JP2012176437A - Method for improving fatigue strength of weld part and weld joint - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fatigue strength improving method for improving fatigue strength of a weld part of a steel structure like a steel bridge suitable for hammer peening by introducing a compressive residual stress without imparting any deformation forming a new stress concentration part in the weld part.SOLUTION: A part of the surface of a base material, which is apart from a weld toe, is struck perpendicularly to the surface of the base material with a flat chipper whose striking face is chamfered. Preferably, striking is moved gradually from the vicinity of the weld toe to the outside so that part of a plastic deformation area caused by striking is overlapped, and the compressive residual stress is introduced into the weld toe. When forming a belt-like plastic deformation area having a described value of a recess characteristics (the product of the maximum depth and width)(mm) while bringing it into contact with the weld toe, the weld toe is struck with the chamfered part of the periphery of the flat part of the chipper striking face, and the base material is struck by the flat part. When the width of the striking face of the chipper used for striking is made B, plastic deformation is performed by striking the portion within B/4 from the weld toe of the surface of the base material.

Description

  The present invention provides a fatigue strength improving method for introducing and improving the fatigue strength of a welded portion in a steel structure such as a steel bridge by introducing compressive residual stress without giving the welded portion a deformation that becomes a new stress concentration portion. The present invention relates to a welded joint suitable for use in hammer peening.

  In recent years, with the aging of steel bridges, reports of damage cases due to corrosion and fatigue are increasing. In order to prevent fatigue damage, it is important to establish an inspection system, to reduce external forces such as passing vehicles, and to improve welding quality from the design and production aspects.

  If the weld has defects such as cracks, or if the shape of the weld toe is inappropriate and becomes a stress concentrated part, the effect of welding residual stress is superimposed on the repeated stress and fatigue cracks tend to occur, resulting in fatigue failure Since there are cases, proposals from various viewpoints have been made to prevent this.

  Patent Document 1 relates to a method for improving fatigue strength of a welded portion and a welded structure using the welded portion. When plastic deformation is performed by a striking device that ultrasonically vibrates the vicinity of a weld toe, a groove having a specific dimension under a predetermined striking condition. It is described that the fatigue strength is stably improved in a short time and without depending on the skill level of the operator.

  Patent Document 2 relates to a laser shock peening method, which uses a pulsed laser beam from a laser light source to instantaneously vaporize a coating on the surface and generate a compressive force locally on a part of the surface by the explosive force. The method describes introducing compressive residual stress into a fan blade of a gas turbine engine.

  Non-Patent Document 1 relates to a method for improving the fatigue strength of a welded joint portion of high strength steel (SM570) by hammer peening and TIG treatment. When hammer peening is applied, the fatigue strength may be reduced. The result of examination of a new hammer peening method for reducing concentration and residual stress is described.

  Normally, hammer peening is performed by holding the chipper of the peening machine with the hand so that the tip of the chipper hits the weld toe from diagonally above and entrusting the load of the peening machine to the weld toe. To reduce the workload.

  Therefore, when hammer peening is performed on the out-of-plane gusset joint in which the rib 2 is erected on the base material 1 shown in FIG. 27, a deep groove 6 serving as a stress concentration point is formed at the weld toe by the tip of the chipper 5 of the peening apparatus. The fatigue crack 7 may be formed from the tip of the weld bead 3.

  Non-Patent Document 1 introduces that it is effective to prevent the occurrence of fatigue cracks if a part of the weld toe is ground in advance with a grinder before hammer peening. is suggesting.

  Further, in Patent Document 3, a hitting pin having a curved tip end portion is used so that a deep groove serving as a stress concentration portion is not formed at the weld toe end, and the distance from the toe end of the weld bead to the hitting center is the hit pin. Describes a method for improving the fatigue characteristics of a butt welded joint that is subjected to hammer peening or ultrasonic impact treatment so that residual plastic deformation of a specific dimension occurs on the surface of the base material that is less than 2.5 times the radius of curvature of the tip. (FIG. 28).

  In Patent Document 4, as a fatigue treatment improving method for fatigue characteristics of a welded joint, a hammer peening process is performed by relatively moving the weld metal in the weld line direction while pressing the impact pin against the surface of the base metal material near the toe of the weld bead. When performing the ultrasonic impact treatment, a hitting pin having a radius of curvature R of 1/2 or less of the thickness of the metal material and 2 to 10 mm is used as the hitting pin, and the center of the hitting position from the toe of the weld bead. The distance is up to 2.5 times the radius of curvature R of the tip of the hitting pin.

  Furthermore, Patent Document 4 discloses that the hitting pin has a groove depth of 0.1 to 2 mm on the surface of the base metal material up to a range where the hitting pin does not touch the weld metal during the hitting process. Residual plasticity having a radius of curvature of the tip of the hitting pin of R or less, 1/10 or less of the thickness of the metal material, and a hitting mark width of 1.5 to 15 mm and 5 times or more of the groove depth. It is also described that hammer peening or ultrasonic impact treatment is applied so that deformation occurs.

  However, in the case of a large structure such as a steel bridge, the weld line length is long, and the weld toe portion is often meandering with a width of about 1 mm instead of a straight line in the weld line direction. It is difficult to perform hammer peening along the weld toe while maintaining a position separated from the weld toe by a minute distance as in the fatigue joint improvement impact processing method described in Patent Document 3.

  The ultrasonic peening method described in Patent Document 1 is expensive and difficult to obtain as compared with a conventional device that drives a chipper with air pressure. The laser shock peening method described in Patent Document 2 requires pretreatment of the material, and the apparatus is expensive and large, and is difficult to apply to steel bridge manufacturing. The processing method using ultrasonic striking treatment described in Patent Document 3 is for butt weld joints, and the effect on fillet weld joints, which is common in steel structures, is unknown.

  In addition, Patent Document 4 has an essential requirement that the tip of the chipper has a shape having a curvature radius R, and the distance from the toe of the weld bead to the center of the hitting processing position is the tip curvature radius R. It cannot be easily applied to the hammer peening method currently used, such as defined by the function of, and is not versatile.

  Therefore, the present invention performs hammer peening using a chipper capable of introducing a compressive residual stress over a wide range without forming a deep groove-like hitting trace that becomes a stress concentration source, and reduces the welding residual stress at the weld toe. It aims at providing the fatigue strength improvement method and welded joint of the welding part which reduce and improve fatigue strength.

The object of the present invention can be achieved by the following means.
1. A method for improving the fatigue strength of a welded portion by improving a fatigue strength by forming a band-shaped plastic deformation region consisting of a plurality of hitting marks by hitting a chipper in the vicinity of a weld toe of the welded portion, The striking surface has a flat portion and a chamfered portion along the flat portion, and the distance between the weld toe portion and the weld toe side of the striking surface is within ¼ of the width of the striking surface. When the belt-shaped plastic deformation region having a predetermined value of the depression characteristic value (product of maximum depth and width) (mm ² ) is formed in the base metal by hitting the vicinity of the weld toe portion, the belt-shaped plastic deformation When forming the region in contact with the weld toe, the fatigue strength of the weld is characterized by hitting the weld toe at the chamfer around the flat part of the striking surface and hitting the base metal at the flat part. How to improve.
2. 2. The method for improving fatigue strength of a weld according to claim 1, wherein the striking surface has a width of 4 mm or more, and the distance between the weld toe and the weld toe side of the striking surface is within 1 mm.
3. The strip-shaped recesses characteristic values in the plastic deformation region (up to the product of the depth and width) (mm ²⁾ is above the nominal stress amplitude that is loaded in the weld is below 150 MPa 0.7 mm ² or more, a nominal stress amplitude 250MPa ^{2. The} method for improving fatigue strength of welds according to 2, wherein the weld strength is 0.8 mm ² or more in the following cases.
4). The maximum depth in the band-shaped plastic deformation region is 0.19 mm or more when the nominal stress amplitude applied to the weld is 150 MPa or less, and 0.22 mm or more when the nominal stress amplitude is 250 MPa or less. The method for improving fatigue strength of a welded portion according to 2 or 3,
5. 5. The method for improving fatigue strength of a weld according to any one of 2 to 4, wherein the hitting by the chipper is a hitting cycle of 80 Hz or more and a moving speed of 10 cm / min or less along the weld toe.
6). The welding line according to any one of 1 to 5, wherein a hitting line is set along the weld toe part, and the vicinity of the hitting line is hit a plurality of times to form a belt-like plastic deformation region. Fatigue strength improvement method.
7). The method for improving fatigue strength of a welded portion according to any one of 1 to 6, wherein the welded portion is a fillet weld joint.
The welded joint which gave the fatigue strength improvement method of the welding part as described in any one of 8.1 thru | or 6.
9. A corner having a band-shaped plastic deformation region formed by hitting the vicinity of the weld toe portion multiple times in a direction perpendicular to the striking surface with a chipper having a flat portion on the striking surface of the tip portion and a chamfered portion along the flat portion Meat welded joints,
When the length of the side perpendicular to the weld toe part (line) of the striking surface is B (mm), the weld toe part side of the band-shaped plastic deformation region is B from the weld toe part to the base metal side. A fillet welded joint in which a base metal is hit so as to be within / 4 (mm) to form a strip-shaped plastic deformation region.
10. The length of the side of the striking surface is 4 mm or more, and the weld toe side of the belt-shaped plastic deformation region is within 1 mm from the weld toe part to the base material side,
The maximum depth in the band-shaped plastic deformation region is 0.19 mm or more when the nominal stress amplitude applied to the weld is 150 MPa or less, 0.22 mm or more when the nominal stress amplitude is 250 MPa or less,
The product of maximum depth and width in the band-shaped plastic deformation region is 0.7 mm ² or more when the nominal stress amplitude applied to the weld is 150 MPa or less, and 0.8 mm ² or more when the nominal stress amplitude is 250 MPa or less. 9. The fillet welded joint according to 9, wherein

  According to the present invention, it is possible to improve the fatigue strength of the welded portion by hammer peening the vicinity of the welded toe portion without causing a notch-shaped defect in the welded toe portion. Fatigue grade is dramatically improved from D grade to A grade. 2. Since both the weld toe and the base metal can be struck and there is no construction restriction that only the base material surface has to be struck, the construction of the welded part, etc. is facilitated and the application structure and application site are limited. It is hard to receive. This is very useful industrially.

The schematic diagram for demonstrating this invention. The figure explaining the shape of a tipper tip part. The figure explaining the theory of brandtl. The figure which shows the FEM analysis conditions for demonstrating the principle of this invention. The figure which shows the FEM analysis result by the conditions of FIG. 4 as three-dimensional residual stress distribution. The figure which shows the FEM analysis result by the thickness direction cross section by the conditions of FIG. 4 (in the case of the chipper which has a flat part in the front-end | tip part) The figure which shows the FEM analysis result by the thickness direction cross section by the conditions of FIG. 4 (in the case of the round chipper which has a hemisphere part in the front-end | tip part) The figure which shows the relationship between the magnitude | size of the residual stress in the board thickness direction cross section by the 1/4 symmetrical model, and the distance from a hit | damage center (tipper which has a flat part in a front-end | tip part (square shape), and a chipper which has a hemisphere part in a front-end | tip part) (For round type)). The figure explaining the effect of this invention by the compressive residual stress distribution in the weld toe vicinity. It is a figure which shows a test body shape, (a) is a top view, (b) is a side view. The figure which shows a fatigue test result. The figure (Example 1) which shows the result of having arranged the number of times of 150MPa fracture | rupture by the hollow characteristic value (maximum depth x width) (mm < ² >) of a plastic deformation area | region. The figure (Example 1) which shows the result of having arranged the 250MPa fracture | rupture frequency | count by the hollow characteristic value (maximum depth x width) (mm < ² >) of the plastic deformation area | region. The figure which shows the relationship between the hollow characteristic value (maximum depth x width) (mm < ² >) of a plastic deformation area | region, and the largest residual stress of a weld toe part. The figure which shows the result of having arranged the 150MPa fracture | rupture frequency by the maximum depth of a plastic deformation area | region (Example 1). The figure which shows the result of having arranged the 250MPa fracture | rupture frequency by the maximum depth of a plastic deformation area | region (Example 1). The figure which shows the relationship between the hollow characteristic value (maximum depth x width) (mm < ² >) measured in the plastic deformation area | region in the edge part of the rib 2 with the largest generated stress, and an actual cross-sectional area. The figure (Example 2) which shows the result which arranged 250 MPa fracture | rupture frequency by the distance (stop distance) from the edge by the side of the weld toe part of a plastic deformation area | region to a weld toe part. The shape of the residual-stress measurement test body welded by the fillet welding used in Example 3 is shown, (a) is sectional drawing, (b) is a top view, (c) is a figure which shows a side view. FIG. 6 is a residual stress distribution diagram in the X direction when the distance from the weld toe to the chipper end is 0 mm; (a) is a square chipper having a flat portion at the tip, and (b) has a hemispherical portion at the tip. The figure which shows the case of a round shape chipper. FIG. 4 is a distribution diagram of residual stress in the X direction when the distance from the weld toe to the tipper end is 1 mm. FIG. 4A is a square chipper having a flat portion at the tip, and FIG. The figure which shows the case of a round shape chipper. FIG. 6 is a residual stress distribution diagram in the X direction when the distance from the weld toe to the end of the chipper is 2 mm; (a) is a square chipper having a flat part at the tip, and (b) has a hemispherical part at the tip. The figure which shows the case of a round shape chipper. FIG. 5 is a distribution diagram of residual stress in the X direction when the distance from the weld toe to the tipper end is 3 mm. (A) is a square chipper having a flat part at the tip, and (b) has a hemispherical part at the tip. The figure which shows the case of a round shape chipper. FIG. 6 is a distribution diagram of residual stress in the X direction when the distance from the weld toe to the chipper end is 4 mm. FIG. 5A shows a square chipper having a flat part at the tip, and FIG. The figure which shows the case of a round shape chipper. The minimum residual stress in the X direction when the distance X from the weld toe side end of the striking surface of the residual stress measurement specimen is 0mm, 1mm, 2mm, 3mm, or 4mm during hammer peening ( The figure which compares the case of a square chipper and the case of a round chipper regarding the maximum compressive residual stress). The side view which shows the positional relationship of the distance X and the test body which were separated from the weld toe part at the time of hammer peening. The figure which shows the damage example of the weld toe part by the conventional hammer peening. Prior art (Patent Document 3).

  In the present invention, the vicinity of the weld toe portion is continuously hit with a chipper (sometimes called a hitting pin), and a band-shaped plastic deformation region having a predetermined size is formed in a specific region, thereby compressing the weld toe portion. It is characterized by improving the fatigue strength of the welded joint by introducing residual stress of.

  FIG. 1 is a conceptual diagram illustrating the present invention. In a side view of a member welded by welding a rib 2 around a base material 1, a weld toe portion of a weld bead 3 (hereinafter sometimes referred to as a toe portion). 4 shows a case where a plastic deformation (indicated by a dotted line) having a width B is generated on the surface of the base material 1 at a specific distance B / 4 away from 4.

  In the present invention, the compressive residual stress generated in the plastic deformation region cancels the tensile residual stress of the weld toe 4 and further introduces the compressive residual stress into the weld toe 4 to apply the plastic deformation. The distance between the formed region and the weld toe is defined to be within ¼ of the width B of the plastically deformed region.

  The width B of the region to be plastically deformed is that a compressive residual stress is introduced into the toe portion 4 and a deformation that becomes a new stress concentration source is caused by plastically deforming the surface of the base material 1 at the toe portion 4. Set so that it does not occur.

  When the surface of the base material 1 is plastically deformed, a press device or a device that repeatedly impacts impact is used. The following description will be made in the case where the surface of the base material 1 is hit with the chipper 5 of the hammer peening apparatus, and the hitting surface at the tip of the chipper 5 has a flat portion and a chamfered portion along the flat portion.

  The shape of the tip of the chipper 5 will be described with reference to FIG. FIG. 2 is a side view of the chipper as viewed from the side. The striking surface is square, and the length of one side perpendicular to the weld toe (line) is between the intersections α1 and α2 of the taper part 51 at the tip of the chipper 5, the extension line of the taper part 52 and the extension line of the flat part 53. The length is L2. The length L2 is substantially equal to a value obtained by adding the length L1 of one side of the flat portion 53 and twice the radius R of the R chamfered portion.

  When hitting the surface of the base material 1 with the chipper 5, the distance from the weld toe portion to the weld toe side of the hitting surface of the chipper 5 is within ¼ of the width B of the hitting surface of the chipper 5. To hit. When the surface of the base material 1 is hit with the chipper 5 having the shape of the tip shown in FIG. 2, the width of the region that is plastically deformed by the hit is substantially equal to the width of the flat portion of the hitting surface.

  In the present invention, the vertical when hitting perpendicularly to the surface of the base material is not limited to the vertical in a strict sense, and the inclination angle in the striking direction is about 80 to 100 ° with respect to the base material surface. If possible, it is acceptable, but 90 ° is desirable.

  Before hitting the surface of the base material with the chipper 5, a recess (r portion), preferably a radius of curvature of 1 mm or more (r portion) is provided at the boundary between the toe portion 4 and the base material 1 by grinding or the like. It is preferable that a larger compressive residual stress can be introduced into the toe portion 4 without affecting the toe portion 4 without deforming the surface of the base material.

  In addition, when a region where the part of the base material surface is hit and plastically deformed is formed, the reason why compressive stress is applied to the weld toe portion away from the region is that of the prandtl shown in FIG. This will be explained by theory (extracted from Non-Patent Document 2).

  When compressive stress q is introduced into the flat plate, a slip line is generated immediately below it, as indicated by a right triangle type ABC having a 45 ° acute angle, a fan type ADC, and BCE. As a result, a force that pushes outward acts on the right-angled triangular ADF and BEG having a hypotenuse of the same length as the length to which the compressive stress is applied, and acts on the plate as a compressive residual stress even after the compressive stress is unloaded.

[Chip tip shape and strike position]
In the present invention, the hitting surface at the tip of the chipper is made flat with a chamfered periphery. 4 to 9 show results of simulating the influence of the shape of the tip of the chipper on the residual stress distribution of the base material by FEM analysis. The FEM analysis conditions were as follows.

The chipper 5 was plastically deformed by applying a load of 26 kN at the maximum with a displacement of 1 mm to the surface of the base material 1 from the vertical direction. The base material 1 had a size of 200 mm × 200 mm, a plate thickness of 12 mm, and a material equivalent to SM490Y. On the other hand, the chipper 5 had a height of 135 mm and a chipper part other than the tip part (general part): 9 mm square. The hitting surface at the tip of the chipper 5 was analyzed for each of a square shape and a round shape. The shape of the striking surface at the tip of the chipper 5 was a square of 4 × 4 (mm ² ) in the case of a square and completely flat without chamfering, and a hemisphere having a diameter of 4 mm in the case of a round shape. Furthermore, the striking surface at the tip of the chipper 5 corresponds to a range in which the base material 1 is subjected to compressive stress.

  FIG. 4 shows an example of a mesh shape for performing FEM analysis. Since the elasto-plastic analysis is performed by the 1/4 symmetry model, the size of the base material 1 and the size of the striking surface at the tip of the chipper 5 are halved.

FIG. 5 shows an example of the outline of the three-dimensional distribution of residual stress obtained in the case of a square chipper 5 having a striking surface of 4 × 4 (mm ² ). 6 and 7 show the outline of the residual stress distribution in a two-dimensional cross section in the plate thickness direction. FIG. 6 shows a case where the hitting surface is a square chipper 5 of 4 × 4 (mm ² ), and FIG. Shows the case of a round chipper 5. 6 and 7, in the case of the rectangular chipper 5 having a striking surface of 4 × 4 (mm ² ), the compressive residual stress is widely distributed in the base metal compared to the case of the chipper 5 having a round striking surface. And the value is large.

  FIG. 8 shows the relationship between the magnitude of the residual stress and the distance from the impact center in the cross section of the base material 1 parallel to the plate thickness direction, obtained from the FEM analysis result. In FIG. 8, X = 0 mm corresponds to the hitting center position, and the dotted line above X = 2 mm corresponds to the end position of the hitting surface.

  In the case of the chipper 5 having a square striking surface indicated by a thick solid line in FIG. 8, compressive residual stress is generated in the striking region, and increases as the distance from the striking center increases, and is about 3 to 4 mm in the horizontal direction from the striking center. A maximum value of about 400 MPa occurs at the position shown as the region b in FIG. Thereafter, the pressure gradually decreases, but a large compressive residual stress exceeding 100 MPa is generated from the end of the hit region to a distance of about 9 mm.

  When the width (B) of the striking surface is 4 mm, that is, at a distance of 2 mm from the striking center in FIG. 8, the distance from the weld toe to the weld toe side of the striking surface is 1-2 mm. When struck, it is possible to introduce a substantially maximum compressive residual stress into the weld toe.

  On the other hand, when the hitting surface indicated by the thin solid line in FIG. 8 is a round chipper 5, tensile residual stress is generated near the center of the hitting surface, and the compressive stress is about 3 mm in the horizontal direction from the center of the hitting surface. In addition, a maximum value of about 350 MPa occurs at the position shown as region a in FIG. Large compressive residual stress exceeding 100 MPa is a region where compressive residual stress is introduced into the base material as compared with the chipper 5 having a flat striking surface from the end of the striking region up to about 6.5 mm away. Is small.

  From the results shown in FIGS. 6 to 8, in the present invention, the striking surface of the tip of the chipper is made flat (beveled to facilitate hammer peening), and the welding toe side of the striking surface from the welding toe part. The distance up to is within 1/4 of the striking face width (B).

  When chamfering the periphery of the flat part to form a plastic deformation region by hitting the chipper in contact with the weld toe, the chamfered part of the striking surface strikes the weld toe, and the flat part is the weld toe. Strike the base material in contact with the part. Since the chamfered portion hits the weld toe, a plastic deformation region is formed in contact with the weld toe without causing a notch-shaped defect in the weld toe. The chamfering may be either C chamfering by cutting off corners or R chamfering by rounding the corners with a radius of curvature R. In the case of C chamfering, the side length is 0.5 mm or less, and in the case of R chamfering, 0.5 mmR or less. Is desirable.

  FIG. 5 shows the FEM analysis results when the flat base metal surface is pressed and plastically deformed. The residual stress when the base metal surface in the vicinity of the weld toe 4 is pressed (struck). An example of the result of the distribution obtained by FEM analysis is shown in FIG. It can be seen that the compressive residual stress distribution shows the same distribution form and size as in the case of the flat plate even when the weld bead 3 is present.

  As described above, in the present invention, when forming a belt-like plastic deformation region consisting of a plurality of hitting marks by hitting the chipper in the vicinity of the weld toe, a hitting line indicating the chipper moving direction in hitting (welding of the hitting surface of the chipper) Is set so that the distance between the weld toe and the striking line is within ¼ of the width of the hitting surface of the chipper.

  At this time, if the striking line is gradually set outward from the vicinity of the toe portion 4 so that a part of the plastic deformation region due to the striking overlaps, a compressive residual stress distribution with small fluctuation is obtained, and the fatigue strength is improved more stably. It is possible to make it.

When the present invention is applied to hammer peening, the size (width B) of the tip of the chipper of the hammer peening apparatus, the applied pressure, and the base material characteristics (in the case of a steel bridge, SS400 to SM570 are used, and the yield stress is 215 N / mm ^2. analyzes of FIG. 4 using a ~450N / mm ^2), the compressive residual stress distribution in the base metal surface seeking (Figure 8), is superimposed on the welding residual stress distribution of the toe portion 4 obtained in advance In such a case, the distance from the weld toe to the weld toe side of the chipper may be defined within B / 4 so that the largest compressive residual stress is obtained at the toe 4. In the present invention, the shape of the hitting surface of the chipper is not limited to a square as long as it has a chamfered flat portion.

  When practicing the present invention, a new chipper is replaced when the shape of the tip of the chipper is damaged. It is desirable to use a chipper with a hardened tip and increased hardness.

A test specimen was prepared by welding a base material 1 having a width of 150 mm, a length of 500 mm, a plate thickness of 12 mm and a material of SM490Y to a rib 2 having a width of 75 mm × height of 50 mm × plate thickness of 12 mm and a material of SM490Y. The rotating welding conditions were wire MXZ200-1, 2Φ, 100% CO ₂ , 240 A, 30 V, 40 CPM, 10, 8 KJ / cm. On the other hand, a chipper having a 4 mm square striking surface at the tip part, which has an R chamfering process of 0.5 mmR and a substantially flat shape with a flat part of 3 mm square, was used. And the base material 1 surface of the said test body was repeatedly hit | damaged perpendicularly along the welding line, and the hammer peening process was performed. At the time of impact, the distance from the weld toe of the specimen to the end on the weld toe side of the hitting surface by the chipper was 1 mm. Then, it used for the fatigue test.

  FIG. 10 shows a plan view and a side view of the test specimen. In hammer peening, a U-shaped region surrounding the longitudinal end portion of the rib 2, which is a region where the welding residual stress is large, in the illustrated specimen is formed from one side surface of the rib to the other side surface along the welded portion. Continuous hitting on the same batting line was performed 1 to 8 times (in the case of an even number, reciprocating).

The conditions of hammer peening are as follows: hammering with a chipper with a striking cycle of 90 ± 10 Hz and a moving speed of 10 cm / min or less in the direction of the welding line. Among the IIW (International Institute of Welding) recommended conditions, air pressure is about 6 kg / cm ² , hammer load 7 kg was changed, and the size of the impact mark by one impact was changed.

The size of the plastic deformation region is measured by measuring the plastic deformation region on the extension of the plate thickness center line of the rib 2 with a laser displacement meter at the tip portion (turned welded portion) of the rib 2 having the largest generated stress in the illustrated specimen. As a characteristic value indicating the shape of the plastic deformation region, a hollow characteristic value (unit: mm ² ) defined by the maximum depth × width in the plastic deformation region was obtained. Moreover, let the maximum depth be the maximum value of the hollow part on the basis of the flat surface of a base material. When hitting a plurality of times, the measurement by the laser displacement meter measures the cross-sectional shape of the plastic deformation region where a part of each hitting mark is superimposed. The maximum residual stress at the weld toe was measured by X-ray diffraction.

  For comparison, a specimen that was not subjected to hammer peening after welding was also prepared and subjected to a fatigue test. The fatigue test was performed by chucking both ends of the base material 1 and repeatedly applying stress to the longitudinal direction of the rib 2 with respect to the test body.

  FIG. 11 shows the fatigue test results when the number of times of movement from end to end on the batting line is 3 or 4 assuming actual construction, as compared with the case of welding. The specimen subjected to hammer peening is grade C line or higher, and the fatigue strength improvement effect of about grade 3 of the fatigue design curve shown in the Japan Steel Structure Association (see Non-Patent Document 3) is recognized compared with the as-welded specimen. It was.

Table 1 and FIG. 12 show the number of breaks when the nominal stress amplitude acting on the base material is 150 MPa, and Table 2 and FIG. 13 show the number of breaks when the nominal stress amplitude is 250 MPa. ^The results organized in ² ) are shown.

When the nominal stress amplitude is 150 MPa or less, 0.7 mm ² or more, and when the nominal stress amplitude is 250 MPa or less, 0.8 mm ² or more with respect to the test results as-welded (when the dent characteristic value is 0 mm ² ) Strength will be higher than grade C line with 2 grades improvement.

Table 3 and FIG. 14 show the relationship between the dent characteristic value (mm ² ) in the plastic deformation region and the maximum compressive residual stress at the weld toe. Maximum compressive residual stress of the weld toe is organized by depression characteristic value (mm ^2), it is recognized that to decrease the indentation characteristic value (mm ²⁾ is increased.

  FIG. 15 shows the result of arranging the 150 MPa fracture times in Table 1 by the maximum depth of the plastic deformation region, and FIG. 16 shows the result of arranging the 250 MPa fracture times in Table 2 by the maximum depth of the plastic deformation region. When the nominal stress amplitude is 150 MPa or less, the maximum depth is 0.19 mm or more, and when the nominal stress amplitude is 250 MPa or less, the maximum depth is 0 with respect to the test results as-welded (in the case of the maximum impact mark depth of 0.00 mm). At 22 mm or more, the fatigue strength is improved by 2 grades or more, and becomes the C grade line or more. The grade of fatigue strength is based on the fatigue design curve shown in the above-mentioned Japan Steel Structure Association.

  In addition, as shown in FIG. 17, the hollow characteristic value (maximum depth × width) measured in the plastic deformation region at the tip end portion (turn weld portion) of the rib 2 having the largest welding residual stress and the actual sectional area of the hollow portion. A good proportional relationship was observed between them. The number of times in the figure indicates the number of times of movement from end to end on the batting line.

  Six test specimens prepared by the same method as in Example 1 were prepared, and the distance from the weld toe side end of the plastic deformation region to the weld toe part was 0 mm with a chipper having a 4 mm square striking surface. Hammer peening was performed so as to be 1 mm, 2 mm, 3 mm, 4 mm, or 5 mm, and then subjected to a fatigue test.

Hammer peening is performed by repeatedly striking the surface of the base material vertically along the weld line to form a plastic deformation region having a dent characteristic value (maximum depth x width) of 0.8 mm ² or more and a maximum depth of 0.20 mm or more. The conditions were adjusted. The setting of the hammer peening striking line and the measurement of the dimensions of the plastic deformation region were in accordance with Example 1. When the distance from the weld toe side end of the plastic deformation region to the weld toe is 0 mm, the weld toe was struck by the chamfered portion of the chipper, but no notch-shaped defect occurred. It was. The fatigue test was performed under the same conditions as in Example 1.

  FIG. 18 shows the result of arranging the 250 MPa fracture times by the distance between the weld toe portion and the plastic deformation region (toe end distance). The distance from the weld toe side end to the weld toe part of the plastic deformation region of the present invention example within a quarter of the striking surface width is 0 mm, and the distance between the weld toe part and the plastic deformation region is 0 mm, Excellent fatigue properties are obtained with the test specimen of 1 mm.

  In the case of hammer peening, the chipper striking surface and the plastic deformation area are almost equal in size, so the distance between the weld toe and the tip of the chipper striking surface is 2 mm or more, and only the base metal is hit. In this case, the fatigue grade calculated from Non-Patent Document 2 is D grade, whereas the construction method according to the present invention is the welding toe portion and the weld toe side of the hitting surface of the chipper. When the distance is within 1 mm and both the weld toe and the boundary between the base metal and the base metal are hit, the fatigue grade becomes A grade, and the fatigue strength of the welded joint is greatly improved.

A test piece for residual stress measurement was prepared by welding a base metal 1 having a width of 150 mm, a length of 150 mm, a plate thickness of 12 mm, and a material of SM400, and a rib 2 of SM400A having a width of 150 mm × height of 50 mm × plate thickness of 12 mm by fillet welding. did. The fillet welding conditions were wires MXZ200-1, 2Φ, 100% CO ₂ , 240A, 30V, 40CPM, 10, 8KJ / cm. On the other hand, the chipper has a rounded chamfered surface of 0.5 mmR at the tip and a flat portion of 4 mm square with a flat shape of 3 mm square, or a tip of the tip has a hemisphere with a diameter of 4 mm It was used. And the base material 1 surface of the said residual-stress measurement test body was repeatedly hit | damaged perpendicularly | vertically along the weld line, and the hammer peening process was performed. At the time of impact, the distance from the weld toe of the residual stress measurement specimen to the end on the weld toe side of the striking surface of each chipper was 0 mm, 1 mm, 2 mm, 3 mm, or 4 mm. Then, it used for the residual stress measurement.

  FIG. 19 shows the shape of the residual stress measurement specimen, (a) is a cross-sectional view, (b) is a top view, and (c) is a side view. Hammer peening was performed on the both sides of the weld toe of the rib in the illustrated test piece, and the straight continuous striking was performed a plurality of times along the weld.

  The condition of hammer peening is that the impact is 90 mm ± 10 Hz, the movement speed is 10 cm / min or less in the direction of the weld line, and the impact surface is a flat 4 mm square so that the impact depth is 0.15 mm or more. In the case of a chipper having a hemispherical hitting surface having a diameter of 4 mm, the chipper having a diameter of 5 was hit twice.

  The dimensions of the plastic deformation region due to impact were measured with a laser displacement meter at a total of three cut surfaces at the residual stress measurement line and at positions 10 mm before and after that. Then, the average value of the maximum depth of the striking traces on the three cut surfaces was defined as the maximum depth in each test specimen, and the average value of the striking trace widths on the same three cut planes was defined as the width in each test specimen. For reference, the maximum depth x width is also shown. Table 4 shows the relationship between the residual stress measurement specimen and the maximum depth.

The maximum residual stress at the weld toe was measured by X-ray diffraction. Table 5 shows the residual stress distribution of a specimen using a square chipper having a flat portion at the tip (sometimes simply referred to as a square chipper), and Table 6 shows a round chipper having a hemispherical portion at the tip (simply round). The residual stress distribution of the test body using a type chipper) is shown. The X-direction residual stress was measured to a position where the distance from the weld toe is about −100 MPa or about 10 mm. Since the measurement position and the measurement result of the residual stress are different depending on the specimen, blanks are generated in Tables 5 and 6.
20 to 24 show the residual stress measurement results (X direction residual stress distribution diagrams when the distance from the weld toe to the tipper end is 0 mm, 1 mm, 2 mm, 3 mm, and 4 mm).

  In each figure, (a) shows the case of a square chipper having a flat part at the tip, and (b) shows the case of a round chipper having a hemispherical part at the tip.

  Table 7 shows the relationship between the distance X from the weld toe side end of the striking surface of the test specimen to the weld toe and the minimum residual stress in the X direction (maximum compressive residual stress) during hammer peening Show. Fig. 25 shows the distance X between the weld toe end of the striking surface of the residual stress measurement test specimen and the weld toe when the distance X during hammer peening is 0 mm, 1 mm, 2 mm, 3 mm, and 4 mm. The X-direction minimum residual stress (maximum compressive residual stress) is shown in comparison with a square chipper having a flat portion at the tip and a round chipper having a hemisphere at the tip. FIG. 26 is a side view showing the positional relationship between the distance X and the specimen.

  20 to 24 and FIG. 25, the compressive residual stress in the vicinity of the weld toe of the residual stress measurement specimen subjected to hammer peening using a square chipper having a flat portion at the tip has a hemispherical portion at the tip. Compared with the compressive residual stress in the vicinity of the weld toe of the residual stress measurement test piece that was hammer peened using a round chipper, the distance from the weld toe side end of the striking surface to the weld toe was In any of 0 mm, 1 mm, 2 mm, 3 mm, and 4 mm, the compressive residual stress is large although the maximum depth of the impact mark is small.

  For example, comparing (a) and (b) of FIG. 22 with the compressive residual stress in the vicinity of the weld toe, in the case of a square chipper having a flat part at the tip part, a round chipper having a hemispherical part at the tip part is obtained. About twice as much as the case. This means that the relationship between the striking trace shape and the residual stress is different between a square chipper having a flat portion at the tip and a round chipper having a hemispherical portion at the tip. That is, in order to introduce a predetermined residual stress in the vicinity of the weld toe, the control value relating to the shape of the damage mark on the weld toe by the square chipper having a flat portion is the weld toe by a round chipper having a hemispherical part. Unlike the management value related to the shape of the hitting mark of the part, it is necessary to manage with a hollow characteristic value (product of maximum depth and width) which is a new management index.

DESCRIPTION OF SYMBOLS 1 Base material 2 Rib 3 Weld bead 4 Weld toe part 5 Chipper 51, 52 Tapered part 53 Flat part 6 Strike part 7 Fatigue crack

JP 2006-175512 A JP 2006-159290 A JP 2010-142870 A JP 2010-29897 A

IMPROVING FATIGUE STRENGTH OF WELDED JOINTS BY HAMMER PEENING AND TIG-DRESSING: Kengo ANAMI, Chitoshi MIKI, Hideki TANI, Haruhito YAMAMouto. / Earthquake Eng. , JSCE, Vol. 17, NO. 1, 57s-68s, 2000 April) Kenzo Kato: Metallic plastic processing, Maruzen, pp. 74-76 Japan Steel Structure Association: Fatigue design guidelines for steel structures and explanations, pp. 1-18, 1993.

Claims (10)

A method for improving the fatigue strength of a welded portion by improving a fatigue strength by forming a band-shaped plastic deformation region consisting of a plurality of hitting marks by hitting a chipper in the vicinity of a weld toe of the welded portion, The striking surface has a flat portion and a chamfered portion along the flat portion, and the distance between the weld toe portion and the weld toe side of the striking surface is within ¼ of the width of the striking surface. When the belt-shaped plastic deformation region having a predetermined value of the depression characteristic value (product of maximum depth and width) (mm ² ) is formed in the base metal by hitting the vicinity of the weld toe portion, the belt-shaped plastic deformation When forming the region in contact with the weld toe, the fatigue strength of the weld is characterized by hitting the weld toe at the chamfer around the flat part of the striking surface and hitting the base metal at the flat part. How to improve.   2. The method for improving the fatigue strength of a welded portion according to claim 1, wherein a width of the striking surface is 4 mm or more, and a distance between the weld toe portion and the weld toe side of the striking surface is within 1 mm. . The strip-shaped recesses characteristic values in the plastic deformation region (up to the product of the depth and width) (mm ²⁾ is above the nominal stress amplitude that is loaded in the weld is below 150 MPa 0.7 mm ² or more, a nominal stress amplitude 250MPa The method for improving the fatigue strength of a welded portion according to claim ^{2, wherein the} following cases are 0.8 mm ² or more.   The maximum depth in the band-shaped plastic deformation region is 0.19 mm or more when the nominal stress amplitude applied to the weld is 150 MPa or less, and 0.22 mm or more when the nominal stress amplitude is 250 MPa or less. The method for improving fatigue strength of a welded portion according to claim 2 or 3.   The method for improving fatigue strength of a welded portion according to any one of claims 2 to 4, wherein the hitting by the chipper is a hitting cycle of 80 Hz or more and a moving speed of 10 cm / min or less along the weld toe portion.   The welding according to any one of claims 1 to 5, wherein a striking line is set along the weld toe, and the vicinity of the striking line is hit a plurality of times to form a belt-like plastic deformation region. Of improving the fatigue strength of parts.   The method for improving fatigue strength of a welded portion according to any one of claims 1 to 6, wherein the welded portion is a fillet weld joint.   The welded joint which gave the fatigue strength improvement method of the welding part as described in any one of Claims 1 thru | or 6. A corner having a band-shaped plastic deformation region formed by hitting the vicinity of the weld toe portion multiple times in a direction perpendicular to the striking surface with a chipper having a flat portion on the striking surface of the tip portion and a chamfered portion along the flat portion. Meat welded joints,
When the length of the side perpendicular to the weld toe part (line) of the striking surface is B (mm), the weld toe part side of the band-shaped plastic deformation region is B from the weld toe part to the base metal side. A fillet welded joint in which a base metal is hit so as to be within / 4 (mm) to form a strip-shaped plastic deformation region. The length of the side of the striking surface is 4 mm or more, and the weld toe side of the belt-shaped plastic deformation region is within 1 mm from the weld toe part to the base material side,
The maximum depth in the band-shaped plastic deformation region is 0.19 mm or more when the nominal stress amplitude applied to the weld is 150 MPa or less, 0.22 mm or more when the nominal stress amplitude is 250 MPa or less,
The product of maximum depth and width in the band-shaped plastic deformation region is 0.7 mm ² or more when the nominal stress amplitude applied to the weld is 150 MPa or less, and 0.8 mm ² or more when the nominal stress amplitude is 250 MPa or less. The fillet welded joint according to claim 9, wherein: