JPH07103437B2 - Ni-based alloy with excellent stress corrosion cracking resistance - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Ni-base alloy for structures used in a cooling water environment in a nuclear reactor, and in particular, has developed a material excellent in stress corrosion cracking resistance for fastening bolts for internal reactor structures. It is intended to be provided.

[0002]

2. Description of the Related Art Conventionally, stainless steel or Ni-base superalloy has been used as an alloy for fastening bolts of structures in nuclear reactors such as light water reactors, and so-called Inconel X- 750 (trade name) (Ni-15.5Cr-
1Nb-0.7Al-2.5Ti-7Fe) etc.
Used with a tensile strength of 00kgf / mm ² level.

However, depending on the heat treatment conditions, Inconel X-750 may have a high sensitivity to stress corrosion cracking in a high temperature and high pressure water environment, and stress corrosion cracking may occur 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, JP-A-62-167836 to 6
In the alloy disclosed in No. 2-167839, the amount of Cr is 20 to 30% by weight (weight% is abbreviated as%, the same applies hereinafter), the amount of Mo is 10% or less, and the amount of By coexisting Al and Ti with Nb of 7% or less and Fe of 15% or less, an intermetallic compound containing Nb is precipitated and strengthened for use.

[0005]

The alloys that have been proposed so far are based on Ni as the alloy base and are useful for improving the corrosion resistance of Cr and Mo.
By adding, it has been attempted to improve the stress corrosion cracking resistance in the environment, but due to the coexistence of a large amount of Nb and Fe for strength improvement, it tends to be a very hard and brittle structure, and the optimum aging conditions The range is narrow and over-aged easily. In particular, the insufficient resistance 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.

The required strength characteristics are 80 kgf / mm.
Precipitation of intermetallic compounds such as γ′-Ni ₃ (Al, Ti) and γ ″ is used to obtain room temperature proof stress of ² levels and room temperature strength of 120 kgf / mm ² level. The structure that is dispersed and precipitated in is hard and poor in ductility, and inferior 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, and has a resistance to stress corrosion cracking due to brittleness and hydrogen of a Ni-based alloy for a structure used in a cooling water environment in a nuclear reactor. In addition, it is intended to improve hot workability.

[0008]

Therefore, the Ni-based alloy of the present invention, C: 0.05% or less, Si: 0.5% or less, Mn: 0.5% or less, Fe: 5% or less, Cr: 18-30%, Mo : 1.5 to 7%, Ta + Nb: 5% or less, Ti: 2% or less, Al: 2%
The following formula 1 is satisfied, and further, one or more kinds of rare earth elements: 0.1% or less, Ca: 0.1% or less, Mg: 0.1% or less, and the balance Ni and unavoidable impurities, In addition, the basic feature is that the intermetallic compound Ni ₃ (Al, Ti, Nb, Ta) is deposited and used in the austenite matrix by heat treatment.

[0009]

[Equation 1] Ta + 1.9Nb + 3.8Ti + 6.7Al ≦ 14

Next, the development process of the present invention having the above configuration will be described. From the above problems, the present inventors have made various studies for the purpose of improving the brittleness of alloys and the resistance to stress corrosion cracking due to hydrogen and the 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 the precipitation of intermetallic compound strengthened phases, especially Ni ₃ Ta and Ni ₃ Nb.If 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 also 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 that caused high cost was unnecessary, and good forging was possible only by primary melting. The possible range of addition was found.

In the present invention, the stress corrosion cracking resistance is improved by limiting the Fe content, and moreover, Ni ₃ T
It limits the total precipitation amount of intermetallic compounds such as a, Ni ₃ Nb, and Ni ₃ (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 deoxidizing agent, but if the content is high, the precipitation of brittle phases 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.
0.5% or less.

Fe: As described above, a large amount of Fe impairs the resistance to stress corrosion cracking caused by hydrogen, so the content is limited to 5% or less.

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. , The amount added 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 if its content is less than 1.5%, its effect is not sufficient. The brittle phase is likely to precipitate, so the addition amount was made 1.5 to 7%.

Ta, Nb: Ta combines with Nb, which is the basic component, together with Nb, and strengthens the parent phase as Ni ₃ (Nb, Ta). However, if these elements are added in excess of 5% in total, ductility and hot workability will be significantly 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 to strengthen 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%.

In addition, the inclusion of 1 to 3 of rare earth elements: 0.1% or less, Ca: 0.1% or less, and Mg: 0.1% or less is expected to improve hot workability by adding deoxidation and desulfurization. However, since the brittle phase is likely to precipitate in the structure if the added amount is large, the added amount is set to 0.1% or less.

Further, the addition amount of Ta, Nb, Ti, and Al is set within the range represented by the above-mentioned mathematical expression 1 because double melting is unnecessary and a good and sound hot forging structure is obtained up to the first melting. Because it is necessary to obtain.

[0024]

EXAMPLES The experiments conducted by the present inventors and the results thereof will be described in detail below. The inventive material and the comparative material whose alloy compositions are shown in the following Table 1 were vacuum induction melted by 50 kg or 150 kg, cast into a metal mold, and cast in vacuum. For materials with inferior torsional rupture characteristics as cast, the electrode was further cut from the ingot and vacuum arc remelting was applied to improve the cast structure φ150 mm
A columnar ingot of × 200 mmh was prepared.

[0025]

[Table 1]

Then, after heating to 1150 ° C. and press-forging into a 50 mm × 50 mm square rod shape in multiple passes so as not to exceed 10% reduction in one pass, it is forged into a φ25 mm round bar shape by 1150 ° C. heating air hammer forging, Heat treatment was applied. 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.

[0027]

[Table 2]

Using the test materials obtained as described above,
For these, an impact test, a tensile test, a torsion test and a low strain rate tensile (SSRT) test were conducted according to the following procedures.

[Test method] Impact test: Charpy impact test piece JIS No. 4 (V notch
It was carried out at 0 ° C. using a 10 mm square full size). 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-cast 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 air to evaluate the environmental embrittlement resistance under 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.

[0030]

[Table 3]

Among the above tests, the results of the impact test and the tensile test are shown in Table 2 above, and 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 FIG. FIG. 3 shows the relationship between the tensile strength and the Charpy absorbed energy.

[0032]

[Table 4]

In the SCC resistance evaluation by the SSRT test, the material having high Cr and Mo and the material having low C have excellent SC resistance.
It is generally known that C property is exhibited, and particularly as shown in FIG. 1, it is necessary to suppress C to 0.05% or less so as not to exhibit brittleness. Moreover, in a system containing hydrogen as in this environment, grain boundary rupture is probably significant due to hydrogen embrittlement, and this tendency can be avoided by suppressing the Fe addition amount to 5% or less, as shown in the same figure. ing.

The intermetallic compound Ni ₃ (N
b, Al, Ti) forming ability has a great influence on the hot workability of the alloy, and if it is too large, it will be satisfactorily hot-worked as cast unless the structure is improved by performing double vacuum melting. It cannot be processed. As a result of various attempts by the present inventors to evaluate this by a hot torsion test, an alloy that counts a torsional breaking value of 3.5 times or more in a hot state does not cause cracking in vacuum melting casting without remelting. It has been found that good castability can be achieved with, and that the G value can be used as one of the excellent parameters for arranging the hot workability.

In FIG. 2, the obtained alloy having a torsional rupture value of 3.5 times or more is shown by an outline plot.
By controlling the value at 14, it is possible to obtain an alloy exhibiting good hot workability.

[0036]

EFFECTS OF THE INVENTION According to the present invention described in detail above, there is provided a Ni-based alloy for a structure used in a cooling water environment in a nuclear reactor, which is excellent in stress corrosion cracking resistance, strength and hot workability. It was clarified that when it was obtained, and particularly when it was used for fastening bolts for the internal structure of the furnace, it exhibited properties superior to those of the current Inconel X-750 alloy.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amounts of Fe and C and environmental embrittlement.

FIG. 2 is a graph showing the relationship among G value, tensile strength and hot workability.

FIG. 3 is a graph showing the relationship between TS and Charpy absorbed energy.

(72) Inventor Yusuke Minami 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd. (72) Inventor Toshio Yonezawa 2-5-1, Marunouchi, Chiyoda-ku, Tokyo Sanryo Heavy Industries Co., Ltd. In-house (72) Inventor Toshihiko Iwamura 2-5-1, Marunouchi, Chiyoda-ku, Tokyo Sanryo Heavy Industries Co., Ltd. (72) In-house companion 2-5-1, Marunouchi, Chiyoda-ku, Tokyo Sanryo Heavy Industries Co., Ltd. (72) The inventor, Kazuhide Aki, 2-5-1, Marunouchi, Chiyoda-ku, Tokyo Kenichi Hara, Examiner, Sanryo Heavy Industries Co., Ltd. (56) Reference JP 63-53233 (JP, A) JP 57-123948 (JP, A) JP 62-167838 (JP, A)

Claims (1)

[Claims] 1. A weight ratio of C: 0.05% or less, Si: 0.5% or less, Mn: 0.5% or less, Fe: 5% or less, Cr: 18 to 30%, Mo:
Contains 1.5 to 7%, Ta + Nb: 5% or less, Ti: 2%
Below, Al: 2% or less and satisfy the condition of the following formula number 1,
Rare earth elements: 0.1% or less, Ca: 0.1% or less, Mg: 0.1
% Or less, including 1 to 2 or more, balance Ni and unavoidable impurities, and characterized by using an intermetallic compound Ni ₃ (Al, Ti, Nb, Ta) precipitated in an austenite matrix by heat treatment. Ni with excellent stress corrosion cracking resistance
Base alloy. [Equation 1] Ta + 1.9Nb + 3.8Ti + 6.7Al ≦ 14