Residual stress reduction method and residual stress reduction device

Abstract

To provide a simple residual stress reduction method.SOLUTION: A residual stress reduction method includes: a cooling step of directly or indirectly bringing a part of a surface of at least one metal member cooled to -100°C or below into contact with a part of a surface of a to-be-treated object having tensile residual stress, thereby locally cooling the to-be-treated object; and a step of returning the to-be-treated object to an ordinary temperature, wherein the cooling step is a step of locally cooling the part of the surface of the to-be-treated object while changing a portion of the surface of the at least one metal member that is to be brought into contact with the to-be-treated object.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a method and an apparatus for reducing residual stress. [Background technology]

[0002] Welding is essential for the construction of large steel structures. However, high tensile residual stress occurs near the welded parts during welding. This residual stress is considered to be one of the causes of fatigue cracks and stress corrosion cracking (SCC), and residual stress is closely related to the soundness of structures. Therefore, it is important to reduce tensile residual stress. Methods for mitigating welding residual stress include peening and post-weld heat treatment (PWHT). Also, a method is known in which the residual stress in a pipe weld is alleviated by forming a temperature gradient by heating the outer surface of the pipe to a high temperature and the inner surface of the pipe to a low temperature (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Publication No. 57-70095 Summary of the Invention [Problem to be solved by the invention]

[0004] However, conventional residual stress relaxation methods can have problems in terms of initial cost and application time, and there is a need for a low-cost, simple application method. The present invention has been made in view of the above circumstances, and provides a simple method for reducing residual stress. [Means for solving the problem]

[0005] The present invention provides a method for reducing residual stress, comprising: a cooling step of locally cooling a surface of an object to be treated having tensile residual stress by directly or indirectly contacting a portion of the surface of at least one metal component that has been cooled to a temperature of -100°C or lower with the object to be treated; and a step of returning the object to room temperature, wherein the cooling step is a step of locally cooling a portion of the surface of the object to be treated while changing the portion of the surface of the at least one metal component that comes into contact with the object to be treated. Effect of the Invention

[0006] Metals have high thermal conductivity and large heat capacity per unit volume (specific heat × density). Therefore, by directly or indirectly contacting a part of the surface of at least one metal member cooled to a temperature of -100°C or lower with a part of the surface of the object to be treated, a large amount of thermal energy can be transferred from the object to be treated to the metal member. In addition, by changing the part of the surface of the metal member that contacts the object to be treated, the thermal energy of the object to be treated can be further transferred to the metal member. By locally cooling the object to be treated while changing the contact part in this way, a part of the object to be treated can be rapidly cooled to a sufficiently low temperature. This rapidly cooled part of the object to be treated (locally cooled part) thermally contracts, so that tensile stress is generated between the locally cooled part and the other part. When this tensile stress reaches the yield stress, tensile plastic strain is generated in the object to be treated. After tensile plastic strain is generated in the object to be treated, when cooling of the object to be treated is stopped, the locally cooled part returns to room temperature. In the process of returning to room temperature, the locally cooled part thermally expands and the tensile stress decreases. Since the object to be treated has tensile plastic strain, the tensile stress disappears at temperatures lower than room temperature. Therefore, even after the tensile stress disappears, the locally cooled part continues to expand due to the temperature rise to room temperature, and compressive stress occurs between the locally cooled part and other parts. Then, this compressive stress remains as residual stress in the object to be treated when it returns to room temperature. The tensile residual stress that the treatment object had before cooling is eliminated or offset by the compressive residual stress that occurs during the cooling cycle described above. Therefore, the method for reducing residual stress of the present invention can eliminate or reduce the tensile residual stress in the treatment object. [Brief description of the drawings]

[0007] [Figure 1] 1 is a schematic perspective view of a processing object and a metal member in a residual stress reducing method according to an embodiment of the present invention. FIG. [Diagram 2] 2 is a schematic cross-sectional view of a processing object and a metal member taken along dashed line AA in FIG. [Diagram 3] 1 is a schematic cross-sectional view of a residual stress reducing device according to one embodiment of the present invention; [Figure 4] FIG. 2 is an explanatory diagram of a mechanism for reducing residual stress by a cooling cycle. [Diagram 5] 1 is a schematic cross-sectional view of a residual stress reducing device according to one embodiment of the present invention; [Figure 6] 1 is a schematic cross-sectional view of a residual stress reducing device according to one embodiment of the present invention; [Figure 7] This is an analytical model for thermal elastic-plastic analysis. [Figure 8] 8 is a schematic cross-sectional view of an analytical model taken along dashed line BB in FIG. 7. [Figure 9] This is the temperature distribution of the object to be treated. [Figure 10] 1 shows the distribution of residual plastic strain in the treated object. [Figure 11] 1 shows the residual stress distribution of a processing object. [Figure 12] 1 is a photograph of the fabricated residual stress reduction device. [Figure 13] FIG. 13 is an exploded perspective view showing the internal structure of the residual stress reducing device shown in FIG. [Figure 14] (a) is an FEM analysis model, (b) is a photograph of the test equipment in which a metal member is brought into contact with the object to be treated, and (c) is a photograph of the test equipment in which liquid nitrogen is brought into contact with the object to be treated. [Figure 15] 1 is a graph showing a change in temperature of an object to be processed. [Figure 16] 13 is a graph showing a change in strain in the X-axis direction of a processing object. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The residual stress reduction method of the present invention includes a cooling step in which a portion of the surface of a treatment object having tensile residual stress is directly or indirectly brought into contact with a portion of the surface of at least one metal component that has been cooled to a temperature of -100°C or lower, thereby locally cooling the treatment object, and a step in which the treatment object is returned to room temperature, and is characterized in that the cooling step is a step of locally cooling a portion of the surface of the treatment object while changing the portion of the surface of the at least one metal component that comes into contact with the treatment object.

[0009] The material of the metal member is a metal having a thermal conductivity of 100 W / (m·K) or more and a thermal conductivity of 2.0 J / (K·cm 3 It is preferable that the metal has a heat capacity (specific heat x density) per unit volume of at least 10 ... The cooling time in the cooling step is preferably 0.1 seconds to 1 minute, and the cooling step is preferably a step of lowering the temperature of a part of the object to be treated to a temperature lower than the temperature at which tensile plastic strain occurs in the object to be treated. This makes it possible to generate compressive residual stress in the object to be treated, and to reduce the tensile residual stress that the object had before the treatment.

[0010] The present invention also provides a residual stress reduction device comprising at least one metal member, a cooling structure configured to cool the at least one metal member with a liquefied gas at -100°C or lower, and a contact structure configured to bring the at least one metal member into direct or indirect contact with a part of a surface of the object to be treated while changing the part of the surface of the at least one metal member that comes into contact with the object to be treated. The metal member preferably has a columnar or cylindrical shape, the contact structure is preferably a structure arranged to rotate the metal member, and the metal member is preferably arranged so that a part of the outer circumferential surface of the metal member directly or indirectly contacts a part of the surface of the object to be treated. By rotating the metal member, the outer circumferential surface of the metal member can be continuously brought into contact with the object to be treated, and the object to be treated can be rapidly locally cooled.

[0011] The at least one metal member preferably includes a first metal member and a second metal member, and the contact structure preferably has a rail provided for suspending and moving the first and second metal members, and a drop structure provided for dropping the suspended first or second metal member into contact with the surface of the object to be treated. By contacting the first metal member with the object to be treated and then further contacting the second metal member with the object to be treated, the object to be treated can be rapidly locally cooled. The at least one metal member preferably includes a plurality of granular metal members, and the contact structure preferably is provided so as to continuously contact the plurality of granular metal members with the object to be treated, thereby enabling rapid localized cooling of the object to be treated.

[0012] The present invention will be described in more detail below with reference to a number of embodiments. The configurations shown in the drawings and the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings and the following description.

[0013] First embodiment FIG. 1 is a schematic perspective view of a treatment object and a metal member in a residual stress reduction method of this embodiment, FIG. 2 is a schematic cross-sectional view of the treatment object and the metal member taken along dashed line AA in FIG. 1, and FIG. 3 is a schematic cross-sectional view of a residual stress reduction apparatus of this embodiment. The residual stress reduction method of this embodiment includes a cooling step of locally cooling the object to be treated 2 by directly or indirectly contacting a portion of the surface of at least one metal member 3 that has been cooled to a temperature of -100°C or lower with a portion of the surface of the object to be treated 2 having tensile residual stress, and a step of returning the object to be treated 2 to room temperature, wherein the cooling step is characterized in that it is a step of locally cooling a portion of the surface of the object to be treated 2 while changing the portion of the surface of the at least one metal member 3 that comes into contact with the object to be treated 2. Moreover, the residual stress reducing method may include a step of cooling the entire metal member 3 to −100° C. or lower by using liquefied gas 8.

[0014] The residual stress reduction device 20 of this embodiment comprises at least one metal member 3, a cooling structure 12 arranged to cool the at least one metal member 3 with a liquefied gas at -100°C or below, and a contact structure 17 arranged to bring the at least one metal member 3 into direct or indirect contact with a portion of the surface of the object to be treated 2 while changing the portion of the surface of the at least one metal member 3 that contacts the object to be treated 2.

[0015] The residual stress reduction method is a method for reducing the tensile residual stress caused by welding or the like. The processing object 2 is a processing object of the residual stress reducing method, and has tensile stress before processing. The processing object 2 is, for example, a metal member having a welded part, a metal structure having a welded part, etc. Furthermore, the temperature of the processing object 2 before cooling can be room temperature (0°C or more and 40°C or less). The cooling structure 12 is a portion provided to cool the metal member 3 with liquefied gas 8 at -100°C or lower. The cooling structure 12 is provided, for example, so as to immerse the metal member 3 in the liquefied gas 8. The cooling structure 12 may include, for example, a liquefied gas container 9. By immersing the metal member 3 in the liquefied gas in this liquefied gas container 9, the metal member 3 can be cooled to approximately the same temperature as the liquefied gas. The cooling structure 12 may also be a structure that cools the metal member 3 by flowing liquefied gas inside a tube of the tubular metal member 3. The cooling structure 12 of the residual stress reduction device 20 shown in FIG. 3 includes a liquefied gas container 9 having an opening at its bottom, and a columnar or cylindrical metal member 3 is fitted into this opening with its outer circumferential surface facing downward. The liquefied gas may be, for example, liquid nitrogen, liquid oxygen, liquid argon or a mixture thereof.

[0016] The metal member 3 is a metal member that is brought into direct or indirect contact with the treatment object 2 after being cooled to -100°C or lower. The material of the metal member 3 is, for example, a metal having a thermal conductivity of 100 W / (m·K) or more and a thermal conductivity of 2.0 J / (K·cm 3 ) or more per unit volume (specific heat × density). This allows a large amount of thermal energy to be transferred from the processing object 2 to the metal member 3, and allows the processing object 2 to be locally and rapidly cooled. For example, the material of the metal member 3 is pure copper, a copper alloy, aluminum, an aluminum alloy, or the like.

[0017] The metal member 3 may be composed of one member or multiple members. The term "surface of at least one metal member 3" refers to the surface of one member when the metal member 3 is composed of one member, and refers to the surfaces of all the multiple members when the metal member 3 is composed of multiple members. The shape of the metal member 3 is not particularly limited as long as a part of the surface of the metal member 3 can be brought into contact with the processing target 2 directly or indirectly. In the drawings exemplified in Figs. 1 to 3, the metal member 3 has a columnar or cylindrical shape.

[0018] Figure 4 is an explanatory diagram of the mechanism by which residual stress is reduced by a cooling cycle. The horizontal axis of Figure 4 is the temperature of the locally cooled portion of the processing object 2, and the vertical axis is the stress generated in the locally cooled portion of the processing object 2. On the vertical axis of Figure 4, positive stress is tensile stress, and negative stress is compressive stress. Also in Figure 4, α is the linear expansion coefficient, E is Young's modulus, and β is a constraint parameter (β<1). After the entire metal member 3 is cooled to -100°C or lower by liquefied gas or the like, a part of the surface of the metal member 3 is directly or indirectly brought into contact with a part of the object to be treated 2. For example, the metal member 3 can be brought into contact with the object to be treated 2 as shown in Figures 1 and 2. For example, the metal member 3 may be brought into direct contact with the object to be treated 2, or the metal member 3 may be brought into contact with the object to be treated 2 via thermally conductive grease. When thermally conductive grease is interposed, the thermally conductive grease can also be cooled by the liquefied gas together with the metal member 3.

[0019] By locally contacting the metal member 3 with the processing object 2, a large amount of heat energy at the contact portion of the processing object 2 can be transferred from the processing object 2 to the metal member 3. This allows the processing object 2 to be locally quenched. In addition, the temperature of the metal member 3 also rises with the quenching of the processing object 2, so it is difficult to cool the processing object 2 to a sufficiently low temperature by one contact. For this reason, in the method of this embodiment, the portion of the surface of the metal member 3 that comes into contact with the processing object 2 is changed. Since the portion of the surface of the metal member 3 that newly comes into contact with the processing object 2 has a sufficiently low temperature, the heat energy of the processing object 2 can be further transferred to the metal member 3. For example, the columnar or cylindrical metal member 3 shown in Figures 1 and 2 is rotated at a predetermined rotation speed without moving the position. This allows the portion of the surface of the metal member 3 that comes into contact with the processing object 2 to be continuously changed, and a part of the processing object 2 can be locally quenched to a sufficiently low temperature.

[0020] A part of the rapidly cooled treatment object 2 (the locally cooled part) thermally contracts, and tensile stress is generated between the locally cooled part and the other parts. When this tensile stress reaches the yield stress, tensile plastic strain is generated in the treatment object. For example, as shown in the graph of Figure 4, when the temperature of the locally cooled part drops rapidly, the tensile stress gradually increases. Then, when the temperature of the locally cooled part drops below the yield temperature T1 (the temperature when the tensile stress reaches the yield stress), the tensile stress becomes almost constant (σ Y) Tensile plastic strain occurs in the locally cooled area, and this plastic strain gradually increases. When the metal member 3 is removed from the treatment object 2 and cooling is stopped, the locally cooled area returns to room temperature. In the process of returning to room temperature, the locally cooled area thermally expands and the tensile stress decreases, as shown in the graph in Figure 4. Because tensile plastic strain has occurred, the tensile stress disappears at temperatures lower than room temperature. Therefore, even after the tensile stress disappears, the locally cooled area continues to expand thermally as the temperature of the locally cooled area increases to room temperature, and compressive stress occurs between the locally cooled area and other parts. This compressive stress increases until it reaches the yield stress. A compressive stress equivalent to this yield stress remains in the treatment object 2 as residual stress. Furthermore, if the locally cooled portion returns to room temperature before the compressive stress reaches the yield stress, the compressive stress generated before the temperature returns to room temperature remains in the treatment object 2 as residual stress. By performing localized rapid cooling of the processing object 2 and heating to room temperature in such a temperature cycle, compressive residual stress can be generated in the processing object 2.

[0021] The tensile residual stress that the processing object 2 had before cooling is removed or offset by the compressive residual stress that occurs due to the above-mentioned temperature cycle. Therefore, the residual stress reduction method of the present embodiment can remove or reduce the tensile residual stress of the processing object 2. The cooling time of the treatment object 2 by the metal member 3 can be from 0.1 seconds to 1 minute, and preferably from 0.5 seconds to 30 seconds. This increases the temperature difference between the locally cooled portion and the other portions, and allows the locally cooled portion to be cooled to a lower temperature. By bringing the metal member 3 into contact with the treatment object 2, the locally cooled portion can be cooled to a temperature lower than the temperature at which tensile plastic strain occurs in the locally cooled portion. This allows compressive residual stress to be generated in the locally cooled portion after the above-mentioned temperature cycle.

[0022] The contact structure 17 is a structure provided so as to bring the metal member 3 into direct or indirect contact with a part of the surface of the object 2 to be treated while changing the part of the surface of the metal member 3 that comes into contact with the object 2 to be treated. 3 is a rotating structure 4 including a motor 5, gears 6a and 6b, and a belt 7. The metal member 3 has a columnar or cylindrical shape and is provided so that a part of the outer circumferential surface of the metal member 3 can be brought into contact with the treatment object 2 directly or indirectly. For example, localized cooling of the treatment object 2 having tensile residual stress is started by bringing the outer circumferential surface of the metal member 3 shown in Fig. 3 into contact with the treatment object 2 having tensile residual stress, and the contact portion of the outer circumferential surface of the metal member 3 is changed by rotating the metal member 3 using a rotating structure 4. Then, the outer circumferential surface of the metal member 3 is separated from the treatment object 2 to end the localized cooling and return the treatment object 2 to room temperature. By this series of operations, the tensile residual stress of the treatment object 2 can be removed or reduced.

[0023] Second embodiment Fig. 5 is a schematic cross-sectional view of a residual stress reduction device 20 according to a second embodiment. The residual stress reduction device 20 according to the second embodiment includes a plurality of metal members 3a to 3c. Although the cooling structure 12 is not shown in Fig. 5, a liquefied gas container 9 is provided as the cooling structure 12. The contact structure 17 has rails 11 provided to suspend and move the multiple metal members 3a-3c, and drop structures 10a-10c provided to drop the suspended multiple metal members 3a-3c and bring them into contact with the surface of the processing target 2. The drop structures 10a-10c are structures that drop the metal members 3a-3c by utilizing the gravity of the metal members 3a-3c. The drop structures 10a-10c may be structures that automatically drop the metal members 3a-3c, or may be structures that manually drop the metal members 3a-3c.

[0024] First, the plurality of metal members 3a to 3c are immersed in the liquefied gas in the liquefied gas container 9, and the plurality of metal members 3a to 3c are sufficiently cooled until the entirety of the plurality of metal members 3a to 3c is at or below -100°C. The cooled plurality of metal members 3a to 3c are attached to a rail 11 arranged on a processing object 2 having a tensile residual stress, and the metal member 3a arranged directly above the area to be cooled of the processing object 2 is dropped to contact the area to be cooled. This allows the thermal energy of the processing object 2 to be transferred to the metal member 3a. After a predetermined time (for example, 1 second) has elapsed, the metal member 3a is lifted, and the metal members 3a to 3c are moved using the rail 11 so that the metal member 3b is directly above the area to be cooled. Then, the metal member 3b is dropped to contact the area to be cooled, and the thermal energy of the processing object 2 is transferred to the metal member 3b. After a predetermined time (for example, 1 second) has elapsed, the metal member 3b is lifted, and the metal members 3a to 3c are moved using the rail 11 so that the metal member 3c is directly above the area to be cooled. Then, the metal member 3c is dropped and brought into contact with the area to be cooled, and the thermal energy of the treatment object 2 is transferred to the metal member 3c. In this way, by cooling the area to be cooled of the treatment object 2 while changing the metal members 3a to 3c, it is possible to rapidly lower the temperature of the area to be cooled to a temperature lower than the temperature at which tensile plastic strain occurs in the treatment object 2. In addition, the number of metal members 3a to 3c can be appropriately changed depending on the cooling temperature of the area to be cooled and the size of the metal members 3a to 3c. Thereafter, the local cooling is terminated by separating the metal member 3c from the processing object 2, and the processing object 2 is returned to room temperature. Through such a series of operations, the tensile residual stress in the processing object 2 can be removed or reduced.

[0025] Other configurations are the same as those of the first embodiment. Furthermore, the description of the first embodiment also applies to the second embodiment unless there is a contradiction.

[0026] Third embodiment Fig. 6 is a schematic cross-sectional view of a residual stress relief device 20 according to a third embodiment. The residual stress relief device 20 according to the third embodiment includes a plurality of granular metal members 3. Although the cooling structure 12 is not shown in Fig. 6, a liquefied gas container 9 is provided as the cooling structure 12. The particle size of the plurality of granular metal members 3 may be, for example, 10 μm or more and 10 cm or less. The shape of the granular metal members 3 may be spherical.

[0027] The contact structure 17 has a duct 16 provided to transport a plurality of granular metal members 3. The duct 16 has an inlet, an outlet, and an opening provided in the pipe wall, and the duct 16 and the object to be treated 2 are connected so that the area to be cooled of the object to be treated 2 closes this opening. The duct 16 is also provided so that the granular metal members 3 flowing inside the duct 16 come into contact with the area to be cooled at this opening, and then flow downstream of the duct 16. The transportation of the metal members 3 in the duct 16 may utilize gravity or a gas flow. For example, the metal members 3 may be sucked together with the gas from the outlet of the duct 16. The material of the duct 16 may be, for example, plastic.

[0028] First, a plurality of granular metal members 3 are immersed in the liquefied gas in the liquefied gas container 9, and are sufficiently cooled until the plurality of granular metal members 3 are cooled to -100°C or less. The duct 16 is connected to the treatment object 2 so that the opening of the duct 16 is blocked in the area to be cooled of the treatment object 2 having tensile residual stress. Then, the plurality of granular metal members 3 that have been sufficiently cooled are poured into the duct 16 from the inlet, and the plurality of granular metal members 3 are caused to flow in the duct 16. The granular metal members 3 that reach the opening of the pipe wall come into contact with the area to be cooled of the treatment object 2, and the thermal energy of the treatment object 2 is transferred to the granular metal members 3. The metal members 3 that have received the thermal energy flow to the outlet of the duct 16, and are collected by a collection unit (e.g., a collection container). By continuously flowing a large number of granular metal members 3 through duct 16, the thermal energy of the area to be cooled of the object to be treated 2 can be continuously transferred to the metal members 3, and the temperature of the area to be cooled can be rapidly reduced to a temperature lower than the temperature at which tensile plastic strain occurs in the object to be treated 2. Thereafter, the supply of the metal member 3 to the inlet of the duct 16 is stopped to terminate the local cooling and return the treatment object 2 to room temperature. Through such a series of operations, the tensile residual stress in the treatment object 2 can be removed or reduced.

[0029] Other configurations are the same as those of the first or second embodiment. Furthermore, the description of the first or second embodiment also applies to the third embodiment unless there is a contradiction.

[0030] FEM thermoelastic-plastic analysis Fig. 7 shows an analytical model for the thermo-elastic-plastic analysis, and Fig. 8 is a schematic cross-sectional view of the analytical model taken along dashed line BB in Fig. 7. The material of the object 2 to be treated was SM490, a rolled steel material for welded structures, and the dimensions of the object 2 to be treated were 150 mm in width, 300 mm in length, and 15 mm in thickness. A thermo-elastic-plastic analysis was also carried out in which arc welding was performed on the welded portion 13 with a current of 400 A, a voltage of 30 V, and a welding speed of 20 mm / sec, and the object 2 to be treated having residual stress resulting from this analysis was used as the analytical model. The welded portion 13 is a central region of the object 2 to be treated, and extends in the longitudinal direction of the object 2 to be treated. The material of the metal member 3 was pure copper, and the dimensions of the metal member 3 were 40 mm in width, 40 mm in length, and 80 mm in thickness.

[0031] An analysis was performed in which the underside (40 mm x 40 mm) of a metal member 3 (cooled by liquid nitrogen) at -196°C was brought into contact with the center (welded portion 13) of the upper surface of a processing object 2 at 20°C for one second to cool the processing object 2, and then the processing object 2 was returned to 20°C (repeated once). In this analysis, the thermal energy of the processing object 2 was transferred to the metal member 3, causing the temperature of the metal member 3 to rise, and therefore the cooling efficiency of the processing object 2 gradually decreased. In addition, an analysis was performed in which the bottom surface (40 mm x 40 mm) of a metal member 3 at -196°C was brought into contact with the center (welded portion 13) of the top surface of a treatment object 2 at 20°C, and the treatment object 2 was cooled for 1 second while the metal member 3 was continuously replaced with a new metal member 3 at -196°C (replaced ∞ times per second), and then the treatment object 2 was returned to 20°C (repeated number of times: ∞). In this analysis, the temperature of the metal member 3 was always -196°C, and the treatment object 2 could be rapidly cooled to a lower temperature.

[0032] 9 to 11 are graphs showing the analysis results. The vertical axis in Figs. 9 to 11 is the position coordinate of the processing object 2 in the Z direction shown by the dashed line CC in Fig. 8. In this position coordinate, the contact surface between the processing object 2 and the metal member 3 is 0 mm, and the bottom surface of the processing object 2 is -15 mm. Fig. 9 shows the temperature distribution, Fig. 10 shows the distribution of residual plastic strain (strain from the processing object 2 before performing the analysis of welding), and Fig. 11 shows the residual stress distribution. In Fig. 11, positive residual stress is tensile residual stress, and negative residual stress is compressive residual stress. As shown in the temperature distribution in Figure 9, in an analysis where the number of repetitions was 1, the contact surface of the treatment object 2 could be cooled to approximately -70°C in 1 second. In an analysis where the number of repetitions was infinite, the contact surface of the treatment object 2 could be cooled to -196°C.

[0033] As shown in the distribution of residual plastic strain in Figure 10, the treatment object 2 had a residual plastic strain of approximately -0.002 at the welded part 13 before cooling, but in an analysis with one repetition, the residual plastic strain approached 0, and in an analysis with an infinite number of repetitions, the residual plastic strain approached 0 even more. In this way, it was found that the plastic strain caused by welding can be reduced by locally cooling the treatment object 2 with the metal member 3.

[0034] As shown in the residual stress distribution in Figure 11, in the treatment object 2 before cooling, tensile residual stress occurred over a wide range from the surface to the inside due to welding, but in an analysis in which the treatment object 2 was locally cooled by the metal member 3 with a single repetition, it was possible to make the residual stress compressive at the contact surface and to reduce the tensile residual stress also inside the treatment object 2. Furthermore, in an analysis in which the treatment object 2 was locally cooled by the metal member 3 with an infinite number of repetitions, it was possible to further reduce the tensile residual stress inside the treatment object 2. It has been found that by locally cooling the processing object 2 using the metal member 3 in this manner, the tensile residual stress caused by welding can be reduced.

[0035] Residual stress reduction device manufacturing experiment A residual stress reduction device as shown in Fig. 3 was fabricated. Fig. 12 is a photograph of the fabricated residual stress reduction device, and Fig. 13 is an exploded view showing the internal structure of the residual stress reduction device. The metal member 3 is cylindrical in shape and is made of pure copper. The liquefied gas is liquid nitrogen. With the fabricated residual stress reduction device, the cylindrical metal member 3 cooled by liquid nitrogen is rotated by a motor 5, thereby enabling rapid localized cooling of a portion of the surface of the treatment object 2 while changing the portion of the outer circumferential surface of the metal member 3 that comes into contact with the treatment object 2.

[0036] Residual stress reduction test An FEM thermal elastic-plastic analysis was performed using an analytical model as shown in Figure 14(a). The material of the processing object 2 was SM490, a rolled steel material for welded structures, and the dimensions of the processing object 2 were 200 mm wide, 200 mm long, and 10 mm thick. The material of the cylindrical metal member 3 was pure copper, and the diameter and height of the metal member 3 were 30 mm and 20 mm. The temperature and strain of the processing object 2 were analyzed when the bottom surface (diameter 30 mm) of the metal member 3 (cooled by liquid nitrogen) at -196°C was brought into contact with the center of the top surface of the processing object 2 at 20°C for 8 seconds to locally cool the processing object 2. The temperature and strain measurement points are shown in Figure 14(a).

[0037] The temperature and strain of the treatment object 2 were measured when the underside of a cylindrical piece of pure copper (diameter 30 mm, height 20 mm) (metal member 3) that had been sufficiently cooled with liquid nitrogen was brought into contact with the center of the top surface of room temperature SM490 rolled steel for welded structures (width 200 mm, length 200 mm, thickness 10 mm) (treatment object 2) to locally cool the treatment object 2. The temperature and strain measurement points were set at the same positions as in Figure 14(a). Figure 14(b) is a photograph of the measurement device.

[0038] A cylinder 15 (inner diameter 30 mm) for holding liquid nitrogen was placed in the center of the top surface of room temperature welded structural rolled steel SM490 (width 200 mm, length 200 mm, thickness 10 mm) (processing object 2), and the temperature of the processing object 2 was measured when the processing object 2 was locally cooled by the liquid nitrogen poured into the cylinder. The depth from the contact surface between the processing object 2 and the liquid nitrogen to the liquid nitrogen surface was kept at 20 mm. The temperature measurement points were set up in the same positions as in Figure 14(a). Figure 14(c) is a photograph of the measurement device.

[0039] The analysis results and the measurement results are shown in Figures 15 and 16. Figure 15 is a graph showing the temperature change of the treatment object 2. In Figures 15 and 16, the time when the metal member 3 or liquid nitrogen is brought into contact with the treatment object 2 is set as 0 seconds. As shown in the graph of Figure 15, when liquid nitrogen was brought into contact with the object to be treated 2 for 1 second, the measured temperature hardly changed, whereas when pure copper was brought into contact with the object to be treated 2 (experiment) and in the FEM analysis, when pure copper was brought into contact with the object to be treated 2, the temperature of the object to be treated 2 decreased in the same manner, reaching approximately -8°C 1 second after contact. It was found that the object 2 can be locally quenched by contacting the metal member 3 with the object 2. Furthermore, the object 2 could not be quenched with liquid nitrogen. This is believed to be because the heat capacity of liquid nitrogen is small and because nitrogen gas is generated between the liquid nitrogen and the object 2.

[0040] Fig. 16 is a graph showing the change in strain in the X-axis direction. As shown in the graph in Fig. 16, it was found that the strain (strain measured in the experiment) generated in the treatment object 2 by bringing cooled pure copper into contact with the treatment object 2 changes in the same way as in the FEM analysis. [Explanation of symbols]

[0041] 2: Processing object 3, 3a to 3c: Metal parts 4: Rotating structure 5: Motor 6a, 6b: Gears 7: Belt 8: Liquefied gas 9: Liquefied gas container 10a to 10c: Drop structure 11: Rail 12: Cooling structure 13: Welded part 15: Liquid nitrogen cylinder 16: Duct 17: Contact structure 20: Residual stress reduction device

Claims

1. A cooling step of directly or indirectly contacting a part of a surface of at least one metal member cooled to a temperature of −100° C. or lower with a part of a surface of the treatment object having tensile residual stress, thereby locally cooling the treatment object; The object to be treated is returned to room temperature, The cooling step is a step of locally cooling a part of the surface of the object to be treated while changing a part of the surface of the at least one metal member that is in contact with the object to be treated, and includes a step of contacting a second part of the surface of the at least one metal member, which is different from the first part, with a contact part that is in direct or indirect contact with a first part of the surface of the at least one metal member, A residual stress reducing method, characterized in that the temperature of the treatment object before cooling in the cooling step is room temperature.

2. The material of the metal member is a metal having a thermal conductivity of 100 W / (m·K) or more and a thermal conductivity of 2.0 J / (K·cm 3 2. The method for reducing residual stress according to claim 1, wherein the metal has a heat capacity per unit volume (specific heat x density) of at least 100000000000.

3. The cooling time in the cooling step is 0.1 seconds or more and 1 minute or less, 3. The residual stress reducing method according to claim 1, wherein the cooling step is a step of lowering a temperature of a part of the object to be treated to a temperature lower than a temperature at which tensile plastic strain occurs in the object to be treated.

4. The cooling structure is provided to cool the at least one metal member by a liquefied gas at -100°C or less, and the contact structure is provided to locally cool the treatment object at room temperature by directly or indirectly contacting the at least one metal member with a part of the surface of the treatment object while changing a part of the surface of the at least one metal member that contacts the treatment object, The contact structure is a residual stress reduction device configured to contact a second portion, different from the first portion of the surface of the at least one metal member, with a contact portion that directly or indirectly contacts a first portion of the surface of the at least one metal member.

5. The metal member has a columnar or cylindrical shape, the contact structure is a structure provided to rotate the metal member, 5. The residual stress reducing device according to claim 4, wherein the metal member is provided so that a part of an outer circumferential surface of the metal member directly or indirectly contacts a part of a surface of the processing object.

6. the at least one metal member includes a first metal member and a second metal member; 5. The residual stress reduction device according to claim 4, wherein the contact structure comprises a rail arranged to suspend and move the first and second metal members, and a drop structure arranged to drop the suspended first or second metal member into contact with the surface of the object to be treated.

7. the at least one metal member includes a plurality of granular metal members; 5. The residual stress reducing apparatus according to claim 4, wherein the contact structure is provided so as to continuously bring a plurality of granular metal members into contact with the object to be treated.

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