JPS61114097A - Heat transfer pipe with excellent resistance to intercrystalline damage and manufacture thereof - Google Patents

Abstract

PURPOSE:To improve the resistance to intercrystalline damage by a method wherein heat transfer pipe for steam generator employed in pressurized water reactor is made of Ni-base alloy containing the specified amount of Cr and Ni and constituted so as to give residual compressive stress to at least some portion of its outer surface layer part. CONSTITUTION:Heat transfer pipe for steam generator is made of Ni-base alloy containing 14-35% by weight of Cr and not least than 30% by weight of Ni and constituted so as to give residual compressive stress to some portion of its outer surface layer part. Concretely, in order to give residual compressive stress to some portion of the outer surface layer part of Ni-base alloy pipe, said portion is held within the temperature range of 300-600 deg.C for no less one minute by heating from the inside surface while being cooled by water from the outside surface and, after that, the heating is stopped. Or, residual compressive stress is given to the surface layer part by holding the Ni-base alloy pipe within the temperature range of 600-800 deg.C for no less than 0.5min in order to precipitate chrome carbide on the boundaries between the crystal grains and, after that, cooling with water.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention has excellent resistance to intergranular damage, that is, resistance to intergranular corrosion and intergranular stress corrosion cracking, and is suitable for use in, for example, pressurized water nuclear reactors. The present invention relates to a heat exchanger tube for a steam generator that exhibits extremely good performance when applied, and a method for manufacturing the same.

<Conventional technology and its problems> In recent years, in order to cope with the ever-increasing energy demand and to reduce dependence on oil for energy, the installation of nuclear power plants has been strongly promoted, and the number of nuclear power plants in operation has been increasing year by year. There is.

By the way, heat transfer tubes for steam generators of pressurized water nuclear reactors currently in operation have a heat exchanger tube of 14 to 3, considering heat resistance and corrosion resistance.
Alloy materials containing 5% by weight of C and 30% by weight or more of Ni, especially as-annealed Attoy600 (75N
i-15Cr-9Fe alloy) material is often used,
Recently, from inside and outside Japan, [Ni 160 like this]
Occurrence of intergranular damage (intergranular corrosion, intergranular stress corrosion cracking, etc.) on the outer surface of the heat exchanger tube in the tube support section and secondary side of the tube plate of a steam generator using a heat exchanger tube made of 0 alloy. Since it has been pointed out that the occurrence of grain boundary damage has been observed, steam oxidation that does not cause these grain boundary damage.

There was a strong desire for not only the realization of heat exchanger tubes for raw materials, but also ``measures to prevent the occurrence and propagation of grain boundary damage'' that could be applied to existing plants with actual furnaces.

1. Means for solving the problems> From the above-mentioned viewpoints, the present inventors have solved the above-mentioned problems found in conventional heat transfer tubes for steam generators such as pressurized water reactors, and Even during use, “grain boundary damage”, especially “
As a result of our research to provide a heat exchanger tube for steam generators that does not cause the progression of intergranular stress corrosion cracking, we have found the following (a)
) to (e). That is, (a) Among "grain boundary damage,""intergranularcorrosion" has a small stress dependence, but "intergranular stress corrosion cracking"
has extremely high stress dependence, and when tensile stress acts, its magnitude increases and the rate of growth of stress corrosion cracking accelerates.However, under compressive stress, corrosion cracking will occur no matter how severe the corrosive environment is. (b) A normal pipe material is heated from the inside with an electric heater or high-frequency heating device while cooling the outside with water to a specific temperature (
Either stop the heating after reaching a temperature as low as 300 to 600°C, or heat the pipe material to a specific temperature,
If the outer surface of the tube is immediately cooled with water after that, it is possible to obtain a tube with compressive stress remaining in its surface layer due to the shrinkage of the outer surface of the tube due to water cooling.However, in this case, the inner surface shrinks. Because the degree is relatively small, tensile stress remains in the middle layer that follows this, but when it reaches the outer surface layer, the tensile stress is relaxed and it is subjected to further shrinkage. A relatively large compressive stress will remain.

(C) High C content as annealed with high grain boundary a scratch sensitivity
(d) In this case, if the above heat treatment is applied to the i-base alloy tube material and compressive stress remains in the surface layer of the outer surface of the tube, an extremely remarkable effect of preventing the progress of grain boundary damage can be obtained; (d) In this case, the If the heating temperature of the heat treatment for imparting residual compressive stress is adjusted (e.g., about 600 to 800°C) to precipitate chromium carbide at the grain boundaries of the pipe material, the grain boundary damage resistance thereof can be further improved.

However, the present inventors have discovered that for pipes made from high Cr-containing Ni-based alloys, a significant effect of preventing the progress of grain boundary damage can be obtained simply by subjecting them to grain boundary precipitation heat treatment of chromium carbide. In other words, as a result of detailed investigation of heat exchanger tubes with grain boundary damage and those without grain boundary damage in actual furnaces,
In heat exchanger tubes with grain boundary damage, almost no chromium carbide is observed at the grain boundaries or inside the grains, whereas in heat exchanger tubes without grain boundary damage, that is, sound tubes, the presence of chromium carbides at the grain boundaries and inside the grains is almost absent. Since the presence of chromium carbides was not observed inside the grains, it was too late to conclude that there was a correlation between grain boundary damage susceptibility and the presence or absence of chromium carbides.
It was confirmed that the development of grain boundary damage in high-Or content Ni-based alloys can be suppressed by applying heat treatment to precipitate chromium carbides mainly at grain boundaries.

However, after such heat treatment for ROM carbide precipitation, the outer surface of the high Cr-containing Ni-based alloy tube is immediately subjected to water cooling, etc., which causes compressive stress to remain in the surface layer of the outer surface of the tube, and to weaken the grain boundaries. When chromium carbide was present in the tube, the grain boundary damage resistance of the outer surface layer of the tube was dramatically improved.

(e) The above-mentioned treatments, such as leaving compressive stress on the outer surface of the tube or causing chromium carbide to exist at the grain boundaries, do not necessarily need to be carried out over the entire length of the tube, but at least part of it. It is possible to obtain a sufficient effect of improving grain boundary damage resistance by applying it locally, that is, to areas where grain boundary damage is likely to occur, and it can also be easily applied to heat exchanger tubes for hot air generators in existing plants. that it is possible.

This invention was made based on the above knowledge, and a heat exchanger tube for a steam generator is made of carbon: 14 to 35% by weight and Ni.
: Made of a Ni-based alloy containing 30% by weight or more,
Moreover, by having a structure in which compressive residual stress is applied to at least a part of the outer surface layer, it not only has excellent mechanical properties required for the heat exchanger tube, but also has particularly excellent grain boundary damage resistance. The outer surface of a Ni-based alloy tube having the following characteristics and further containing Cr: 14 to 35% by weight and Ni: 30% by weight or more is water-cooled;
At least a part of it is heated from the inside while
By maintaining the temperature in the temperature range of 00 to 600°C for 1 minute or more and then stopping the heating to apply compressive residual stress to at least a part of the outer surface layer, or by heating the Ni-based alloy pipe. 600-800 at least part of it
℃ for 0.5 minutes or more to precipitate chromium carbide at grain boundaries, immediately cool the outer surface of the tube with water and apply compressive residual stress to at least a portion of the outer surface layer. The present invention is characterized in that the heat exchanger tube for a steam generator having excellent grain boundary damage resistance is manufactured.

In addition, in this invention, the reason why the Cr content and NL content m of the Ni-based alloy constituting the heat exchanger tube for a steam generator are limited as described above is that the high performance heat exchanger tube for a steam generator has
Normally, a Ni-based alloy with the above composition is used, and if the Cr content is less than 14g11%, the high temperature corrosion resistance and oxidation resistance necessary for a heat exchanger tube for a steam generator cannot be secured, and conversely, if the Cr content exceeds 35% by weight, Even if it contains, the property improvement effect is $ 1! , 3tL th i(: 1!: l-7
This is because if the amount is less than 30E'C1Ni*@5i#130t4%, the high temperature corrosion resistance will be insufficient.

However, although this type of Ni-based anhydride is a material with high sensitivity to grain boundary damage, if compressive residual stress is applied to at least the surface layer of its outer surface, the sensitivity to grain boundary damage becomes extremely low.
Practical inconveniences are almost completely eliminated.

The required thickness of the "tube outer surface surface layer" to which compressive stress is applied varies depending on the grain boundary damage environment, but it is usually sufficient to consider the depth of about 0.3 mm from the outer surface.

Furthermore, this term does not necessarily refer to the surface layer over the entire length of the tube, but also includes the case where only a portion of the tube in the longitudinal direction is referred to. Alternatively, even a heat exchanger tube in which compressive residual stress is only applied to the surface layer of the outer surface of the tube in the portion that becomes the "tube support plate" (where grain boundary damage is likely to occur) has sufficient resistance to grain boundary damage. It is something that you have.

One advantageous means for manufacturing such a Ni-base alloy heat exchanger tube for a steam generator is as shown in FIG. This is a method in which at least a portion of the material in the longitudinal direction is maintained in a temperature range of 300 to 600° C. for 1 minute or more by heating with a heater 2 or the like, and then the heating is stopped.

In addition, what is shown by the reference numeral 3 in FIG. 1 is water for cooling the outer surface of the tube.

According to the above method, it is possible to treat heat exchanger tubes after assembly for a steam generator or heat exchanger tubes in an existing plant, as well as local parts of heat exchanger tubes (for example, tube supports that form gaps with tube support plates and tube sheets). It is easy to understand that it is possible to extremely easily take measures to prevent grain boundary damage even only for the tube plate portion and the tube plate portion.

Now, in this method, the heating temperature is 300 to 600°C.
The reason why the heating and holding time was set at 1 minute or more is as follows. That is, if the heating temperature is less than 300°C or the heating holding time is less than 1 minute, the desired compressive residual stress will not be imparted to the surface layer of the Ni-based alloy tube, ensuring good intergranular damage resistance. On the other hand,
This is because even if the heating temperature exceeds 600°C, the residual compressive stress does not show any further increasing tendency.

Figure 2 shows an outer diameter of 22.24I11. Thickness: 1.27a
Attov600 (75Ni) as annealed with m
-15Cr-9Fe alloy) while cooling the outer surface of the pipe material with water,
This is a graph showing the relationship between the heating temperature and the residual stress on the outer surface of the tube when the inner surface is heated with a heater and held at various temperatures for 3 minutes, and then the heating is stopped. It is clear that sufficient compressive stress remains on the tube surface at 300-600°C (preferably 450-550°C).

Another method for manufacturing a Ni-base alloy heat exchanger tube for a steam generator proposed by the present inventors is to “heat a Ni-base alloy tube,
600 to 800°C at least in part in the longitudinal direction, etc.
□ is a method in which chromium carbide is precipitated at the grain boundaries by holding the tube in the temperature range for 0.5 minutes or more, and immediately cooling the outer surface of the tube with water. Any method can be used to heat the tubes, but heat transfer tubes after the steam generator is assembled or heat transfer tubes in an existing plant can be easily used as target materials, so heating from the inner surface of the tubes using a heater etc. is recommended. It is preferable to adopt

Now, in this method, the heating temperature is 600 to 800°C.
The reason why the heating and holding time was set at 0.5 minutes or more is as follows. That is, in this case, the heating temperature is less than 600°C or the heating holding time is 0.5°C.
If the heating temperature is less than 800°C, it will be difficult to secure the desired grain boundary damage resistance, and on the other hand, even if the heating temperature exceeds 800°C, the grain boundary damage resistance will tend to deteriorate. It is best to adjust the temperature to 650-750°C.

In addition, the heat exchanger tube for a steam generator, which is obtained by this method and has excellent grain boundary damage resistance and has compressive residual stress applied to at least a part of the outer surface layer of the tube, has chromium carbide (mainly This is due to the precipitation of CrCr23O, but if such grain boundary carbides exist,
□ due to the superimposed action of grain boundary carbides and compressive residual stress in the surface layer of the outer surface of the tube. □Ah, □-1°. Eh, ゎ ゛・°゛ means 5. Note that no other means are required to precipitate chromium carbide at grain boundaries;
It is sufficient to hold the i-based alloy tube in a temperature range of 600 to 800°C for 0.5 minutes or more.

Figure 3 shows outer diameter: 22.2am, 7gT: 1
．． At as-annealed which is 2am+! oy600
(75Ni-15Cr-9Fe alloy) pipe material is heated to various temperatures, held for various times, and then its outer surface is immediately cooled with water. The effects of heating temperature and heating holding time on the occurrence of grain boundary damage are shown. It is a graph. The grain boundary damage test was conducted in high-temperature water (325°C) containing 40% NaOH and magnetite sludge, and the broken lines in Figure 3 indicate that chromium carbide was precipitated at the grain boundaries. For comparison, the test results are shown for a case in which only the tube was covered with a compressive residual stress and no compressive residual stress was applied to the surface layer of the outer surface of the tube.

From this figure 3, it can be seen that when heated to 600-800℃,
It is clear that tubes that are water-cooled immediately after being held for more than a minute exhibit superior intergranular damage resistance. Further, although good grain boundary damage resistance can be obtained only by grain boundary precipitation treatment of chromium carbide, it is also clear that in this case, holding in a relatively high temperature range for a long time is required.

Next, the present invention will be specifically explained using examples and comparing with comparative examples.

<Examples> Example 1 First, an as-annealed pipe material having a chemical composition as shown in Table 1, an outer diameter of 22.2 m + φ, and a wall thickness of 1.2 mm was prepared. As shown in Table 2, a coil heater made of nichrome wire is inserted into the inner surface of the tube while the outer surface of the tube is cooled with water, and the tube is heated to the temperature shown in Table 2, and maintained at that temperature for the time shown in Table 2. After that, heating was stopped.

A stress corrosion cracking test was conducted on the tube material to which compressive residual stress was applied to the surface layer of the outer surface of the tube in this way, and the results are also shown in Table 2.

The test material used had a length of 30 mm, as shown in Figure 4.
One end of the above-mentioned 0I tube was welded (4) and the center part was cut by 0.2 am from the width of 20 IMR to make the wall thickness 0.8 m+.The internal pressure was applied to this tube with argon gas. The tube was subjected to a test by applying a tensile stress of 30 to 9/- due to the pressure difference between the inside and outside of the tube (differential pressure).

In the Ct-high temperature water stress corrosion cracking test (C1-8 CC resistance test), a cylindrical backing material with a fluororesin sheet sandwiched between the 0.8 am+ wall thickness of the test material was fitted to create a gap, and the test material was placed in an autoclave container. 5001 at 300℃ using
) Ell C,t - (as Na C1) by immersion in a non-degassed solution for 1000 hours.

Alkali stress corrosion cracking test (alkali SCC resistance test)
was heated at 325°C for 1 hour using the same autoclave container.
It was carried out by immersion in a degassed solution of 0% NaOH for 500 hours.

The grain boundary damage resistance test in an alkaline solution containing iron oxide (magnetite) was conducted using degassed 0.4% NaOH
(PH13) or 0.04% Na OH (PH12>) with magnetite (Fe 304 ) added, and immersion was carried out for 5000 hours or 1000 hours, respectively.

Evaluation after the test was performed by observing the crack depth of the test material after each test using an optical microscope.

As is clear from the results shown in Table 2, according to the method of the present invention, pipe materials not only have excellent C1-8CC resistance and alkali SCC resistance, but also have excellent intergranular damage resistance. In contrast, the conventional method that does not apply a compressive residual stress (-20 to -25 to 9/-> to the surface layer of the outer surface of the tube, or the comparative method whose processing conditions are outside the scope of the present invention) It can be seen that the tube material obtained by the method does not exhibit good intergranular damage resistance.

Example 2 The chemical composition is as shown in A in Table 1 above, and the outer diameter is 22
．． Prepare an as-annealed tube material with a diameter of 2 #φ and a wall thickness of 1.2 am, insert a coil heater made of nichrome wire into the inner surface of the tube material, and heat the tube to the temperature shown in Table 3.
Similarly, the temperature was maintained for the time shown in Table 3, and some of the tubes were allowed to cool as they were (no compressive residual stress imparting treatment), but the remaining portions were immediately cooled with water from the outer surface of the tube (compressive residual stress imparting treatment). carried out.

A stress corrosion cracking test was conducted on the tube material thus obtained, and the results are also shown in Table 3.

The test material used was subjected to the same treatment as in Example 1 (as shown in FIG. 4), and subjected to the test by applying internal pressure with argon gas to give a tensile stress of 30 #/-.

The Ct-high temperature water stress corrosion cracking test (Ct-8CC resistance test) was carried out in the same manner as in Example 1.

Degassed high temperature stress corrosion cracking test (Degassed high temperature water SCC test)
As shown in Fig. 5, a reverse rebend test piece with a 10% tensile prestrain (a U-bend processed so that tensile stress is applied with the inner surface of the tube as the outside) was used to test the autoclave container. Among them, 50001) I Bs+
+1E)I)1m Li'' (7) IIQ gas solution at 60°C) for 1000 hours.

The grain boundary damage resistance test was conducted on the test material shown in Figure 4 at 30K.
After applying a tensile stress of g/-, the samples were immersed in a degassed 40% Na OH or 10% Na OH solution to which iron oxide (magnetite: Fe304) was added for 200 hours or 1000 hours, respectively.

Evaluation after the test was performed by observing the crack depth of the test material or specimen after each test using an optical microscope.

As is clear from the results shown in Table 3, when the test tube is heated to 600 to 800°C to precipitate chromium carbide at the grain boundaries, it will not react in a highly corrosive [high concentration NaOH + magnetite] solution environment. The effect of suppressing grain boundary damage is observed, but if the outer surface of the tube is water-cooled immediately after this heat treatment to impart compressive residual stress (-20 to -25 phantom/-) to the outer surface layer, the heating holding time will be reduced. It can be seen that the grain boundary damage property is dramatically improved even if the time is extremely short.

1 Moreover, since the heat treatment in this method takes only a very short time, the degassing-resistant high-temperature water SCC next to the steam generator
No adverse effect of deteriorating the properties or secondary side Ct-high temperature water SCC properties was observed.

Any of the methods described above in the two embodiments can be fully applied as a means for improving the grain boundary illumination resistance of heat exchanger tubes that are highly sensitive to grain boundary damage in existing plants with actual reactors, and the scope of their application is Needless to say, this applies not only to heat exchanger tubes for steam generators such as nuclear reactors, but also to general heat exchanger tube materials for the chemical industry.

<Summary and Effects> As described above, according to the present invention, it is possible to easily and easily produce a heat exchanger tube for a steam generator that does not cause grain boundary damage even after long-term use and has excellent heat resistance properties. In addition, extremely useful effects can be brought about industrially, such as heat exchanger tubes after steam generator assembly and heat exchanger tubes in actual or existing plants can be used as materials.

[Brief explanation of the drawing]

+1 Figure 1 is a schematic explanatory diagram of the method of the present invention for manufacturing a heat exchanger tube for a steam generator with excellent grain boundary damage resistance by water-cooling the outer surface and heating from the inner surface. A graph showing the relationship between heating temperature and residual stress on its outer surface. Figure 3 is a graph showing the influence of tube heating temperature and heating holding time on the occurrence of grain boundary damage. Figure 4 is a graph showing the relationship between heating temperature and residual stress on the outer surface. Figure 5 is a schematic vertical cross-sectional view showing the shape of the test material used in the stress corrosion cracking test.
FIG. 2 is a schematic perspective view of a test piece used in the test. In the drawings: 1... Heat exchanger tube, 2... Heater. 3...Cooling water, 4...Welding part. 5... Test piece, 6... Bolt. 7...Natsuto. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo et al.

Claims (3)

[Claims] (1) It is made of a Ni-based alloy containing 14 to 35% by weight of Cr and 30% by weight or more of Ni, and is characterized by having compressive residual stress applied to at least a portion of the outer surface layer of the tube. Heat exchanger tubes for steam generators with excellent grain boundary damage resistance. (2) A Ni-based alloy tube containing Cr: 14 to 35% by weight and Ni: 30% by weight or more is heated from the inside while cooling the outside with water, so that at least a part of it is heated to 300 to 300% by weight.
A method for manufacturing a heat exchanger tube for a steam generator with excellent resistance to intergranular damage, characterized by maintaining the temperature in a temperature range of 600° C. for 1 minute or more and then stopping heating. (3) Heating a Ni-based alloy tube containing Cr: 14-35% by weight and Ni: 30% by weight or more, and holding at least a portion of it in a temperature range of 600-800°C for 0.5 minutes or more. A method for manufacturing a heat exchanger tube for a steam generator with excellent grain boundary damage resistance, which is characterized in that the outer surface of the tube is immediately water-cooled after chromium carbide is precipitated at grain boundaries.