JPH02187275A - Manufacture of joint for piping of different materials - Google Patents

Abstract

PURPOSE:To prevent the corrosion resistance of a welding joint from lowering and to reduce the residual stress by welding a part obtained by previous solid phase joining of a stainless steel pipe and a carbon pipe to a steel pipe of the material side of the respective same kind. CONSTITUTION:A stainless steel pipe 2 and a carbon steel pipe 1 or a low alloy steel pipe 1 are connected for piping. Then, the parts obtained by joining these steel pipes 1, 2 previously through solid phase joining are connected respectively to the steel pipes on the sides of the same kinds of material by welding. Steel pipes are joined by solid phase joining due to previous hot isostatic pressing to different materials and parts restraining generation of different phases due to the mutual diffusion of both different metals on the joining surface are welded respectively to the steel pipes on the sides of the same kinds of material. Solid phase welding is carried out by hot isostatic pressing joining under the interpolation of intermediate material 3 such as Fe, Ni, or Ni alloy having a thickness exceeding 50mum. Thus, a peeling defect can prevented from occurring in the rolling laminated structure.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to the structure of a dissimilar-metal joining pipe of a stainless steel pipe and a carbon steel pipe or a low-alloy steel pipe for high-temperature water piping in nuclear power generation equipment.

[Conventional technology] As described in JP-A-62-144883, conventional dissimilar metal joints are made by interposing Ni foil or high Ni alloy foil between a stainless steel plate and carbon steel, etc. After rolling, a dissimilar material part is cut out in the thickness direction to form a tubular shape. The parts are interposed between stainless steel piping and carbon steel piping and welded with the same material.

In addition, as related to this type, for example, Japanese Patent Application Laid-open No. 6
No. 1-99590 discloses a technique for increasing the joint strength by overlapping dissimilar material joints in the tube axis direction to increase the joint area.
This method involves joining by hot isostatic pressing. In this method, the joint has a slope from the center of the shaft,
Moreover, this part has a thick-walled double-tube structure. This is to compensate for the weak strength of the joint and to lengthen the joint boundary to improve seal accuracy.

[Problem to be solved by the invention]

In the above-mentioned prior art, the rolled structure of the cross section of the plate is exposed to a high-temperature water environment in order to make a joint component by cutting out a joint plate stacked in a plate shape into a tubular shape in the plate thickness direction. In this rolled structure, inclusions such as impurities in the steel are elongated by rolling and exist in a layered structure, resulting in a decrease in corrosion resistance and strength in a hot water environment as a joint part. In addition, dissimilar metal joint parts are ultimately co-metallically welded to the piping system, but conventional parts cut out in the plate thickness direction are joints where the maximum welding stress acts in the plate thickness direction, so the rolled layer in the weld heat affected zone There is a great risk of peeling cracks occurring in the structure, and no consideration is given to preventing this cracking. In addition, when dissimilar metal joints are joined at high temperatures, a decarburized layer is formed on the carbon steel side of the joint boundary, a carburized layer is formed on the stainless steel side, and various foreign phases such as chromium carbide are generated, which impairs the corrosion resistance and strength properties of this part. cause a decline.

This problem was prevented by using an appropriate intermediate material, but it was not perfect. Furthermore, residual stress is generated at this boundary after joining due to the difference in linear expansion coefficient between the two metals. In the worst case, this residual stress and the different phase of the joint may cause stress corrosion cracking (SCC) to occur in a high-temperature water environment. This residual stress varies depending on the joining method, the material and shape of the intermediate material, but these factors are not taken into account. Furthermore, parts used in a limited space such as a nuclear power plant must be made as small as possible, and a joint with a double pipe joint is inappropriate. Conventional dissimilar material joining techniques have not taken this intended use into account, and have limited applicability.

The purpose of the present invention is to prevent the formation of different phases and reduce the generation of residual stress at the joint of dissimilar materials between carbon steel piping and stainless steel piping.
The purpose is to prevent SCC at joints of dissimilar materials, and it is also used in cold water installations in nuclear facilities, etc.

The purpose of the present invention is to provide suitable dissimilar material joint piping parts for piping systems that are used repeatedly at various temperatures such as hot water and high-temperature water.

[Means to solve the problem]

In order to achieve the above purpose, carbon steel pipes and stainless steel pipes of the same shape are used.

The end faces in the axial direction are butted and joined by hot isostatic pressing (HIP). In this case, depending on the carbon content of the carbon steel pipe, an intermediate material such as Fe, Ni, or Ni alloy is interposed to join the pipes. If no intermediate material is used, carbon i
The carbon content of the bolt is less than 0.05% or less than the carbon content of stainless steel. When an intermediate material is used, the carbon content of the intermediate material must also be within the above-mentioned value. Further, the thickness of this intermediate material is preferably 20 mm or more, and the maximum thickness is preferably 30% or less of the carbon steel pipe wall thickness. Further, hot isostatic pressurization during bonding is performed at an appropriate temperature of 600° C. or higher at the start of heating, but during cooling, pressurization is continued until cooling to substantially room temperature. This method prevents foreign phases from occurring at the joint, reduces residual stress at the joint, and prevents corrosion, SCC, etc. in high-temperature water.

[Effect]

First, as shown in FIG. 1, a carbon steel pipe 1 and a stainless steel pipe 2 are butted together. An intermediate material is interposed in the joint portion 3 as necessary. It is set in a vacuum container 4 for performing diffusion bonding by isostatic pressure. The intermediate material uses Fe, Ni, and Ni alloy. It is desirable that this intermediate material has fewer impurities than the steel pipes to be joined.

In particular, the amount of carbon must be strictly regulated and must be less than 0.05%.
The carbon content is always lower than that of stainless steel pipes. The purpose of the intermediate material is to prevent carburization from the carbon steel side to the stainless steel side as the joining temperature increases, and to reduce the generation of residual stress at the joint.

The thickness of this intermediate material varies depending on the bonding temperature and time, and is approximately 30 to 40μ when the bonding temperature is 700℃ and the bonding time is 5 minutes.
Since the decarburization and carburization width is 50 μm, a minimum width of 50 μm is required.

Since this intermediate material is a low carbon material, it has low strength and generates small residual stress, but if the thickness of this intermediate material is thick, the joint strength will decrease. Therefore, the intermediate material strength is 80% of the steel pipe strength.
In this case, the thickness of the intermediate material is 30% or less of the steel pipe wall thickness, and if the strength is the same, there is no restriction in principle, but it is not necessary to increase the thickness from an economical point of view.

The bonding conditions are determined by the bonding temperature and bonding time9 isostatic pressure (hereinafter referred to as pressure), but in general, the bonding temperature is 700 to 1050°C, and the bonding time (time to maintain the bonding temperature) is 5 minutes to 5 minutes. 3 hours, pressurizing force is 3 to 20 kgf/a
It is #. Pressurization is preferably carried out at a temperature at which the vacuum container is softened by heating and has no substantial strength, and for carbon steel containers, a temperature of 650° C. or higher is preferable, but there is no limitation. This pressure is applied after joining.
It must be kept until cooled to room temperature. When the pressure is removed at high temperature, the residual stress increases.

After the joining is completed, the vacuum container is removed and the specified joint parts are processed. Figure 2 shows the procedure for welding the joint parts to the piping.

Dissimilar metal joint parts are welded together at the respective welds.

The distance Q from the dissimilar metal joining boundary to the welded part is 50 μm or more,
Alternatively, a diameter greater than or equal to the tube diameter is required. This is to avoid the risk of peeling due to stress generated due to differences in linear expansion coefficients due to the high temperature of the joint when welding with the same metal. 7. Joints should not be heated to temperatures above 500°C. .

Furthermore, the joint shown in FIG. 1, which is a double-pipe part that simultaneously joins Ni inside the pipe and the Ni alloy pipe, is effective in preventing corrosion of the joined parts of the materials to be joined. This is effective in preventing corrosion caused by minute defects on the surface that cannot be detected by non-destructive inspection at the joining boundaries of the materials to be joined.

〔Example〕

The present invention will be explained in detail below.

<Example 1> Figure 3 shows overlay welding on the end face of a carbon steel pipe by covered arc welding using a low carbon steel welding rod, and the joint surface is machined flat and aligned with the end face of a stainless steel II pipe 5US304. This is a joint type in which the bonding material is placed inside a vacuum container and the containers are joined by hot isostatic pressure. The outer diameter of each tube is 1
14am, and the wall thickness is 11++n+. The length of the tube is 12
0 + m. The carbon content of carbon steel pipe is 0.18%, that of weld metal is 0.048%, and that of stainless steel pipe is 0.
．． It is 063%. Three layers of overlay welding were performed, and the thickness of the overlay layer after zero machining was approximately 7 mm. Substantially no dilution of carbon from the carbon steel was made at the overlay joint surface. The bonding conditions by hot isostatic pressing are bonding temperature 850℃, bonding time -
The test was carried out at a pressure of 4 kgf/aJ.

After bonding, the hardness distribution at the bond boundary was measured. There is no local increase in hardness at the boundary, and no decarburization or carburization layer occurs. The fractured part of this joint in the tensile test was on the carbon steel side and not at the joint interface. Also, the maximum residual stress on the inner surface of this joint is a tensile stress of 12 kgf/mm.The position where this maximum stress is exhibited is on the stainless steel pipe side about 10 degrees from the joint boundary.The joint boundary rather has a compressive residual stress. show.

<Example 2> In the same manner as in Example 1, Ni alloy was overlay-welded on the end face of a stainless steel pipe 5US304. This Ni alloy is 0,0
3%C275%Ni, 16%Cr.

The main component is 8% Fe. The bonding conditions are also the same as in Example 1. There was no decarburization or carburization of the joint, and the maximum residual stress was almost the same as in Example 1.

<Example 3> In the same manner as in Example 1, overlay welding 8 was performed on the end face of a stainless steel pipe 5US304 using an industrial pure Ni welding material using the TIG welding method. This Ni welding material contains 0.02% C,
It consists of 1% or less of Mn, Fe, Cu, A(l) and impurities. The bonding conditions are also the same as in Example 1. The results are also the same.

As described above, overlay welding is applied to the joints of carbon steel pipes or stainless steel pipes, and the joints are joined by hot isostatic pressing.

In both cases, the purpose of the present invention, which is to prevent the formation of foreign phases and reduce residual stress, was simultaneously achieved. In the example, this overlay welding was performed only on the carbon steel side or the stainless steel side, but it is determined that it is also possible to overlay both sides and join the end faces of the overlay parts.

<Example 4> An industrial pure Fe plate with a thickness of 2 m was used as an intermediate material in the joint type shown in FIG. This carbon content is 0.02%.

The chemical composition and shape of the carbon steel pipe and stainless steel pipe are the same as in Example 1. The bonding conditions are a bonding temperature of 1100℃,
The bonding time was 30 minutes and the pressure was 12 kgf/d. After joining, there is no hardened portion at the boundary even in the measurement results of the hardness distribution of this cross section. The maximum residual stress at the joint is approximately 20+u++ from the boundary.
It was the + stainless steel side. In the figure, 5 is a deaeration pipe.

<Example 5> The intermediate material was made by changing the intermediate material in the same manner as in Example 4, and the main component was Ni composite plywood with a carbon content of 0.04%.

75% Ni, 16% Cr, 8% Fe. There is no hardened portion in the hardness distribution measurement results of this cross section after bonding. The maximum residual stress is approximately 4511I11 from the boundary on the stainless steel side.

<Example 6> The Ni composite plywood of Example 5 was changed to industrial pure Ni. The carbon content is 0.015%. The results are similar to those in Example 5.

<Example 7> The dissimilar metal joint parts up to Example 6 were connected to test piping of a high-temperature water corrosion test device by mutual metal welding, and a corrosion test was conducted.

This welding was covered arc welding, and first, the stainless steel side was completely welded, and then the carbon steel side was welded. After welding, we conducted a dye flaw detection test and an X-ray inspection of the welded area, including the joint of dissimilar materials, but cracks were found.

No peeling defects are detected. A corrosion test was conducted by circulating pure water containing 8 ppm of oxygen through this piping. The maximum temperature of the circulating water is 180°C, and the minimum temperature is 15°C. This temperature cycle is 48 hours. The trial lasted for six months. Thereafter, the dissimilar metal joint was removed from the test equipment and the corrosion status of the joint surface was inspected, but there was no abnormal corrosion on the boundary or stainless steel side. Although a slight oxide film and pit-like corrosion were observed on the carbon steel side surface, there was no abnormal corrosion.

Cross-sectional inspection revealed no local corrosion at the joint boundary, and it was found that the product completely fulfilled its intended use as a nuclear power piping component. That is, the dissimilar material piping of the present invention can also be applied to thermal power plants and the like that have similar corrosive environments.

<Example 8> With the same quality and shape as in Example 5, a Ni alloy tube 9 with a wall thickness of 3 m was provided as shown in FIG. 4, and welding was performed under the joining conditions of Example 4. After bonding, N is applied in the process of removing the vacuum container 4.
The wall thickness of the i-alloy tube 9 was processed to be 0.5 strokes. The thickness of this Ni alloy tube 9 needs to be 50 μm or more in consideration of the diffusion of carbon from the materials to be joined. There is no particular limit to the maximum thickness, but considering the inner diameter of the pipe, the shape of the welding back, etc., it should be 11! It is sufficient if it is about m. In this example, Ni
Although alloy tubes are used, Ni tubes may also be used, and similar effects can be expected even if the tubes are filled with powder having the same composition as these tubes. This dissimilar material piping component was processed and brought into contact with the piping at 7ff at the welded portion 6.7, as shown in FIG. 5, and the same corrosion test as in Example 7 was conducted. As a result, no abnormal corrosion was observed, and it was found that the product exhibited excellent properties as a dissimilar metal joint component for high-temperature water, and was suitable for the intended use.

〔Effect of the invention〕

According to the present invention, the rolled layered structure on the inner and outer surfaces of the tube of the dissimilar metal joint component is not exposed to a corrosive environment, and no peeling defects occur in the rolled layered structure even when welded to the pipe. The carbon of the carbon steel does not combine with the chromium of the stainless steel to form a brittle foreign phase, and the corrosion resistance does not deteriorate. Furthermore, residual stress at the joint can be reduced.

[Brief explanation of the drawing]

Fig. 1 is a side and center sectional view of an original drawing of joining dissimilar metal joint parts according to an embodiment of the present invention, Fig. 2 is a schematic partial sectional view of welding dissimilar metal joint parts of the present invention to a piping system, and Fig. 3 The figure is a partial cross-sectional view of a dissimilar metal joint part showing the state of weld overlay in an example.
Figure 4 is a partial cross-sectional view of a tube in which a Ni tube, etc. is joined to the inner surface of the tube.
FIG. 5 is a partial sectional view of a pipe welded structure in which a double pipe is welded with a Ni pipe joined to the inner surface of the pipe. 1... Carbon steel pipe, 2... Stainless steel pipe, 3...
Intermediate material.

Claims (1)

[Claims] 1. In piping that connects stainless steel pipes and carbon steel pipes or low alloy steel pipes, the pipe is characterized in that parts of these steel pipes joined in advance by solid phase welding are connected to respective steel pipes made of the same material by welding. Dissimilar material piping joint structure. 2. In piping that connects stainless steel pipes and carbon steel pipes, these steel pipes are joined in advance by solid phase welding using hot isostatic pressure, and the generation of different phases due to mutual diffusion of the dissimilar metals at the joint interface is avoided. A dissimilar material piping structure characterized by connecting the suppressed parts to steel pipes made of the same material by welding. 3. In claim 1 or 2, Fe, N
1. A dissimilar material piping component for high-temperature water, characterized in that it is solid phase joined by hot isostatic pressure welding with one or more intervening intermediate materials such as Ni or Ni alloy plates having a thickness of 50 μm or more or weld overlays. 4. The length of one side of the dissimilar material in the pipe axis direction of the dissimilar material piping component of patent claim 3 shall be at least 50 mm from the joint interface to the outer diameter of each pipe, and the total length shall be the thickness of the intermediate material and the outer diameter of the pipe. Dissimilar material piping parts characterized in that the diameter is twice the diameter or 100 mm is added. 5. Ni and Ni alloy pipes with a wall thickness of 50 μm or more are joined to the entire inside of the pipe parts of dissimilar material piping parts according to claims 1 to 4 by hot isostatic pressure in an area that will not be melted during welding of the pipes. Dissimilar material piping joint parts with special features. 6. A piping structure made of different materials, characterized in that piping made of stainless steel is welded to carbon steel using the method described in claim 5.