JPH0153341B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is 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 with excellent stress corrosion cracking resistance (hereinafter also referred to as SCC resistance). significantly improved sex
Concerning Ni-based high Cr alloy. Nickel ^- based alloys, which are said to have excellent stress corrosion cracking resistance, are used for tubes, containers, and their accessories in nuclear or chemical plants that are used in stress corrosion cracking environments containing Cl - ions. is often used. However, the conventionally commonly used 30%Cr-60%Ni
It has been reported that stress corrosion cracking cannot be avoided even with alloys based on the usage environment. The object of the present invention is to provide an alloy with excellent corrosion resistance, particularly stress corrosion cracking resistance, which is used in the form of thick plates, round bars, or pipes for tubes, containers, and accessory parts of nuclear or chemical plants, etc. The goal is to provide the following. Therefore, the present inventors developed a 30% Cr-
Focusing on the fact that 60% 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, When we investigated the relationship between morphology and 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 Cl ^- ions, so it is known as an effective element for improving pitting corrosion resistance.
When Mo, W, and V were added to strengthen the coating, it was discovered that, together with the aforementioned carbide precipitation effect, the corrosion resistance, that is, stress corrosion cracking resistance, of the resulting alloy was significantly improved. , the present invention has now been completed. Here, the gist of the present invention is that the weight%
So, 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%) One or more of Mo, W and V in total 0.5-5.0%, balance, substantial Cl ^- is composed of Fe and has a recrystallized grain structure in which at least most of the undissolved Cr carbides precipitate within the grains.
This is a Ni-Cr alloy with excellent stress corrosion cracking resistance in an ion-containing high-temperature water environment. Thus, according to the present invention, the stress corrosion cracking resistance of Ni-based high Cr alloys, which had been considered a problem in the past, is significantly improved. In addition to restricting
When final annealing is performed at a relatively low temperature of 900℃ to 975℃, Ti
Since Nb has a greater C fixing effect than Nb, fewer Cr carbides precipitate at grain boundaries, and at the same time, the synergistic effect of adding at least one of Mo, W, and V to strengthen the film. it is conceivable that. 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, Mn, Al: All of these elements are deoxidizing elements, and are added in appropriate amounts depending on the melting conditions.
When Si, Mn, and Al exceed the upper limits of 1.0%, 1.0, and 0.5%, respectively, they degrade the cleanliness of the alloy. Note that Al has no effect if it is less than 0.1%. Ni: Ni is an effective element for improving corrosion resistance, especially acid resistance and resistance in high-temperature water containing Cl ^- ions.
Improve SCC properties. For this purpose, Ni is required to be 40% or more, and the upper limit is set to 70% or less, taking into account the addition ratio of alloying elements such as Cr, Mo, W, and V. Cr: Cr is an essential element for improving corrosion resistance, but 25%
If it is less than that, the effect of improving SCC resistance will be 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 if it exceeds 0.030%, 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%, like P, it is harmful to acid resistance and hot workability. Ti: Since no significant effect can be obtained even if P and S are controlled below the above values, in the present invention, Ti is set at 0.05%.
By adding the above, predetermined hot workability is ensured. On the other hand, if it 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 higher), Nb has a greater carbon fixation effect than Ti. Therefore, the Nb content should be 0.2% or more and 5.0% or less and Nb/C 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, even if the content exceeds 5%, the effect (carbon fixation effect) will be saturated, and
The upper limit was set to 5.0 because it significantly deteriorated hot workability.
%. Mo, W, V: These elements are effective for improving pitting corrosion resistance, and particularly improve pitting corrosion resistance in high-temperature water containing Cl ^- 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, when 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, the present invention has a recrystallized grain structure in which at least most of the undissolved Cr carbides are precipitated within the grains. Annealing is performed at 975°C. For example, if the annealing temperature is lower than 900°C, recrystallization will not occur, resulting in high strength and insufficient corrosion resistance. On the other hand, when the temperature exceeds 975°C, the carbon in the alloy is completely dissolved during annealing, so that carbides no longer exist in the grains. Therefore, 975℃
For example, if annealing is performed at a temperature exceeding 600℃
When time sensitization treatment is performed, all carbides precipitate at grain boundaries, which deteriorates intergranular corrosion resistance.
Therefore, in order to prevent all the carbides from precipitating at the grain boundaries, at least most of the undissolved
In the present invention, the final annealing is performed at a temperature of 900 to 975°C so that Cr carbides are precipitated within the grains. Next, the present invention will be explained in more detail with reference to Examples. Example: 17 alloys with the chemical composition shown in Table 1.
Kg vacuum furnace, forged under normal conditions, hot rolled and heat treated, then 30% cold worked,
Subsequently, annealing was performed at various temperatures. Furthermore, after performing sensitization treatment by heat treatment at 600℃ x 5 hours, which was set based on the conditions that predicted the lifespan under actual use, the
40mm intergranular corrosion test piece and 2mm thick x 10mm wide x
A stress corrosion crack test piece with a length of 75 mm was taken. 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 then stacked one on top of the other and bent into a U shape to form a double U-bend test piece. Using an autoclave (high temperature and high pressure vessel), this was heated to 1000 ppm Cl ^- (as NaCl) at 330°C. It was immersed in the solution for 1500 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% HNO ₃ + 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 shows a graph summarizing the intergranular corrosion test results for a 25%Cr-55%Ni alloy with approximately 0.6% each of Mo, W, and V added, and with various changes in Nb/C. It is something. 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. It can be seen that alloys with Nb/C of less than 10 have very poor intergranular corrosion resistance, but when Nb/C exceeds 10, intergranular corrosion resistance rapidly improves. This means that if a sufficient amount of Nb is not added, Cr carbides will precipitate at the grain boundaries during sensitization treatment, creating a Cr-deficient layer near the grain boundaries, which will deteriorate corrosion resistance. it is conceivable that. Therefore, in order to fix carbon, a sufficient amount of Nb, that is, a large Nb/C ratio, is required, and the value is 10
More than twice as much is required. Figure 2 shows Nb/C between 12 and 20 and Cr: 25%.
and 0.6% each of Mo, W and V.
This figure shows the SCC (stress corrosion cracking) test results of alloys in which Ni was added and the Ni content was varied in the range of 18 to 75%. In this case, the test material is heated to 950℃.
It was annealed by heating 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 Mo, V, and W, the amount of Ni is 40%.
It was found that stress corrosion cracking occurs if the amount is less than 100%. Therefore, if the Ni content is 40% or more,
It became clear that the susceptibility to SCC was high. Figure 3 shows a Cr− with Nb/C of 12 or more and Ni of 40% or more in a high-temperature, high-pressure solution containing 1000 ppm ^Cl- ions.
The presence or absence of pitting corrosion in V-W alloys is summarized in a graph. In this case, the test material was annealed by heating at 950°C for 30 minutes, and then water-cooled. In the figure, circles indicate when V is added, and triangles indicate W.
indicates the case where it is added. Even if Nb/C is 12 or more, if the Cr content is less than 25%, V
Pitting occurs even when the content of 0.6% or more of W and 0.6% or more, respectively. Further, even if Cr is 25% or more, pitting corrosion occurs if the amount of V and W is less than 0.5%. Therefore, the area where pitting corrosion does not occur is 25% Cr or more and the value of V or W is 25% or more.
It has been found that 0.5%, preferably 0.6% is necessary. 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 V and W
It is thought that adding 0.5% or more of seeds in total has the effect of strengthening the passive film and preventing attack from Cl ^- ions. Therefore, it can be seen that a synergistic effect can be obtained by adding Cr, V and/or W in order to improve pitting corrosion resistance. This synergistic effect is further enhanced by Mo or Mo+W.
Data regarding the pitting corrosion resistance when +V is added is summarized in a table in FIG. 4 in the same manner as in FIG. 3. 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 the case where Mo+W+V are added. From the graph shown, in order to improve pitting corrosion resistance, the Cr content must be 25% or more, and the Mo content must be at least 25%.
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, annealing was performed at various annealing temperatures within the range of 850 to 1050°C, and then heated to 600°C for 5 hours to sensitize the material. After the chemical treatment, it was 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.

【table】

[Table] (Note) *: Outside the scope of the present invention

[Brief explanation of drawings]

1 to 5 are graphs summarizing the experimental results of the embodiments of the present invention.

Claims (1)

[Claims] 1% by 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 to 70%, Cr: 25-35%, Al: 0.1-0.5%, Ti: 0.05-1.0%, Nb/C: 10-125 (however, Nb: 0.2-5.0%) One or more of Mo, W and V in total 0.5 to 5.0%, the remainder essentially consists of Fe, and has a recrystallized grain structure in which at least most of the undissolved Cr carbides precipitate within the grains, Cl ^-
A Ni-Cr alloy with excellent stress corrosion cracking resistance in high-temperature water environments containing ions.