JPH09263826A - Manufacture of alloy excellent in intergranular corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacture of an inexpensive alloy having excellent intergranular corrosion resistance even under a severe corrosive environment, particularly under the environment where intergranular corrosion, caused by the chromium-deficient layer at crystalline grain boundaries resultant from the precipitation of chromium carbide into the crystalline grain boundaries, is brought about. SOLUTION: An Fe-base and Ni-base alloy has a composition containing, by weight, >=20.0% Ni and >=14.0% Cr. This alloy is hot-worked, held at a temp. of 600-700 deg.C where chromium carbide is precipitated, cold-worked at >=20% draft, and further heat-treated at a temp. between the recrystallization temp. and the temp. at which chromium carbide is allowed to enter into solid solution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an alloy having excellent intergranular corrosion resistance, and particularly to various single acids such as nitric acid and mixed acids, as well as chromium carbide precipitation at grain boundaries. The present invention relates to a method for producing an alloy exhibiting excellent intergranular corrosion resistance in an environment that causes intergranular corrosion due to a chromium deficient layer near the intergranular boundaries.

[0002]

2. Description of the Related Art Conventionally, SUS310S and NCF800,
Alloys having a high Cr or Ni content, such as NCF600, have excellent corrosion resistance and are used in all corrosive environments. Since the solid solubility of C in the solution temperature range is low and the precipitation of chromium carbide (Cr ₂₃ C ₆ ) is fast, grain boundary carbides are likely to precipitate even after the solution treatment even if the C content is low, and the vicinity of grain boundaries The intergranular corrosion resistance deteriorates due to the chromium-deficient layer formed on the surface. As shown in many documents (for example, the Stainless Steel Handbook) for solving this, the following two methods are generally used.

[0003] One is to sufficiently dissolve C by increasing the solid solubility of C in the matrix by holding it at a high temperature for a long time during the solution treatment, and then the C dissolved in cooling is converted into chromium carbide again. Immediately use a large amount of water, etc.
This is a method of suppressing the precipitation of chromium carbide by cooling at a higher cooling rate than that used in US304 and SUS316.

On the other hand, as represented by stabilized stainless steel such as SUS321 and SUS347, Ti and Nb are used.
It is a method of adding an element that fixes C such as. By adding these elements in appropriate amounts, they are precipitated as TiC and NbC which are more thermally stable than chromium carbides, and by reducing the C content, the precipitation of chromium carbides at grain boundaries is suppressed and the grain boundary resistance of the chromium-deficient layer is improved. It suppresses corrosive deterioration.

[0005]

However, in an alloy having a high Ni content and originally a low solid solubility of C,
In order to perform solution treatment at high temperature and to perform cooling faster than the normal cooling rate by the above method that is performed to improve intergranular corrosion resistance, a heating furnace that can be held at high temperature for a long time and immediate cooling It is necessary to have a cooling device with a special layout that makes it possible, and in the case of a continuous furnace that is normally used, it is necessary to increase the transfer speed for the purpose of improving cooling capacity,
In order to secure the holding time, it is necessary to make the furnace length several times the current length, resulting in enormous cost in terms of equipment and manufacturing. Further, as the scale of the heating furnace becomes larger, it becomes difficult to control the solution treatment conditions such as temperature setting and atmosphere setting, and thus it becomes difficult to properly control the material.

Further, the method of adding an element for fixing carbides such as Ti and Nb can secure corrosion resistance by controlling the composition of the alloy, but these alloys are expensive, so that the cost is increased. Further, depending on the composition of the alloy, there is a problem that hot workability is hindered and the yield is lowered, and other adverse effects are exerted. Also, for example, T
Depending on the environment in which it is exposed, such as iC being dissolved in a strongly oxidizing acid, it may accelerate corrosion, which is not desirable in some cases.

[0007]

In view of the problem that the intergranular corrosion resistance is lowered in the above-mentioned alloy having a high Ni content and in which chromium carbide is likely to precipitate, the intergranular corrosion resistance of the alloy is improved. Then, a method for manufacturing at low cost was examined. As a result, cold working, recrystallization and chromium carbide precipitation state were investigated, and a method for detoxifying the chromium deficient layer harmful to intergranular corrosion resistance by cold working degree after chromium carbide precipitation and subsequent heat treatment. I found it. The present invention was made based on such findings,
It is characterized by having the following configuration.

That is, the gist of the present invention is that
In Fe-based and Ni-based alloys containing Ni: 20.0% or more and Cr: 14.0% or more by weight, after the hot working, the workability is maintained at 600 to 700 ° C. where chromium carbide precipitates. A method for producing an alloy having excellent intergranular corrosion resistance is characterized in that cold working of 20% or more is performed, and further heat treatment is performed at a temperature of a recrystallization temperature or higher and a solid solution of chromium carbide or lower.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for defining the alloy in the present invention will be described below. Hereinafter,% means% by weight. Ni: Ni is a heavy element for obtaining a stable austenite structure and corrosion resistance. When the Ni content is less than 20.0%, the solid solution of C can be achieved relatively easily by the ordinary solution treatment, but when the Ni content is 20% or more, the solid solubility of C in the matrix is lowered and the grain boundaries are increased. Chromium carbide precipitation is promoted to facilitate the formation of a chromium-deficient layer, which lowers the intergranular corrosion resistance. Therefore, the Ni content of the alloy intended in the present invention is set to 20.0% or more.

Cr: Cr is a main element that exhibits an effect of improving and improving the corrosion resistance under various environments, particularly an effect of excellent corrosion resistance under an oxidizing environment represented by nitric acid. However, the content in Fe-based and Ni-based alloys is 1
If it is less than 4.0%, the above effect is small, so the Cr content is set to 14.0% or more.

Further, according to the present invention, the alloy of the above chemical composition has a precipitation of chromium carbide of 600 to 7 after hot working.
After being held at 00 ° C., cold working with a working ratio of 20% or more is performed, and further heat treatment is performed at a temperature of not less than the recrystallization temperature and not more than the solid solution of chromium carbide. The manufacturing process will be described below. First, a conceptual diagram of the change in structure in the manufacturing process of the present invention is shown in FIGS.
FIGS. 1 (d) and 1 (e) show conceptual diagrams of (c) and the chromium carbide and the surrounding chromium-depleted layer (chromium concentration distribution).
As shown in FIG. 1 (a), after hot working, after normal solution treatment or after carbide precipitation treatment, chromium carbide is precipitated and remains at grain boundaries. It has a groove-like structure, and a chromium-deficient layer exists near the grain boundaries. Figure 1 shows the cold-worked state.
(B). In FIG. 1B, the crystal grains have a deformed structure due to processing, but the carbide remains present at the grain boundaries. When this is heat-treated at a temperature above the recrystallization temperature and below the solid solution temperature of the carbide, the deformed crystal grains cause recrystallization and
Although the grain size is as shown in (c), since the chromium carbide originally existed is hardly dissolved, it is scattered inside the grain size-controlled grains.

However, since this carbide is not newly deposited, there is no chromium depletion layer in the periphery. In addition, chromium carbide is a compound containing a large amount of chromium, which is generally represented by Cr ₂₃ C _6, and as a result, it exhibits excellent corrosion resistance in most environments. Therefore,
This recrystallized structure has no chromium-deficient layer and exhibits excellent intergranular corrosion resistance. Further, since the carbide precipitation treatment is performed first, the amount of solid solution C in the matrix is very small, and there is no new carbide precipitation even in a so-called sensitizing environment, and the intergranular corrosion resistance does not deteriorate. Absent.

The reasons for limiting each process will be described below. In alloys with a high Cr content, and especially with a high Ni content, the C solid solubility is low near the solution temperature, so the carbides cannot be completely dissolved after hot working or after solution treatment, and during cooling. Grain boundary carbide easily precipitates. In the present invention, a temperature range in which chromium carbide, which is called sensitization, precipitates, that is, 600 to 7
By maintaining the temperature at 00 ° C., chromium carbide is preferentially precipitated at the crystal grain boundaries, and a state in which solid solution C in the matrix is small is obtained. The holding time cannot be decided unconditionally because the precipitation rate of chromium carbide differs depending on the alloy, but it is desirable that the holding time is long. However, considering the processability and cost, for example, 2
The purpose can be achieved even in about time.

In the present invention, the uniform recrystallization process after cold working is an important factor, and it is necessary to suppress the solid solution of carbide at the same time as performing recrystallization. Generally, the recrystallization temperature is set to about one half of the melting point of the alloy, but the cold reworking can lower the recrystallization temperature.
Therefore, using rod-shaped and tubular test pieces, cold working was performed at a working ratio of 5 to 80%, and the recrystallization condition and the carbide condition after heat treatment were investigated. FIG. 2 shows the cold workability and the recrystallized state after the heat treatment. As a result, after undergoing cold working, it is necessary to carry out cold working with a working rate of 20% or more in order to obtain uniform crystal grains by heat treatment and to obtain a microstructure in which solid solution of carbide can be suppressed. It became clear. Therefore,
The cold workability was 20% or more.

Next, the reason why the heat treatment is performed at a temperature not lower than the recrystallization temperature and not higher than the solid solution of chromium carbide is that in this heat treatment, recrystallization is performed while suppressing the solid solution of the previously precipitated grain boundary carbide. Then, the purpose is to create a crystal state in which a chromium-deficient layer does not exist in the newly generated crystal grain boundary.
Since this treatment temperature differs depending on the composition of the alloy and the degree of cold work, it cannot be clearly specified here, but it is almost 750 to 1
It is often in the region of 000 ° C. Further, this treatment has an advantage that fine crystal grains are obtained and the strength is secondarily improved.

In the alloy produced by the combination of the above-mentioned processes, there is no grain boundary carbide, so that there is no chromium depletion layer, and the previously precipitated carbide phase exists in the matrix without a chromium depletion layer. , Intergranular corrosion is significantly suppressed. Further, since the amount of solid solution C is small, further sensitization does not occur even if the temperature is kept in the sensitization temperature range. However, since the recrystallized grain boundaries may be stopped by the carbide that has previously precipitated during the recrystallization, carbides may also exist at the crystal grain boundaries in the structure observation. However, since the carbides existing on the grain boundaries after the final heat treatment were not generated by the final heat treatment, there is no chromium deficient layer, and it does not affect the intergranular corrosion like the carbides existing inside the crystal grains. .

[0017]

EXAMPLE Alloys having the chemical composition shown in Table 1 were melted in a vacuum high frequency melting furnace, and the obtained steel ingot was hot worked at 1230 ° C. to obtain rods having outer diameters of 25 mm and 50 mm. All alloys to which the present invention is applied have Ni ≧ 20.0% and Cr ≧ 14.0.
% Is included. A rod with an outer diameter of 25 mm is 1050 ° C
After a solid solution treatment for 10 minutes with water cooling and a sensitization treatment at 675 ° C., a rod having an outer diameter of 21 mm was formed by turning, and cold working was performed by drawing to have a workability of 5 to 50%. Similarly outer diameter 50m
The rod of m has an outer diameter of 45 m by turning and punching after solid solution treatment with water cooling at 1050 ° C for 10 minutes and sensitization treatment at 675 ° C.
A tube having a wall thickness of 5 mm was used and subjected to cold working by 50 to 80% by cold drawing. The cold-worked rods and tubes were heat-treated at a temperature between 800 and 1000 ° C. for 10 minutes, and then corrosion test pieces of 14 × 30 × 1.5 mm were sampled. For the corrosion test piece, the surface of 0.2 mm was removed by electrolytic polishing to remove the processing layer at the time of processing the test piece, and then the intergranular corrosion resistance was evaluated in boiling sulfuric acid / ferric sulfate (JIS G 0572). went. In addition, in the material during each process, 10
The structure was evaluated by a% oxalic acid etch test (JIS G 0571). For comparison, the alloys shown in Table 1 were used after being hot-worked and subjected to a solution treatment by water cooling at 1050 ° C. for 10 minutes.

[0018]

[Table 1]

In Table 2, the alloys shown in Table 1 are subjected to a solution treatment in the conventional method of water cooling at 1050 ° C. for 10 minutes, and as an example of the method of the present invention, a sensitizing treatment at 675 ° C. followed by 50% cold working. The results of the structure determination in the state of being subjected to the heat treatment and the recrystallization heat treatment are shown. Judgment method was according to JIS G 0571 oxalic acid etch test, and it was classified into judgment criteria A: step structure, B: mixed structure, and C: groove structure in the state of crystal grain boundaries. As a result, in the conventional solid solution state, the solid solution of carbide was insufficient and a mixed structure or a groove structure was exhibited, but in the state treated by the process of the present invention, any alloy showed a step structure. . In addition, Table 2 also shows the results of the intergranular corrosion resistance test of the alloy shown in Table 1 in the conventional process (solution treatment) and after the manufacturing process according to the present invention. JIS G widely used for evaluation of intergranular corrosion resistance in corrosion tests
The 0572 sulfuric acid / ferric sulfate corrosion test was used. The evaluation method was evaluated by the corrosion weight loss per unit time and unit area. From these results, it can be seen that any of the alloys produced by the process of the present invention exhibits excellent intergranular corrosion resistance as compared with the conventional solution treatment.

[0020]

[Table 2]

[0021]

As described above, according to the production method of the present invention, it is possible to cause the intergranular corrosion due to the harsh corrosive environment, especially the chromium-deficient layer at the crystal grain boundary due to the precipitation of chromium carbide at the crystal grain boundary. Even under various environments, it is possible to provide an inexpensive alloy having excellent intergranular corrosion resistance, which is an extremely excellent effect.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a microstructure change in each manufacturing process of the present invention and a chromium carbide and a surrounding chromium deficient layer.

FIG. 2 is a diagram showing a cold workability and a recrystallized state after heat treatment.

Claims (1)

[Claims] 1. By weight%, Ni: 20.0% or more, C
In an Fe-based and Ni-based alloy containing r: 14.0% or more, precipitation of chromium carbide after hot working is 600 to
After maintaining at 700 ° C., cold working with a working degree of 20% or more is performed, and further heat treatment is performed at a temperature of not less than the recrystallization temperature and not more than the solid solution of chromium carbide, which is excellent in intergranular corrosion resistance. Production method.