JP5433928B2 - 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 A weld metal having a toughness of 2.5% by mass or more (see Patent Document 3) or a weld metal having a fracture toughness having an arrest performance (K _Ca value) of 2000 N / mm ^1.5 or more (Patent Document 4) 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 has been found that even if an effective method is applied, a sufficient effect may 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 JP 2004-130315 A US Pat. No. 6,467,321 Welded Structure Symposium Abstracts 2006, p. 195-202

In the present invention, when a steel plate is butt welded in multiple passes, even if a brittle crack occurs in the welded joint, the brittle crack hardly propagates in the longitudinal direction of the welded joint, and even if it propagates. It is an object of the present invention to form a welded joint excellent in the characteristics to stop, that is, the brittle crack propagation resistance.
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, a layer having higher toughness than other weld metal parts is formed between the surface weld layer and the back surface weld layer of the multi-layer welded joint, and the toughness distribution in the plate thickness direction of the weld joint is divided by the layer. If this is the case, the layer having the high toughness functions as a fracture resistance layer, and even if a brittle crack occurs, the propagation of the crack in the longitudinal direction of the welded joint can be suppressed or stopped. I found.
This invention was made | formed based on the said knowledge, and the summary is as follows.

(1) In a multi-layer butt-welded joint in which welded layers are continuously formed by welding butt V-grooves of steel sheets having a thickness of 50 mm or more in multiple passes, an intermediate weld layer between the surface weld layer and the back surface weld layer, fracture modified layer compressive residual stress by ultrasonic striking treatment has been applied are the forms, reforming layer is formed intermittently in the longitudinal direction of the welded joint, the longitudinal direction of the intermittent reforming layer A multilayer having excellent brittle crack propagation characteristics, characterized in that the length is 200 mm or more, the intermittent interval is 400 mm or less, and the modified layer functions as a fracture resistance layer that suppresses or stops the propagation of brittle cracks. Butt weld joint.
(2) The multilayer butt-welded joint having excellent brittle crack propagation characteristics as described in (1) above, wherein the modified layer is formed at a plurality of intervals in the thickness direction of the welded joint. .
(3) Furthermore, a fracture behavior modified layer to which a compressive residual stress is applied by an ultrasonic impact treatment is formed on either one or both of the front surface weld layer and the back surface weld layer. The multilayer butt-welded joint according to (1) or (2).
(4) A welded structure having a multi-layer butt-welded joint portion of the steel sheet, wherein at least a brittle crack is generated in the weld joint portion, and the weld joint portion that is likely to propagate is the above (1) to (3) A welded structure having the multilayer butt-welded joint according to any one of (3).

  According to the present invention, in multi-pass butt welding of steel sheets, it is possible to form a welded joint having excellent brittle crack propagation characteristics, and as a result, using the steel sheet, welding having excellent brittle crack propagation characteristics. A structure can be built.

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 in multiple layers by multi-layer welding.
At that time, the brittle crack propagation behavior different from that of the weld layers 2a, 2b and the intermediate weld layer 2c is exhibited by performing ultrasonic striking treatment in the weld layer between the front weld layer 2a and the back weld layer 2b. The modified layer 3x is formed.

  The modified layer 3x to which the compressive residual stress is applied by performing the ultrasonic striking treatment exhibits a crack propagation behavior apparently different from the weld layers 2a, 2b, and 2c in the other tensile residual stress states, and has high toughness. It has the same effect as the case where the weld layer which has this is formed. In the present invention, the modified layer formed on the weld layer by this ultrasonic striking treatment is defined as a “destructive behavior modified layer” in order to distinguish it from other weld layer portions.

  As shown in Patent Document 5, for example, the ultrasonic hitting process mechanically vibrates an ultrasonic vibration terminal having a diameter of about 2 to 8 mm under conditions of a frequency of 5 to 60 kHz and an output of 500 W to 10 kW. A process for modifying the physical properties of the treated part by striking the treated part at the front surface of the ultrasonic vibration terminal is performed by an apparatus as shown in Patent Document 6, for example.

  Such ultrasonic striking treatment is performed on the weld bead as shown in FIG. A compressive residual stress having a depth of several millimeters in the plate thickness direction can be applied to the surface layer of the weld bead hit by the ultrasonic vibration terminal 4 as shown in FIG. In the portion 3x ′ to which the compressive residual stress is applied, the surface is remelted and disappears at the time of the next upper layer welding, but the lower portion is a modified layer to which the compressive residual stress is applied as shown in FIG. Remains as 3x.

  In general, a weld layer formed by welding is thermally contracted by cooling after solidification, and as a result of restraining this contraction from the surroundings, tensile residual stress is introduced into the weld layer. In the region where the tensile residual stress of the weld layer is introduced, a brittle crack is likely to occur, and the propagation growth of the brittle crack is also facilitated. On the other hand, in the modified layer 3x in which the ultrasonic wave is applied to the weld layer, a compressive residual stress is applied to a depth range of several millimeters from the surface layer, and thus has a large resistance to brittle fracture and is brittle. Cracks are unlikely to occur and brittle cracks are difficult to propagate. For this reason, in the modified layer 3x, the propagation of the brittle crack is delayed as compared with the other weld layers, and acts to suppress the propagation of the crack.

As shown in FIG. 7, in the situation where a brittle crack propagates and grows in the longitudinal direction of the weld line of the weld joint, tensile stress acts on the weld joint, but in the region of the modified layer 3x to which compressive residual stress is applied. Since the principal stress direction is inclined from a direction perpendicular to the normal plate thickness, a fracture surface inclined more than other weld layers is exhibited.
For this reason, even if a brittle crack occurs in the welded joint, the fracture behavior improved layer 3x can suppress or stop the propagation of the crack in the longitudinal direction of the welded joint, as will be described below. it can.

  By disposing the fracture behavior modified layer 3x in the multi-layered weld layer constituting the weld joint, the toughness distribution in the plate thickness direction of the weld joint changes sharply at the fracture behavior modified layer 3x. The shape is divided up and down. That is, in the welded joint shown in FIG. 1, there are relatively low toughness weld layers 2c above and below the fracture behavior modified layer 3x.

For example, when impact stress is applied to the welded joint, the fracture behavior modified layer 3x is not easily brittle fractured. Therefore, the brittle crack is, as shown in FIG. It is generated by branching into a brittle crack Y on the back side of the plate thickness, and propagates through the weld layer 2c on each side.
As shown in FIG. 6, the brittle crack X has both ends reaching the joint surface and the fracture behavior modified layer 3x, but the brittle crack Y is still present 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 fracture behavior modified layer 3x as shown in FIG. 7, in the fracture behavior modified layer 3x, Plastic deformation occurs in the ductile fracture region Z between the brittle cracks X 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. 8, the brittle cracks X and Y are stopped when they are propagated to some extent on the plate thickness front side and the plate thickness rear surface side.

As shown in FIG. 4, the fracture behavior modified layer 3x as described above is superior to the toughness of the weld layers 2a, 2b and 2c among the plurality of weld layers between the front surface weld layer 2a and the back surface weld layer 2b. and that to form the fracture behavior modification layer 3x and 3y comprises a toughness. As shown in FIG. 4, the weld layers forming the fracture behavior modified layers 3x and 3y may be weld layers that are continuous in the vertical direction, or the weld layers that are above and below the weld layers that do not form the modified layer therebetween. Good. By providing a plurality of modified layers, the performance of suppressing the propagation of brittle cracks is enhanced.

  Furthermore, after welding, either one of the surface weld layer 2a and the back surface weld layer 2b, or both weld layers are subjected to ultrasonic impact treatment, and the fracture behavior is improved by applying compressive residual stress to the surface layer. A quality layer may be formed. FIG. 5 shows a case where the fracture behavior modified layer 3z is formed on the surface weld layer 2a.

  In addition to the above-described effects due to the application of compressive residual stress when ultrasonic welding is applied to either one of the welded surface layer 2a and the welded surface layer 2b of the welded joint, or both welded layers, ultrasonic waves Since the surface layer region of the surface layer and / or the back layer subjected to the striking treatment is refined by ultrasonic processing, resistance to brittle fracture is significantly improved. Therefore, brittle cracks can be buried inside the plate thickness, and the ability to suppress the propagation of brittle cracks is further enhanced. And since the shape discontinuity part and welding defect in a weld toe part are eliminated, the effect that generation | occurrence | production of the fatigue crack from a welded joint can be delayed is also acquired.

  In addition, it is preferable to perform an ultrasonic hit | damage process in the state whose temperature of a weld bead is 300 degrees C or less. A temperature of 300 ° C. or higher is not preferable because the yield stress of the weld metal is low when striking with an ultrasonic vibration terminal.

The fracture behavior modified layer formed by the ultrasonic hitting process as described above is desirably continuously present in the longitudinal direction of the welded joint, but may be present intermittently. If a fracture behavior modified layer having a length of 200 mm or more is ensured in the longitudinal direction of the welded joint, the fracture behavior modified layer can cause brittle cracks generated in the weld joint to Branches to the back surface side and exhibits the function of suppressing or stopping the propagation of cracks in the longitudinal direction of the welded joint.
However, in the case where the fracture behavior modified layer is intermittently formed, if the intermittent interval exceeds 400 mm, an initially generated crack propagates and the length of one finally formed crack is 400 mm or more. There is a possibility. When the crack length is 400 mm or more, the energy of the crack becomes excessive, and the fracture behavior modified layer does not function sufficiently, and it becomes difficult to branch a brittle crack in the fracture behavior modified layer. The intermittent interval is limited to 400 mm or less.

  The mechanism by which the brittle crack stops when the fracture behavior modified layer is intermittently present is the same as that when the fracture behavior modified layer exists continuously. That is, even if a brittle crack occurs in the joint part where the fracture behavior modified layer does not exist and propagates in the length direction of the welded joint, if the fracture behavior modified layer is encountered, the plate thickness front side and plate thickness back side It branches to the side, propagates on each side, and stops when it propagates to some extent.

  The welded joint that is the subject of the present invention is a multi-layer butt weld of steel plates. In the present invention, among the plurality of weld layers in the multi-layer butt-welded joint of steel sheets, one or a plurality of the weld layers are subjected to ultrasonic striking treatment to form the fracture behavior modified layer, whereby the above-described welding is performed. The effect of improving the propagation stopping characteristics of the brittle crack in the length direction of the joint is exhibited. In order to sufficiently exhibit this effect, in the multi-layer butt welding by the MIG or MAG welding method, the thickness of the steel plate for securing three or more weld layers is preferably 50 mm or more. Further, in multi-layer butt welding by the TIG welding method, since the weld layer can be about several millimeters, a sufficient effect can be obtained when the plate thickness of the steel plate is 10 mm or more.

As a further, in forming the welded joint, welding position, heat input, the number of paths, with such a multi-pass welding process, the present invention is not limited in particular.

  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.

  Welded joints were formed under the steel plate conditions and welding pass conditions shown in Table 1, and the brittle crack propagation characteristics were measured by a brittle crack propagation test. The results are shown in Table 2. In addition, Table 3 shows the component composition of the steel sheet steel used.

No. In the invention examples of 2 to 15, it can be seen that the brittle crack has a short length even if it propagates and stops immediately.
Of the inventive examples, No. Nos. 10 to 15 are examples of the invention in which a fracture behavior modified layer was provided inside the thickness of the welded joint, and the propagation path of the brittle crack changed before and after the modified layer in the thickness direction, so the number of brittle cracks after branching. Became 3 or more, and the propagation of cracks in the longitudinal direction of the weld line stopped. In addition, No. 2, 3, 8 are invention examples in which a plurality of fracture behavior modification layers are provided continuously on the surface layer portion and the back layer portion of the welded joint, and a ductile fracture region Z is formed on the surface layer portion and the back layer portion, There were no brittle crack branches in the thickness direction, the number of cracks was 1, and the propagation of cracks in the longitudinal direction of the weld line stopped.
On the other hand, Nos. 16 to 25, which are welded joints deviating from the conditions of the present invention, could not immediately stop the propagation of the brittle crack in the longitudinal direction of the weld line.

  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. 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 a high toughness modified layer is formed in the middle of multilayer butt welding. It is a figure which shows the state in which the high toughness modified layer was formed in the middle of multilayer butt welding. It is a figure which shows another aspect of the multilayer butt-welding joint of this invention. 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 Weld metal 2a Surface weld layer 2b Back surface weld layer 2c Intermediate weld layer in multi-layer welding 3x 'Portions to which compressive residual stress is applied 3x, 3y, 3z Fracture behavior modified layer 4 Ultrasonic vibration terminal X, Y Brittle crack Z Ductile fracture region

Claims (4)

In a multi-layer butt-welded joint in which welded layers are continuously formed by welding butt V-grooves of steel sheets having a thickness of 50 mm or more in multiple passes,
In the intermediate weld layer between the front surface weld layer and the back surface weld layer, a fracture behavior modified layer to which compressive residual stress is applied by ultrasonic impact treatment is formed,
Reforming layer is formed intermittently in the longitudinal direction of the welded joint, the longitudinal direction length of the intermittent reforming layer is more than 200 mm, intermittent interval is at 400mm or less,
A multilayer butt-welded joint excellent in brittle crack propagation characteristics, wherein the modified layer functions as a fracture resistance layer that suppresses or stops the propagation of brittle cracks.   The multilayer butt weld joint having excellent brittle crack propagation characteristics according to claim 1, wherein the modified layer is formed at a plurality of intervals in the thickness direction of the weld joint.   Furthermore, the fracture behavior improvement layer to which the compressive residual stress was given by ultrasonic striking treatment is formed in either one or both of the surface weld layer and the back surface weld layer. 2. A multi-layer butt-welded joint according to 2.   It is a welded structure which has the multilayer butt-welding joint part of the said steel plate, Comprising: At least a brittle crack of the said weld joint part generate | occur | produces, The welded joint part which may propagate is any one of Claims 1-3. A welded structure characterized in that it is a multilayer butt-welded joint according to item 1.

Images