JP2007283369A - Method for improving fatigability of fillet welded zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fatigability improving method by which D-grade or higher fatigability level can be stably secured by improving fatigue strength both in a root and a toe of a weld bead in existing fillet welding. <P>SOLUTION: In a bead including a toe of at least one existing regular weld bead which is formed by fillet welding of a base material and a member to be welded, there is newly formed an overlaid weld bead. Then, an ultrasonic impact treatment is performed on the regular weld bead, the toe of the regular weld bead appearing on the surface of the overlaid weld bead, the toe of the overlaid weld bead, and the toe of the boundary between the regular weld bead and the overlaid weld bead. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for improving fatigue performance of a welded part in a steel structure including a fillet welded part such as a bridge, a tank, and a ship.

Steel structures such as bridges, tanks, ships, etc. are mainly manufactured by combining many steel members by welding. These steel structures are fatigue designed in consideration of fatigue strength in welding.
Since fatigue cracks occurring in welds have a significant effect on the reliability of the entire structure, various methods for improving fatigue characteristics have been attempted. The portion where fatigue cracks are likely to occur is a welded portion, and this includes reasons such as the presence of a stress concentration portion in the welded portion and the occurrence of residual tensile stress.
On the other hand, Non-Patent Document 1 discloses that stress concentration is reduced by a method of smoothing the welded portion by a mechanical method, or a method of applying decorative welding (dressing) to the welded portion by TIG welding. Yes.
In addition, a method has also been proposed in which peening is applied to a welded portion to introduce compressive stress into a portion where fatigue occurs, and stress concentration is reduced at the same time (see, for example, Patent Document 1).
Recently, a method has been proposed in which residual stress is reduced by utilizing transformation expansion of a weld metal to improve fatigue strength (see Patent Documents 2 and 3).

US Pat. No. 6,171,415 JP 2000-288728 A JP 11-138290 A Steel Bridge Fatigue Japan Road Association

In the welding of these steel structures, full penetration welding without a root portion requires a large amount of work and is expensive, and as shown in FIG. 8, the base material 1 and the material 2 to be welded are fillet welded. The welding method in which the main welding bead 5, the toe portion 3 and the root portion 4 are present is employed. In such fillet welding, most of the cracks due to fatigue occur from the root portion 4 or the toe portion 3.
In addition, the fatigue performance of steel structures, for example, for bridges, is determined by the Japan Steel Structure Association (JSSC) “Fatigue Design Guidelines and Explanations for Steel Structures” (referred to as JSSC fatigue design guidelines). Fatigue performance grade D level or higher should be secured on the basis of the fatigue life curve (a curve that plots how many times it can withstand a certain amount of stress repeatedly for each grade, also called SN curve) This is sufficient to surpass the fatigue strength of flange welds and web neck welds.
In addition, although the base material 1 is distinguished for the time being from the main structural material and the material to be welded 2 being welded thereto, this relationship is relative and may be reversed depending on the form of the structure. For convenience.

Under such circumstances, the root part fatigue strength cannot be improved only by the method using the above-described peenings, although the effect of improving the fatigue strength is provided on the surface of the welded portion including the toe portion. For this reason, it is not always possible to reliably achieve the fatigue performance level D grade or higher.
In addition, with TIG dressing, it is difficult to make the toe part completely smooth, and tensile residual stress remains. Therefore, even if the toe fatigue performance is improved, the fatigue performance level F grade is ensured. Is the limit. Of course, it is impossible to improve the fatigue performance of the route.
On the other hand, in the method using the low temperature transformation welding material, the improvement of the fatigue strength of the toe portion is insufficient and not stable, and it is finally possible to secure the fatigue performance level D grade, and in some cases, the E grade is obtained. There is also. In particular, when a bead is added, the boundary portion between the main weld bead and the additional weld bead tends to be a weak point.

Thus, in fillet welding conventionally applied to steel structures, the fatigue strength of both the root part and the toe part of the welded part cannot be improved at the same time, and the fatigue performance level D It was difficult to ensure a grade higher than that stably.
In view of the above situation, the present invention improves the fatigue strength of both the root part and the toe part of the existing welded part (main weld bead) by repairing and stabilizes the fatigue performance level D grade or higher. It is an object of the present invention to provide a method for improving the fatigue performance that can be ensured.

In view of the above problems, the present inventors have reached the following technical idea.
In other words, when improving the fatigue performance of welds in existing fillet welds at both the toe and root parts by repair, it is considered appropriate to take measures to improve each part of the welds. It was.
First, in order to improve the fatigue performance of the toe part, it is considered appropriate to reduce the stress concentration at the toe part and to apply compressive stress, so that the toe part of the weld bead is subjected to ultrasonic impact treatment. It was decided.
Next, it was considered that the fatigue strength of the root portion depends on the size of the throat thickness, and the fatigue strength of the root portion was improved by increasing the throat thickness.
For this purpose, in addition to the main welding bead formed by the existing fillet welding, a part of or the whole of the main welding bead is newly overlapped and additionally welded. (Hereinafter, the weld bead formed by the incremental build-up welding is referred to as an additional build-up weld bead).

As a result of repeated studies based on the above idea, the following solution has been reached.
The gist is that
(1) In the method for improving fatigue performance of a welded part in fillet welding, a new bead including at least one toe part of an existing main weld bead formed by fillet welding a base material and a member to be welded is newly added. After forming the increased weld bead, the main weld bead and the toe portion of the main weld bead appearing on the surface of the additional weld bead, the toe portion of the additional weld bead, and the main weld bead It is characterized by applying ultrasonic shock treatment to the boundary toe with the weld bead,
(2) In the method for improving fatigue performance of a welded part in fillet welding according to (1), the increased weld bead includes the board side toe part and the welded member side toe part of the main weld bead. It is characterized by being formed in both
(3) In the method for improving fatigue performance of a welded part in fillet welding according to (1), the increased weld bead is the board side toe part or the welded member side toe part of the main weld bead. The length of the main weld bead on the side where the increased weld bead is not formed is L, and the substrate on the side where the increased weld bead is formed from the boundary toe, or The increased weld bead is formed so that L ′ ≧ 1 / 3L, where L ′ is a perpendicular distance to the member to be welded.

  In the existing fillet welding, the fatigue strength of both the root portion and the toe portion of the welded portion can be improved by the method of the present invention, and the fatigue performance level D grade or higher can be secured stably.

Embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic cross-sectional view of an existing fillet weld in a bridge or the like. The substrate 1 and the member 2 to be welded are fillet welded at their corners to form a main weld bead 5. Thereby, the toe part 3 (31, 32) is formed in the board | substrate 1 side and the to-be-welded member 2 side, respectively. In the figure, L represents the leg length of the main welding bead 5, and a represents the throat thickness. Note that the main welding bead 5 is formed along the corner portion over a portion where the substrate 1 and the member 2 to be welded are in contact (perpendicular to the paper surface of the drawing).

FIG. 2 is a schematic cross-sectional view of a welded portion showing the first embodiment of the present invention. In the present embodiment, a newly increased weld bead is added to the bead including the toe 31 on the substrate 1 side of the existing main weld bead 5 formed by fillet welding the base material 1 and the member 2 to be welded. 6 is formed. Furthermore, the toe end 32 of the main welding bead 5, the toe end 33 of the additional welding bead 6, and the main welding bead 5 and the additional welding bead appearing on the surfaces of the main welding bead 5 and the additional welding bead 6. 6 is subjected to ultrasonic impact treatment. It should be noted that the formation of the increased weld bead 6 and the ultrasonic treatment of the end portion are performed over a range where the fillet welded substrate 1 and the member 2 to be welded are in contact with each other along the corner portion. Is called.
As a result, the original throat thickness a increases to the throat thickness a ′, and the occurrence of fatigue cracks in the root portion can be suppressed, and the toe portion appearing on the bead surface is subjected to ultrasonic impact treatment, so that The occurrence of fatigue cracks at the end can also be suppressed.
It should be noted that it is preferable to perform an ultrasonic impact treatment on the toe end 31 on the substrate 1 side of the main welding bead 5 before performing the incremental build-up welding because the fatigue performance may be further improved.

FIG. 3 is a schematic cross-sectional view of a welded portion showing a second embodiment of the present invention. In this embodiment, the increased weld bead 6 is formed on both the board 1 side toe 31 and the member to be welded 2 side toe 32 of the main weld bead 5. Further, ultrasonic shock treatment is applied to the toe portion appearing on the surface of the main welding bead 5 and the additional welding bead 6. In the embodiment of FIG. 3, the entire surface of the main welding bead 5 is increased and the upper welding bead 6 is added. The toe ends appearing on the surface are two places, that is, the increased weld bead 35 on the substrate 1 side and the toe end portion 36 of the increased weld bead on the welded member 2 side. An ultrasonic impact treatment is applied to the location.
As a result, the original throat thickness a is increased to the throat thickness a ″, and the occurrence of fatigue cracks at the root portion can be suppressed, and the toe portion appearing on the bead surface is subjected to ultrasonic impact treatment. The occurrence of fatigue cracks at the end can also be suppressed.
In addition, if performing the ultrasonic impact treatment on the toe end 31 on the substrate 1 side and the toe end 32 on the welded member 2 side in the main welding bead 5 before performing the incremental welding, fatigue performance is improved. It may improve more and is preferable.

Here, as shown in FIG. 1, most of the main weld beads in fillet welding have leg lengths L that are equal on both the substrate 1 side and the welded member 2 side.
On the other hand, the spread of stress from the toe portion 3 to the main weld bead 5 is often narrower than 45 degrees, and therefore, all parts of the throat thickness of the main weld bead 5 are not necessarily effective. I may not work.
For example, as shown in FIG. 4 (a), when considering the case of a regular weld bead with a leg length L = 6 mm and an equal leg, the throat thickness a is 4.2 mm, but the stress spread is 30 degrees. As shown in FIG. 4B, the throat thickness (referred to as the effective throat thickness) a ₂ that actually works effectively decreases to 3 mm.

Therefore, it is important to form an increased weld bead so that this effective throat thickness a ₂ is sufficient to sufficiently improve the fatigue performance.
For example, if the stress spread is 30 degrees, the effective throat thickness a ₂ is 3 mm in the main weld bead 5 of FIG. 1, whereas the effective throat thickness a ₂ is on the toe 31 side of the substrate 1 as shown in FIG. If the increased weld bead 6 is formed at an angle of 30 degrees, the effective throat thickness a ₂ is substantially equal to the throat thickness a ′, and thus a ₂ > 4.2 mm, which is 40% or more.

As a result of examining the formation condition of the increased weld bead that can cope with the spread of stress that varies depending on the condition of the welded portion that is symmetric, by adjusting the relationship between the leg length L and the formation position of the increased weld bead I found a solution.
Details will be described with reference to FIG.

That is, the increased build-up weld bead 6 is formed only on one side of the main weld bead 5 on the substrate 1 side toe end portion 31 or the welded member 2 side toe end portion 32 side (formed on the 31 side in the figure). The length of the leg of the main weld bead 5 on the side where the weld bead 6 is not formed is L, and the perpendicular distance from the boundary toe 34 to the substrate 1 or the member 2 to be welded 2 on the side where the increased weld bead 6 is formed. When L ′ is set (perpendicular to 1 in the figure), the additional weld bead 6 is formed so that L ′ ≧ 1 / 3L.
Accordingly, the effective throat thickness a ′ can be set to a thickness sufficient to sufficiently improve the fatigue performance, and the occurrence of fatigue cracks at the boundary toe portion 34 and the root portion 4 can be suppressed.

(Comparative Examples 1 and 2): Examples in which the actual welding bead is used, and examples in which only the ultrasonic impact treatment is applied to the toe portion of the main welding bead.
A fatigue test body having an I-shaped cross section was manufactured and subjected to a fatigue test. That is, FIG. 5 is a figure which shows the outline | summary of the test body of a fatigue test, (a) is a side view, (b) is a top view, (c) is sectional drawing.
As shown in FIG. 5, a plurality of in-plane gussets 12 to be welded members were subjected to main welding by fillet welding to an I-shaped steel 11 to be a substrate used as a bridge girder.
The size of the I-shaped steel is 700 mm (A) × 200 mm (B) × 9 mm (t1) × 25 mm (t2) × 5500 mm (L), and the size of the in-plane gusset 12 is 50 (w) × 200 mm (L). ) × 9 mm (t). The I-shaped steel is obtained by welding assembly using a steel plate having a tensile strength of 400 MPa.
The main welding was performed using the most common 500 MPa class steel as the welding material, and using CO ₂ arc welding with a welding leg length of 6 mm. As a result, a main welding bead 5 having a cross-sectional shape as shown in FIG. 1 was formed. The leg length was determined by the strength design, and the throat thickness a was about 4.2 mm because of the equal leg length. Further, the effective throat thickness a ₂ of the main welding bead 5 is considered to be about 3 mm because of the stress spread in the main case of 30 degrees.

Next, ultrasonic impact treatment was performed on the toe portion of the main weld bead that was main welded. This ultrasonic impact treatment (hereinafter also referred to as UIT (Ultrasonic Impact Treatment)) is a process of hitting an object using a device that vibrates ultrasonically the tip pin with an amplitude of 20 to 60 μm and a frequency of 15 kHz to 60 kHz ( This device includes, for example, a transducer that generates ultrasonic waves, a wave guide that is attached to the front of the transducer, and a pin that is vibrated by ultrasonic waves that is attached to the tip of the wave guide. The head is equipped with.
Using this processing apparatus, ultrasonic vibration of 27 kHz was generated in the transducer from the output power of 1 kw, and processing was performed at a speed of about 15 cm per minute.

After this treatment, a fatigue test was performed. The fatigue test was performed by bending the above-mentioned test body at four points and setting an amplitude condition of 135 MPa (the generated stress of the lower flange in the equal bending section of the central portion 1500 mm of the test body was 135 MPa). This is referred to as Comparative Example 1.
In addition, an experiment under the same conditions as Comparative Example 1 was also performed except that UIT was not applied, and Comparative Example 2 was obtained. The result is shown in FIG.

In Comparative Example 2 where the UIT was not applied, cracks occurred at 1 × 10 ⁵ cycles, and the fatigue performance grades were GF levels.
On the other hand, in Comparative Example 1 to which UIT was applied, some in-plane gussets with low generated stress in the test specimens had fatigue performance grade D level, but fatigue performance in which cracks occurred in 4 × 10 ⁵ cycles. There were many grade E grades.
Although the effect of improving the fatigue performance was recognized to some extent by the UIT of the toe portion, the fatigue performance grade D level could not be achieved stably.
In the results shown in FIG. 6, all the cracks occurred from the toe in Comparative Example 2, and all the cracks occurred from the root in Comparative Example 1 subjected to the UIT process.

Example 1
Next, as an example, a test was performed in which heap welding was performed on the main welding bead, and further, the toe end portion of the bead surface was subjected to UIT treatment.
That is, as in the case of the test for confirming the effect of improving the fatigue performance by the UIT of the toe part, a plurality of in-plane gussets 12 are fillet welded using I-shaped steel used as a bridge girder. The main welding was performed. The dimensions of the test body 10 and the in-plane gusset 12 are the same as in FIG.
Also, the main welding bead 5 was formed under the same conditions as in the case of FIG.
Further, an additional build-up welding was performed on the main welding bead. Welding was performed under a heat input condition of 1.7 kJ / mm using the most common 500 MPa class steel low hydrogen welding rod for welding.
A main welding bead 5 and an additional welding bead 6 as shown in FIG. 2 were formed by main welding and incremental welding.
In forming the increased build-up weld bead 6, L ′ was formed to 4 mm as shown in FIG.

Next, ultrasonic impact treatment was applied to the toe end portion 32 of the main weld bead 5 appearing on the bead surface, the toe end portion 33 of the additional weld bead 6, and the boundary toe end portion 34. This ultrasonic impact treatment was performed at a speed of approximately 15 cm per minute by generating a 27 kHz ultrasonic vibration in the transducer from an output power of 1 kw.
After this treatment, the specimen was subjected to a fatigue test under the same conditions as described above.

As a result, even if it exceeded 2 × 10 ⁶ , cracks did not occur, the average stress (nominal stress) was also at a high level, and it was confirmed that the fatigue performance grade was D or higher.
In addition, when only the formation of the increased weld bead 6 is performed and no UIT is applied, the toe end portion 32 of the main weld bead 5 and the toe end portion 33 of the increase weld bead 6 are obtained in 1.2 × 10 ⁵ cycles. A crack occurred from any one of the boundary toe portions 34.
Based on the above, it was confirmed that the fatigue performance of the root portion of the welded portion and the fatigue performance of the toe portion can be improved by combining the formation of the increased weld bead and ultrasonic impact treatment. It was done.

(Example 2)
In Example 1 described above, L ′ = 4 mm was obtained by incremental welding, but a test was performed in which the length of L ′ was changed.
A test body having an out-of-plane gusset 13 as shown in FIGS. In this test body, the main plate 15 is 100 mm × 600 mm × t12 mm and gussets of 100 mm × 50 mm × t9 mm are joined by fillet welding. The material is steel with a tensile strength of 400 MPa.
About this test body, the overlap thickness L 'shown in FIG. 2 was changed, and the increased welding bead was formed. In addition, this welding bead was made into equal leg length and leg length L was 6 mm.
Next, an ultrasonic impact treatment was performed on at least the toe ends of these welds, that is, the main weld bead and the increased weld bead. In this ultrasonic impact treatment (UIT), 27 kHz ultrasonic vibration was generated in the transducer from the output power of 1 kw, the treatment was performed at a speed of about 15 cm per minute, and the same conditions as in the above test were used. The specimen was bent at four points and subjected to a fatigue test under an amplitude condition of 135 MPa.
After the fatigue test, cracks or fractures were examined by visual observation. The results are shown in Table 1.

Thus, when L ′ was less than 2 mm, the boundary toe 34 of the main weld bead 5 and the additional weld bead 6 was broken. On the other hand, when L ′ was 2 mm or more, it did not break at the boundary toe, and the fatigue performance was improved.
That is, it can be seen that when L ′ is 1/3 or more of L (= 6 mm), the effect of improving the fatigue performance becomes remarkable.

It is a cross-sectional schematic diagram which shows the shape of the weld bead in fillet welding. It is a cross-sectional schematic diagram which shows the shape of the welding bead which consists of the main welding bead in the first embodiment of this invention, and an additional build-up welding bead. It is a cross-sectional schematic diagram which shows the shape of the weld bead which consists of the main weld bead in the second embodiment of this invention, and an additional build-up weld bead. It is a figure explaining the concept of the effective throat thickness which concerns on this invention. It is a schematic diagram which shows the structure of the test body in Example 1 and Comparative Examples 1 and 2 of this invention, (a) is a side view, (b) is a top view, (c) shows sectional drawing. It is a figure which shows the fatigue test result in the comparative examples 1 and 2. It is the fatigue test body which examined when changing L 'in Example 2 of this invention. (A) is a bird's-eye view, (b) is a top view, and (c) is a front view. It is a cross-sectional schematic diagram which shows generation | occurrence | production of the crack in this welding bead.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Base material 2 To-be-welded material 3 Toe part 31 The board | substrate side toe part in this welding bead 32 The to-be-welded member side toe part 33, 35 in the main welding bead Substrate side toe part in an additional welding bead 36 Toe part to be welded in bead 34 Boundary toe part 4 Root part 5 This weld bead 6 Additional weld bead 7 Root crack 8 Toe part crack 9 Welded part 10 Specimen 11 I-shaped steel (base material)
12 In-plane gusset 13 Out-of-plane gusset 14 Rib

Claims (3)

  After forming a newly increased weld bead on a bead including at least one toe portion of an existing main weld bead formed by fillet welding a base material and a member to be welded, the main weld bead and the Apply ultrasonic shock treatment to the toe of the main weld bead appearing on the surface of the additional weld bead, the toe of the additional weld bead, and the boundary toe of the main weld bead and the additional weld bead. A method for improving fatigue performance of a welded part in fillet welding characterized by the following.   2. The welded portion in fillet welding according to claim 1, wherein the increased weld bead is formed on both the board-side toe portion and the to-be-welded member-side toe portion of the main welding bead. To improve fatigue performance   The incremental welding bead is formed only on one side of the substrate side toe end portion or the welded member side toe end portion of the main welding bead, and the main welding on the side where the increasing heap welding bead is not formed. When the leg length of the bead is L and the distance of the perpendicular from the boundary toe to the substrate or the member to be welded on the side where the increased weld bead is formed is L ′, L ′ ≧ 1 / 3L The method for improving fatigue performance of a welded part in fillet welding according to claim 1, wherein the increased weld bead is formed.

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