JPS6347557B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to welded structures capable of reducing welding residual stress through stress relief annealing treatment and improving mechanical strength such as brittle fracture strength and fatigue strength, and a method for manufacturing the same. Generally, tensile residual stress is generated in butt welds, T-joints, and fillet welds of steel structures after welding, and stress relief annealing treatment is usually performed to remove or reduce this stress. This stress relief annealing treatment is performed to reduce tensile stress and improve the structure of the weld heat-affected zone to increase strength. It is performed at a temperature at which αγ transformation occurs in normal steel (particularly preferably 600 to 600°C). 650°C) for approximately 1 hour per inch of wall thickness of the structure, and then gradually cooled to a temperature of approximately 300°C (especially
(preferably by furnace cooling) at a cooling rate of 50°C/hr or less. Materials that can be stress-relieved annealed by this treatment are carbon steel, low alloy steel, and stainless steel (high alloy steel). Since the weld metal of the same type as the material to be welded is usually deposited on the welding part, the coefficients of thermal expansion of the material to be welded and the weld metal are approximately the same. Therefore, even if stress relief annealing is applied to the weld zone (including the weld heat affected zone), tensile stress remains on the surface of the weld metal, although it is less than 10 kg/ ^mm2 . This is because heating the structure, including welds, during the stress relief annealing process
They expand thermally. When they are cooled, tensile stress remains on the surface of the weld metal restrained by the welded part. Of course, the magnitude of the tensile stress is reduced to one-third to one-fourth of that without stress-relieving annealing, but there is still a risk of brittle fracture and fatigue fracture occurring in the weld. There is sex. Such welds are also sensitive to stress corrosion cracking. An object of the present invention is to provide a welded structure in which the brittle fracture strength and fatigue strength of the welded part are improved by applying compressive stress to the surface of the welded metal, and the stress corrosion cracking resistance is improved, and a method for manufacturing the same. be. In order to achieve the above object, the present invention relates to a welded structure in which a weld metal with a large coefficient of thermal expansion and a weld metal with a small coefficient of thermal expansion are appropriately overlapped.
Compressive stress is generated on the surface of the weld metal by subjecting it to stress relief annealing treatment. More specifically, the present invention includes a weld metal welded to a base metal surface within a region defined by the welded part of a metal structure so as to be restrained by the welded part. The weld metal includes a first weld metal layer and a second weld metal layer continuous thereto, the second weld metal layer having a thermal expansion coefficient greater than that of the first weld metal layer, The welded structure and its manufacturing method are characterized in that the first welded metal layer has an open surface within the above region. According to the principle of the present invention, the first weld metal layer and the second weld metal layer should be overlapped in the thickness direction of the base metal, and the former should cover the latter on at least one side of the welded part. It is necessary to do so. Hereinafter, the present invention will be explained with reference to Examples. The present invention is applicable to various types of welding, examples of which are shown in FIGS. 1A to 1E. Figure 1A is applied to an I-shaped groove, in which a first weld with a thermal expansion coefficient of α 3 is first applied to the I-shaped groove with a base material of thermal expansion coefficient α ₁ and thickness _{t 1 separated by x 1 .} The metal 3 is welded, then the second weld metal 2 with a coefficient of thermal expansion α ₂ is welded to a height h ₁ , and finally the first weld metal 3' is welded. When the welded part thus obtained is subjected to stress relief annealing treatment, the second weld metal 2 having a large coefficient of thermal expansion expands thermally more than the first weld metal 3 during heating. When the weld is cooled, the weld metal, which has a large coefficient of thermal expansion, contracts more. Due to this contraction, the first weld metal layer receives a compressive force, and compressive stress remains on the open surface A. Similarly, Figure 1B
Figure 1 shows a K-shaped groove, Figure 1 C shows a V-shaped groove, and Figure 1 D shows a U-shaped groove.
The mold groove, FIG.
6 as the first weld metal 6, 6', 9, 12, 15,
15' and subjected to stress relief annealing treatment, compressive stress can be generated on the open surface of the first weld metal layer. The thermal expansion coefficients of the base metals 1, 4, 5, 8, 11, and 14 in FIGS. 1A to 1E are not determined by the relationship between the thermal expansion coefficients with the first and second weld metal layers. From the viewpoint of general steel materials and their welding, it is common to use materials for the base material and the first weld metal layer that are of the same type, that is, have approximately the same coefficient of thermal expansion. The magnitude of the compressive force differs depending on the magnitude of the difference in coefficient of thermal expansion between the first and second weld metals and other causes. In terms of steel materials, martensitic steel has a coefficient of thermal expansion of 11 to 13 x 10 ^-6 /°C, and austenitic steel has a coefficient of thermal expansion of 16 to 19 x 10 ^-6 /°C. In order to expect the effects of the present invention, there is little point in overlapping weld metals of the same type with only a slight difference in coefficient of thermal expansion. Practically speaking, the second weld metal layer preferably has a thermal expansion coefficient greater than that of the first weld metal layer by 1×10 ^-6 /°C or more, particularly preferably by 3×10 ^-6 /°C or more. Next, what significantly influences the generation of residual compressive stress is the cross-sectional area ratio (this may also be the ratio of the layer thicknesses) of the first and second weld metal layers stacked in the groove. . The influence of the second weld metal layer is
In order to cause this to occur on the open surface of the weld metal layer, the cross-sectional area or layer thickness of the second weld metal layer must be at least a certain amount. Although the amount varies depending on the base material, weld metal, groove shape, welding stress relief annealing treatment conditions, etc., it is possible to use the specific information presented by the present invention. do,
It is possible to determine individual area ratios. In FIGS. 1A and 1E, welded structures were fabricated using base metals and welding materials having chemical components, mechanical strengths, and coefficients of thermal expansion shown in Tables 1 and 2. All welding was covered arc welding, and the welding conditions were a preheating temperature of 150°C, an interpass temperature of 150°C, and a heat input of 18000 J/cm.

【table】

[Table] In the case of the I-type groove shown in Fig. 1A, the plate thickness t ₁ of the base material to be welded is 100 mm, the groove gap x ₁ is 25 mm, and
Using a backing metal (not shown), a first weld metal layer (martensitic) 3' is formed, then a second weld metal layer (austenitic) 2 is formed, and finally the first A weld metal layer 3 was formed. In the case of the X-shaped groove shown in Fig. 1E, the plate thickness t ₂ of the material to be welded (base material) is
100mm, and groove gap x ₃ is 5mm. First, using the second welding material 16 (austenitic material to be welded), build up on both sides by h ² /2 from the center of the butt part, and then Weld metal layer 15, 1 on both sides using 1 welding material (contentite type welding material)
5' was formed. Note that in FIGS. 1A and 1E, the structures of the first and second weld metal layers were designed to be symmetrical in the thickness direction of the base metal. The butt welded joints produced as described above were subjected to stress relief annealing treatment at 600° C. for 3 hours after welding. After that, welding material 3 and welding material 1
The residual stress on the surface of No. 5 was measured using a resistance wire strain gauge. Figure 2 shows the relationship between the stress in the weld zone in Figure 1A, the thickness _h1 of the second weld metal layer, and the thickness _t1 of the base metal, and Figure 3 shows the relationship between the stress in the weld zone in Figure 1A, and the relationship between the weld zone in Figure 1E. 2 is a graph showing the relationship between the stress at the part, the thickness _h2 of the second weld metal layer, and the thickness _t2 of the base material. From these graphs, the thickness h ₁ , h ₂ of the second weld metal layer and the thickness t ₁ of the base metal,
It can be seen that the larger the ratio of t ₂ is, the larger the compressive force on the open surface (surface) of the first weld metal layer can be. Also, according to FIGS. 2 and 3, FIG.
In the case of I-shaped groove and It can be seen that compressive stress is generated on the surface of the first weld metal. If the size of the second weld metal layer is constant, the amount (area) of the second weld metal layer is smaller in an X-shaped groove than in an I-type groove, so in order to generate compressive stress, It is necessary to increase the length of the metal layer a little. Next, a welded structure was produced using Inconel or carbon steel as the material to be welded, Inconel as the first welding material, and austenitic material as the second welding material. The chemical composition, mechanical properties and coefficient of thermal expansion of these welding materials are shown in Tables 3 and 4, respectively.

【table】

[Table] Welding was performed at room temperature with a heat input of 1500 J/m. In Figures 1A and 1B, after the second welding material (austenitic) is built up to the same height on both sides from the center of the plate, a One welding material (Inconel type) was built up on the second welding material. Here, the height of the second welding material relative to the plate thickness is 20% in Figure 1A and 20% in Figure 1E.
It was set at 40%. After welding, stress relief annealing was performed at 600°C for 3 hours, and then residual stress was measured on both the front and back sides of the plate using a resistance wire strain gauge. The measurement results are shown in Table 5.

[Table] According to Table 5, the height of the second weld material relative to the plate thickness is 20% for the I-shaped groove (Fig. 1 A) and 40% for the X-shaped groove (Fig. 1 E). This indicates that compressive residual stress is occurring. Next, a welded structure was fabricated using the materials shown in Table 1 (base metal, first welding material, second welding material) in the V-shaped groove (60 degree groove) shown in FIG. 4A. The welding method, stress relief annealing treatment method, and stress measurement were as described above. The results of this stress measurement test are shown in Figure 4B.
Shown below. According to FIG. 4B, it can be seen that compressive stress occurs when the height h ₃ of the second welding material 19 with respect to the plate thickness t ₃ of the base material 17 exceeds 20%. However, the increasing trend is not as pronounced as for FIGS. 2 and 3. This is thought to be because the cross-sectional shape of FIG. 4A is asymmetrical in the thickness direction of the base material, which causes bending stress to occur in the welded portion and reduces compressive stress. Hereinafter, similarly, using the materials shown in Table 1 (base metal, first welding material, second welding material),
Figure A, Figure 6A, Figure 7A, Figure 8A and Figure 9
A welded structure shown in Figure B was produced, and after stress-relieving annealing treatment, stress measurements were performed. In FIG. 5A, the second weld metal layer 22 is sandwiched between the first weld metal layers 21 and 23. As shown in FIG. 5B, the residual stress of this structure shows a tendency almost similar to that in FIG. 4B. In FIG. 6A, the second weld metal layer 25 is inserted only near the surface of the first weld metal layer 26. Note that the ratio of the distance d from the surface of the first weld metal layer 26 to the second weld metal layer 25 to the plate thickness t ₆ of the base material is 5%. According to FIG. 6B, which shows the residual stress of this structure, although the proportion of the second weld metal layer 25 to the total weld metal is not large,
This shows that compressive stress is sufficiently expressed. FIG. 7B is a graph showing the compressive stress obtained for the 60 degree V-shaped groove shown in FIG. 7A. Depth from the surface of the welded structure to the second weld metal layer 28
The ratio of d' to the base metal _t7 is 10%, and 29 indicates the first weld metal layer. FIG. 8B is a graph showing the compressive stress obtained with the I-shaped groove shown in FIG. 8A. In FIG. 8A, 32 is a first weld metal layer, and 31 is a second weld metal layer. The first and second weld metal layers are arranged symmetrically in the thickness direction of the base material. In Fig. 8B, the ratio of the depth d of the first weld metal layer to the base material plate thickness _t8 is 5% (〇), 10% (△),
It was set as 15% (×). According to this, compressive stress can be generated on the open surface even if the proportion of the second weld metal layer is not so large. FIG. 9B is a graph showing the compressive stress obtained with the welded structure of FIG. 9A. The welded structure shown in FIG. 9A is asymmetrical in the thickness direction of the base material, and therefore the degree of compressive stress generated is smaller than that of other symmetrical structures. In addition, in FIG. 9B, the ratio of the depth d of the second weld metal layer 34 to the plate thickness t ₉ of the base material 33 is 10% (〇), 20% (△), and 30% (x). . Taking the above embodiments together, a symmetrical weld is more effective in applying compressive stress, and a symmetrical weld is usually advantageous for reducing bending deformation in the weld. In the above embodiments, I-type, K-type, V-type, U-type, and It can also be applied to fillet welding. Further, in the embodiments of the present invention, two types of weld metals are used, but it is obvious that three or more types of weld metals may be combined. The welded structure according to the present invention can be applied to water turbine runners, nuclear reactor pressure vessels, various thick-walled pipes, and the like. As described above, according to the present invention, it is possible to apply compressive stress to the surface of the weld metal, improve the fracture strength and fatigue strength of the weld, and prevent stress corrosion cracking.

[Brief explanation of the drawing]

1A to 1E are partial sectional views of welded parts showing examples of joints to which the present invention is applied, and FIGS. 2 and 3 are weld metals in an I-shaped groove and an X-shaped groove. Graphs showing the relationship between surface residual stress and the cross-sectional shape of the welded part; FIGS. 4A to 9A are partial cross-sectional views of welded parts showing other examples of joints to which the present invention is applied; Figures B to 9B are graphs showing the relationship between residual stress and the cross-sectional shape of the welded parts corresponding to the various welded parts shown in Figures 4A to 9A. 1, 4, 5, 8, 11, 14, 17, 20, 2
4, 27, 30, 33... material to be welded (base metal), 3,
6,9,12,15,18,21,23,26,
29, 32, 35...first weld metal layer, 2, 7,
10, 13, 16, 19, 22, 25, 28, 3
1, 34...Second weld metal layer.

Claims (1)

[Scope of Claims] 1. In a metal structure having a weld metal welded to a base metal surface so as to be restrained by the welded part within a region defined by the welded part, The weld metal includes a first weld metal layer and a second weld metal layer continuous therewith, the second weld metal layer having a coefficient of thermal expansion greater than that of the first weld metal layer, and A welded structure, wherein the weld metal layer has an open surface within the region. 2. The welded structure according to claim 1, wherein the first weld metal layer has a coefficient of thermal expansion smaller than that of the second weld metal layer by 1×10 ^-6 /°C or more. 3. The welded structure according to claim 1, wherein the welded portion has a butt structure. 4 The butt structure is I type, U type, V type, K type,
The welded part structure according to claim 3, characterized in that it is X-shaped or Y-shaped. 5. The welded part structure according to claim 1, wherein the welded part includes at least one of low alloy steel, carbon steel, and high alloy steel. 6. The welded part structure according to claim 1, wherein the first weld metal is made of substantially the same material as the part to be welded. 7. Within the groove formed in the welded part, so that the first weld metal layer has an open surface at the welded part,
A first weld metal layer and a second weld metal layer are formed in series, the second weld metal layer has a larger coefficient of thermal expansion than the first weld metal layer, and is subjected to stress relief annealing treatment. A method for manufacturing a welded structure characterized by applying compressive stress to the surface of the welded metal. 8. The manufacturing method according to claim 7, wherein the stress annealing treatment is performed by slowly cooling the material constituting the welded portion after maintaining the material at a transformation point.