JPS6324062B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a Ni-based alloy material that exhibits excellent intergranular corrosion resistance and stress corrosion cracking resistance, as well as good strength, particularly 0.2% proof stress. Generally, JIS NCF600 (75%Ni-15%Cr-8%
Ni-based alloys containing compatible C and Cr such as Fe) have been recognized as heat-resistant materials because they exhibit excellent high-temperature strength, but they also have strong resistance to stress corrosion cracking. Therefore, it has recently been reconsidered as a corrosion-resistant material. However, since the C solid solubility limit of Ni-based alloys is extremely low, under certain usage conditions a Cr-deficient layer may be formed due to the precipitation of Cr carbides, resulting in intergranular corrosion type stress corrosion cracking in high-temperature, high-pressure water. is also known. Therefore, in order to prevent such stress corrosion cracking, it is necessary to reduce the amount of intergranular precipitation of Cr carbides as much as possible to weaken the intergranular corrosion susceptibility. Conventionally, in order to reduce the intergranular corrosion susceptibility of Ni-based alloys containing C and Cr as mentioned above, the C content was kept as low as possible and the final annealing process was
Prevent carbide precipitation at grain boundaries by heating to 900-1050°C and then rapidly cooling. A process of heating to 1080-1120°C, soaking to 850-900°C, and then rapidly cooling. (Japanese Unexamined Patent Publication No. 106811/1983), it has been proposed to apply such means as When thermal processing is performed, a thermal history from the melting temperature to room temperature remains in the weld heat affected zone, and in the area heated to the sensitization temperature, Cr carbide is reprecipitated at the grain boundaries, reducing grain resistance. There was a built-in fear that this would lead to the inconvenience of deterioration due to corrosive properties. Furthermore, in such cases, if an attempt is made to improve intergranular corrosion resistance by annealing to dissolve carbides into solid solution, the Cr carbides will decrease, leading to a significant decrease in the strength of the material, especially the 0.2% yield strength. There was a problem. On the other hand, to solve this problem, Ni
Heating the base alloy to 800-875℃, then 600-675℃
A method of performing stabilization treatment was also proposed, but this method also had the disadvantage that the proof stress decreased by 0.2% if the hot rolling end temperature was high, and the stabilization treatment required a long time. . From the above-mentioned perspective, the inventors have determined that 0.2%
In order to obtain a metal material that exhibits a yield strength of 25 Kgf/mm ² or more and has excellent intergranular corrosion resistance and stress corrosion cracking resistance that are fully satisfactory even in environments such as high-temperature and high-pressure water, we have particularly developed high-temperature strength. Focusing on Ni-based alloys, which exhibit excellent corrosion resistance as well as excellent corrosion resistance, we searched for ways to improve their properties through repeated trial and error. (a) C: 0.15% or less (hereinafter referred to as % weight indicating component proportions) %). Si: 0.50% or less, Mn: 1.00% or less, Cr: 14-17%, Fe: 6-10%, Ni: 72%
Ni-based alloys containing C and Cr, such as the JIS NCF600 alloy with the above composition, are subjected to strong working in a temperature range below the recrystallization temperature to completely destroy the crystal structure, and then the When kept at a high temperature within the crystal temperature range, carbides precipitate rapidly and in large quantities on the slip lines remaining in the crystal grains, reducing the amount of carbide precipitated at old grain boundaries, and dispersing the carbides uniformly as a whole. (b) When this Ni-based alloy is recrystallized in a specific temperature range, a recrystallized structure is obtained in which the previously precipitated carbides are not re-dissolved and there is no concentration of precipitated carbides at grain boundaries. It has excellent intergranular corrosion resistance, and because of the uniformly precipitated carbides.
We have come to the knowledge shown in (a) and (b) above that a material that is fully satisfactory with 0.2% proof stress can be realized. This invention was made based on the above knowledge, and after processing a Ni-based alloy containing C and Cr at a processing rate of 20% or more in a temperature range below the recrystallization temperature, Excellent intergranular corrosion resistance and stress corrosion cracking resistance are achieved by holding the material in a temperature range below the recrystallization temperature to precipitate carbides, then heating it to a temperature range of 850 to 950°C for a short time, and then rapidly cooling it. It is also characterized by the ability to efficiently manufacture Ni-based alloy materials that have a high value of 0.2% proof stress in a relatively short time. In addition, the Ni targeted in the method of this invention
It should be clear from the above description that the base alloy may be of any type as long as it contains C and Cr as typified by JIS NCF600. Next, the reason why the manufacturing conditions for the Ni-based alloy material are limited as described above in the method of the present invention will be explained. A Processing of 20% or more below the recrystallization temperature This process is performed to achieve a recrystallized structure without grain boundary carbide precipitation in the material as processed, and to improve intergranular corrosion resistance. The purpose is to completely destroy the crystal structure. For example, the recrystallization temperature of the JIS NCF600 alloy is
The temperature is 850 to 900°C, but in order to completely destroy the crystal structure, it is necessary to perform processing at a temperature lower than this temperature and at a processing degree of 20% or more, preferably 30% or more. If the working degree is less than 20%, the crystal structure will not be completely destroyed and a recrystallized structure with carbide precipitated at the grain boundaries will appear in some parts, so 20% or more is added in the temperature range below recrystallization. I set the conditions. B Holding at 650℃ or higher and lower than the recrystallization temperature This temperature range is targeted by the method of this invention.
This is a temperature range at which Ni-based alloys do not recrystallize and rapidly precipitate Cr carbides. If this holding temperature is lower than 650℃, a sufficient amount of carbides cannot be quickly precipitated on the slip lines remaining in the grains, whereas if held above the recrystallization temperature, carbides will concentrate at the grain boundaries. Since a recrystallized structure precipitated by this process appears and deteriorates intergranular corrosion resistance, the holding temperature was set at 650°C or higher and lower than the recrystallization temperature. Note that the longer the holding time at this temperature, the more advantageous it is to precipitation of carbides, so it is preferable.
The effect was sufficiently recognized even with soaking at a rate of about 5 minutes/mm. C Short-time heating at 850-950°C This heating is to make the Ni-based alloy a recrystallized structure and remove processing strain, etc. However, if the temperature is below 850°C, a recrystallized structure cannot be obtained. I can't do it,
On the other hand, heating above 950°C promotes solid solution of precipitated carbides, resulting in a 0.2% reduction in yield strength, so the heating temperature was set at 850 to 950°C. However, in order to maintain strength, it is preferable to keep the heating temperature as low as possible, so the heating temperature should be set at 850 to 900℃.
It is recommended that you select In addition, since the purpose of this heating is to recrystallize as mentioned above, a short soaking time of 1 to 5 minutes/mm is sufficient for the heating time to complete recrystallization. This is because if the heating time is less than 1 minute/mm, recrystallization cannot be completed as described above, whereas if the heating time exceeds 5 minutes/mm, carbides will precipitate and tend to become a solid solution. It is. D Rapid cooling after short-time heating at 850 to 950°C The reason for rapid cooling after heating in the second stage heat treatment is as follows.
Since carbides show a slight tendency to form a solid solution at temperatures above 900°C, the aim is to prevent redecipitation of carbides at grain boundaries during cooling. Water cooling, which is commonly used, is sufficient as the rapid cooling means. Next, the present invention will be explained by examples and in comparison with comparative examples. Example First, the component composition shown in Table 1, arrangement 850℃
A 3-ton ingot was obtained by melting a Ni-based alloy with a recrystallization temperature of .

[Table] Next, this ingot was press cast and then hot rolled into a 25mm thick plate, which was further hot rolled at varying processing rates under the conditions shown in Table 2 to achieve the desired dimensions. Manufactured plate materials. The processing rate of 850℃ or less and the structure state of the obtained plate material were determined in the second
It is also shown in the table. Next, each of the obtained steel plates was subjected to heat treatment under the conditions shown in Table 3, and then subjected to a tensile test and an intergranular corrosion test. The results are also shown in Table 3.

【table】

【table】

【table】

[Table] As is clear from Tables 2 and 3, rolled material A
is recrystallized as rolled, and 2 in the method of the present invention.
Although the 0.2% yield strength is low even after step heat treatment, rolled material B has a working rate of 20% or more at 850°C or less,
Since C and D have non-recrystallized structures, the two-step heat treatment in the method of the present invention reduces the
The % proof stress in all cases is as high as 25Kgf/mm ² or more. Although this example shows rolling in a non-recrystallized region by hot rolling, similar results can be obtained by cold rolling at room temperature. Comparative methods 1 and 2 in Table 3 are obtained by subjecting the recrystallized material to the heat treatment of the present invention, but the 0.2% yield strength is low, and the degree of corrosion in the intergranular corrosion test is 0.154.
It shows a high value of more than g/m ² ·hr. On the other hand, the materials obtained by methods 3 to 20 and 22 of the present invention have a 0.2% yield strength of 25 kgf/mm ² or more, and a corrosion degree of 0.075 g/m ² in the intergranular corrosion test. It can be seen that it shows a low value of less than hr. Especially 700℃
The degree of corrosion after the sensitization treatment was shown to be sufficiently lower than that of the comparative method, and it is clear that the method has excellent intergranular corrosion resistance. The material obtained by Comparative Method 24 has an unrecrystallized structure.
Although the 0.2% proof stress is extremely high, the materials obtained by Comparative Methods 26 to 30 having a recrystallized structure have a low 0.2% proof stress. Moreover, it is clear that the intergranular corrosion resistance of these materials is significantly inferior to that of the materials obtained by the method of the present invention. As described above, according to the present invention, a Ni-based alloy material having excellent intergranular corrosion resistance, extremely good stress corrosion cracking resistance, and high strength can be obtained efficiently and at low cost. This brings about industrially useful effects, such as further improving the performance of equipment used in harsh environments such as places that come into contact with high-temperature, high-pressure water.

Claims (1)

[Claims] 1 A Ni-based alloy containing C and Cr is processed at a processing rate of 20% or more in a temperature range below the recrystallization temperature, and then held at a temperature range of 650°C or higher and below the recrystallization temperature to precipitate carbides. A high-strength Ni-based alloy material with excellent intergranular corrosion resistance is produced by heating to a temperature range of 850 to 950°C for a short time of 1 to 5 minutes/mm, and then rapidly cooling it. Production method.