JPH05140707A - Heat treating method for improving corrosion resistance of solid-solution strengthened ni base alloy - Google Patents

Abstract

PURPOSE:To provide the heat treating method for improving the corrosion resistance of a solid-soln. strengthened Ni base alloy for a nuclear reactor for solving the serious problem of the corrosive damage of 'Inconel(R)' 600 used for a structure in a BWR(boiling water reactor) caused by the sensitization of material in a reactor water environment of BWR. CONSTITUTION:This method is characterized in that in heat treatment in the final stage, for obtaining high corrosion resistance and good mechanical properties of a solid solution strengthened Ni base alloy, setting its heating temp. to 1000 to 1100 deg.C, allowing >=60% of the C content therein to enter into solid solution, regulating the grain size to fine one of >=4 grain size number and thereafter executing rapid cooling from the heating temp. to 300 deg.C at >=200 deg.C/sec cooling rate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for improving the corrosion resistance of a solid solution strengthened Ni-based alloy in a solution heat treatment state or a final heat treatment state, and particularly to a grain boundary which is of concern in a high temperature and high pressure water environment such as a light water reactor. TECHNICAL FIELD The present invention relates to a heat treatment method for improving corrosion resistance of a solid solution strengthened Ni-based alloy and a solution-strengthened Ni-based alloy, which are suitable for preventing type stress corrosion cracking (hereinafter referred to as IGSCC).

[0002]

2. Description of the Related Art Ni-based alloys containing 10% or more of Cr, such as Inconel 600 (15% Cr, 7% Fe,
Ni) is used as a structural material of equipment requiring high temperature strength and corrosion resistance, except for impurities. In general, it can be said that this type of alloy has excellent corrosion resistance due to its high Cr content, but it is known that corrosion and stress corrosion cracking (hereinafter referred to as SCC) occur in an acid or alkaline environment or a reactor water environment. There is. Most of these corrosion damages are intergranular corrosion or IGSCC, and it is considered that Cr carbide precipitation and intergranular segregation of impurities at the grain boundaries caused by heat treatment are the main causes. That is, of the solid solution strengthened Ni-based alloys, for example, Inconel 600 has both corrosion resistance in a non-oxidizing acid due to Ni and corrosion resistance in an oxidizing acid due to Cr, and is also a decisive factor for austenitic stainless steel. It has excellent resistance to stress corrosion cracking due to chloride, which is a weak point, and is a trump card against stress corrosion cracking. However, conventional solution treatment or final heat treatment (MA: mill annealing treatment) It is known that when applied, since sensitization caused by heat treatment conditions is not taken into consideration, sensitization occurs in the heat treatment process, and intergranular corrosion resistance and high-temperature high-pressure water IGSCC resistance decrease.

1050 ° C. in order to improve intergranular corrosion resistance
It has been considered to carry out a solution treatment at about 1200 ° C. for about 0.5 h, but the intergranular corrosion resistance has not been improved yet. In JP-A-57-9861, when the solution treatment temperature is set to T degrees Celsius and the solution treatment time is set to t hours, if the solution heat treatment satisfying the condition of 18260 / T ≦ logt + 12.00 is performed, Inconel Although the intergranular corrosion resistance of 600 is improved, the intergranular corrosion susceptibility and the high temperature high pressure pure water SCC susceptibility have not been completely eliminated. Further, Japanese Patent Application Laid-Open No. 61-284558 discloses a method for producing a Ni-based alloy having excellent hydrogen crack resistance.
Although it has been shown that the cooling rate in the solution heat treatment is 50 ° C./sec or more (60 ° C./sec, 80 ° C./sec in the specific examples), it is feared in a high temperature and high pressure water environment such as a light water reactor. No consideration has been given to the problem of intergranular stress corrosion cracking.

Therefore, in order to ensure the soundness and reliability of the nuclear reactor equipment made of the corresponding alloy, the intergranular corrosion and the high temperature high pressure pure water SC based on the intergranular corrosion mechanism or the SCC mechanism are used.
The establishment of C prevention technology became important.

[0005]

Among the solid solution strengthened Ni-based alloys, for example, Inconel 600 exhibits intergranular corrosion susceptibility and high temperature high pressure pure water SCC susceptibility in the as-received state or in the state of solution treatment. It was thought that the main cause of this was grain boundary segregation of impurities caused by heat treatment, but no experimental facts were found to prove it and grain boundary corrosion and high temperature water IG
It was difficult to establish SCC prevention technology.

The object of the present invention is to provide an appropriate solution treatment temperature or final heat treatment temperature according to the C content based on the intergranular corrosion occurring in the solution treatment state or the final heat treatment state and the generation mechanism of high temperature high pressure pure water SCC. Another object of the present invention is to obtain a solid solution strengthened Ni-based alloy having excellent corrosion resistance by a heat treatment method that combines an appropriate crystal grain size and an appropriate cooling method. According to the present invention, it is possible to provide a solution strengthened Ni-based alloy having excellent corrosion resistance without impairing the mechanical properties of the alloy in the solution treatment state or the final heat treatment state.

[0007]

SUMMARY OF THE INVENTION The present invention is a solid solution strengthened type N
The present invention relates to a heat treatment method for preventing intergranular corrosion or high temperature high pressure pure water IGSCC that occurs in a solution treatment state or final heat treatment state of an i-based alloy. It has been discovered that since the solid solution strengthened Ni-based alloy has a low C solid solubility, sensitization (Cr deficiency) may occur in the conventional solution treatment method or the final heat treatment method, resulting in a decrease in corrosion resistance. Therefore, it has been found that it is beneficial to establish a heat treatment technique for suppressing sensitization in the solution treatment state or the final heat treatment state in order to improve the corrosion resistance.

That is, the present invention is a solid solution strengthened Ni which is a Ni-based alloy containing Cr and which is used in high temperature high pressure water.
In the heat treatment method for improving the corrosion resistance of a base alloy, a heating step of heating the Ni-based alloy to form a solid solution in an amount of 60% or more of the C content, and a crystal grain size after the heating step
G 0552) is characterized by including a heating and holding step of holding it for a certain period of time so as to make fine particles of 4 or more, and a rapid cooling step of cooling to 300 ° C. at a cooling rate of 200 ° C./sec or more.

Further, the present invention is a heat treatment method for improving the corrosion resistance of a Cr-containing Ni-base alloy which is used in high-temperature high-pressure water, wherein the Ni-base alloy is heated to 1000 to 1100 ° C. A heating step, a heating and holding step of holding the crystal grain size for 4 hours or more so that the crystal grain size becomes finer than the crystal grain size number 4 after the heating step, and a rate of not depleting Cr in the crystal grain boundaries during the cooling process up to 300 ° C. And a quenching step of cooling. Here, the quenching step is preferably performed in a water, oil or gas atmosphere at 10 ° C. or lower.

Further, according to the present invention, in a Ni-based alloy containing Cr, which is used in high-temperature high-pressure water, a solution-strengthened Ni-based alloy, the concentration of Cr in the crystal grain boundaries and in the vicinity of the grain boundaries is almost equal. It is characterized by being like. Here, the weight ratio of the Ni-based alloy is C: 0.15% or less, Si: 0.5% or less, Mn: 1.0% or less, Cr:
It is preferable to contain 14 to 17% and Fe: 6 to 10% with the balance being Ni and inevitable impurities. Alternatively, the weight ratio of the Ni-based alloy is C: 0.1% or less, Si: 0.5% or less, Mn: 0.5% or less, Cr: 20-23%, Mo :.
8 to 10%, Nb: 3.15 to 4.15%, Ti: 0.
It is preferable that the content is 4% or less, Al: 0.4% or less, Fe: 5% or less, and the balance is Ni and inevitable impurities.
Alternatively, the weight ratio of the Ni-based alloy is C: 0.05% or less,
Si: 0.5% or less, Mn: 0.5% or less, Cr: 27
.About.31%, Fe: 7 to 11%, with the balance being Ni and inevitable impurities.

[0011]

The solution treatment temperature or the final heat treatment temperature is 1000 ° C. to 1100 ° C. at which 60% or more of the C content is dissolved to obtain good mechanical properties. 1 more
The crystal grain size under the heating condition of 000 ° C. to 1100 ° C. is kept at that temperature for 1 minute or more in order to make fine grains having a crystal grain size number of 4 or more. Furthermore, the cooling rate after heating and holding is 200 ° C./sec from the heating and holding temperature to 300 ° C. so that grain boundary Cr deficiency cannot occur in the cooling process.
Above, in particular, it is set to exceed 200 ° C./sec. When the above heat treatment in which the heating conditions, the crystal grain size, and the cooling rate are combined is carried out, solid solution strengthened Ni excellent in corrosion resistance is obtained.
A base alloy is obtained. This heating rate is the cooling rate up to the center, and the surface has a higher cooling rate. In the present invention, controlling the temperature of the refrigerant enables the cooling rate of a larger member to be increased. As for the cooling method, it can be cooled by dipping it in a refrigerant or spraying a refrigerant. The temperature of the refrigerant is preferably 10 ° C. or lower for cooling, and particularly preferably kept at 0 ° C. for cooling.

[0012]

EXAMPLES The present invention will be described in detail below with reference to examples. The test materials are 2 heats (sample symbols A and B) and the chemical compositions are shown in Table 1. All of these are commercially available materials that have been subjected to mill annealing (MA).

[0013]

[Table 1]

In the experiment, the processed / heat-treated materials shown in Table 2 (Sample Code A) and Table 3 (Sample Code B) were used. The corrosion resistance of the test material was evaluated by the intergranular corrosion test, high temperature water SCC test and SCC test device shown in FIG. (Grain boundary corrosion test) Test: striker test Test liquid: 400 ml H ₂ O + 236 ml H ₂ SO ₄ +5
0 g Fe (SO ₄ ) ₃ (boiling aqueous solution) Specific liquid amount: 20 ml / cm ² Test time: 24 h Evaluation: Maximum intergranular erosion depth (μm) (high temperature water SCC test) Temperature: 288 ° C Pressure: 85 Kg / cm ² Dissolved oxygen: 32 ppm (O ₂ gas continuous injection) Electric conductivity: 0.8 μs / cm or less Flow rate: 20 liters / h Gap: Graphite wool Applied strain: max 2.0 to 3.0% Test time: 640 h 1 and 2 show the results of component analysis of the crystal grain boundaries and the vicinity of the grain boundaries of the heat-treated material of the present invention and the conventional heat-treated material. The results of the tests conducted on corrosion resistance and mechanical properties are summarized in Tables 4 and 5.

[0015]

[Table 2]

[0016]

[Table 3]

[0017]

[Table 4]

[0018]

[Table 5]

A solid solution strengthened Ni-based alloy used as a material for a reactor plant is required to have good corrosion resistance and good mechanical properties. However, since the corrosion resistance and mechanical properties of the corresponding alloy change intricately depending on the combination of the heating temperature, the grain size, and the cooling rate, it is particularly important to select a suitable combination of these. As shown in FIG. 4, the heating temperature is a temperature at which the content of C is 60% or more so as to form a solid solution with the content of C to obtain good mechanical properties (C: 0.07%, 1000 ° C.). Above). on the other hand,
Since the corrosion resistance tends to decrease with the coarsening of the crystal grains when the heating and holding temperature is increased more than necessary, it is preferable to make the crystal grains as fine as possible in order to obtain good corrosion resistance. Further, as shown in FIG. 5, since the corresponding alloy has a low C solid solubility, sensitization occurs in the cooling process from the heating and holding temperature to about 300 ° C. Therefore, in order to obtain high corrosion resistance, the sensitization should be prevented. Selection of rapid cooling rate is also essential. In this case, the corrosion resistance is good at 150 ° C./sec or more, but further improved at 200 ° C./sec or more.

The present invention relates to a heat treatment method for obtaining good corrosion resistance and good mechanical properties of a solid solution strengthened Ni-based alloy. Below, the examination results of various characteristics by the combination of the above will be explained.

Since the corresponding alloy equipment for a nuclear reactor plant is mainly made of cold-worked materials (pipes) or forged materials (bolts, nuts, rings etc ...), this point was also evaluated. Tables 4 and 5 evaluate the relationship between the intergranular corrosion resistance, the high-temperature high-pressure pure water SCC property, and the mechanical properties of the processed and heat-treated materials of sample symbol A and sample symbol B that were subjected to the conventional heat treatment and the heat treatment of the present invention. The results are shown. Regardless of whether cold working is performed or not, if the heating temperature is less than 1000 ° C., the solid solution of C becomes insufficient and the mechanical properties deteriorate (A-1 to A-
4, A-17, A-18, B-1), the heating temperature for obtaining good mechanical properties is preferably 1000 ° C. or higher. In this case, good mechanical properties require that the C content be 60% or more to form a solid solution. However, when the heating temperature is 1150 ° C. or higher, the crystal grain size becomes coarse (grain size number: 1-2), and the cooling rate from the heating temperature is 20%.
Even if adjusted to 0 ° C / sec, the corrosion resistance (grain boundary corrosion resistance and high temperature high pressure pure water SCC resistance) decreases (A-14 to A-16, A-
25, A-26, B-5 to B-7, B-11 to B-1
3) Do.

Therefore, the heating temperature for obtaining good corrosion resistance and good mechanical properties irrespective of the plastic working conditions before heat treatment is from 1000 ° C. to 11 ° C. at which 60% or more of the C content is dissolved.
The crystal grain size at the heating temperature must be set to a range of 00 ° C., and the fine grain size of grain size number 4 or more must be set.

On the other hand, the heating temperature is 1000 ° C., 1050
It was shown that the corrosion resistance depends on the cooling rate from the heating / holding temperature to 300 ° C. even when adjusted to 1 ° C. and 1100 ° C. (FIG. 5). That is, the cooling rate from the heating / holding temperature to 300 ° C. is 100 ° C./sec or less (100 ° C./sec, 50 ° C./se even when heated and held at a temperature of 1000 ° C. to 1100 ° C.
In the case of c), the corrosion resistance decreases (A-5, A-6, A-
8, A-9, A-11, A-12, A-19, A-2
1, A-23, A-27, A-29, A-31, B-
2, B-3, B-8, B-9) were observed. Therefore,
The cooling rate from the heating temperature to 300 ℃ is 200 ℃ /
It must be sec or more. In this case, a cooling rate of 200 ° C./sec or more was obtained by rapidly putting the sample in ice water at 0 ° C. Strictly speaking, the cooling rate is determined by the shape of the sample and the temperature and capacity of the refrigerant, but in the present invention, the cooling rate of the sample surface is specified. However, if the surface is to be ground by mechanical processing after heat treatment, the finished surface area should be 200
It is preferable to adjust the cooling rate to not less than ° C / sec. Further, in order to obtain a cooling rate of 200 ° C./sec or more in a gas atmosphere, it is desirable to use hydrogen gas. As shown in FIG. 2, the samples that were inferior in intergranular corrosion resistance and high-temperature high-pressure high-pressure pure water SCC resistance in the conventional heat-treated material were sensitized (Cr
Deficiency) has become apparent. The cooling rate in this case is about 100 ° C./sec.

Samples (A-7,
A-10, A-13, A-20, A-22, A-24,
A-28, A-30, A-32, B-4, B-10)
Indicates the difference in the C content (sample code A: 0.03% C, sample code B: 0.07% C) and whether cold working is performed (cold workability: 0, 20%, 30%, 40% ), 1000
Under the heating condition of ℃ to 1100 ℃
After the fine grains were adjusted as described above, the heating and holding temperature to 300 ° C. was rapidly cooled at a cooling rate of 200 ° C./sec or more, so that no sensitization could occur at the grain boundaries as shown in FIG. Good corrosion resistance and good mechanical properties were obtained.

According to the heat treatment method of the present invention, there has been a high temperature and high pressure alkaline environment which has been a problem until now, or an acidic environment due to a decrease in PH which may occur in the gap in the reactor water, or a high temperature and high pressure water environment containing chlorine ions. Corrosion resistance of the corresponding alloy equipment members used underneath, etc. is remarkably improved, and eventually reliability and long life of the reactor plant can be achieved. The present invention is also effective as a method for improving the corrosion resistance in the welded part of the corresponding alloy. The present invention also relates to BWR and P
It can also be applied to equipment parts of the corresponding alloys used in chemical plants, boilers, etc. including WR nuclear plants. FIG. 6 shows an example of a Ni-based alloy part of the present invention in a BWR. In the figure, 1 is a water supply nozzle thermal sleeve, 2
Is an instrument nozzle, 3 is a shroud head bolt, 4 is a shroud support, 5 is a jet pump lower ring, 6
Is a core differential pressure instrumentation nozzle, 7 is a lower mirror patterning, 8 is an in-core housing, and 9 is a CRD stub tube, and these are made of the above Ni-based alloy.

[0026]

According to the present invention, the solution treatment temperature for achieving sufficient solid solution strengthening is set, and the crystal grain size at the solution treatment temperature important for obtaining the corrosion resistance is specified.
Further rapid cooling prevents sensitization that may occur during the cooling process, that is, since the heating temperature, the crystal grain size, and the cooling rate are suitably combined, a solid material having both good corrosion resistance and good mechanical properties can be obtained. A solution strengthened Ni-based alloy can be provided.

By subjecting the alloy of interest to the heat treatment of the present invention, it is possible to provide a nuclear reactor equipment member having excellent corrosion resistance and mechanical properties, and thus to achieve high reliability and a long life of the nuclear reactor plant.

[Brief description of drawings]

FIG. 1 is a diagram showing a result of component analysis of a crystal grain boundary and a portion near the grain boundary of a heat treated material of the present invention.

FIG. 2 is a diagram showing a result of component analysis of a crystal grain boundary and a portion near the grain boundary of a conventional heat-treated material.

FIG. 3 is a schematic view of a high temperature water SCC test apparatus.

FIG. 4 is a diagram showing the relationship between the C content of various alloys and the solid solution temperature.

FIG. 5 is a diagram showing a relationship between a cooling rate at 300 ° C. and sensitization.

FIG. 6 is a partially cutaway perspective view showing an example of a Ni-based alloy part of the present invention in a BWR.

[Explanation of symbols]

 1 Water supply nozzle Thermal sleeve 9 CRD stub tube

Claims (7)

[Claims] 1. In a heat treatment method for improving the corrosion resistance of a Ni-based alloy containing Cr, which is used in high-temperature high-pressure water, in a solution-solution strengthened Ni-based alloy, the Ni-based alloy is heated to contain C.
A heating step of forming a solid solution of 60% or more of the amount, and a heating holding step of holding the crystal grain size for a certain period of time after the heating step so that the grain size becomes fine grains having a grain size number (JIS G 0552) of 4 or more, and 200 ° C / A solid solution strengthened type N characterized by including a quenching step of cooling to 300 ° C. at a cooling rate of sec or more.
A heat treatment method for improving the corrosion resistance of an i-based alloy. 2. A heat treatment method for improving the corrosion resistance of a Ni-based alloy containing Cr, which is used in high-temperature and high-pressure water, in a heat treatment method for improving the corrosion resistance of the Ni-based alloy.
A heating step of heating to 00 ° C., a heating and holding step of holding the crystal grain size for a certain period of time so that the crystal grain size becomes finer than the crystal grain size number 4 after the heating step, and a rate at which Cr deficiency does not occur in the crystal grain boundaries during the cooling process A rapid cooling step of cooling to 300 ° C .; and a heat treatment method for improving corrosion resistance of a solid solution strengthened Ni-based alloy. 3. The quenching step according to claim 1 or 2,
A heat treatment method for improving corrosion resistance of a solid solution strengthened Ni-based alloy, which is carried out in a water, oil or gas atmosphere at 0 ° C or lower. 4. A solution-strengthened Ni-based alloy containing Cr, which is used in high-temperature high-pressure water,
A solid solution strengthened Ni-based alloy characterized in that the concentration of Cr in the grain boundaries and in the vicinity of the grain boundaries is substantially uniform. 5. The Ni-based alloy according to claim 4, wherein the weight ratio of C: 0.15% or less, Si: 0.5% or less, and Mn:
1.0% or less, Cr: 14 to 17%, Fe: 6 to 10%
A solid solution strengthened Ni-based alloy containing Ni and the balance Ni and inevitable impurities. 6. The Ni-based alloy according to claim 4, wherein the weight ratio of C: 0.1% or less, Si: 0.5% or less, and Mn:
0.5% or less, Cr: 20-23%, Mo: 8-10
%, Nb: 3.15 to 4.15%, Ti: 0.4% or less, Al: 0.4% or less, Fe: 5% or less, and the balance is Ni and unavoidable impurities. Ni-based alloy. 7. The Ni-based alloy according to claim 4, wherein the weight ratio of C: 0.05% or less, Si: 0.5% or less, and Mn:
0.5% or less, Cr: 27 to 31%, Fe: 7 to 11%
A solid solution strengthened Ni-based alloy containing Ni and the balance Ni and inevitable impurities.