JPS63213639A - Stainless steel for heat transfer pipe-supporting plate in steam generator - Google Patents

Abstract

PURPOSE:To obtain an inexpensive stainless steel for heat-transfer pipe- supporting plate in nuclear reactor steam generator excellent in corrosion resistance, strength, workability, and impact toughness, by specifying a composition consisting of C, Si, Mn, Cr, Ni, Al, Mo, Cu, and Fe. CONSTITUTION:The ferritic stainless steel has a composition consisting of, by weight, >0.02-0.08% C, <=1.00% Si, <=1.00% Mn, 11.50-14.50% Cr, <0,60% Ni, 0.10-0.30% Al, 0.30-1.00% Mo, 0.30-1.00% Cu, and the balance essentially Fe and is suitable for a heat transfer pipe-supporting plate in a nuclear reactor steam generator. The above-mentioned stainless steel shows superior corrosion resistance and, particularly, causes no denting to the above heat-transfer pipe, and further, it has excellent stress corrosion cracking resistance.

Description

[Detailed description of the invention] <Industrial field of application> The present invention has excellent corrosion resistance, strength, workability (including weldability and machinability), and impact toughness, and is particularly applicable to nuclear reactor steam. The present invention relates to a ferritic stainless steel suitable for use as a heat exchanger tube support plate in a generator.

Background technology Among the currently used nuclear reactors, pressurized wood reactors (PWRs) do not have to worry about "bubbles generated by boiling," which is a negative reaction factor, so from a control point of view, they are better than boiling water reactors ( However, in Europe and the United States, it has recently been discovered that an inconvenient phenomenon called ``dending'' occurs in the heat transfer tubes that make up the steam generators of pressurized water reactors. , there was an urgent need for improvement measures.

"Dending" of heat transfer tubes constituting the steam generator of a pressurized wooden nuclear reactor is, as shown in FIG.
04) is deposited, and its growth not only fills the gap between the heat exchanger tube 1 and the support plate 2, but also locally compresses and deforms the heat exchanger tube 1. The cause of this phenomenon is that [chloride mixed into the water on the secondary side of the steam generator due to seawater leakage from the condenser is concentrated in the gap between the heat exchanger tube and the support plate, and that part of the support plate material is rapidly damaged. However, when such dending occurs, stress is generated on the inner surface of the heat transfer tube, which can cause intergranular stress corrosion cracking, which may lead to leakage of contaminated water. This could have led to

Fortunately, in Japan, all atomic couplers undergo volatile chemical treatment (AVT) and water quality is well controlled, so no denting phenomenon has been detected to date. Learn from the measures,
Heat exchanger tube support plate material replaced with conventional carbon steel 5US4
Measures have been taken to change to 05 stainless steel.

However, a subsequent report from a U.S. nuclear power organization stated that ``there remains some concern that the application of SUS 405 stainless steel alone as described above will prevent denting of reactor steam generator heat transfer tubes.'' is also served.

By the way, taking these issues into consideration, the main features required for support plate materials for reactor steam generator heat transfer tubes are: (a) Corrosion resistance (crevice corrosion resistance, general corrosion resistance, stress corrosion cracking resistance) (b) have good mechanical properties (e.g. strength, impact properties, etc. at room temperature and high temperatures of about 300'C); (C1 physical properties (specific gravity); , coefficient of linear expansion, Young's modulus, etc.), (dl) excellent wear resistance (flenting properties), (el) good weldability, (f) machinability (drillability) , broaching workability, etc.); and satisfactory price from an industrial standpoint. However, apart from its high corrosion resistance, M5 US405 stainless steel was also known as a material that satisfies most of the above requirements.

<Purpose of the Invention> Therefore, the present inventors took advantage of the advantages of 5US405 stainless steel, which has been considered suitable as a support plate material for heat exchanger tubes in nuclear reactor steam generators, while developing new applications other than those based on actual results. Taking into account the special characteristics of the nuclear power-related field, which must be approached with extreme caution, it is necessary to demonstrate even better corrosion resistance and to be particularly concerned about the occurrence of denting in the steam generator heat exchanger tubes. The reliability of nuclear power-related equipment and equipment can be solidly improved by creating materials that are free of stress corrosion cracking and can be even more reliable in terms of stress corrosion cracking resistance. In order to achieve this goal, we carried out research to achieve this goal.At that time, the target setting for the above-mentioned denting and stress corrosion cracking was that the amount of corrosion of the heat exchanger tube support plate against the denting phenomenon was approximately 60 mg. /cI11, the lifespan of the reactor was assumed to be approximately 40 years, the estimated amount of corrosion over 40 years in the AVT environment was smaller than the critical corrosion amount, and stress corrosion cracking was assumed to be impossible. .

<Means for Solving Conventional Problems> As a result of the research conducted by the present inventors based on the above-mentioned viewpoints, the following matters were clarified.

That is, ■ 5US405 stainless steel, which has been used as a support plate material for steam generator heat exchanger tubes in the nuclear power field, where new applications other than those based on actual results is cautious, has excellent mechanical properties, physical properties, and durability. It is an excellent material that is well-balanced in terms of abrasion resistance, weldability, machinability, and cost as well as the support plate material. It is disadvantageous to significantly change from 5US405; ■ Cr plays an important role in the corrosion resistance of the stainless steel mentioned above (general corrosion resistance in sulfuric acid-copper sulfate solution); As a supporting plate material, it is absolutely necessary to ensure a C content Wt of 11.50% or more (hereinafter, % representing the component ratio is expressed as weight %) and to reduce C and Mn. The addition of trace amounts of Ni and Mo is essential; ■ The impact value of the stainless steel mentioned above and the
It is necessary to add Ni and ensure a minimum amount of C in order to improve the strength at
, it is preferable to reduce Mn; (1) the combined addition of trace amounts of Cu and Mo is extremely effective for general corrosion resistance in high-temperature water in the presence of C2- ions; A phenomenon occurs in which these substances become concentrated in the extremely superficial layer of the passive film on the surface of the component, and this concentrated layer, together with other corrosion resistance improving components, improves general corrosion resistance and resistance in high temperature water in the presence of C1- ions. Because it significantly improves crevice corrosion resistance, when applied to the support plate material of steam generator heat exchanger tubes, it is possible to reduce dending to a virtually non-problematic level.■ Therefore, the amount of C in 5US405 stainless steel CdJ is reduced to the extent that
SUS 405 stainless steel has However, while retaining or surpassing the excellent mechanical properties, physical properties, wear resistance, weldability, machinability, and cost characteristics of support plate materials for steam generator heat transfer tubes, To obtain a ferritic stainless steel capable of suppressing the above-mentioned denting phenomenon to such an extent that it does not pose a problem at all throughout the life of the nuclear reactor equipment itself.

In addition, Fig. 2 shows carbon steel (C: 0.17%), which has been conventionally used as a support plate material for steam generator heat exchanger tubes.

Si: 0.21%, Mn: 0.66%, Cr: 01
10%, balance: substantially Fe), recently switched 5US405 stainless steel (C: 0.04%,
Si: 0.25%.

Mn: 0.25%, Ni: 0.5%, Cr:
13.0%, p, i: 0.15%, N; 0.02%
, balance: substantially Fe), and the stainless steel of the present invention (C: 0.03%, Si: 0.25%,
Mn: 0.25%, Ni: 0.57%, C
r: 13.2%.

Al: 5%, Mo: 0.50%, Cu: 0
．． 50%, N: 0.02%, balance: substantially Fe) is a grab showing the results of a general corrosion investigation in a 5 ppm Cf-solution (300 ° C., pH 9, non-degassing).
From this figure 2, we can adjust the composition of 5US405 stainless steel and add trace amounts of Ni, Mo and Cu to it.
It can be seen that the corrosion resistance is unexpectedly improved by containing .

This invention was made based on the above-mentioned knowledge, and provides a stainless steel for a support plate of a heat exchanger tube in a steam generator of a nuclear reactor.C: more than 0.02% to 0.08%.

si: 1.00% or less, Mn: 1.00% or less.

Cr: 11.50-14.50%.

Nt: less than 0.60%, 8β: 0.10-0.3
0%.

Mo: 0.30-1.00%, Cu: 0.3
0 to 1.00%, and the remainder is substantially F.
By configuring the component composition consisting of e, it is possible to sufficiently eliminate factors that cause heat exchanger tube densation and other problems from the heat exchanger tube support plate.

Next, the reason why the content ratio of each component in the stainless steel of the present invention is limited as described above will be explained.

8) C The C component has the effect of ensuring the strength and impact value of steel.
If the content is less than 0.02%, it will be difficult to secure the desired strength and impact value, and on the other hand, the higher the content, the greater the tendency for corrosion resistance to deteriorate. Since the desired corrosion resistance improvement effect cannot be obtained, the C content is set at more than 0.02% to 0.08%.

B) 5t Si is usually added to steel as a deoxidizing agent, but if it is contained in excess of 1.00%, it will cause a decrease in the toughness of the steel, so the Si content should be 1.00% or less. It was determined that

C) Mn Mn is also usually added to steel as a deoxidizing agent,
If the Mn content exceeds 1.00%, the corrosion resistance and weldability of the steel will deteriorate, so the Mn content should be 1.00%.
% or less.

D) Cr The Cr component is essential for maintaining sufficient corrosion resistance as stainless steel and preventing disadvantages due to corrosion during use of the heat exchanger tube support plate, and for this purpose, it must be at least 11.50%. It is necessary to ensure the content of 1
If the Cr content exceeds 4.50%, the toughness and workability of the weld zone will deteriorate, so the Cr content should be 11.50%.
~14.50%.

E) Ni Ni not only improves corrosion resistance and prevents denting of heat exchanger tubes due to corrosion of the support plate, but also improves the mechanical properties of steel, and even a small amount of Ni has a noticeable effect. However, containing 0.60% or more leads to a significant increase in the cost of the steel, increases tempering resistance, increases hardness, and impedes hot workability and increases weld cracking susceptibility. Because it may also increase the
The Ni content was determined to be less than 0.60%.

F) Al Al is a strong ferrite-forming element and has the effect of improving the toughness of the weld heat affected zone, but if its content is 0.10
If it is less than 0%, the desired effect cannot be obtained;
．． If the content exceeds 30%, the cleanliness of the steel will deteriorate and the quenched hardness will decrease, so the l content should be 0.10 to 0.
．． It was set at 30%.

G) M.

When added in a small amount, the Mo component has the effect of improving corrosion resistance and preventing denting of the heat exchanger tube due to corrosion of the support plate, but if its content is less than 0.30%, the desired effect may not be achieved. On the other hand, Mo content exceeding 1.00% leads to a decrease in the toughness of the steel.
It was set at 30-1,00%.

H) Cu Cu component also has an important effect of improving corrosion resistance when added in small amounts together with other components and preventing denting of the heat exchanger tube due to corrosion of the support plate, but its content is If the Cu content is less than 0.30%, the desired effect cannot be obtained, while if the Cu content exceeds 1.00%, the hot workability of the steel will be inhibited. 30~
It was set at 1.00%.

Next, the present invention will be explained using Examples and comparing with Comparative Examples.

<Example> First, steel having the chemical composition shown in Table 1 was melted by a generally employed method, and was made into a plate material by forging and hot rolling.

Next, for stainless steel excluding carbon steel, 982℃
After heating and holding for 60 minutes, normalizing treatment with water cooling is performed.
A tempering treatment of heating and holding at 60° C. for 90 minutes and then cooling was performed, and corrosion test pieces were cut out from these plates and subjected to various tests.

Among these, the "general corrosion test" and the "double bibend (stress corrosion cracking) test" were conducted as corrosion tests, and the test conditions were as follows.

[Full surface corrosion test]

Test solution: 5ppm (J!-aqueous solution, pH 9, non-degassed, Test temperature: 300℃, Test time: 500 hours.

[Double U bend test]

Test liquid inlet: 500 ppm C1-aqueous solution.

Test temperature = 300℃, Test time: 500 hours.

The test results obtained are also shown in Table 1.

As is clear from the results shown in Table 1, the steel of the present invention has good corrosion resistance (denting resistance) and mechanical properties, whereas the conventional steel has slightly lower corrosion resistance. I understand. In addition, the results of physical property measurements, wear resistance tests, weldability tests, and machinability tests conducted on the steel of the present invention were all sufficiently satisfactory, and taking these into consideration, the present invention was confirmed. It is clear that steel has extremely excellent performance and practicality as a support plate material for heat exchanger tubes in nuclear reactor steam generators.

<Summary of Effects> As explained above, according to the present invention, excellent corrosion resistance and excellent corrosion resistance without the risk of causing denting of steam generator heat exchanger tubes can be achieved without requiring the addition of large amounts of special ingredients or special treatment. We provide a low-cost material that has excellent strength, good workability (including weldability and machinability), and satisfactory impact toughness, and is particularly suitable for use as heat exchanger tube support plates in nuclear reactor steam generators. This will bring about extremely excellent industrial effects, such as making it possible to further improve the reliability of nuclear equipment.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram for explaining the denting phenomenon that occurs in heat exchanger tubes of a nuclear reactor steam generator, and FIG. 2 is a 5 ppm C1-solution (30
2 is a graph showing the results of investigating general corrosion properties at 0° C., pH 9, non-deaeration. In the drawings, 1... Heat exchanger tube, 2... Support plate, 3... Corrosion product.

Claims (1)

[Claims] In terms of weight percentage, C: more than 0.02 to 0.08%, Si: 1.00% or less, Mn: 1.00% or less, Cr:
11.50 to 14.50%, Ni: less than 0.60%, Al: 0.10 to 0.30%,
Mo: 0.30-1.00%, Cu: 0.30-1.0
1. A stainless steel for a support plate of a steam generator heat exchanger tube in a nuclear reactor, characterized in that the stainless steel contains 0% Fe, and the remainder consists essentially of Fe.