JPH08239739A - Heat tratment for ni-base alloy excellent in corrosion resistance - Google Patents

Abstract

PURPOSE: To improve stress corrosion cracking resistance under high-temp. and high-concentration alkaline environments by specifying the composition of an Ni-base alloy and the heating and cooling conditions at the final annealing, respectively. CONSTITUTION: The Ni-base alloy has a composition consisting of, by weight, 0.015-0.05% C, <=0.5% Si, <=0.015% P, >35-40% Cr, 50-57% Ni, <=0.5% Al, and the balance essentially Fe. As the conditions for subjecting an ingot of this Ni-base alloy to rolling and to final annealing, a treatment, consisting of heating up to a temp. between 1000-1200 deg.C, holding there for 1-60min, and cooling through the temp. region between 900 and 500 deg.C at (1 to 100) deg.C/sec cooling rate, is preformed. By the control of cooling velocity at cooling at the time of final annealing, grain boundary Cr carbides can be sufficiently precipitated and the occurrence of Cr-deficient layer can be prevented and, as a result, excellent stress corrosion cracking resistance can be obtained. Further, treatment time can be shortened because the necessity of two-stage heat treatment can be obviated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for a Ni-base alloy having excellent resistance to stress corrosion cracking, which is used as a tube material of a heat exchanger tube used in a pressurized water nuclear power plant.

[0002]

2. Description of the Related Art At present, as a tube material for a heat exchanger heat transfer tube of a chemical plant or a nuclear power plant (light water reactor) which is exposed to high-temperature and high-pressure water, Cr as disclosed in, for example, JP-A-59-232246 is used. : 25-35%, Ni: 4
0-70% Alloy690 alloy (trade name, 60% N
Ni-based alloys such as i-30% Cr-9% Fe alloy) have been used.

However, the water quality environment in an actual plant is a high temperature weak alkaline environment such as a pH of 9.2 ° C. to 9.5 at about 280 ° C., but alkali concentration in the gap between the heat transfer tube and the tube support plate. Under such a high temperature and high concentration alkaline environment, it cannot be said that even the above alloys can sufficiently secure the stress corrosion cracking resistance (SCC resistance).

In order to improve the resistance to stress corrosion cracking under such a high temperature and high concentration alkaline environment, Japanese Patent Laid-Open No. 59-59-
In the inventions disclosed in JP-A-85850 and JP-A-60-50134, a special heat treatment after the final heat treatment is a two-stage heat treatment of holding at 600 to 750 ° C. for 0.1 to 100 hours and then gradually cooling. Are doing.

The above-mentioned two-step heat treatment requires a long period of time for precipitating Cr carbide in the grain boundaries and for recovering the Cr-deficient layer near the grain boundaries due to the precipitation of Cr carbide. Also, this heat treatment is the final treatment, so that it can be used in the plant as it is after treatment.
In order to prevent the formation of an oxide film, heat treatment is usually performed in vacuum. As described above, since a two-step heat treatment that requires a long time is required and a vacuum treatment is performed, an increase in manufacturing cost cannot be avoided.

[0006]

As described above, the conventional heat treatment method for Ni-base alloys having excellent resistance to stress corrosion cracking is as follows.
Both require a two-step heat treatment, which requires a long time for treatment.
There has been a problem that manufacturing costs have risen.

The present invention eliminates the problems found in the above-mentioned conventional methods and provides a heat treatment method for a Ni-base alloy which is excellent in stress corrosion cracking resistance under a high temperature and high concentration alkaline environment.

[0008]

In order to achieve the above object, as a result of studying various alloys, as a result, in a Ni-based alloy having a Cr content of more than 35%, a two-step heat treatment is not required unlike the conventional method. It was found that grain boundary Cr carbide can be precipitated and a remarkable Cr-deficient layer can be suppressed by controlling the cooling rate during cooling in the final annealing. Further, it was found that by controlling the heating rate at the time of temperature rise, the precipitation amount of grain boundary Cr carbide can be increased and the stress corrosion cracking resistance can be improved. The present invention
It was completed based on the above findings.

That is, the first heat treatment method for a Ni-base alloy having excellent corrosion resistance according to the present invention is C: 0.015% by weight.
~ 0.05%, Si: 0.5% or less, P: 0.015%
Below, Cr: more than 35% and 40% or less, Ni: 50 to 5
7%, Al: 0.5% or less, and the balance of 1000 to 1000 in the final annealing during the production of the Ni-based alloy consisting essentially of Fe.
After heating to a temperature range of 1200 ° C and holding for 1 to 60 minutes, a temperature range of 900 to 500 ° C is cooled at a cooling rate of 1 to 100.
It is to cool at ° C / sec.

The second corrosion resistant Ni of the present invention is also used.
The heat treatment method for the base alloy is C: 0.015 by weight%.
0.05%, Si: 0.5% or less, P: 0.015% or less, Cr: more than 35% and 40% or less, Ni: 50 to 57.
%, Al: 0.5% or less, the balance is substantially 15% in the final annealing during the production of the Ni-based alloy consisting essentially of Fe.
After heating in the temperature range of 1000 to 1200 ° C at 0 to 500 ° C / min and holding for 1 to 60 minutes, 900 to 500 ° C
In the temperature range of 1 to 100 ° C./sec.

[0011]

According to the present invention, grain boundary Cr carbide is sufficiently precipitated by controlling the cooling rate at the time of cooling in the final annealing without performing the two-step heat treatment, and a remarkable Cr-deficient layer is generated. Can also be suppressed and excellent stress corrosion cracking resistance can be obtained. Further, by controlling the heating rate at the time of temperature rise, the precipitation amount of grain boundary Cr carbide can be increased, and the stress corrosion cracking resistance can be further improved. Moreover, since the two-step heat treatment is not required, the processing time is saved and the manufacturing cost is effectively reduced.

The reason for limiting the chemical composition of the alloy used in the present invention is as follows. C is Cr at the grain boundary
It is an essential element for precipitating carbide, and 0.015% or more is necessary to obtain the required amount of precipitation. However, if it exceeds 0.05%, the strength becomes too high, the pipe formability is deteriorated, and a Cr deficient layer is formed due to the precipitation of grain boundaries of Cr carbide in the final annealing to deteriorate the stress corrosion cracking resistance. Therefore, it is limited to 0.015 to 0.05%.

Si is an element necessary for deoxidation,
If it exceeds 0.3%, the precipitation of the a'phase of high Cr ferrite is accelerated, so the upper limit was made 0.3%. However, if it is too small, the deoxidizing effect will be insufficient, so it is desirable to contain 0.05% or more.

Mn is an element necessary for deoxidation like Si, and contains 0.5% or less.

Although P is contained as an impurity, it segregates at the grain boundaries and deteriorates the stress corrosion cracking resistance.
% Or less.

Cr is an essential element for maintaining the stress corrosion cracking resistance of the Ni-based alloy, and if it is less than 35%, the corrosion resistance required for a heat exchanger heat transfer tube exposed to high temperature and high pressure water can be secured. If it exceeds 40%, the precipitation of the a'phase of high Cr ferrite is accelerated, so that 35-40
Limited to%.

Ni is a main component of the alloy, and it is necessary to contain Ni in an amount of 50% or more in order to stabilize the structure and suppress the precipitation of the a'phase of high Cr ferrite. Although it is not necessary to set the upper limit from the viewpoint of corrosion resistance, the upper limit was set to 57% in consideration of the addition ratio of other alloy elements such as Cr, and was limited to 50 to 57%.

Al is an element having a deoxidizing action like Si and Mn, but if it exceeds 0.5%, the cleanliness of the alloy is lowered, so the content is limited to 0.5% or less. But,
If the addition amount is too small, the deoxidizing effect is not sufficient and the hot workability is deteriorated. Therefore, it is preferable to contain 0.05% or more.

Although Ti is a component element other than the above,
Ti is an element effective in improving the hot workability and the strength by fixing B as TiN or Ti (C, N) by combining with N, and it is desirable to contain 0.5% or less.

Next, the reason for limiting the heat treatment in the present invention will be explained. If the annealing temperature is less than 1000 ° C, a sufficient annealing effect cannot be obtained, and the amount of undissolved carbides is large and the strength becomes too high, and the amount of solute C decreases, so that the grain boundary is controlled by controlling the cooling rate. Since the amount of carbides deposited on the steel does not occur, sufficient corrosion resistance cannot be ensured.
If the temperature exceeds 0 ° C, the crystal grain size becomes too large and the strength decreases, so the temperature was limited to 1000 to 1200 ° C. Also,
If the annealing time at this time is less than 1 minute, a solid solution of C cannot be sufficiently obtained, and therefore it is necessary to hold for 1 minute or more. But,
Since the annealing effect does not change even if held longer than 60 minutes, it was set to 1 to 60 minutes.

It is necessary to control the cooling rate after annealing in the temperature range of 900 to 500 ° C. at which Cr carbides easily precipitate. If the cooling rate is less than 1 ° C./sec, carbide precipitation in the grain boundaries increases, but a Cr-deficient layer is formed in the vicinity of the grain boundaries due to the precipitation, so a cooling rate of 1 ° C./sec or more is required. . However, if it exceeds 100 ° C./sec, the precipitation of Cr carbide at the grain boundaries becomes insufficient and the corrosion resistance decreases.
Therefore, the cooling rate in the temperature range of 900-500 ° C is 1
Limited to -100 ° C / sec.

As described above, the cooling rate during annealing is the most important from the viewpoint of carbide precipitation, but by controlling the rate of temperature increase during heating to the annealing temperature, the precipitation amount of Cr carbide is further increased. The corrosion resistance can be improved. In general,
In annealing in a continuous furnace, the temperature rise is relatively slow (100
(° C / min), but 500-900 when the temperature rise is slow.
Cr carbides precipitate during the temperature rise in the temperature range of ° C, and then recrystallization and grain growth occur. For this reason, a large amount of Cr carbide is precipitated in the grains. Therefore, in order to increase the amount of carbide precipitated at the grain boundaries and further improve the corrosion resistance, it is necessary to set the heating rate at the final annealing to 150 ° C./min or more. However, when the temperature exceeds 500 ° C / min, the effect is saturated and the cost of the temperature raising device becomes high. Therefore, the upper limit is 500 ° C.
/ Min.

[0023]

EXAMPLES Alloys a, b, c and d of the present invention whose chemical compositions are shown in Table 1 and comparative alloys e, f and g which are not the subject of the present invention
Forging, hot rolling, intermediate heat treatment, cold rolling, etc., are applied to the ingot melted by vacuum melting to obtain a final plate thickness of 3
Finished into a cold rolled plate of mm. Then, final annealing was performed under various conditions shown in Table 2. Further, a corrosion test piece having a size of 2 × 10 × 40 (mm) and a low strain rate stress corrosion cracking test piece having a dimension of a parallel portion of the test piece of 2 × 3 × 20 (mm) were prepared from these treated materials, An intergranular corrosion test and a low strain rate stress corrosion cracking test (SSRT) were carried out as follows.

[0024]

[Table 1]

[0025]

[Table 2]

(1) Grain boundary corrosion test C at the grain boundary formed by grain boundary precipitation of Cr carbide
To detect the r-deficient layer, 65% HNO ₃ +0.2 g /
A 1Cr ⁶⁺ solution was used to carry out a boiling and 24-hour immersion test. In this test, the corrosion rate increases when there is a Cr-depleted layer.

(2) Low strain rate stress corrosion cracking test To detect intergranular stress corrosion cracking resistance, 20% NaOH was used.
A constant potential low strain rate stress corrosion cracking test was performed in an aqueous solution at 320 ° C. The potential was kept at the highest level of stress corrosion cracking susceptibility (corrosion potential +100 mV). In this environment, the slower the pulling rate, the higher the sensitivity to stress corrosion cracking.
The test was conducted by setting the strain rate to 8 × 10 ^-7 S ^-1 . The fracture surface after fracture was observed with a scanning electron microscope. The intergranular stress corrosion cracking resistance was evaluated by the intergranular fracture surface ratio of the fracture surface.

The test results are shown in Table 2. The e alloy containing a small amount of C given as Comparative Example 11 has a large intergranular fracture surface ratio and a poor intergranular stress corrosion cracking resistance even when subjected to the appropriate annealing as in the present invention. In addition, the f alloy containing a large amount of C in Comparative Example 12
The precipitation of Cr carbide is increased, the corrosion rate in the intergranular corrosion test is high, and the intergranular fracture surface ratio is also larger in the formation along the Cr deficiency. Furthermore, g with a low Cr content in Comparative Example 13
The alloy has a large corrosion rate and a high intergranular fracture surface ratio, and no improvement in intergranular stress corrosion cracking resistance is observed.

Further, using the b alloy of the present invention, Comparative Example 14 in which the annealing temperature deviates from the annealing conditions of the present invention, Comparative Example 15 in which the annealing time is short, and the cooling rate after annealing is 0.5 ° C. /
Comparative Example 16 slow as sec and cooling rate of 200 ° C./se
In Comparative Example 17 where c is large, the intergranular fracture surface ratio is large and no improvement in intergranular stress corrosion cracking resistance is observed.

On the other hand, in Examples 1 to 10 using the alloys a to d of the present invention and annealed under the annealing conditions of the present invention, the intergranular fracture surface ratio is small and the intergranular stress corrosion cracking resistance is low. It can be seen that the improvement has been achieved.

[0031]

According to the present invention, a Ni-base alloy having excellent intergranular stress corrosion cracking resistance can be obtained, and a heat exchanger heat transfer tube for a chemical plant or a nuclear (light water reactor) plant exposed to high-temperature high-pressure water. It is possible to provide an optimal alloy for the pipe material of.
Further, since the two-step heat treatment is not necessary, the processing time can be saved and the manufacturing cost can be reduced.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Haruhiko Kajimura 4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Mamoru Inoue 4-chome Kitahama, Chuo-ku, Osaka City, Osaka Prefecture No. 33 Sumitomo Metal Industries, Ltd.

Claims (2)

[Claims] 1. C: 0.015 to 0.05 by weight%
%, Si: 0.5% or less, P: 0.015% or less, C
r: more than 35% and 40% or less, Ni: 50 to 57%, A
l: 0.5% or less, the balance is 1000 to 1200 in the final annealing during the production of the Ni-based alloy consisting essentially of Fe.
After heating in the temperature range of ℃ and holding for 1 to 60 minutes, 900
~ 500 ℃ temperature range cooling rate 1-100 ℃ / sec
A method for heat treatment of a Ni-based alloy having excellent corrosion resistance, which comprises cooling with an alloy. 2. C: 0.015 to 0.05 by weight
%, Si: 0.5% or less, P: 0.015% or less, C
r: more than 35% and 40% or less, Ni: 50 to 57%, A
l: 0.5% or less, the balance is substantially 50% in the final annealing during the production of the Ni-based alloy consisting essentially of Fe
After being heated to a temperature range of 1000 to 1200 ° C. at 00 ° C./min and held for 1 to 60 minutes, the temperature range of 900 to 500 ° C. is cooled at a cooling rate of 1 to 100 ° C./sec. Excellent Ni-based alloy heat treatment method.