JPH0770680A - Alloy excellent in hot workability and corrosion resistance in high temperature water - Google Patents

Abstract

PURPOSE:To provide an alloy excellent in hot workability and corrosion resistance in high temp. water. CONSTITUTION:This alloy excellent in hot workability and corrosion resistance in high temp. water is the one having a compsn. contg., <=0.03% C, <=1.0% Si, <=1.0% Mn, >35 to 43% Cr, 40 to 57% Ni, 0.05 to 0.5% Al, Ti: N(%)X5 to 0.5% and Ca and Mg: by >=0.0005% independently and 0.001 to 0.03% in total, and in which, in impurities, the content of B is regulated to <=0.001%, O to <=0.002%, S to <=0.001% and N to <=0.02%. In addition to the same components, either or both components selected from the following two groups may be incorporated: the primary group... 0.01 to 0.1% in total of one or >=two kinds among Y, Ce and La and the secondary group... 0.5 to 5.0% in total of one or >= two kinds among Mo, W and V. Thus, the alloy excellent in SCC resistance and pitting corrosion resistance and having good hot workability can be obtd.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to corrosion resistance, particularly stress corrosion cracking resistance (corrosion resistance), which is suitable as a material for heat exchanger tubes used in pressurized water nuclear power plants and structures used in nuclear reactors. Hereinafter, it will be referred to as SCC resistance) and a hot workability Ni-Cr alloy.

[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 transfer tubes of heat exchangers of plants and chemical plants.
Alloy, all wt%) and modified Alloy 690 alloy (60%, same)
Ni-30% Cr-1.8% Nb-7% Fe alloy, all by weight) 25-35% by weight of Cr (hereinafter,
Simply referred to as%), Ni of 40-70% is used. Such alloys are disclosed, for example, in JP-A-59-232246 and JP-A-60-50134.

However, although the water quality environment in an actual plant is a high temperature weak alkaline environment where the pH at 280 ° C. is 9.2 to 9.5, alkali concentration occurs in the gap between the heat transfer tube and the tube support plate. There is. In an environment where a high concentration of alkali is concentrated at such a high temperature,
The SCC resistance is not perfect even with the above alloys.

In the above heat exchanger and the like, seawater used as cooling water leaks from the condenser and is mixed with Cl
^- 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 sex.

In order to improve the SCC resistance in an alkaline environment as described above, the inventors of the present invention in Japanese Patent Application No. 4-27825 have proposed an alloy containing 38 to 45% of Cr and 0.05% or more of N. Proposed. This alloy has good SCC resistance even in the harshest possible alkaline environment, but has a thin material such as a heat transfer tube (for example, thickness 1.
(27 mm), the hot workability is inferior.

In order to improve hot workability, addition of Ca is generally
Alternatively, it is said that the addition of Y or the like is effective as disclosed in Japanese Patent Publication No. 584862. However, when only such a component is added, satisfactory hot workability cannot be obtained when manufacturing the thin-walled heat transfer tube as described above from a high Cr, high Ni alloy.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention, the starting point of the stress corrosion cracking (SCC) occurred, Cl ^- high resistance to pitting corrosion in a high temperature water environment, including, hot weakly alkaline environment In order to provide an alloy having excellent SCC resistance and good hot workability.

[0008]

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

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

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

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

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

[0013] The term "high-temperature water" in the present invention, for example, refers to a high-temperature high-pressure water, such as the secondary side of a pressurized water nuclear reactor, in particular at about 300 ° C. approximately as described above, Cl ^- contaminated Or environmental water in which alkali concentration occurs.

[0014]

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

Cr is more than 35% and 43% or less, and B is 0.00
When it is 1% or less, the corrosion resistance can be improved.

Further, if Mo, W and V are added,
The pitting corrosion resistance can be further improved.

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

To further improve hot workability, A
It is more effective to add them in combination in relation to the contents of O, S, and N, than to add them individually from among 1, l, Ti, Ca, Mg, Y, Ce, and La.

The reason why the chemical composition of the alloy of the present invention is limited as described above will be explained below. % Means% by weight.

C: 0.03% or less C is chromium carbide (Cr ₂₃
C ₆ ) is an element that precipitates and causes deterioration of 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 made 0.03% or less.

Si and Mn: 1.0% or less for each Si and Mn are elements that act as a deoxidizing agent for the alloy, and it is necessary to contain them to some extent.
However, in all cases, if the content exceeds 1.0%, the weldability and cleanliness of the alloy will deteriorate, so the content of each will be 1.0%.
Below. On the other hand, if the content is too small, the deoxidizing effect will be insufficient, so it is desirable that the content be 0.05% or more.

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

Ni: 40 to 57% Ni is an element effective for improving the corrosion resistance, and particularly improves the acid resistance and the SCC resistance in high temperature water containing chloride ions (Cl ⁻ ). To obtain this effect, the Ni content needs to be 40% or more. On the other hand, it is not necessary to set an upper limit from the viewpoint of only corrosion resistance, but considering the contents of other elements such as Cr, it is sufficient to set the upper limit of the Ni content to 57%.

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

Ti: N (%) × 5-0.5% Ti combines with N to fix N as TiN or Ti (C, N),
It is an effective element for improving hot workability. To obtain this effect, the Ti content needs to be 5 times or more the N content. On the other hand, if it exceeds 0.5%, the effect is saturated, so the upper limit was made 0.5%.

Ca and Mg: 0.0005% or more alone, and 0.001 to 0.03% in total Ca and Mg are elements having a large affinity for S and O, which are harmful to hot workability. To improve hot workability,
It is necessary to fix S and O as CaS and MgO, respectively, by reducing S and O and by containing Ca and Mg in combination. To get this effect, type C
0.0005% or more for a and Mg respectively, and 0.00 in total
Must be contained at 1% or more. On the other hand, 0.03 in total
%, Not only the effect is 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 such as the present invention, and is 0.1% when it is high.
004% or more is included. However, B deteriorates the SCC resistance due to segregation at the grain boundaries or deteriorates the corrosion resistance by promoting the precipitation of Cr ₂₃ (C, B) ₆ , so the B content must be reduced to 0.001% or less. I have to. Desirable is 0.0005% or less.

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

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

N: 0.02% or less N is an element that improves pitting corrosion resistance, but it is desirable that N is low from the viewpoint of hot workability. Especially, the high Cr
Considering 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 fix Ti by adding Ti in the above range.

In addition to the above components, the alloy of the present invention may further contain a component selected from one or both of the following two groups, if necessary.

First group: 0.01 to 0.1% in total of one or more of Y, Ce, La Second group: Total of one or more of Mo, W, V in total
0.5 to 5.0% Y, Ce, La: These elements fix S like Ca and S
This has the effect of suppressing the adverse effects of the above and improving the hot ductility of the alloy itself. Therefore, it is effective to improve the hot workability by reducing the S content and adding these rare earth elements in addition to the composite addition of Ca and Mg in appropriate amounts. To obtain this effect, the content of one of these or the total content of two or more is 0.01%.
The above is necessary. On the other hand, if it exceeds 0.1%, not only the above effect is saturated, but also the inclusions in the alloy increase and become the starting point of the occurrence of pitting corrosion, so 0.1% was made the upper limit.

Mo, W, V: These elements are effective for improving pitting corrosion resistance. Although the pitting corrosion resistance is improved by making the Cr content to exceed 35% as described above, when further pitting corrosion resistance is required, the content of each one of these elements is Alternatively, it is 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 significantly deteriorated.

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

In this heat treatment, annealing for adjusting the mechanical properties is performed, and then Cr ₂₃ C ₆ to the grain boundaries, which is subsequently applied if necessary.
And the heat treatment for recovering the Cr deficient layer.

The annealing temperature is preferably 1000-1200 ° C. If the annealing temperature is lower than 1000 ° C, the tensile strength, yield strength, hardness, etc. will be unnecessarily high. On the other hand, when the temperature exceeds 1200 ° C., the crystal grains are remarkably coarsened, and predetermined properties 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 it is further heat-treated at a temperature of 800 ° C. or lower for 0.1 hour or longer, 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] In order to investigate hot ductility from these plate materials after melting alloys having chemical compositions shown in Table 1 (1) and Table 1 (2) by vacuum melting method and finishing by forging to thickness of 40 mm The test piece of was collected. Specimens for corrosion testing are hot rolled,
It was prepared by cold rolling into a plate material having a thickness of 4.9 mm and then subjecting it 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 above test pieces, an intergranular stress corrosion cracking resistance (SCC resistance) test, a pitting corrosion test and a hot workability test were conducted.

(1) Grain boundary stress corrosion cracking resistance (SCC resistance)
Test Using a plate-shaped tensile test piece with a parallel section of 3 mm x 20 mm, which was made from the plate solution with a thickness of 4.9 mm that had been subjected to the above solution treatment, at 320 ° C
A constant potential low strain rate tensile stress cracking (SSRT) test was conducted in a 20% aqueous NaOH solution. The potential is (corrosion potential + 100m
V) and the tensile speed was 4.2 × 10 ^-6 s ^-1 . The fracture surface after fracture was observed with a scanning electron microscope, and the SCC resistance was evaluated by the grain boundary fracture surface ratio of the fracture surface. The results are shown in Table 2.

As can be seen from Table 2, in Comparative Alloys a to c, the grain boundary fracture rate is 10% or more because the Cr content is low,
SCC resistance is inferior. In the alloy of the present invention having an appropriate Cr content (for example, No. 1), the grain boundary fracture surface ratio is 10% or less, and the SCC resistance
It has excellent properties. Even if the Cr content is within the range specified in the present invention, if the B content is 0.001% or more (comparative alloy s)
, The grain boundary fracture surface ratio exceeds 10%, which causes deterioration of SCC resistance.

(2) Pitting corrosion test Using a plate-shaped test piece of 2 mm in thickness × 15 mm in width × 15 mm in length, which was prepared from the plate material having a thickness of 4.9 mm subjected to the solution treatment, 10
Aqueous solution containing 00ppm Cl ^- ion [320 ℃, pH3.0 (H
_The pitting potential was measured by the polarization curve in ₂ SO ₄ ). The pitting potential was the potential at which the current density was 100 μA / cm ² . The results are shown in Table 2.

From Table 2, comparative alloys a to c with low Cr content
Shows that the pitting corrosion potential is low and the pitting corrosion resistance is poor. Further, even among the alloys of the present invention, the 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.

(3) Hot workability test Using a test piece cut out from a plate material finished by forging to a thickness of 40 mm, a tensile test is conducted at 1200 ° C. and the elongation at that time is measured to investigate the high temperature ductility. Then, the 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 the high Cr alloy having a Cr content of 35% or more, the contents of O (oxygen) and S have a great influence on the hot workability. Further, high-temperature ductility is improved by containing Y, La, and Ce in appropriate amounts.

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

As shown in FIG. 1, even if the O content is reduced to 0.002% or less, the high temperature ductility is deteriorated 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 defined by the present invention, high temperature ductility deteriorates unless Ca and Mg are added in appropriate amounts in combination. The alloy of the present invention (No.
Nos. 1, No. 2 and No. 10 to 12) show good hot ductility.

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

[0049]

[Table 1 (1)]

[0050]

[Table 1 (2)]

[0051]

[Table 2]

[0052]

The alloys of the present invention exhibits, SCC resistance in a high temperature weakly alkaline environment, Cl ^- is excellent in pitting resistance in a high temperature water environment containing, having good hot workability. This alloy is suitable as a material for heat exchanger tubes of nuclear power plants (light water reactors) and chemical plants that are exposed to high-temperature and high-pressure water.

[Brief description of drawings]

FIG. 1 is a diagram showing an influence of contents of S and O on hot ductility.

FIG. 2 is a diagram showing the influence of the contents of Mg and Ca on hot ductility.

FIG. 3 is a diagram showing the influence of Ti and N contents on hot ductility.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Makoto Ito 2-1-1, Niihama, Arai-cho, Takasago-shi, Hyogo Sanbishi Heavy Industries, Ltd. Takasago Laboratory (72) Inventor Dai Suwa Wadazaki-cho, Hyogo-ku, Hyogo-ku, Hyogo Prefecture 1-1-1 Mitsubishi Heavy Industries, Ltd. Inside Kobe Shipyard

Claims (4)

[Claims] 1. By 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 to 0.5%, Ti: N (%) x 5 to 0.5% and Ca, Mg: 0.0005% or more for each, and in total
It contains 0.001 to 0.03% and the balance is Fe and unavoidable impurities. B in the impurities is 0.001% or less and O (oxygen) is
0.002% or less, S 0.001% or less, N 0.02% or less, an alloy with excellent hot workability and corrosion resistance in high temperature water. 2. In addition to the components according to claim 1, a total of one, two or more of Y, Ce and La is 0.1% by weight.
An alloy excellent in hot workability and corrosion resistance in high temperature water according to claim 1, which is contained in an amount of 01 to 0.1%. 3. In addition to the components according to claim 1, one or more of Mo, W and V are added in total in weight%.
An alloy excellent in hot workability and corrosion resistance in high temperature water according to claim 1, containing 0.5 to 5.0%. 4. In addition to the components according to claim 1, the total amount of one, two or more of Y, Ce and La is 0.1% by weight.
An alloy excellent in hot workability and corrosion resistance in high-temperature water according to claim 1, containing 01 to 0.1% and one or more of Mo, W and V in a total amount of 0.5 to 5.0%.