JP6606730B2 - Weld reinforcement method - Google Patents

Description

  The present invention relates to a method for reinforcing a welded portion and a welded structure obtained thereby, and more specifically, a simple welded portion that can be applied to various steel materials and can dramatically improve the fatigue strength of the welded portion. The present invention also relates to an effective reinforcing method and a welded structure obtained thereby.

  In recent years, application of high-strength steel to large-sized welded structures such as ships, offshore structures and bridges has progressed. Here, although the fatigue strength of the base material is improved by increasing the tensile strength of the steel material, the reliability and safety of the entire welded structure are limited by the characteristics of the welded portion having the lowest fatigue strength.

  On the other hand, friction stir processing is attracting attention as a means for modifying the welded part, and a method for removing defects (blowholes, cracks, etc.) mainly in the welded part has been studied. For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2006-239734) proposes a welded joint in which a weld metal part is friction stir using a probe of a friction stir welding tool.

  And in the welded joint described in the said patent document 1, while the weld defect, such as the blowhole of a weld metal part, a welding defect, and a crack, has decreased, the crystal grain of a weld metal part is about 20 micrometers, for example. It is said that the fatigue characteristics such as fatigue strength and elongation of the welded joint will be improved.

  Moreover, in patent document 2 (Unexamined-Japanese-Patent No. 2008-246501), in the welding structure comprised by joining the member by the welding part which consists of a welding material made from a nickel base alloy or austenitic stainless steel, the surface of a welding part Or, the friction stir processing unit that performs the friction stir processing by moving the rotating tool in a state of being crimped to the surface of the welded portion and the member in the vicinity of the weld portion by a load load in the surface vertical direction There has been proposed a method for improving the progress of stress corrosion cracking of a welded structure, characterized in that the columnar crystal direction is the in-plane direction.

  In the method for improving the progress of stress corrosion cracking of a welded structure described in Patent Document 2, the occurrence of stress corrosion cracking in the welded portion is caused by setting the columnar crystal direction of the friction stir processing portion to the in-surface direction. Even if stress corrosion cracking occurs in the weld zone, the crack growth rate in the depth direction is such that the columnar crystal direction is perpendicular to the stress corrosion cracking direction. Can be reduced to about 1/10 compared with the case where stress corrosion cracking occurs along the columnar crystal direction. Thereby, it is supposed that the service life of a welding part can be lengthened and the lifetime of a welded structure can be lengthened.

JP 2006-239734 A JP 2008-246501 A

  However, the welded joint disclosed in Patent Document 1 basically uses aluminum as a target material, and does not disclose any effects related to steel materials. In addition, it is necessary to press-fit the probe of the friction stir welding tool into the welded portion, and preferably to friction stir a portion of more than half the thickness of the material to be joined. It is limited to light metal materials that can be easily stirred.

  Further, the method for improving the stress corrosion cracking progress of the welded structure disclosed in Patent Document 2 is characterized in that the columnar crystal direction of the welded portion is the surface in-plane direction, and the effective target material is It is limited to welds made of nickel-base alloy and austenitic stainless steel welds. In addition, no effect on fatigue strength when not in a corrosive environment is disclosed. Moreover, since the depth which performs a friction stirring process is so deep that it is desirable, Preferably it is 2 mm or more, The range applicable practically will be very limited by restrictions, such as a tool lifetime. In addition, when it is necessary to apply friction stir processing to all welds, the practically applicable range is extremely limited due to constraints such as tool life.

  Furthermore, in the basic aspect of the technique disclosed in Patent Document 1 and Patent Document 2, a hole formed when the tool probe is pulled out at the end of the friction stirring process remains. The drawn hole not only has a problem in appearance, but also greatly decreases the strength and reliability of the welded structure depending on the depth of the hole.

  In view of the problems in the prior art as described above, an object of the present invention is to provide a welding portion reinforcing method and a welded structure obtained thereby, and more specifically, welding that can be applied to various steel materials. An object of the present invention is to provide a simple and effective method for reinforcing a welded portion capable of dramatically improving the fatigue strength of the welded portion, and a welded structure obtained thereby.

  In order to achieve the above object, the present inventor conducted extensive research on the friction stir processing conditions and the like of the welded portion effective in improving fatigue strength, and as a result, a thin friction stir processing region is formed in a part of a specific welded portion. Has been found to be extremely effective, and the present invention has been achieved.

That is, the present invention
A method of reinforcing a welded portion that performs friction stir processing on a welded portion formed by fusion welding of two metal workpieces,
The region where the friction stir treatment is performed includes a tensile residual stress concentration portion of the weld,
The bottom surface of the friction stir tool used for the friction stir processing has a substantially flat surface or a probe having a length of 1 mm or less;
A method for reinforcing a welded portion is provided.

  Generally, regardless of the joint shape, there is a tensile residual stress due to welding thermal stress in the weld, and there is a region where the residual stress is concentrated depending on the welding state. In particular, in the weld toe portion or the like, in addition to the welding thermal stress, stress concentration caused by the cross-sectional shape change occurs. As a result of superimposing these stresses or the like, a region where the tensile residual stress is locally increased is often formed in the weld toe portion or the like.

  Various methods known in the art can be used for specifying the tensile residual stress concentration portion in the welded portion. For example, residual stress measurement by X-ray or neutron diffraction, stress analysis of FEM finite element analysis, or the like is used. Can do.

  Since the friction stir processing region formed in the welding portion reinforcement method of the present invention is only in the vicinity of the surface of the welded portion, the processing is performed using a friction stir tool having a probe whose bottom surface is substantially flat or whose length is 1 mm or less. Preferably it is done. By using the friction stir tool, it is possible to suppress a decrease in strength and reliability of the welded structure due to a hole formed when the probe of the tool is pulled out at the end of the friction stir processing. In the case of a mode in which a friction stir tool is inserted into the through hole of a member having a through hole (shoulder portion) and the friction stir tool is rotated at a high speed and press-fitted into the processing target portion (so-called stationary tool) The press-fitting depth of the friction stir tool may be 1 mm or less.

  In the method for reinforcing a welded portion of the present invention, it is preferable that the region to be subjected to the friction stir processing is a corner welded portion of a T-shaped joint. The corner welded part of the T-joint has a residual stress concentration part (part that determines the fatigue strength) as compared with other welds, and by applying friction stir processing to the residual stress concentration The fatigue strength of the welded portion can be improved extremely effectively.

  Friction stir processing may be applied to the entire welded part, but by applying it only to a limited area including the residual stress concentration part, the number of materials that can be processed with one friction stirrer tool is greatly improved. Can be made. In particular, this is an extremely effective embodiment for a steel material that has been put into practical use due to insufficient life of the friction stir tool.

  Moreover, it is preferable to perform a friction stirring process with respect to both the fusion | melting solidification part in a welding part, and the boundary of the said fusion | melting solidification part and a heat affected zone. In addition, when a convex part is formed in a welding part by a friction stirring process, it is preferable to perform a friction stirring process again with respect to the said convex part, and to smooth a welding part.

  The number of friction stir treatments applied to the welded portion is not particularly limited, and there is no problem even with one friction stir processing. However, when the structure of the welded portion is further refined by applying a plurality of friction stir processings, 2 It is preferable to superimpose the friction agitation process more than once. As a result of the structure refinement of the welded portion, the rigidity of the region is increased. As a result, when the external stress is applied, the displacement of the entire welded portion is reduced, and the fatigue life is increased.

  In the method for reinforcing a welded portion according to the present invention, at least one of the two metal workpieces is preferably a steel material that undergoes transformation during fusion welding, and more preferably includes a high-tensile steel material. The method for reinforcing a welded portion of the present invention is effective for various metal welds, but in a welded portion of a steel that causes transformation compared to steel that does not undergo transformation during welding, such as austenitic steel. Since tensile residual stress is likely to occur, friction stir processing is effective. Further, the friction stir processing is particularly effective for high-tensile steel materials having high tensile strength and high fatigue strength of the base material.

  The welded portion reinforcing method of the present invention also provides a welded structure having a welded portion reinforced by the welded portion reinforcing method of the present invention. Since the welded portion that controls the mechanical properties of the entire welded structure is reinforced, a welded structure that can sufficiently exhibit the mechanical properties of the structural material used can be obtained.

  According to the present invention, a method for reinforcing a welded portion and a welded structure obtained thereby are provided, and more specifically, the fatigue strength of a welded portion that can be applied to various steel materials can be dramatically improved. It is possible to provide a simple and effective reinforcing method for a welded portion, and a welded structure obtained thereby.

It is process drawing of the reinforcement method of the welding part of this invention. It is a schematic front view of the tool for friction stirring. It is a top view of a T-shaped joint. It is a side view of a T-shaped joint. It is a top view of the corner turning welding part in the T-shaped joint which performed the friction stirring process. It is a side view of the corner turning welding part in the T-shaped joint which performed the friction stirring process. It is a SEM photograph of the sample material used in the example. (A) The SEM photograph of the cross section of the test material which gave the friction stirring process to the surface of the TIG welding part, and (b) The SEM photograph of the cross section of the test material after TIG welding. It is EBSD mapping of the cross section of the test material which gave the friction stirring process to the surface of the TIG welding part, and the test material after TIG welding. (A) Front view of tensile test piece and (b) Schematic showing cut-out position. It is a graph which shows a tensile characteristic. (A) The schematic perspective view of the test piece for a bending test and a bending fatigue test, (b) The schematic which showed the cutting-out position. It is the schematic which showed the mode of the bending test. It is a graph which shows a bending characteristic. It is a graph which shows a bending fatigue characteristic. 3 is a graph showing the relationship between the number of cycles and the displacement of a test piece during a fatigue test. It is an external appearance photograph of the test piece after a bending fatigue test ((a) TIG welding and friction stirring process, (b) TIG welding only). It is a graph which shows hardness distribution ((a) TIG welding and friction stirring process, (b) TIG welding only). 4 is an EBSD mapping of a weld reinforcement obtained in Example 2. It is a graph which shows a bending fatigue characteristic. 3 is a graph showing the relationship between the number of cycles and the displacement of a test piece during a fatigue test.

  Hereinafter, representative embodiments of the method for reinforcing a welded portion of the present invention will be described in detail with reference to the drawings, but the present invention is not limited to these. In the following description, the same or corresponding parts are denoted by the same reference numerals, and redundant description may be omitted. Further, since the drawings are for conceptually explaining the present invention, the dimensions and ratios of the components shown may be different from the actual ones.

(A) Method for reinforcing welded portion FIG. 1 is a process diagram of a method for reinforcing a welded portion according to the present invention. In the method for reinforcing a welded portion of the present invention, the first step (S01) for specifying the tensile residual stress concentration portion of the welded portion and the region including the tensile residual stress concentration portion specified in the first step (S01) And a second step (S02) for applying a friction stirring process.

≪First step: Identification of tensile residual stress concentration part≫
The conventionally known various methods can be used for specifying the tensile residual stress concentration portion in the welded portion. For the identification, for example, as described above, residual stress measurement by X-ray or neutron diffraction, stress analysis of FEM finite element analysis, or the like can be used.

  Moreover, when the tensile residual stress concentration part used as the starting point of the fracture progress with respect to a specific joint is known conventionally, the 1st process (S01) may be abbreviate | omitted using the said knowledge. For example, in a corner weld of a gusset plate in a T-shaped joint, in addition to a large stress concentration due to a change in cross-sectional shape at the weld toe, generation of tensile residual stress due to welding thermal stress overlaps, It is known that the tensile residual stress is extremely high locally at the weld toe. Therefore, the corner welded portion of the T-shaped joint can be the tensile residual stress concentration portion of the present invention.

≪Second process: Friction stirring process≫
The second step (S02) is a step of subjecting the region including the tensile residual stress concentration portion of the weld specified in the first step (S01) to friction stir processing. The friction stir processing region does not need to be applied to all of the welded portions, and may include a tensile residual stress concentration portion. Further, it is not necessary to make the friction stir processing region deeper than necessary, and it is sufficient that the friction stir processing region is formed in the vicinity of the surface of the weld.

  Friction stir processing is the application of Friction Stir Welding (FSW: Friction Stir Welding) to surface modification of metal materials. Basically, friction stir welding is performed except that the shape of the tool used may differ. Is the same technology. Specifically, it is a method of obtaining a friction stir processing region by inserting a protrusion (probe) provided at the tip of the rotary tool into the material to be processed and moving the rotary tool while rotating it.

  FIG. 2 is a schematic front view of a friction stir tool used in the method for reinforcing a weld according to the present invention. The bottom surface of the friction stir tool 1 used in the method for reinforcing a welded portion of the present invention preferably has a probe 2 having a substantially flat surface (FIG. 2 (b)) or a length of 1 mm or less (FIG. 2 (a)). ). Here, it is more preferable to use the friction stir tool 1 having a substantially flat bottom surface and no probe 2 (FIG. 2B). When the friction stir tool 1 having the probe 2 is press-fitted and moved into a steel material having a high melting point and high temperature deformation resistance, the friction stir tool 1 often breaks from the base of the probe 2 and reaches the life of the friction stir tool 1. In contrast, by using the friction stirring tool 1 having a substantially flat bottom surface, it is not necessary to consider the tool life due to the fracture of the probe 2.

  In addition, by setting the length of the probe 2 to 1 mm or less, it is possible to greatly reduce the probability of the probe 2 being broken during the friction stir processing. The shape of the probe 2 is not particularly limited, and a simple columnar shape, a taper shape with a thick root, and a thin tip can be used. The probe 2 may be threaded or chamfered, but it is preferable not to perform such machining from the viewpoint of tool life.

  By making the bottom surface of the friction stir tool 1 substantially flat, the range of materials that can be used as the material of the friction stir tool 1 can be widened. In the case where the probe 2 is not provided, the shape of the friction stir tool 1 is basically a cylindrical shape, so that it is also possible to use a hardly-sinterable material or a difficult-to-work material. The friction stir tool 1 that can be used in the present invention includes one having a concave bottom surface.

The material of the friction stir tool 1 is, for example, tool steel such as SKD61 steel standardized by JIS, cemented carbide made of tungsten carbide (WC), cobalt (Co), nickel (Ni), cobalt (Co ) A base alloy, a refractory metal such as iridium (Ir) and its alloy, or a ceramic such as Si ₃ N ₄ . Here, when the metal workpiece is a steel material such as high-tensile steel, tungsten carbide (WC), cemented carbide made of cobalt (Co), cobalt (Co) -based alloy, iridium (Ir), etc. It is preferable to use a melting point metal and its alloy, or ceramics such as Si ₃ N ₄ .

  The friction stir processing in the present invention includes (1) a mode in which the friction stir tool 1 is rotated and moved in the processing direction, (2) a mode in which the friction stir tool 1 is rotated and not moved at the processing position, 3) A mode in which the processing regions formed in (1) are overlapped, a mode in which the processing regions formed in (4) and (2) are overlapped, and the processing in (5) (1) to (4) are arbitrarily combined. Embodiments.

  The metal welded material to which the method for reinforcing a welded portion of the present invention can be applied is not particularly limited, and various conventionally known metal materials can be used as long as friction stir processing is possible. It is preferable that at least one of the workpieces to be welded is a steel material that undergoes transformation during fusion welding. From the viewpoint of clearly appearing, it is more preferable to use a high-tensile steel material.

  High-strength steel is a steel whose strength is improved over that of general structural steel by addition of alloy components and structure control. While the guaranteed value of the tensile strength of a general SS400 material is 400 MPa, a tensile strength of approximately 490 MPa or more is called a high-tensile steel material. In the present invention, a high-strength steel material has a tensile strength of 490 MPa. It means the above steel materials.

  3 and 4 show a plan view and a side view of the T-shaped joint, respectively. In the T-shaped joint 10, a gusset plate 14 is welded perpendicularly to the flat plate 12 by using conventional fusion welding. It is configured.

  In addition to an increase in the stress concentration due to the change in the cross-sectional shape at the weld toe 20 of the corner weld 18, the effect of the generation of tensile residual stress due to the welding thermal stress is superimposed, resulting in a weld toe. At 20 the tensile force is very high locally. That is, in the T-shaped joint 10, the weld toe portion 20 of the corner welding portion 32 becomes a tensile residual stress concentration portion.

  5 and 6 are a plan view and a side view, respectively, of a corner welded portion in a T-shaped joint subjected to the friction stir processing. As described above, in the T-shaped joint, the weld toe 20 of the corner welded portion 18 becomes a tensile residual stress concentration portion, so that the friction stir processing is performed so that the weld toe 20 is included in the friction stir processing region 22. It has been subjected.

  The friction stir processing region 22 only needs to include the weld toe 20, and the friction stir processing region 22 may include the flat plate 12. The friction stir processing may be performed without any pre-treatment on the corner welded portion 18 including the weld toe portion 20, but the pre-processing is performed such that the region where the friction stir processing is performed is polished and processed into a substantially flat shape. You may give it.

  As described above, it is not necessary to make the depth of the friction stir processing region 22 unnecessarily deep. From the viewpoint of the life of the friction stir tool 1 and ease of processing, it is preferable to set the depth to 100 to 1000 μm. More preferably, the depth is 100 to 500 μm. In addition, when the friction stirring tool 1 having a substantially flat bottom surface with a diameter of about 10 to 15 mm is used, it is possible to suitably obtain the friction stirring processing region 2 having a depth of 100 to 500 μm, depending on processing conditions. it can.

(B) Welded structure in which welded part is reinforced The welded structure of the present invention provides a welded structure having a welded part reinforced by the method for reinforcing a welded part of the present invention. Since the welded portion that controls the mechanical characteristics of the entire welded structure is reinforced, it is possible to obtain a welded structure that can sufficiently exhibit the mechanical characteristics of the structural material used.

  In the welded structure of the present invention, it is not necessary that all welds be reinforced, but a weld toe that controls the mechanical properties of the welded structure, for example, a weld toe of a round welded part of a T-joint. It is preferable that the part is subjected to a friction stirring process. The depth of the friction stir processing region is preferably 100 to 1000 μm, and more preferably 100 to 500 μm. Here, when the weld is thick, it is preferable to relatively deepen the depth of the friction stir processing region. For example, when the thickness of the welded portion (molten portion) is 5 mm or more, the depth of the friction stir treatment region is preferably 10 to 40% of the thickness of the welded portion, and more preferably 20 to 30%. .

  Moreover, in the welded structure of the present invention, it is preferable that there is no pull-out hole of the friction stir tool 1 due to the friction stir processing. However, it is acceptable if the depth of the extraction hole is about 1 mm or less.

  Furthermore, in the welded structure of the present invention, it is preferable that at least one of the two metal workpieces forming the welded portion subjected to the friction stir processing is a high-tensile steel material. By performing friction stir processing on the welded portion in which at least one of the metal workpieces is a high-tensile steel material, a welded structure that can express the excellent mechanical properties inherent in the high-tensile steel material can be obtained.

  As mentioned above, although typical embodiment of this invention was described, this invention is not limited only to these, Various design changes are possible and these design changes are all contained in the technical scope of this invention. It is.

Example 1
An SS400 steel plate (150 mm × 150 mm × 5 mm) was welded to produce a steel plate having a weld. The composition of the steel material used is shown in Table 1, and the SEM photograph of the cross section is shown in FIG. TIG welding was used for welding, and welding conditions were arc length: 2 mm, welding current: 150 A, welding speed: 2 mm / s, and atmosphere: Ar.

  Next, using a friction stir tool made of cemented carbide having a shoulder diameter of 12 mm, a probe diameter of 4 mm, and a probe length of 0.8 mm, the welded portion was subjected to a friction stir treatment, and an embodiment of the present invention was performed. A reinforced weld was obtained. The friction stirring conditions were as follows: tool load: 1500 kg, tool rotation speed: 400 rpm, tool movement speed: 140 mm / s, tool advance angle: 3 °, atmosphere: Ar.

[Evaluation]
(1) Microstructure observation About the reinforcement weld part produced as mentioned above, the cross-sectional SEM observation was performed. The obtained SEM photograph is shown in FIG. Moreover, EBSD mapping was acquired in the position of the outermost surface, the depth of 1 mm, and the depth of 2 mm in the said cross section. The obtained results are shown in FIGS. The EBSD measurement was performed under the conditions of an acceleration voltage of 15 kV and a step size of 0.25 μm using OIM data Collection ver 5.31 manufactured by TSL.

(2) Tensile test Tensile characteristics were evaluated for the reinforced welded part produced as described above. A front view of the tensile test piece used and a schematic diagram showing the cut-out position are shown in FIG. All the parallel parts of the test piece are included in the friction stir processing unit, and have a shape capable of evaluating the tensile properties of the friction stir processing unit. The strain rate in the tensile test was 1 × 10 ⁻³ / s. The obtained stress-strain curve is shown in FIG.

(3) Three-point bending test The bending characteristics of the reinforced welded part produced as described above were evaluated. FIG. 12 shows a schematic perspective view of the test piece used and a schematic view showing the cut-out position. The central part of the test piece is a friction stir processing part, and has a shape that can evaluate the bending characteristics of the friction stir processing part. The strain rate in the bending test was 5 × 10 ⁻⁵ / s. A schematic diagram showing the bending test is shown in FIG. Measurement is performed in such a manner that a test piece is placed on a jig with the friction stir processing section down and stress is applied from the opposite side of the friction stir processing section. The obtained stress-strain curve is shown in FIG.

(4) Three-point bending fatigue test Fatigue properties of the reinforced welded part produced as described above were evaluated. The test pieces used and the arrangement of the test pieces are the same as in the case of the three-point bending test. The fatigue test was performed under the conditions of a load frequency of 20 Hz, a control mode: stress control, a stress ratio: 0.1, and a stress range: 600 to 800 MPa. The obtained bending fatigue characteristics are shown in FIG. Further, a graph showing the relationship between the number of cycles and the displacement of the test piece during the fatigue test is shown in FIG. 16, and the external shape of the test piece after the test is shown in FIG.

(5) Hardness test The cross section hardness of the reinforced welded part produced as described above was measured. The hardness was measured with a load of 0.1 kgf and a load time of 15 s. The measurement position was the outermost surface, the depth was 2 mm, and the depth was 4 mm, and the horizontal hardness profile at each depth was measured. The obtained result is shown in FIG.

<< Example 2 >>
A weld bead was formed on the surface of a 490 MPa class high-tensile steel plate (KA36 steel plate, 150 mm × 150 mm × 20 mm) using MAG welding. MX-Z200 manufactured by Kobe Steel, Ltd. was used as the welding wire. After removing the surplus, in order to make residual stress again in the weld metal, the weld was subjected to TIG welding and remelted and solidified. Table 2 shows the composition of the steel materials used. The TIG welding conditions were arc length: 2 mm, welding current: 150 A, welding speed: 2 mm / s, and atmosphere: Ar.

  Next, using a friction stirrer tool made of cemented carbide with a shoulder diameter of 12 mm, a probe diameter of 4 mm, and without a probe, a friction stirrer is applied to the center of the width of the welded portion, and the embodiment of the present invention A reinforcement weld was obtained (hereinafter referred to as a Crown FSP weld reinforcement). The friction stirring conditions were as follows: tool load: 1500 kg, tool rotation speed: 400 rpm, tool movement speed: 140 mm / s, tool advance angle: 3 °, atmosphere: Ar.

  Further, in order to examine the influence of the place where the friction stir processing is performed on the fatigue characteristics, the friction stir processing is performed on each of the two boundaries between the melt-solidified portion and the heat affected zone of the welded portion. A reinforcement weld was obtained (hereinafter referred to as Toe FSP weld reinforcement). In addition, the friction stir processing was further performed on the overlapped portion of the friction stir processing of the Toe FSP weld reinforcing portion to obtain a weld reinforcing portion which is an example of the present invention (hereinafter referred to as Toe + Crown FSP weld reinforcing portion).

[Evaluation]
(1) Microstructure observation About the area | region of about 50 micrometers of surface layers of the reinforcement welding part produced as mentioned above, the cross-sectional SEM-EBSD observation was performed. FIGS. 19A and 19B show EBSD mapping of the Crown FSP weld reinforcement and the Toe + Crown FSP weld reinforcement, respectively. The EBSD measurement was performed under the conditions of an acceleration voltage of 15 kV and a step size of 0.25 μm using OIM data Collection ver 5.31 manufactured by TSL.

(2) Three-point bending fatigue test Fatigue characteristics were evaluated for each reinforced welded part produced as described above. The test pieces used and the arrangement of the test pieces are the same as in the three-point bending test of Example 1 above. The fatigue test was performed under the conditions of a load frequency of 20 Hz, a control mode: stress control, a stress ratio: 0.1, and a stress range: 600 to 800 MPa. The obtained bending fatigue characteristics are shown in FIG. FIG. 21 shows a graph showing the relationship between the number of cycles and the displacement of the test piece during the fatigue test. For comparison, the results regarding a welded portion (without friction stirring treatment) in which the welded portion was subjected to TIG welding and remelted and solidified were shown as MAG + TIG. Further, the Toe FSP weld reinforcement was evaluated when the load point was in the center of the weld (Load point: WM) and in the case of a heat affected zone (Load point: HAZ).

≪Comparative example≫
A welded portion, which is a comparative example of the present invention, was obtained in the same manner as in the example except that the friction stirring treatment was not performed. In addition, various evaluations were performed in the same manner as in the examples. The SEM photograph of the weld cross section is shown in FIG. 8B, the EBSD mapping is shown in FIGS. 9D to 9F, the stress-strain curve of the tensile test is shown in FIG. 11, and the stress-strain curve of the bending test is shown in FIG. FIG. 15 shows the fatigue characteristics, FIG. 16 is a graph showing the relationship between the number of cycles and the displacement of the test piece in the fatigue test, FIG. 17 shows the external shape of the test piece after the test, and FIG. 18 shows the hardness profile. , Respectively.

  In Example 1, the maximum depth of the friction stir processing region is about 0.8 mm, which is substantially the same depth as the probe length of the friction stir tool used (FIG. 8). Further, the average crystal grain size of the base material in the friction stir processing region is 1.5 μm, and it can be seen that it is remarkably refined as compared with the region not subjected to the friction stir processing (FIG. 9). .

  Regarding the tensile properties, no clear difference is observed with or without the friction stir treatment (FIG. 11). This is because in the present invention, the friction stir processing is limited to the extreme surface layer portion of the welded portion. On the other hand, regarding bending characteristics, the bending strength is improved by about 30% by the friction stir processing. In the bending test, a test piece is set so that a tensile stress is applied to the welded portion, and the reliability and mechanical characteristics of an actual welded structure can be evaluated.

In addition, regarding the fatigue characteristics, a clear improvement in characteristics is recognized by the friction stir processing. When the stress amplitude is 270 MPa, the number of repetitions of fracture is increased by 170%, and when the number of repetitions of fracture is 2.1 × 10 ⁵ , the stress amplitude is increased by 13%.

  In the relationship between the number of cycles in the fatigue test and the displacement of the test piece, even when the same stress amplitude is applied, the test piece subjected to the friction stir treatment is displaced compared to the test piece not subjected to the friction stir treatment. The amount is suppressed (FIG. 16). This is due to the fact that the strength in the vicinity of the surface of the welded portion has been improved by the refinement of the base material crystal grains. In the hardness profile, a significant increase in hardness is recognized in the friction stir processing region (FIG. 18).

  In the test piece after the fatigue test, the crack progresses linearly when the friction stir treatment is not performed, whereas the crack progresses zigzag when the friction stir treatment is performed. That is, when the friction stir processing is performed, the crack propagation is deflected and the fatigue characteristics are improved.

Further, as shown in FIG. 20, in Example 2, in the MAG + TIG material (Δ), the fatigue life increases as the stress amplitude decreases, and the lowest stress amplitude is 292.5 MPa (maximum stress 650 MPa, minimum stress 65 MPa). Then, it broke in about 4 × 10 ⁵ times. On the other hand, when the stress amplitude is lower than 315 MPa in the Crown FSP weld reinforcement (◯), it does not break even 1.5 × 10 ⁶ times, and an increase in fatigue life of about an order of magnitude is obtained compared to the MAG + TIG material. It was.

As a result of the tensile test, the fracture occurred from the vicinity of the boundary between the weld metal and the heat-affected zone. Therefore, the Toe FSP weld reinforcement (）) in which the friction stir treatment was applied to that portion broke at 10 ⁴ times. Cracks have progressed from the protrusion (near the bending load point) that occurs at the intersection of the first pass friction stir processing and the second pass friction stir processing, which is the result of the surface shape formed by Toe FSP. it is conceivable that. In fact, the result of shifting the load point to the center of the friction stir processing region (♦) shows the same result as that of the Crown FSP (◯) material.

In the Toe + Crown FSP weld reinforcement (□), when the stress amplitude is lower than 337.5 MPa, it does not break even 1.5 × 10 ⁶ times, and at a high stress amplitude of 337.5 MPa, it is one digit or more compared with the MAG + TIG material. Increased fatigue life is obtained.

  FIG. 21 is a plot of displacement with respect to the number of repetitions for the maximum stress and the minimum stress at a stress amplitude of 337.5 MPa with respect to Example 2. In the case where the friction stir processing is applied to the welded portion in a plurality of patterns (open plot), the displacement is reduced from the MAG + TIG material (△) in the Crown FSP weld reinforcing portion (◯) and the Toe FSP weld reinforcing portion (◇), and Toe + Crown. It can be seen that the displacement is further reduced in the FSP weld reinforcement (□), and the surface modification by the friction stir treatment improves the rigidity of the modified region and significantly increases the fatigue life. Note that the Toe FSP weld reinforcement (♦) shifts the load point from the center of the weld to the heat-affected zone in a soft region, so the displacement seems to be larger than other evaluation materials. . In other words, the fatigue characteristics can be effectively improved by applying the friction stir processing to the region where the displacement increases with the application of the external stress.

  In the area of the surface layer of about 50 μm of the Crown FSP weld reinforcement, uniform crystal grain refinement occurs, and the average crystal grain size of the crystal grains is about 1.6 μm (FIG. 19A). On the other hand, the Toe + Crown FSP weld reinforcement has a non-uniform microstructure in which ultrafine grains and crystal grains of about 6 μm are mixed (FIG. 19B). Here, the inside of the crystal grain of about 6 μm is also divided into several regions having different contrasts, and it is suggested that a finer structure is formed by overlapping the friction stir processing a plurality of times. . It is considered that the surface layer of the Toe + Crown FSP weld reinforcement becomes more rigid due to the refinement of the structure, and the characteristics of the region improve the fatigue life of the entire weld.

  From the above results, the mechanical properties (bending strength, fatigue strength, etc.) of the welded portion are greatly improved by applying the friction stir treatment region only to the vicinity of the surface of the welded portion or only to a part of the region. I understand. Therefore, a welded structure that can fully exhibit the mechanical properties of structural materials by applying friction stir processing to the welded toe of the T-joint that controls the mechanical properties of the entire welded structure. Things can be obtained very efficiently.

1 ... Tool for friction stirring,
2 ... Probe,
10 ... T-shaped joint,
12 ... Metal plate,
14 ... Gusset board,
16: welded portion on the lower side surface,
18 ... cornering weld,
20 ... weld toe,
22: Friction stir processing region.

Claims (5)

A method for reinforcing welds subjected to friction stir processing to weld formed by the two molten weld metal workpieces,
At least one of the two metal workpieces is a steel material,
The region where the friction stir treatment is performed is a part of the weld including the tensile residual stress concentration portion of the weld,
The bottom surface of the friction stir tool used for the friction stir processing has a probe that is substantially flat or 1 mm or less in length,
The base material crystal grains of the friction stir processing region formed by the friction stir processing are equiaxed grains that are finer than the untreated region,
The friction stir processing region has a depth of 100 to 1000 μm,
A method for reinforcing a welded portion characterized by the above. The region to be subjected to the friction stir processing is a corner weld of a T-shaped joint;
The method for reinforcing a welded portion according to claim 1. At least one of the two metal workpieces is a steel material that undergoes transformation during fusion welding;
The method for reinforcing a welded portion according to claim 1, wherein: At least one of said two metal workpieces are high-tensile steel material,
The method for reinforcing a welded portion according to any one of claims 1 to 3.   A welded structure having a welded portion reinforced by the method for reinforcing a welded portion according to claim 1.