JPH05345959A - Fe-cr-ni-si shape memory alloy excellent in resistance to intergranular corrosion and stress corrosion cracking - Google Patents

Abstract

PURPOSE:To obtain an Fe-Cr-Ni-Si shape memory alloy excellent in shape memory characteristics and resistance to intergranular corrosion and stress corrosion cracking by specifying the compsn. consisting of Cr, Si, Ni, Ti, Zr, Hf, V, Nb, Ta and Fe. CONSTITUTION:This shape memory alloy consists of, by weight, 16.0-21.0% Cr, 3.0-7.0% Si, 11.0-21.0% Ni, one or more among 0.01-1.0% Ti, 0.01-2.0% Zr, 0.01-2.0% Hf, 0.01-1.0% V, 0.01-2.0% Nb and 0.01-2.0% Ta and the balance Fe with inevitable impurities or further contains prescribed amts. of Mn, Cu, N, Mo and W. The above-mentioned components satisfy inequalities Ni>={0.67[Cr+1.2(Si+Ti+Zr+Hf+V+Nb+Ta)]-3} and 0.02<=[Ti+V+0.5(Zr+Nb)+0.25(Hf+ Ta)]<=2.0%. This shape memory alloy is excellent in various characteristics.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an Fe-Cr-Ni-Si-based shape memory alloy having excellent intergranular corrosion resistance and stress corrosion cracking resistance, and has a grain boundary resistance in nitric acid such as in a nuclear fuel reprocessing plant. Fe-Cr-Ni-Si with excellent corrosion resistance and intergranular erosion resistance in high temperature high pressure pure water such as light water reactor
It is intended to obtain a system shape memory alloy.

[0002]

2. Description of the Related Art A shape memory alloy is produced by subjecting an alloy to plastic deformation at a predetermined temperature in the vicinity of a martensitic transformation point, and then heating the alloy to a predetermined temperature equal to or higher than a temperature at which the alloy is reverse transformed into its matrix phase. An alloy exhibiting a property of recovering the shape before plastic deformation, and by subjecting this shape memory alloy to plastic deformation at a predetermined temperature, the crystal structure of the alloy transforms from its matrix phase to martensite.

When the alloy, which has been plastically deformed as described above, is heated to a predetermined temperature higher than the temperature at which the matrix is reversely transformed, the martensite is transformed back to the original matrix, that is, the alloy is The alloy exhibits shape memory characteristics, whereby the alloy plastically deformed is restored to its original shape before being plastically deformed. As an alloy having such a shape memory characteristic, many non-ferrous shape memory alloys have been known so far (for example, “Shape Memory Alloy” edited by Teruyasu Funakubo, 1984 Industrial Book).

However, among these conventional non-ferrous shape memory alloys, Ni--Ti type and Cu type shape memory alloys have already been put to practical use, and pipe joints, clothing, medical devices, actuators, etc. It is manufactured from a non-ferrous shape memory alloy, and in addition to this, the development of a technique in which this shape memory alloy is applied to various uses has been actively conducted in recent years.

However, from the viewpoint of application to structural members, Ni--Ti type shape memory alloys produce hydrogen compounds remarkably when used in high temperature pure water, and cannot be used in such an environment. Is. Further, the Cu-Zn-Al-based shape memory alloy lacks corrosion resistance. Moreover, since these non-ferrous shape memory alloys are expensive, they are economically limited. Under such circumstances, an iron-based shape memory alloy, which is cheaper than a non-ferrous shape memory alloy, is being developed, and in place of the non-ferrous shape memory alloy, which has restrictions in terms of economical efficiency, the application range of the iron-based shape memory alloy is changed. Expansion is expected.

From the viewpoint of its crystal structure, fct (face centered tetragonal), bct (body centered cubic) and hcp (martensite, which is an iron-based shape memory alloy transformed from its matrix phase by applying plastic deformation, are added. Dense hexagonal), which is an iron-based shape memory alloy that transforms from its matrix phase to dense hexagonal ε martensite by adding plastic deformation, and also as an alloy with excellent corrosion resistance. 77554 (hereinafter "First
“Prior art”) has been proposed. That is, according to the first conventional technique, Cr: 5.0 to 2.0 wt%, Si: 2.0
~ 8.0 wt%, at least one element selected from the group consisting of: Mn: 0.1-14.8 wt%, Ni: 0.1-20.0 wt%
%, Co: 0.1-3.0 wt%, Cu: 0.1-0.3 wt%, N:
It is a system containing 0.001 to 0.400 wt% and has excellent shape memory characteristics and corrosion resistance.

Further, in Japanese Patent Laid-Open No. 2-301514, as a high Mn type shape memory alloy having a high Cr content and improved corrosion resistance,
Cr: 10-17 wt%, Si: 3.0-6.0 wt%, Mn: 6.0-
25.0 wt%, Ni: 7.0 wt% or less, Co: 2.0-10.0 wt%
Then, an alloy system further containing Ti, Zr, V, Nb, Mo, Cu, etc. has been proposed (hereinafter referred to as "second conventional technology").

On the other hand, as an iron-based alloy having excellent resistance to stress corrosion cracking, for example, BEWILDE, CORROSION-NACE (1986), V
ol.42, No.11, p.678. That is, in this report, C
r: 17.0 to 19.0 wt%, Si: 0.35 to 4.79 wt%, N
i: 8.83 to 9.07 wt%, Mn: 1.30 to 1.53 wt%, C
u: 0.009 to 0.20 wt%, N: 0.011 to 0.040 wt
%, Mo: 0.019 to 0.21 wt%, and exhibits excellent stress corrosion cracking resistance in high temperature pure water (hereinafter referred to as "third prior art").

[0009]

When a shape memory alloy is used in nitric acid in a nuclear fuel reprocessing plant or in high temperature pure water (primary cooling water) in a light water reactor or the like, the characteristics of the alloy are the shape memory characteristic and the durability. It is necessary to have excellent intergranular corrosion resistance and stress corrosion cracking resistance, but none of the above-mentioned conventional techniques satisfies such requirements.

The iron-based shape memory alloy disclosed in the first prior art is an iron-based alloy to which both elements of Cr and Si are added for the purpose of improving shape memory characteristics and corrosion resistance, and further Mn, Ni. Although at least one element of Co, N and Co is added to the alloy, it has the following problems. That is, although it is a shape memory alloy with excellent corrosion resistance,
The corrosion resistance was evaluated by an atmospheric exposure test for 2 years, and the intergranular corrosion resistance in nitric acid and the stress corrosion cracking resistance in high temperature pure water cannot be said to be sufficient. As seen in the examples, the basic alloy system is Fe-13Cr.
-6Si system and Fe-18Cr-2Si system are roughly classified. The former alloy system has a Cr addition amount of 15.1 wt% or less, and the latter alloy system has a Si addition amount of 2.8 wt% or less. is there. For this reason,
The effects of improving the intergranular corrosion resistance in nitric acid and the stress corrosion cracking resistance in high temperature pure water, which are expected to be the effects of Cr and Si addition, are insufficient.

In the first prior art, the C and N contents are
Although the amount is limited to 0.1 wt% or less, according to the study by the present inventors, in the alloy system in which the total content of C and N exceeds 0.01 wt%, the processing which is indispensable for improving the shape memory property is performed. Heat treatment (for example, 500-700 ° C after deformation at room temperature)
When heat treatment (heating) is applied, Cr carbide or Cr
Due to the deficiency of Cr from the grain boundary accompanying the precipitation of the nitride at the crystal grain boundary, or even if the precipitate does not exist, the segregation of C or N at the grain boundary causes resistance to grain boundary resistance in nitric acid. It causes corrosion and deterioration of stress corrosion cracking resistance in high temperature pure water. However, in order to reduce the total content of C and N to 0.01 wt% or less in the alloy component system defined by the prior art, the current manufacturing technology has no choice but to use expensive raw materials, which is very expensive. Become.

Further, although Co is added as an arbitrary element in the first prior art, the Co content is 1.0 wt% or more as described in the examples. Therefore, it is not suitable for use in high-temperature pure water (primary cooling water) in the nuclear field from the viewpoint of activation, and its application range is limited.

The iron-based shape memory alloy disclosed in the second prior art has a high content of Cr for the purpose of improving the corrosion resistance and for the addition of Ti, Zr, V, and Nb to improve the shape memory characteristics. Although it is of the Mn type, this second conventional technique has the following problems. That is, first, the content of Cr is 10 to 1
Although it is 7 wt%, the Cr content is 16 wt in the embodiment.
It is less than%. Therefore, the effect of improving the intergranular corrosion resistance in nitric acid and the stress corrosion cracking resistance in high temperature pure water, which is expected as an effect of Cr addition, is insufficient.

Further, since the Mn content is 6.0 wt% or more, the increase in non-metallic inclusions deteriorates the stress corrosion cracking resistance in high temperature pure water, and at the same time, the intergranular corrosion resistance in nitric acid is also increased. to degrade. Further, since the Co content is 2.0 wt% or more, it is unsuitable for use in high temperature pure water (primary cooling water) in the nuclear field from the viewpoint of activation, and the application range is limited.

Further, the alloy disclosed in the third prior art is an alloy excellent in stress corrosion cracking resistance, but has a Si content of 2.9.
Alloys with wt% or less and alloys with Si content of 3.8 wt% or more are roughly classified. Since the former has a Si content of 2.9 wt% or less, the intergranular corrosion resistance and the stress corrosion cracking resistance in the above-mentioned environment are insufficient. In the latter case, the ratio of the total content of the austenite forming elements to the total content of the ferrite forming elements is not appropriate, so that the shape memory characteristic is insufficient.

Therefore, iron excellent in shape memory characteristics, intergranular corrosion resistance and stress corrosion cracking resistance that can be used in nitric acid such as nuclear fuel reprocessing plant and in high temperature pure water (primary cooling water) such as light water reactor Although development of a base shape memory alloy is strongly desired, such an iron base shape memory alloy has not been obtained yet.

[0017]

Means for Solving the Problems The present invention has been studied to solve the technical problems in the prior art as described above, and in any of shape memory characteristics, intergranular corrosion resistance and stress corrosion cracking resistance. We have succeeded in providing an excellent Fe-Cr-Ni-Si-based shape memory alloy.

When plastic deformation is applied to an hcp-type iron-based shape memory alloy at a predetermined temperature, the alloy phase is transformed into its parent phase,
That is, it transforms from austenite to ε martensite. The alloy whose parent phase has thus been transformed into ε-martensite is then subjected to an austenite transformation point (hereinafter “Af
Point)) and heating to a temperature near the Af point,
The ε-martensite undergoes a reverse transformation to its matrix phase, ie, austenite, so that the alloy subjected to plastic deformation recovers to its original shape before plastic deformation.

In order for the above hcp-type iron-based shape memory alloy to exhibit excellent shape memory characteristics, it is necessary to satisfy the following conditions.

(1) It is necessary that the matrix phase of the alloy before being plastically deformed at a predetermined temperature is mainly composed of austenite. The above-mentioned predetermined temperature, when plastic deformation is applied to the alloy at that temperature,
It is the temperature at which the matrix phase can transform into ε martensite.

(2) The stacking fault energy of austenite must be low. Furthermore, it is necessary to transform the alloy from its parent phase to only ε-martensite by applying plastic deformation to the alloy, and not to α′-martensite.

(3) The yield strength of austenite must be high. Further, when plastically deforming the alloy, no slip deformation should occur in the crystal structure of the alloy.

The present invention satisfies each of the above conditions,
Moreover, the present invention succeeds in solving the above-mentioned problems of the prior art, and is as follows.

(1) wt%, Cr: 16.0 to 21.0%, Si:
It contains 3.0 to 7.0%, Ni: 11.0 to 21.0%, and Ti: 0.01 to 1.0%, Zr: 0.01 to 2.0%, Hf: 0. .
01-2.0%, V: 0.01-1.0%, Nb: 0.01-2.0
%, Ta: 0.01 to 2.0%, and one or more of them is contained and Niwt% ≧ {0.67 (Cr + 1.2 (Si + Ti + Zr +
Hf + V + Nb + Ta))-3} wt% and 0.02 wt% ≤
{Ti + V + 0.5 (Zr + Nb) +0.25 (Hf + Ta)} wt% ≦ 2.0
A Fe-Cr-Ni-Si-based shape memory alloy excellent in intergranular corrosion resistance and stress corrosion cracking resistance, characterized in that it is wt% and the balance is Fe and inevitable impurities.

(2) wt%, Cr: 16.0 to 21.0%, Si:
It contains 3.0 to 7.0%, Ni: 11.0 to 21.0%, and Mn: 0.5 to 5.0%, Cu: 0.1 to 1.0%, N: 0. .00
1-0.1%, Mo: 0.1-3.0%, W: 0.1-3.0%, and any one or more of them are contained, and Ti: 0.01-
1.0%, Zr: 0.01 to 2.0%, Hf: 0.01 to 2.0%,
V: 0.01 to 1.0%, Nb: 0.01 to 2.0%, Ta: 0.01
To 2.0% of any one kind or two or more kinds, and (Ni + 0.5Mn + 0.06Cu + 0.002 (C + N)) wt% ≧
{0.67 (Cr ＋1.2 (Si ＋ Ti ＋ Zr ＋ Hf ＋ V ＋ Nb ＋ Ta) ＋ Mo ＋
W) −3} wt% and 0.02wt% ≦ {Ti + V + 0.5
(Zr + Nb) + 0.25 (Hf + Ta)} wt% ≤ 2.0 wt%, the balance being Fe and unavoidable impurities, Fe- excellent in intergranular corrosion resistance and stress corrosion cracking resistance
Cr-Ni-Si type shape memory alloy.

[0026]

With respect to the iron-based shape memory alloy of the present invention as described above, the reason why the chemical composition is limited to the above-mentioned range is as follows.

(1) Cr: Cr has an action of lowering stacking fault energy of austenite and increasing yield strength of austenite, and improves shape memory characteristics.
Further, Cr has an effect of improving the intergranular corrosion resistance and the stress corrosion cracking resistance of the alloy. The content of Cr is 16.0 wt%
If the amount is less than the above range, desired effects cannot be obtained, so the lower limit is set to 16.0 wt%. On the other hand, the Cr content is 21.
Since exceeding 0 wt% is economically disadvantageous, the Cr content should be limited to the range of 16.0 to 21.0 wt%.

(2) Si: Si improves the shape memory property because it has the effects of lowering the stacking fault energy of austenite and increasing the yield strength of austenite. Furthermore, Si also has the effect of enhancing intergranular corrosion resistance and stress corrosion cracking resistance. However, if the Si content is less than 3.0 wt%, the desired results cannot be obtained for the above-mentioned action. on the other hand,
If the Si content exceeds 7.0 wt%, the ductility of the alloy is significantly reduced, and the hot workability and cold workability of the alloy are significantly deteriorated. Therefore, the Si content is 3.0 to 7.0.
Limit to within wt% range.

(3) Ni: Ni is a strong element that forms austenite, and this Ni has an action of mainly converting the matrix phase of the alloy into austenite before the alloy is plastically deformed. If the Ni content is less than 11.0 wt%, the desired effect due to the above-described action cannot be obtained,
The lower limit was set to 11.0 wt%. On the other hand, the Ni content is 21.0 wt%
Above, the transformation point of ε martensite (hereinafter referred to as “Ms point”)
Is significantly shifted to a low temperature region, the temperature at which the plastic deformation of the alloy is applied is significantly lowered, and the shape memory property is deteriorated. Therefore, the upper limit is set to 21.0 wt%.
Therefore, it is necessary to limit the Ni content within the range of 11.0 to 21.0 wt%.

In the present invention, in addition to the above-mentioned Cr, Si and Ni, at least one kind of the following elements can be added.

(4) Mn: Mn is a strong element that forms austenite, and this Mn has an action of mainly converting the matrix before plastic deformation of the alloy into austenite. However, if the Mn content is less than 0.1 wt%, such an effect cannot be properly obtained, while the Mn content is 5.0 wt%.
%, The intergranular corrosion resistance is deteriorated, and the formation of the σ phase is significantly facilitated to deteriorate the shape memory property.
The upper limit was set to 5.0 wt%. That is, the Mn content is 0.1 to 5.0
It is necessary to limit it within the range of wt%.

(5) Cu: Cu is an austenite forming element, and Cu has a function of mainly converting the matrix phase of the alloy before applying plastic deformation into austenite.
Further, the addition of a small amount of Cu has the effect of improving the pitting corrosion resistance of the alloy. However, if the Cu content is less than 0.1 wt%, the desired effects due to these actions cannot be obtained.
On the other hand, when the Cu content exceeds 1.0 wt%, the formation of ε-martensite is hindered and the shape memory characteristic is deteriorated. The reason is that Cu has a function of increasing the stacking fault energy of austenite, so that the Cu content should be limited to the range of 0.1 to 1.0 wt%.

(6) N: N is an austenite forming element, and this N has a function of mainly converting the matrix phase of the alloy into austenite before the alloy is plastically deformed.
Furthermore, the addition of a small amount of N improves the pitting corrosion resistance of the alloy,
It has the effect of increasing the yield strength of austenite. If the N content is less than 0.001 wt%, these effects cannot be properly obtained, while if the N content exceeds 0.100 wt%, nitrides of Cr and Si are likely to be formed and the alloy The shape memory characteristics of are deteriorated. At the same time, the intergranular corrosion resistance in nitric acid and the stress corrosion cracking resistance in high temperature pure water deteriorate, and Ti, Zr, Hf, V, Nb and Ta described below are added within the range of the present invention. However, sufficient improvement cannot be obtained. Therefore, the N content is limited to the range of 0.001 to 0.100 wt%.

(7) Mo: Mo is an element effective in improving intergranular corrosion resistance and stress corrosion cracking resistance, and is 0.1
If less than wt%, these effects are insufficient, so lower limit
It was set to 0.1 wt%. However, addition of more than 3.0 wt% deteriorates the shape memory characteristics. Therefore, the upper limit is set to 3.0 wt%.

(8) W: W is an element effective for improving intergranular corrosion resistance and stress corrosion cracking resistance, and
If it is less than 1 wt%, its effect is insufficient, while it is 3.0 wt%.
Over 0.1 deteriorates the shape memory property, so 0.1-0.1
It was set to 3.0 wt%.

(9) Ti, Zr, Hf, V, Nb, Ta: All of these elements are strong C and N stabilizing elements and suppress the precipitation of Cr carbide or Cr nitride at the grain boundaries. This has the effect of suppressing deterioration of intergranular corrosion resistance and stress corrosion cracking resistance. Further, the present inventors have found that when the total content of C and N exceeds 0.01 wt%, the total content is sufficiently low (for example, 0.02 wt%) and precipitation of Cr carbide or Cr nitride does not occur. Even if it is necessary, the thermomechanical heat treatment (for example,
By heat treatment of heating to 500 to 700 ° C. after deformation at room temperature), intergranular corrosion resistance in nitric acid and stress corrosion cracking resistance in high temperature pure water deteriorate, and However, it has been found that the addition of C and N stabilizing elements is effective. In order to obtain a sufficient improvement effect by the addition of these elements, it is necessary to add all of them in an amount of 0.01 wt% or more, and 0.02 wt% ≦ {Ti + V + 0.5
(Zr + Nb) +0.25 (Hf + Ta)} wt% ≦ 2.0 wt% must be satisfied. However, since all of these elements are ferrite-forming elements, addition of a large amount deteriorates the shape memory characteristics and the hot workability and weldability. Therefore, Ti and V have an upper limit of 1.0 wt% and other elements. The upper limit of the elements is 2.0 wt%.

(10) Since P and S, which are impurity elements, are deteriorated in hot workability and toughness when they are contained in a large amount, it is necessary that the content of each of them be 0.1 wt% or less. Regarding C, which is an impurity element, when 0.1 wt% or more is contained, Ti, Zr,
Even if Hf, V, Nb, and Ta are added within the range of the present invention, the intergranular corrosion resistance in nitric acid and the stress corrosion cracking resistance in high temperature pure water cannot be sufficiently improved. It should be 0.1 wt% or less. When N is contained as an impurity element, it is necessary to limit it to 0.1 wt% or less for the same reason as the above C. Considering the problem of activation in the environment such as high temperature pure water (primary cooling water) represented by the nuclear field, Co, which is an impurity element, is preferably 0.1 wt% or less.

(11) Ratio of total content of austenite forming elements to total content of ferrite forming elements:
In the present invention, as described above, it is absolutely necessary that the matrix phase of the alloy before it is plastically deformed at a predetermined temperature is mainly composed of austenite.
Therefore, in the present invention, it is necessary to satisfy the following formula in addition to the above-mentioned limitation on the chemical composition. Ni wt% ≧ {0.67 (Cr + 1.2 (Si + Ti + Zr + Hf + V + Nb + Ta))-3} wt% (Ni + 0.5Mn + 0.06Cu + 0.002 (C + N)) wt% ≧ {0.67 (Cr + 1.2 (Si + Ti + Zr + Hf + V + Nb + Ta) + Mo + W) -3} wt% That is, by satisfying the above equation, the matrix phase of the alloy before being plastically deformed at a predetermined temperature,
It can be mainly austenite.

[0039]

EXAMPLES Specific examples of the present invention will be described. The present inventors have prepared alloy steels (No. 1 to 20) according to the present invention as shown in Table 1 below and comparative alloy steels (No. 21 to 31) having chemical composition outside the scope of the present invention, respectively. It was melted in a melting furnace under vacuum and cast into an ingot. The ingot obtained in this way is placed at 1100 to 1200 ° C.
Was heated to a temperature within the range of, and then hot-rolled to a thickness of 12 mm to obtain a specimen. The shape memory characteristics, intergranular corrosion resistance, and stress corrosion cracking resistance of each sample were examined by the tests described below. The results of these tests are shown in Table 2.
As shown in.

[0040]

[Table 1]

[0041]

[Table 2]

The characteristics in Table 2 are as follows.

(1) Shape memory characteristic A round bar-shaped test piece having a diameter of 6 mm and a gauge length of 30 mm was cut out from each of the present invention matchmaking Nos. 1 to 20 and comparative alloys No. 21 to 30. 4% tensile strain was applied to each of the test pieces cut out in this manner at -196 ° C,
Then, each test piece is heated to a predetermined temperature (300 ° C. or higher) near the Af point and in the vicinity of the Af point, the tensile strain is applied, and the gauge length between the test points after heating is measured, Based on the measurement results between the gauge points, the shape recovery rate was calculated by the following formula, and the shape memory characteristics of each match money were evaluated (Strictly speaking, the Ms point is slightly different for each match money). Although different, the optimum temperature for applying plastic deformation was -196 ° C and unified for each test).

The results of the above-mentioned shape memory characteristic test are shown in FIG. 1 in the column of “shape memory characteristic” in Table 2 above and in the alloys of the present invention Nos. 1 to 8 and comparative alloys Nos. 21 to 24. The evaluation criteria of the shape memory characteristic used here are as follows. ⊚: Shape recovery rate is 70% or more ◯: Shape recovery rate is 30 to less than 70% ×: Shape recovery rate is less than 30% However, L ₀ : initial gauge length of test piece L ₁ : gauge distance of test piece after tensile strain is applied L ₂ : gauge distance of test piece after heating

In FIG. 1, the horizontal axis represents the Si content (wt%),
The vertical axis indicates the Cr content (wt%), and the range surrounded by the dotted line in FIG. 1 indicates that the Cr content and the Si content are within the range of the present invention, and the “⊚” mark Has a shape recovery rate of 7
0% or more was recognized, and the “○” mark shows a shape recovery rate of 30.
% Or more and less than 70% is recognized, "x"
The mark indicates that the shape recovery rate is less than 30%. As is apparent from FIG. 1, the Ni content in the range of 11.0 to 21.0 wt%, the Cr content in the range of 16.0 to 21.0 wt% and the 3.
Matchings with Si content in the range of 0-7.0 wt% are
It exhibits excellent shape memory characteristics.

Comparative alloy "23," containing 1.8 wt% Si, which is outside the scope of the present invention, has very low shape memory properties. Further, the comparative alloy "24" containing 2.6 wt% of Si has a shape memory characteristic indicated by "○", but is inferior to the evaluation of the alloy of the present invention, and the intergranular corrosion resistance or Inferior in stress corrosion cracking. It should be noted that although sufficient shape memory characteristics are obtained for the comparative alloys "21" and "22" which are out of the scope of the present invention in FIG. 1, the intergranular corrosion resistance or stress corrosion cracking resistance described later is inferior. ..

(2) Intergranular corrosion resistance With respect to each of the alloys of the present invention Nos. 1 to 20 and comparative alloys No. 21 to 30, a plate-shaped test piece having a thickness of 4 mm, a width of 20 mm and a length of 100 mm was cut out and subjected to room temperature. 4% tensile strain at 600 ° C
The heating in was repeated 3 times. Thereafter, a plate test piece for corrosion test having a thickness of 2 mm, a width of 15 mm and a length of 20 mm was cut out from the plate test piece, the surface was wet-polished to # 600, and immersed in boiling 40% nitric acid. After immersion for 5 days, the cross section of the test piece was observed with an optical microscope, and the intergranular corrosion resistance was evaluated by examining the maximum erosion depth of the grain boundary. The results of the intergranular corrosion resistance test are shown in FIG. 2 in the column of “Grain boundary corrosion resistance” in Table 2 above and in the alloys of the present invention Nos. 1 to 8 and comparative alloys Nos.

The evaluation criteria for the intergranular corrosion resistance are as follows. ⊚: Maximum erosion depth is less than 10 μm ◯: Maximum erosion depth is 10 μm or more and less than 30 μm ×: Maximum erosion depth is 30 μm or more

As is clear from Table 1, Table 2 and FIG. 2, the Ni content in the range of 11.0 to 21.0 wt%, 16.0 to
Alloys with a Cr content in the range of 21.0 wt% and a Si content in the range of 3.0 to 7.0 wt% show excellent intergranular corrosion resistance.

Cr less than 16.0 wt% outside the scope of the present invention
Comparative alloys containing "21" and "22", 1.8 wt%
Comparative alloy “23” containing Si, comparative alloys “25”, “26” and “27”, and 5, in which the addition amount of C and N stabilizing elements such as Ti is insufficient or not added.
The comparative alloy "28" in which Mn was added in excess of 0 wt% was inferior in intergranular corrosion resistance. The comparative alloy "24" having a Si content of 2.6 wt% is evaluated as "○", but the stress corrosion cracking resistance described later is inferior. Also, Mo and W are 3.0
Comparative alloys "29" and "30" added in wt% or more are
Shows excellent intergranular corrosion resistance, but has insufficient shape memory characteristics.

(3) Stress corrosion cracking resistance With respect to each of the alloys of the present invention No. 1 to 20 and the comparative alloys No. 21 to 30, a plate-shaped test piece having a thickness of 4 mm, a width of 20 mm and a length of 100 mm was cut out and kept at room temperature. 4% tensile strain at 600 ° C
The heating in was repeated 3 times. Then, the test piece shown in FIG. 3 is cut out from the plate-shaped test piece, and each of the test pieces cut out in this way is set in the holder 3 shown in FIG. 4, and then the strain gauge 2 is attached to the test piece 1. Then, the tightening bolt 4 is pushed in to give a strain corresponding to a predetermined stress (yield stress). However, the stress corrosion cracking resistance of each alloy was evaluated by immersing it under the stress corrosion cracking test conditions shown in the following Table 3 and confirming the occurrence of cracks on the surface of the test piece after 3000 hours.

[0052]

[Table 3]

The results of the stress corrosion cracking test described above are also shown in Table 2 under the column "Stress corrosion cracking resistance" and Alloy No. 1 of the present invention.
8 and Comparative Alloys Nos. 21 to 24 are shown in FIG. 3, the evaluation criteria for crack initiation in the stress corrosion cracking test are as follows. ◯: No crack is observed (no crack). X: Cracking is recognized (cracking).

FIG. 5 shows Fe-Cr-Ni according to an embodiment of the present invention.
The influence of Cr and Si contents in the -Si-based shape memory alloy on the intergranular corrosion resistance is shown.
The horizontal axis shows the Si content (wt%), and the vertical axis shows the Cr content (wt%).
%), The range surrounded by a dotted line in FIG. 5 indicates that the Cr content and the Si content are within the range of the present invention. Further, in FIG. 5, the mark “◯” indicates that no crack was observed, and the mark “x” indicates that crack was observed.

As is apparent from Tables 1 and 2 and FIG. 5, the Ni content in the range of 11.0 to 21.0 wt% is 16.0.
All alloys having a Cr content in the range of 21.0 wt% and a Si content in the range of 3.0 to 7.0 wt% show excellent resistance to stress corrosion cracking. In contrast, the comparative alloy “21” containing less than 16.0 wt% Cr, which is outside the scope of the present invention
And "22", a comparative alloy "2" containing 1.8 wt% Si.
3 ”, comparative alloy“ 24 ”containing 2.6 wt% of Si, comparative alloys“ 25 ”,“ 26 ”and“ 2 ”with insufficient or no addition of C and N stabilizing elements such as Ti.
7 ”and the comparative alloy“ 28 ”in which Mn is added in an amount of more than 5.0 wt% are inferior in stress corrosion cracking resistance. Also, Mo
Comparative alloy containing 2 and more W and 3.0 wt% respectively [2
9 ”and“ 30 ”show excellent stress corrosion cracking resistance, but the shape memory characteristics are not sufficient.

[0051]

As described above, the Fe--Cr of the present invention is used.
-Ni-Si type shape memory alloys are excellent in both shape memory characteristics, intergranular corrosion resistance in nitric acid, and stress corrosion cracking resistance in high temperature pure water, and are therefore suitable for nuclear fuel reprocessing plants. High-temperature pure water in nitric acid and light water reactors (primary cooling water)
It is suitable for use as a structural material in the middle, etc., and is also suitable for use in a wide range of fields such as pipe joints and various tightening devices, actuators, biomaterials, etc. in addition to the nuclear field, and its manufacture. It is an invention that has the effect of being able to reduce the cost and has a great effect industrially.

[Brief description of drawings]

FIG. 1 is a table showing the influence of Cr and Si contents in shape memory characteristics in a Fe—Cr—Ni—Si type shape memory alloy according to an example of the present invention.

FIG. 2 is a table showing the influence of Cr and Si contents in the intergranular corrosion resistance in the Fe—Cr—Ni—Si shape memory alloy according to the example of the present invention.

FIG. 3 is an explanatory view of the shape of a stress corrosion cracking test piece used in the examples of the present invention.

FIG. 4 is an explanatory diagram of a stress loading method for a stress corrosion cracking test piece used in an example of the present invention.

FIG. 5 is a table showing the influence of Cr and Si contents in the stress corrosion cracking resistance in the Fe—Cr—Ni—Si shape memory alloy according to the example of the present invention.

[Explanation of symbols]

 1 Test piece 2 Strain gauge 3 Holder 4 Tightening bolt

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] March 25, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Claims

[Correction method] Change

[Correction content]

[Claims]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0025

[Correction method] Change

[Correction content]

(2) wt%, Cr: 16.0 to 21.0%, Si:
3.0 to 7.0% Ni: contained 11.0 to 21.0%, yet, Mn:. 0 1 ~5.0% , Cu: 0.1~1.0%, N: 0 0.001-
0.1%, Mo: 0.1 to 3.0%, W: 0.1 to 3.0%, and any one or more of them are contained, and Ti: 0.01 to 1.0. %, Zr: 0.01 to 2.0%, Hf: 0.01
-2.0%, V: 0.01-1.0%, Nb: 0.01-2.0%,
Ta: contains 0.01 to 2.0% of any one kind or two or more kinds, and (Ni + 0.5Mn + 0.06Cu + 0.002 (C + N))
wt% ≧ {0.67 (Cr + 1.2 (Si + Ti + Zr + Hf + V + Nb + Ta)
+ Mo + W) -3} wt%, and 0.02wt% ≦ {Ti + V
+0.5 (Zr + Nb) +0.25 (Hf + Ta)} wt% ≤ 2.0 wt%, with the balance consisting of Fe and unavoidable impurities, excellent in intergranular corrosion resistance and stress corrosion cracking resistance Fe
-Cr-Ni-Si type shape memory alloy.

 ─────────────────────────────────────────────────── ─── Continued front page (72) Inventor Naotake Ito 1-1-1, Wadasaki-cho, Hyogo-ku, Kobe-shi, Hyogo Mitsubishi Heavy Industries, Ltd. Kobe Shipyard (72) Toru Inazumi 1-chome, Marunouchi, Chiyoda-ku, Tokyo No. 1-2 Nihon Steel Tube Co., Ltd. (72) Inventor Yutaka Moriya 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Haruo Suzuki 1-1-Chome, Marunouchi, Chiyoda-ku, Tokyo No. 2 Japan Steel Pipe Co., Ltd. (72) Inventor Katsumi Masamura 1-1-2 Marunouchi, Chiyoda-ku, Tokyo No. 2 Nihon Steel Pipe Co., Ltd. (72) Takemi Yamada 1-1-Chome, Marunouchi, Chiyoda-ku, Tokyo No. 2 Japan Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. In wt%, Cr: 16.0 to 21.0%, Si: 3.
0 to 7.0%, Ni: 11.0 to 21.0%, Ti: 0.01 to 1.0%, Zr: 0.01 to 2.0%, Hf: 0.01
-2.0%, V: 0.01-1.0%, Nb: 0.01-2.0%,
Ta: contains 0.01 to 2.0% of any one kind or two kinds or more, and Niwt% ≧ {0.67 (Cr + 1.2 (Si + Ti + Zr + Hf +
V + Nb + Ta))-3} wt% and 0.02 wt% ≦ {Ti +
V + 0.5 (Zr + Nb) +0.25 (Hf + Ta)} wt% ≦ 2.0 wt% with the balance being Fe and inevitable impurities, excellent intergranular corrosion resistance and stress corrosion cracking resistance Was
Fe-Cr-Ni-Si based shape memory alloy. 2. Cr: 16.0 to 21.0% by weight, Si: 3.
0-7.0%, Ni: 11.0-21.0%, Mn: 0.5-5.0%, Cu: 0.1-1.0%, N: 0.001 ~
0.1%, Mo: 0.1 to 3.0%, W: 0.1 to 3.0%, and any one or more of them are contained, and Ti: 0.01 to 1.0. %, Zr: 0.01 to 2.0%, Hf: 0.01
-2.0%, V: 0.01-1.0%, Nb: 0.01-2.0%,
Ta: contains 0.01 to 2.0% of any one kind or two or more kinds, and (Ni + 0.5Mn + 0.06Cu + 0.002 (C + N))
wt% ≧ {0.67 (Cr + 1.2 (Si + Ti + Zr + Hf + V + Nb + Ta)
+ Mo + W) -3} wt%, and 0.02wt% ≦ {Ti + V
+0.5 (Zr + Nb) +0.25 (Hf + Ta)} wt% ≤ 2.0 wt%, with the balance consisting of Fe and unavoidable impurities, excellent in intergranular corrosion resistance and stress corrosion cracking resistance Fe
-Cr-Ni-Si type shape memory alloy.