JPH0299294A - Equipment member having excellent nitric acid resistance - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance in nitric acid and to extend the service life of the above member by composing the base metal consisting of an austenitic stainless steel in such a manner that the weld metal formed by melting and solidifying of the base metal has the solidification form to first crystallize a delta ferrite phase at the time of solidification. CONSTITUTION:The compsn. of the base metal of the austenitic stainless steel consists, by weight, of <=0.05% C, <=1.0% Si, 0.2 to 2.5% Mn, <=0.035% P, <=0.035% S, 8 to 12% Ni, 17 to 21% Cr, 0.1 to 3.0% Mo, and the balance Fe and unavoidable impurities. Further, the compsn. satisfies Cr equiv./Ni equiv. 1.6 in the Cr equiv. and Ni equiv. calculated by the formula. The welding material for welding at least from the root layer to the 3rd layer at the time of welding has the Cr equiv. and Ni equiv. within the region A and the welding is executed by the welding material having the above mentioned equiv. within the region B.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a new equipment component used in a nitric acid solution environment containing highly oxidizing ions, and in particular to austenitic stainless steel in a nuclear fuel reprocessing plant. Concerning steel welded joints.

[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 Cr6+, Ce'+, Fe3+, etc. A nuclear fuel reprocessing plant is one of the plants that handle such harsh environments.

A second major process in a nuclear fuel reprocessing plant that extracts useful uranium, plutonium-11, from spent nuclear fuel.
As shown in the figure. Among the important equipment, for example, the dissolution 1,
Waste liquid concentration evaporator 2, acid recovery evaporator 3, acid recovery rectification column 4,
This is a waste liquid storage tank 5, and FIG. 3 shows the concentrating evaporator 2, and FIG. 4 shows a configuration diagram of the storage tank 5. These devices are
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.
2 Proceedings of the Co-located Food and Prevention Discussion Group, p317 (1985-
8) 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 Ru'0 is oxidized to Ru♂+ in nitric acid and volatilized as Ru0i.

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 stainless steel members, the base material has a uniform austenite 1-(γ) phase, whereas the fused joint has a solidified structure with high heterogeneity in chemical composition and metal structure.

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, Maruzen (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 1 deteriorates the corrosion resistance of the weld metal, and the welded parts of ordinary austenitic steel have poor cracking resistance due to the inclusion of the δ ferrite phase. This has both the advantage of improving corrosion resistance and the disadvantage of reducing corrosion resistance.

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

Furthermore, it is generally known that crevice corrosion occurs when gaps exist, and as a countermeasure to this, Austety 1-Alloy 1, for example 5US316 steel, to which MO is added is generally used. It is known that when Mo is added, it exhibits sufficient corrosion resistance against local corrosion and pitting corrosion when gaps exist.

Austenitic alloys containing C 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 Application Laid-Open No. 59-222559 discloses that the P content should be limited to 0.005% or less in order to improve corrosion resistance.

However, increasing the Si content may impose restrictions on the processability and manufacturability of the material.

Furthermore, limiting the P content to a very low value of 0.005% or less poses a problem in terms of steel manufacturing, but it cannot be said that reducing P alone is necessarily sufficient.

In addition, the corrosion resistance of welded parts is generally lower than that of the base metal, especially in the case of SUS 316 series austenitic alloys, which contain MO and are expected to have better corrosion resistance against crevice corrosion. Currently, the value is lower than that of wood.

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 components for nuclear fuel reprocessing plants, but also for chemical plants, industrial plants, and nuclear power plants that are used in nitric acid environments containing highly oxidizing ions.

[Does the invention try to solve the problem? Title a] As mentioned above, welding joints of austenitic alloys or melt F! A surface treatment layer accompanied by solidification tends 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 a welded joint made of Austety 1-stainless steel which has excellent corrosion resistance in a nitric acid environment.

[Means to solve the problem]

The present invention provides an equipment member having a welded part with a base metal made of austenitic stainless steel, characterized in that the molten and solidified weld metal of the base metal has a solidification form in which a δ-ferrite phase is first crystallized during solidification. Equipment components made of welded parts with excellent nitric acid resistance.

The base material used for the welded portion has a weight percentage of c: 0. os
% 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
:0.1 to 3.0%, the remainder being Fe and unavoidable impurities. Furthermore, this austenitic stainless steel is preferably characterized in that the Cr equivalent and Ni equivalent calculated by the following formula satisfy Cr equivalent/Ni equivalent≧1.6.

In addition, when performing welding work, first, the Cr of the welding material is
The equivalent and Ni equivalent are within region A shown in Figure 1, and at least the first to third layers are welded, and then welded to region B.
It is characterized by welding using the welding material contained within.

Cr equivalent = %Cr + %Mo + 1, 5 x %Si
-(1) Ni equivalent = %Ni+30x%C+ 0
, 5 x% M n -(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 and purifying the nitric acid solution used for dissolution. This is a nuclear fuel reprocessing plant environment with a system for reusing radioactive waste and a system for concentrating and storing radioactive waste liquid generated from each process.

It was discovered that the corrosion resistance of austenitic stainless steel joints in nitric acid containing highly oxidizing ions is significantly affected by the solidification form of the molten joint alloy when heated above the melting point. It was clarified that this is basically achieved by first crystallizing the δ-ferrite phase and then crystallizing the austenite phase during solidification.

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, we found that when an alloy in a liquid state is solidified by being heated above its melting point in a weld, 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 H in nitric acid containing highly oxidizing ions, it was found that the corrosion resistance of the solidified part of Type I was higher than that of Type U. I just found out that it's significantly better.

Table 1 Corrosion test of austenitic stainless steel joints in nitric acid containing highly oxidizing ions.

Corrosion occurs unevenly along the dendritic dendrite layer generated during solidification, giving the appearance that the δ-ferrite phase has been corroded. Conventionally, methods have been taken to reduce the amount of δ-ferrite, assuming that the δ-ferrite phase is being corroded. However, 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.
By controlling this, a joint with excellent corrosion resistance can be obtained.

For this purpose, it is essential to control the composition of the welding material and select the solidification form, and to similarly control the composition of the base material used. This is because the welding material and the base metal are mixed during melting, so that there is a region having a composition intermediate between the two.

In such welded joints, the composition range of the austenitic stainless steel base metal that is particularly effective in obtaining high corrosion resistance is, in weight percent, C: 0.05% or less, Si: 1.0
% or less, Mn: 0.2 to 2.5%.

P: 0.035% or less, S: 0.035% or less, Ni:
8-12%, Cr: 17-21%, MO2 0.1-3.
0%, the balance consists of Fe and unavoidable impurities, and further,
Preferably, in the Cr equivalent and Ni equivalent calculated by the above formulas (1) and (2), Cr equivalent/Ni equivalent ≧1
．． The composition satisfies 6. In addition, welding materials for welding at least the first layer to the third layer during welding are those whose Cr equivalent and Ni''j amount are within region A shown in Figure 1, followed by welding materials which are within region B. It is characterized by welding.

Next, the reason for limiting the range of chemical composition of the austenitic stainless steel of the present invention and its welded joint will be explained.

C: If C exceeds 0.05%, Cr carbide precipitates will increase in the austenitic alloy joint, 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 a major austenite-forming element, and along with Cr it is an important element in austenite alloys. 6Ni is used to maintain corrosion resistance in nitric acid and to improve the structure of the solidified part (the ratio of the δ ferrite 1-phase to the austenite phase). ) is preferably in the range of 8 to 12%, in balance with the amount of Cr, which is a main ferrite-forming element for regulating the ferrite-forming element.

Cr: Cr is a ferrite-forming element and is an essential and important element in order to obtain corrosion resistance in nitric acid. In order to ensure corrosion resistance in nitric acid and at the same time control the structure of the coagulated part, it is preferable that the Ni content be 17 to 21%, corresponding to the Ni content. That is, 1
7% is necessary for corrosion resistance in nitric acid, while 21%
If it exceeds this, 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 AQ and Ca at 0.5% or less each may be added, and the effects of the present invention will still be obtained in this case.

In addition to the composition of the austenitic stainless steel base metal mentioned above, the ratio of Cr equivalent to Ni equivalent is further limited to 1.6 or more, and the composition of the welding material used for at least the first to third layers is determined as shown in Figure 1. Range A of Cr equivalent and Ni equivalent shown in
By selecting a composition within the range, the most effective and highly corrosion resistant welded joint can be obtained.

This is the result of the discovery that by determining the Ni equivalent and Cr equivalent, it is possible to crystallize the δ ferrite phase prior to the desired austenite phase.

In the composition of the austenitic stainless steel base material, the reason why the ratio of Cr equivalent to Ni equivalent is limited to 1.6 or more is to control the solidification form of the base material itself to Type I. By regulating in this way, the solidification form of the molten part can be controlled to Ty even when the composition is mixed with the welding material.
It is possible to hold it in pe I.

Next, the reason for limiting areas A and B shown in FIG. 1 will be described below.

[Area A] In this area, the welding material undergoes compositional mixing with the base metal during melt solidification during welding, and the solidification form of the welding material Type
In order to prevent (primary δ phase type) from changing to Type (primary γ phase type), welding materials used for at least the first to third layers during welding are specified.

(2) Line AB: At a high Cr equivalent below this line, the amount of δ ferrite becomes too large, which may cause problems in weldability, workability, etc., so this line is set as the limit.

■ Line BC: Due to the non-uniformity of the chemical composition of the weld metal, a Ni equivalent above this line is required to prevent martensitic structure from forming.

(2) Line CD: Above this line, changes in the solidification form due to compositional mixing that occur during melt solidification during welding cannot be prevented, so a higher Cr equivalent and lower Ni equivalent than this line are preferred.

■ Line DA: If the Cr equivalent is larger than this line, the weldability and workability will deteriorate, and problems such as embrittlement will occur, so the Cr equivalent is set to 24% or less as a practical range.

[Area B] This area specifies the welding material to be used when welding is performed successively after welding at least the first to third layers with the welding material of area A during welding.

■Line EF: T below this line, that is, on the low Ni equivalent side
The initial product of ype I has a solidification form of δ phase type, and high corrosion resistance is obtained.

■ Line FC: In order to obtain corrosion resistance against general corrosion in nitric acid, the Cr content should be 18% or more, and it should be higher than this line to prevent the formation of martensitic structure due to the heterogeneity of chemical components. Let it be Ni equivalent.

■Line CD: If a large amount is used, it is better not to exceed this line because embrittlement problems may occur on the low Ni equivalent side exceeding this line.

■Line DE: Weldability increases as the Cr equivalent increases.

Processability decreases and problems such as embrittlement occur. As a practical range, The Cr equivalent is 24% or less.

For the above reasons, the Cr equivalent and Ni
By welding at least the first to third layers using a welding material that takes into account the compositional mix with the base metal, the solidification form of the weld metal can be changed to the Type I primary δ phase type. control and achieve higher corrosion resistance than conventional materials.

Furthermore, the joining joint or surface treatment layer of an austenitic alloy whose solidification mode is controlled to Type I according to the present invention can be made of, for example, 9N at 100°C containing 1100pp of Ru.
In nitric acid solution, the corrosion rate is 2. It has corrosion resistance of less than Onm/y.

The excellent corrosion resistance of the austenitic stainless steel joining joint according to the present invention described above does not depend on the joining method 1 and the surface treatment method. That is, sufficiently high corrosion resistance can be achieved by commonly used arc welding, electron beam welding, laser welding, etc. Basically, the present invention can be applied to any melting/solidifying portion where the solidification mode control according to the present invention can be performed, regardless of the method.

Furthermore, it is presumed that the difference in solidification mode affects the degree of segregation of impurity elements P, Si, etc.

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 determined that the impurity element P,
The Si content is 1]≦0.01%.

It has been found that good corrosion resistance can be obtained by simultaneously regulating SiS to 2.05%. The effect is small unless both P and Si are reduced at the same time.

Weld metal refers to the part where the deposited metal from the welding material and the base metal are melted and solidified.

〔Example〕

[Example 1 Tables 2 and 3 show the chemical components of the test materials used in this example. The base material used to manufacture the welded joints was commercially available austenitic stainless steel type 4 M (BCI, BO
2, BPI, BF2).

The welding materials used at this time were four types of commercially available welding wires (WCI, WO2, WPI, WF2).The solidification form was ryρe■ for the base metal BCI and BO2, and BPI
and BF2 are Type I. Also, all welding materials are Type! It is.

For each of these test materials, a U-shaped groove butt welded joint was fabricated by TIG welding, and a corrosion test piece was taken from that part. Corrosion test at 100°C containing 100ppm Ru
It was immersed for 500 hours in a 9NHN○3 solution. After the immersion test, the erosion depth of the specimen was measured2, and then the corrosion rate (
Table 4 shows the corrosion rate of the welded metal parts of the conventional welded joint and the welded joint according to the present invention. Fourth
As can be seen from the table, base material B is not subject to any particular regulation of ingredients.
CI or BO2, and welding material in area B in Figure 1, WCI
or 4 types of welding test pieces with WO2, CWJl, -
No. 4 has a corrosion rate of 5.0 mm/y or more.

On the other hand, PWJ 1 to 4, which are test pieces welded with welding materials with the same composition as above, in which the ratio of Cr equivalent to Ni equivalent was controlled to 1.6 or more in the base metal composition, were all 1 to 5 nrn. / indicates the corrosion rate of Y,
The material of the present invention showed better corrosion resistance.

Table 4 [Example 2] Table 5 shows the corrosion rates of conventional welded joints and joints produced by the welding method according to the present invention. The conventional material showed the same results as the test piece CWJ1.2 shown in Table 4 for comparison. During welding, the materials PWJ5 to PWJ8 of the present invention are first welded from the first layer to the third layer using the welding material WPI or WF2 of area A in FIG.
The final layer was welded using O2. The corrosion rate of conventional materials was 5. Onn+ /y or more, whereas
The corrosion resistance of the material of the present invention was 1.0 mm/y or less, which was better than that of the conventional material.

As described above, according to the method of the present invention, it is possible to produce a welded joint having good corrosion resistance even when using a conventional base material whose components are not particularly regulated.

Table 5 [Example 3] Table 6 shows the corrosion rate of a joint manufactured by combining a conventional welded joint, a base material whose components are regulated according to the present invention, and a welding method according to the present invention. For conventional materials, the results of test pieces CWJI and CWJ3 shown in Table 4 are shown for comparison. The present invention material uses base material BPi or BF2 with regulated composition,
As in Example 2, the first to third layers were first welded using the welding material WPI or WF2 of area A, and then welded to the final layer using the welding material WC] or WO2 of area B.

The corrosion rate of the material of the present invention is 1.0 mm/y or less.

It was shown that it has better corrosion resistance than conventional materials.

Table 6 [Effects of the Invention] According to the present invention, since corrosion resistance in nitric acid containing highly oxidizing ions can be improved, a joint with high reliability and a long service life can be obtained.

The present invention is effective in nitrogen environments containing highly oxidizing ions, and is suitable for chemical plants used in such environments.
It can be used for industrial plants, nuclear power plant equipment, and their constituent parts. In particular, it shows remarkable effects when applied to nuclear fuel reprocessing plan 1 to equipment and its constituent members.

Here, highly oxidizing ions include Fe 8+, Cr 8
+.

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), and in this region Provided are devices and their constituent parts that exhibit high corrosion resistance.

[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... Dissolution tank, 2... Waste liquid concentration evaporator, 3... Acid recovery evaporator, 4... Acid recovery rectification column, 5... Waste liquid storage tank. 6... Cooling piping, 7... Waste liquid storage container, 8...
Heat exchanger tube, 9...evaporation container.

Claims (1)

[Scope of Claims] 1. In an equipment member having a welded portion on a base metal made of austenitic stainless steel, the weld metal of the base metal that has been molten and solidified has a solidification form in which a δ ferrite phase is first crystallized during solidification. An equipment member having excellent nitric acid resistance. 2. In claim 1, the nitric acid environment includes a system for dissolving spent nuclear fuel with a nitric acid solution and extracting uranium and plutonium through a chemical separation process, as well as recovering and purifying 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 reuse and a system for concentrating and storing radioactive waste liquid generated from each process. 3. In claim 1 or 2, the base material is C
: 0.05% or less, Si: 1.0% or less, Mn: 0.2
~2.5%, P: 0.035% or less, S: 0.035%
Below, Ni: 8-12%, Cr: 17-21%, Mo:
An equipment member with excellent nitric acid resistance made of austenitic stainless steel containing 0.1 to 3.0%, the balance being Fe and unavoidable impurities. 4. In claim 3, the austenitic stainless steel as the base material has a Cr equivalent and N calculated by the following formula.
An equipment member having excellent nitric acid resistance and having an i equivalent (Cr equivalent/Ni equivalent) ratio of 1.6 or less. [Cr equivalent=%Cr+%Mo+1.5x%SiNi equivalent=%Ni+30x%C+0.5x%Mn] 5, Claim 3
In this process, the austenitic stainless steel that is the base metal is first welded from the first layer to the third layer using a welding material whose Cr equivalent and Ni equivalent calculated by the above formula are within the region A shown in FIG. is welded, and then welded with the welding material in area B to produce equipment parts with excellent nitric acid resistance. 6. In claim 5, the austenitic stainless steel as the base material has a Cr equivalent and N calculated by the above formula.
An equipment member having excellent nitric acid resistance and having an i equivalent (Cr equivalent/Ni equivalent) ratio of 1.6 or less. 7. In any one of claims 1 to 6, the welded portion is 9
NHNO_3+100ppmRu, an equipment member with excellent nitric acid resistance and a corrosion rate of 2.0 mm/y or less at 100°C. 8. The device member according to any one of claims 1 to 7, having excellent nitric acid resistance, wherein the welded portion is formed by any one of arc welding, electron beam welding, and laser beam welding. 9. A chemical plant, industrial plant, or nuclear power plant configured by the equipment member according to any one of claims 1 to 8. 10. The welding material for nitric acid-resistant equipment made of the austenitic stainless steel according to claim 5 or 6. 11. In claim 5, the welding material in area A in FIG.
Mn0.2-2.5%, P0.01% or less, S0.03
5% or less, Ni8-12%, Cr17-21%, Mo0
．． An equipment member with excellent nitric acid resistance, made of austenitic stainless steel consisting of 1 to 3.0% Fe and unavoidable impurities.