steel.

The present invention adopts the FCAW method to comprehensively solve the problems in welding 9% Ni steel, and uses the Inconel-based and Hastelloy-based welding materials to maximize the advantages of each material.

That is, the present invention uses a combination of Hastelloy-based and Inconel-based welding materials in FCAW welding of 9% Ni steel, which is a base metal, to provide a Hastelloy-based welding material for the initial welding including, for example, a root path in which the restraint stress is large. After that, the layer can be prevented from cracking by welding the entire layer by FCAW method using Inconel-based welding material and preventing the reduction of strength of the weld metal compared to the base metal.

In addition, the present invention enables the construction of the entire welded portion by semi-automatic welding only by using a combination of Hastelloy-based and Inconel-based welding materials in FCAW welding of 9% Ni steel, thereby significantly improving economical efficiency and workability at the same time. It is.

According to the present invention, when welding 9% Ni steel, which is the base metal, by using the FCAW method, it corresponds to 5 to 50% of the number of dedicated layers including the root pass (starting layer of the shortest part in the base joint). Hastelloy-based welding materials are used for the initial welding, and Inconel-based welding materials are used for the other welding layers.

When the use ratio of Hastelloy-based welding material is less than 5%, there is no effect of preventing the occurrence of cracking, and if it exceeds 30%, the economic effect of the present invention is lost, so the range is preferably limited to 5-50%.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example

Inconel-based welding materials and Hastelloy-based welding materials having the chemical composition shown in Table 1 are used, Hastelloy-based welding materials in the amount of Table 2 are used for initial welding, and Inconel-based welding materials are used for the other welding layers. Welding was performed.

That is, as shown in Figures 1 and 2, the invention example of Table 3 is a case of welding the base metal 9% Ni steel (1) (2) in accordance with the present invention of the initial weld portion 3 including the root path The case where the Hastelloy-based welding material 4 is used for welding and the intonel-based welding material 6 is used for the welding of the other welding portion 5, and the conventional example shows the case where the welding is performed using only the inconel-based welding material. .

After welding as described above, the crack generation of the weld metal was investigated, and the results are shown in Table 2 below, and the mechanical properties of the weld metal were examined, and the results are shown in Table 3 below.

In addition, the macro photograph of the weld part macro photograph for the example of the invention and the first layer of the welding material for the conventional example was observed, and the results are shown in FIGS. 3 and 4, respectively.

Unit (wt%) C Mn Si P S Ni Cr Mo W Fe Ti Cu Nb Inconel Type 0.03 3.10 0.33 0.003 0.006 70.12 21.47 - - 1.90 0.28 0.04 2.49 Hastelloy 0.01 0.28 0.26 0.007 0.003 61.9 12.7 15.2 3.05 5.78 - - 0.02

Hastelloy-based Welding Materials / Allied Metals (%) Cracked fusion metal 4 Occur 10 Not Occurred 30 Not Occurred

Welding material All-fusion Metal Tensile Test Joint tension test Impact value (-196 ℃) Yield strength (Mpa) Tensile Strength (Mpa) Elongation (%) Tensile Strength (Mpa) Judgment position Fusion Metal Part (J) Heat affected zone (J) Inconel Welding Material (Prior Example) 400 660 43.3 680 Welding metal 71 108 395 670 40.2 685 Welding metal Hastelloy-based + Inconel-based welding materials (invention example) 450 723 42.3 740 Welding metal 90 107 482 738 40.6 744 Welding metal

As shown in Table 2, when the application rate of the Hastelloy-based welding material is less than 5%, cracks occur in the weld metal, whereas when the application rate of the Hastelloy-based welding material conforms to the present invention, the welding is performed. It can be seen that no cracking occurs in the metal.

In addition, as shown in Table 3, it can be seen that when in accordance with the present invention can be made a stable weld without a decrease in strength of the weld metal.

3 and 4, no crack was observed in the case of welding according to the present invention (invention example), but in the case of the conventional example, it was seen that the crack was observed.

As described above, the present invention not only prevents crack resistance and strength reduction of the welded portion by mixing Inconel-based and Hastelloy-type welded materials, but also enables the entire welded portion to be constructed by semi-automatic welding, thereby improving productivity related to the welded material. And there is an effect that can achieve an economic improvement.

Claims (1)

In the method of flux-core arc welding 9% Ni steel which is a base material, Weld using a Hastelloy-based welding material for initial welding of 5-50% of the number of dedicated layers, including the root pass, and Inconel-based welding material for welding other welds. Flux cored arc welding method of 9% Ni steel