KR102625068B1 - Weld metal having excellent impact toughness at low temperature and welding method for forming it - Google Patents

Abstract

We propose a weld metal with excellent impact toughness even in a low-temperature environment and a welding method to form it through flux-cored arc welding (FCAW). The welding method for forming the weld metal with excellent low-temperature impact toughness includes a placement step, an attachment step, an initial layer welding step, a removal step, a filling layer welding step, and a surface layer welding step, and the weld metal with excellent low-temperature impact toughness is weighted. In % C: 0.05%, Si: 0.23~0.32%, Mn: 0.67~0.74%, Cr: 0.026~0.032%, Ni: 3.17~3.39%, Mo: 0.01% or less, B: 0.003~0.004%, Nb: 0.010~0.013%, V: 0.01%, N: less than 0.001%, P: less than 0.01%, S: less than 0.01%, the remainder includes Fe and inevitable impurities.

Description

Weld metal having excellent impact toughness at low temperature and welding method for forming it {Weld metal having excellent impact toughness at low temperature and welding method for forming it}

The present invention relates to a weld metal with excellent low-temperature impact toughness and a welding method for forming the same. More specifically, the present invention relates to a weld metal with excellent low-temperature impact toughness even in a low-temperature environment of -60°C through flux cored arc welding (FCAW). It relates to this excellent weld metal and the welding method for forming it.

Recently, the design temperature of the LPG carrier cargo hold has been lowered to -52 ~ -55℃ due to the ethane content from shale gas mining, and the impact toughness of the weld zone 5℃ lower than the design temperature is required. In other words, according to the IGC Code (International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk), welded joints with guaranteed -60℃ impact toughness are required.

In particular, in the case of LPG cargo holds, low-temperature steel with a thickness of 8 to 25 mm is used, of which steel with a thickness of 11 mm and 12 mm is most commonly used. Steel materials of this thickness have a relatively slow cooling rate compared to thick plates when making butt joints. In addition, when testing the impact toughness of welded joints, there is no distinction between the surface and back sections and the impact toughness is greatly affected by the upper layer, so it is not easy to guarantee the impact toughness at -60°C.

In order to solve these problems, methods such as limiting welding heat input, limiting interlayer temperature, and dividing the fill layer and surface layer into several passes to weld (see Figure 1) are currently being proposed in the field.

However, when performing flux cored arc welding, which is a semi-automatic welding technique, unlike automatic welding such as submerged arc welding, welding conditions are likely to change depending on the welder's work method, so welding must be done according to the guidelines suggested by the engineer. In many cases, it is not performed. Even if performed according to the instructions, welding productivity is poor due to various constraints.

Republic of Korea Patent Publication No. 10-0666788 (registered on January 3, 2007) (one-side welding method for ship welding butt joints)

Therefore, the problem to be solved by the present invention is to provide a weld metal with excellent impact toughness even in a low temperature environment (-60°C) through flux cored arc welding.

Another problem to be solved by the present invention is to provide a sound weld metal without high-temperature cracking by selecting an appropriate nickel content with low high-temperature cracking susceptibility.

Another problem to be solved by the present invention is to provide a welding method for forming a weld metal with excellent impact toughness even in a low temperature environment (-60°C) through flux cored arc welding.

Another problem to be solved by the present invention is to provide a welding method for forming a weld metal with excellent impact toughness even in a low temperature environment (-60°C) by reducing the number of welds compared to the existing method in consideration of welding productivity.

In order to achieve the above-described task, the weld metal with excellent low-temperature impact toughness according to the present invention and the welding method for forming the same are a method of forming a weld metal with excellent low-temperature impact toughness by butt welding two base materials to be welded with FCAW. wherein the weld metal is made of a first pass, a second pass, and a third pass, and the butt welding includes an arrangement step of arranging the two base materials in a form facing each other; An attachment step of attaching a ceramic backing material to the bottom of the butted base material; An ultra-layer welding step of performing ultra-layer welding on the ceramic backing material to form the first layer, which is the first pass; A removal step of removing the ceramic backing material after the first pass is cooled; A fill layer welding step of welding an upper part of the first pass to form a fill layer that is the second pass; and performing a surface layer welding step of forming the third pass surface layer by welding the top of the fill layer.

In addition, the butt welding is characterized in that two base materials with a thickness of 9 to 12 mm are placed at intervals of 4 to 6 mm.

In addition, the butt welding is performed by downward welding, and the first pass is performed at a current of 180 to 190 A, a voltage of 24 to 25 V, a speed of 13 to 14 CPM, a heat input of 19 to 24 KJ/CM, and a preheating temperature of 0° C. or higher; The second pass has a current of 230 to 270A, a voltage of 25 to 26V, a speed of 16 to 26CPM, a heat input of 13 to 27KJ/CM, and an interlayer temperature of 250°C or less; The third pass is characterized in that the current is 240 to 270A, the voltage is 25 to 26V, the speed is 18 to 22CPM, the heat input is 16 to 23KJ/CM, and the interlayer temperature is 250℃ or less.

In addition, the butt welding is performed by vertical upward welding, and the first pass is performed at a current of 180 to 190 A, a voltage of 21 to 22 V, a speed of 7 to 8 CPM, a heat input of 28 to 33 KJ/CM, and a preheating temperature of 0° C. or higher; The second pass has a current of 190 to 200A, a voltage of 22 to 23V, a speed of 15 to 16CPM, a heat input of 16 to 18KJ/CM, and an interlayer temperature of 250°C or less; The third pass is characterized in that the current is 190 to 200A, the voltage is 22 to 23V, the speed is 14 to 16CPM, the heat input is 17 to 20KJ/CM, and the interlayer temperature is 250℃ or less.

In addition, the weld metal is C: 0.05%, Si: 0.23-0.32%, Mn: 0.67-0.74%, Cr: 0.026-0.032%, Ni: 3.17-3.39%, Mo: 0.01% or less, B : 0.003~0.004%, Nb: 0.010~0.013%, V: 0.01%, N: less than 0.001%, P: 0.01% or less, S: 0.01% or less, the remainder including Fe and inevitable impurities. .

As described above, according to the weld metal with excellent low-temperature impact toughness according to the present invention and the welding method for forming the same, impact toughness can be secured even in a low-temperature environment of -60°C, thereby preventing rework due to failure in the production test. You can.

In addition, by selecting an appropriate nickel content with low high-temperature crack susceptibility, weld metal without high-temperature cracking can be formed, thereby lowering high-temperature crack susceptibility and producing a sound weld.

In addition, considering welding productivity, it is possible to secure fast welding productivity by reducing the number of welds compared to before.

Figure 1 is a cross-sectional view showing the shape of the weld part of a butt weld joint according to the prior art.
Figure 2 is a process diagram showing a welding method for forming a weld metal with excellent low-temperature impact toughness according to the present invention.
Figure 3 is a diagram showing a weld metal with excellent low-temperature impact toughness according to the present invention and welding conditions for forming the weld metal.
FIG. 4 is a cross-sectional view showing the shape of weld metal formed as the welding conditions shown in FIG. 3 are met.
Figure 5 is a diagram showing the chemical components contained in the weld metal with excellent low-temperature impact toughness according to the present invention.
Figure 6 is a diagram for explaining the appropriate content of nickel shown in Figure 5.

In order to fully understand the present invention, its operational advantages, and the objectives achieved by practicing the present invention, reference should be made to the accompanying drawings illustrating exemplary embodiments of the present invention and the contents described in the accompanying drawings.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description with reference to the accompanying drawings, identical or corresponding components will be assigned the same drawing numbers and overlapping descriptions thereof will be omitted. do.

Figure 2 is a process diagram showing a welding method for forming a weld metal with excellent low-temperature impact toughness according to the present invention.

Referring to Figure 2, the welding method for forming a weld metal with excellent low-temperature impact toughness according to the present invention is to butt weld two base materials 10 to be welded through flux cored arc welding (FCAW) in a low temperature environment (-60°C). ℃) to provide a weld metal with excellent impact toughness, including the placement step (110), the attachment step (120), the first layer welding step (130), the removal step (140), the filling layer welding step (150), and the surface layer. A welding step 160 may be included.

In the arrangement step 110, the two base materials 10 to be welded are placed facing each other. At this time, the base material 10 is a metal that is used as a material for welding or gas cutting, and in the present invention, a material with a thickness of 9 to 12 mm can be used. In addition, it is preferable that the two base materials 10 are placed facing each other with a gap of 4 to 6 mm. In other words, in the present invention, butt welding is performed by arranging two base materials with a thickness of 9 to 12 mm at intervals of 4 to 6 mm.

In the attachment step 120, a ceramic backing material 20 is attached to the bottom of the butted base material 10 to prevent melting of the first layer 30 during butt joint welding. Here, the ceramic backing material 20 is for forming a back bead, and may be made of a ceramic material with a certain thickness, and may be attached to the bottom of the base material 10 butted by the aluminum tape 21. there is.

At this time, since the ceramic backing material 20 and the aluminum tape 21 are conventional, detailed description thereof will be omitted.

In the first layer welding step 130, the first layer, which is the first pass 30, is formed by performing first layer welding on the upper part of the ceramic backing material 20 attached in the attachment step 120. At this time, the first layer welding step 130 may be performed by conventional flux core arc welding (FCAW). Additionally, the welding posture applied in the first layer welding step 130 may be any one of the downward-facing, vertically upward, and horizontal welding postures.

At this time, when applying the downward view and vertical upward welding posture in the first layer welding step 130, it is desirable to meet the welding conditions as shown in FIG. 3.

Figure 3 is a diagram showing a weld metal having excellent low-temperature impact toughness according to the present invention and welding conditions for forming the weld metal. Referring to Figures 2 and 3, a bottom view welding posture in the first layer welding step 130. When forming the first layer, which is the first pass 30, the conditions of the welding posture as shown below are current 180 to 190A, voltage 24 to 25V, speed 13 to 14CPM, heat input 19 to 24KJ/CM, and preheating temperature 0℃ or more. It is desirable to meet.

In contrast, when forming the first layer, which is the first pass 30, through the vertical upward welding posture in the first layer welding step 130, the conditions of the vertical upward welding posture are current 180 to 190 A, voltage 21 to 22 V, and speed 7 to 8 CPM. , it is desirable to meet heat input of 28 ~ 33KJ/CM and preheating temperature of 0℃ or higher.

Meanwhile, the horizontal welding position has good impact toughness even when using the existing welding method, so it will not be described separately.

Referring again to FIG. 2, in the removal step 140, after the first pass 30 welded in the first layer welding step 130 is cooled, the ceramic backing material 20 is separated and removed from the bottom of the base material 10 to which it is butted. do. At this time, the ceramic backing material 20 is preferably removed in the removal step 140, but is not limited to this. That is, the ceramic backing material 20 may be removed after the first layer welding, or after forming the second pass 40, a filling layer, depending on the welder's preference.

In the filling layer welding step 150, the upper portion of the first pass 30 formed in the first layer welding step 130 is welded to form a filling layer, which is the second pass 40. At this time, the filling layer welding step 150 may be performed by conventional flux core arc welding, like the first layer welding step 130. In addition, the welding posture applied in the filling layer welding step 150 may be any of the downward-facing, vertically upward, and horizontal welding postures.

At this time, when forming the second pass 40 by applying the downward and vertical upward welding postures in the filling layer welding step 150, it can be formed by satisfying the welding conditions as shown in FIG. 3.

In more detail with reference to FIG. 3, when forming the filling layer of the second pass 40 through the downward welding posture in the filling layer welding step 150, the conditions for the downward welding posture are a current of 230 to 270 A. , it is desirable to meet the voltage of 25 ~ 26V, speed of 16 ~ 26CPM, heat input of 13 ~ 27KJ/CM, and interlayer temperature of 250℃ or less.

In contrast, when forming the second pass 40 filling layer through the vertical upward welding posture in the filling layer welding step 150, the conditions of the vertical upward welding posture are current 190 to 200 A, voltage 22 to 23 V, and speed 15. It is desirable to meet ~ 16CPM, heat input of 16 ~ 18KJ/CM, and interlayer temperature of 250℃ or less.

Meanwhile, the horizontal welding position has good impact toughness even when using the existing welding method, so it will not be described separately.

In the surface layer welding step 160, the surface layer, which is the third pass 50, is formed by welding the upper part of the fill layer, which is the second pass 40. At this time, the surface layer welding step 160 may be performed by conventional flux core arc welding, similar to the first layer welding step 130 and the filling layer welding step 150. In addition, the welding posture applied in the surface layer welding step 160 may be any one of downward facing, vertical upward, and horizontal welding postures.

At this time, when forming the third pass 50 by applying the downward and vertical upward welding postures in the surface layer welding step 160, it can be formed by satisfying the welding conditions as shown in FIG. 3.

Specifically, as shown in FIG. 3, when forming the surface layer, which is the third pass 50, through the downward welding posture in the surface layer welding step 160, the conditions of the downward welding posture are current 240 to 270 A, voltage It is desirable to meet 25 ~ 26V, speed 18 ~ 22CPM, heat input 16 ~ 23KJ/CM, and interlayer temperature 250℃ or less.

In contrast, when forming the surface layer of the third pass 50 through the vertical upward welding posture in the surface layer welding step 160, the conditions of the vertical upward welding posture are current 190 to 200 A, voltage 22 to 23 V, and speed 14 to 16 CPM. , it is desirable to meet heat input of 17 ~ 20KJ/CM and interlayer temperature of 250℃ or less.

Meanwhile, the horizontal welding position has good impact toughness even when using the existing welding method, so it will not be described separately.

Therefore, according to the weld metal and welding method with excellent low-temperature impact toughness according to the present invention, unlike the existing method of welding in a total of 6 passes, welding can be done in 3 passes, and impact toughness can be secured at -60°C. There will be.

FIG. 4 is a cross-sectional view showing the shape of weld metal formed as the welding conditions shown in FIG. 3 are met.

The first pass 30, the second pass 40, and the third pass 50 formed by satisfying the welding conditions shown in FIG. 3 are formed to have a cross section as shown in FIG. 4 according to the downward-looking posture and the vertical upward posture. do. That is, the weld metal 60 formed according to the downward viewing posture may be formed in the form shown in FIG. 4A. Additionally, the weld metal 60 formed according to the vertical upward posture may be formed in the form shown in FIG. 4B.

That is, as shown in FIG. 4, welding one layer in one pass is different from the conventional welding method shown in FIG. 1.

To be more specific, as in the conventional welding method shown in Figure 1, it is advantageous to secure impact toughness by dividing one layer into two passes and welding, but as in the welding method according to the present invention, chemical composition that can secure impact toughness is used. One layer can be welded in one pass by applying welding materials. For this reason, when using a base material 10 having a thickness of 9 to 12 mm, such as the base material 10 used in the present invention, welding can be completed in a total of 3 passes. Accordingly, welding productivity can be increased.

Meanwhile, the chemical composition is formed as the flux cored wire and the base material 10 are melted and solidified by a welding arc.

It is preferable that the first pass 30, the second pass 40, and the third pass 50 are welded using a welding material having the same chemical composition.

At this time, the chemical composition of the welding metal 30, 40, 50 has the greatest influence on the chemical composition of the welding material, but the dilution of the base material 10 and the difference in weld metal recovery rate depending on welding conditions also have an influence. It's crazy.

That is, since the base material dilution rate and welding conditions of the first pass 30, second pass 40, and third pass 50 are different, the chemical composition at each location may be somewhat different.

Accordingly, the chemical composition of the weld metal 60 described later represents the value measured in the surface layer, which is the third pass 50.

As such, the chemical composition measured on the surface of the third pass 50 will be described in detail with reference to FIGS. 5 and 6.

Figure 5 is a diagram showing the chemical components contained in the weld metal with excellent low-temperature impact toughness according to the present invention, and Figure 6 is a diagram explaining the appropriate content of nickel shown in Figure 5.

Referring to Figures 5 and 6, the weld metal with excellent low-temperature impact toughness according to the present invention is C: 0.05%, Si: 0.23-0.32%, Mn: 0.67-0.74%, Cr: 0.026-0.032%, Ni: 3.17-3.39%, Mo: 0.01% or less, B: 0.003-0.004%, Nb: 0.010-0.013%, V: 0.01%, N: less than 0.001%, P: 0.01% or less, S: 0.01% or less, The remainder includes Fe and inevitable impurities.

With reference to Figures 5 and 6, the reason for limiting the numerical value of each component is explained as follows. Hereinafter, it should be noted that the unit of content of each ingredient is weight% unless specifically mentioned.

Carbon (C): 0.05% by weight

Carbon is an essential element to ensure the strength of weld metal. In addition, by forming delta-ferrite in an appropriate range, the high-temperature cracking resistance of the weld zone can be improved by increasing the solubility of impurity elements. In order to form the minimum tensile strength of 490 MPa or more required by the weld metal of the present invention and to make the primary solidified phase delta-ferrite considering the nickel content, the content range of 0.05% is preferable.

Silicon (Si): 0.23~0.32% by weight

Silicon acts as a deoxidizer in the weld metal, participates in the fluidity of the molten metal, and maintains the strength of the weld metal. Additionally, silicon affects slag fluidity and improves bead appearance. However, if added excessively, it may accelerate the transformation of island-phase martensite (M-A constituent), which may have a negative effect on the impact toughness and crack susceptibility. Therefore, in the present invention, it is preferable to limit the silicon content to 0.23 to 0.32%.

Manganese (Mn): 0.67~0.74% by weight

Manganese is an element that improves the deoxidation effect and strength of weld metal. It is formed in the form of MnS around TiO oxide and plays a role in promoting the formation of needle-shaped ferrite, which is advantageous for impact toughness. In addition, since MnS is formed before FeS, a low-melting point compound, it contributes to improving high-temperature crack resistance. In the present invention, a content range of 0.67 to 0.74% is preferable considering combination with other elements.

Chromium (Cr): 0.026~0.032% by weight

Chromium is an element that contributes to strength improvement, but if it is excessive, it forms carbides and nitrides, which may reduce impact toughness. Accordingly, in the chemical composition of the present invention, a content range of 0.026 to 0.032% is preferable.

Nickel (Ni): 3.17~3.39% by weight

Nickel is an element that improves the strength and toughness of the matrix through solid solution strengthening. It is an austenite stabilizing element that can lower the transition temperature of impact toughness and simultaneously stabilize impact toughness. If the nickel content of the weld metal is less than 3.17%, the impact toughness stabilization effect cannot be achieved during high heat input welding considering productivity, and if the nickel content of the weld metal is more than 3.40%, weldability may be reduced due to lower meltability.

Meanwhile, as shown in Figure 6, the primary solidified phase of the welded metal with a nickel content of 3.17 to 3.39% is delta-ferrite with a BCC structure, but the primary solidified phase of the welded metal with a nickel content of 4.00% or more is auste with an FCC structure. It's a night. In this case, S and P that are not dissolved in the austenite resin phase, which has a small solubility of impurity elements, are organized in the liquid phase to form a low melting point compound, making it very sensitive to high temperature cracking. Therefore, when considering all -60℃ impact toughness guarantee, high-temperature crack resistance, and welding workability, a content range of 3.17 to 3.39% is preferable.

Molybdenum (Mo): 0.01% by weight or less

Molybdenum plays a role in improving the strength of the matrix, but if it exceeds 0.01%, hardenability increases under welding conditions with a fast cooling rate, which can create a low-temperature transformation structure, which poses a risk of low-temperature cracking. Additionally, impact toughness may be reduced. Therefore, it is desirable to limit the molybdenum content to 0.01% or less.

Boron (B): 0.003~0.004% by weight

Boron is an element that improves hardenability and segregates at grain boundaries to suppress the formation of grain boundary ferrite. However, if added excessively, it is undesirable because it greatly increases weld hardenability and promotes the creation of low-temperature transformation tissue, resulting in low-temperature cracks and lowering toughness. Therefore, it is desirable to limit the boron content to 0.003-0.004%.

Niobium (Nb): 0.010~0.013% by weight

Niobium precipitates at grain boundaries and increases the strength and hardness of the weld zone, but can reduce the toughness and crack resistance of the weld metal. Therefore, it is desirable to limit the niobium content to 0.010-0.013%.

Vanadium (V): 0.01% by weight

Vanadium precipitates at grain boundaries and increases the strength and hardness of the weld zone, but can reduce the toughness and crack resistance of the weld metal. Therefore, it is desirable to limit the vanadium content to 0.01%.

Nitrogen (N): less than 0.001% by weight

If the nitrogen content is excessive, it may cause a decrease in toughness due to an increase in the amount of dissolved nitrogen present in the weld metal. Therefore, it is desirable to limit the nitrogen content to less than 0.001%.

Phosphorus (P): 0.01% by weight or less

Phosphorus is an impurity element that promotes high-temperature cracks in welds, so it must be kept as low as possible. Therefore, it is desirable to limit the phosphorus content to 0.01% or less.

Sulfur (S): 0.01% by weight or less

Sulfur can cause high-temperature cracking by forming low-melting point compounds such as FeS. Therefore, it is desirable to limit the sulfur content to 0.01% or less.

The remaining components of the present invention include iron (Fe) and inevitable impurities.

Therefore, the weld metal 60 made of the above-mentioned chemical composition can complete the welding in a total of three passes by welding one layer in one pass, different from the conventional welding method, thereby increasing welding productivity.

In addition, the weld metal 60 composed of the above-mentioned chemical components has excellent impact toughness even in a low-temperature environment.

Although the invention made by the present inventor has been described in detail according to the above-mentioned embodiments, the present invention is not limited to the above-described embodiments and, of course, can be changed in various ways without departing from the gist of the invention.

10: Base material
20: Ceramic backing material
30: 1st pass
40: 2nd pass
50: Third pass
60: Weld metal
110: Placement step
120: Attachment step
130: First layer welding step
140: Removal step
150: Filling layer welding step
160: Surface layer welding step

Claims (5)

In the method of butt welding two base materials to be welded with FCAW to form a weld metal with excellent low-temperature impact toughness,
The weld metal is
Consisting of a first pass, a second pass and a third pass,
The butt welding is
A placement step of arranging the two base materials in a form facing each other;
An attachment step of attaching a ceramic backing material to the bottom of the butted base material;
An ultra-layer welding step of performing ultra-layer welding on the ceramic backing material to form the first layer, which is the first pass;
A removal step of removing the ceramic backing material after the first pass is cooled;
A fill layer welding step of welding an upper part of the first pass to form a fill layer that is the second pass; and
A surface layer welding step is performed to form the third pass surface layer by welding the top of the fill layer,
The butt welding is performed by placing two base materials with a thickness of 9 to 12 mm at intervals of 4 to 6 mm,
When the butt welding is performed by downward welding, the first pass is performed at a current of 180 to 190 A, a voltage of 24 to 25 V, a speed of 13 to 14 CPM, a heat input of 19 to 24 KJ/CM, and a preheating temperature of 0° C. or higher; The second pass has a current of 230 to 270A, a voltage of 25 to 26V, a speed of 16 to 26CPM, a heat input of 13 to 27KJ/CM, and an interlayer temperature of 250°C or less; The third pass is performed under the conditions of current 240 to 270A, voltage 25 to 26V, speed 18 to 22CPM, heat input 16 to 23KJ/CM, and interlayer temperature 250℃ or less,
When the butt welding is performed by vertical upward welding, the first pass uses a current of 180 to 190 A, a voltage of 21 to 22 V, a speed of 7 to 8 CPM, a heat input of 28 to 33 KJ/CM, and a preheating temperature of 0° C. or higher; The second pass has a current of 190 to 200A, a voltage of 22 to 23V, a speed of 15 to 16CPM, a heat input of 16 to 18KJ/CM, and an interlayer temperature of 250°C or less; The third pass is a weld metal with excellent low-temperature impact toughness, characterized in that it is made under the conditions of a current of 190 to 200 A, a voltage of 22 to 23 V, a speed of 14 to 16 CPM, a heat input of 17 to 20 KJ/CM, and an interlayer temperature of 250 ℃ or less. Welding method for forming. delete delete delete In claim 1, the weld metal is,
By weight %, C: 0.05%, Si: 0.23-0.32%, Mn: 0.67-0.74%, Cr: 0.026-0.032%, Ni: 3.17-3.39%, Mo: 0.01% or less, B: 0.003-0.004%, Nb : 0.010~0.013%, V: 0.01%, N: less than 0.001%, P: 0.01% or less, S: 0.01% or less, the remainder containing Fe and inevitable impurities. A weld metal with excellent low-temperature impact toughness. and a welding method for forming it.

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