Methods for improving fatigue strength of welded joints

Abstract

To provide a method of improving the fatigue strength of a weld part which can improve the fatigue life of a weld part in a large sized weld structure, such as a floating type offshore wind power generation facility.SOLUTION: A method of improving the fatigue strength of a weld part 1 includes: forming a linear striking mark 41 extending in a weld bead extension direction 2, in a base material part 3 adjacent to a weld stop end 1a of the weld part, with hammer peening processing; and forming a similar linear striking mark 42 in the base material part adjacent to the weld stop end after a predetermined period.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for improving the fatigue strength of welds, particularly welds in large welded structures, by hammer peening. [Background technology]

[0002] In recent years, offshore wind power generation has been attracting attention from the perspective of preventing global warming. In addition, in order to reduce power generation costs and increase power generation efficiency, the size of wind power generation facilities is increasing, and the thickness of the steel plates used in welds is also trending toward being thicker, at 30 mm or more.

[0003] Generally, fatigue cracks in welded joints initiate in the thickness direction from stress concentration points such as the weld toe. In particular, in large welded structures such as offshore wind power generation facilities, which use thick materials, the distribution of compressive residual stress in the thickness direction is thought to be important for suppressing the initiation and propagation of fatigue cracks.

[0004] On the other hand, as welded structures become larger, the strength of the materials used is being increased in order to reduce their weight. As the strength of materials increases, the fatigue strength of the base material increases, but there is concern that the fatigue strength of the welds will decrease. Therefore, various methods have been proposed to improve the fatigue properties of welds.

[0005] For example, Patent Document 1 proposes a peening method for improving the fatigue performance of welds in structures subjected to repeated loads. This method involves moving a striking tool with a flat tip up and down at a frequency of 100 Hz or less, continuously along the weld bead, to form a continuous band-like impact mark in the base metal near the weld toe. In this method, the distance between the impact mark and the weld toe is adjusted so as not to strike the weld toe. Preferably, the impact mark is formed by two or more successive strikes, or four or more successive strikes. The technology in Patent Document 1 can impart a compressive residual stress of at least 50% of the base metal's yield stress at the surface layer near the weld toe. Furthermore, since no notch is formed at the weld toe compared to conventional peening, fatigue strength is significantly improved.

[0006] Furthermore, Patent Document 2 proposes a fatigue reinforcement method for a girder structure with flange gussets. In this technology, ultrasonic impact treatment is applied to the flange-side toes of the welds at the flange gusset ends, over a range of at least 1t (t: gusset plate thickness) from the flange gusset end to the top and bottom surfaces. The technology described in Patent Document 2 is said to improve the toe shape, suppress stress concentration, and improve the fatigue performance of girder structures with flange gussets.

[0007] Furthermore, Patent Document 3 proposes a method for managing the useful life of a group of metal parts that are subject to fatigue-related failure, primarily targeting components such as gas turbines and jet engines. This technique involves measuring the compressive residual stress on the surface of selected metal parts in high-stress concentration areas using X-ray diffraction technology. If the measured value exceeds a predetermined value, the metal part is returned to service. If the measured value is equal to or less than the predetermined value, the metal part is either permanently removed from service or reworked to increase the compressive residual stress to above the predetermined value and then returned to service. This process is then preferably repeated periodically for each individual part. This method for managing a group of metal parts is said to safely and effectively extend the useful life of the parts. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 4895407 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-167516 [Patent Document 3] Special Publication No. 10-503839 Summary of the Invention [Problem to be solved by the invention]

[0009] However, it remains unclear whether the technology of Patent Document 1 is effective in improving the fatigue strength of welds that join steel plates with a thickness of 30 mm or more.

[0010] Furthermore, in the technology of Patent Document 2, in order to perform ultrasonic impact treatment to strike the surface of the welded portion of the flange gusset, a special tool must be used, and the tip of the tool must be ultrasonically vibrated with an amplitude of 20 to 60 μm and a frequency of 15 to 60 kHz, which is disadvantageous in terms of construction costs.

[0011] Furthermore, the technology in Patent Document 3 targets machine parts and measures the compressive residual stress on the part surface using X-ray diffraction technology, which makes it difficult to apply to large welded structures such as offshore wind power generation facilities.

[0012] Furthermore, in the field of offshore wind power generation, floating wind turbines have recently been considered due to their ease of installation. Floating offshore wind turbines are constantly subjected to external forces from waves. Therefore, there is a concern that the absolute value of the compressive residual stress introduced around the weld toe will fall below the level required to prevent the initiation and propagation of fatigue cracks due to the long-term action of these external forces, thereby eliminating the intended fatigue strength improvement effect.

[0013] However, none of Patent Documents 1 to 3 considers measures to prevent the reduction in compressive residual stress due to the long-term action of external force.

[0014] The present invention aims to provide a method for improving the fatigue strength of welds that advantageously solves the problems of the conventional technology and can improve the fatigue life of welds in large welded structures such as floating offshore wind power generation facilities. [Means for solving the problem]

[0015] In order to achieve the above object, the present inventors have conducted extensive research into a method for maintaining, for a long period of time, the fatigue strength improving effect of compressive residual stress introduced by hammer peening under the above conditions before use in welded structures in environments where external forces are constantly acting, such as the floating offshore wind power generation equipment. As a result, the following new findings have been obtained. (a) It is important to perform hammer peening again after a specified period of time has passed since the start of service. (a) The impact position for the hammer peening treatment must be the base metal part in the vicinity of the weld toe. (c) This makes it possible to maintain the compressive residual stress at a suitable level (-400 to -200 MPa) on the surface of the base material in the vicinity of the weld toe (hereinafter simply referred to as "toe").

[0016] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows. [1] A method for improving the fatigue strength of a weld, comprising: forming linear impact marks in the direction of extension of the weld bead by hammer peening on a base metal portion near the weld toe of the weld; and then forming similar linear impact marks again on the base metal portion near the weld toe after a predetermined period of time. [2] The method for improving the fatigue strength of a welded portion according to [1] above, wherein the predetermined period is 3 years or less. [3] The method for improving the fatigue strength of a welded portion according to [1] above, wherein the predetermined period is 0.5 years or more and 3 years or less. [4] The method for improving the fatigue strength of a welded portion according to any one of [1] to [3] above, wherein the distance between the impact mark and the weld toe is 1.0 mm or less. [5] In any one of the above [1] to [4], the maximum depth of the impact mark (D P ) and width (W P ) product (D P ×W P ) is 3.0 to 5.0 mm 2 A method for improving the fatigue strength of a welded portion, characterized by: [6] A method for improving the fatigue strength of a weld in any one of [1] to [5], characterized in that the weld is a weld that joins the tower and base of a floating offshore wind power generation facility. [7] The method for improving the fatigue strength of a welded portion according to any one of the above [1] to [6], wherein the base material portion is a steel material having a plate thickness of 30 mm or more. [Effects of the Invention]

[0017] According to the present invention, compressive residual stress can be maintained on the surface of the base material near the toe of a weld in a welded structure that is subjected to external forces such as waves, and the fatigue strength of the weld can be improved over the long term. [Brief explanation of the drawings]

[0018] [Figure 1] 1A and 1B are explanatory views schematically illustrating an example of an embodiment of the present invention, in which (a) shows the state after the first hammer peening treatment and (b) shows the state after the second hammer peening treatment. [Figure 2] (a) is a partial cross-sectional view of FIG. 1(b), and (b) is a partial enlarged view of FIG. 2(a). [Figure 3] 1A, 1B, and 1C are schematic views showing an example of the shape of a chipper, in which FIG. 1A is a perspective view, FIG. 1B is an XZ cross-sectional view, and FIG. 1C is a YZ cross-sectional view. [Figure 4] FIG. 2 is an explanatory diagram schematically showing a fillet welded joint in an embodiment. [Figure 5] FIG. 2 is an explanatory diagram schematically illustrating a fatigue test method in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention relates to a method for improving the fatigue strength of a welded joint subjected to repeated loads by performing hammer peening again after a predetermined period of time. An example of this embodiment is shown in Fig. 1. Fig. 1 shows a welded joint 1 of a fillet welded joint formed by joining a horizontal member 3a and a vertical member 3b as base material 3 by fillet welding, but the subject matter of this invention is not limited to welded joints of fillet welded joints, and may also be welded joints of butt welded joints (not shown).

[0020] [Hammer peening treatment] In the hammer peening process (hereinafter also referred to as "HP process"), it is preferable to use a chipper having a tip shape as shown in FIG. 3 as the impact tool. The tip shape of the chipper is such that the chipper travel direction is the Y direction, the chipper length direction is the Z direction, and the chipper width direction is the X direction (FIG. 3(a)). The XZ cross section is a semicircle with a diameter a and a curvature radius r (FIG. 3(b)), and the YZ cross section is a trapezoid with a base b (FIG. 3(c)). The diameter a and the base b are both preferably 1.0 to 10.0 mm, more preferably 5.0 to 9.5 mm. The curvature radius r is preferably 1.0 to 10.0 mm. If r is less than 1.0 mm, deformation that causes stress concentration may occur at the toe 1a. On the other hand, if r exceeds 10.0 mm, the contact area with the base material 3 becomes too large, and compressive residual stress may not be introduced into the region of the toe 1a within a depth of 3.0 mm from the surface of the base material 3.

[0021] The chipper is connected to a striking device (not shown), which is driven by air pressure, high-frequency current, ultrasonic waves, or the like. For example, a preferred method is to strike the aforementioned area by operating the tip of the chipper with air pressure. Here, the striking frequency is preferably a low frequency of 100 Hz or less, more preferably 50 Hz or more and 100 Hz or less.

[0022] Furthermore, the inclination angle of the chipper's striking direction is permissible if it is about 80 to 100° relative to the base material surface, but it is preferable to set it at a nearly perpendicular 90°, as this reduces variation in each treatment.

[0023] [Selection of a specified period] In the present invention, a structure is first welded, and then the welded joint of the welded structure is subjected to the first HP treatment. Here, "after welding" refers to immediately after welding. "in service" refers to the time when the welded structure is installed at the site.

[0024] After a predetermined period of time has passed since the start of service, the welded joint is subjected to a second HP treatment. The absolute value of the compressive residual stress introduced by the first HP treatment decreases due to the long-term application of external forces such as waves. However, the second HP treatment can increase the absolute value of the compressive residual stress, thereby improving the fatigue strength of the weld.

[0025] In the present invention, the predetermined period is preferably three years or less, the reason for which will be explained below.

[0026] When selecting the predetermined period, it is preferable to consider the period during which the absolute value of the compressive residual stress imparted by the first HP treatment decreases due to the action of repeated external forces. Therefore, the predetermined period is preferably selected based on the SN curve obtained in a fatigue test of a welded joint having the target weld. In the case of a floating offshore wind power generation facility, the predetermined period can be associated with the number of repetitions (cycles) in a fatigue test of a fillet welded joint simulating repeated stress loading conditions caused by waves. An example of this number of repetitions is 300,000 cycles at a stress amplitude of 200 MPa. This corresponds to three years of service for the floating offshore wind power generation facility. Therefore, the predetermined period is preferably less than this, i.e., three years or less. To avoid economic disadvantages due to an unnecessary increase in the number of HP treatments, the predetermined period is more preferably between 0.5 and 3 years. Furthermore, it is even more preferably between 2 and 3 years. Here, the predetermined periods of 0.5 years and 2 years correspond to 50,000 cycles and 200,000 cycles, respectively, in terms of the number of repetitions at a stress amplitude of 200 MPa.

[0027] [Strike position] In the first HP treatment, as shown in Figure 1(a), linear impact marks 41 are formed in the base material 3 near the weld toe 1a of the weld 1, extending in the weld bead extending direction 2. In the second HP treatment, after the predetermined period has elapsed, similar linear impact marks 42 are again formed in the base material 3 near the weld toe 1a.

[0028] The vicinity of the weld toe, which is the impact position, will be explained using Figures 1 and 2(a). In this example, the toe near the weld toe is the toe 1a on the horizontal member 3a side, but the same applies to the toe 1b on the vertical member 3b side.

[0029] The vicinity of the weld toe refers to a region (not shown) on the base metal 3 side where the distance L1 from the toe 1a is greater than 0.0 mm and less than 2.0 mm. Regions with a distance of 0.0 mm or less from the toe 1a are excluded from the impact position because they would damage the weld 1 due to impact. Furthermore, regions with a distance of more than 2.0 mm from the toe 1a are too far from the toe 1a, and impacting these regions would not introduce compressive residual stress into the surface of the base metal 3 near the toe 1a. Therefore, the distances L1 and L2 between the impact marks 41 and 42 and the toe 1a shown in Figure 2(a) should be greater than 0.0 mm and less than 2.0 mm, preferably greater than 0.0 mm and less than 1.0 mm. By setting the distances L1 and L2 to greater than 0.0 mm and less than 1.0 mm, fatigue strength can be improved even under more severe external force conditions.

[0030] In the present invention, the impact by the HP process may be repeated any number of times, such as a second time, a third time, a fourth time, etc. The impact position (not shown) for the third time and thereafter is the same as that for the first and second times.

[0031] 1 shows the case where the impact position is near the toe 1a of the horizontal member 3a, but it may be near the toe 1b of the vertical member 3b. Whether the impact position near the weld toe is on the horizontal member 3a side, the vertical member 3b side, or both sides is determined based on the state of external forces acting on the welded structure when in service.

[0032] [Product of maximum depth and width of impact mark] In the present invention, the maximum depth of the impact mark (D P ) and width (W P ) product (D P ×W P ) is 3.0 to 5.0 mm 2 It is preferable that the maximum depth of the impact mark (D P ) and width (W P ) is defined in a cross section perpendicular to the direction of extension of the weld bead in each HP treatment, and is expressed in mm. For example, Figure 2(b) shows the maximum depth Dp1 and width Wp1 of the first impact mark 41 and the maximum depth Dp2 and width Wp2 of the second impact mark 42.

[0033] D of each impact mark P ×W P is 3.0 mm 2 If the impact depth is less than 1000 sq.m., it is difficult to impart sufficient compressive residual stress. P ×W P is 5.0 mm 2 If the impact strength is more than D, fatigue cracks may occur at the impact indentation. P ×W P is 3.0~4.0mm 2 is.

[0034] Maximum depth D P and width W P are measured as follows: maximum depth is measured using a depth gauge and width is measured using a vernier caliper.

[0035] [Welded structures] The present invention has a significant effect of improving fatigue strength when applied to welds in welded structures that are used in environments where external forces are constantly acting. An example of such a weld is the weld that joins the tower and base of the aforementioned floating offshore wind power generation facility (not shown), which is constantly subjected to external forces from waves.

[0036] The welding method for forming the welded portion is not particularly limited, and any of shielded metal arc welding, carbon dioxide gas arc welding, etc. may be used.

[0037] [Base material] In the present invention, it is preferable that the base material portion 3 is a steel material having a plate thickness of 30 mm or more. By using steel material for the horizontal members 3a and vertical members 3b shown in Fig. 1, the material is relatively inexpensive, welding is easy, and the strength of the welded structure can be easily ensured.

[0038] However, if the thickness of the steel material (for example, the thicknesses ta and tb of the horizontal member 3a and the vertical member 3b in FIG. 1) is less than 30 mm, it may be difficult to ensure the strength of the floating offshore wind power generation facility described above, so the thickness of the steel material is preferably 30 mm or more. Note that the upper limit of the thickness is not particularly limited in the present invention, and depends on the upper limit of the range of thicknesses that can normally be manufactured. [Example]

[0039] Six fillet welded joints, shown in Figure 4, were fabricated using 100 mm thick YP470-grade steel plate (Young's modulus E: 206 GPa, Poisson's ratio ν: 0.3) to simulate the welded joint between the tower and base of a floating offshore power generation facility. These joints are designated joints No. 1 to No. 6 in Table 1. Gas-shielded arc welding was used for fillet welding, with welding current of 230 A, welding voltage of 30 V, and welding speed of 34 m / min (heat input: approximately 12.2 kJ / cm). Carbon dioxide (100% CO2) was used as the shielding gas. 1.2 mm diameter YP470-grade wire was used.

[0040] Next, for each fillet weld joint, HP treatment was performed on the base material portion 3 near the weld toe on the side of the cross member 3a, forming an impact mark 41 (see Figure 1(a)). The distance between the impact mark 41 and the toe 1a was 1.0 mm (see Table 1). For HP treatment, a chipper with a tip shape as shown in Figure 3 and dimensions a = 9.0 mm and b = 5.0 mm was connected to an impact device (not shown), and the impact direction was perpendicular to the steel plate surface, and the treatment was performed using air pressure at an impact frequency of 70 Hz. After the treatment, the maximum depth (D P ) x width (W P The results are shown in Table 1. Here, D P and W P The average value of N measurement data obtained using a depth gauge and a vernier caliper at N=5 locations in the extension direction of the impact mark was used.

[0041] Joint No. 1 is a comparative example in which the HP treatment was performed only once, and was used to measure the residual stress after the first HP treatment.

[0042] Joint No. 2 is a comparative example in which HP treatment was performed only once, and was used for measuring residual stress after the first HP treatment and a fatigue test for a predetermined number of cycles were performed in sequence.

[0043] Joint No. 3 is a comparative example in which HP treatment was performed twice, but the impact position in the second HP treatment was removed from the vicinity of the toe, i.e., the distance between the toe 1a and the impact mark 42 (see Figure 1(b)) was set to 3 mm (see Table 1), exceeding 2 mm. This joint was used to measure residual stress after the first HP treatment, a fatigue test for a specified number of cycles, and the second HP treatment were performed in that order.

[0044] Joints Nos. 4 to 6 are examples of the present invention in which the treatment was performed twice, and the second impact position was the same as the first, near the toe, i.e., the distance between the toe 1a and the impact mark 42 (see Figure 1(b)) was within 2 mm (see Table 1). These were used to measure residual stress after the first HP treatment, fatigue testing for a specified number of cycles, and second HP treatment were performed in that order.

[0045] The second HP treatment was performed in the same manner as the first, except that the impact position was as shown in Table 1. After the second HP treatment, the maximum depth (D P ) x width (W P The results are shown in Table 1.

[0046] The fatigue test was carried out by a tension-compression fatigue test method as shown in FIG. The predetermined number of cycles was set to 300,000 (see Table 1). As mentioned above, this corresponds to the predetermined period of three years for the floating offshore wind power generation facility to be in service. The second HP treatment was carried out after the predetermined number of cycles had elapsed.

[0047] The residual stress was measured using X-rays on the surface of the base material 0.2 mm away from the toe. The results are shown in Table 1.

[0048] Table 1 reveals the following. Specifically, the first HP treatment introduced compressive residual stress of the preferred level (-400 to -200 MPa) into the target region (Joint No. 1, Comparative Example). However, the absolute value of the compressive residual stress decreased after a fatigue test with a predetermined number of cycles (Joint No. 2, Comparative Example). In this case, even if a second HP treatment was performed, if the impact position was outside the range of the present invention, the compressive residual stress did not improve (Joint No. 3, Comparative Example). In contrast, by performing a second HP treatment at an impact position within the range of the present invention, the compressive residual stress improved to the preferred level (Joints Nos. 4 to 6, Inventive Examples).

[0049] In this way, it has been verified that the present invention can maintain the compressive residual stress on the surface of the base material near the weld toe of a welded structure that is subjected to repeated external forces at a suitable level, thereby improving fatigue strength.

[0050] [Table 1] [Explanation of symbols]

[0051] 1 Welded section 1a, 1b Weld toe (toe) 2 Weld bead extension direction 3 Base material part 3a Horizontal member 3b Vertical member 41, 42 Impact marks (1st, 2nd)

Claims

1. A method for improving the fatigue strength of a weld, comprising: forming linear impact indentations that are continuous in the direction of extension of the weld bead by hammer peening on a base metal portion in the vicinity of a weld toe of the weld; and then forming similar linear impact indentations again on the base metal portion in the vicinity of the weld toe after a predetermined period of time that is not less than 0.5 years and not more than 3 years.

2. 2. The method for improving fatigue strength of a welded portion according to claim 1, wherein the distance between the impact mark and the weld toe is 1.0 mm or less.

3. The maximum depth of the impact mark (D P ) and width (W P ) product (D P ×W P ) is 3.0 to 5.0 mm 2 3. The method for improving the fatigue strength of a welded portion according to claim 1, wherein:

4. 3. The method for improving fatigue strength of a weld according to claim 1, wherein the weld is a weld that joins a tower and a base of a floating offshore wind power generation facility.

5. 4. The method for improving fatigue strength of a weld according to claim 3, wherein the weld is a weld that joins a tower and a base of a floating offshore wind power generation facility.

6. 3. The method for improving fatigue strength of a welded portion according to claim 1, wherein the base material portion is a steel material having a plate thickness of 30 mm or more.

7. 4. The method for improving fatigue strength of a welded portion according to claim 3, wherein the base material portion is a steel material having a plate thickness of 30 mm or more.

8. 5. The method for improving fatigue strength of a welded portion according to claim 4, wherein the base material portion is a steel material having a plate thickness of 30 mm or more.

9. 6. The method for improving fatigue strength of a welded portion according to claim 5, wherein the base material portion is a steel material having a plate thickness of 30 mm or more.

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