JP2834982B2 - Alloy with excellent hot workability and corrosion resistance in high temperature water - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a corrosion resistance, particularly a stress corrosion cracking resistance suitable for a heat exchanger tube used in a pressurized water nuclear power plant and a structure used in a nuclear reactor. Hereinafter, referred to as SCC resistance) and a Ni—Cr alloy excellent in hot workability.

[0002]

2. Description of the Related Art Nuclear power (light water reactor) exposed to high temperature and high pressure water
Alloy 690 alloy (trade name, 60% Ni-30% Cr-9% Fe) as a material for heat exchanger tubes in heat exchangers of plants and chemical plants
Alloy, all by weight) and modified Alloy 690 alloy (60%,
Ni-based alloys such as Ni-30% Cr-1.8% Nb-7% Fe alloys, all of which are in weight%), have a Cr content of 25-35% by weight (hereinafter, referred to as
Simply referred to as%), and those with 40 to 70% Ni are used. Such alloys are disclosed in, for example, JP-A-59-232246 and JP-A-60-50134.

[0003] However, the water quality environment in an actual plant is a high-temperature weakly alkaline environment, for example, a pH of about 280 ° C and a pH of 9.2 to 9.5, but alkali concentration occurs in a gap between a heat transfer tube and a tube support plate. There is. Under such an environment where high concentration of alkali concentration occurs at high temperature,
Even with the aforementioned alloys, the SCC resistance is not perfect.

In the above-mentioned heat exchangers and the like, seawater used as cooling water leaks from the condenser and mixes in.
^- the environment is expressed pitting the heat transfer tube is produced, since the occurrence of SCC is promoted it as a starting point be used heat transfer tube of the alloy as described above, Cl ^- SCC resistance in high temperature water containing
There is a problem with gender.

The present inventors have disclosed in Japanese Patent Application No. Hei 4-27825 an alloy containing 38 to 45% of Cr and 0.05% or more of N in order to improve the SCC resistance in an alkaline environment as described above. Proposed. This alloy has good SCC resistance even under the most severe alkaline environment assumed, but has a small thickness such as a heat transfer tube (for example, a thickness of 1.
In consideration of processing to 27 mm), there is a problem that hot workability is inferior.

[0006] To improve the hot workability, generally, the addition of Ca,
Alternatively, as disclosed in JP-B-58-4862, addition of Y is said to be effective. However, when such a thin-walled heat transfer tube is produced from a high Cr high Ni alloy only by adding such components, satisfactory hot workability cannot be obtained.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a high resistance to pitting corrosion in a high temperature water environment containing Cl ^- which is a starting point of stress corrosion cracking (SCC), and a high temperature weak alkali environment. An object of the present invention is to provide an alloy having excellent SCC resistance and good hot workability.

[0008]

The gist of the present invention is as follows.
It is in alloys (1) to (4).

(1) By weight%, C: 0.03% or less, Si: 1.0
%, Mn: 1.0% or less, Cr: more than 35% and 43% or less, N
i: 40 to 57%, Al: 0.05 to 0.5%, Ti: N (%) × 5 to 0.5
% And Ca, Mg: 0.0005% or more each alone and 0.001 to 0.03% in total, with the balance being Fe and unavoidable impurities, B in the impurities is 0.001% or less, and O (oxygen) is 0.002% or less. , S is 0.001% or less, N is 0.02% or less. Alloy with excellent hot workability and corrosion resistance in high temperature water.

(2) In addition to the components described in (1) above,
The alloy having excellent hot workability and corrosion resistance in high-temperature water as described in (1) above, which contains one or more of Y, Ce and La in total of 0.01 to 0.1% by weight.

(3) In addition to the components described in (1) above,
The alloy having excellent hot workability and corrosion resistance in high-temperature water according to the above (1), containing one or more of Mo, W and V in a total amount of 0.5 to 5.0% by weight.

(4) In addition to the components described in (1) above,
(1) The above-mentioned (1) containing, by weight, one or more of Y, Ce and La in total of 0.01 to 0.1% and one or more of Mo, W and V in total of 0.5 to 5.0%. Alloy with excellent hot workability and corrosion resistance in high temperature water.

The term "high-temperature water" as used in the present invention means, for example, high-temperature high-pressure water such as the secondary side of a pressurized water reactor. Specifically, at about 300 ° C. as described above, Cl ^- is mixed. Or environmental water in which alkali concentration occurs.

[0014]

[Action] Hot workability and SC resistance under the above high temperature water environment
As a result of examining the influence of alloying elements on C properties and pitting corrosion resistance, the following findings were obtained.

Cr is more than 35% and 43% or less, and B is 0.00
By setting the content to 1% or less, an improvement in corrosion resistance can be obtained.

Furthermore, if Mo, W and V are added,
Pitting corrosion resistance can be further improved.

From the viewpoint of improving hot workability, it is essential to suppress the impurities O, S and N to a certain value or less.

In order to further improve hot workability, A
It is more effective to add them in combination with the contents of O, S, and N than to add them alone from among l, Ti, Ca, Mg, Y, Ce, and La.

Hereinafter, the reasons for limiting the chemical composition of the alloy of the present invention as described above will be described. % Means% by weight.

C: 0.03% or less C is chromium carbide (Cr ₂₃
C ₆ ) is an element that precipitates and deteriorates corrosion resistance at grain boundaries. If the C content exceeds 0.03%, the corrosion resistance at the grain boundaries deteriorates, so the C content was set to 0.03% or less.

Si and Mn: 1.0% or less, respectively Si and Mn are elements that act as deoxidizing agents for the alloy, and it is necessary to contain each of them to some extent.
However, if the content exceeds 1.0%, the weldability and cleanliness of the alloy will be reduced.
It was as follows. On the other hand, if the content is too small, the deoxidizing effect becomes insufficient, so that the content is desirably set to 0.05% or more.

Cr: more than 35% and less than 43% Cr is an essential element for maintaining SCC resistance. If the content is 35% or less, the corrosion resistance required for the above-mentioned applications cannot be secured. On the other hand, if it exceeds 43%, the hot workability is deteriorated due to the precipitation of a high Cr ferrite phase. Therefore, the range of the Cr content is set to more than 35% and 43% or less.

[0023] Ni: from 40 to 57% Ni is an element effective in improving the corrosion resistance, particularly acid resistance and chlorine ions ^- improving the SCC resistance in a high temperature water containing (Cl). To obtain this effect, the Ni content needs to be 40% or more. On the other hand, from the viewpoint of corrosion resistance alone, there is no particular need to set an upper limit, but considering the content of other elements such as Cr, setting the upper limit of the Ni content to 57% is sufficient.

Al: 0.5% or less Al is an element effective as a deoxidizing agent like Si and Mn.
If the content exceeds 0.5%, the cleanliness of the alloy is reduced, so the content is set to 0.5% or less. However, if the content is too small, the deoxidizing effect becomes insufficient and the hot workability deteriorates, so the lower limit is preferably set to 0.05%.

Ti: N (%) × 5 to 0.5% Ti is combined with N to fix N as TiN or Ti (C, N).
It is an element effective for improving hot workability. To obtain this effect, the Ti content must be at least five times the N content. On the other hand, if it exceeds 0.5%, the effect will be saturated, so the upper limit is set to 0.5%.

Ca, Mg: 0.0005% or more each alone, and 0.001 to 0.03% in total Ca and Mg are elements having a high affinity between S and O, which are harmful to hot workability. To improve the hot workability,
It is necessary to fix S and O as CaS and MgO, respectively, by reducing S and O and simultaneously containing Ca and Mg. To achieve this effect, C
a, Mg individually 0.0005% or more, and 0.00% in total
Must be contained at least 1%. On the other hand, a total of 0.03
%, The effect is not only saturated, but also the inclusions in the alloy increase, so the upper limit of the total content is 0.03%
And

B: 0.001% or less B is an element contained as an impurity in Ni, and is generally 0.002% or more in a Ni-based alloy as in the present invention, and is 0.
004% or more. However, since B deteriorates SCC resistance by segregation at grain boundaries or accelerates precipitation of Cr ₂₃ (C, B) ₆ , it deteriorates corrosion resistance. Therefore, the B content must be reduced to 0.001% or less. Must. Desirable is 0.0005% or less.

O: 0.002% or less O is an element that deteriorates hot workability and is suppressed to 0.002% or less.

S: 0.001% or less S is an element that degrades hot workability and is suppressed to 0.001% or less.

N: 0.02% or less N is an element for improving the pitting corrosion resistance, but from the viewpoint of hot workability, a lower N is desirable. In particular, the high Cr height of the present invention
In consideration of the improvement of hot workability in the Ni alloy, it is essential to suppress the N content to 0.02% or less, and it is necessary to further contain Ti in the above range to fix N.

In the alloy of the present invention, in addition to the above components, if necessary, components selected from one or both of the following two groups can be contained.

First group: One or two or more of Y, Ce and La in total 0.01 to 0.1% Second group: One or two or more of Mo, W and V in total
0.5-5.0% Y, Ce, La: These elements fix S like Ca,
This has the effect of suppressing the adverse effect of the alloy and also improving the hot ductility itself of the alloy. Therefore, it is effective to reduce the S content and to add the rare earth elements in addition to the above-described complex addition of Ca and Mg in an appropriate amount to improve hot workability. In order to obtain this effect, the content of one type or two or more types is 0.01% in total.
The above is necessary. On the other hand, if the content exceeds 0.1%, not only the above effect is saturated, but also the inclusions in the alloy increase to become the starting point of pitting corrosion, so the upper limit was made 0.1%.

Mo, W, V: These elements are effective in improving pitting resistance. Although the pitting corrosion resistance is improved by setting the Cr content to more than 35% as described above, when further pitting corrosion resistance is required, the content of each of these elements is one. Alternatively, they are added so that the total content of two or more kinds is 0.5 to 5.0%. If it is less than 0.5%, the passivation film on the surface is not strengthened, so that sufficient improvement in pitting corrosion resistance cannot be expected. On the other hand, if it exceeds 5.0%, not only this effect is saturated, but also the hot workability is remarkably deteriorated.

The alloy of the present invention contains the above-mentioned components. This alloy has sufficient corrosion resistance in the as-annealed complete solid solution structure or incomplete solid solution structure, but the desirable structure is a structure in which Cr ₂₃ C ₆ is precipitated at the crystal grain boundaries to strengthen the grain boundaries, More desirable is a structure in which a Cr-deficient layer near the grain boundaries generated by the grain boundary precipitation of Cr ₂₃ C ₆ has been recovered.
For this reason, it is desirable to perform an appropriate heat treatment.

In this heat treatment, annealing for adjusting the mechanical properties and Cr ₂₃ C ₆
And a heat treatment for recovering the Cr-deficient layer.

The temperature of the above annealing is preferably 1000 to 1200 ° C. If the annealing temperature is lower than 1000 ° C., the tensile strength, proof stress, hardness and the like become unnecessarily large. On the other hand, when the temperature exceeds 1200 ° C., the crystal grains are remarkably coarsened, and predetermined characteristics such as tensile strength, proof stress, and hardness cannot be obtained.

As described above, the alloy of the present invention is sufficiently excellent in corrosion resistance even in such an annealed state. However, when the alloy is further heat-treated at a temperature of 800 ° C. or less for 0.1 hour or more, the Cr ₂₃ C with ₆ precipitates at the grain boundaries, since Cr is diffused into the grain boundaries of Cr grain boundaries near the Cr-depleted zone is recovered from the crystal grains, hot weak even if the grain boundary precipitation of Cr ₂₃ C ₆ Grain boundary type SCC does not occur in an alkaline environment.

[0038]

EXAMPLE An alloy having the chemical composition shown in Table 1 (1) and Table 1 (2) was melted by a vacuum melting method, then forged to a thickness of 40 mm, and the hot ductility of these sheets was investigated. Of test pieces were collected. Specimens for corrosion tests were further hot-rolled,
A sheet material having a thickness of 4.9 mm was formed by cold rolling, and then subjected to a solution treatment at 1100 ° C. In Table 1, Nos. 1 to 15 are alloys of the present invention, and symbols a to s are comparative alloys.

Using the test pieces described above, an intergranular stress corrosion cracking resistance (SCC resistance) test, a pitting corrosion test and a hot workability test were performed.

(1) Intergranular stress corrosion cracking resistance (SCC resistance)
Test Using a plate-like tensile test specimen of 3 mm x 20 mm with a parallel portion made of a 4.9 mm thick plate material subjected to the solution treatment described above, at 320 ° C
A potentiostatic low strain rate tensile stress cracking (SSRT) test was performed in a 20% aqueous NaOH solution. Potential is (corrosion potential + 100m
V), and the tensile speed was 4.2 × 10 ⁻⁶ s ⁻¹ . The fracture surface after the fracture was observed with a scanning electron microscope, and the SCC resistance was evaluated by the grain boundary fracture ratio of the fracture surface. Table 2 shows the results.

As can be seen from Table 2, the comparative alloys a to c have a grain boundary fracture rate of 10% or more due to the low Cr content.
SCC resistance is poor. In the alloy of the present invention having an appropriate Cr content (eg, No. 1), the grain boundary fracture rate is 10% or less, and the SCC resistance
Excellent in nature. Even when the Cr content is within the range specified in the present invention, the B content is 0.001% or more (Comparative alloy s)
In this case, the grain boundary fracture rate exceeds 10%, leading to deterioration of SCC resistance.

(2) Pitting corrosion test Using a plate-shaped test piece of 2 mm thick x 15 mm wide x 15 mm long prepared from the 4.9 mm thick plate material subjected to the solution treatment described above,
00ppm of Cl ^- solution containing ions [320 ℃, pH3.0 (H
_The pH was adjusted with ₂ SO ₄ ). The pitting potential was a potential at which the current density became 100 μA / cm ² . Table 2 shows the results.

From Table 2, it can be seen that the comparative alloys a to c having a low Cr content
Shows that the pitting potential is low and the pitting resistance is poor. Further, even in the alloys of the present invention, alloys containing Mo, W, and V (Nos. 4 to 6, No. 8, and No. 13) have high pitting potential and excellent pitting resistance. It is clear that.

(3) Hot workability test Using a test piece cut out from a plate material finished to a thickness of 40 mm by forging, a tensile test was performed at 1200 ° C., and the hot ductility was investigated by measuring the elongation at that time. Then, hot workability was evaluated. The results are shown in Table 2 and FIGS.
◎ means 70% or more, ○ means 55% to less than 70%, Δ means 40% to less than 55%, and × means less than 40%.

As can be seen from Table 2, in a high Cr alloy having a Cr content of 35% or more, the contents of O (oxygen) and S in particular have a great effect on hot workability. When Y, La, and Ce are contained in appropriate amounts, the high-temperature ductility is improved.

FIG. 1 shows the effect of S and O on the hot ductility in FIG.
FIG. 3 shows the effect of Mg and Ca, and FIG. 3 shows the effect of Ti and N, respectively.

As shown in FIG. 1, even when the O content is reduced to 0.002% or less, the high-temperature ductility decreases when the S content is high.
It can be seen that in the case of a high Cr high Ni alloy such as the alloy of the present invention, simultaneous reduction of O and S is necessary. As shown in FIG. 2, even if the contents of O and S are within the range specified in the present invention, the high-temperature ductility is deteriorated unless Ca and Mg are added in an appropriate amount in combination. The alloy of the present invention (No.
1, No. 2, No. 10 to 12) show good hot ductility.

The hot workability is related to the contents of Al, Ti and N. The comparative alloy r having a low Al content shown in Table 2 has low hot ductility. As shown in FIG. 3, when the N content is suppressed to 0.02% or less and the Ti content is set to 5 times or more of the N content, the high-temperature ductility is improved.

[0049]

[Table 1 (1)]

[0050]

[Table 1 (2)]

[0051]

[Table 2]

[0052]

The alloy of the present invention is excellent in SCC resistance under a high-temperature weak alkaline environment, pitting corrosion resistance in a high-temperature water environment containing Cl- ^and has good hot workability. This alloy is suitable as a material for heat exchanger tubes of a nuclear (light water reactor) plant or a chemical plant exposed to high-temperature and high-pressure water.

[Brief description of the drawings]

FIG. 1 is a view showing the effect of the contents of S and O on hot ductility.

FIG. 2 is a graph showing the effect of Mg and Ca contents on high-temperature ductility.

FIG. 3 is a view showing the effect of the contents of Ti and N on hot ductility.

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hitomi Ito 2-1-1 Shinhama, Arai-machi, Takasago City, Hyogo Prefecture Inside the Takasago Research Laboratory, Mitsubishi Heavy Industries, Ltd. (72) Inventor Dai Dai Sunami 1 No. 1-1, Kobe Shipyard, Mitsubishi Heavy Industries, Ltd. (56) References JP-A-62-180028 (JP, A) JP-A-60-50134 (JP, A) JP-A-59-232246 (JP, A) JP-A-60-211029 (JP, A) JP-A-58-1042 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 30/00 C22C 19/00-19/05

Claims (4)

(57) [Claims] (1) In weight%, C: 0.03% or less, Si: 1.0% or less, Mn: 1.0% or less, Cr: more than 35% and 43% or less, Ni: 40
~ 57%, Al: 0.05 ~ 0.5%, Ti: N (%) x 5 ~ 0.5% and Ca, Mg: 0.0005% or more each alone and in total
0.001 to 0.03%, the balance consists of Fe and inevitable impurities, B in the impurities is 0.001% or less, and O (oxygen) is
Alloy with excellent hot workability and corrosion resistance in high-temperature water with 0.002% or less, S at 0.001% or less, and N at 0.02% or less. (2) In addition to the components described in (1), one or more of Y, Ce and La may be added in a total of 0.1% by weight.
2. The alloy according to claim 1, wherein the alloy has excellent hot workability and corrosion resistance in high-temperature water. 3. In addition to the component of claim 1, one or more of Mo, W and V in total by weight.
2. The alloy according to claim 1, which contains 0.5 to 5.0% and has excellent hot workability and corrosion resistance in high-temperature water. (4) In addition to the components described in (1), one or more of Y, Ce and La may be added in a total of 0.1% by weight.
2. The alloy according to claim 1, wherein said alloy contains 01 to 0.1% and one or more of Mo, W and V in a total amount of 0.5 to 5.0%.