JP5052976B2 - Multilayer butt-welded joint and welded structure with excellent brittle crack propagation characteristics - Google Patents

Description

  The present invention relates to a multi-layer butt-welded joint that is excellent in suppressing or stopping the propagation of a brittle crack generated in a welded joint, that is, an excellent brittle crack propagation characteristic, and a welded structure having the welded joint.

  When building a welded structure by welding steel plates, a large heat input welding method is usually used to reduce the construction cost and improve the welding efficiency, but in a welded joint formed by the large heat input welding method, In addition, the toughness of the weld heat affected zone (hereinafter sometimes referred to as “HAZ zone”) decreases, the width of the HAZ zone increases, and the fracture toughness value Kc (an index related to brittle fracture) also decreases.

  Fracture of a welded joint occurs when stress concentrates on a defect formed at the time of welding and a crack is generated starting from the defect, and this crack propagates inside the joint. If the fracture toughness value Kc of the welded joint is low, cracks are likely to occur and the propagation of the cracks is fast, so that the welded joints are suddenly broken. That is, the welded joint breaks brittlely.

  In order to prevent brittle fracture of a welded joint, it is necessary to (i) suppress the generation of cracks and (ii) suppress or stop the propagation of the generated cracks. In view of the above (i), the present applicant has found a technique for sufficiently increasing the fracture toughness value Kc, and has proposed a high heat input butt weld joint having excellent brittle fracture resistance (Patent Document 1 and 2), and a verification method for accurately verifying the brittle fracture resistance of the high heat input butt weld joint based on the fracture toughness value Kc was proposed (see Patent Document 2).

The weld joint proposed by the present applicant is useful in that brittle fracture is unlikely to occur and the safety of the welded structure can be improved. In addition, the verification method described above causes brittle fracture. This is useful in designing difficult welded joints.
However, even in a welded joint designed so as not to generate a crack in a normal stress load environment, a crack may occur due to sudden or impact stress or irregular and complicated stress.

Conventionally, in butt welded joints of TMCP steel sheets with a plate thickness of about 25 mm, brittle cracks are deflected to the base metal due to the action of residual stress inside the welded joints, so the brittle crack propagation characteristics of the base metal are reduced. If increased, it was thought that a brittle crack generated inside the welded joint could be guided to the base metal and stopped.
However, the brittle cracks generated in butt welded joints in the case of high-strength thick steel plates that have started to be used because the design stress can be set high with the increase in the size and simplification of the welded structure in recent years. As a result of the inventor's destructive test, it was found that the weld joint does not deviate to the base metal side and propagates along the HAZ portion depending on the degree of the fracture resistance value of the welded joint (see Non-Patent Document 1).

  Further, the present inventor has found that when a plate thickness is, for example, a steel plate of 70 mm or more, a large toughness distribution is formed in the welded joint, and a brittle crack crosses the welded joint to reinforce it. Even if the plate was welded with fillet, it was not captured by the reinforcing plate, but propagated along the weld metal part or the HAZ part, and the weld joint was found to be broken.

Therefore, the present inventor, based on the above-mentioned facts, removes part or all of the region where the butt weld joint of the vertical member and the fillet weld joint of the horizontal member intersect, and contains Ni having compressive residual stress by repair welding Weld metal with excellent toughness with an amount of 2.5 mass% or more (see Patent Document 3) or weld metal with excellent fracture toughness with arrest performance (K _Ca value) of 2000 N / mm ^1.5 or more (Patent Document) 4), and even if the brittle crack propagates along the longitudinal direction of the butt weld of the vertical member, the crack propagation direction is diverted around the weld metal with high toughness or arrest performance, and the horizontal member We proposed a welded joint and welding method with excellent brittle crack propagation stopping ability to stop at the base metal part.

  These methods mainly displace cracks propagating along FL (the boundary between the weld metal and the base metal heat-affected zone) in the one-pass large heat input butt welded joint to the steel plate side where the arrest performance is high. By stopping, the safety of the butt weld joint is improved.

However, from the results of large-scale butt-welded joints, the inventors have found that in multi-layer butt-welded joints, cracks propagate inside the weld metal, so conventional one-pass large heat input butt-welded joints are used. It was confirmed that even if an effective method was applied, a sufficient effect could not be obtained with a multilayer butt-welded joint.
Therefore, there is a need for a technique for reliably stopping a brittle crack generated in a multi-layer butt welded joint of steel plates having a thickness of 50 mm or more and avoiding large-scale damage of the butt welded joint.

JP 2005-144552 A JP 2006-088184 A JP 2005-111520 A JP 2006-078874 A Welded Structure Symposium Abstracts 2006, p. 195-202

In the present invention, when a steel plate having a thickness of 50 mm or more is butt welded in multiple passes, even if a brittle crack occurs in the welded joint, the brittle crack is difficult to propagate in the longitudinal direction of the welded joint, and It is an object of the present invention to form a welded joint excellent in properties that will stop even if propagated, that is, in brittle crack propagation properties.
And this invention solves the said subject, and aims at providing the welded structure which has the welded joint which was excellent in the brittle crack propagation characteristic, and this welded joint.

The present inventor has intensively studied a method for solving the above problems based on the above-mentioned facts. As a result, if at least one weld layer having better toughness than the other weld layers is formed between the front surface weld layer and the back surface weld layer of the multi-layer welded joint, the weld layer having excellent toughness will have a fracture resistance. It has been found that even if a brittle crack occurs, it functions as a layer, and the propagation of the crack in the longitudinal direction of the welded joint can be suppressed or the propagation can be stopped.
This invention was made | formed based on the said knowledge, and the summary is as follows.

In multi-pass butt welded joint (1) thickness 50mm or more of the steel sheet, of the surface weld layer and back surface weld layers of weld layer, the entire layer of at least one welding layer is, during the welding of the upper layer of the welding layer A brittle-proof flaw characterized by being formed as a welded layer that has a better tempering effect and has better toughness, and functions as a fracture-resistant layer that suppresses or stops the propagation of brittle cracks in the longitudinal direction of welded joints. Multi-layer butt weld joint with excellent crack propagation characteristics.
(2) The weld layer functioning as the fracture resistance layer is formed at a plurality of intervals in the thickness direction of the welded joint, and has excellent brittle crack propagation characteristics as described in (1) above Multi-layer butt weld joint.
(3) The weld layer functioning as the fracture resistance layer is formed continuously in the longitudinal direction of the welded joint, and has the brittle crack propagation characteristics as described in (1) or (2) above Excellent multi-layer butt weld joint.
(4) The weld layer functioning as the fracture resistance layer is formed intermittently in the longitudinal direction of the welded joint, with the brittle crack propagation characteristics as described in (1) or (2) above Excellent multi-layer butt weld joint.
(5) The multi-layer excellent in brittle crack propagation characteristics according to (4) above, wherein the weld layer functioning as the fracture resistance layer has a length of 200 mm or more and an intermittent interval of 400 mm or less. Butt weld joint.
(6) A welded structure having a welded joint part obtained by multi-layer butt welding of steel plates, wherein at least a brittle crack is generated in the welded joint part and may be propagated in the longitudinal direction of the welded joint A welded structure characterized in that the joint portion is the multilayer butt-welded joint according to any one of (1) to (5).

  According to the present invention, in multi-pass butt welding of steel plates having a thickness of 50 mm or more, a welded joint having excellent brittle crack propagation characteristics can be formed.As a result, using a steel plate having a thickness of 50 mm or more, A welded structure with excellent brittle crack propagation characteristics can be constructed.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows an embodiment of the multi-layer butt weld joint of the present invention formed by welding the butt V groove of the steel plate 1 by multiple passes.

In the embodiment shown in FIG. 1, each weld layer is formed by a plurality of passes except for the back surface weld layer 2b, and such weld layers are sequentially stacked by multi-layer welding.
At that time, a weld layer (hereinafter referred to as a high toughness weld layer) having toughness superior to the weld layers 2a, 2b and other intermediate weld layers 2c in the weld layer between the front surface weld layer 2a and the back surface weld layer 2b. Also referred to as)) 2x is formed.
In the present invention, such a high toughness weld layer 2x is a layer that has been given a tempering effect during welding of the upper weld layer 2c and has a better toughness.

In order to form the high toughness weld layer 2x, first, the heat input of each weld pass of the layer (hereinafter sometimes referred to as a specific weld pass) is welded to 70% or less of the heat input of the normal weld pass. . Then, as shown in FIG. 2, the weld layer is formed by a weld pass having a throat thickness Lx smaller than a throat thickness Lc of a normal weld pass (other intermediate weld pass). Then, when the upper layer 2c is welded subsequently to the layer, each welding pass is returned to the normal heat input, and welding is performed with a thickness Lc that is larger than Lx.
As a result, the entire layer of the welding path formed with a small throat thickness is tempered by the heat effect of the upper layer welding path.

As described above, the weld layer 2x is tempered under the influence of the heat of the upper layer weld path, and the toughness is improved by the tempering effect, so that the weld layer 2x has a large resistance to brittle fracture.
For this reason, even if a brittle crack is generated inside the welded joint, it is possible to suppress or stop the propagation of the crack in the longitudinal direction of the welded joint by the high-toughness weld layer 2x as described below. it can.

  By disposing the high toughness weld layer 2x in the multilayer weld layer constituting the weld joint, the toughness distribution in the plate thickness direction of the weld joint changes sharply at the weld layer 2x, so it is divided vertically. It becomes the shape that was made. That is, in the welded joint shown in FIG. 1, there are relatively low toughness weld layers 2c above and below the high toughness weld layer 2x.

For example, when impact stress acts on the welded joint, the high toughness weld layer 2x is difficult to brittle fracture, so that the brittle crack is composed of the brittle crack X on the plate thickness surface side and the plate as shown in FIG. It splits into a brittle crack Y on the thick back surface side, and propagates through the weld layer 2c on each side.
As shown in FIG. 4, when both ends of the brittle crack X reach the joint surface and the high toughness weld layer 2x, the brittle crack Y still exists in the weld layer 2c. Both the two brittle cracks X and Y propagate in the longitudinal direction of the welded joint while maintaining a narrow crack in the plate thickness direction. As a result, the stress intensity factor at the crack tip is reduced and the driving force for propagating the crack is reduced, so that the crack propagation is likely to stop.

When the brittle crack Y gradually propagates in the plate thickness direction and propagates through the brittle fracture region of the weld layer 2c and reaches the high toughness weld layer 2x as shown in FIG. 5, the brittle crack X in the weld layer 2x. Plastic deformation occurs in the ductile fracture region Z between Y and Y, and the propagation energy of the brittle cracks X and Y is absorbed while ductile fracture occurs.
As a result, as shown in FIG. 6, the brittle cracks X and Y stop when they propagate to some extent on the plate thickness front side and plate thickness back side.

  For this reason, the high toughness weld layer 2x is formed in the welded joint part which has a possibility that at least a brittle crack occurs and propagates in the longitudinal direction of the welded joint among the welded joint parts welded by multi-layer butt welding. Thus, even when a brittle crack occurs, this high toughness weld layer 2x functions as a fracture resistance layer, and can suppress or stop the propagation of the crack along the longitudinal direction of the welded joint. The brittle crack propagation characteristics of the welded structure having the welded joint portion can be improved.

  The high toughness weld layer 2x as described above is not limited to only one layer as shown in FIG. 1, but as shown in FIG. 3, among the plurality of weld layers between the front surface weld layer 2a and the back surface weld layer 2b. The high toughness weld layers 2x and 2y having toughness superior to the toughness of the weld layers 2a, 2b, and 2c can be formed with the weld layer 2c interposed therebetween. In this case, the ability to suppress the propagation of brittle cracks is enhanced.

  In addition, it is desirable that the high toughness weld layer is continuously present in the longitudinal direction of the welded joint. However, if a length of 200 mm or more is secured in the longitudinal direction of the welded joint, the brittleness generated inside the welded joint is ensured. Since it exhibits the function of suppressing or stopping the propagation of cracks, it can also exist intermittently in the longitudinal direction of the welded joint. When forming intermittently, it is necessary to switch the welding conditions each time, and the layer thickness may change in the middle of the layer, so a high toughness weld layer should be formed over the entire length in the longitudinal direction of the welded joint. Is more preferable.

  Furthermore, when making it intermittent, the length of the part which does not become a high toughness weld layer needs to be 400 mm or less. If the intermittent interval exceeds 400 mm, the crack generated at the initial stage may propagate and the length of one finally formed crack may be 400 mm or more. When the crack length is 400 mm or more, the energy of the crack becomes excessive, and the high toughness weld layer does not function sufficiently, and it becomes difficult to branch the brittle crack in the high toughness weld layer.

  The welded joint that is the subject of the present invention is a butt welded steel plate having a thickness of 50 mm or more. If the height in the plate thickness direction from the groove bottom portion of the welded joint is 50 mm or more, it is more preferable to form a high toughness weld layer in the plurality of weld layers, and more preferably to put another weld layer in between. By forming a plurality of high-toughness weld layers by sandwiching them, the above-described function of the high-toughness weld layer can be fully exhibited. In particular, when the thickness of the steel plate is 70 mm or more, the function of the high toughness weld layer can be more fully exhibited.

  In the present invention, the shape of the groove forming the welded joint is not particularly limited, such as a V-type, an X-type, a K-type, and a ladle. Further, the present invention is not particularly limited with respect to the welding posture, the number of passes, the multi-layer welding method, and the like when forming the welded joint.

  Next, examples of the present invention will be described. The conditions of the examples are one example of conditions used to confirm the feasibility and effects of the present invention, and the present invention is limited to this example. It is not a thing. As long as the object of the present invention is achieved without departing from the gist of the present invention, the present invention can adopt various conditions.

  Test specimens having welded joints formed under the steel plate conditions and welding pass conditions shown in Table 1 were prepared, and the brittle crack propagation characteristics were measured by a brittle crack propagation test. The results are shown in Table 2. In addition, the composition of the steel plate steel type used at that time is shown in Table 3, and the composition of the welding material is shown in Table 4, respectively.

In producing the welded joint, a steel plate having a thickness of 50 mm to 100 mm was used, and all the welding passes were performed by carbon dioxide arc welding using the same welding material.
As the welding material shown in Table 4, two types of flux cored wires having chemical compositions designed so that the yield strength of the weld metal sufficiently satisfies 36 kg / mm ² and 47 kg / mm ² were used.
The groove at the time of welding is formed by applying a V, X or K type groove to the end of the steel sheet by machining according to the thickness and yield strength of the steel sheet, applying the heat input shown in Table 1, and welding length A weld joint with a thickness of 2500 mm was formed.

In Table 1, the specific welding pass is a welding pass for forming a high toughness weld layer, and the heat input amount during each welding is shown in Table 1 for the specific welding pass and the other basic welding passes.
Also, among the basic welding passes, the welding pass that forms a layer immediately above the specific welding pass layer gives a tempering effect (thermal effect) to the specific welding pass layer and improves the toughness of that layer. The amount of heat and the temperature between passes are shown separately from other welding passes.
Note that the welding was performed so that the temperature between all the passes was in the range of 80 ° C to 150 ° C.

Since the specific welding pass needs to receive a sufficient heat capacity from the pass of the layer immediately above the entire region of the weld bead, the throat thickness Lx was suppressed from becoming too large. In this example, in addition to the amount of heat input, the size of the weld bead, that is, the temperature between passes, which is a factor for determining the throat thickness Lx, is particularly limited not to exceed 80 ° C.
In addition, the interpass temperature of the weld pass that forms a layer immediately above the specific weld pass is the upper limit pass temperature in this embodiment for all joints in order to give sufficient tempering (thermal effect) to the specific weld pass. It was set to 150 ° C.

The specific welding pass performed to form the high toughness weld layer was in the range of 1 to 5 layers in one joint.
In Table 1, the length in the longitudinal direction is “continuous” indicates that the weld bead forming the high toughness weld layer is applied over the entire weld length of 2500 mm. Further, the numerical values described in the columns of the longitudinal length and the longitudinal interval are respectively the length at which the high toughness weld layer exists in the weld longitudinal direction of the joint, and the longitudinal interval between adjacent high toughness weld layers. Is shown.

In Table 1, Nos. 1 to 8 are joints into which a high toughness weld layer is introduced in accordance with the present invention, and the distribution form in the joint longitudinal direction of the high toughness weld layer is variously changed upon introduction.
This No. In the inventive examples 1 to 8, it can be seen that the brittle crack has a short length even if it propagates and is immediately stopped (arrested).

On the other hand, Nos. 9 to 18 are comparative examples, and in particular, Nos. 9 to 13 and Nos. 16 to 18 are joints in which the high toughness weld layer according to the present invention is not formed at all. Nos. 14 and 15 are joints in which two or three high-toughness weld layers are formed in the joint, but the length in the longitudinal direction is up to 200 mm and the interval exceeds 400 mm.
In the comparative examples of Nos. 9 to 18, in all cases, the brittle crack easily propagated over the entire length of the joint, and the specimen was broken without being arrested.

  As described above, according to the present invention, a welded structure having a welded joint having excellent brittle crack propagation characteristics can be constructed using a steel plate having a thickness of 50 mm or more. Therefore, the present invention has great applicability in the field of construction of welded structures.

It is a figure which shows the one aspect | mode of the multilayer butt-welding joint of this invention. It is a figure explaining the process in which the multilayer butt-welding joint of this invention is formed. It is a figure which shows another aspect of the multilayer butt-welding joint of this invention. It is a figure which shows the aspect in which the brittle crack produced | generated in each of the plate | board thickness surface side and the plate | board thickness back surface side in the welded joint of this invention. It is a figure which shows the aspect which a plastic deformation produced and ductile fractured in the area | region between two brittle cracks. It is a figure which shows the aspect which a brittle crack propagates to some extent and stops on a plate | board thickness surface side and a plate | board thickness back surface side.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Welded metal part 2a Surface weld layer 2b Back surface weld layer 2c Middle weld layer in multi-layer welding 2x, 2y High toughness weld layer Lx High toughness weld layer throat thickness Lc Middle weld layer throat thickness X, Y Brittleness Crack Z Ductile fracture region

Claims (6)

In multi-pass butt welded joint having a thickness of 50mm or more of the steel sheet, the whole layer of at least one welding layer of the surface weld layer and back surface weld layers of welding layer, a tempering effect during the welding of the upper layer of the welding layer Brittle crack propagation characteristics characterized by being formed as a welded layer that has a superior toughness and that functions as a fracture resistant layer that suppresses or stops the propagation of brittle cracks in the longitudinal direction of welded joints Multi-layer butt-welded joint with excellent resistance.   The multi-layer butt welding having excellent brittle crack propagation characteristics according to claim 1, wherein a plurality of weld layers functioning as the fracture resistance layer are formed at intervals in the thickness direction of the weld joint. Fittings.   The multi-layer butt welding with excellent brittle crack propagation characteristics according to claim 1 or 2, wherein the weld layer functioning as the fracture resistance layer is formed continuously in the longitudinal direction of the weld joint. Fittings.   The multi-layer butt welding having excellent brittle crack propagation characteristics according to claim 1 or 2, wherein the weld layer functioning as the fracture resistance layer is formed intermittently in the longitudinal direction of the weld joint. Fittings.   The multilayer butt-welded joint having excellent brittle crack propagation resistance according to claim 4, wherein the weld layer functioning as the fracture resistance layer has a length of 200 mm or more and an intermittent interval of 400 mm or less. .   A welded structure having a welded joint part obtained by multi-layer butt welding of steel plates, wherein at least a brittle crack is generated in the welded joint part and a welded joint part that may propagate in the longitudinal direction of the welded joint is provided. A welded structure comprising the multilayer butt-welded joint according to any one of claims 1 to 5.

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