JPS6154518B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for welding a tubular member made of austenitic stainless steel, which can improve quality by reducing the area affected by welding heat. Generally austenitic stainless steel and
When Ni-Cr-Fe alloys are welded, carbides precipitate in the heat-affected zone near the weld due to welding heat input, reducing corrosion resistance and toughness. This causes cracking, and this cracking is a major problem in pipe welding for nuclear reactors and other equipment. Conventionally, in order to eliminate these defects and guarantee the quality of the welded part, the interlayer temperature in multi-layer welding of alloy steel has been limited to approximately 150°C in order to reduce the weld decay area. . In the conventional metal welding method, the welded part is air-cooled with compressed air until the next weld bead is placed, or the welded part is left to cool in the atmosphere to a predetermined interlaminar temperature. However, in parts such as nuclear reactor parts that come into contact with high-temperature, high-pressure coolant (light water), carbide precipitation cannot be avoided due to the effects of heat near the weld, and stress corrosion cracking may occur. . The present invention has been made in view of the above circumstances, and its object is to provide a tubular member that improves the quality of the welded part by welding the back side of the metal welded part while cooling it with a liquid cooling medium. A welding method is provided. An outline of the configuration to which the method of the present invention is applied will be explained below with reference to the drawings. FIG. 1 shows the case of butt joint welding. First, as shown in FIG. 1, a liquid supply pipe 8c is connected to an annular tube body 8a which has a large number of holes 8b, . Place the attached liquid spray nozzle device 8. 8d is a positioning member of the liquid spray nozzle device 8. When butt joint welding of tubular members is performed, the two tubular members 1 are butted together and root pass welding 6 is performed, and then a liquid spray nozzle device 8 is placed inside the welded part 7 and a liquid supply pipe is injected from the outside. body 8c
This liquid is injected from the holes 8b, . While this cooling is being performed, the welding portion 7 is welded. Fig. 2 shows a case where two tubular members 1 and 10 with different diameters are aligned and welded with a fillet joint. First, an annular tubular body 8a is formed inside the tubular member 1, and a number of annular tubular bodies 8a are formed at predetermined intervals on the surface side of the tubular member 1. A liquid spray nozzle device 8 having holes 8b, . . . and a liquid supply pipe 8c attached to the annular pipe 8a is disposed,
After performing root pass welding, the liquid is supplied to the liquid supply pipe 8c of the liquid spray nozzle device 8,
This liquid passes through the annular tube 8a and is injected from the holes 8b, . . . to cool the back side of the welded portion. Then, both tubular members 1, 1 are cooled while cooling the back side of the welded part with the liquid.
Welding is performed on the welded portion 11 of No. 0, that is, the weld fillet portion. Next, an example of actual welding using the method of the present invention will be shown. That is, austenitic stainless steel pipes with a thickness of 16 mm were butted together with a U-shaped groove, and multilayer welding was performed using a tungsten inert gas welding method. The chemical composition of the base material is shown in Table 1, and the welding process was performed under the conditions shown in Table 2.

【table】

[Table] After root pass welding was performed by butting the pipes at the U-shaped groove as described above, water was sprayed from the liquid spray nozzle inserted inside the pipes in the second and subsequent passes, and the results were shown in Figure 3. As shown in Figure 2, the width of the heat-affected zone near the weld was small, and the precipitation of harmful carbides was very slight, resulting in a high-quality weld. On the other hand, in order to compare with the method of the present invention, tungsten welding was performed under the same conditions as described above without any rapid cooling during welding after the second pass, and the width of the heat affected zone near the weld was as shown in Figure 4. As shown, there were many very large and harmful carbide precipitates. It was also confirmed that the welded parts welded by the method of the present invention were less prone to stress corrosion cracking, and the reliability of the welded parts was extremely high. The reason for this will be explained below. In general, austenitic stainless steels have the drawback of causing stress corrosion cracking under certain conditions, as described above. This stress corrosion cracking is particularly likely to occur near the weld. By the way, various analyzes have been conducted on the causes of stress corrosion cracking, and the factors include (i) changes in material properties that make stress corrosion cracking more likely to occur, so-called sensitization of materials; (ii) The presence of stresses such as residual stresses and stresses due to external loads. Note that the stress that causes stress corrosion cracking is tensile stress. (iii) An environment prone to stress corrosion cracking. There is. When these factors overlap, stress corrosion cracking occurs. Since nuclear reactor components are exposed to high-temperature, high-pressure coolant, ie, water containing dissolved oxygen, the environment described in (iii) above is an environment where stress corrosion cracking is likely to occur. In addition, in the welded part, the sensitization of the material described in (i) above is likely to occur due to welding heat input. This sensitization of the material is due to the fact that chromium within the crystal grains of austenitic stainless steel precipitates at the grain boundaries in the form of chromium carbide, and the amount of chromium in the vicinity of the grain boundaries decreases within the grains. % or less, the corrosion resistance will be significantly reduced and the material will become more sensitive. And, in the case of austenitic stainless steel, this chromium precipitation occurs between approximately 400°C and 800°C.
Occurs in a temperature range of °C. FIG. 5 shows the results of measuring the distribution of temperature rise at each point on the inner surface at different distances from the center of the weld for each pass when welding was performed in multiple passes using the conventional method. As is clear from Fig. 5, approximately 400 mm is applied during welding within a range of approximately 30 mm from the center of the weld.
Heated above ℃. In addition, parts that reached a temperature of approximately 800°C or higher during welding will be cooled down to 400°C to 800°C by subsequent cooling.
The temperature will be ℃. Therefore, in the conventional method, chromium precipitates within a range of 30 mm on both sides of the welded part, which may cause the material to become sensitized. Furthermore, according to the conventional method, as is clear from Figure 5, the temperature rise in the vicinity of the weld increases with each pass, and with a large number of passes, the range in which the material becomes sensitive may expand. . On the other hand, the distribution of the temperature rise near the weld when welding is performed by the method of the present invention is shown in the sixth figure.
As shown in the figure. As is clear from FIG. 6, according to the method of the present invention, the range where the temperature rises above about 400°C is about 10 mm from the center of the weld. Further, according to the method of the present invention, since the cooling rate is high, even if passes are repeated, the temperature rise in the vicinity of the weld does not increase, but on the contrary becomes smaller. Therefore, even if the number of passes increases, there is no possibility that the range in which the material becomes sensitized will expand. Further, since the weld metal melted during welding solidifies in a few seconds from the molten state, residual stress is generated in the welded part. FIG. 7 shows the results of measuring the residual stress at different points from the center of the weld on the inner surface near the weld when welded using the conventional method. As is clear from FIG. 7, according to the conventional method, tensile residual stress is generated on the inner surface near the weld over a range of about 30 mm from the center of the weld. The cause of such tensile residual stress is considered to be as follows. That is, when the molten weld metal is cooled and solidified, the heat radiated to the outside is directly radiated into the air from the weld metal. On the other hand, the heat radiated inward is first transmitted through the base material or previously welded metal by conduction, and is radiated into the air from the inner surface.
However, since austenitic stainless steel has low thermal conductivity, the amount of heat radiated inward from the molten weld metal is smaller than the amount of heat radiated outward. Therefore, the outer part of the molten weld metal cools down faster, and the outer part solidifies first, followed by the inner part. Contraction occurs when molten metal solidifies, so when the inner part of the weld metal solidifies and contracts, compressive stress is generated in the outer part that has solidified first, and tensile stress is generated in the inner part. Occur. Therefore, tensile residual stress is generated on the inner surface near the weld. In contrast, according to the method of the present invention, the distribution of residual stress generated on the inner surface near the weld is as shown in Figure 8, and compressive residual stress is generated over a range of approximately 70 mm from the center of the weld. ing. In other words, since the method of the present invention brings a liquid cooling medium with a large cooling capacity into contact with the inside of the weld, the amount of heat radiated inward from the molten weld metal is extremely large, which is contrary to the conventional method. The inner part of the molten metal solidifies first, and compressive residual stress is generated in the inner part, contrary to the conventional method. Therefore, according to the conventional method, the material becomes sensitized within a range of about 30 mm from the center of the weld, and tensile residual stress is generated, which is a factor that causes stress corrosion cracking when high-temperature, high-pressure coolant comes into contact with the inner surface. All of these will overlap, increasing the probability that stress corrosion cracking will occur. On the other hand, according to the method of the present invention, the material becomes sensitized within a range of about 10 mm from the center of the weld, but
Compressive residual stress occurs on the inner surface over a range of about 70 mm. This compressive residual stress does not become a cause of stress corrosion cracking, and even if tensile stress is generated due to an external load, it cancels out this tensile stress, so it rather works to prevent stress corrosion cracking.
Therefore, according to the present invention, stress corrosion cracking can be reliably prevented. It should be noted here that according to the method of the present invention, the range in which material sensitization occurs is approximately 10 mm.
However, the range in which compressive residual stress occurs is approximately 70 mm, which is extremely large compared to the range in which material sensitization occurs, and there is sufficient margin to prevent stress corrosion cracking. This is definitely preventable. In other words, welding work is often carried out on-site under poor working conditions, and the shapes and dimensions of the parts to be welded vary widely, so these conditions can affect the range of sensitization of the material and the compressive residual stress. Considerable variation occurs in the range in which this occurs, and unless there is sufficient margin as described above, the effect of preventing stress corrosion cracking cannot be reliably guaranteed. Further, in order to obtain the above-mentioned effects, it is necessary to bring the liquid cooling medium into reliable and sufficient contact with the inside of the welded part at least until the molten weld metal solidifies and cools down to a certain extent. In the method of the present invention, a liquid spray nozzle device, that is, a liquid cooling medium jetting nozzle is provided in the tubular member to be welded, and the liquid cooling medium is jetted toward the inside of the welding part. Since the liquid cooling medium is in constant contact with the entire circumference, the quality of the welded part can be reliably guaranteed even in the case of on-site welding work under poor working conditions. Furthermore, since the method of the present invention performs welding in multiple passes,
Compressive residual stress is generated on the inner surface of the weld with each pass, and this compressive residual stress is accumulated, so it is possible to generate large compressive residual stress over a wide range, ensuring the prevention of stress corrosion cracking. It can be done. Note that the present invention is not limited to the above embodiments. For example, the liquid cooling medium is not limited to water, but may also be oil or other medium. As detailed above, in the method for welding a tubular member according to the present invention, when welding a tubular member made of austenitic stainless steel, first, root pass welding is performed over the entire circumference of the tubular member to be welded in gas. Next, a liquid cooling medium jetting nozzle is installed inside the tubular member to be welded, and the liquid cooling medium is jetted toward the inside of the welded part, and the welded part is welded while rapidly cooling the inside of the welded part with the liquid cooling medium. So,
The width of the heat-affected zone near the weld becomes extremely small, making it possible to obtain a high-quality weld. In addition, since there is little thermal influence from welding, the precipitation of carbides is slight, and since no tensile residual stress is generated within the weld, on the contrary, compressive residual stress is generated, the occurrence of stress corrosion cracking and intergranular corrosion cracking is extremely small. At the same time, continuous welding is possible because the interlaminar temperature does not rise above 100°C, which contributes to a significant reduction in welding work and reduces costs.

[Brief explanation of the drawing]

Figures 1 to 4 are diagrams explaining the method of the present invention. Figure 1 is a partially cutaway sectional view explaining butt joint welding, and Figure 2 shows another example of a configuration to which the method of the present invention is applied. The figures are a partially cutaway cross-sectional view to explain fillet joint welding, Fig. 3 is a diagram showing the structural change of the welded part when butt joint welding is performed using the method of the present invention, and Fig. 4 1 is a diagram showing the structural change of a welded part due to conventional butt joint welding. Furthermore, Fig. 5 shows the temperature distribution near the weld when welding is performed using the conventional method, and Fig. 6 shows the temperature distribution near the weld when welding is performed using the method of the present invention. Figure 7 shows the distribution of residual stress on the inner surface of the weld when welding is performed using the conventional method, and Figure 8 shows the residual stress distribution on the inner surface of the weld when welding is performed using the method of the present invention. It is a figure showing distribution of stress. 1...Tubular member, 6...Root pass welding, 7...
... Welded part, 8 ... Liquid spray nozzle device (liquid cooling medium ejection nozzle), 10 ... Tubular member, 11
……welded part.

Claims (1)

[Claims] 1 In a method for welding a tubular member made of austenitic stainless steel, after root pass welding is performed around the entire circumference of the tubular member to be welded in gas,
A state in which a liquid cooling medium jetting nozzle is inserted inside this tubular member and positioned at a corresponding position inside the welded part, and then liquid cooling medium is jetted from the liquid cooling medium jetting nozzle over the entire circumference inside the welded part. A method for welding a tubular member, characterized in that the tubular member to be welded is welded from the outside in multiple passes.