JP2006274374A - Production method of ni-based alloy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a production method of an Ni-based alloy for uniformly forming a chromium oxide. <P>SOLUTION: The production method of the Ni-based alloy is characterized in that the Ni-based alloy containing Cr is heated in a gaseous mixture atmosphere composed of carbon dioxide and at least one kind selected from carbon monoxide, gaseous nitrogen and gaseous hydrocarbon and an oxide film composed of a chromium oxide is formed on the surface of the Ni-based alloy. The gaseous mixture atmosphere may further include ≤5vol% gaseous oxygen. The concentration of the carbon dioxide of the gaseous mixture atmosphere is preferably 0.0001 to 50vol%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a Ni-base alloy that causes little elution of Ni even when used for a long period of time in a high-temperature water environment, and particularly relates to a method for producing a Ni-base alloy suitable for applications such as members for nuclear power plants.

  Ni-based alloys are used as various members because of their excellent mechanical properties. In particular, since the members of the nuclear reactor are exposed to high-temperature water, Ni-based alloys having excellent corrosion resistance are used. For example, a 60% Ni-30% Cr-10% Fe alloy is used for a steam generator of a pressurized water reactor (PWR).

  These members will be used in the environment of high-temperature water at around 300 ° C., which is the reactor water environment for several years to several decades. Ni-based alloys have excellent corrosion resistance and a slow corrosion rate, but trace amounts of Ni are eluted from the base material after long-term use.

  The eluted Ni is transported to the core in the process of circulating the reactor water and irradiated with neutrons in the vicinity of the fuel. When Ni is irradiated with neutron, it is converted to radioactive Co by nuclear reaction. Since this radioactive Co has a very long half-life, it continues to emit radiation for a long time. Therefore, when the amount of Ni elution increases, the exposure dose of workers who perform periodic inspections and the like increases.

  Reducing the exposure dose is a very important issue for long-term use of light water reactors. Therefore, measures have been taken to prevent elution of Ni in the Ni-base alloy by improving the corrosion resistance of the material side and controlling the water quality of the reactor water.

In Patent Document 1, a Ni-based alloy heat transfer tube is annealed in a temperature range of 400 to 750 ° C. in an atmosphere of a vacuum degree of 10 ⁻² to 10 ⁻⁴ Torr to form an oxide film mainly composed of chromium oxide. A method for improving the overall corrosion resistance is disclosed.

In Patent Document 2, after the solution treatment of the Ni-based precipitation strengthening type alloy, a heat treatment is performed in which an age hardening treatment and at least a part of the oxide film formation treatment are performed in an oxidizing atmosphere at 10 ⁻³ Torr to atmospheric pressure. A method for manufacturing a member for a nuclear power plant is disclosed.

  Patent Document 3 discloses a method for producing a Ni-based alloy product in which a Ni-based alloy product is heat-treated in a mixed atmosphere of hydrogen or hydrogen and argon having a dew point of −60 ° C. to + 20 ° C.

  Patent Document 4 discloses a method of forming a chromium-enriched layer by exposing an alloy workpiece containing Ni and Cr to a gas mixture of water vapor and at least one non-oxidizing gas.

JP-A-64-55366 JP-A-8-29571 Japanese Patent Laid-Open No. 2002-121630 JP 2002-322553 A

  However, since the film formed by the method disclosed in Patent Document 1 has an insufficient thickness, there is a problem that the elution prevention effect is lost because the film is damaged by long-term use. is there.

  The method disclosed in Patent Document 2 has a problem in that oxidized Ni is easily taken into the film, and this Ni is eluted during use.

  And in the method of forming an oxide film by controlling the amount of water vapor (dew point) as in the methods disclosed in Patent Documents 3 and 4, a uniform oxide film can be formed on the inlet side and the outlet side of water vapor. Have difficulty. This is due to the following reasons.

For example, in the case of continuous processing such as an oxide film on a long tube, the thickness of the oxide film to be generated is determined not only by the oxygen potential but also by the diffusivity through the concentration boundary layer of the oxidizing gas on the surface of the material to be processed. Is done. Here, the concentration boundary layer refers to the boundary layer of the gas concentration distribution at the surface of the material to be processed and at a position away from the surface (for example, near the central axis inside the tube). This diffusivity is affected by physical properties such as gas diffusion coefficient and kinematic viscosity coefficient, and oxidation treatment conditions such as gas concentration and flow velocity. Since water vapor (H ₂ O) has a higher diffusivity than other oxidative gases such as CO ₂ , when performing an oxidation treatment in a water vapor atmosphere, it is uniform on the water vapor inlet and outlet sides. It becomes difficult to form an oxide film.

  The present invention has been made to solve these problems, and provides a method for producing a Ni-base alloy that can form a chromium oxide uniformly and uniformly on the surface of the Ni-base alloy at low cost. Objective.

  The gist of the present invention is a method for producing a Ni-base alloy shown in the following (1) to (7).

  (1) Ni-base alloy containing Cr by heating in a mixed gas atmosphere consisting of at least one gas selected from carbon dioxide gas, carbon monoxide gas, nitrogen gas and hydrocarbon gas A method for producing a Ni-base alloy, comprising forming an oxide film made of chromium oxide on a surface.

  (2) The method for producing a Ni-based alloy as described in (1) above, wherein the mixed gas atmosphere further contains 5% by volume or less of oxygen gas.

  (3) The method for producing a Ni-based alloy as described in (1) or (2) above, wherein the concentration of carbon dioxide gas in the mixed gas atmosphere is 0.0001 to 50 Vol.

  (4) The Ni-based alloy is, by mass%, C: 0.15% or less, Si: 1.00% or less, Mn: 2.0% or less, P: 0.030% or less, S: 0.030% or less, Cr: 10.0-40.0%, Fe: 15.0% or less, Ti: 0.5% or less, Cu: 0.50% or less, and Al: 2.00% or less, any of (1) to (3) above, wherein the balance consists of Ni and impurities A method for producing a Ni-based alloy according to claim 1.

  (5) The Ni-based alloy contains Nb and / or Ta as a single element or a total of 3.15 to 4.15% in mass%, instead of a part of Ni, as described in (4) above Of manufacturing Ni-based alloys.

  (6) The Ni-based alloy as described in (4) or (5) above, wherein the Ni-based alloy contains 8 to 10% of Mo by mass% instead of a part of Ni. Method.

  (7) The method for producing a Ni-based alloy according to any one of (1) to (6), wherein the Ni-based alloy is used as a member for a nuclear power plant.

The “oxide film made of chromium oxide” means an oxide film mainly composed of Cr ₂ O ₃ , and is an oxide other than Cr ₂ O ₃ , for example, MnCr ₂ O ₄ , TiO ₂ , Al ₂ O. ₃ and oxides such as SiO ₂ may be contained. Further, if the surface of the Ni-based alloy has an oxide film made of chromium oxide, another oxide layer is formed on the upper layer (outer layer) and / or lower layer (inner layer) of the chromium oxide layer. It may be.

  According to the present invention, it is possible to uniformly form chromium oxide on the surface of the Ni-based alloy at a low cost, so that even when used for a long time in a high-temperature water environment, for example, a high-temperature water environment in a nuclear power plant. Ni-based alloys with extremely low Ni elution can be manufactured. Therefore, this Ni-based alloy is most suitable for steam generator tubes and components for nuclear power plants such as spacer springs, coil springs, finger springs, channel fasteners and lid nozzles used in high-temperature water. is there.

1. Regarding Mixed Gas Atmosphere In the method for producing a Ni-based alloy of the present invention, the Ni-based alloy containing Cr is at least one gas selected from carbon dioxide gas, carbon monoxide gas, nitrogen gas and hydrocarbon gas. The main feature is that an oxide film made of chromium oxide is formed on the alloy surface by heating in a mixed gas atmosphere comprising

Carbon dioxide gas has the effect of forming an oxide film made of chromium oxide on the surface of the Ni-based alloy in a high temperature environment. That is, in an atmosphere composed of carbon dioxide gas, as shown in the following reaction formula, CO ₂ is adsorbed to the Ni-based alloy, and O (oxygen) is directly taken into the Ni-based alloy from CO ₂ , and chromium oxidation Things are generated.
CO ₂ + Metal → CO + Metal-O

  Carbon dioxide gas acts as an oxidizing gas in a heated atmosphere, and forms the above chromium oxide if it is contained even in a trace amount. Therefore, although the lower limit of the carbon dioxide gas concentration is not set, the effect becomes remarkable at 0.0001 Vol.% Or more. On the other hand, the upper limit of the carbon dioxide gas concentration is not particularly limited, but from the viewpoint of reducing the production cost, it is preferably 50 Vol.% Or less, and more preferably 10 Vol.% Or less. At this time, the non-oxidizing gas concentration may be increased and the carbon dioxide gas concentration may be decreased.

  As described above, Patent Documents 3 and 4 disclose a method of forming an oxide film by heating in a water vapor atmosphere. In this method, a uniform oxide film is formed on the inlet side and the outlet side of water vapor. Difficult to do.

  However, since carbon dioxide is less diffusive than water vapor, the thickness of the formed oxide film is not easily affected by the oxidation treatment conditions such as the gas concentration and flow rate to be supplied. For this reason, a more uniform oxide film can be formed on the alloy surface than the oxidation treatment performed in the conventional steam atmosphere. An advantage of using carbon dioxide gas is that a desired oxidation treatment atmosphere can be created at a lower cost than a method in which the moisture concentration is controlled by a conventional dew point generator.

  The mixed gas atmosphere includes at least one gas selected from carbon monoxide gas, nitrogen gas, and hydrocarbon gas in addition to carbon dioxide gas. These gases are non-oxidizing gases that do not contribute to the formation of Cr oxides, and the concentration of carbon dioxide gas can be appropriately adjusted by adjusting the concentration of these gases.

  The mixed gas atmosphere may contain 5% or less of oxygen gas in addition to the above gas. This is because oxygen gas also contributes to the formation of Cr oxide. However, when a large amount of oxygen gas is contained, the formation of an oxide film is promoted, the Cr concentration in the base material is lowered, and the corrosion resistance is deteriorated. Therefore, when oxygen gas is present, it is desirable to limit its concentration to 5% by volume.

2. Heat treatment temperature and heat treatment time Heating temperature: 500-1250 ° C
The heating temperature only needs to be within a range where appropriate thickness and composition of the oxide film and strength characteristics of the alloy can be obtained. Specifically, when the heating temperature is less than 500 ° C., the oxidation of chromium may be insufficient, but when it exceeds 1250 ° C., the strength of the Ni-based alloy material may not be ensured. Therefore, the heating temperature is preferably in the range of 500 to 1250 ° C.

Heating time: 10 seconds to 35 hours The heating time may be set within a range in which an appropriate oxide film thickness and composition can be obtained. That is, in order to form an oxide film mainly composed of chromium oxide, it is desirable to heat for 10 seconds or more. However, even if it is heated for more than 35 hours, the oxide film hardly forms. Therefore, the heating time is desirably in the range of 10 seconds to 35 hours.

  Since the heating time can be shortened as the heating temperature increases, for example, when the heating temperature is in the range of 1000 to 1200 ° C., the heating time may be in the range of 10 seconds to 60 minutes.

  As described above, the thickness and composition of the coating can be adjusted by appropriately adjusting the heating temperature, heating time, and gas concentration conditions.

3. Ni-base alloy to be treated As the Ni-base alloy used in the production method of the present invention, for example, in mass%, C: 0.15% or less, Si: 1.00% or less, Mn: 2.0% or less, P: 0.030 %: S: 0.030% or less, Cr: 10.0-40.0%, Fe: 15.0% or less, Ti: 0.5% or less, Cu: 0.50% or less, and Al: 2.00% or less, with the balance being Ni and impurities There are Ni-based alloys. The reasons for limiting each element are as follows. In the following description, “%” for the content means “% by mass”.

C: 0.15% or less If C exceeds 0.15%, the stress corrosion cracking resistance may deteriorate. Therefore, when C is contained, the content is desirably 0.15% or less. More desirable is 0.06% or less. C has the effect of increasing the grain boundary strength of the alloy. In order to obtain this effect, the C content is desirably 0.01% or more.

Si: 1.00% or less
Si is used as a deoxidizer during smelting and remains as an impurity in the alloy. At this time, it is necessary to limit to 1.00% or less. If the content exceeds 0.50%, the cleanliness of the alloy may decrease, so it is desirable to limit the Si content to 0.50% or less.

Mn: 2.0% or less
If Mn exceeds 2.0%, the corrosion resistance of the alloy is lowered, so it is desirable to make it 2.0% or less. Mn has a lower free energy of formation of oxide than Cr and precipitates as MnCr ₂ O ₄ by heating. Further, since the diffusion rate is relatively fast, usually, Cr ₂ O ₃ is preferentially generated in the vicinity of the base material by heating, and MnCr ₂ O ₄ is formed as an upper layer outside thereof. If the MnCr ₂ O ₄ layer is present, the Cr ₂ O ₃ layer is protected in the usage environment, and even if the Cr ₂ O ₃ layer is destroyed for some reason, the MnCr ₂ O ₄ repairs the Cr ₂ O ₃ Is promoted. Such an effect becomes remarkable when the content is 0.1% or more. Therefore, the desirable Mn content is 0.1 to 2.0%, and more desirably 0.1 to 1.0%.

P: 0.030% or less P is an element present as an impurity in the alloy. If its content exceeds 0.030%, corrosion resistance may be adversely affected. Therefore, it is desirable to limit the P content to 0.030% or less.

S: 0.030% or less S is an element present as an impurity in the alloy. If its content exceeds 0.030%, corrosion resistance may be adversely affected. Therefore, it is desirable to limit the S content to 0.030% or less.

Cr: 10.0-40.0%
Cr is an element necessary for generating an oxide film made of chromium oxide. In order to form such an oxide film on the alloy surface, it is desirable to contain 10.0% or more. However, if it exceeds 40.0%, the Ni content is relatively reduced, which may reduce the corrosion resistance of the alloy. Accordingly, the Cr content is desirably 10.0 to 40.0%. In particular, when Cr is contained in 14.0 to 17.0%, it is excellent in corrosion resistance in an environment containing chloride, and in the case where Cr is contained in 27.0 to 31.0%, it is further excellent in corrosion resistance in pure water and alkaline environments at high temperatures. .

Fe: 15.0% or less
If Fe exceeds 15.0%, the corrosion resistance of the Ni-based alloy may be impaired. Therefore, it shall be 15.0% or less. Further, since it is an element that can be used in place of a part of expensive Ni dissolved in Ni, it is desirable to contain 4.0% or more. The Fe content may be determined from the balance between Ni and Cr. If the Cr content is 14.0 to 17.0%, the content is 6.0 to 10.0%. If the Cr content is 27.0 to 31.0%, the content is 7.0 to 11.0%. Is desirable.

Ti: 0.5% or less
If the Ti content exceeds 0.5%, the cleanliness of the alloy may be deteriorated, so the content is desirably 0.5% or less. More desirable is 0.4% or less. However, from the viewpoint of improving the workability of the alloy and suppressing grain growth during welding, it is desirable to contain 0.1% or more.

Cu: 0.50% or less
Cu is an element present as an impurity in the alloy. If the content exceeds 0.50%, the corrosion resistance of the alloy may be lowered. Therefore, it is desirable to limit the Cu content to 0.50% or less.

Al: 2.00% or less
Al is used as a deoxidizer during steelmaking and remains as an impurity in the alloy. The remaining Al becomes oxide inclusions in the alloy, which deteriorates the cleanliness of the alloy and may adversely affect the corrosion resistance and mechanical properties of the alloy. Therefore, it is desirable to limit the Al content to 2.00% or less.

  The above Ni-based alloy may contain any of the above elements, and the balance may be made of Ni and impurities, but for the purpose of improving performance such as corrosion resistance and strength, an appropriate amount of Nb, Ta, or Mo may be added. Good.

Nb and / or Ta: 3.15 to 4.15% either alone or in total
Nb and Ta are effective in improving the strength of the alloy because they easily form carbides. Moreover, since C in the alloy is fixed, there is an effect of suppressing Cr deficiency at the grain boundary and improving the corrosion resistance of the grain boundary. Accordingly, one or both of these elements may be contained. The above effect becomes significant when the content of one element is contained when any one element is contained, and when the total content is 3.15% or more when both elements are contained.

  However, when the content of Nb and / or Ta is excessive, hot workability and cold workability may be impaired and sensitivity to heat embrittlement may be increased. Therefore, when either one element is contained, the content of the single element is desirable, and when both elements are contained, the total content is desirably 4.15% or less. Therefore, the content when one or both of Nb and Ta is contained is desirably 3.15 to 4.15% as a single substance or in total.

Mo: 8-10%
Mo has an effect of improving pitting corrosion resistance, and may be contained as necessary. The above effect becomes remarkable at 8% or more, but when it exceeds 10%, an intermetallic compound may be precipitated and the corrosion resistance may be deteriorated. Therefore, the content when Mo is contained is desirably 8 to 10%.

  Two typical types of Ni-based alloys are as follows.

  (a) C: 0.15% or less, Si: 1.00% or less, Mn: 2.0% or less, P: 0.030% or less, S: 0.030% or less, Cr: 14.0 to 17.0%, Fe: 6.0 to 10.0%, Ti: 0.5 % Ni, Cu: 0.50% or less and Al: 2.00% or less, the balance being Ni-based alloy consisting of Ni and impurities.

  (b) C: 0.06% or less, Si: 1.00% or less, Mn: 2.0% or less, P: 0.030% or less, S: 0.030% or less, Cr: 27.0-31.0%, Fe: 7.0-11.0%, Ti: 0.5 % Ni, Cu: 0.50% or less and Al: 2.00% or less, the balance being Ni-based alloy consisting of Ni and impurities.

  The alloy (a) contains 14.0 to 17.0% of Cr and contains about 75% of Ni, so that it has excellent corrosion resistance in an environment containing chloride. In this alloy, the content of Fe is preferably 6.0 to 10.0% from the viewpoint of the balance between the Ni content and the Cr content.

  The alloy (b) contains 27.0 to 31.0% of Cr and contains about 60% of Ni, so that it is excellent in corrosion resistance in high-temperature pure water or alkaline environments in addition to chloride-containing environments. In this alloy as well, the Fe content is preferably 7.0 to 11.0% from the viewpoint of the balance between the Ni content and the Cr content.

  Using alloys A to G shown in Table 1, tubes having a diameter of 20 mm and a wall thickness of 1.5 mm were prepared, and these tubes were cut to a length of 200 mm to obtain test materials. These test materials were heat-treated under the conditions shown in Table 2. The heat treatment was performed by setting a test material in the quartz tube, venting a gas having a predetermined concentration in the quartz tube at a constant flow rate, and heating by a heater installed on the outer periphery of the quartz tube.

When both ends of the test material after heat treatment were cut out and the coating composition on the inner surface of the tube was investigated with EDX (Energy Dispersive X-ray micro-analyzer), it was found that an oxide coating consisting of chromium oxide was formed. did. Its cross-section scanning electron microscope; measuring the thickness of the oxide coating was observed pipe inner surface side (SEM Scanning Electron Microscope), oxidizing the thickness of the oxide film of the gas upstream t _in, the gas downstream side the thickness of the coating as t _out, was to evaluate the variation of both the thickness as _{│t in -t out │ / t in} . In Table 2, the case of 0.5 or less is shown as “◯”, and the case of exceeding 0.5 is shown as “X”.

  As shown in Table 2, the oxide film formed under the condition using carbon dioxide gas or further oxygen gas as the oxidizing gas had little variation in the oxide film thickness in the longitudinal direction of the tube. On the other hand, the oxide film formed under the condition using water vapor as the oxidizing gas had a considerable variation.

According to the present invention, it is possible to uniformly form chromium oxide on the surface of the Ni-based alloy at a low cost, so that even when used for a long time in a high-temperature water environment, for example, a high-temperature water environment in a nuclear power plant. Ni-based alloys with extremely low Ni elution can be manufactured. Therefore, this Ni-based alloy is used for nuclear power plant components such as steam generator tubes, spacer springs, coil springs, finger springs, channel fasteners and lid nozzles used in high-temperature water. Is optimal.

Claims (7)

  A Ni-based alloy containing Cr is heated in a mixed gas atmosphere composed of at least one gas selected from carbon dioxide gas, carbon monoxide gas, nitrogen gas and hydrocarbon gas, and chromium is formed on the surface of the Ni-based alloy. A method for producing a Ni-based alloy, comprising forming an oxide film made of an oxide.   2. The method for producing a Ni-based alloy according to claim 1, wherein the mixed gas atmosphere further contains 5 vol.% Or less of oxygen gas.   The method for producing a Ni-based alloy according to claim 1 or 2, wherein the concentration of carbon dioxide gas in the mixed gas atmosphere is 0.0001 to 50 Vol.% Or less.   The Ni-based alloy is, by mass%, C: 0.15% or less, Si: 1.00% or less, Mn: 2.0% or less, P: 0.030% or less, S: 0.030% or less, Cr: 10.0-40.0%, Fe: 15.0 % Or less, Ti: 0.5% or less, Cu: 0.50% or less, and Al: 2.00% or less, with the balance being Ni and impurities. Ni-based alloy manufacturing method.   5. The Ni-based alloy according to claim 4, wherein the Ni-based alloy contains Nb and / or Ta alone or in a total amount of 3.15 to 4.15% in mass% instead of a part of Ni. Manufacturing method.   The method for producing a Ni-based alloy according to claim 4 or 5, wherein the Ni-based alloy contains 8 to 10% of Mo in mass% instead of a part of Ni. The method for producing a Ni-based alloy according to any one of claims 1 to 6, wherein the Ni-based alloy is used as a member for a nuclear power plant.