JPH0674475B2 - Ni-based alloy with excellent stress corrosion cracking resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a structure used in a cooling water environment in a nuclear reactor.
Regarding Ni-based alloys, we will develop and submit those with excellent stress corrosion cracking resistance, especially for fastening bolts for internal structure.

[Conventional technology]

Conventionally, stainless steel and Ni-base superalloys have been used as alloys for fastening bolts of structures in nuclear reactors such as light water reactors, and so-called Inconel X-750 (trade name) is used particularly in parts with severe stress conditions. (Ni-15.5Cr-1Nb-0.7Al-
2.5Ti-7Fe), etc. are used after being given a tensile strength of 100 kgf / mm ² level by precipitation aging treatment. However, Inconel X-750 may have high stress corrosion cracking susceptibility under high temperature and high pressure water environment depending on heat treatment conditions, and may cause stress corrosion cracking during use as a fastening member.
Several alloys have been developed and new alloys have been proposed for the purpose of improving these defects of the alloy and improving the safety and reliability of the material.

For example, in the alloys disclosed in JP-A-62-167836 to 62-167839, the amount of Cr is 20 to 30 for the purpose of improving corrosion resistance.
% By weight (abbreviation of% by weight, all the same below), Mo content less than 10%, small amount of Al, Ti and 7% or less of Nb, 15
%, An intermetallic compound containing Nb is precipitated and strengthened by the coexistence of Fe and used.

[Problems to be solved by the invention]

The alloys that have been proposed hitherto are tried to improve the resistance to stress corrosion cracking in the environment by adding Cr and Mo, which are useful for improving the corrosion resistance, with Ni as an alloy base, but to improve the strength. Due to the coexistence of a large amount of Nb and Fe, a very hard and brittle structure is likely to be formed, and the range of optimum aging conditions is narrow and overaging is likely to occur. In particular, the susceptibility to stress corrosion cracking caused by hydrogen is found in alloys including Inconel X-750 currently in use. This tendency is a serious problem especially when used in high-temperature high-pressure water in the presence of hydrogen. For example, in a pressurized water type light water reactor primary cooling water environment, a small amount of hydrogen is usually mixed in the environment. Despite the importance of the material design concept for avoiding stress corrosion cracking caused by hydrogen, conventional materials have not been designed for alloys from this viewpoint.

Further, as the required strength properties, 80 kgf / mm ² level at room temperature yield strength, in order to obtain the room-temperature strength of 120 kgf / mm ² level gamma '
-The use of precipitation of intermetallic compounds such as Ni ₃ (Al, Ti) and γ ″ is used, but the structure of these dispersed and precipitated in the matrix phase is hard and poor in ductility, and poor in hot workability. In order to obtain a good hot forged structure, it is necessary to perform double melting such as vacuum induction melting-vacuum arc remelting or vacuum induction melting-electroslag remelting to appropriately improve hot workability during processing. It was

The present invention was devised in view of the above problems of the prior art, reduction of susceptibility to stress corrosion cracking due to brittleness and hydrogen of Ni-based alloys for structures used in the reactor cooling water environment, In addition, it is intended to improve hot workability.

[Means for solving problems]

Therefore, the Ni-based alloy of the present invention, C: 0.05% or less, Si: 0.5%
Below, Mn: 0.5% or less, Fe: 5% or less, Cr: 18-30%, Mo: 1.
Including 5 to 7%, Ta + Nb: 5% or less, Ti: 2% or less, Al: 2%
It is basically the following that satisfies the following condition and consists of the balance Ni and unavoidable impurities, and it is basically used by precipitating the intermetallic compound Ni ₃ (Al, Ti, Nb, Ta) in the austenite matrix by heat treatment. It has a feature.

Ta + 1.9Nb + 3.8Ti + 6.7Al ≦ 14 Next, the development history of the present invention having the above-described structure will be described.

Due to the above problems, the present inventors have made various studies for the purpose of reducing brittleness of alloys and susceptibility to stress corrosion cracking caused by hydrogen, and improving hot workability. As a result, hydrogen embrittlement resistance is remarkably improved by reducing the amount of Fe in the Ni-based alloy, and the aging condition sensitivity for age hardening by the γ ′ phase and γ ″ phase is also weakened. It was discovered that stable performance can be obtained over a long period of time, that is, when Fe is dissolved in Ni-based austenite, it probably promotes diffusion of hydrogen in austenite, Is an element that increases the sensitivity to stress corrosion cracking due to
Is an element that promotes precipitation of intermetallic compound strengthening phase, especially Ni ₃ Ta and Ni ₃ Nb. When the alloy matrix phase contains Fe as a solid solution, hardening and embrittlement due to aging seem to progress more rapidly. become.
Therefore, various studies were conducted on the addition amounts of Ta, Nb, Ti, and Al, which are the strengthening elements, and the required strength level was satisfied, and double melting, which was costly, was unnecessary and good forging was possible only by primary melting. The possible range of addition was found.

In the present invention, by limiting the content of Fe,
Improves resistance to stress corrosion cracking and improves Ni ₃ Ta, Ni ₃ Nb, Ni
₃ Limits the total precipitation amount of intermetallic compounds such as (Al, Ti) to reduce the sensitivity to aging conditions and prevent the material from becoming brittle easily. As a result, it is possible to provide a high-strength and high-corrosion-resistant alloy that is less likely to undergo environmental embrittlement as a whole and has good hot workability.

The reasons for limiting each component specified in the present invention will be described below.

C: Since it is an element that deteriorates SCC resistance, it is limited to 0.05% or less.

Si: It may be added as a deoxidizer, but if the content is high, the precipitation of brittle phase is promoted, so the content is made 0.5% or less.

Mn: The presence of a small amount enhances the stability of the structure, but excessive addition promotes precipitation of a brittle phase, so the content is made 0.5% or less.

Fe: As described above, a large content of Fe increases the susceptibility to stress corrosion cracking caused by hydrogen, so the content is 0.5%.
Limited to:

Cr: An element that imparts corrosion resistance and stress corrosion cracking resistance to the alloy, but if it is less than 18%, its effect is not sufficient, and if it exceeds 30%, a brittle phase tends to precipitate in the structure, so the addition amount Was in the range of 18 to 30%.

Mo: An element that imparts stress corrosion cracking resistance to the alloy and solid-solution strengthens the matrix phase, but its effect is not sufficient if it is less than 1.5%, and if it is added in excess of 7%, a brittle phase is formed in the structure. Since it easily precipitates, the addition amount was set to a range of 1.5 to 7%.

Ta, Nb: Ta combines with Nb together with Ni, which is a basic component, to form Ni ₃
Strengthen the parent phase as (Nb, Ta). However, if these elements are added in excess of 5% in total, the ductility and hot workability are markedly reduced, so the addition amount was made Ta + Nb ≦ 5%.

Ti: Ti combines with Ni and strengthens the matrix phase as Ni ₃ Ti. However, if added in excess of 2%, ductility and hot workability are significantly reduced, so the addition amount was made Ti ≤ 2%.

Al: Al combines with Ni and strengthens the matrix phase as Ni ₃ Al. However, if added in excess of 2%, the ductility and hot workability are significantly reduced, so the addition amount was made Al ≦ 2%.

Further, the amount of addition of Ta, Nb, Ti, Al is set within the range represented by the following formula in order to obtain a good and sound hot forging structure up to the first melting without double melting. Because it is necessary for.

Ta + 1,9Nb + 3.8Ti + 6.7Al ≦ 14 [Example] The experiments conducted by the inventors and their results will be described in detail below.

Inventive materials and comparative materials whose alloy compositions are shown in Table 1 below
kg or 150 kg vacuum induction melting, casting in a metal mold and casting in vacuum. As for the material with inferior torsional fracture characteristics as cast, the electrode was further scraped from the ingot and subjected to vacuum arc remelting to obtain a φ150 mm × 200 mmh cylindrical ingot with an improved cast structure.

Then, after press forged to 50 mm ^□ square rod with multiple paths so as not to exceed one pass of 10% reduction by heating to 1150 ° C., 1150 ° C.
It was forged into a 25 mm round bar shape by hot air hammer forging and heat treated. Regarding the heat treatment conditions at this time, each alloy was examined in detail so as to obtain the maximum tensile strength, and the heat treatment conditions shown in the following Table 2 were determined.

Using the test materials obtained as described above, an impact test, a tensile test, a torsion test, and a low strain rate tensile (SSRT) test were performed on them by the following procedures.

〔Test method〕

Impact test: Charpy impact test piece JIS4 (V notch 10mm
Square full size) was performed at 0 ° C.

Tensile test: A test piece having a parallel part shape of φ6 mm × 30 mml was used at room temperature. The pulling rate was 0.1 mm / min until the yield strength and 3 mm / min until the break.

Torsional test: The hot workability of the as-forged alloy was evaluated. Using a test piece having a parallel portion of φ10 mm × 30 mml, the test piece was held at 1150 ° C. and a torsion test was performed at a rotation speed of 100 rpm, and the rotation speed before breaking was evaluated.

Low Strain Rate Tensile (SSRT) Test: A low strain rate tensile test was performed in the environment and in the atmosphere to evaluate the environmental embrittlement resistance in a light water reactor environment. The test conditions at this time are shown in Table 3 below. The environment simulated the primary cooling water of a pressurized water type light water reactor. The evaluation was made based on the fracture ductility of the fractured material tested in the atmosphere and the presence or absence of a brittle fracture surface of the fractured material in the environment.

Of the above tests, the results of the impact test and tensile test are the second
The results of the SSRT test are shown in Table 4 below. The relationship between the Fe and C contents and the environmental embrittlement in the SSRT test is shown in Fig. 1, and the relationship between the G value in Table 1 and the tensile strength and hot workability by the torsion test is shown in the second.
FIG. 3 shows the relationship between tensile strength and Charpy absorbed energy.

In the SCC resistance evaluation by the SSRT test, it is generally known that a material with high Cr and Mo and a material with low C show excellent SCC resistance. In particular, as shown in Fig. 1, C is 0.05% or less. Suppression is necessary to show no brittleness. Moreover, in a system containing hydrogen as in this environment, grain boundary rupture is probably prominent due to hydrogen embrittlement, and this tendency can be avoided by suppressing the Fe addition amount to 5% or less. ing.

Also, the intermetallic compound Ni ₃ (Nb, Al, Ti) forming ability shown as the G value has a great influence on the hot workability of the alloy, and if it is too large, the double vacuum melting causes the microstructure. If it is not improved, it is not possible to perform good hot working as it is. As a result of various attempts by the present inventors to evaluate this by a hot torsion test, in the torsional rupture value during hot, an alloy that counts 3.5 times or more does not cause cracking in the vacuum melting forging without remelting. It was found that good forging can be carried out by the above method, and that the G value can be used as one of the excellent parameters for arranging the hot workability.

In Fig. 2, the obtained alloys with a torsional rupture value of 3.5 times or more are shown in outline plots. By controlling the G value at 14 in this way, an alloy showing good hot workability is obtained. be able to.

〔The invention's effect〕

According to the present invention described in detail above, regarding the Ni-based alloy for the structure used under the cooling water environment in the reactor, the stress corrosion cracking resistance,
A product with excellent strength and hot workability is obtained, and especially when used for fastening bolts for in-core structures, the current Inconel X-
It has become clear that it shows superior properties to 750 alloy.

[Brief description of drawings]

Fig. 1 is a graph showing the relationship between the Fe and C contents and environmental embrittlement.
FIG. 3 is a graph showing the relationship between G value, tensile strength and hot workability, and FIG. 3 is a graph showing the relationship between TS and Charpy absorbed energy.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Yusuke Minami Marunouchi 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Toshio Yonezawa 2-5-1 Marunouchi, Chiyoda-ku, Tokyo 3 Hishiju Heavy Industries Co., Ltd. (72) Inventor Toshihiko Iwamura 2-5-1, Marunouchi, Chiyoda-ku, Tokyo San Ryoju Kogyo Co., Ltd. (72) Inventor Banzo 2-5-1, Marunouchi, Chiyoda-ku, Tokyo Sanbishi Heavy Industries Co., Ltd. (72) Inventor, Kazuhide Aki, 2-5-1, Marunouchi, Chiyoda-ku, Tokyo Sanryo Heavy Industries Co., Ltd. In-house referee body Kouji Matsuura Judge, Asahi Asahi Judge, Mitsuo Teramoto (56) References Kai 62-167838 (JP, A) JP 60-131958 (JP, A) JP 63-53234 (JP, A) JP 57-101634 (JP, A)

Claims (1)

[Claims] 1. A weight ratio of C: 0.05% or less, Si: 0.5% or less, M
n: 0.5% or less, Fe: 5% or less, Cr: 18-30%, Mo: 1.5-7%
Including Ta + Nb: 5% or less, Ti: 2% or less, Al: 2% or less, and the balance of Ni and unavoidable impurities, and the intermetallic compound Ni in the austenite matrix by heat treatment. A nickel-base alloy with excellent resistance to stress corrosion cracking, characterized by being used by precipitating ₃ (Al, Ti, Nb, Ta). Ta ＋ 1.9Nb ＋ 3.8Ti ＋ 6.7Al ≦ 14