JP2011131260A - Method for increasing fatigue strength of weld zone, and weld joint - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fatigue strength increasing method favorable when it is used for the hammer peening, and capable of increasing the fatigue strength of a weld zone in a steel structure such as a steel bridge by introducing the compressive residual stress without imparting any deformation forming a new stress concentration part in the weld zone. <P>SOLUTION: A part of a surface of a base material distant from a toe of weld is pressed normally to the surface of the base material, preferably, a part of plastically deformed zones is moved gradually to the outer side from the side in a vicinity of the toe so as to overlap each other, and subjected to plastic deformation to introduce the compressive residual stress in the toe. Favorably, a fore end of a member to be used for pressing is substantially rectangular and has a flat part of the width of ≥4 mm, a portion of the surface of the base material exceeding 3 mm from the toe is plastically deformed, and further preferably, a recess having the radius of curvature of ≥1 mm is formed in the toe in advance before the pressing is performed. <P>COPYRIGHT: (C)2011,JPO&INPIT

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. 11, 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.

JP 2006-175512 A JP 2006-159290 A

IMPROVING FATIGUE STRENGTH OF WELDED JOINTS BY HAMMER PEENING TIG-DRESSING / Earthquake Eng. , JSCE, Vol. 17, NO. 1,57s-68s, 2000 April)

  However, in the case of a steel bridge, since there are many welds and the weld length is long, it is not preferable to perform hammer peening a plurality of times because it may be a burden on the operator.

  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.

  Therefore, the present invention provides a method for improving the fatigue strength of a welded portion by reducing the welding residual stress at the weld toe and improving the fatigue strength without causing a deformation to the weld toe and forming a new stress concentration portion. The purpose is to provide.

The object of the present invention can be achieved by the following means.
1. A portion of the base metal surface away from the weld toe is pressed perpendicularly to the base metal surface to cause plastic deformation, and compressive residual stress is introduced into the weld toe. Fatigue strength improvement method.
2. When pressurizing a part of the base material surface vertically, a part of the base material surface separated from the weld toe part is pressurized so that deformation of the base material surface does not reach the weld toe part. The method for improving the fatigue strength of the welded portion.
3. Using a pressure member having a cross-sectional shape perpendicular to the pressing direction of the distal end portion being substantially rectangular and having a flat portion with a width of 4 mm or more at the distal end portion, 3 mm from the weld toe portion on the surface of the base material. 3. The method for improving fatigue strength of a welded portion according to 1 or 2, wherein a portion exceeding the limit is plastically deformed.
4). 4. The method according to any one of 1 to 3, wherein pressurization is performed so as to gradually move the pressurization portion from the vicinity of the weld toe portion so as to partially overlap the plastic deformation region. To improve the fatigue strength of welds.
5. 5. The method for improving fatigue strength of a welded portion according to any one of 1 to 4, wherein a depression having a radius of curvature of 1 mm or more is previously formed in the weld toe before pressurization.
The welded joint which performed the fatigue strength improvement method of the welding part as described in any one of 6.1 thru | or 5.

  According to the present invention, it is possible to improve the fatigue strength of the welded portion stably, which is extremely useful industrially.

The schematic diagram for demonstrating this invention. 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. 2 as three-dimensional residual stress distribution. The figure which shows the FEM analysis result by the conditions of FIG. 2 as a compressive residual stress distribution curve of the base material surface. The figure which shows the FEM analysis result by the conditions of FIG. 2 as a compressive residual stress distribution on the surface of a base material, and the deformation | transformation state of a plate surface. The figure explaining the theory of brandtl. The figure explaining the effect of this invention by the compressive residual stress distribution in a weld toe part and a base material. The figure explaining the other example of this invention. The figure which shows a test body shape. The figure which shows a fatigue test result. The figure which shows the damage example of the weld toe part by the conventional hammer peening.

  The present invention is characterized in that a part of the base material surface separated from the weld toe portion is pressed perpendicularly to the base material surface to be plastically deformed, and compressive residual stress is introduced into the weld toe portion. .

  FIG. 1 is a conceptual diagram for explaining the present invention. In a side view of a member welded by welding a rib 2 around a base material 1, a distance d is separated from a weld toe portion (hereinafter, toe portion) 4 of a weld bead 3. The case where the surface of the base material 1 is plastically deformed (indicated by a dotted line) is generated by pressing the surface of the base material with a chipper 5 having a width B at the tip (flat portion) in the direction perpendicular to the surface of the base material. . Pressurization of the surface of the base material by the chipper 5 is performed by a press device or a device that repeatedly impacts.

  In the present invention, the term “perpendicular when pressing perpendicularly to the surface of the base material” is not limited to the strict sense of verticality, and the inclination angle in the pressing direction is 80 to 100 ° with respect to the surface of the base material. Although it is acceptable up to about 90 °, 90 ° is desirable.

  In the present invention, the position at which the surface of the base material 1 is plastically deformed by the chipper 5 (width B) (specified by the distance d) is the tensile residual stress due to welding of the toe portion 4 by the compressive residual stress generated in the base material 1 due to plastic deformation. To cancel and compressive residual stress is introduced. More preferably, a compressive residual stress is introduced into the toe portion 4 and a new stress concentration is generated at the toe portion 4 due to the deformation of the base material surface caused by plastic deformation of the surface of the base material 1. The distance d is set so as not to exist.

  Before pressurizing 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 formed at the boundary between the toe portion 4 and the base material 1 by grinder grinding or the like. When it is provided, it is possible to introduce a larger compressive residual stress into the toe portion 4 without affecting the toe portion 4 without causing deformation of the surface of the base material.

  2 to 5 are diagrams for explaining the principle of the present invention. Residual stress distribution is simulated by FEM analysis when the base material 1 is pressed in the direction perpendicular to the surface of the base material by the chipper 5 to cause plastic deformation. The results are shown.

FIG. 2 shows FEM analysis conditions. A load of 26 kN is applied to the base material 1 corresponding to SM490Y from a direction perpendicular to the surface of the base material by a chipper 5 having a rectangular flat portion with a pressing surface (applying compressive stress) of 4 × 4 (mm ² ). The elasto-plastic analysis was performed using a quarter-symmetric model for the case of plastic deformation.

  3 to 5 are analysis results, FIG. 3 is a three-dimensional analysis result, FIG. 4 is a stress distribution on the base material surface, and FIG. 5 is a deformation state of the base material surface. From the stress distribution on the surface of the base metal (curve a in FIG. 4), the pressed region is a compressive residual stress of about 50 MPa, and the position is about 4 mm away from the center of the pressing surface in the horizontal direction (distance C from the center in FIG. 4). ), A maximum value of compressive stress of about 350 MPa (point b of curve a in FIG. 4) is generated and then gradually decreases, but a large compressive residual stress of about 30 MPa is generated from the center to a position of about 12 mm. It is recognized that That is, a large compressive residual stress exceeding 30 MPa is introduced from the end of the pressurized region to a position about 10 mm away.

  In addition, since the pressurized region is plastically deformed to form a recess (residual dent amount: 0.06 mm), the periphery thereof is a section having a width of about 3 mm from the end of the region (a range of about 5 mm from the center). Inner) is slightly deformed (FIG. 5).

  Accordingly, the distance d overlaps the stress distribution on the surface of the base material (curve a in FIG. 4) and the weld residual stress distribution of the toe part 4 separately obtained so that a compressive residual stress can be obtained at the toe part 4. Stipulate. More preferably, a compressive residual stress is obtained at the toe portion 4 and the base material surface is deformed to prevent a new stress concentration from occurring due to the deformation of the base material surface at the toe portion 4. The distance d is set so as to be outside the portion.

  When the base metal surface is pressed and plastically deformed under the conditions satisfying the FEM analysis conditions of FIG. 2, in order to realize stable fatigue strength improvement, the distance d exceeds 3 mm and is 10 mm or less (2.5 B of width B). Double or less: In this analysis, the width B is up to 4 mm), so that a relatively large compressive residual stress exceeding 30 MPa is introduced (FIG. 4). There is no impact (Figure 5).

  When plastic deformation is introduced by pressing (striking) the surface of the base material by hammer peening, the shape of the cross section perpendicular to the pressing direction of the flat portion at the tip of the chipper is rectangular, and the side having a width of 4 mm or more It is desirable to strike the surface of the base material substantially perpendicularly so that is perpendicular to the weld line. The cross-sectional shape of the flat end portion of the tip portion of the chipper is preferably a rectangular shape or a substantially rectangular shape with r added to a corner portion of the rectangle, but may be another shape such as a circle or an ellipse.

  When pressurizing (striking) the surface of the base material, if the deformation state of the base material surface shown in FIG. 5 is not affected, the pressurization (striking) direction is strictly perpendicular to the base material surface as described above. There is no need.

  FIG. 2 described above shows the FEM analysis results when the flat base metal surface is pressed and plastically deformed. The base metal surface in the vicinity of the weld toe 4 of the structure having the weld bead 3 is shown. An example of the FEM analysis result of the residual stress distribution at the time of pressurization (striking) 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.

  The reason why the stress distribution (curve a in FIG. 4) on the surface of the base material is obtained when a part of the surface of the base material is blown or pressed to impart plastic deformation is that the principle of the brandtl ( Non-Patent Document Metal Plasticity Processing (excerpted by Kenzo Kato, Maruzen) (Figure 6).

  When compressive stress q is introduced into a flat plate, a slip line is generated immediately below it, as indicated by a right triangle ABC having a 45 ° acute angle, a fan-shaped ADC, and BCE. As a result, a force pushing outward acts on the right triangles ADF and BEG having hypotenuses having 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.

  When plastic deformation is caused on a part of the surface of the base material 1 according to the present invention, as shown in FIG. 8, the maximum value of the compressive residual stress (at the point b of the compressive residual stress distribution shown by the curve a in FIG. 4). The distance d is defined so that the maximum compressive residual stress) is introduced into the toe portion 4, and the plastic deformation region is pressurized so as to gradually move outward from the vicinity of the toe portion 4 so that a part thereof overlaps. As a result, a compressive residual stress distribution with small fluctuation can be obtained, and the fatigue strength can be improved more stably.

When the present invention is applied to hammer peening, the chipper size (width B) of the actual hammer peening machine, pressurizing force, base material characteristics (in the case of a steel bridge, SS400 to SM570 are used, yield stress 215 N / mm ^{2 to} 450 N / mm ² is used to obtain a compressive residual stress distribution (curve a in FIG. 4) on the surface of the base material and superimpose it on the weld residual stress distribution of the toe portion 4 obtained in advance. In this case, the distance d may be defined so that a compressive residual stress can be obtained at the toe portion 4.

A rib 2 (SM490Y) of 75 mm × 50 mm × plate thickness 12 mm is wound on a base material 1 (SM490Y) of width 150 mm × length 500 mm × plate thickness 12 mm and welded (wire MXZ200-1.2Φ, 100% CO2, 240A, 30V). , 40 CPM, 10.8 KJ / cm), and hammer peening (IIW recommended conditions: air pressure of about 6 kg / cm ² , hammer load of 1.7 kg, frequency of 43 Hz, moving speed of 1.2 mm / second) After going on, it was subjected to a fatigue test. The hammer peening conditions were performed under the following conditions (1) and (2) by applying the present invention.
(1) A tipper having a flat portion with a 4 mm square at the tip, except for a region that is within 3 mm from the weld toe, repeatedly hitting the surface of the base material vertically, with a width of about 6 mm and a maximum depth of about 0.1 mm It processed so that the hollow plastic deformation of this might arise (this invention 1).
(2) Before hammer peening, the turned welded part was finished with a grinder so that a recess with a radius of curvature of 1 mm or more was formed, and then struck under the condition (1) (the present invention No. 2).

  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. For comparison, a fatigue test was also performed on a specimen that was welded without hammer peening.

  FIG. 9 shows a plan view and a side view of the specimen, and FIG. 10 shows the fatigue test results. The joint specimen subjected to hammer peening condition (1) has a fatigue design curve shown by the Japan Steel Structure Association as compared to the as-welded specimen (see Non-Patent Document: Japan Steel Structure Association ) Of 3 grades of fatigue strength was confirmed (Invention No. 1). Further, the joint specimen subjected to the hammer peening condition (2) was confirmed to have an effect of improving the fatigue strength of about 4 grades of the fatigue design curve shown in the Japan Steel Structure Association (the present invention No. 2).

1 Base Material 2 Rib 3 Weld Bead 4 Weld Toe 5 Chipper

Claims (6)

  A portion of the base metal surface away from the weld toe is pressed perpendicularly to the base metal surface to cause plastic deformation, and compressive residual stress is introduced into the weld toe. Fatigue strength improvement method.   When pressurizing a part of the base material surface vertically, a part of the base material surface separated from the weld toe part is pressurized so that deformation of the base material surface does not reach the weld toe part. Item 2. The method for improving fatigue strength of welds according to Item 1. Using a pressure member having a cross-sectional shape perpendicular to the pressing direction of the distal end portion being substantially rectangular and having a flat portion with a width of 4 mm or more at the distal end portion, 3 mm from the weld toe portion on the surface of the base material. The method for improving fatigue strength of a welded portion according to claim 1, wherein a portion exceeding the limit is plastically deformed.   4. The pressurization is performed such that the pressurization portion is gradually moved outward from the vicinity of the weld toe portion so that a part thereof overlaps the plastic deformation region. 5. The method for improving fatigue strength of welds as described in 1.   The method for improving fatigue strength of a welded portion according to any one of claims 1 to 4, wherein a depression having a radius of curvature of 1 mm or more is formed in advance at the weld toe before pressurization.   The welded joint which gave the fatigue strength improvement method of the welding part as described in any one of Claims 1 thru | or 5.