JPS59232246A - Ni-cr alloy having excellent resistance to stress corrosion cracking - Google Patents

Abstract

PURPOSE:To provide an Ni-Cr alloy having excellent resistance to stress corrosion cracking by annealing an alloy consisting of a specific ratio of C, Si, Mn, P, S, Ni, Cr, Al, Ti, Nb, Mo, W, V and Fe at a specific temp. CONSTITUTION:An alloy consisting, by weight %, <=0.04% C, <=1.0% Si, <=1.0% Mn, <=0.030% P, <=0.02% S, 40-70% Ni, 25-35% Cr, 0.1-0.5% Al, 0.2-1.0% Ti, 10-125 Nb/C (where 0.2-5.0/ Nb), 0.5-5.0% total of 1 or >=2 kinds among Mo, W and V and the balance substantially Fe is annealed at 900-975 deg.C. An unsolutionized carbide is thus precipitated in the grains and the surface film is strengthened, by which the Ni-base high Cr alloy having considerably improved resistance to stress corrosion cracking is obtd.

Description

Detailed Description of the Invention The present invention provides a Ni-based high Cr alloy with excellent stress corrosion cracking resistance (hereinafter also referred to as SCC resistance), in particular, a Ni-based high Cr alloy that precipitates undissolved carbides in the grains and strengthens the surface film. This invention relates to a Ni-based high-Cr alloy that has significantly improved stress corrosion cracking resistance.

Used in stress corrosion cracking environments containing CI-ions,
For example, Nisogel-based alloys, which are said to have excellent stress corrosion cracking resistance, are often used in duplexes, containers, and their accessory parts for nuclear power or chemical plans 1 to 1. However, it has been reported that stress corrosion cracking cannot be avoided depending on the usage environment even with the conventionally used 30% Cr-60% Ni alloy.

The object of the present invention is to improve the corrosion resistance, especially the stress corrosion cracking resistance, of tubes, containers, and metal parts used in nuclear or chemical plants in the form of thick plates, round bars, or pipes. The goal is to provide the alloys that were given to us.

Therefore, the present inventors made the above-mentioned 30%Cr-60%
Focusing on the fact that Ni-based alloys are finally annealed at a relatively high temperature of 980 to 1150°C depending on the C content and are used in the absence of undissolved carbides, we investigated the morphology of carbides in the alloy structure. When we investigated the relationship with corrosion resistance, we found that actively precipitating carbides within grains is more effective in improving stress corrosion cracking resistance. In addition, it has been reported that stress corrosion cracking occurs starting from pitting corrosion in a high-temperature water environment containing CI- ions. When we tried to strengthen the film by using the above-mentioned method, the corrosion resistance of the obtained alloy was improved due to the carbide precipitation effect mentioned above.
In other words, the present invention was completed by discovering that the stress corrosion cracking resistance was significantly improved.

Here, the gist of the present invention is that in weight %, C:
0.04% or less, Si: 1.0% or less, Mn
: 1.0% or less, P: 0.030% or less,
s: o, o2% or less, Ni: 40-70%, C
r: 25-35%, 8l: 0.1
~0.5%, Ti: 0.2~1.0%, Nb/C: 10~125 (however, Nb: 0.2~5
．． Stress corrosion cracking resistance obtained by annealing at 900-975°C, consisting of a total of 0.5-5.0% of one or more of MO%W and It is a Ni-Cr alloy with excellent properties.

Thus, according to the present invention, Ni
The stress corrosion cracking resistance of base-heavy Cr alloys is significantly improved, and such an unexpected effect was demonstrated when the C content was reduced to 0.0.
Limit to 4% or less, and 900℃ to 975℃
When final annealing is performed at a relatively low temperature of
In Ni-based alloys containing 0% or more, Nb has a higher C than Tt.
This is thought to be the result of a synergistic effect resulting from the large fixing effect, which reduces the amount of C'r carbide precipitated at the grain boundaries, and at the same time, the addition of at least one of Mo, W, and V to strengthen the film.

The reason why the alloy composition and annealing temperature are limited as described above in the present invention is as follows.

C: Since C is an element harmful to SCC resistance, its content is limited to 0.04% or less.

Si, MnXAl: All of these elements are deoxidizing elements, and are added in appropriate amounts depending on the melting conditions.
, , A I is 1.0%, 1.0% and 0.5 respectively
Exceeding the upper limit of % will degrade the cleanliness of the alloy. Note that AI has no effect if it is less than 0.1%.

Ni: Ni is an effective element for improving corrosion resistance, and particularly improves acid resistance and SCC resistance in high-temperature water containing C1- ions. For this purpose, Ni needs to be at least 40%,
Further, the upper limit is set to 70% or less, taking into consideration the addition ratio of alloying elements such as Cr%Mo, W, and ■.

Cr: Cr is an essential element for improving corrosion resistance, but if it is less than 25%, the effect of improving SCC resistance is small. On the other hand, when it exceeds 35%, hot workability deteriorates significantly.

Therefore, in the present invention, Cr is limited to 25 to 35%. - P: P exists as an impurity in the alloy, and has a content of 0.
If it exceeds 0.30%, it is harmful to acid resistance and hot workability.

S: Similarly, S is a type of impurity, and if present in an amount exceeding 0.02%, it is harmful to acid resistance and hot workability like P.

Ti: Even if P and S are controlled to below the above values, no significant effect can be obtained. Therefore, in the present invention, by adding 0.05% or more of Ti, a predetermined hot workability is ensured. On the other hand, if the content exceeds 1.0%, the effect will be saturated, so the upper limit is set at 1.0%.

Nb: In Ni-based alloys (40% Ni or more), Nb has a greater carbon fixing effect than Ti. Therefore, the Nb content is 0.2% to 5.0% and Nb/C is 10 to 125%.
Double. If it is less than 0.2%, the effect of fixing carbon is small, resulting in sensitization and SCC (stress corrosion cracking). On the other hand, if the content exceeds 5%, the effect (carbon fixation effect) is saturated and hot workability is significantly deteriorated, so the upper limit is set at 5.0%.

MOLW, V: These elements are effective for improving pitting corrosion resistance, and particularly improve pitting corrosion resistance in high-temperature water containing CI- ions. If the total content of at least one of these elements is less than 0.5%, the passive film on the surface is not strengthened, causing pitting corrosion, which deteriorates stress corrosion cracking resistance. On the other hand, if the total content of at least one of these elements exceeds 5.0%, the effect of improving pitting corrosion resistance is saturated, and hot workability is significantly deteriorated.

Next, in the present invention, annealing is performed at a temperature of 900 to 975°C, but if the annealing temperature is less than 900°C, recrystallization will not occur, so the strength will not be high and the corrosion resistance will not be sufficient. On the other hand, if the temperature exceeds 975°C, the carbon in the alloy will be completely dissolved during annealing, so carbides will exist within the grains.
It disappears. Therefore, when annealing is performed at a temperature exceeding 975° C., for example, when a sensitization treatment is performed at 600° C. for 5 hours, all the carbides precipitate at the grain boundaries, which deteriorates the intergranular corrosion resistance. Therefore, in the present invention, the final annealing is 9
It is carried out at a temperature of 00-975°C.

Next, the present invention will be explained in more detail with reference to Examples.

A 17 kg alloy having the chemical composition shown in Table 1 was melted in a vacuum furnace, forged under normal conditions, hot rolled and heat treated, then cold worked by 30%, and then Then, annealing was performed at various temperatures. In addition, the temperature was set at 600℃
After sensitization treatment by time heat treatment, thickness 3×
A test piece for intergranular corrosion with a width of 10 mm x a length of 40 mm and a stress corrosion cracking test piece with a thickness of 2 mm x width of 10 + w x length of 75 + n + a were collected. These intergranular corrosion test pieces and stress corrosion crack test pieces were polished with No. 320 emery paper and used in the experiments described below.

First, two stress corrosion cracking test pieces were polished and stacked together.
A double U-bent test piece bent into a U shape was heated at 330℃ for 10 minutes using an autoclave (high temperature and high pressure container).
15 in a solution of 00 ppm CI- (as NaCl)
It was soaked for 00 hours. After the test was completed, the depth of the crack in the inner specimen was measured using a microscope.

On the other hand, the intergranular corrosion test piece was 60% HNO3 + 0.1% HF.
It was immersed in a boiling solution for 4 hours, and the corrosion weight loss at that time was measured.

Figure 1 is a graph summarizing the intergranular corrosion test results for a 25%Cr-55%Ni alloy with approximately 0.6% each of Mo, W, and ■ added, and with various changes in Nb/C. This is what is shown. In this case, the test gold was annealed by heating at 950° C. for 30 minutes, water-cooled, sensitized by heating at 600° C. for 5 hours, and then air-cooled. Nb/C
Alloys with a value of less than 10 have poor intergranular corrosion resistance, but those with a value of 1
It can be seen that when the value becomes 0 or more, the intergranular corrosion resistance improves rapidly. This is because if a sufficient amount of Nb is not added, Cr carbides will precipitate at the grain boundaries during the sensitization treatment, resulting in a Cr-depleted layer near the grain boundaries.
It is thought that corrosion resistance deteriorates. Therefore, in order to fix carbon, a sufficient amount of Nb, that is, a large Nb/C
A ratio of 10 times or more is required.

In Figure 2, Nb/C is 12 to 20 and Cr: 25%.
This figure shows the SCC (stress corrosion cracking) test results of alloys in which 0.6% each of Mo, W, and ■ were added, and the Ni content was varied in the range of 18 to 759A. In this case, the sample material was annealed by heating at 950° C. for 30 minutes, and then water-cooled.

Even in a 25% Cr alloy with Nb/C of 10 or more and containing 0.6% each of M(+, V, and W), the Ni content is 4.
It was found that stress corrosion cracking occurs if the content is less than 0%. Therefore, if the Ni content exceeds 40%, SCC
It was revealed that the sensitivity was high.

FIG. 3 is a graph summarizing the occurrence of pitting corrosion in a Cr-V-W alloy with Nb/C of 12 or more and Ni of 40% or more in a high-temperature, high-pressure solution containing 1000 ppm CI- ions. In this case, the sample material was annealed by heating at 950° C. for 30 minutes and then cooled with water. In the figure, circles indicate the case where ■ is added, and triangle marks indicate the case where W is added. Even if Nb/C is 12 or more, Cr
If the content is less than 25%, pitting corrosion will occur even if the V and W contents are each 0.6% or more. In addition, even if Cr is 25% or more, the amount of V and W is 0.5%.
If it is less than that, pitting corrosion will occur as well. Therefore, it has become clear that for a region where pitting corrosion does not occur, it is necessary to have a C'r of 25% or more and a value of ■ or W of 0.5%, preferably 0.6%. . From this, it can be seen that a passive film of only 25% Cr is insufficient to prevent the occurrence of pitting corrosion, and that at least one of
It is thought that adding 0.5% or more of 1 amount has the effect of strengthening the passive film and preventing attack from C1- ions. Therefore, in order to improve pitting corrosion resistance, Cr
, V and/or W can provide a synergistic effect.

FIG. 4 shows data compiled in the same manner as in FIG. 3 regarding the pitting corrosion resistance when Mo or MO+W+■ is further added to reflect such a synergistic effect. The sample material in this case was annealed by heating at 950° C. for 30 minutes and then water-cooled.

In the figure, circles indicate the case where Mo is added, and diamond marks indicate M.

The cases in which +W+V are added are shown.

From the graph shown, it is clear that in order to improve pitting corrosion resistance,
Cr content is required to be 25% or more, and M.

Alternatively, it can be seen that the total amount of Mo+V+W is 0.5% or more, preferably 0.6% or more.

FIG. 5 is a graph showing the influence of annealing temperature on stress corrosion cracking. In this case, using the test materials of alloy numbers 1 and 12 shown in Table 1, the annealing temperature was set to 850 to 10.
After annealing at various temperatures within the range of 50°C,
The sample was sensitized by heating to 00°C for 5 hours and then air cooled. A stress corrosion cracking test was conducted on each sample material thus obtained, and the crack depth was measured. As is clear from the data shown, those annealed at 900 to 975°C have excellent stress corrosion cracking resistance. The reason for this is thought to be that NbC precipitates and fixes the solid solution carbon.

[Brief explanation of drawings]

1 to 5 are graphs summarizing the experimental results of the embodiments of the present invention. Applicant: Sumitomo Metal Industries, Ltd. Mitsubishi Heavy Industries, Ltd., Agent Patent Attorney: Akira Hirose - House l leap Nb1c ratio #2巳Niyuaritsu (y,) L3 Figure 7, ■-Content (〃#i times Mo , Moive content (γ. Small rod 25 Concave cheeks and nose (0C) Mitsubishi Heavy Industries, Ltd. Takasago Research Institute, 2-1-1 Niihama, Arai-cho, Takasago City 0 Inventor: Toshio Yonezawa Niihama, Arai-cho, Takasago City No. 2-1-1 Mitsubishi Heavy Industries, Ltd., Takasago Research Center 0 Author: Shinya Sasaguri 2-1-1 Niihama, Arai-cho, Takasago City, Mitsubishi Heavy Industries, Ltd. Takasago Research Center ■ Applicant: Mitsubishi Heavy Industries, Ltd. 2, Marunouchi, Chiyoda-ku, Tokyo Chome 5-1

Claims (1)

[Claims] In weight %, C: 0.04% or less, Si: 1.0% or less,
Mn: 1.0% or less, P: 0.030% or less, S: 0.02% or less, Ni: 40-70%
, Cr: 25-35%, Al: 0.1-0.
5%, Ti: 0.2-1.0%, Nb/C: 10-125 (however, Nb: 0.2-5
．． 0%) Stress corrosion cracking resistant, consisting of one or more of Mo, W and V in a total of 0.5 to 5.0%, the remainder substantially Fe, and obtained by annealing at 900 to 975°C. Ni-Cr alloy with excellent properties.