Method for suppressing fatigue crack generation in welded parts and method for manufacturing welded joints

Abstract

To provide a method for suppressing fatigue crack initiation of a weld zone to improve fatigue strength, by executing a striking hammer peening process to a base material part having residual compression stress region in a plate thickness direction defined to apply the residual compressive stress up to a certain depth or more and suppressing fatigue crack initiation of a weld zone, and a method for manufacturing a weld joint that is executed using the method.SOLUTION: In a method for suppressing fatigue crack initiation of a weld zone, a base material part 1 near a weld toe 3 is struck continuously along weld bead of the weld zone so as to form a struck mark 5 and residual compression stress is formed in a region where a depth (Ds) from a surface layer of the base material part is equal to 3.0 mm or more. Preferably, a distance (L) between the weld toe and the struck mark is within 1.0 mm and product (DP×WP) of a maximum depth (Dp) and a width (Wp) of the struck mark is equal to 3.0 mm2 or more.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for suppressing fatigue crack generation in a welded portion and a method for manufacturing a welded joint. In particular, the present invention relates to a method for suppressing fatigue crack generation in a welded portion and a method for manufacturing a welded joint by performing a hammer peening impact treatment on a base material portion to impart compressive residual stress to a certain depth or more, thereby suppressing fatigue crack generation in a welded portion of, for example, a monopile in the offshore wind power field, and improving fatigue strength. Furthermore, the present invention relates to a method for manufacturing a welded joint constructed using the method. [Background technology]

[0002] In recent years, the monopile type has been considered in the offshore wind power field to reduce construction costs. In addition, generators are becoming larger in size to reduce power generation costs and increase power generation efficiency, and the monopiles that support them are also becoming larger. The thickness of the steel plate used in these monopiles ranges from about 30 mm to 100 mm, for example. However, since there is a limit to the loading capacity of the laying ships that transport the monopiles, there is an increasing demand for reducing the thickness by using high-strength materials in the monopiles. The use of high-strength materials would enable a design with higher stress than the current design stress, but in that case, there is a concern that the fatigue strength of the monopile's circumferential butt welds will decrease.

[0003] As a technique for improving the fatigue strength of welds, Patent Document 1 discloses a hammer peening method in which a base metal part is struck. It describes that the base metal part of a welded joint with a plate thickness of 12 mm is formed so that the distance between the impact mark and the weld bead toe is more than 0 mm and not more than 3 mm, and that the maximum compressive residual stress in the surface layer of the weld toe is 50% of the yield stress of the base metal. However, no verification has been done for steel plates with a plate thickness of 30 mm or more, such as those used in the above-mentioned monopiles. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4895407 Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, fatigue cracks in welded joints generally occur in the plate thickness direction from the stress concentration point at the weld toe, and therefore the compressive residual stress region in the plate thickness direction is considered to correlate with the fatigue crack initiation life. In addition, when the plate thickness is large, such as in the case of steel plates for monopiles, the compressive residual stress region in the plate thickness direction becomes important for suppressing fatigue cracks that occur in the plate thickness direction. In other words, since Patent Document 1 does not define the magnitude of the compressive residual stress in the plate thickness direction and the compressive residual stress region given in the plate thickness direction is not clear, there is a problem that it cannot suppress fatigue crack initiation in welded joints of thick materials with a plate thickness of 30 mm or more.

[0006] As described above, even if the conventional base metal impact hammer peening treatment focusing on the compressive residual stress in the surface layer was performed, it was unclear whether the fatigue strength would be improved in the case of a welded joint made of thick material.

[0007] That is, the present invention aims to provide a method for suppressing the occurrence of fatigue cracks in a welded joint and improving its fatigue strength by performing impact hammer peening on a base material portion defining a compressive residual stress region in the plate thickness direction, thereby imparting compressive residual stress to a certain depth or more, and further aims to provide a method for manufacturing a welded joint constructed using the method. [Means for solving the problem]

[0008] In order to clarify the effect of improving fatigue strength by hammer peening treatment of base metal of thick-walled butt-welded joints, the inventors measured compressive residual stress in the plate thickness direction induced by various hammer peening treatments by a hole drilling method. Based on the results, they investigated the optimum conditions for hammer peening treatment of base metal to suppress fatigue crack generation in thick-walled butt-welded joints. As a result, they found that applying compressive residual stress from the surface layer of the base metal near the weld toe to a depth of 3 mm or more contributes significantly to suppress fatigue crack generation in thick-walled butt-welded joints.

[0009] The present invention was completed based on these findings and through further investigation, and the gist of the present invention is as follows. [1] A method for suppressing the occurrence of fatigue cracks in a weld, comprising the steps of: striking a base metal portion in the vicinity of a weld toe continuously along a weld bead of the weld to form an impact mark; and reducing compressive residual stress to a depth (D S ) is formed in an area of ​​3.0 mm or more. [2] The method for suppressing fatigue crack occurrence in a weld according to [1] above, characterized in that the distance (L) between the weld toe and the impact mark is within 1.0 mm. [3] In the above [1] or [2], the maximum depth of the impact mark (D P ) and width (W P ) product (D P ×W P ) is 3.0mm 2 A method for suppressing the occurrence of fatigue cracks in a welded portion, characterized by the above. [4] A method for suppressing the occurrence of fatigue cracks in a welded portion according to [1] or [2] above, characterized in that the base material portion of the welded portion is a butt welded joint having a plate thickness of 30 mm or more. [5] The method for suppressing fatigue crack occurrence in a welded portion according to [3] above, characterized in that the base material portion of the welded portion is a butt welded joint having a plate thickness of 30 mm or more. [6] A method for manufacturing a welded joint, characterized in that the method for suppressing fatigue crack occurrence in a welded joint described in [1] or [2] above is used. [7] A method for manufacturing a welded joint, characterized in that it is carried out using the method for suppressing fatigue crack occurrence in a welded portion described in [3] above. [8] A method for manufacturing a welded joint, characterized in that it is carried out using the method for suppressing fatigue crack occurrence in a welded portion described in [4] above. [9] A method for manufacturing a welded joint, characterized in that the method for suppressing fatigue crack occurrence in a welded portion described in [5] above is used. Effect of the Invention

[0010] According to the present invention, a compressive residual stress region is imparted not only to the surface layer but also to a certain depth in the plate thickness direction, thereby making it possible to improve the fatigue strength of thick-walled materials, which is extremely effective in industry. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a schematic diagram showing a hammer peening method according to the present invention. [Diagram 2] FIG. 2 is a schematic diagram illustrating a residual stress test method using a hole drilling method. [Diagram 3] FIG. 2 is a schematic diagram illustrating a four-point bending fatigue test method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The present invention is a method for improving the fatigue strength of a welded joint by continuously striking the base material in the vicinity of the weld toe and introducing compressive residual stress into the weld toe.

[0013] Hereinafter, embodiments of the present invention will be described.

[0014] [Area where compressive residual stress is applied] As described above, the present invention introduces compressive residual stress near the weld toe in order to improve the fatigue strength of the welded joint. Specifically, the depth (D S The compressive residual stress is imparted to a region where the depth (D S In the region where the depth (D ) is less than 3.0 mm, compressive residual stress is not sufficiently imparted, so that the fatigue strength of the welded joint cannot be improved. S ) is 3.0~5.0mm.

[0015] The compressive residual stress is measured based on the ASTM E837 standard (hole-drilling method), which is a measurement technique that determines the residual stress by drilling holes in a test specimen and evaluating the amount of deformation before and after the stress is released.

[0016] Here, as the residual stress, a range of -400 to -200 MPa is preferable for suppressing the occurrence of fatigue cracks.

[0017] From the surface layer of the above-mentioned base material part to a depth (D S ) of 3.0 mm or more, in order to apply compressive residual stress, it is important to define the product of the impact position by the chipper, the maximum depth of the impact mark which is the cross-sectional area in the depth direction of the impact mark, and the width of the impact mark within a specific range.

[0018] The following will be described with reference to FIG. 1.

[0019] First, it is preferable that the distance (L) between the welding termination part 3 and the impact mark 5 is within 1.0 mm. That is, the distance (L) from the welding termination part 3, which is the boundary between the weld metal 2 and the base material 1 of the welded joint formed by butt-welding the base material (hereinafter also referred to as the "base material part") 1, to the position where the tip of the chipper 4, which is an impact pin, contacts the base material 1 is within 1.0 mm. If the distance (L) exceeds 1.0 mm, the impact mark 5 will be separated from the welding termination part 3, making it difficult to introduce compressive residual stress in the vicinity of the welding termination part 3, and thus not improving the fatigue strength of the welded joint. Also, in order for the impact of the chipper 4 not to hit the welding termination part 3, since the distance (L) is preferably greater than 0 mm, it is more preferable that the distance (L) is 0 mm < L ≤ 1.0 mm.

[0020] Next, the product of the maximum depth (D P ) of the impact mark 5, which is the cross-sectional area in the depth direction of the impact mark 5, and the width (W P ) of the impact mark (D P ×W P ) is preferably 3.0 mm 2 or more. If the product of the maximum depth (D P ) and the width (W P ) of the impact mark 5 (D P ×W P ) is less than 3.0 mm 2 , sufficient compressive residual stress cannot be applied. More preferably, the product (D P ×WP ) is 3.0~5.0mm 2 It is.

[0021] [Welded joint manufacturing method, welding method] Next, a specific example of the manufacturing method and welding method for a welded joint according to the present invention will be described. The type of steel for the steel plate that becomes the base material 1 is, for example, SM490, and the tensile strength is preferably in the range of 400 MPa to 720 MPa.

[0022] The plate thickness is intended to be a thick steel plate suitable for monopiles, which is the object of the present invention, and specifically, is preferably 30 mm or more, and more preferably 50 to 100 mm.

[0023] The base material is double-sided multi-layer welded by gas-shielded arc welding using carbon dioxide gas under welding conditions of a heat input of 0.6 to 1.0 kJ / mm to produce a butt welded joint.

[0024] [Hammer peening method] The obtained butt welded joint is subjected to a hammer peening treatment to impart compressive residual stress to the base material 1 in the vicinity of the weld toe.

[0025] After the above-mentioned welded joint is formed, as shown in Fig. 1, the base material 1 in the vicinity of the weld toe 3 is struck (peened) to form an impact mark 5. As described above, the distance (L) between the weld toe 3 and the impact mark 5 is preferably within 1.0 mm, and the maximum depth (D P ) and width (W P ) product (D P ×W P ) is 3.0mm 2 By striking this range, the compressive residual stress is increased from the surface layer of the base material to a depth (D S ) can be formed in an area having a depth (D S If the gap is less than 3.0 mm, the range in which compressive residual stress is introduced is small, and the effect of preventing fatigue cracks from the weld toe 3 cannot be exhibited. Here, the treatment is preferably carried out so that the residual stress falls within the range of -400 to -200 MPa.

[0026] [Chipper] As shown in Fig. 1, the chipper 4, which is an impact tool, is preferably a rectangular prism with a semicircular curved surface at the bottom end, and the area of ​​the impact mark 5 is preferably struck with the arcuate curved surface. The tip of the chipper 4 may be spherical, rectangular, or of a similar shape. The thickness (width) of the tip is preferably in the range of 1 mm to 10 mm.

[0027] The chipper 4 is connected to an impact device (not shown), which is driven by air pressure, high-frequency current, ultrasonic waves, etc. For example, a preferred method is to operate the semi-cylindrical tip with air pressure to strike the aforementioned area near the weld toe 3. Here, the impact frequency is preferably a low frequency of 100 Hz or less.

[0028] In addition, the maximum depth of the impact mark 5 (D P ) and width (W P ) product (D P ×W P ) is 3.0mm 2 It is preferable to control the temperature so as to fall within the above range.

[0029] Furthermore, the inclination angle (θ) of the chipper's striking direction is permissible if it is about 80 to 100° with respect to the base material surface, but it is preferable that it is approximately perpendicular, that is, 90°. EXAMPLES

[0030] The test material used was SM490B. The plate thickness was 50 mm. The test material was double-sided welded by gas shielded arc welding under the following welding conditions (heat input 0.8 kJ / mm) to produce a butt welded joint. Young's modulus E and Poisson's ratio v were measured as properties of the welded joint. Test pieces were taken from the butt welded joint, and one was treated with hammer peening and the other was not. The compressive residual stress in the plate thickness direction was measured by the hole drilling method, and a four-point bending fatigue test was conducted on each test piece.

[0031] [Gas shielded arc welding conditions] Shielding gas: CO2 Welding heat input: 0.8kJ / mm, welding current: 310A, welding voltage: 40V Welding speed: 29.5cm / min Welding wire composition: C: 0.07wt%, Si: 0.37wt%, Mn: 1.11wt%, P: 0.013wt%, S: 0.009wt%, Mo: 0.37wt% Welding wire diameter: 1.2mm

[0032] [Method of measuring Young's modulus and Poisson's ratio] The Young's modulus and Poisson's ratio were measured in accordance with ASTM E8M "Method of tensile testing for metallic materials."

[0033] [Measurement of compressive residual stress by hole drilling method] The compressive residual stress was measured by a hole-drilling method based on the ASTM E837 standard. In this hole-drilling method, as shown in Figure 2, first a hole is made with a drill 6 at the location of the test specimen where the residual stress is to be measured, the hole diameter before stress release is measured, and then the stress is released by trepanning. After stress release, the hole diameter is measured again, and the amount of deformation before and after stress release is evaluated to measure the residual stress.

[0034] [Four-point bending fatigue test] The four-point bending fatigue test method used a fatigue testing device 10 as shown in Fig. 3, in which stress was repeatedly applied to a fatigue test piece 11, and a fatigue test was performed by bending the fatigue test piece 11 at four points (two upper supports 12 on the upper side and two lower supports 13 on the lower side). That is, an impact was applied from above the fatigue testing device 10, and the number of loading cycles until the fatigue test piece 11 broke was determined. The nominal stress range (MPa) is the difference between the maximum stress and the minimum stress.

[0035] The measurement and test results are shown in Table 1.

[0036] [Table 1]

[0037] Nos. 1 to 5 are test results of examples of the present invention that were subjected to hammer peening, and Nos. 6 to 8 are comparative examples that are test results of as-welded samples that were not subjected to hammer peening.

[0038] The test results for No. 1 to 5 where the hammer peening was applied showed that the fatigue life was more than three times longer (one grade improvement on the fatigue design curve) than the test results for the as-welded specimens. Table 1 also shows the measurement results of the compressive residual stress area in the plate thickness direction, which is thought to contribute to improving fatigue strength. The product of maximum depth and width (D P ×W P ) is 3.0mm 2 By forming impact indentations of this magnitude or greater, compressive residual stress was imparted from the surface to a depth of 3.0 mm or more.

[0039] On the other hand, the maximum depth x width product is 2.5 mm 2 In the case of No. 9, which was a conventional method using base metal impact, the fatigue life was improved by about 1.2 times compared to the case without hammer peening, which was not enough to contribute to an improvement of one grade on the fatigue design curve.

[0040] As described above, the present invention is a method for manufacturing a slit having a maximum depth and width of 3.0 mm. 2By forming impact indentations of this magnitude, compressive residual stress could be imparted from the surface to a depth of 3.0 mm or more, which demonstrated the effect of improving fatigue strength. [Explanation of symbols]

[0041] 1 Base metal (base metal part) 2. Weld metal 3 Weld Toe 4. Chipper 5 Impact marks 6. Drill 10. Fatigue Testing Equipment 11 Fatigue test specimen 12 Upper Support Point 13 Lower fulcrum 14 Loading fixture 15 Receiving jig L: Distance between the weld toe and the impact mark D P Maximum impact depth W P Width of impact mark t Plate thickness θ The angle of the chipper's impact direction

Claims

1. A method for suppressing the occurrence of fatigue cracks in a welded portion, comprising the steps of: striking a base material portion in the vicinity of a weld toe continuously along a weld bead of the welded portion to form an impact mark; forming a compressive residual stress in a region of the base material portion having a depth (D S ) of 3.0 mm or more from the surface layer thereof; a distance (L) between the weld toe and the impact mark being within 1.0 mm; and Ｐ ) and width (W Ｐ ) product (D Ｐ ×W Ｐ ) is 3.0 mm ２ A method for suppressing the occurrence of fatigue cracks in a welded portion, characterized by the above.

2. 2. The method for suppressing fatigue crack occurrence in a welded portion according to claim 1, wherein the base material portion of the welded portion is a butt welded joint having a plate thickness of 30 mm or more.

3. A method for manufacturing a welded joint, comprising the method for suppressing fatigue crack initiation in a welded portion according to claim 1 or 2.

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