JP2015182130A - Method of manufacturing welded joint and welded joint - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve fatigue life of a welded joint.SOLUTION: In manufacturing a welded joint 1, firstly, a first member 21 and a second member 22 are welded, such that a welded portion 3 is formed. Then, a predetermined target region 32 at a weld toe 31 of the welded portion 3 is subjected to surface melting treatment, so that a surface of the target region 32 is smoothed. The surface melting treatment is, for example, TIG welding without a welding rod. Further, burnishing of continuously rolling a ball along the surface of the target region 32 while pressing the ball against the surface the target region 32 is applied to the target region 32, so that compressive residual stress is imparted to the target region 32. Thus, stress concentration is decreased by smoothing the surface of the target region 32 and compressive residual stress is imparted to the smoothed target region 32, and thereby fatigue life of the welded joint 1 is improved.

Description

  The present invention relates to a method for manufacturing a welded joint and a welded joint.

  Conventionally, welding is frequently used for large steel structures such as road bridges and pressure vessels. In such a structure, when a load is repeatedly applied to the weld toe portion of the welded portion, fatigue cracks due to tensile residual stress occur. Therefore, mechanical treatment is applied to the weld toe by shot peening or hammer peening to introduce compressive residual stress. In addition, improvement of fatigue strength by ultrasonic impact treatment (UIT treatment) or laser peening has been attempted (for example, see Patent Document 1 for laser peening). In Non-Patent Document 1, it is described that TIG processing is used as a toe portion processing method for a welded joint.

JP 2012-153777 A

Kengo Anami, Chitoshi Miki, “Improvement of fatigue strength of welded joints of high-strength steel-especially additional welding with low-temperature layer transformation welding rods” Year, No.I-A076

  By the way, the improvement of the fatigue life of the welded joint by the above method may be insufficient, and a new method for improving the fatigue life of the welded joint is required.

  This invention is made | formed in view of the said subject, and aims at improving the fatigue life of a welded joint.

  Invention of Claim 1 is a manufacturing method of a welded joint, Comprising: a) The process of welding a 1st member and a 2nd member and forming a welding part, b) The welding toe part of the said welding part Smoothing the surface of the target region by surface melting treatment or grinding treatment on the predetermined target region, and c) applying compressive residual stress to the target region of the weld toe .

  Invention of Claim 2 is a manufacturing method of the welded joint of Claim 1, Comprising: In the said c) process, while pressing a ball | bowl or a roller against the said surface of the said object area | region, it is continuous along the said surface. Compressive residual stress is applied to the target region.

  Invention of Claim 3 is a manufacturing method of the welded joint of Claim 1, Comprising: In the said c) process, it compresses with respect to the said target area | region by making many particle | grains collide with the said target area | region. Residual stress is applied.

  Invention of Claim 4 is the manufacturing method of the welded joint in any one of Claim 1 thru | or 3, Comprising: In the said b) process, the said surface of the said object area | region is smoothed by the said surface melting process. The surface melting treatment is TIG tanning.

  A fifth aspect of the present invention is the method for manufacturing a welded joint according to any one of the first to fourth aspects, wherein the welded joint is a gusset welded joint.

  The invention according to claim 6 is manufactured by the method for manufacturing a welded joint according to any one of claims 1 to 5.

  According to the present invention, the fatigue life of a welded joint can be improved by reducing stress concentration by smoothing the surface of the target region and applying compressive residual stress to the smoothed target region.

It is a top view which shows a welded joint. It is a front view which shows a welded joint. It is a figure which expands and shows a part of welded joint. It is a figure which shows the flow of the process which manufactures a welded joint. It is a figure which shows the surface shape of the area | region vicinity of the welding toe part before TIG tanning. It is a figure which shows the surface shape of the target area | region vicinity of the welding toe part after TIG tanning. It is a figure which shows a part of burnishing apparatus. It is a figure for demonstrating burnishing. It is a figure which shows the relationship between the depth from the surface of an object area | region, and the residual stress value in the said depth. It is a photograph of the section in the neighborhood of the object field of the welded joint in which the processing of the comparative example was performed. It is a photograph of the section in the neighborhood of the object field of a welded joint. It is a SN diagram obtained by the fatigue test with respect to a welded joint.

  FIG. 1 is a plan view showing a welded joint 1 according to an embodiment of the present invention, and FIG. 2 is a front view showing the welded joint 1. In FIG. 1 and FIG. 2, three directions orthogonal to each other are indicated by arrows as an X direction, a Y direction, and a Z direction. Since the welded joint 1 shown in FIGS. 1 and 2 is a specimen for a fatigue test to be described later, in the first member 21 that is a base plate, the width in the X direction of the central portion in the Y direction is narrowed. However, in the actual welded joint 1, the shape of each component may be changed as appropriate.

  The welded joint 1 includes a first member 21, a second member 22, and a welded part 3. The first member 21 is made of metal and has a plate shape parallel to the X direction and the Y direction. The second member 22 is also made of metal and has a plate shape parallel to the Y direction and the Z direction. In the present embodiment, the first member 21 and the second member 22 are formed of the same type of metal, and specifically, the metal is high-tensile steel (for example, HT780). The welded portion 3 is a weld bead formed by a welding process to be described later, and is obtained by mainly melting and solidifying a filler material during welding. As shown in FIG. 1 and FIG. 2, the welded joint 1 is a gusset plate so as to protrude from the (+ Z) side main surface (hereinafter simply referred to as “main surface”) of the first member 21 that is a base plate. A certain second member 22 is fillet welded to the first member 21, and is a so-called gusset weld joint (out-of-plane gusset weld joint).

  FIG. 3 is an enlarged view showing a part of the welded joint 1 and shows the vicinity of the end of the second member 22 in the Y direction. The welded portion 3 has a portion where the surface intersects with the main surface of the first member 21 and a weld toe portion 31 that is in the vicinity thereof. When viewed along the Z direction, the weld toe portion 31 has an annular shape surrounding the second member 22. In the welded joint 1 of FIG. 3, each end portion in the Y direction of the weld toe portion 31, that is, a portion formed by rotary welding is defined as a target region 32 (shown by a thick solid line in FIG. 3). It has been. In the welded portion 3, a region including the target region 32 is subjected to a first processing mark 33 by a surface melting process described later and at least one (three in FIG. 3) second processing marks 34 by a burnishing described later. Is formed. In FIG. 3, the first processing mark 33 and the second processing mark 34 are given parallel oblique lines having different directions.

  FIG. 4 is a diagram showing a flow of processing for manufacturing the welded joint 1. When manufacturing the welded joint 1, first, the first member 21 and the second member 22 are fillet welded in a state where the second member 22 is erected on the main surface of the first member 21. 3 (see FIG. 3) is formed (step S11). In the process of step S11 in the present embodiment, arc welding is performed using a high-strength steel mag welding solid wire (for example, trade name: MGS-80 (manufactured by Kobe Steel, Ltd.)) as a filler material. The welded portion 3 is one that is mainly solidified after the filler material has melted. Subsequently, a surface melting process is performed on the target region 32 in the weld toe portion 31 of the welded portion 3, and the surface of the target region 32 is smoothed (step S12). The surface melting process in the present embodiment is a process of melting the surface by TIG welding without using a filler material, that is, TIG tanning. The first processing mark 33 shown in FIG. 3 is formed by TIG tanning.

  5 and 6 are diagrams showing a surface shape in the vicinity of the target region 32 of the weld toe 31 in a cross section parallel to the YZ plane, FIG. 5 shows a surface shape before TIG tanning, and FIG. The surface shape after TIG tanning is shown. 5 and 6, the horizontal axis indicates the position in the Y direction, and the vertical axis indicates the position in the Z direction. Here, in each of the surface shapes of FIGS. 5 and 6, when the vicinity of the target area 32 is approximated by an arc A1 indicated by a broken line, the radius of curvature of the arc A1 is represented by ρ, and the bottom and inclined portions of the target area 32 are defined. Angles formed by the two straight lines A2 and A3 when approximated by two straight lines A2 and A3 indicated by broken lines (that is, the arc A1 is roughly included in the four regions divided by the two straight lines A2 and A3). The stress concentration coefficient Kt in the target region 32 is obtained by Equation 1, where θ is an opening angle that is an angle formed by the two straight lines A2 and A3 in the region to be obtained.

  In the target region 32 before TIG tanning shown in FIG. 5, the radius of curvature ρ is 0.6 mm (millimeters), the opening angle θ is 135 degrees, and the stress concentration factor is 3.8. In the target region 32 after TIG tanning shown in FIG. 6, the radius of curvature ρ is 12 mm, the opening angle θ is 174 degrees, and the stress concentration factor is 1.2. In this way, a surface shape with a low stress concentration factor is obtained in the target region 32 of the weld toe 31 by TIG tanning. In other words, the TIG tanning is a process for reducing the stress concentration in the target region 32. In the process of step S12, the surface of the target region 32 is preferably smoothed so that the stress concentration coefficient is 1.5 or less.

  Subsequently, compressive residual stress is applied to the target region 32 (step S13). Specifically, burnishing using the burnishing device 81 shown in a simplified manner in FIG. In the burnishing device 81, for example, a ball 83 having a diameter of 6 mm is accommodated in a recess 821 provided at the tip of a main body 82, and a high-pressure liquid (for example, a liquid pressurized to 40 MPa (megapascal)) is stored in the recess 821. Is retained. In the present embodiment, the posture of the main body 82 with respect to the target region 32 is set so that the main body 82 stands upright at an angle of about 90 degrees with respect to the target region 32. Note that the diameter, material, and the like of the ball 83 may be appropriately determined according to the material of the welded portion 3 and the like.

  FIG. 8 is a view for explaining burnishing, and is a plan view of a region where burnishing is performed in the welded joint 1. For example, as shown in FIG. 8, when burnishing is performed on a rectangular region having sides parallel to the X direction and the Y direction, the main body 82 is predetermined in the X direction while pressing the ball 83 against the surface of the target region 32. , For example, at a speed of 500 mm per minute (see the arrow labeled B1 in FIG. 8). When the main body 82 moves by the distance, the main body 82 moves by a predetermined distance, for example, 0.2 mm in the Y direction perpendicular to the X direction (see the arrow labeled B2 in FIG. 8). In FIG. 8, the main body 82 continuously moves to the side opposite to the immediately preceding continuous movement (see the arrow labeled B3 in FIG. 8). By repeating the continuous movement in the X direction and the intermittent movement in the Y direction as described above, the region including at least a part of the target region 32 is burned, and compressive residual stress is applied. Is done. In the present embodiment, the burnishing is repeated for a plurality of regions, whereby a plurality of second processing marks 34 shown in FIG. 3 are formed, and compressive residual stress is applied to the entire target region 32. With the above processing, the manufacture of the welded joint 1 is completed.

  Next, another processing example in step S13 will be described. In this processing example, a shot peening apparatus is used. The shot peening apparatus is a direct pressure type apparatus using air, for example, and projects and collides a relatively large number of particles (for example, steel balls) onto the target region 32. As a result, compressive residual stress is applied to the region including the entire target region 32. The average particle diameter of the particles in the present embodiment is, for example, 0.3 mm. In addition, the projection speed in the shot peening apparatus, the particle size, the material, or the like of the particle may be appropriately determined according to the material of the welded portion 3 or the like.

  Here, the compressive residual stress applied to the target region 32 by the above-described processing will be described. FIG. 9 is a diagram illustrating a relationship between the depth from the surface of the target region 32 and the residual stress value at the position of the depth. FIG. 9 also shows the result of the process of the comparative example in which only the shot peening is performed on the target region 32 without performing the TIG tanning in step S12. In the process of the comparative example, as indicated by the line denoted by reference numeral C1 in FIG. 9, the magnitude (absolute value) of the compressive residual stress on the surface where the depth is 0 is about 200 MPa, and in the vicinity of the depth of 25 μm, The magnitude of the compressive residual stress becomes maximum at about 350 MPa. The compressive residual stress becomes almost zero at a depth of about 200 μm. That is, the compression depth at which compressive residual stress occurs is approximately 200 μm. When TIG tanning is performed in step S12 and shot peening is performed in step S13, the relationship between the depth and the residual stress value is compared as shown by the line labeled C2 in FIG. It is almost the same as the processing in the example. The relationship between depth and compressive stress value varies depending on various conditions in shot peening, but in shot peening, the maximum value of compressive residual stress is 300 MPa or more, and the compressive depth (compressed residual stress layer thickness) It is preferable that the processing conditions are set so that the thickness becomes 200 μm or more.

  On the other hand, when TIG tanning is performed in step S12 and burnishing is performed in step S13, as shown by the line labeled C3 in FIG. The magnitude of the stress is about 850 MPa, and the compressive residual stress is zero at a depth of about 1500 μm. In this way, by performing burnishing, it is possible to increase the compressive residual stress on the surface and increase the compression depth. The relationship between depth and compressive stress value varies depending on various conditions in burnishing, but in burnishing, the processing conditions are such that the maximum value of the compressive residual stress is 700 MPa or more and the compression depth is 1300 μm or more. It is preferably set.

  Next, the surface state of the target region 32 obtained by the process of FIG. 4 will be described. Here, a comparative example in which compressive residual stress is applied to the target region by peening that causes the tip of the needle (hemisphere having a diameter of 3 mm) to collide with the entire target region without performing TIG tanning in step S12. Processed. When a cross section in the vicinity of the target region of the welded joint subjected to the processing of the comparative example (cross section perpendicular to the weld toe) is observed with an optical microscope, there are minute cracks on the surface as shown on the right side of FIG. To do. In the peening in the processing of the comparative example, the surface of the target region has a shape in which a large number of grooves are arranged. On the other hand, when performing TIG tanning in step S12 and performing burnishing or shot peening in step S13, even if the cross section in the vicinity of the target region 32 of the welded joint 1 is observed with an optical microscope, There are no such cracks. In FIG. 11, the photograph by the optical microscope of the cross section of the weld joint 1 in the vicinity of the object area | region 32 which performed both TIG tanning and burnishing is shown.

  FIG. 12 is an SN diagram obtained by a fatigue test on the welded joint 1 formed as a test body. The vertical axis in FIG. 12 indicates the stress range (nominal stress range) in the fatigue test, and the horizontal axis indicates the number of repetitions leading to fracture. In the fatigue test, stress is applied in the Y direction while holding both ends of the first member 21 in the Y direction. The repetition frequency in the fatigue test is 10 Hz.

  Table 1 shows the number of repetitions when the specimen is broken when the stress ranges are 350 MPa and 300 MPa, respectively. In FIG. 12 and Table 1, “unprocessed” indicates a test body in which the processes of steps S12 and S13 are omitted, and “shot” indicates a test body in which only shot peening is performed without performing TIG tanning. “TIG + shot” indicates a specimen subjected to both TIG tanning and shot peening, and “TIG + burnishing” indicates a specimen subjected to both TIG tanning and burnishing. In Table 1, “TIG” indicates a test body in which only TIG tanning is performed and the processing in step S13 is omitted. Fatigue Strength Improvement-Especially Additive Welding with Low Temperature Layer Transformation Welding Rod " Based on FIG. 6, the strength class D is calculated. Further, in the case of “TIG + burnishing” in the case where the stress range is 300 MPa, the fracture did not occur even when the number of repetitions was 5157572 times.

  As is apparent from Table 1, the test specimens subjected to both TIG tanning and shot peening and the specimens subjected to both TIG tanning and burnishing were specimens subjected only to shot peening. In addition, the number of repetitions leading to breakage, that is, the fatigue life, is improved as compared with the specimen subjected only to TIG tanning. In particular, in a specimen subjected to both TIG tanning and burnishing, when the stress range is 300 MPa, the fatigue life is 100 times or more that of an untreated specimen. As shown in FIG. 12, in the specimen subjected to both TIG tanning and shot peening, and the specimen subjected to both TIG tanning and burnishing, the fatigue limit is determined by the Japan Steel Structure. It exceeds the A grade in the fatigue strength grade (fatigue design curve) stipulated by the Association (JSSC), and can be treated as having a fatigue strength equivalent to that of the base material in design.

  As described above, in the method for manufacturing the welded joint 1, the surface of the target region 32 is smoothed by the surface melting process on the predetermined target region 32 in the weld toe 31 of the welded portion 3, and then welding is performed. A compressive residual stress is applied to the target region 32 of the toe 31. As described above, the fatigue life of the welded joint 1 is improved by reducing the stress concentration in the target region 32 by smoothing the surface of the target region 32 and applying compressive residual stress to the smoothed target region 32. can do.

  Further, since the surface melting process in step S12 is TIG tanning, the surface melting process can be easily performed. Further, in the process of step S13, burnishing that rolls continuously along the surface while pressing the ball 83 against the surface of the target region 32 or shot peening that causes a large number of particles to collide with the target region 32 is performed. Thereby, the surface shape of the target region 32 is not significantly changed (without a stress concentration shape), and the impact is accompanied by a minute crack (crack) such as hammer peening or ultrasonic impact treatment. Without performing the treatment, compressive residual stress can be applied to the target region 32, and the fatigue life of the welded joint 1 can be greatly improved. In the process of step S13, when burnishing is employed, the fatigue life of the welded joint 1 can be dramatically improved, and when shot peening is employed, compressive residual stress can be easily applied. It is possible to easily adjust the amount of compressive residual stress applied without depending on the skill of the operator by adjusting the burnishing device 81 for burnishing and by adjusting the shot peening device for shot peening. .

  Various modifications are possible in the manufacture of the weld joint 1.

  The smoothing of the surface of the target region 32 may be, for example, surface melting treatment (so-called laser tanning) by laser light irradiation other than TIG tanning. In the surface melting treatment by laser light irradiation, it is possible to reduce the influence of the skill difference between workers compared to TIG tanning. Further, the smoothing of the surface of the target region 32 may be realized by a grinding process (grinding). Also in this case, the stress concentration in the target region 32 can be reduced, that is, the target region 32 can have a shape with a reduced stress concentration coefficient.

  Depending on the design of the burnishing device 81 used in the burnishing, compressive residual stress is applied to the target region 32 by continuously rolling the roller along the surface while pressing the roller against the surface of the target region 32 in the burnishing. May be. Further, in the process of step S13, both burnishing and shot peening may be performed.

  In step S13 for applying the compressive residual stress, the compressive residual stress is applied to the target region 32 without greatly changing the surface shape of the target region 32 and without performing a striking process involving a minute crack. If possible, processes other than burnishing and shot peening may be performed.

  The first member 21 and the second member 22 in the welded joint 1 may be metals other than high-tensile steel, and may be, for example, rolled steel for welded structures such as SM490. Further, the first member 21 and the second member 22 may be different types of metals. Furthermore, in the formation of the welded portion 3, a filler material other than the MAG welding solid wire for high-strength steel may be used.

  The said manufacturing method of the welded joint 1 may be used for manufacture of welded joints (for example, a cross joint) other than a gusset welded joint. Moreover, the object area | region 32 in the welding toe part 31 may be other than the part formed by rotation welding.

  The configurations in the above-described embodiments and modifications may be combined as appropriate as long as they do not contradict each other.

DESCRIPTION OF SYMBOLS 1 Welded joint 3 Welding part 21 1st member 22 2nd member 31 Welding toe end part 32 Target area 83 Ball S11-S13 Step

Claims (6)

A method for manufacturing a welded joint, comprising:
a) welding the first member and the second member to form a weld;
b) smoothing the surface of the target region by surface melting treatment or grinding treatment for a predetermined target region at the weld toe of the weld;
c) applying compressive residual stress to the target region of the weld toe;
A method for manufacturing a welded joint, comprising: A method for producing a welded joint according to claim 1,
In the step c), the welding is characterized in that a compressive residual stress is applied to the target region by continuously rolling the ball or roller against the surface of the target region along the surface. A method for manufacturing a joint. A method for producing a welded joint according to claim 1,
In the step c), a compressive residual stress is applied to the target region by causing a large number of particles to collide with the target region. A method for producing a welded joint according to any one of claims 1 to 3,
In the step b), the surface of the target region is smoothed by the surface melting treatment,
The method for manufacturing a welded joint, wherein the surface melting treatment is TIG tanning. A method for manufacturing a welded joint according to any one of claims 1 to 4,
The method for manufacturing a welded joint, wherein the welded joint is a gusset welded joint.   A welded joint manufactured by the method for manufacturing a welded joint according to claim 1.