JP6756241B2 - Ultrasonic impact treatment method - Google Patents

Description

The present invention relates to an ultrasonic impact treatment method for improving the fatigue characteristics of a weld toe in a steel structure having a weld.

When a repeated load is applied to the weld toe of a steel structure having a weld, fatigue cracks may occur and fracture may occur. Fatigue fracture of such a weld toe has become a problem.

As one of the methods for improving the fatigue characteristics of the weld toe, a method of relaxing the stress concentration in the weld by applying a grinder to smooth the weld is disclosed in, for example, Patent Document 1.

Further, as another method for improving the fatigue characteristics of the weld toe, an ultrasonic impact treatment (UIT) has been recently developed for the purpose of improving the fatigue strength of the weld. Ultrasonic shock processing is the process of applying ultrasonic shock of several tens of kHz generated from an ultrasonic generator to an object through a tool such as a pin to give an ultrasonic impact to the object. This is a process that causes plastic deformation. As a result, the surface shape is improved, and at the same time, the residual stress near the surface is improved.

For example, Patent Document 2 discloses a method of improving fatigue strength by applying ultrasonic impact treatment to a welded portion and a machined hole. Further, Patent Document 3 by the present inventors discloses a method of improving the fatigue strength of a butt welded joint by ultrasonically striking the vicinity of the weld toe of the butt welded joint. Further, Patent Document 4 discloses a method of improving fatigue strength by applying ultrasonic impact treatment away from the weld toe.

In addition, when an ultrasonic impact is applied to the weld toe from an ultrasonic impact device, the metal in the vicinity of the weld toe may plastically flow due to the impact of the pin, causing a fold defect. Patent Document 5 discloses a method for ultrasonic impact treatment of a weld toe portion in which the radius of curvature of the tip portion of the pin is less than 2.0 mm in order to suppress the occurrence of this fold defect.

Japanese Unexamined Patent Publication No. 2010-29897 U.S. Pat. No. 6,338,765 JP-A-2010-142870 Japanese Unexamined Patent Publication No. 2012-11462 JP-A-2007-283355

However, even when the ultrasonic impact treatment is performed at a position away from the weld toe or the weld toe using the ultrasonic impact treatment method disclosed in the above patent document, the structure is compared with the structure as it is welded. Although the fatigue life is improved, fatigue cracks occur at the weld toe and fatigue failure occurs after about 2 million times in the fatigue test. Therefore, it has been required to further extend the fatigue life of the welded structure.

Therefore, the present invention has been made in view of the above problems, and an object of the present invention is a new and improved ultrasonic impact treatment capable of extending the fatigue life of a welded structure for a longer period of time. To provide a method.

The present inventors have diligently studied a method for further improving the fatigue characteristics of the weld toe even when a minute crease occurs on the surface of the weld toe to which the ultrasonic impact treatment is applied. did. The present invention made as a result is as follows.

(1) An ultrasonic impact treatment method for improving the fatigue strength of the weld toe by bringing a pin into contact with the weld toe of a steel structure having a weld to give an ultrasonic impact. ,
A step of performing a first ultrasonic impact treatment using a first pin having a first radius of curvature that is equal to or less than the radius of curvature of the weld toe.
After the first ultrasonic shock treatment, a second ultrasonic shock treatment is performed using a second pin having a second radius of curvature of 1.0 mm or more.
Ultrasonic shock treatment method having.
(2) The ultrasonic impact treatment method according to claim 1, wherein the second radius of curvature is 3.0 mm or more.
(3) The ultrasonic impact treatment method according to (1) or (2), wherein the radius of curvature of the weld toe is measured before the first ultrasonic impact treatment.
(4) In the second ultrasonic impact treatment, the weld toe portion forms a processing band obtained by continuously forming a plurality of treatment marks along the direction in which the weld toe portion is formed. Formed in parallel along the direction orthogonal to the direction in which they are formed,
In the processing region composed of the plurality of processing bands, at least three edges of the processing marks are present on the cross section in the direction orthogonal to the direction in which the welding toe is formed, (1) to (1). The ultrasonic impact treatment method according to any one of 3).

As described above, according to the present invention, the fatigue life of the welded structure can be extended longer even when a minute crease is generated on the surface of the weld toe to which the ultrasonic impact treatment is applied. Is possible.

It is a figure which shows an example of the state of the vicinity of a weld toe portion in the conventional ultrasonic impact processing. It is a photograph which imaged the cross section of the weld toe part after ultrasonic shock treatment. It is the schematic of the ultrasonic shock processing method which concerns on this embodiment. It is a figure for demonstrating the 1st example of the processing band formed along the direction in which a weld toe portion is formed. It is a figure for demonstrating the 2nd example of the processing band formed along the direction in which the weld toe is formed.

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the present specification and the drawings, components having substantially the same functional configuration are designated by the same reference numerals, so that duplicate description will be omitted.

(Mechanism of fatigue fracture at weld toe after conventional ultrasonic impact treatment)
First, the present inventors conducted a fatigue test on each of the welded structure subjected to the ultrasonic impact treatment and the welded structure not subjected to the ultrasonic impact treatment, and the fractured portion of the fractured structure was described in detail. Analyzed. Then, the present inventors investigated the mechanism of fatigue fracture at the weld toe after the conventional ultrasonic impact treatment based on the analysis result.

The present inventors prepared a cross-welded joint in which these steel plates were welded using a rolled steel plate for welding SM490 (yield strength YP = 345 MPa, tensile strength = 531 MPa) having a plate thickness of 16 mm as a specimen. Then, a specimen in which the weld toe was subjected to ultrasonic impact treatment and a specimen in which the welding toe was not subjected to ultrasonic impact treatment were prepared, and a fatigue test was performed on each specimen. The fatigue test was carried out by repeatedly applying a load to the weld toe of each specimen. The conditions of the fatigue test were a stress range ΔS = 220 MPa, a stress ratio R = 0.1, and a frequency f = 10 Hz. In this fatigue test, the number of repetitions of the repeating load applied until the specimen breaks is evaluated as the fatigue characteristic. In addition, if the specimen did not break even if the number of repetitions of the repeated load exceeded 5 million times, the fatigue test was stopped. After the fatigue test was completed, the cross section of the fractured specimen was observed in detail, and the depth of the fatigue crack occurrence site was investigated.

The present inventors investigated the relationship between the depth of the fatigue crack occurrence site after the fatigue test and the hardness distribution in the depth direction from the surface of the weld toe. As a result, in the specimen subjected to the ultrasonic impact treatment, the location where the fatigue crack is generated is deeper than that in the specimen not subjected to the ultrasonic impact treatment, and the location where the fatigue crack is generated is deeper. It was found that the hardness is the same as the hardness of the original weld toe after welding and before ultrasonic impact treatment. On the other hand, the surface of the weld toe after the ultrasonic impact treatment showed higher hardness than the surface of the original weld toe. It is considered that this is due to work hardening caused by ultrasonic impact treatment. However, since the hardness of the fatigue crack generation portion generated at the weld toe portion subjected to the ultrasonic impact treatment did not increase, it is considered that work hardening by the ultrasonic impact treatment did not occur so much. Therefore, it is probable that fatigue fracture occurred even at the weld toe portion subjected to the ultrasonic impact treatment starting from the original weld toe portion having low hardness.

Therefore, the present inventors have conducted the following experiments to study a method of causing work hardening by ultrasonic impact treatment at the original weld toe where fatigue cracks occur.

First, the present inventors investigated the mechanism by which the portion where fatigue cracks occur becomes deeper than the surface of the weld toe when the ultrasonic impact treatment is applied to the weld toe. FIG. 1 is a schematic view showing the behavior of the weld toe when the weld toe is subjected to ultrasonic impact treatment using a pin. As shown in FIG. 1, at the initial stage of the ultrasonic impact treatment, there is a space 3 surrounded by the base material 1, the weld metal (weld bead) 2, and the pin 10 for the ultrasonic impact treatment. When the weld toe 4 is ultrasonically impact-treated using a pin 10 having a radius of curvature larger than the radius of curvature of the weld toe 4, which is the contact point between the surface of the base metal 1 and the weld metal 2, the ultrasonic impact is applied. The base metal 1 and the weld metal 2 plastically flow in the directions shown by the arrows in FIG. Then, from the welding toe (original welding toe) 4 before the ultrasonic impact treatment to the contact point 5 (hereinafter, contact point 5) on the surface of the base metal and the weld metal after the plastic flow. A minute fold flaw 6 occurs in the region.

At this time, as shown in FIG. 1, the weld toe portion 4 before the ultrasonic impact treatment is not in contact with the pin 10. Therefore, since the welding toe portion 4 before the ultrasonic impact treatment is substantially not subjected to the ultrasonic impact treatment, the effect of processing and hardening by the ultrasonic impact treatment is the welding toe portion 4 (and the welding toe portion 4 before the ultrasonic impact treatment). It is thought that it is unlikely to occur in the vicinity). That is, since the position deeper than the contact point 5 (specifically, the vicinity of the original weld toe portion 4) is not work-hardened, it can be a place where fatigue cracks occur. Therefore, even if the crease 6 itself is so small that it does not adversely affect the fatigue strength, fatigue fracture can occur through the fold flaw 6 starting from the location where the fatigue crack occurs as described above. Conceivable.

When the weld toe 4 is subjected to ultrasonic impact treatment using a pin 10 having a radius of curvature larger than the radius of curvature of the weld toe 4, a cross section is cut out around the contact point 5 and the structure is observed. It is shown in FIG. It can be seen that minute fold flaws 6 are generated in a region from the surface of the welded portion to a depth of about 150 μm.

(Ultrasonic shock treatment method according to this embodiment)
Therefore, the present inventors minimize the space created between the base metal, the weld metal, and the pins used for ultrasonic impact treatment, and bring a part of the pins into contact with the weld toe to make ultrasonic waves. An ultrasonic shock treatment method for shock treatment was investigated. In order to bring a part of the pin into contact with the weld toe, a pin having a radius of curvature smaller than the radius of curvature of the weld toe before the ultrasonic impact treatment is used at the time of starting the ultrasonic impact treatment. It is required that the weld toe is subjected to ultrasonic impact treatment. That is, the pin can be brought into contact with the weld toe by performing the ultrasonic impact treatment using a pin having a radius of curvature equal to or less than the radius of curvature of the weld toe before the ultrasonic impact treatment. As a result, the effect of work hardening by ultrasonic impact treatment can be produced on the weld toe.

However, the radius of curvature of the weld toe before the ultrasonic impact treatment is generally very small, about 0.2 mm to 1.0 mm. Therefore, when ultrasonic impact treatment is performed using a pin having a radius of curvature smaller than the radius of curvature of the weld toe, the radius of curvature of the weld toe becomes even smaller, and the treated weld toe The stress concentration coefficient of is likely to be higher than before the ultrasonic impact treatment. Therefore, it is difficult to relax the stress concentration of the weld toe only by performing ultrasonic impact treatment using a pin having a radius of curvature smaller than the radius of curvature of the weld toe.

On the other hand, as in the conventional case, when the welding toe is subjected to ultrasonic impact treatment using a pin having a radius of curvature larger than the radius of curvature of the welding toe, welding after the ultrasonic impact treatment is performed. The radius of curvature of the toe is larger than that of the weld toe before ultrasonic impact treatment. Therefore, the stress concentration at the weld toe after the ultrasonic impact treatment is reduced. However, as described above, fold flaws occur inside the weld toe. Since the work hardening effect of ultrasonic impact treatment is unlikely to occur in the fold flaw, it can be a starting point for the occurrence of fatigue cracks. Therefore, the present inventors considered that the fatigue strength of the weld toe can be further improved by pre-work hardening the portion that can be the fatigue starting point by ultrasonic impact treatment.

Therefore, using a pin having a radius of curvature smaller than the radius of curvature of the weld toe, the pin was brought into contact with the weld toe and the surface near the weld toe was processed and hardened by ultrasonic impact treatment. Later, it was examined to apply ultrasonic impact treatment to the weld toe using a pin having a radius of curvature larger than the radius of curvature of the weld toe. Hereinafter, the ultrasonic impact treatment method according to the present embodiment will be described.

The ultrasonic shock treatment method according to the present embodiment includes a first ultrasonic shock treatment step and a second ultrasonic shock treatment step. First, it is required to obtain the value of the radius of curvature of the weld toe before performing the ultrasonic impact treatment according to the present embodiment. This is to set the radius of curvature of the pin used in the first ultrasonic impact treatment. In order to obtain the value of the radius of curvature of the weld toe, for example, it is preferable to measure a region within 200 μm around the weld toe including the weld toe using a laser displacement meter. The laser displacement meter can measure and evaluate the uneven shape of the weld toe of a structure having a complicated shape with high accuracy, easily, and non-destructively and non-contact. It is possible to obtain the value of the radius of curvature of the weld toe from the unevenness information measured using the laser displacement meter. The radius of curvature of the weld toe may be measured by a palpation-type uneven shape evaluation method, or another evaluation method using various microscopes capable of measuring the uneven shape. Further, when spatter or slag is present at the weld toe, it is preferable to measure after removing these spatters or to measure the portion where these are not present. If there are variations in the shape of the weld toe, or if variations are expected, evaluate multiple points of the weld toe and use the minimum value as the radius of curvature of the weld toe. Is preferable.

If the radius of curvature of the weld toe is known in advance, the uneven shape of the weld toe is measured, and if the radius of curvature can be estimated from the uneven shape, the radius of curvature of the weld toe is calculated. The step of measuring may be omitted. Further, the measurement step may be omitted for a structure having a welded portion manufactured under the same welding conditions as the welded portion having a weld toe whose radius of curvature has already been measured.

FIG. 3 is a schematic view of the ultrasonic impact processing method according to the present embodiment. Referring to FIG. 3, in the ultrasonic impact treatment method according to the present embodiment, first, a first pin 10A having a first radius of curvature smaller than the radius of curvature of the weld toe 4 is used to use the weld toe. Ultrasonic impact treatment is applied to the part 4 (first ultrasonic impact treatment). After performing the first ultrasonic impact treatment, the welding toe 4 is subjected to the second pin 10B having a radius of curvature having a second radius of curvature larger than the radius of curvature of the weld toe 4. And ultrasonic shock treatment (second ultrasonic shock treatment).

In the first ultrasonic impact treatment, the ultrasonic impact treatment is performed on the weld toe 4 by the first pin 10A having a radius of curvature smaller than the radius of curvature of the weld toe 4. Since the radius of curvature of the first pin 10A is smaller than the radius of curvature of the weld toe portion 4, a space is unlikely to be created between the base metal 1, the weld metal 2, and the first pin 10A. Therefore, the first pin 10A can be brought into contact with the weld toe portion 4. As a result, the impact of ultrasonic waves is directly applied to the weld toe portion 4. Therefore, the surface in the vicinity of the weld toe portion 4 is work-hardened, and the hardness in the vicinity of the weld toe portion 4 increases.

After that, in the second ultrasonic impact treatment, the ultrasonic impact treatment is performed on the weld toe 4 by the second pin 10B having a radius of curvature larger than the radius of curvature of the weld toe 4. As a result, the base metal 1 and the weld metal 2 to which the impact of ultrasonic waves is applied are plastically flowed in the direction in which the weld toe 4 is present. As a result, the radius of curvature of the weld toe 4 increases due to the plastic flow, and from the point where the two metals collide on the surface due to the plastic flow to the vicinity of the surface of the fold flaw 6 generated from the original weld toe 4. , Compressive residual stress occurs.

Since the radius of curvature of the weld toe 4 is generally about 0.2 mm to 1.0 mm, the radius of curvature of the second pin 10B used for the second ultrasonic impact treatment is 1.0 mm or more. Is preferable. Further, it is considered that the larger the radius of curvature of the pin, the greater the effect of relaxing the stress concentration and the effect of introducing the residual compressive stress.

As a result of a series of ultrasonic impact treatments, a crease 6 is generated at the weld toe portion 4, and the crease 6 becomes a fatigue starting point. However, since the deep region 7 of the fold flaw 6 (corresponding to the region near the original weld toe 4) is work-hardened by the above-mentioned first ultrasonic impact treatment, the fatigue strength is improved. .. Further, the second ultrasonic impact treatment introduces the residual compressive stress to the region below the surface (the region separated from the surface by 1 mm or more), so that the fatigue strength is improved. Further, since the radius of curvature of the weld toe 4 is increased and the surface shape is smoothed by the second ultrasonic impact treatment, the stress concentration in the welded structure is relaxed.

The second ultrasonic impact treatment introduces residual compressive stress to the lower region of the surface (region 1 mm or more away from the surface), but at that time, the lower region (region 2 mm or more away from the surface). May generate tensile residual stress. Therefore, in the first ultrasonic impact treatment, it is desirable that the treatment is performed so that the depth of the treatment mark is within 1 mm.

As described above, according to the ultrasonic impact treatment method according to the present embodiment, in addition to alleviating stress concentration in the structure by ultrasonic impact treatment and introducing residual stress near the surface of the weld toe, folds occur. Improvement of fatigue strength is achieved by work hardening inside the flaw. Therefore, the fatigue characteristics of the weld toe can be further improved as compared with the conventional case. That is, in a steel structure having a welded portion, particularly in an actual steel structure having a very large welded portion, the probability of occurrence of fatigue failure is reduced, so that the fatigue life of these steel structures can be extended longer.

(Modification example)
Next, a modified example of this embodiment will be described. In this modification, in the second ultrasonic impact treatment, a plurality of second pins 10B are used to continuously form a plurality of treatment marks along the direction in which the weld toe portion 4 is formed. The obtained processing strips are formed in parallel along the direction orthogonal to the direction in which the weld toe portion 4 is formed. In such processing, in the processing region composed of the plurality of processing bands, at least three edges of the processing marks are present on the cross section in the direction orthogonal to the direction in which the welding toe portion 4 is formed. , These processed bands are formed. This makes it possible to further extend the fatigue life of the steel structure. Hereinafter, this modification will be described.

When dents (treatment marks) are formed on the metal surface by ultrasonic impact treatment, the peripheral portion (edge) of the treatment marks is raised and formed, and compressive residual stress is introduced into the lower layer of the metal surface. This compressive residual stress is not uniform in the region inside the treatment mark and tends to be higher at the edge than at the center of the treatment mark.

Therefore, the present inventors have conceived that by introducing more edges of such treatment marks, more such compressive residual stress can be introduced into the lower layer of the metal surface. 4 and 5 are views for explaining the first example and the second example of the processing band formed along the direction in which the weld toe portion 4 is formed.

The processing band 100 shown in FIG. 4 is composed of a plurality of processing marks 11 obtained by ultrasonically impact-treating one second pin 10B along the welding toe portion 4, and the processing marks 11 are formed. It has one streak. An edge 11a is formed around the processing mark 11. In such a processing band 100, two lines including edges 11a of a plurality of processing marks 11 are formed on both sides of the processing band 100 along the processing direction (the direction of welding processing, that is, the direction in which the welding toe portion 4 is formed). Straight edges 12A and 12B are formed.

On the other hand, the processing region 200 shown in FIG. 5 is a three-row streak composed of a plurality of processing marks 13 obtained by ultrasonically impact-treating the three second pins 10B along the weld toe portion 4. It is composed of the processing band 201 of. In such a processing region 200, four linear edges 14A to 14D including edges of a plurality of processing marks 13 are formed on both sides and inside of the processing region 200 along the processing direction. Since the edges 14B and 14C are formed by overlapping the edges of adjacent processing marks, the absolute value of the compressive residual stress in the direction orthogonal to the processing direction increases. As a result, a higher compressive residual stress can be introduced in the processing region 200 shown in FIG. 5 than in the processing zone 100 shown in FIG. Therefore, as described in the previous embodiment, even if a fold scratch occurs, the effect of work hardening by the first ultrasonic impact treatment and relaxation of stress concentration by the second ultrasonic impact treatment. , And the effect of introducing residual compressive stress can be obtained, and the effect of enhancing the compressive residual stress in the direction perpendicular to the work zone can be obtained by overlapping the edges of the plurality of work bands. Due to such an enhancing effect, the fatigue life can be further extended.

When one processing band is formed, there are only two edges at most in the direction orthogonal to the processing direction, but when two or more processing bands are formed, When the edges of adjacent processing bands are formed so as to overlap each other, there may be three or more edges on the cross section in the direction orthogonal to the processing direction. In this treatment, in order to obtain the effect of compressive residual stress at the edge of the processing band, it is preferable to perform the treatment so as not to crush the edge of the adjacent processing mark. The arrangement of the processing marks shown in FIG. 5 is merely an example, and is composed of a plurality of processing marks and the plurality of processing marks so that there are three or more edges on the cross section in the direction orthogonal to the processing direction. As long as the processing band is formed, the formation position and arrangement direction of the processing marks are not particularly limited.

When forming a plurality of processing bands, for example, a plurality of second pins 10B are arranged at an angle not parallel to the processing direction and scanned along the direction in which the welding toe portion 4 is formed. By performing the ultrasonic impact treatment, the above-mentioned processing zone (processing region 200) is formed. The present invention is not limited to this example, and a processed band may be formed by, for example, one second pin 10B. However, by simultaneously processing in parallel using a plurality of second pins 10B, the effect of enhancing the residual compressive stress can be easily obtained, and the processing time can be shortened.

The present modification has been described above.

A cross-welded joint as a test piece was prepared from the following steel materials, and the service life of the joint material was evaluated by a fatigue test under various conditions to verify the effect of the present invention.

As the steel material, 50 k steel (SM490, yield strength YP = 345 MPa, tensile strength TS = 531 MPa) is used, and the plate thickness 16 mm × width 100 mm × length 700 mm is made of the same material on both central sides of the plate. A vertical plate of 100 mm × 40 mm in height was arranged in a load-non-transmission cross joint shape. Shielded Metal Arc Welding (SMAW, welded material for 50k steel JIS Z 3211 D4316) or arc welding with flux (Flux Cored Arc Welding: FCAW, JIS Z 3313 YFW-C50DR for 50k steel) as welding methods for these steel materials. ) Was used. The welding conditions were shield gas: carbon dioxide gas, no preheating, and heat input 15 to 20 kJ / cm ² . The above steel material was fillet welded with a leg length of 7 mm to prepare a cross-welded joint, which was used as a specimen.

For the specimens after welding, the radius of curvature of the weld toe was measured for 13 specimens using a laser displacement meter. The radius of curvature of the weld toe was 0.25 mm. After measuring the radius of curvature of the weld toe with a laser displacement meter, these specimens were subjected to ultrasonic impact treatment on the weld toe. As described above, the ultrasonic impact treatment was performed twice or once by changing the radius of curvature of the pin tip. The common conditions for ultrasonic impact processing were a resonance frequency of 27 kHz, an amplitude of 30 μm, a pin diameter of φ5 mm, and a processing speed (welding direction) of 30 cm / min.

Ultrasonic impact treatment was performed twice on 10 of the specimens after welding (Examples 1 to 7 and Comparative Examples 1 to 3). The remaining three bodies were subjected to ultrasonic impact treatment only once (Comparative Examples 4 to 6).

For the specimens of Examples 1 to 4 and Comparative Examples 1 and 2, a first pin having a first radius of curvature (0.2 mm) smaller than 0.25 mm, which is the radius of curvature of the weld toe after welding. The weld toe was subjected to ultrasonic impact treatment using. Then, the specimens of Examples 1 to 4 and Comparative Examples 1 and 2 were subjected to ultrasonic impact treatment on the weld toe using a second pin having a second radius of curvature. The second radius of curvature was 5 mm, 2 mm, 1.2 mm, 1 mm, 0.8 mm and 0.5 mm, respectively.

For the specimens of Examples 5 to 7, a first pin having a first radius of curvature (0.2 mm) smaller than 0.25 mm, which is the radius of curvature of the weld toe after welding, is used to stop welding. The edges were subjected to ultrasonic shock treatment. Then, the specimens of Examples 5 to 7 were subjected to ultrasonic impact treatment on the weld toe using three second pins having a second radius of curvature. The second radius of curvature was 5 mm, 3 mm and 1 mm, respectively. In each embodiment, three second pins are arranged so as to be inclined with respect to the direction in which the weld toe is formed and scanned along the direction to superimpose the weld toe. It was subjected to ultrasonic shock treatment.

For the specimen of Comparative Example 3, a first pin having a radius of curvature (1.5 mm) larger than the radius of curvature of the weld toe after welding, which is 0.25 mm, was used with respect to the weld toe. Ultrasonic impact treatment was applied. Then, the specimen was subjected to ultrasonic impact treatment on the weld toe using a second pin having a radius of curvature of 3 mm.

For the specimen of Comparative Example 4, ultrasonic impact treatment was applied to the weld toe using a pin having a radius of curvature (3 mm) larger than the radius of curvature of the weld toe after welding of 0.25 mm. gave.

For the specimen of Comparative Example 5, an ultrasonic impact was applied to the weld toe using a pin having a radius of curvature (0.2 mm) smaller than the radius of curvature of the weld toe after welding of 0.25 mm. Processed.

Regarding the specimen of Comparative Example 6, three pins having a radius of curvature (3 mm) larger than the radius of curvature of the weld toe after welding of 0.25 mm were placed in the direction in which the weld toe was formed. The weld toe was arranged so as to be inclined, and the weld toe was scanned to perform ultrasonic impact treatment.

These 13 specimens were subjected to ultrasonic impact treatment and then a fatigue test. The fatigue test was a tensile-tensile test of axial force, and the number of repeated life times N until the specimen broke was evaluated under the conditions of a stress range ΔS = 220 MPa, a stress ratio R = 0.1, and a frequency of 10 Hz. When the number of repetitions exceeded 10 million times, the fatigue test was stopped.

Table 1 shows the radius of curvature of the weld toe and each pin, the number of ultrasonic impact treatments, and the number of repeated life times N until the specimen breaks in the fatigue test.

In Comparative Example 4, the specimen was subjected to ultrasonic impact treatment by a pin having a radius of curvature larger than the radius of curvature of the weld toe as in the conventional case. The number of repetitions N in Comparative Example 4 was 2304956.

In Examples 1 to 4, each specimen is subjected to the first ultrasonic impact treatment on the weld toe by a pin having a radius of curvature of 0.2 mm, which is smaller than the radius of curvature of the weld toe. After that, a second ultrasonic impact treatment was applied to the weld toe by a pin having a radius of curvature of 1 mm or more. The number of repeated life times N in each example was 823,201 times, 6350,311 times, 3609744 times, and 313,090 times, respectively. That is, in Examples 1 and 2, it was unbroken when the number of repetitions was 5 million. From the above, it can be seen that the fatigue life of the structures in Examples 1 to 4 is longer than that in Comparative Example 4. It is considered that this is because the original weld toe was work-hardened by the first ultrasonic impact treatment, and the fatigue characteristics of the weld toe were improved.

In Examples 5 to 7, each specimen is subjected to the same first ultrasonic shock treatment as in Examples 1 to 4, and then in the second ultrasonic shock treatment, three threads are processed using three pins. Bands were formed, and four edges of the processing marks were observed in any cross section along the direction orthogonal to these processed bands. The fatigue life was more than 10 million times in Examples 5 and 6, and more than 8 million times in Example 7.

On the other hand, in Comparative Examples 1 and 2, for each specimen, the first ultrasonic impact treatment was applied to the weld toe by a pin having a radius of curvature of 0.2 mm, which is smaller than the radius of curvature of the weld toe. After that, a second ultrasonic impact treatment was applied to the weld toe with a pin having a radius of curvature smaller than 1 mm. The number of repetition times N in each comparative example was 2360002 times and 1743237 times. That is, when a pin having a radius of curvature smaller than 1 mm was used in the second ultrasonic impact treatment, the repeat life was equal to or less than that of Comparative Example 4.

Further, when Examples 1 to 4 and Comparative Examples 1 and 2 were compared, the number of repetitions N decreased as the radius of curvature of the pin in the second ultrasonic impact treatment decreased. This is because the radius of curvature of the pin in the second ultrasonic impact treatment is reduced, so that the fatigue strength due to work hardening of the weld toe is improved, but the stress concentration at the weld toe is difficult to be relaxed. And it is considered that the cause is that the compressive stress introduced into the weld toe does not increase. On the other hand, it was confirmed that the fatigue life was extended by increasing the radius of curvature of the pin used for the second ultrasonic impact treatment to more than 1.0 mm. In particular, when the second ultrasonic impact treatment (Examples 1 and 2) was performed using a pin having a radius of curvature of 3 mm or more, a result exceeding the measurement upper limit of the fatigue life was obtained.

In Comparative Example 3, in the specimen, each ultrasonic shock treatment and the second ultrasonic shock treatment were performed with respect to the weld toe by a pin having a radius of curvature larger than the radius of curvature of the weld toe. Ultrasonic impact treatment was applied. The number of repetitions N in Comparative Example 3 was 2308280. The number of repetitions N in Comparative Example 3 was about the same as the number of repetitions in Comparative Example 4. That is, in Comparative Example 3, similarly to Comparative Example 4, only the same effect as the effect of the conventional ultrasonic impact treatment was obtained. When observing the weld toe portion of the specimen of Comparative Example 3, a fold flaw was generated after the first ultrasonic impact treatment was performed after the completion of the first ultrasonic impact treatment. It is considered that this is because the ultrasonic impact treatment was performed using a pin having a radius of curvature of 1.5 mm, which is larger than the radius of curvature of the weld toe. Therefore, it is considered that the original weld toe was not work-hardened and the effect of improving the fatigue strength could not be obtained. Therefore, it is considered that the fatigue life was not extended.

In Comparative Example 5, the specimen was subjected to ultrasonic impact treatment only once on the weld toe by a pin having a radius of curvature smaller than the radius of curvature of the weld toe. The number of repetitions N in Comparative Example 5 was 750520 times. The number of repetition times N in Comparative Example 5 is significantly smaller than that in Comparative Example 4. When observing the weld toe of the specimen of Comparative Example 5, no creases were found. It is considered that this is because the ultrasonic impact treatment was performed using a pin having a radius of curvature of 0.2 mm, which is smaller than the radius of curvature of the weld toe. As a result of the ultrasonic impact treatment, it is considered that the weld toe was work-hardened and the fatigue strength was improved. However, it is probable that the stress concentration at the weld toe was not relaxed as described above because the ultrasonic impact treatment was performed using a pin having a radius of curvature smaller than the radius of curvature of the weld toe. Therefore, it is considered that the fatigue life is remarkably lowered as compared with Comparative Example 4.

In Comparative Example 6, three processed bands were formed on the specimen by using three pins having a radius of curvature larger than the radius of curvature of the weld toe, but the second ultrasonic impact treatment was performed. There wasn't. As a result, the number of repetitions N in Comparative Example 5 was 2993855. Even when the result of Comparative Example 4 in which the ultrasonic impact treatment was performed using only one pin having a radius of curvature of 3 mm was compared, no significant improvement was observed. When observing the weld toe of the specimen of Comparative Example 6, the weld toe at the bottom of the fold scratch was not work-hardened. Therefore, it is considered that the effect of improving the fatigue strength could not be obtained.

In this embodiment, the test was carried out using a structure having a radius of curvature of the weld toe portion of 0.25 mm before the ultrasonic impact treatment, but in general, the weld toe portion as welded is used. The radius of curvature is in the range of 0.2 mm to 1.0 mm. Therefore, in this embodiment, the effect of improving the fatigue life by ultrasonic impact treatment using the first pin having a radius of curvature of 0.2 mm, which is the minimum value of the radius of curvature that can be measured at the weld toe as it is welded. Therefore, it is considered that the effect of improving the fatigue life can be obtained even for the weld toe having a radius of curvature other than 0.25 mm.

Although the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, the present invention is not limited to such examples. It is clear that a person having ordinary knowledge in the field of technology to which the present invention belongs can come up with various modifications or modifications within the scope of the technical ideas described in the claims. , These are also naturally understood to belong to the technical scope of the present invention.

1 Base material (steel plate)
2 Welded metal (welded bead)
3 Space 4 Weld toe before ultrasonic impact treatment (original weld toe)
5 Contact point on the surface between the base metal and the weld metal after plastic flow 6 Folded flaw 7 Deep region 10 Pin 11, 13 Treatment mark 12, 14 Treatment mark edge 100, 201 Processing band 200 Processing area

Claims (4)

An ultrasonic impact treatment method for improving the fatigue strength of a weld toe by bringing a pin into contact with the weld toe of a steel structure having a weld and applying an ultrasonic impact.
A step of performing a first ultrasonic impact treatment using a first pin having a first radius of curvature that is equal to or less than the radius of curvature of the weld toe.
After the first ultrasonic shock treatment, a second ultrasonic shock treatment is performed using a second pin having a second radius of curvature of 1.0 mm or more.
Ultrasonic shock treatment method having. The ultrasonic impact treatment method according to claim 1, wherein the second radius of curvature is 3.0 mm or more. The ultrasonic impact treatment method according to claim 1 or 2, wherein the radius of curvature of the weld toe is measured before the first ultrasonic shock treatment. In the second ultrasonic impact treatment, the weld toe is formed on a processing zone obtained by continuously forming a plurality of treatment marks along the direction in which the weld toe is formed. Formed in parallel along the direction orthogonal to the direction in which they are
Claims 1 to 3 include at least three edges of the processing marks on a cross section in a direction orthogonal to the direction in which the welding toe is formed in a processing region composed of the plurality of processing bands. The ultrasonic impact treatment method according to any one of the above items.