KR102046952B1 - Welded joint with excellent ctod properties in welding heat affected zone - Google Patents

Abstract

According to an exemplary embodiment of the present invention, in a welding part obtained by multi-layer welding of steel, when the welding heat affected portion formed by the welding is divided into an upper region, a center region, and a lower region along the thickness direction, the center region is a fine grain region ( Fine Grain Heat Affected Zone (FGHAZ) and the upper region and the lower region relates to a welded portion having excellent CTOD characteristics of the weld heat affected zone including a coarse grain heat affected zone (CGHAZ) and a manufacturing method thereof. . According to the present invention, it is possible to provide a welded portion and a method of manufacturing the improved welded heat affected zone CTOD characteristics by controlling the welding wire angle and the bead overlapping length appropriately in the multilayer welding of the ultra-thick steel.

Description

WELDED JOINT WITH EXCELLENT CTOD PROPERTIES IN WELDING HEAT AFFECTED ZONE}

The present invention relates to a welded portion and a method of manufacturing the same having excellent weld heat affected zone CTOD (Crack Tip Opening Displacement) characteristics, and more particularly, to a weld wire angle and a bead overlap length during multilayer welding of an ultra-thin steel. The present invention relates to a welded part having excellent weld heat-affected zone CTOD characteristics and to a method of manufacturing the same.

In recent years, the application of high strength, ultra-thick shipbuilding steel has increased with the increase of container ships. In the case of large ships using such ultra-thick steels, there is always a risk of brittle crack propagation that can lead to hull failure in a short time, even if small defects occur inside the steel or especially the weld due to external stress during operation or continuous fatigue environment.

Therefore, in order to prevent the risk of brittle crack propagation in advance, in large ships including the Norwegian German Register (DNVGL), the welding heat affected zone and the weld metal are used to prevent crack initiation in the weld zone when the ultra-thick steels are applied. It requires a CTOD characteristic.

 On the other hand, for the welding of thick plates, the shipbuilding industry uses submerged arc welding (SAW) with excellent welding efficiency and flux cored arc welding (FCAW) with excellent welding workability at various angles. ) Is mainly applied. In addition, multi-pass welding is mainly applied for welding ultra-thick steels. The multi-layer welding has an advantage of low weld heat input for each pass, which is superior in strength and toughness of the weld metal part or the heat affected part compared to the high heat input welding method.

For example, Patent Document 1 discloses a method of using multilayer welding as a welding method for structural steel materials containing Cr: 1.5 to 3.5%, Mo: 0.5 to 1.5%, and V: 0.15 to 0.5% by mass.

However, even during multi-layer welding, the grains of the base material grow by welding heat input at the part adjacent to the fusion line, which is the boundary line between the weld metal and the base material, and the cooling rate is relatively high at this part, so the low temperature transformation structure is mainly generated. Coarse grain HAZ (CGHAZ) is formed. Since the CGHAZ formed as a cause of deterioration of CTOD characteristics, it is necessary to control the microstructure of the weld heat affected zone.

Korean Unexamined Patent Publication No. 10-2014-0142226

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide a welded portion having improved CTOD characteristics of welded heat affected zones and a method of manufacturing the same, by appropriately controlling the welding wire angle and the length of bead overlap during multilayer welding of ultra-thick steel materials. .

In order to solve the above problems, the present invention, in the welding portion obtained by welding the steel in multiple layers, the weld heat affected portion formed by the welding divided the steel into the upper region, the center region and the lower region along the thickness direction In this case, the central region is composed of a fine grain heat affected zone (FGHAZ) alone, and the upper region and the lower region have a weld heat affected zone CTOD characteristic including a coarse grain heat affected zone (CGHAZ). Provides excellent welds.

The lengths of the upper region, the center region and the lower region satisfy the following relations (1) to (3).

0.2 × T <H ₁ <0.5 × T --- (1)

0 <H ₂ ≤ 0.3 × T --- (2)

0.2 × T <H ₃ <0.5 × T --- (3)

However, in the relations (1) to (3), T represents the thickness of the steel, H ₁ to H ₃ represents the length (based on the steel thickness direction) of the upper region, the center region and the lower region, respectively.

The FGHAZ constituting the center region may be formed by reheating the CGHAZ formed by the preceding pass welding by the trailing pass welding in direct contact with the preceding welding pass, and may be composed of grains having a relatively smaller size than the CGHAZ. have.

The upper region and the lower region may further include FGHAZ.

At least a portion of the central region may be disposed on the steel thickness centerline.

The steel may be a very thick steel having a thickness of 50mm ~ 200mm.

According to the present invention, it is possible to control the weld wire angle and the length of bead overlap in the multi-layer welding applied to the ultra-thick steels so that the grain structure in the center region (based on the steel thickness direction) of the weld heat affected zone is relatively excellent in CTOD characteristics in CGHAZ. By transforming to FGHAZ, the CTOD characteristics of the weld heat affected zone can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic cross-sectional view of steel and welded parts welded in a multilayer according to a conventional method.
2 is a schematic cross-sectional view of a steel and welded parts welded in multiple layers according to an embodiment of the present invention.
3 is a schematic diagram showing a process of forming a weld bead using a welding wire.
Figure 4 is a photograph showing the weld heat affected portion of the intermediate region welded in accordance with Example 1 and the weld bead formed in contact with it.

Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

 The present inventors have conducted studies to secure the CTOD characteristics in the multi-layer welding process of the ultra-thick steel. As a result, the part which most influences the CTOD characteristics of the weld heat affected zone is near the center of the weld heat affected zone (based on the steel thickness direction). The present invention was confirmed that the CTOD characteristics of the weld heat affected zone can be improved by adjusting the welding wire angle and the bead overlap length when forming the weld bead contacting the steel in the vicinity of the center.

First, a process of identifying a part that affects the CTOD characteristics of the welding heat affected zone will be described in detail with reference to FIG. 1. 1 is a cross-sectional view of the ultra-thick steel and welded portions welded by multilayer welding according to a conventional method. However, FIG. 1 does not show weld beads stacked in multiple layers by multilayer welding.

In general, when welding a very thick steel, a single bevel groove (Single groove) or K-groove (K-groove, K-shaped groove) is applied a lot, the steel is processed with a single bevel groove in Figure 1 Is shown

Specifically, in order to investigate the CTOD characteristics of the weld heat affected zone, first, the submerged arc welding (SAW) or the flux cored arc welding is performed on the ultra-thick steel material 10 processed with a single bevel groove. ; FCAW) multi-layer welding. Subsequently, after processing the notch 30 in the weld heat affected zone generated by the welding to generate a fatigue crack, the CTOD characteristics of the test piece cooled to a predetermined temperature by a bending tester are evaluated. At this time, the notch 30 was generated in the steel thickness direction within about 5 mm of the melting line 31, as shown in FIG.

In the CTOD characteristic test, crack generation in the fatigue crack 30 is generally started. In addition, in the case where a special defect does not exist at the tip of the fatigue crack 30, the starting point of the crack is usually the center of the weld heat affected portion in which the stress is largely applied to the surface portion (based on the thickness direction of the steel) due to triaxial stress action or the like. In the thickness direction of steel). That is, near the point M where the notch line 30 and the steel thickness centerline (black horizontal dotted line) cross each other, the crack initiation point of the weld heat affected zone may be used. Therefore, in order to improve the CTOD characteristic of a welding heat affected zone, it can be said that the characteristic control of the M point vicinity which is a starting point of a crack is important.

In addition, when the CGHAZ having the most toughness in the weld heat affected zone on the notch line 30 is present, it becomes vulnerable to crack initiation when the CTOD property of the weld heat affected zone is evaluated.

In view of these points, the present inventors can control the microstructure so that CGHAZ is not generated near the center of the weld heat affected zone (based on the steel thickness direction) where the crack initiation point can be located to improve the CTOD characteristics of the weld heat affected zone. It was recognized that there was a need.

Hereinafter, with reference to FIG. 2 will be described in detail with respect to the weld portion excellent in the heat affected zone CTOD characteristics according to the present invention. 2 is a cross-sectional view schematically showing the extreme thick steel and welded portion welded by multilayer welding according to an embodiment of the present invention.

Specifically, the welded portion having excellent CTOD characteristics of the welded heat-affected portion of the present invention is obtained by multi-layer welding the steel 10, and the welded heat-affected portion formed by the welding is formed in the upper region along the thickness direction of the steel 10. 23), when divided into the central region 24 and the lower region 25, the central region 24 is composed of a fine grain heat affected zone (FGHAZ) alone, the upper region 23 and the lower region Zone 25 is characterized by including a coarse grain heat affected zone (CGHAZ).

The FGHAZ constituting the center region 24 may be formed by reheating the CGHAZ formed by the preceding pass welding by the trailing pass welding in direct contact with the preceding welding pass, and having a relatively smaller size than the CGHAZ. Can be configured. Specifically, some of the structure of the preceding pass welding part is transformed by the thermal effect during the trailing pass welding in the multi-layer welding process, wherein the CGHAZ structure near the heat melting line 31 formed during the preceding pass welding is formed by the trailing pass welding. It can be reheated and transformed into FGHAZ. The FGHAZ is a fine grain region in which CGHAZ composed of coarse grains is reheated to an ac3 transformation point or more, and has excellent mechanical properties such as toughness.

The tissue transformation from CGHAZ to FGHAZ as described above is possible by adjusting the welding conditions in the unstructured control region 22 (yellow display portion-) shown in FIG. 2, and the welding conditions will be described later. Method ”.

However, as the microstructure control region 22 increases, the work time increases and the number of welders increases by requiring detailed work of the operator, so that the length of the microstructure control region 22 (based on steel thickness direction) Is less than 30% of the thickness of the steel. In addition, it is desirable that CGHAZ is not present as close to the thickness centerline 40 as there may be a crack initiation point.

Thus, the length of the upper region 23, the center region 24 and the lower region 25 preferably satisfies the following relations (1) to (3).

0.2 × T <H ₁ <0.5 × T --- (1)

0 <H ₂ ≤ 0.3 × T --- (2)

0.2 × T <H ₃ <0.5 × T --- (3)

However, in the relations (1) to (3), T represents the thickness of the steel, H ₁ to H ₃ represents the length (based on the steel thickness direction) of the upper region, the center region and the lower region, respectively.

At least a portion of the central region 24 may be disposed on the steel thickness centerline 40, and more preferably, only the FGHAZ tissue may be disposed on the thickness centerline extension line 40 without CGHAZ.

On the other hand, the classification including DNVGL, there is a provision that should include more than 15% of the fraction of the CGHAZ structure on the notch wire 30 when the CTOD evaluation of the weld heat affected zone. In the present invention, the CGHAZ is a tissue existing only in the upper region 23 and the lower region 25 of the welding heat affected zone. When the CGHAZ fraction of at least 15% is secured in these regions, the FGHAZ and CGHAZ are different in the remaining portions of the regions. By transforming to coexist, it is possible to manufacture welds suitable for the class standards and exhibiting excellent weld heat affected zone CTOD characteristics. That is, the upper region 23 and the lower region 25 may further include FGHAZ in addition to CGHAZ. In this case, the FGHAZ may be included in an appropriate fraction as necessary, and may be included continuously or discontinuously in the regions.

The welded portion provided according to the present invention can be preferably applied to ultra-thick steels having a thickness of 50 mm to 200 mm. Since the weld shows excellent weld heat affected zone CTOD characteristics, the weld structure provided with the weld of the present invention can significantly reduce the risk of brittle fracture.

In order to fabricate the weld as described above, a microstructure composed of FGHAZ alone is required in the center region 24 of the weld heat affected zone. In the present invention, the microstructure may be controlled by controlling a lamination method for multilayer welding. Can be.

Specifically, the method of manufacturing a welded portion having excellent heat affected zone CTOD characteristics according to the present invention is a method of obtaining a welded portion by welding a steel multilayer, and when the steel is divided into an upper region, a center region, and a lower region along a thickness direction, When the welding bead is formed in contact with the steel in the center region, the welding wire angle is controlled in the range of 0 ° to 10 °, and the bead of the welding bead (B) formed after the welding bead (A) and the welding bead (A) The overlap length is characterized in that it is adjusted to 1/2 or less (excluding 0mm) of the weld bead width.

3 schematically shows a process of forming a weld bead using a welding wire. With reference to Figure 3 looks at in more detail with respect to the welded manufacturing method according to the present invention.

When the angle of the welding wire is increased, the contact area between the welding bead and the melting line is increased, thereby increasing the height of the welding bead, thereby increasing the CGHAZ fraction. Thus, in the present invention, the welding wire angle is limited to within 10 °. In addition, as the wire angle is closer to 0 ° within the range of 0 ° to 10 °, the contact area between the weld bead and the melting line is reduced because the wire is positioned parallel to the molten surface, thereby reducing the CGHAZ fraction.

And, bead overlap length indicates the length of the weld bead (A) formed in contact with the melting line and the weld bead (B) welded after the weld bead (A) overlap, if the overlap length at this time increases the weld bead ( When the height of the trailing pass welding bead C stacked on A) is increased, and the overlap length is reduced, the height of the welding bead C is lowered. At this time, the lower the height of the weld bead (C) is the CGHAZ fraction is reduced.

In the present invention, the center of the steel material by limiting the welding wire angle to 0 ° ~ 10 ° and limiting the bead length to less than 1/2 of the weld bead width when forming the weld bead in contact with the steel in the center region 24 The CGHAZ fraction in the vicinity was minimized.

The welding bead may be formed to be in contact with the steel is a plurality of continuously stacked. As described above, the weld metal region 22 in which the weld bead is formed by adjusting the welding wire angle and the bead overlapping length is a region formed to control the microstructure of the weld heat affected zone. It is named microstructure control region 22 and is indicated by a yellow portion in FIG. In addition, the region except for the microstructure control region 22 in the welder 20 is referred to as the microstructure uncontrolled region 21.

In addition, the grain size of the central region 24 can be refined by the formation of the weld bead in the microstructure control region 22 as described above, and thus FGHAZ alone is formed near the center of the weld heat affected zone. The central region 24 may be formed.

In addition, the upper region 23 and the lower region 25 in order to further include FGHAZ in addition to the CGHAZ, in the present invention, the welding wire when forming a weld bead in contact with the steel even in a portion of the upper region and the lower region Welding can be performed at an angle (0 ° to 10 °) and a bead overlap length (half or less of the weld bead width).

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

Example 1

First, a 100 mm thick steel material prepared by a single bevel groove was prepared, and multi-layer welding was performed through submerged arc welding (SAW).

At this time, the steel is divided into an upper region of 32.5mm, a center region of 21mm and a lower region of 46.5mm along the thickness direction, the welding wire angle 0 °, bead width 15mm, bead overlap when forming a weld bead contacting the steel in the center region Weld beads were formed under the condition of 7 mm in length. In this way, the central region of the weld heat affected zone was composed of FGHAZ tissue without CGHAZ.

 However, the upper region and the lower region formed weld beads under the conditions of a welding wire angle of 7 °, a bead width of 15 mm, and a bead overlap length of 10 mm. In these regions, CGHAZ and FGHAZ coexisted in crystal structure.

Examples 2,3 and Comparative Examples 1-7

The same as in Example 1 except that the wire angle, bead width, bead overlap length and the center region length (based on the steel thickness direction) were adjusted as shown in Table 1 when the weld bead was formed in contact with the steel in the central region. Multi-layer welding was performed by the method.

<Evaluation of CTOD Characteristics of Weld Heat Affected Zone>

After performing the multi-layer welding according to the above Examples 1 to 3 and Comparative Examples 1 to 7, after making the notch in the steel thickness direction at the position of the notch line 30 shown in Figure 2 (5mm point toward the steel in the melting line), The weld metal CTOD value was measured using a CTOD tester at −10 ° C., and the results are shown in Table 1 below.

Wire angle
(deg.) Bead width (mm) Bead overlap length
(mm) Center area length
(mm) Weld Heat Affected Zone CTOD @ -10 ℃ (mm) Example 1 0 15 7 25 0.45 Example 2 2 13 5 23 0.61 Example 3 10 12 5 24 0.58 Comparative Example 1 4 12 4 42 0.42 Comparative Example 2 4 12 5 50 0.42 Comparative Example 3 5 11 7 10 0.25 Comparative Example 4 13 14 5 15 0.21 Comparative Example 5 12 10 7 10 0.18 Comparative Example 6 15 12 7 12 0.19 Comparative Example 7 10 11 8 17 0.18

Looking at Table 1, the welding wire angle is in the range of 0 ° ~ 10 °, the bead overlap length is less than 1/2 of the bead width, the ratio of the length of the center region (microstructure control region) to the steel thickness 30% or less In Examples 1 and 2 welded under the condition of, it can be seen that the CTOD value of the weld heat affected zone is high.

In addition, referring to FIG. 4, in which a photograph of a weld heat-affected portion of the intermediate region welded according to the first embodiment and a weld bead contacted thereto is shown, it can be seen that CGHAZ does not exist on the notch line. However, in Examples 2 and 3, the weld heat affected zone and the weld bead shape, which were almost similar to those of Example 1, were not attached as separate drawings, and CGHAZ was not present on the notched line.

On the other hand, in Comparative Examples 1 and 2 in which the wire angle is in the range of 0 ° to 10 ° and the bead overlap length is less than 1/2 of the bead width, the CTOD value of the welding heat affected zone is relatively higher than in Comparative Examples 3 to 7 Although it has, compared to the embodiment is inadequate state, the ratio of the length of the central region to the steel thickness exceeds 30%, the working time is considerably longer, it can be said that the practical effect of the present invention is reduced.

Also, in Comparative Example 3, the wire angle was in the range of 0 ° to 10 °, but the bead overlap length was more than 1/2 of the bead width, and in Comparative Example 4, the bead overlap length was not more than 1/2 of the bead width, but the wire angle was Since it is 10 degrees or more, CGHAZ also exists in a center region, and the CTOD value of the weld heat affected zone fell. In Comparative Examples 5 to 7, the CANGAZ was present in the center region because the wire angle and the bead overlap length were outside the optimum ranges, and the region was also widened, resulting in a decrease in the CTOD value of the weld heat affected zone.

As described above, the embodiments disclosed in the present invention are not intended to limit the technical idea of the present invention but to explain the present invention, and the scope of the present invention should be interpreted by the following claims, and all technical ideas within the scope equivalent thereto. Should be construed as being included in the scope of the present invention.

10: steel 20: welded portion
21: microstructure non-control area 22: microstructure control area
23: upper region 24: central region
25: lower region 30: notch line
31: melting line 40: steel thickness centerline
T: Steel thickness M: Crack starting point

Claims (5)

In the welding part obtained by welding a steel multilayer,
When the welding heat affected portion formed by the welding is divided into the upper region, the center region and the lower region along the thickness direction,
The length of the upper region, the center region and the lower region satisfies the following relations (1) to (3),
The central region is composed of only a fine grain heat (Fineaz Heat Affected Zone (FGHAZ)),
The upper region and the lower region are welded parts having excellent CTOD characteristics of the weld heat affected zone including a coarse grain heat affected zone (CGHAZ).
0.2 × T <H ₁ <0.5 × T --- (1)
0 <H ₂ ≤ 0.3 × T --- (2)
0.2 × T <H ₃ <0.5 × T --- (3)
(However, in the relations (1) to (3), T represents the thickness of the steel, H ₁ to H ₃ represents the length (based on the steel thickness direction) of the upper region, the center region and the lower region, respectively. The method of claim 1,
The FGHAZ constituting the center region is formed by reheating the CGHAZ formed by the preceding pass welding by the trailing pass welding in direct contact with the preceding pass welding, and the heat effect of the welding consisting of grains having a relatively smaller size than the CGHAZ. Weldment with excellent CTOD characteristics. The method of claim 1,
The upper region and the lower region are welded parts having excellent CTOD characteristics of the welding heat affected zone further comprising FGHAZ. The method of claim 1
At least a portion of the central region has a weld heat-affecting zone CTOD characteristic disposed on the steel thickness centerline. The method of claim 1,
The steel is a welded material having excellent CTOD characteristics of the welded heat affected zone, which is an extremely thick steel having a thickness of 50 mm to 200 mm.

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