JP6319022B2 - Railway vehicle bogie frame and manufacturing method thereof - Google Patents

Description

  The present invention relates to a bogie frame that is a skeleton of a bogie constituting a railway vehicle, and a method for manufacturing the bogie frame for the railway vehicle.

  A railway vehicle is composed of a vehicle body and a carriage that supports the vehicle body. The skeleton of the bogie is the bogie frame.

  Generally, a bogie frame is manufactured through the following series of steps. Various steel materials processed into a predetermined shape are prepared. Next, the prepared various steel materials are combined, and the steel materials are joined to each other by gas shield arc welding (eg, MAG welding) to form a bogie frame.

  FIG. 1 is an enlarged perspective view of a welding portion of the bogie frame. FIG. 1 illustrates a portion where the steel materials 1A and 1B are joined by fillet welding. A weld bead 2 is formed between the steel materials 1A and 1B by gas shield arc welding.

  Subsequently, the surface of the weld bead 2 is prepared by a grinder in a region where high stress is expected to be generated from the strength design. After cleaning with a grinder, the bogie frame is annealed to remove residual stress. After the heat treatment, the trolley frame is shot blasted to remove scale on the surface of the trolley frame. After shot blasting, paint the bogie frame. Through such a series of steps, the bogie frame is completed.

  Japanese Patent Laid-Open No. 6-270810 (Patent Document 1) discloses a method for improving the fatigue strength of a welded joint in a bogie frame. In this method, the necessity of the heat treatment step, the shot blasting step, and the coating step after the welding step described above is unknown, but after the welding step by MAG welding, the fatigue strength of the welded joint is reduced by going through the following steps. It can be improved. The shape of the weld toe of the weld bead formed by MAG welding is improved by TIG welding. Thereafter, shot peening is performed on the entire surface of the weld bead including the molten metal portion formed by TIG welding.

  By the way, in the above-described method for manufacturing a bogie frame, appearance inspection is performed between processes after arc welding. During this appearance inspection, a slight welding defect may be found in a part of the weld bead 2. In this case, the defective part is repaired by TIG tanning. TIG tanning melts the weld bead 2 and the steel material 1A at the defective portion without using a filler rod.

  FIG. 2 is an enlarged perspective view showing a welded part of the bogie frame and repaired by TIG tanning. FIG. 2 illustrates a portion where the steel materials 1A and 1B are joined by fillet welding in the same manner as in FIG. Between the steel material 1A and the weld bead 2, a molten metal portion 3 is locally formed by TIG tanning.

Japanese Patent Laid-Open No. 6-270810

  Although the repaired portion by TIG tanning is local, there is heat input during TIG tanning. For this reason, it is considered that some tensile residual stress is generated in the repaired portion. In the above-described method for manufacturing a bogie frame, when a defective part of a weld bead is found before heat treatment and repair by TIG tanning is performed before heat treatment, the tensile residual stress of the repaired part is determined by heat treatment in a subsequent process. Since it is removed, there is no problem.

  However, if a defective part of the weld bead is found after heat treatment and repair by TIG tanning is performed after the subsequent shot blasting process, the remaining process is only a painting process, so the tensile residual stress of the repaired part May remain in the bogie frame as a product. For this reason, there exists a possibility that the fatigue strength of a welded joint may fall.

  Such a problem also occurs in the method disclosed in Patent Document 1. This is because the situation does not change when the defective part of the weld bead is discovered and repaired by TIG tanning after the heat treatment is performed after the process of Patent Document 1 described above.

The present invention has been made in view of the above problems, and an object thereof is to provide a railcar bogie frame having the following characteristics and a method for manufacturing the same.
The fatigue strength of the welded joint can be reliably ensured even when the defective part of the weld bead is repaired by TIG tanning.

(I) A method for manufacturing a railcar frame for a railway vehicle according to an embodiment of the present invention includes:
Welding process of joining steel materials by gas shielded arc welding and forming a bogie frame;
A heat treatment step of performing an annealing heat treatment on the carriage frame;
A shot blasting process for shot blasting the carriage frame;
And a painting process for painting the bogie frame.
The manufacturing method of this bogie frame is:
When there is a defective part in a part of the weld bead after the shot blasting process,
A repairing process for repairing the defective part by TIG tanning;
It is a region including the intersection of the molten metal portion formed in the repair process, the weld bead, and the unmelted portion of the steel material, and is 10 mm or more along the weld toe of the weld bead from the intersection. The region further includes a striking process in which the region is subjected to a striking process by ultrasonic striking or hammer peening.

  In the manufacturing method of the bogie frame described above, it is preferable that a depth of 0.1 to 0.3 mm and a radius of curvature be 1.0 to 3.0 mm in the hitting trace formed in the hitting process.

  In the manufacturing method of the bogie frame, it is preferable that the compressive residual stress at the bottom of the impact mark formed in the impact processing step is 100 MPa or more.

  In the manufacturing method of the bogie frame, the height of the raised portion in the region of the end portion in the direction along the weld bead in the hitting mark formed in the hitting process is 25% of the depth of the hitting mark. It is preferable that it is less than.

(II) A bogie frame for a railway vehicle according to an embodiment of the present invention is:
A rail car carriage frame formed by joining steel materials by gas shielded arc welding,
A molten metal part formed by TIG tanning on a part of the weld bead;
Ultrasonic striking in an area including an intersection of the molten metal part, the weld bead, and the unmelted part of the steel material, and an area of 10 mm or more from the intersection along the weld toe of the weld bead Or a hitting mark formed by hammer peening.

  The bogie frame preferably has a depth of 0.1 to 0.3 mm and a radius of curvature of 1.0 to 3.0 mm in the hitting mark.

  The bogie frame preferably has a compressive residual stress of 100 MPa or more at the bottom of the hitting mark.

  In the bogie frame, the height of the raised portion is preferably less than 25% of the depth of the hitting mark in the end region in the direction along the weld bead in the hitting mark.

The railcar bogie frame and the manufacturing method thereof according to the present invention have the following remarkable effects:
The fatigue strength of the welded joint can be reliably ensured even when the defective part of the weld bead is repaired by TIG tanning.

FIG. 1 is a perspective view showing a welded portion of the bogie frame. FIG. 2 is a perspective view showing a welded part of the bogie frame and repaired by TIG tanning. FIG. 3A is a plan view of a reference test piece. FIG. 3B is a side view of a reference test piece. FIG. 4 is a plan view of a test piece subjected to TIG tanning. FIG. 5 is an SN diagram showing the fatigue test results of the reference specimen and the TIG tanned specimen. FIG. 6 is a photograph showing a fracture surface of a TIG tanned test piece subjected to fatigue failure. FIG. 7 is a plan view of a test piece that has been subjected to a striking process. FIG. 8 is an SN diagram showing the fatigue test results of the impact imparting test piece. FIG. 9A is a perspective view showing a welded part of the bogie frame that is repaired by TIG tanning and further subjected to a striking process. 9B is a cross-sectional view taken along line AA in FIG. 9A. 9C is a cross-sectional view taken along the line BB in FIG. 9A. FIG. 10 is a diagram illustrating a state of an end portion of the hitting trace formed by the hitting process. FIG. 11 is a perspective view showing another example of the welded portion of the bogie frame, which is repaired by TIG tanning and further subjected to a striking process.

  In order to confirm the influence of heat input when a defective part of a weld bead is repaired by TIG tanning in a bogie frame for a railway vehicle, the inventors conducted the following tensile fatigue test.

  3A and 3B are views showing the appearance of a reference test piece. Among these figures, FIG. 3A shows a plan view and FIG. 3B shows a side view. As shown in FIGS. 3A and 3B, a cross weld joint used for standard evaluation of a weld joint in a bogie frame was adopted as the test piece 10A. In the cross welded joint test piece 10A, a pair of main steel plates 11A and 11B to be evaluated are arranged on the same plane, and a sub steel plate 11C is arranged between the main steel plates 11A and 11B. 11B and the secondary steel plate 11C are joined by fillet welding. A reference test piece (hereinafter also referred to as “reference test piece”) 10A is formed by forming only the weld bead 2 between the main steel plates 11A and 11B and the sub steel plate 11C. Many such reference test pieces 10A were prepared.

  Here, the thickness of the main steel plates 11A and 11B was 9 mm in the region of the end portion on the side to be welded, and 12 mm in the region of the opposite end portion (the end portion that became the grip portion in the fatigue test). The width of the main steel plates 11A and 11B was 27 mm in the region of the end portion on the side to be welded. The material of the main steel plates 11A and 11B and the sub steel plate 11C was SM490YB (rolled steel for welded structure defined by JIS G3106).

  Moreover, TIG tanning was performed on some of the above-mentioned reference test pieces 10A, assuming a state in which a defective portion of the weld bead in the bogie frame was repaired by TIG tanning.

  FIG. 4 is a plan view showing an appearance of a test piece subjected to TIG tanning. As shown in FIG. 4, a test piece 10B subjected to TIG tanning (hereinafter also referred to as “TIG tanning test piece”) 10B is one of the weld toes 2a of the weld bead 2 adjacent to the main steel plates 11A and 11B. The molten metal part 3 by TIG tanning is formed in the part. Many such TIG tanning test pieces 10B were prepared.

  Here, regarding TIG tanning conditions, the current value was about 200 A, and the time was about 5 seconds. About the dimension of the molten metal part 3 formed by TIG tanning, the length of the direction in alignment with the weld bead 2 was 7.5 mm, and the width | variety was 10.2 mm.

A tensile fatigue test was performed using a plurality of reference test pieces 10A and TIG tanning test pieces 10B prepared as described above. In the fatigue test, an electrohydraulic servo type fatigue tester was used, and a repeated load having a stress ratio of 0.05 was applied in the longitudinal direction of the test pieces 10A and 10B. The test frequency was 10 Hz, and the number of repetitions was 1 × 10 ⁷ times.

  FIG. 5 is an SN diagram showing the fatigue test results of the reference specimen and the TIG tanned specimen. The vertical axis in the figure indicates the nominal stress range obtained by dividing the load range by the cross-sectional area on the welded base material side of the test piece. As shown in FIG. 5, it was found that when the results of the reference test piece and the TIG tanning test piece were compared, the fatigue strength of the TIG tanning test piece was lowered. This suggests that the heat input during TIG tanning affects the decrease in fatigue strength. Therefore, when the defect part of the weld bead is repaired by TIG tanning, it has been clarified that the fatigue strength of the welded joint is lowered unless some means are added.

  FIG. 6 is a photograph showing a fracture surface of a TIG tanned test piece subjected to fatigue failure. As shown in FIG. 6, the starting point of fatigue failure is 2 mm along the weld toe of the weld bead from the intersection of the molten metal portion formed by TIG tanning, the weld bead, and the unmelted portion of the main steel plate. It was an outer position (hereinafter also referred to as “specific position”).

  Residual stress was measured at specific locations using TIG tanned specimens not provided for testing. The residual stress was measured by the X-ray diffraction sin 2ψ method. At that time, the X-ray irradiation region was a 1 mm square region, and the direction of residual stress was the longitudinal direction of the test piece. The residual stress at a specific position in the TIG tanned specimen was a tensile residual stress of 83 MPa at the maximum.

  From these results, it was found that due to the influence of heat input during TIG tanning, a tensile residual stress was generated at a specific position, which caused a decrease in the fatigue strength of the TIG tanned specimen. Therefore, when the defective part of the weld bead is repaired by TIG tanning in the bogie frame, it is clear that the residual stress at a specific position needs to be converted into compressive stress in order to ensure the fatigue strength of the welded joint. Became.

  Therefore, as a method for converting the residual stress at a specific position into a compressive stress, application of a hammering process by ultrasonic hammering or hammer peening was examined. In these striking processes, a striking pin that vibrates at a high frequency is pressed against an object, and a tensile residual stress is applied to a region of a striking trace formed thereby.

  Some of the TIG tanned specimens not provided for testing were subjected to a hammering process.

  FIG. 7 is a plan view showing the external appearance of a test piece that has been subjected to a striking process. As shown in FIG. 7, a test piece (hereinafter also referred to as “blow imparting test piece”) 10 </ b> C that has been subjected to the hammering process is a weld toe portion of the weld bead 2 adjacent to the main steel plates 11 </ b> A and 11 </ b> B, and this weld The striking process is performed along the weld toe end of the weld bead 2 at the boundary between the toe end and the main steel plates 11A and 11B, whereby the hitting marks 4 are formed. The region of the hit mark 4 completely includes the specific position described above, and also includes a part of the molten metal portion 3 by TIG tanning. Many such impact imparting test pieces 10C were prepared.

  Here, the striking pin used in the striking processing was 3 mm in diameter, and the tip portion was a spherical surface having a radius of 3 mm.

  A tensile fatigue test was performed using a plurality of impact imparting test pieces 10C prepared as described above. The fatigue test was performed under the same conditions as the test of the reference test piece 10A and the TIG tanning test piece 10B described above.

  FIG. 8 is an SN diagram showing the fatigue test results of the impact imparting test piece. FIG. 8 also shows the results of the TIG tanning test piece shown in FIG. As shown in FIG. 8, it was found that the fatigue strength of the impact imparting test piece was improved by comparing the results of the impact imparting test piece and the TIG tanned test piece. This suggests that the hammering process contributes to the improvement of fatigue strength. Therefore, when the defective part of the weld bead is repaired by TIG tanning, it has been clarified that the fatigue strength of the welded joint is improved by performing a hammering process at a specific position.

  Residual stress was measured at a position corresponding to the above-mentioned specific position, which was an area at the bottom of the hitting mark, using a hitting test piece not provided for the test. The measurement of the residual stress was performed by the same method as the measurement of the residual stress of the TIG tanning test piece 10B described above. The maximum residual stress at the specific position (bottom of the hitting mark) in the hitting test piece was -147 MPa. That is, a compressive residual stress of at least 147 MPa was applied to the specific position.

  From these results, it was found that by applying the hammering process, a compressive residual stress was generated at a specific position, and this resulted in an improvement in the fatigue strength of the hammering test piece. Therefore, in the bogie frame, when the defective part of the weld bead is repaired by TIG tanning, it is clear that the fatigue strength of the welded joint can be surely ensured by performing a hammering process on the region including the specific position. It was.

  The railcar bogie frame and the manufacturing method thereof according to the present invention have been completed based on the above knowledge. Below, embodiment of the bogie frame for rail vehicles of the present invention and its manufacturing method is described.

  The manufacturing method of the bogie frame of the present embodiment includes a series of processes including a welding process, a care process, a heat treatment process, a shot blasting process, and a painting process as standard processes.

  In the welding process, the prepared various steel materials are combined, the steel materials are joined to each other by gas shield arc welding (eg, MAG welding), and a bogie frame is formed. The steel material is, for example, cut from a thick steel plate and hot-bended into a predetermined shape.

  Subsequently, in the care process, a part of the surface of the weld bead formed in the welding process is smoothed by a grinder. This grinder care is performed mainly on the weld toe at the boundary with the steel material in the surface of the weld bead where high stress is expected to be generated from the strength design. This is to ensure the fatigue strength of the welded joint.

  Next, in the heat treatment step, the bogie frame is subjected to annealing heat treatment. This is because residual stress generated in the carriage frame due to heat input in the welding process is removed.

  Next, in the shot blasting process, shot blasting is performed on the carriage frame. This is because the scale generated on the surface of the carriage frame in the heat treatment process is removed. Moreover, it is for smoothing the unevenness | corrugation of the surface of a trolley | bogie frame. It is also for tightening the surface of the weld bead.

  In the painting process, the bogie frame is painted. This is to make the bogie frame beautiful as a product and to protect the surface of the bogie frame. As a standard, the bogie frame is completed through such a series of steps.

  Here, in the series of processes described above, the appearance inspection is performed between the processes after the welding process, that is, immediately after the welding process, immediately after the care process, immediately after the heat treatment process, immediately after the shot blasting process, and immediately after the coating process. Is called. During this appearance inspection, a slight welding defect may be found in a part of the weld bead.

  When a welding defect is found in the appearance inspection immediately after the shot blasting process, a repairing process and a hammering process are added before the painting process. In the repair process, the defective part found by the appearance inspection is repaired by TIG tanning. Subsequently, in the striking process, a striking process by ultrasonic striking or hammer peening is performed on the repaired part by TIG tanning. Below, the specific aspect of a striking process is demonstrated.

  FIG. 9A to FIG. 9C are enlarged views showing the welded portion of the bogie frame that has been repaired by TIG tanning and further subjected to a striking process. Among these drawings, FIG. 9A shows a perspective view, FIG. 9B shows an AA cross-sectional view of FIG. 9A, and FIG. 9C shows a BB cross-sectional view of FIG. 9A. 9A to 9C exemplify portions where the steel materials 1A and 1B are joined by fillet welding in the same manner as in FIGS. 1 and 2.

  The striking processing is performed along the weld toe 2a of the weld bead 2 at the weld toe 2a of the weld bead 2 adjacent to the steel 1A and the boundary between the weld toe 2a and the steel 1A. The More specifically, the hammering process is an area including the intersecting portion 5 of the molten metal portion 3, the weld bead 2, and the unmelted portion of the steel material 1 </ b> A formed in the repair process. 2 is applied to a region of 10 mm or more along the weld toe portion 2a. A hitting mark 4 is formed in a region where the hitting process is performed. The region of the hitting mark 4 completely includes the specific position (a position 2 mm outside the weld toe 2 a of the weld bead 2 from the intersection 5), and a part of the molten metal part 3 by TIG tanning is also included. Including.

  As described above, when a defective portion existing in the weld bead is repaired by TIG tanning immediately after the shot blasting process, a compressive residual stress is generated at a specific position by performing the striking process. For this reason, although the process to leave is a coating process, the fatigue strength of a welded joint can be ensured reliably.

  In the present embodiment, the repair process and the striking process are added immediately after the shot blasting process, but the repair process and the striking process may be added immediately after the painting process. In this case, since the coating is peeled off on the surface of the molten metal portion formed in the repairing process and the area of the hitting trace formed in the striking process, it is only necessary to apply the coating only to the peeled area. In addition, if a weld defect is found in the appearance inspection prior to the shot blasting process, the repair by TIG tanning immediately after is put on hold, and after the shot blasting process, the repairing process and the hammering process may be added. .

  In addition, when the repair by TIG tanning is implemented before the heat treatment process, the addition of the striking process is not necessarily required. This is because the tensile residual stress in the repaired portion is removed by a heat treatment in a later process. Further, even when the repair by TIG tanning is performed after the heat treatment process and before the shot blasting process, the addition of the striking process is not necessarily required. This is because the tensile residual stress in the repaired portion is removed by shot blasting in a later process.

  Here, in the present embodiment, as shown in FIGS. 9A and 9C, the length s from the intersecting portion 5 to the end portion 4a of the hitting mark 4 is 10 mm or more. This is because the specific position is completely included in the region of the hitting mark 4. The length s is not particularly limited as long as it is 10 mm or more. However, if the length s is too long, only the time required for the impact processing is increased. For this reason, the upper limit of the length s is preferably 20 mm.

  Moreover, as shown to FIG. 9B, it is preferable that the depth d of the hitting mark 4 is 0.1-0.3 mm, and the curvature radius r of the hitting mark 4 is 1.0-3.0 mm. The reason is as follows. If the depth d of the hitting mark 4 is too small, the application of compressive residual stress is insufficient. If the radius of curvature r is too small, the effect of reducing stress concentration becomes insufficient. On the other hand, if the depth d of the hitting mark 4 is too large, the hitting mark 4 itself becomes a stress concentration source. If the radius of curvature r is too large, the application of compressive residual stress will be insufficient. Therefore, if it is out of the above range, the fatigue strength may decrease.

  Moreover, it is preferable that the compressive residual stress of the bottom of the impact mark 4 is 100 MPa or more. This is because if the compressive residual stress at the bottom of the hitting mark 4 is too small, the fatigue strength cannot be ensured sufficiently. On the other hand, the upper limit of the compressive residual stress at the bottom of the hitting mark 4 is not particularly limited. However, if the compressive residual stress is too large, deformation may occur during use. For this reason, the upper limit of the compressive residual stress at the bottom of the impact mark 4 is 1000 MPa.

  Further, in the present embodiment, the hitting mark 4 is formed by the hitting process, but strictly, the periphery of the hitting mark 4 is raised accordingly. When the degree of this bulge is significant, the residual stress in the bulged part turns into tensile stress. In this regard, the portion of the hitting mark 4 that is raised in the region of the end 4a in the direction along the weld bead 2 is disposed at the boundary between the weld bead 2 and the unmelted portion of the steel material 1A. For this reason, when the residual stress of the raised portion is a tensile stress, the raised portion may reduce the fatigue strength.

  FIG. 10 is a diagram illustrating a state of an end portion of the hitting trace formed by the hitting process. As shown in FIG. 10, when the ratio h / d between the height h of the raised portion 6 and the depth d of the hitting mark 4 is, for example, about 0.5 in the region of the end 4 a of the hitting mark 4. The residual stress of the raised portion 6 becomes a tensile stress. When the ratio h / d decreases to 0.25, the residual stress of the raised portion 6 becomes 0 or less. For this reason, it is preferable that the height h of the raised portion 6 at the end 4 a of the hitting mark 4 is less than 25% of the depth d of the hitting mark 4. Thereby, the residual stress of the raised portion 6 can be maintained as a compressive stress, and a decrease in fatigue strength due to the raised portion 6 can be prevented. In the actual striking process, once the striking trace 4 is formed, the end 4a of the striking trace 4 is again subjected to a striking process, and the raised portion 6 is crushed.

  In addition, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, as shown in FIG. 11, the region of the hit mark 4 may be separated by a portion of the molten metal portion 3 by TIG tanning. This is because it is sufficient if the above-mentioned specific position (a position 2 mm outside from the intersection 5 along the weld toe 2 a of the weld bead 2) is included in the region of the impact mark 4.

  The present invention can be effectively used for all railway vehicles.

1A, 1B: Steel, 2: Weld bead, 2a: Weld toe,
3: Molten metal part, 4: Striking mark, 4a: End part of striking mark,
5: Intersection, 6: Raised part,
10A: reference specimen, 10B: TIG tanned specimen,
10C: Impact test specimen,
11A, 11B: Main steel plate, 11C: Sub steel plate,
s: the length from the intersection to the end of the hitting mark, d: the depth of the hitting mark,
r: radius of curvature of hitting mark, h: height of the raised portion

Claims (8)

A method for manufacturing a bogie frame for a railway vehicle,
Welding process of joining steel materials by gas shielded arc welding and forming a bogie frame;
A heat treatment step of performing an annealing heat treatment on the carriage frame;
A shot blasting process for shot blasting the carriage frame;
A series of processes including a coating process for coating the bogie frame,
When there is a defective part in a part of the weld bead after the shot blasting process,
A repairing process for repairing the defective part by TIG tanning;
It is a region including the intersection of the molten metal portion formed in the repair process, the weld bead, and the unmelted portion of the steel material, and is 10 mm or more along the weld toe of the weld bead from the intersection. The manufacturing method of the bogie frame for rail vehicles further including the striking process process which performs the striking process process by ultrasonic striking or hammer peening in an area | region. It is a manufacturing method of the bogie frame for rail vehicles according to claim 1,
The manufacturing method of the bogie frame for rail vehicles whose depth is 0.1-0.3mm and whose curvature radius is 1.0-3.0mm in the hitting trace formed at the said hitting process. It is a manufacturing method of the bogie frame for rail vehicles according to claim 1 or 2,
The manufacturing method of the bogie frame for rail vehicles whose compressive residual stress of the bottom of the impact mark formed in the said impact processing process is 100 Mpa or more. It is a manufacturing method of the bogie frame for rail vehicles according to any one of claims 1 to 3,
A bogie frame for a railway vehicle in which the height of the raised portion is less than 25% of the depth of the hitting mark in the region of the end in the direction along the weld bead in the hitting mark formed in the hitting process. Manufacturing method. A rail car carriage frame formed by joining steel materials by gas shielded arc welding,
A molten metal part formed by TIG tanning on a part of the weld bead;
Ultrasonic striking in an area including an intersection of the molten metal part, the weld bead, and the unmelted part of the steel material, and an area of 10 mm or more from the intersection along the weld toe of the weld bead Or a railcar frame for railway vehicles, comprising striking marks formed by hammer peening. It is a bogie frame for railway vehicles according to claim 5,
A bogie frame for a railway vehicle, wherein the hitting mark has a depth of 0.1 to 0.3 mm and a radius of curvature of 1.0 to 3.0 mm. A bogie frame for a railway vehicle according to claim 5 or 6,
A bogie frame for a railway vehicle, wherein a compressive residual stress at the bottom of the hitting mark is 100 MPa or more. A bogie frame for a railway vehicle according to any one of claims 5 to 7,
A bogie frame for a railway vehicle, wherein a height of a raised portion is less than 25% of a depth of the hitting mark in an end region in a direction along the weld bead in the hitting mark.

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