JPH01268848A - Equipment member excellent in nitric acid resistance - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance of a welded joint, etc., to a highly oxidiz ing nitric acid solution by forming a welded joint or surface treatment layer made of austenitic alloy in equipment made of austenitic stainless steel for use under a nitric-acid environment containing highly oxidizing ions into a specific structure. CONSTITUTION:E.g., for equipment members used for recovering U, Pu, and nitric acid from a nitric acid solution containing highly oxidizing ions in a nuclear fuel reprocessing plant, etc., an austenitic stainless steel having a composition containing, by weight, <0.05% C, <0.05% Si, 0.2-2.5% Mn, <0.01% P, <0.035% S, 8-12% Ni, 17-21% Cr, and 0.13% Mo is used as a base metal, and, as to a weld zone and a surface treatment layer, an austenitic stainless steel which has a composition almost identical with that of the above base metal and in which the relationship between Ni equivalent and Cr equivalent represented by equations I lies in the first crosshatched region is formed by means of arc welding, etc., and further formed into a structure in which a delta-ferrite phase is first crystallized out and then an austenitic phase is crystallized out at the time of the solidification of weld metal, by which corrosion resistance in the weld zone and the surface-treated zone to a highly oxidizing nitric acid solution can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a new component used in a nitric acid solution environment containing highly oxidizing ions, particularly for austenitic alloy joining joints or surfaces in nuclear fuel reprocessing plants. Regarding the processing layer.

[Conventional technology]

Austenitic alloys have good corrosion resistance in nitric acid. However, it is known that corrosion is significantly oxidized in nitric acid containing highly corrosive highly oxidizing ions such as Cr'+, Ce'+, Fea, etc. A nuclear fuel reprocessing plant is one of the plants that handle such harsh environments.

Figure 2 shows the main steps in a nuclear fuel reprocessing plant that extracts useful uranium and plutonium from spent nuclear fuel. Among the important equipment, for example, dissolution 1. Waste liquid concentration evaporator 2. Acid recovery evaporator 3, acid recovery rectification column 4. This is the waste liquid storage tank 5, and the concentration evaporator 2 and the fourth tank are shown in FIG.
A configuration diagram of the storage tank 5 is shown in the figure. The former is equipment for evaporating and concentrating waste liquid, and the latter is equipment for storing radioactive waste liquid for a long period of time.

Each device is provided with wiring 6, 8, and the nitric acid solution in each evaporator and tank is heated by supplying a heating medium to the piping 6.8. In particular, it is preferable to boil the inside of the device under reduced pressure from the viewpoint of corrosion.

In this process, a nitric acid solution is used to dissolve the fuel after the dissolution step. In addition to various corrosion products, the highly oxidizing nitric acid solution also contains large amounts of fission products. Among these, ruthenium (Ru) is a highly corrosive element, and is the third most corrosive element.
Proceedings of the 2nd Corrosion Prevention Conference, p31.7 (19858
) shows that Ru has a strong corrosion promoting effect in nitric acid.

In addition, the collection of abstracts of the 25th Annual Meeting of the Atomic Energy Society of Japan (Volume ■)
, p160 (1987-4) describes that Ru3+ is oxidized to Ruδ+ in nitric acid and volatilized as Ru14.

Austenite alloys are currently used in the parts of nuclear fuel reprocessing plant equipment, but since they corrode when exposed to this highly corrosive environment, there is a strong desire to improve the corrosion resistance of these parts.

Among austenitic alloy members, the base material has a uniform austenite (γ) phase, but the chemical composition and metal structure are highly heterogeneous during fusion joints or surface treatment layers that involve melting (overlay welding, etc.). Has an organization.

In arc welding, which is generally widely used industrially, in the case of certain types of austenitic steel joints, the molten solidified part contains about 5 to 10% of δ-ferrite phase (abbreviated as δ phase) in addition to γ phase. It has a two-phase mixed structure. For example, welding handbook (edited by the Welding Society of Japan).

According to the third revised edition, Marukubi (Showa 52-3), it is stated that the presence of 5 to 10% ferrite in the weld metal is effective in improving the cracking resistance during welding. However, as described in the above literature, it is generally said that the presence of ferrite deteriorates the corrosion resistance of weld metal.
Ordinary austenitic steel welds have the advantage of improved cracking resistance due to the inclusion of the δ-ferrite phase, and the disadvantage of reduced corrosion resistance.

For this reason, appropriate δ
It has become common sense to select the amount of ferrite.

In addition, it is generally known that crevice corrosion occurs when gaps exist, and as a countermeasure to this, austenitic alloys with MO added, such as SUS 316 steel, are commonly used. It is known that austenitic alloys containing MO have sufficient corrosion resistance against localized corrosion and pitting corrosion, and austenitic alloys containing MO are often used as highly corrosion-resistant materials.

In a nitric acid environment containing highly corrosion-resistant ions, corrosion of the material is accelerated as described above, but there is a concern that the acceleration of corrosion will be particularly pronounced in molten and solidified welds for the reasons described above.

Until now, as a method for improving corrosion resistance in a nitric acid environment, Japanese Patent Application Laid-Open No. 54-124820. Japanese Patent Publication No. 55-91960
discloses a highly corrosion-resistant welding rod with increased Si content.

Furthermore, Japanese Patent Laid-Open No. 59-222559 discloses that the P content is limited to 0.005% or less in order to improve corrosion resistance.

However, increasing the Si content may impose restrictions on the material's workability and manufacturability, and limiting the P content to a very low value of o, o o s% or less is difficult for steel manufacturing. Although there is also the problem of lowering P, it cannot be said that the effect alone is sufficient.

In addition, the corrosion resistance of welded parts is generally lower than that of the base metal, especially in the case of 5US316 series austenitic alloys that contain Mo and are expected to have better corrosion resistance against crevice corrosion. The current situation is that it shows a lower value.

Equipment parts used in a nitric acid environment containing highly oxidizing ions, especially equipment parts for nuclear fuel reprocessing plants, are required to have high corrosion resistance in order to have a long service life.

Furthermore, the above-mentioned tendency is a common problem not only for equipment members for nuclear fuel reprocessing plants, but also for chemical plants, industrial plants, and even atomic couplants that are used in a nitric acid environment containing highly oxidizing ions.

[Problem to be solved by the invention]

As mentioned above, austenitic alloy joints or surface treatment layers that involve melting and solidification tend to be corroded in a nitric acid environment containing highly oxidizing ions.

An object of the present invention is to provide an equipment member having excellent corrosion resistance in a nitric acid environment containing highly oxidizing ions and having an austenitic alloy joint or a welded part with a surface treatment layer.

[Means to solve the problem]

The present invention provides an equipment member having a welded portion on a base material made of austenitic stainless steel, in which the welded portion has a solidification structure in which a δ-ferrite phase is first crystallized and then an austenite phase is crystallized during solidification. Features include nitric acid-resistant equipment parts.

The welded portion contains C: 0.05% or less and Si: 1.
0% or less 2Mn: 0.2-2. ! 3%, P: 0.0
35% or less, S: 0.035% or less, Ni: 8 to 12%
, Cr: 17-21%, Mo: 0.1-3.0%, the balance being Fe and unavoidable impurities.

The nitric acid environment includes a system for dissolving spent nuclear fuel with a nitric acid solution and extracting U and Pu through a chemical separation process, as well as a system and each process for recovering, purifying and reusing the nitric acid solution used for dissolution. This is a nuclear fuel reprocessing plant environment with a system for concentrating and storing radioactive waste liquid produced by nuclear fuel.

The base material has C: 0.05% or less, Si: 0.0 by weight
5% or less, Mn: 0.2-2.5%, P: 0.01
% or less, S: 0.035% or less, Ni: 8 to 12%, C
It is composed of an austenitic alloy consisting of r: 17 to 21%, Mo: 0.1 to 3.0%, the balance being Fe and inevitable impurities.

[Effect]

The corrosion resistance of the austenitic alloy joint or surface treated IIJA layer in nitric acid containing such highly oxidizing ions is as follows:
It has been discovered that the solidification form of the alloy in the joint or surface treatment layer that has been heated above the melting point and melted has a significant effect on solidification,
It was clarified that high corrosion resistance can basically be achieved by first crystallizing the δ-ferrite phase and then crystallizing the austenite phase at the same time.

As a result of a detailed study of the corrosion behavior and corrosion morphology of various austenitic alloy welds in nitric acid containing highly oxidizing ions, it was found that when the alloy in the liquid state is heated above the melting point in the weld and solidifies, the third There are two types of processes shown in the table, and as a result of examining the corrosion resistance of the solidified parts of both Type I and Type ■ in nitric acid containing highly oxidizing ions, it was found that the corrosion resistance of the solidified part of Type I is similar to that of Type II. It has been newly discovered that it is significantly better.

When a corrosion test of an austenitic alloy joint or a surface treatment layer is performed in nitric acid containing highly oxidizing ions, corrosion occurs unevenly along the dendritic dendrite layer formed during solidification, as if the δ-ferrite phase was corroded. Conventionally, the method of reducing the amount of the δ ferrite phase was considered to be corroded because it is visible, but the present invention is completely different from this idea. Corrosion is not the amount of δ ferrite, but the form in which δ ferrite crystallizes during solidification is important. Corrosion resistance of solidified austenite alloy parts can be improved by controlling the solidification mode to Type I to create joints or joints with excellent corrosion resistance. A surface treatment layer is obtained.

The composition range of the austenitic alloy that is effective for obtaining particularly high corrosion resistance in such joints or surface treatment layers is as follows:
By weight: C: 0.05% or less, Si: 1.0% or less, M
n: 0.2-2.5%, P: 0.035% or less, S
: 0.035% or less* N x : 8 to 12%, Cr:
This is an austenitic alloy joint or surface treatment layer characterized by comprising 17 to 21% Mo, 0.1 to 3.0% Mo, and the balance Fe and unavoidable impurities.

Next, the reason for limiting the range of chemical composition of the austenitic alloy joint or surface treatment layer of the present invention will be explained.

C: If C exceeds 0.05%, the amount of Cr carbide precipitated in the austenitic alloy joint will increase, resulting in a lack of Cr near these precipitates, resulting in a decrease in corrosion resistance, so the upper limit is 0.05%. It was determined that

Si and Mn: Si and Mn are elements used as deoxidizing agents in steel manufacturing. Therefore, industrially, Si is 1.
It is necessary to add Mn in an amount of 0% or less, and the range of Mn is preferably 0.2 to 2.5% from the viewpoint of stability of the austenite phase.

Ni: Ni is the main austenite-forming element,
Along with Cr, it is an important element in austenitic alloys. Ni is used to maintain corrosion resistance in nitric acid, and also to improve the structure of the solidified part (ratio of δ ferrite phase to austenite phase).
Cr is the main ferrite-forming element to regulate
A range of 8 to 12% is preferable in balance with the amount of .

Cr: Cr is a ferrite-forming element and is an essential element in order to obtain corrosion resistance in nitric acid. N is added to ensure corrosion resistance in nitric acid and to control the structure of the solidified part.
Corresponding to the i amount, 17 to 21% is good, i.e. 17%
is necessary from the viewpoint of corrosion resistance in nitric acid, and on the other hand, if it exceeds 21%, there is a concern that the solidified portion will become brittle.

Mo: Mo is a ferrite-forming element, and is said to have the effect of strengthening the passive film formed on the surface, and has been shown to increase resistance to crevice corrosion. A range of 3.0% is preferable.

In addition, Ti of 1% or less each as necessary.

Appropriate amounts of carbide-forming elements such as Nb, V, and Zr, and elements having a deoxidizing effect such as A Q p Ca of 0.5% or less each may be added, and the effects of the present invention can also be obtained in this case.

In addition to the structure of the austenitic alloy joint or surface treatment layer described above, if the composition is selected within the Ni equivalent and Cr equivalent range shown in Figure 1, the most effective highly corrosion-resistant joint or surface treatment layer can be obtained. It will be done.

Here, Ni equivalent = %Ni + 30X%C + 0.5%%MnCr equivalent = %Cr + %Mo + 1, 5 This is the result of the discovery that it is possible to crystallize the ferrite phase.

The reasons for limiting the area ABCD shown in FIG. 1 are as follows.

■Line AB: T below this line, that is, on the low Ni equivalent side
The solidification mode is the primary δ phase type of ype I, and high corrosion resistance is obtained.

(2) Line CD: If the Ni equivalent value exceeds this line, problems such as embrittlement may occur, so it is preferable to keep the line up to line CD.

(2) Line DA: In order to obtain corrosion resistance against general corrosion in nitric acid, the Cr equivalent is set to 19% or more. If it is less than this, the corrosion resistance as an austenitic alloy cannot be obtained. Further, the above Cr equivalent is necessary to prevent the formation of martensitic structure due to non-uniformity of chemical components.

■ Line BD: As the Cr equivalent increases, the weldability and workability of the material decrease, and problems such as embrittlement occur. From this point of view, the Cr equivalent is preferably 24% as a practical range.

For the above reasons, higher corrosion resistance than conventional materials can be obtained by controlling the solidification mode to Type I "initial δ phase type" in the region ABCD of FIG.

Furthermore, the austenitic alloy joint or surface treatment layer in which the solidification mode is controlled to Type I according to the present invention has an excellent corrosion rate of 2 Orrn/y or less in a 9N nitric acid solution at 100°C containing 1100 pp of Ru2+, for example. It has corrosion resistance.

The excellent corrosion resistance of the austenitic alloy joint or surface treatment layer according to the present invention as described above is due to the joining method 2.
Sufficiently high corrosion resistance can be achieved regardless of the surface treatment method, that is, by commonly used arc welding, electron beam welding, laser welding, etc. Basically, the present invention can be applied to any melting/solidification part where solidification mode control according to the present invention can be performed, regardless of the method6.As mentioned above, the difference in solidification mode is due to impurity elements P, Si
It is inferred that this will affect the degree of segregation. Therefore, reducing the impurity elements themselves that reduce corrosion resistance is effective in improving corrosion resistance in any solidification mode. That is, the present inventors have found that good corrosion resistance can be obtained by simultaneously regulating the impurity element P and Si contents to P≦0.01% and SiS 2.05%.

The effect is /j% unless both P and Si are reduced at the same time.
Sai.

The above description has focused on the austenitic alloy joining joint or surface treatment layer of the present invention, but the present invention can also be applied to the welding rod itself used in manufacturing them to obtain similar effects. That is, for example, as a welding rod when performing arc welding, it is a welding rod having a chemical composition as described above, or a welding rod having a chemical composition as well as Ni equivalent and Cr equivalent in the range ABCD shown in FIG. , excellent corrosion resistance can be obtained by using these welding rods.

[Example 1] Table 1 shows the chemical components of the test materials used in this example. The results of corrosion tests of welded parts are shown for a total of 16 types, including 3 conventional materials, 2 inventive materials, 11 inventive materials, and 2 comparative materials.

For each of these test materials, a V-shaped groove butt welded joint was fabricated by TIG welding, and a corrosion test piece was taken from that part. A commercially available 5US316L material was used as the base material. Corrosion tests were carried out in 9 N HNOs + 100 ppm Ru solution (100
℃) for 500 hours. After the immersion test, the corrosion depth of the test piece was measured, and the corrosion rate (no/y) was calculated from it. The solidification mode is classified into the types shown in Table 2, and the conventional material and comparative material are Type n, and the inventive material is Type n.
It was eI.

As can be seen from Table 1, the corrosion rate of conventional materials C1 to C3 with a solidification mode of Type ■ is 4.0 mm/y or more, and the corrosion rate of comparative materials R1 and R2 is 1% higher than that of the conventional material. On the other hand, the corrosion rate of the material of the present invention was 0.7 mm/y or less, and 1.0 to 1.0 mm/y.
There are some in the range of 3 mm/y, but all of them showed significantly better corrosion resistance than conventional materials and comparative materials. Among the samples of the present invention, the samples with chemical components shown in Table 2 P1 to P7 were shown to have particularly excellent corrosion resistance.

[Example 2] The chemical composition of the test material is shown in Table 3, the solidification mode is "primary δ phase type", and the amount of P and Si is P≦0.01%.

SiS reduced to 2.05% (sample NQPP-1
), one with normal content (N [lPP-2), and one with "primary γ phase type" with similarly reduced P and Si content (&PP
There are a total of four types: -3) and one with normal content (hcc-1). A TIG welded fleshy part of these components was formed, and a corrosion test piece was taken from that part. The corrosion test conditions are the same as in Example 1.

The test results are shown in Table 3. Among the materials of the present invention, the "primary δ phase type" has a low corrosion rate of 1.0 m/y or less even if the P and Si contents are normal, and when P and Si are reduced, Corrosion rate is noticeably reduced. On the other hand, in the "primary γ phase type", P, Si
shows a high corrosion rate of about 10.5mw+/y with normal content, while that with reduced P and Si content is 2.0a
It showed a remarkable corrosion rate of less than m/y.

As mentioned above, even in the "primary γ phase type", P.

It has been known that reducing the Si content provides excellent corrosion resistance.

[Example 3] As in Example 1, a 5US316L base material was used, and an overlay weld layer with a thickness of about 3 m was formed on the surface by TIG welding using the welding material shown in Table 1 P5.

A corrosion test was conducted on the overlay weld layer in the same manner as described above, and as a result, the amount of corrosion was the same as in Example 1.

〔Effect of the invention〕

According to the present invention, since corrosion resistance in nitric acid containing highly oxidizing ions can be improved, a joint or a surface treatment layer with high reliability and a long service life can be obtained.

The present invention is effective in a nitric acid environment containing highly oxidizing ions, and can be used in chemical plants, industrial plants, atomic couplant equipment, and their constituent members used in such environments. In particular, it shows remarkable effects when applied to nuclear fuel reprocessing plant equipment and its constituent parts.

Here, the highly oxidizing ions are F e "+, Cr'+
.

It represents ions of a higher number group such as Ce'+ and Ru' 10, and is effective in a nitric acid solution environment containing one or a combination of these highly oxidizing ions. Quantitatively showing this nitric acid solution containing highly oxidizing ions, it is effective in the overpassive region where the corrosion potential of stainless steel is 0.95V or more compared to the reference comparison electrode (SHE).

The present invention provides equipment and its constituent members that exhibit high corrosion resistance in this area.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the range of Cr equivalent and Ni equivalent according to the present invention, FIG. 2 is a block diagram of a nuclear fuel reprocessing plant,
FIG. 3 is a block diagram of the waste liquid concentration evaporator, and FIG. 4 is a block diagram of the waste liquid storage tank. 1... Melting, 2... Waste liquid concentration evaporator, 3... Acid recovery evaporator, 4... Acid recovery rectification column, 5... Waste liquid storage tank, 6° 8... Piping. Ward 1

Claims (1)

[Claims] 1. In an equipment member having a welded portion on a base material made of austenitic stainless steel, the welded portion has a solidification structure in which a δ-ferrite phase is first crystallized during solidification, and an austenite phase is then crystallized. An equipment member having excellent nitric acid resistance. 2. The device member according to claim 1, wherein the welded portion is a butt welded portion or a surface treatment layer made of austenitic stainless steel, and has excellent nitric acid resistance. 3. In claim 2, the nitric acid environment includes a system for dissolving spent nuclear fuel with a nitric acid solution and extracting U and Pu through a chemical separation process, as well as recovering, purifying and reusing the nitric acid solution used for dissolution. Equipment components with excellent nitric acid resistance that are used in the environment of a nuclear fuel reprocessing plant, including a system for use and a system for concentrating and storing radioactive waste liquid generated from each process. 4. In claim 2 or 3, the welded portion has a weight of C:
0.05% or less, Si: 1.0% or less, Mn: 0.2~
2.5%, P: 0.035% or less, S: 0.035% or less, Ni: 8-12%, Cr: 17-21%, Mo: 0
．． An equipment member with excellent nitric acid resistance made of an austenitic alloy consisting of 1 to 3.0%, the balance being Fe and unavoidable impurities. 5. An equipment member having excellent nitric acid resistance according to claim 4, wherein the austenitic alloy has Ni equivalent and Cr equivalent calculated by the following formula within the range shown in FIG. Ni equivalent=%Ni+30×%C+0.5×%MnCr equivalent=%Cr+%Mo+1.5×%Si6, claims 1 to 5
In any of the above, the welded portion is 9NHNO_3+1
00ppmRu, corrosion rate at 100℃ is 2.0mm/
Equipment parts with excellent nitric acid resistance that is less than y. 7. In any one of claims 1 to 6, the base material is C: 0.05% or less, Si: 0.05% or less, Mn:
0.2-2.5%, P: 0.01% or less, S: 0.03
5% or less, Ni: 8-12%, Cr: 17-21%, M
An equipment member with excellent nitric acid resistance, made of an austenitic alloy consisting of o: 0.1 to 3.0%, the balance being Fe and unavoidable impurities. 8. The device member according to claim 1, wherein the welded portion is formed by any one of arc welding, electron beam welding, and laser beam welding, and has excellent nitric acid resistance. 9. A chemical plant, industrial plant, or nuclear power plant constructed of the equipment member according to any one of claims 1 to 8. 10. The welding material for nitric acid equipment made of the austenite alloy according to claim 4 or 5. 11. The device member with excellent nitric acid resistance according to any one of claims 1 to 9, wherein the surface treatment layer is a built-up welding layer.