JP2015055005A - Austenite stainless steel and radioactive waste liquid treatment equipment using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenite stainless steel having excellent corrosion resistance in presence of oxidized acid and radioactive waste liquid treatment equipment using the same.SOLUTION: There is provided an austenite stainless steel having a composition consisting of, by mass%, C: 0.02% or less, Si: 1.0% or less, Mn: 2.0% or less, Ni: 9.0 to 13.0%, Cr: 18.0 to 20.0% and N: 0.1% or less and the balance Fe with inevitable impurities, a ratio of Cr equivalent/Ni equivalent is 1.45 to 1.80 inclusive and the austenite phase is a single crystal structure.

Description

  The present invention relates to austenitic stainless steel and radioactive liquid waste treatment equipment using the same.

Conventionally, austenitic stainless steels such as SUS304 series and SUS316 series have been widely used as materials for various equipment such as nuclear reactor core members and pipes, and supporting parts and fastening parts provided in nuclear power generation facilities. ing. Austenitic stainless steel is known as a material having good mechanical properties and workability, a high chromium content ratio, and excellent corrosion resistance.
However, austenitic stainless steels may cause sensitization when heated and maintained in a temperature range of around 700 ° C. when the carbon content is high, and after that, under certain conditions, intergranular corrosion and Intergranular Stress Corrosion Cracking (IGSCC) is likely to occur. Further, it is known that irradiation induced stress corrosion cracking (IASCC) is induced in an environment where irradiation is performed for a long period of time as the cumulative irradiation amount increases. That is, austenitic stainless steel has the characteristic that under irradiation of neutrons, this irradiation-induced stress corrosion cracking is induced and the sensitivity to grain boundary cracking is further increased.
Therefore, as a steel material that can be used under neutron irradiation in a nuclear reactor or the like, the development of single crystals that eliminate the presence of grain boundaries that are the root cause of corrosion and reduce the potential for occurrence of intergranular corrosion and intergranular cracking has been promoted. Yes.

  For example, Patent Document 1 discloses a technique for providing an austenitic stainless steel having a stable structure and excellent in corrosion resistance, stress corrosion cracking resistance and mechanical properties. The single phase or ferrite phase is 10 volume% or less in the austenite matrix, and the austenite matrix consists of sub-crystal grains, and further consists of a single crystal of a grain boundary with a high degree of order with little deviation from the corresponding orientation relationship. An austenitic stainless steel is disclosed.

  Patent Document 2 discloses a technique for providing a single crystal austenitic stainless steel having a stable structure and having excellent stress corrosion cracking resistance, strength, and irradiation embrittlement. After homogenization by crystallization treatment, cold working or hot working is performed in the temperature range below the recrystallization temperature, and further, the precipitates are dispersed by aging heat treatment in the temperature range below the recrystallization temperature, Pre-annealed single crystal stainless steel is disclosed.

  Further, in Patent Document 3, as a technique for providing an austenitic stainless steel having a stable structure and excellent toughness and irradiation stress corrosion cracking resistance, C0.1% or less, Si1% or less, Mn2% by weight Hereinafter, in an austenitic steel containing Ni 9 to 15%, Cr 10 to 18.5% and Mo 1 to 3% and Fe being 65.433% or more, the steel has a total austenite phase, and the austenite phase is a single crystal. Austenitic steel with excellent stress corrosion cracking resistance, characterized by being C0.1% or less, Si1% or less, Mn2% or less, Ni9 to 15%, Cr10 to 18.5% and Ti1% by weight Or an austenitic steel containing not more than 1.5% Nb and Fe not less than 65.433%, the steel should be composed of a single crystal all austenitic phase. Excellent in stress corrosion cracking resistance austenitic steel, characterized is described.

Japanese Patent Laid-Open No. 11-080905 JP-A-10-204586 Japanese Patent No. 2557417

  At nuclear fuel reprocessing facilities that collect and refuel spent fuel used in nuclear power generation, reusable uranium (U) and plutonium (Pu) contained in spent fuel are separated. And recover and produce raw materials for uranium fuel and MOX (Mixed Oxide) fuel. In reprocessing, the sheared spent fuel is dissolved in nitric acid and then separated into uranium, plutonium, fission products, etc. by solvent extraction. The separated uranium and plutonium are converted into the form of oxide powder, which is the raw material of uranium fuel and MOX fuel, through steps such as concentration, denitration treatment, and heat treatment, and fission products, transuranium elements, etc. are converted. The contained radioactive liquid waste is concentrated and volume-reduced, then vitrified and disposed of as high-level radioactive waste.

  Thus, in the process of reprocessing, there are many processes for handling nitric acid, which is an oxidizing acid. In particular, the waste liquid generated in the solvent extraction process contains chemical species such as fission reaction products and transuranium elements together with nitric acid, and some of them also promote corrosion. It is known to show. When stainless steel, which is the main material of equipment, is exposed to such radioactive liquid waste, corrosion is accelerated, and corrosion involving grain boundary erosion becomes dominant in an environment that extends to a hyperpassive zone. For this reason, when using stainless steel in a highly corrosive environment, it is necessary to consider the occurrence of so-called tunnel corrosion in which corrosion progresses along the metal flow and metal thinning that accompanies degranulation.

According to the austenitic stainless steel single crystal described in Patent Document 1, a member to be used in an oxidizing environment such as a nuclear fuel reprocessing facility is provided. However, since the austenitic stainless steel single crystal disclosed in Patent Document 1 (see No. 1 and No. 2 in Table 1) has a high nitrogen content that reduces weldability, it is applied as a material for equipment. The range is limited.
On the other hand, the stainless steel disclosed in Patent Document 2 contains a high proportion of Ni that stabilizes the austenite phase, and many of them are SUS316 stainless steel containing Mo (No. S2 in Table 1). To S8). Moreover, the austenitic stainless steel disclosed in Patent Document 3 contains Mo or Nb (see No. 3 to No. 13 in Table 1). However, when the Ni content is high, pit-like overall corrosion tends to occur, and when the Mo and Nb content is high, intergranular corrosion may be promoted.
Therefore, a stainless steel with a single crystal structure that has excellent corrosion resistance even in an environment in which the presence of a high concentration of an oxidizing acid is assumed and grain boundary erosion is dominant, even though the composition of these elements is reduced. Steel is sought.
Accordingly, an object of the present invention is to provide an austenitic stainless steel having excellent corrosion resistance in the presence of an oxidizing acid and a radioactive liquid waste treatment facility device using the same.

  In order to solve the above problems, the austenitic stainless steel according to the present invention is, in mass%, C: 0.02% or less, Si: 1.0% or less, Mn: 2.0% or less, Ni: 9.0. ~ 13.0%, Cr: 18.0 to 20.0%, N: 0.1% or less, the balance is composed of Fe and inevitable impurities, and Cr equivalent / Ni equivalent ratio is 1.45 or more 1.80 or less, and the austenite phase has a single crystal structure.

  Moreover, the radioactive waste liquid treatment equipment according to the present invention uses the austenitic stainless steel.

  ADVANTAGE OF THE INVENTION According to this invention, the austenitic stainless steel which has the outstanding corrosion resistance in presence of an oxidizing acid, and a radioactive waste liquid processing equipment apparatus using the same can be provided.

It is a schematic sectional drawing which shows an example of a single crystal manufacturing apparatus. It is a schematic diagram showing an example of radioactive waste liquid processing equipment. (A) is the fragmentary sectional view of the apparatus which encloses radioactive waste liquid, concentrates and reduces volume, (b) is the elements on larger scale of the A area | region in (a). It is a figure showing the corrosion degree of the austenitic stainless steel which concerns on an Example.

  The austenitic stainless steel according to an embodiment of the present invention and radioactive waste liquid treatment equipment using the same will be described in detail below.

The stainless steel according to the present embodiment is an Fe—Cr—Ni austenitic stainless steel containing at least iron (Fe), chromium (Cr), and nickel (Ni), and the main phase is a single austenite. It has a crystal structure consisting of a crystal structure.
In general, austenitic stainless steel is known as a steel type having excellent properties such as corrosion resistance, workability, and weldability, and is used in various applications ranging from various plant facilities to industrial equipment. In addition to these properties, the austenitic stainless steel according to the present embodiment has corrosion resistance especially against corrosion caused by exposure to an oxidizing acid, and the occurrence of corrosion thinning and tunnel corrosion is reduced. Special steel.
Hereinafter, the elemental composition of the austenitic stainless steel according to the present embodiment will be described.

Chromium (Cr) is a component that mainly forms a passive film on stainless steel and contributes to an improvement in corrosion resistance, as well as an improvement in strength and heat resistance. Since Cr is an element forming a ferrite phase, if the Cr content is excessively high, the ferrite phase is likely to precipitate, and it becomes difficult to obtain an austenite phase that is a single crystal structure.
Therefore, in the austenitic stainless steel according to the present embodiment, the Cr content is in the range of 18.0% by mass to 20.0% by mass, preferably 18.0% by mass to 19.5% by mass. Range.

Nickel (Ni) is a component that mainly contributes to improvements in toughness, strength and heat resistance. Further, since it is an austenite phase forming element, it is a component that contributes to the stable formation of the austenite phase, which is a single crystal structure. Moreover, it has the effect | action which improves tolerance with respect to the corrosion in the active region by a non-oxidizing acid. However, if the Ni content is excessively high, pit-like corrosion tends to occur on the surface of the austenitic stainless steel. Moreover, the raw material cost of stainless steel tends to be expensive.
Therefore, in the austenitic stainless steel according to the present embodiment, the Ni content is in the range of 9.0% by mass to 13.0% by mass, preferably 9.0% by mass to 11.0% by mass. Range.

Manganese (Mn) is a component that contributes mainly to the improvement of mechanical properties, workability, and toughness, in addition to being added as a deoxidizer or desulfurizer. Further, since it is an austenite phase forming element, it is a component that contributes to the stable formation of the austenite phase, which is a single crystal structure. Moreover, there is a tendency that the raw material cost of the stainless steel material can be reduced compared with Ni. However, since it is not advantageous from the viewpoint of corrosion resistance, in the stainless steel according to the present embodiment, Mn is preferably suppressed to a content rate derived from the added deoxidizer or desulfurizer.
Therefore, in the austenitic stainless steel according to the present embodiment, the content ratio of Mn is 2.0 mass% or less, preferably 0.5 mass% or more and 1.5 mass% or less.

Carbon (C) is a component that lowers the workability and weldability in addition to causing a decrease in corrosion resistance due to the formation of chromium carbide.
Therefore, in the austenitic stainless steel according to the present embodiment, the C content is set to a range of 0.02% by mass or less. However, since C contributes to improvement in strength in the range of low content, the content of C may be in the range of 0.002% by mass to 0.02% by mass. The formation of carbide can be reduced by adding an element capable of stably forming a carbide such as Ti, Nb or Ta.

Silicon (Si) is a component that contributes to the improvement of heat resistance in addition to being added as a deoxidizer. Moreover, there exists an effect which provides tolerance with respect to the corrosion in the superpassive region by oxidizing acids, such as nitric acid, and the effect which improves the hot water flow at the time of welding. However, since Si is a ferrite phase-forming element, if the Si content is excessively high, the ferrite phase is likely to precipitate, making it difficult to obtain an austenite phase that is a single crystal structure. Therefore, in the stainless steel according to the present embodiment, Si is preferably suppressed to a content rate derived from the added deoxidizer.
Therefore, in the austenitic stainless steel according to the present embodiment, the Si content is in the range of 1.0 mass% or less, preferably in the range of 0.2 mass% or more and 0.8 mass% or less.

Nitrogen (N) is a component that lowers the workability and weldability in addition to causing a decrease in corrosion resistance due to the formation of chromium nitride.
Therefore, in the austenitic stainless steel according to the present embodiment, the N content is in the range of 0.1% by mass or less, and preferably in the range of 0.05% by mass or less. Thus, by making the content rate of N into the range of 0.1 mass% or less, it can be set as the austenitic stainless steel with favorable weldability, and the application range in radioactive waste liquid processing equipment is expanded. In particular, since the austenitic stainless steel according to the present embodiment is limited in size to be formed as a single crystal, it is expected to be used as a small member in radioactive waste liquid treatment equipment. Even in applications where welding between such members is required at a high frequency, since the weldability is good, the local corrosion resistance of the radioactive liquid waste treatment equipment can be improved. However, since N contributes to improvement of resistance to pitting corrosion in the range of a low content rate, it does not hinder aggressive addition, and may be in a range of 0.001% by mass to 0.1% by mass. Good.

Molybdenum (Mo) generally has a function of improving resistance to corrosion in an active region caused by a non-oxidizing acid, and is a component that contributes to improvement of strength at high temperatures. However, it is a component that causes a decrease in corrosion resistance against corrosion in a perpassive region due to an oxidizing acid. For this reason, in the austenitic stainless steel according to the present embodiment, the content of Mo is restricted as an impurity that can be inevitably mixed, and no positive addition is performed.
Therefore, in the austenitic stainless steel according to the present embodiment, the Mo content is preferably in the range of 0.2% by mass or less, more preferably in the range of 0.1% by mass or less.

Niobium (Nb) is a component that generally contributes to the improvement of toughness and has the effect of improving the corrosion resistance by stably forming carbides and nitrides. However, it is a component that causes a decrease in corrosion resistance against corrosion in a perpassive region due to an oxidizing acid. For this reason, in the austenitic stainless steel according to the present embodiment, the content of Nb is restricted as an impurity that can inevitably be mixed, and positive addition is not performed.
Therefore, in the austenitic stainless steel according to the present embodiment, the Nb content is preferably in the range of 0.1% by mass or less, more preferably in the range of 0.05% by mass or less.

Phosphorus (P) is an impurity inevitably mixed from ferrochrome or the like, which is a raw material for stainless steel, and is a component that mainly causes corrosion.
Therefore, in the austenitic stainless steel according to this embodiment, the P content is preferably in the range of 0.015% by mass or less. However, since P has the effect of improving the hot water flow during welding, it does not hinder aggressive addition.

Sulfur (S) is an impurity inevitably mixed from ferronickel or the like, which is a raw material for stainless steel, and is a component that combines with iron to produce iron sulfide, which mainly causes cracking and corrosion.
Therefore, in the austenitic stainless steel according to the present embodiment, the S content is preferably in the range of 0.015% by mass or less. However, since S has the effect of increasing the penetration depth during welding, positive addition is not hindered.

  The austenitic stainless steel according to the present embodiment has an elemental composition represented by the above-described content, and the remainder of the elemental composition is mainly composed of iron (Fe). Note that the austenitic stainless steel according to the present embodiment is allowed to contain a trace amount of other inevitable impurities resulting from the raw materials used in the production, equipment used in the production process, or auxiliary materials. As other impurities, in addition to the aforementioned Mo, Nb, P, S, alkali metals or alkaline earth metals such as Na, K, Mg, Ca, Sr, Ba, Ti, V, Co, Cu, Zn, Examples include metals such as Zr, Ag, Cd, W, Ce, and Nd, and other typical elements such as B, Al, Ga, Sn, As, Sb, Te, Cl, Br, O, and Ar. Elemental species are generally present at low concentrations and are not particularly limited. In the austenitic stainless steel according to the present embodiment, the content of inevitable impurities is 0.5% by weight or less.

In the austenitic stainless steel according to the present embodiment, the austenite phase as a main phase is substantially composed of a single crystal structure, and at room temperature, a single phase of a single crystal austenite phase or a compound containing a single crystal austenite phase. It has a phase metallographic structure. The single crystal structure of the austenite phase of the main phase is realized by controlling the elemental composition and the content of the elements described above, and the occurrence of intergranular corrosion is caused by the absence of grain boundaries in the austenite phase of the main phase. Is reduced. Therefore, the austenitic stainless steel according to the present embodiment exhibits excellent corrosion resistance even in an environment in which the presence of a high concentration of oxidizing acid is assumed and intergranular corrosion is dominant.
In general, in an austenitic stainless steel, depending on the elemental composition, a ferrite phase may be precipitated with the austenite phase as a parent phase. However, in the austenitic stainless steel according to the present embodiment, since the austenite phase has an elemental composition that is stably formed as a single crystal structure, the ferrite phase is dispersed in the austenite phase of the parent phase even if precipitated. The island-like distribution is scattered. Therefore, the phase boundary between the austenite phase and the ferrite phase has a short line segment and a discontinuous shape, and corrosion and cracks starting from the phase boundary are difficult to continuously progress, and corrosion elution is suppressed. The

In the austenitic stainless steel according to the present embodiment, the cross-sectional area ratio of the precipitated ferrite phase is 10% or less, preferably 3% or less. In the elemental composition of the austenitic stainless steel according to the present embodiment, the cross-sectional area ratio of the ferrite phase in the cast steel is usually 10% or less. This cross-sectional area ratio can be further reduced by solidifying the precipitated phase by performing a solid solution treatment. The solution treatment may be performed by heating the cast steel material to a temperature range of about 1050 to 1150 ° C. and holding it for a predetermined time, followed by rapid cooling.
By setting the cross-sectional area ratio of the precipitated ferrite phase to 10% or less in this way, it is possible to suppress the influence of the ferrite phase that is likely to cause corrosion elution.
In addition, the cross-sectional area ratio of a ferrite phase is an area ratio with respect to the total cross-sectional area of stainless steel measured using a point calculation method, and is a value calculated | required according to the method of JIS G 0555 annex 1.

In the austenitic stainless steel according to the present embodiment, in order to stably form the single crystal structure of the austenite phase that is the main phase, in particular, the ratio of the ferrite forming element such as Cr and the austenite forming element such as Ni is predetermined. The element composition is limited within the range. Specifically, in the austenitic stainless steel according to this embodiment, the Cr equivalent / Ni equivalent ratio is 1.45 or more and 1.80 or less, preferably 1.55 or more and 1.80 or less.
When the Cr equivalent / Ni equivalent ratio exceeds 1.80, an austenite-forming element is not secured at a sufficient ratio, so that it becomes difficult to form an austenite phase composed of a single crystal structure. Further, if the Cr equivalent / Ni equivalent ratio is less than 1.45, the ratio of Cr to Ni in the general concentration range becomes excessively small, so that the formation of a passive film is impaired, and the corrosion resistance may be deteriorated. There is. Therefore, by setting the Cr equivalent / Ni equivalent ratio in the range of 1.45 or more and 1.80 or less, the austenitic phase has a single crystal structure, and an austenitic stainless steel having excellent corrosion resistance due to the action of the passive film is obtained. be able to.

The austenitic stainless steel according to the present embodiment is mainly composed of Fe, Cr, and Ni, and the content ratios of C, Si, Mo, Nb, N, Mn, and the like are limited. Therefore, Fe, Cr, and The content of elements other than Ni is a relatively small value. Therefore, as the specific numerical value of the Cr equivalent / Ni equivalent ratio, a numerical value calculated using the content rate (concentration) of only Cr as the Cr equivalent and the content rate (concentration) of only Ni as the Ni equivalent is adopted. doing. However, when the conditions for the content of each element are satisfied, the specific value of the Cr equivalent / Ni equivalent ratio is a ratio calculated from the Cr equivalent and Ni equivalent estimated by other methods. Conversion to the value of is allowed.
Other methods include, for example, Schaeffler's structure chart that predicts the state of alloy structure in austenitic stainless steel based on chemical composition, and DeLong's structure chart that considers nitrogen composition. There is a way. In the Schaeffler structure chart, each of ferrite forming elements other than Cr and austenite forming elements other than Ni is considered, and Cr equivalent (%) and Ni equivalent (%) are
Cr equivalent =% Cr +% Mo + 1.5% Si + 0.5 ×% Nb (Formula 1)
Ni equivalent =% Ni + 30 ×% C + 30 ×% N + 0.5 ×% Mn (Formula 2)
It is given according to each equation. Therefore, the Cr equivalent / Ni equivalent ratio of the austenitic stainless steel according to the present embodiment can be converted into a value calculated by such a method. For example, when converted into a numerical value in the Schaeffler structure chart, a preferable Cr equivalent The range of the / Ni equivalent ratio is about 1.41 to about 1.75.

  As a manufacturing method of the austenitic stainless steel according to the present embodiment, a general orientation control casting method such as a Bridgeman method, a Tamman method, a Czochralski method, a floating zone melting method, etc. can be used. A single crystal casting method using a selector for selecting is a preferable method.

FIG. 1 is a schematic cross-sectional view showing an example of a single crystal manufacturing apparatus.
The single crystal manufacturing apparatus 10 is an apparatus that casts a stainless steel single crystal by vertically controlling the solidification of the stainless steel by pulling down the molten metal in the vertically downward direction. The single crystal manufacturing apparatus 10 is an austenitic stainless steel according to this embodiment. Suitable for manufacturing.
As shown in FIG. 1, the single crystal manufacturing apparatus 10 mainly includes a main body 4 including a mold 3, a selector 5, and a starter 6, a water-cooled chill 9, and a mold heating furnace 8.

The mold 3 provided at the upper part of the main body 4 is formed with a substantially cylindrical mold corresponding to the shape of the austenitic stainless steel to be cast. The selector 5 provided below the mold 3 includes the mold 3 and the starter 6. And a mold that is bent several times in a zigzag manner is formed inside. In addition, a space for growing a seed crystal is formed in the starter 6 provided at the lowermost part of the main body 4.
The water-cooled chill 9 has a top plate portion on which the main body 4 is placed, and has a refrigerant flow path through which water for heat exchange with the top plate portion circulates, and can be raised and lowered.
The main body 4 is fixed to the top plate portion of the water-cooled chill 9 and the outer periphery thereof is covered with the mold heating furnace 8, and the main body 4 can be moved relative to the mold heating furnace 8 in the vertical direction. Has been.

  In the production of the single crystal of austenitic stainless steel according to the present embodiment, the main body 4 is heated to a predetermined temperature by the mold heating furnace 8, and melted from the casting port 7 formed at the upper end of the mold heating furnace 8. 1 is cast into the main body 4. The molten metal 1 is obtained by melting the austenitic stainless steel ingot according to the present embodiment, which has been adjusted to a predetermined elemental composition, by heating it to a melting point or higher in the melting furnace 2, and the heating temperature is, for example, The temperature is 1500-1600 ° C. The melting furnace 2 is, for example, a high-frequency melting furnace.

  The cast molten metal 1 is cooled by a water-cooled chill 9 in a starter 6 to generate a large number of crystals. Accordingly, the main body 4 is pulled down at a predetermined speed so that the predetermined crystal growth rate is maintained, and the molten metal 1 is gradually solidified to narrow the crystal growing by the selector 5 to one, while the growth direction is vertical. A single crystal controlled in the direction is cast in the mold 3 to produce an austenitic stainless steel single crystal according to the present embodiment. The pulling speed is preferably 100 cm / h or less in order to properly grow the single crystal.

The cast austenitic stainless steel according to the present embodiment may have a double-phase metal structure including a single crystal austenite phase and other precipitated phases. Therefore, the cast stainless steel is preferably subjected to a solution treatment in order to solidify the precipitated phase. The solution treatment may be performed, for example, by allowing the heating temperature to be 1050 to 1150 ° C. and holding for a certain period of time, and then allowing to cool or cool in an inert gas atmosphere such as argon gas.
By performing such a solution treatment, the precipitated ferrite phase re-dissolves in the austenite phase of the parent phase, so that a single crystal structure of the austenite phase closer to the single phase can be obtained, and the cross-sectional area of the ferrite phase The rate can be about 3% or less.

  According to the austenitic stainless steel according to this embodiment, an austenitic stainless steel material having excellent corrosion resistance is provided in an environment where an oxidizing acid such as nitric acid is present at a high concentration. In particular, a steel material having reduced weld thinning and tunnel corrosion caused by intergranular erosion and excellent weldability is provided.

When the austenitic stainless steel according to the present embodiment is manufactured as a stainless steel single crystal having a columnar shape as described above, it can be generally manufactured as a single crystal steel material having an outer diameter of 50 mm or less.
Therefore, the austenitic stainless steel according to the present embodiment is a component of radioactive waste liquid treatment equipment installed in a nuclear fuel reprocessing facility, equipment installed in a nuclear reactor, equipment installed in a chemical plant, etc. Alternatively, it can be applied as a high corrosion resistance stainless steel material for constituting a member, and based on its excellent characteristics such as corrosion resistance, the reliability of the equipment can be improved and the life of the equipment can be extended.
Examples of equipment include containers, pipes, valves, pumps, instrumentation, and the like. Moreover, it can also be applied as parts or members such as various support plates, supports, fasteners, etc., as part of large structural materials, or as small structural materials. The austenitic stainless steel according to the present embodiment is particularly preferably used as a material for a component or member used in an environment where there is contact with an oxidizing acid such as nitric acid. There are a thermometer protection tube cap, a thermometer protection tube support and the like attached to a thermometer installed in a container or the like containing radioactive liquid waste.

FIG. 2 is a schematic diagram illustrating an example of radioactive waste liquid treatment equipment. (A) is the fragmentary sectional view of the apparatus which encloses radioactive waste liquid, concentrates and reduces volume, (b) is the elements on larger scale of the A area | region in (a).
In the process of reprocessing in the nuclear fuel reprocessing facility, the radioactive liquid waste generated from the solvent extraction step contains the radioactive liquid waste as shown in FIG. 2 (a) and concentrates and reduces the volume (waste liquid concentration / volume reduction apparatus 20). ) To reduce the volume of the waste liquid.

  The lower part of the waste liquid concentration / volume reduction device 20 is covered with a heating jacket 21, and the waste liquid concentration / volume reduction device 20 is provided with a coil-type heating tube (not shown). The radioactive waste liquid to be treated is supplied from the waste liquid supply port 210 and heated by the heating jacket 21 or a heating tube in the apparatus to be evaporated and concentrated. When the evaporation / concentration is completed, the radioactive liquid waste is transferred from the waste liquid discharge port 211 to the subsequent process.

The waste liquid concentration / volume reduction apparatus 20 is provided with a thermometer for measuring the temperature in the apparatus, and the thermometer 33 is isolated from the radioactive waste liquid by a stainless steel thermometer protection tube 30 to be corroded or contaminated. Protected from. The thermometer protection tube 30 is fixedly supported inside the waste liquid concentration / volume reduction device 20 by a thermometer protection tube support 40, and has a bottomed cylindrical shape as shown in FIG. A thermometer protection tube cap 31 is attached. The thermometer protection tube cap 31 has a structure joined to a protection tube body 32 constituting a part of the thermometer protection tube 30 by welding via a welding portion 301. Conventionally, the thermometer protection tube cap 31 may be manufactured by plastic processing of a plate material or tube material if it is thin, but is generally manufactured by cutting out from a stainless steel bar or forged steel material. The way to do is taken.
The thermometer protection tube cap 31 and the thermometer protection tube support 40 are members that come into contact with radioactive waste liquid containing a high concentration of oxidizing acid. By manufacturing the thermometer protective tube cap 31 and the thermometer protective tube support 40 with the austenitic stainless steel single crystal according to the present embodiment, it becomes possible to suppress the progress of corrosion, and the reliability of equipment and equipment. Can be improved.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail using the Example of this invention, the technical scope of this invention is not limited to this.

First, the austenitic stainless steel according to the present embodiment was manufactured, and the crystallinity of the metal structure was confirmed.
Austenitic stainless steel was manufactured by a direction control casting method using the manufacturing apparatus 10 in total of six types (No. 1 to No. 6) having different elemental compositions.
For the production of austenitic stainless steel, the single crystal production apparatus 10 shown in FIG. 1 is used. For each austenitic stainless steel of each elemental composition, the mold temperature in the mold heating furnace 8 is 1560 ° C., the casting temperature is 1560 ° C., Casting was performed. The cast molten metal 1 is held at 1560 ° C. for 5 minutes, and then the water-cooled chill 9 is lowered together with the main body 4 at a speed of 50 cm / h until the mold 3 is pulled down to the outside of the mold heating furnace 8. Unidirectionally solidified. At this time, the mold temperature in the mold heating furnace 8 was maintained at 1560 ° C. until solidification was completed.

  The analytical values of the elemental composition of the produced austenitic stainless steel (No. 1 to No. 6) are shown in Table 1 below. In addition, No. 7 is an austenitic stainless steel “R-SUS304ULC” defined in the Japan High Pressure Technology Association Nuclear Fuel Reprocessing Equipment Standard Material Standard (HPIS C 108). This R-SUS304ULC has a polycrystalline structure whose direction is not controlled. In Table 1, the numerical value of each element is the element content [% by mass], and the numerical value of Cr / Ni is a numerical value obtained by calculating the Cr element concentration / Ni element concentration ratio.

About each manufactured austenitic stainless steel, the structure of the metal structure was confirmed using the macro etching by aqua regia method (refer JIS G 0553) and the X-ray back reflection Laue method. As a result, no. 1-No. For No. 5, it was confirmed that an austenite main phase composed of a single crystal structure was formed. However, no. For No. 6, a single crystal of the main phase austenite phase was not obtained.
Therefore, in order to obtain an austenitic stainless steel having a single crystal structure as the main phase, the austenite phase needs to have a composition ratio of austenite forming elements of a certain level or more, and Cr element concentration / Ni element concentration It was confirmed that the ratio must be 1.80 or less.

Next, the corrosion resistance of the austenitic stainless steel according to this embodiment was evaluated based on the Corio test. The Corio test is one of corrosion tests for evaluating the corrosion resistance against high concentrations of hot nitric acid.
The specimens were produced by subjecting the produced austenitic stainless steel (No. 1 to No. 6) and R-SUS304ULC (No. 7) to a solid solution treatment under an argon gas atmosphere. The solution treatment temperature was 1050 ° C., and the treatment time was 2 hours. Then, three test pieces each having a size of 12 mm × 8 mm × 2 mm were cut out from each of these test materials, and the surface of each was polished up to # 600 in order. In addition, as for the test piece dimension, the cross-sectional area of the orthogonal surface in the casting direction is 12 mm × 8 mm, and the thickness in the casting direction is 2 mm. No. For No. 7, test specimens were taken perpendicular to the rolling direction.
Further, as the corrosive test solution, a solution prepared by dissolving chromic anhydride in an 8 mol / L nitric acid solution so that the Cr ⁶⁺ concentration was 1 g / L was used.

  In the Corio test, each of the three test pieces was immersed in a boiled corrosive test solution, and the boiled state was immersed for 24 hours in one cycle, and a total of four cycles of the test were performed. At this time, after each cycle, each test piece was subjected to weight measurement and appearance observation with a microscope, and the degree of corrosion was calculated based on the measured weight loss of corrosion.

FIG. 3 is a diagram illustrating the corrosion degree of the austenitic stainless steel according to the example.
The vertical axis in FIG. 3 is the corrosion degree [g / m ² · h] (corrosion weight loss per unit area / time). In the immersion test, depending on the test material, an increase in the degree of corrosion was observed as the cycle progressed, so the average degree of corrosion of the three test pieces was calculated for the fourth cycle where the degree of corrosion settled down substantially constant, This is shown in FIG.

As shown in FIG. In No. 7, the highest degree of corrosion was observed among the test materials. As a result of appearance observation, No. As for the surface of 7, the unevenness | corrugation by degranulation has generate | occur | produced and it was confirmed that the intergranular corrosion has progressed. In addition, No. of the comparative example which could not be single crystallized. In No. 6, no. Although not as high as 7, a relatively high degree of corrosion was observed. No. No. 6 is controlled in one direction and has a crystal structure with a large grain size, but has not reached a single crystal structure, and therefore, it was confirmed that intergranular corrosion has progressed to a certain degree.
On the other hand, in the example No. 1 in which the austenite phase of the main phase has a single crystal structure. 1-No. In No. 4, the degree of corrosion was kept low, and the improvement in corrosion resistance by single crystallization was clearly recognized. As a result of appearance observation, No. 1-No. The surface of 4 was confirmed to be relatively smooth.
On the other hand, although the austenite phase of the main phase is composed of a single crystal structure, the comparative example No. 1 has a small Cr element concentration / Ni element concentration ratio. In No. 5, no. Although not about 6, a relatively high degree of corrosion was observed. As a result of appearance observation, No. On the surface of No. 5, it was confirmed that pit-like overall corrosion occurred at a high number density. The cause of this form of corrosion is no. 5 was considered to be a high Ni element concentration. Therefore, it was confirmed that a numerical value of 1.45 or more was required as the Cr element concentration / Ni element concentration ratio in order to maintain corrosion resistance against corrosion including overall corrosion.
Therefore, from the above results, the austenitic stainless steels (No. 1 to No. 4) according to the examples have a property of appropriately forming a main phase composed of a single crystal structure by having a predetermined element composition. It was confirmed that In particular, it has a characteristic that is excellent in corrosion resistance against corrosion caused by an oxidizing acid accompanied by grain boundary erosion, and it has been confirmed that it is effective as a corrosion-resistant structural material with reduced corrosion thinning and occurrence of tunnel corrosion.

Next, using the austenitic stainless steel according to the example, a cap was manufactured that was supposed to be used at the tube end of the thermometer protection tube of the radioactive liquid waste treatment facility.
As the steel material to be processed, the elemental composition is No. 1 in Table 1. A cylindrical austenitic stainless steel single crystal having an outer diameter of 40 mm and a length of 200 mm manufactured by the direction control casting method using the manufacturing apparatus 10 was used.
The cap was manufactured by performing machining on the steel material to be processed and cutting it into a bottomed cylindrical shape having an outer diameter of 34 mm. At this time, a total of two tensile test pieces were collected from the remaining steel material to be processed and subjected to a tensile test at 23 ± 5 ° C.

As a result, the 0.2% proof stress of the austenitic stainless steel single crystal used as the work material was 195 MPa, the tensile strength was 492 MPa, and the elongation was 42%.
That is, the mechanical properties of the above, stainless steel rod of SUS304L standard defined in JIS G 4303, yield strength, 175 N / mm ² or more, tensile strength, 480N / mm ² or more, elongation more than 40% And SCS19 standard of stainless steel cast steel specified in JIS G 5121, 0.2% proof stress is 185 N / mm ² or more, tensile strength is 390 N / mm ² or more, and elongation is 33%. All of the above conditions were satisfied.

Subsequently, the manufactured cap was joined to the protective tube body by TIG (Tungsten Inert Gas) welding, and the weldability was evaluated.
As the protective tube, a pipe material made of R-SUS304ULC was used, and the weldability was evaluated by a penetrant test.
As a result, no crack was observed in the weld.

From the results of the above machining, tensile test and penetration test, it was confirmed that the austenitic stainless steel according to the present embodiment has good workability and weldability and is excellent in mechanical properties. .
Therefore, it is a steel material suitable for application as a component or member of radioactive waste liquid treatment equipment that can come into contact with high-concentration oxidizing acids, and contributes to the expansion of the scope of application to components or members that require improved corrosion resistance. It was confirmed that the steel material can be used.

DESCRIPTION OF SYMBOLS 1 Molten metal 2 Melting furnace 3 Mold 4 Main body 5 Selector 6 Starter 7 Casting port 8 Mold heating furnace 9 Water-cooled chill 10 Single crystal manufacturing device 20 Waste liquid concentration / volume reduction device 21 Heating jacket 30 Thermometer protection tube 31 Thermometer protection tube cap 32 protection tube body 33 thermometer 40 thermometer protection tube support 210 waste liquid supply port 211 waste liquid discharge port 301 welded portion

Claims (6)

In mass%, C: 0.02% or less, Si: 1.0% or less, Mn: 2.0% or less, Ni: 9.0 to 13.0%, Cr: 18.0 to 20.0%, N: 0.1% or less, with the balance being composed of Fe and inevitable impurities,
Cr equivalent / Ni equivalent ratio is 1.45 or more and 1.80 or less,
An austenitic stainless steel characterized in that the austenitic phase has a single crystal structure. The austenitic system according to claim 1, characterized in that P: 0.015% or less, S: 0.015% or less, Mo: 0.2% or less, Nb: 0.1% or less in mass%. Stainless steel. The austenitic stainless steel according to claim 1 or 2, wherein the mass ratio is Ni: 9.0 to 11.0% and Cr: 18.0 to 19.5%. The austenitic stainless steel according to any one of claims 1 to 3, wherein the mass ratio is Mo: 0.1% or less and Nb: 0.05% or less. In the austenite phase, the ferrite phase is distributed in islands,
The austenitic stainless steel according to any one of claims 1 to 4, wherein a cross-sectional area ratio of the ferrite phase is 10% or less. A radioactive waste liquid treatment facility equipment using the austenitic stainless steel according to any one of claims 1 to 5.

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