JP3531351B2 - Welded structure and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to pipes, water pumps, heat exchangers or dam gates used in freshwater environments such as lake water and river water, or members such as marine structures used in seawater or the whole. And a method for manufacturing the same.

[0002]

[Prior art]

1) When stainless steel is used in an aqueous solution containing chlorine ions, local corrosion such as pitting and crevice corrosion may be a problem. It has been conventionally considered that the concentration of chloride ion in a solution and the temperature of the solution are important factors for the occurrence of pitting corrosion and crevice corrosion of stainless steel in an aqueous solution containing these chloride ions.

2) However, recently, there have been reported cases in which so-called microbial corrosion is a problem that cannot be explained only by these factors. Microbial corrosion is the corrosion induced by the action of microorganisms existing in the environment, and for example, the case of accelerating the corrosion of carbon steel by sulfate reducing bacteria in an anaerobic environment is well known.

[0004] In addition, even in stainless steel having sufficient corrosion resistance in a test in a solution in which a chlorine ion concentration or the like is adjusted to artificially reproduce a corrosive environment, even in actual environmental water such as seawater or fresh water. Local corrosion may occur. This is also considered to be due to the influence of microorganisms existing in the natural environment. In other words, in a natural environment, it is considered that the corrosion potential of stainless steel shifts to the noble side due to the action of microorganisms present in the natural environment, which exceeds the potential for local corrosion to occur, leading to the occurrence of local corrosion. .

It has been reported that corrosion of stainless steel due to such microorganisms is more likely to occur in the welded portion than in the base material (material to be welded). Further, it is known that the selective dissolution of the austenite phase in the melt-solidified structure remarkably occurs.

As a welding material for stainless steel, for example, SUSY308 is generally used for general-purpose SUS 304 steel, and SUS 316L of common metal type having the same composition as the base metal is generally used for SUS 316L steel. In the structure of the weld metal welded using these welding materials, about several% of the δ ferrite phase is precipitated, and in such a structure, the austenite phase is selectively melted, and the skeleton-like δ ferrite phase is generated. Many cases of remaining microbial corrosion have been reported (GJ Licina;
CORROSION, 88 (1988), No.268). SUSY308 and SUSY316L
In existing welding materials, such as Cr, Mo, etc., elements that are effective in improving corrosion resistance are set to be somewhat higher than those in the base metal, but such minor measures are not sufficient for microbial corrosion resistance. Is.

[0007] Generally, "a portion consisting of a solidified structure" obtained as a result of welding a base metal using a welding material is called "weld metal".
Say. The weld metal includes most of the material transferred from the welding material and a partially melted base metal. Of the weld metal, the alloy component transferred from the welding material may be referred to as "welding metal". That is, the weld metal is not the composition of the welding material itself, but is composed of the weld metal diluted with the molten base material with a small ratio. The degree of dilution depends on the welding method, especially the welding heat input, and is commonly used for welding stainless steel such as gas / tungsten / arc welding (GTA).
When the heat input is relatively small as in W), the dilution is not large.

"Welded part" means "weld metal and weld heat affected zone (HAZ)". HAZ means a portion that does not melt during welding. The “bond” refers to a portion including a molten portion of the base material near the boundary between the deposited metal and the base material. Normally, the bond is treated as included in the HAZ because it is greatly influenced by the chemical composition of the base metal, but in the present specification, the “molten part of the bond” is also included in the “weld metal”. .

3) In order to impart the same corrosion resistance as the base metal to the weld metal having poor corrosion resistance in a severe corrosive environment,
Conventionally, measures have been taken to make the weld metal contain more Mo, which is effective in improving pitting corrosion resistance, than the base metal. In addition to measures to increase the amount of alloying elements, Alloy 625 (61% N
In some cases, expensive high alloy materials such as i-9% Mo-22% Cr-3.4% Nb) are used (Ogawa et al., Welding Society, 9 [1] (1991),
p154).

One of the reasons why such a Ni-based alloy is used is that even if the welding material contains more Mo than the base metal, the Fe-based alloy causes a large segregation of Mo in the solidified weld metal. , It is difficult to obtain the effect of Mo, but the Ni-based alloy has a small solidification segregation of Mo.

Most of the conventional tests for deterioration of corrosion resistance due to the non-uniformity of the structure of the weld metal, including the case of using Alloy 625 described above, are mostly conducted in a severe corrosion environment (low pH, high pH) in a FeCl ₃ solution. Cl ^-) of the work was performed in. Such harsh environments, the occurrence of microbial corrosion is mixed in seawater or seawater neutral in question freshwater or freshwater (Number ppmCl ^-) essentially corrosion mechanism in relatively mild environments such as Is different.

4) Unnecessarily high alloying of the welding material on the premise of a harsh environment is not preferable from the economical point of view, and there is an increasing demand for a versatile and appropriate welding material excellent in microbial corrosion resistance. There is.

[0013]

The present invention includes an inexpensive weld metal having excellent microbial corrosion resistance in relatively mild natural environment water in which microbial corrosion such as sea water, lake water or river water is a problem. An object of the present invention is to provide a welded structure and a method for manufacturing the welded structure.

[0014]

The mechanism of occurrence of microbial corrosion of stainless steel in a natural environment can be summarized as follows.

1) When stainless steel is immersed in natural environment water in which microorganisms are present, a film of the attached microorganisms, that is, a so-called biological film is rapidly formed on the surface thereof. It was revealed that the presence of this biofilm caused the corrosion potential of stainless steel to shift to the noble side, and that the cathodic reaction (oxygen reduction reaction) was significantly promoted on the surface (Amatani et al .: The Japan Institute of Metals). Bulletin, 35 (1996), 231). This is because active oxygen species such as hydrogen peroxide are produced as an intermediate product of the metabolic reaction of microorganisms, and these active oxygen species promote the oxidative action.

2) With such a noble corrosion potential, in an environment in which chlorine ions are present, a part of the passive film on the stainless steel surface is destroyed by chlorine ions to cause local corrosion such as pitting corrosion. To occur. Thus, even in an environment that is considered to be relatively mild as long as it is judged only from the temperature or the chloride ion concentration, the presence of microorganisms causes severe corrosion.

3) In the case of actual pipes and structures, microbial corrosion based on this mechanism occurs intensively in the welded part, especially in the weld metal. The reason is that in weld metal,
It is conceivable that a stagnation portion of the solution is likely to occur due to the shape, and thus microorganisms are likely to adhere. That is,
In the weld metal, the oxidative action of the active oxygen species is promoted. On the other hand, although it is conceivable to grind the weld bead portion smoothly, it is not a practical method because it is difficult on the inner surface of the pipe or the like and it takes time and labor.

4) Further, in the weld metal, (a) the oxide scale generated during welding is present on the surface, and (b) the composition is in the as-solidified structure and therefore has non-uniform components corresponding to the microstructure. As a result, the susceptibility to microbial corrosion is higher than that of the base metal. That is, when the corrosion potential becomes noble due to the oxidizing environment caused by the action of microorganisms, the stability of the passivation film of the weld metal is lower than that of the passivation film of the base metal. The selective destruction of the passive film causes corrosion. From a microstructural point of view, the fact that microbial corrosion in the weld metal results in selective dissolution of the austenite phase is due to the nonuniform composition of the δ ferrite phase and the austenite phase in the solidified structure.

The present inventor conducted tests and studies on the influence of the alloy composition of the weld metal and the microstructure on the corrosion of the stainless steel weld metal caused by such microorganisms. We have found a welded structure with excellent properties, in particular weld metal, and a method for producing it, especially welding materials.

The gist of the present invention is a welded structure including a weld metal in which an alloying element such as Cr is increased by a certain amount as compared with the base material stainless steel, and a method for manufacturing the welded structure.

[0021] (1) Ri Do from the weld metal of stainless steel base material and stainless steel, used in an environment in contact with the natural environment water
A welded structure to use, Cr of the weld metal, Mo, N
And Ni contents are Cr, Mo, N and N of the base material.
A welded structure which is a weld metal satisfying the following formulas, formulas and formulas with respect to the content of i, and having a microstructure δ ferrite volume ratio of 7% or less and the balance substantially consisting of austenite ([Invention 1 ] And)).

Here,% of the alloying element is defined as% by weight, and the contents of Cr, Mo, N and Ni of the base material are Cr ⁰ , Mo ⁰ ,
With N ⁰ and Ni ⁰ , the contents of Cr, Mo, N and Ni in the weld metal are Cr ^J , Mo ^J , N ^J and Ni ^J.
As Cr ^J (%) − Cr ⁰ (%) ≧ 2.35
・ ・ ・ ・ ・ ・ ・ ・ ・ Mo ^J (%)-Mo ⁰ (%) ≧ 0.27
・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ {Cr ^J (%) + 1.1Mo
^J (%) + 12N ^J (%) + 0.1Ni ^J (%)}-{Cr ⁰ (%)
+ 1.1Mo ⁰ (%) + 12N ⁰ (%) + 0.1Ni ⁰ (%)}
≧ 2.85 ········· (2) stainless steel Ri Do from the weld metal of the weld material and the stainless steel, the natural environment
In manufacturing a welded structure used in an environment in contact with water, as a material to be welded, in% by weight, C: 0.08% or less,
Si: 0.1-1.5%, Mn: 0.1-2.5%, N
i: 3.5 to 45%, Cr: 15 to 30%, Mo: 0 to
7%, Cu: 0 to 3%, N: 0.002 to 0.3%, N
b: 0-0.5%, Ti: 0-0.5% and Al: 0
For stainless steel containing up to 0.2%, Cr, Mo, N and Ni are used as welding materials in the following formula:
Formula and formula are satisfied, and Cr, Mo, N, N
The method for producing a welded structure according to (1) above, which comprises welding using stainless steel in which i, Si, C and Mn also satisfy the formula (referred to as [invention 2]).

Here, the contents of Cr, Mo, N and Ni of the material to be welded are Cr 2 ^{O 3} , Mo 2 ^{O 3} , N 2 ^O and Ni 2 ^O, and the contents of the welding materials are Cr, Mo, N, Ni, Si, C and Mn. the content of the ^{Cr W, Mo W, N W} , Ni W, Si W, C W and ^{Mn W, Cr W (%)} - Cr O (%) ≧ 2.5 ·········・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ Mo ^W (%)-Mo ^O (%) ≧ 0.3 ^{{Cr W (%) + 1.1Mo} W (%) + 12N W (%) + 0.1Ni W (%)} - {Cr O (%) + 1.1Mo O (%) + 12N O (%) + 0.1Ni O ( %)} ≧ 3 ... 1.34 Cr ^W (eq) −Ni ^W (eq) ≦ 14 ..... However, Cr ^W (eq) = Cr ^W (%) + Mo ^W ( %) + 1.5
Si ^W (%) Ni ^W (eq) = Ni ^W (%) + 30 C ^W (%) + 30 N ^W (%) +
0.5 Mn ^W (%) The "weld metal" in [Invention 1] is not only the weld metal welded by GTAW and GMAW, but also the shielded metal arc welding (using a welding rod coated with flux ( So-called hand welding, SMA
W), a SAW using a flux, and a weld metal manufactured by a welding method using a flux-cored composite wire. Further, the "welded structure" includes all welded steel members, such as welded steel pipes, seamless steel pipes welded as pipes or welded steel plates or steel rods.

The "welding material" in [Invention 2] is SM.
Welding rod core steel rod used for AW, GTAW and gas.
Solid wire or welded steel rod for metal arc welding (GMAW) and submerged arc welding (SAW). In the case of a flux-cored composite wire, it refers to the remaining steel portion excluding the flux.

In the case of SMAW or SAW, the flux is always used, but if the steel portion other than the flux corresponds to [Invention 2], the method of the present invention is used.

The weight% of the alloying elements of the welding material steel is SM
In the case of AW, it is the weight% in the steel of the core of the welding rod, and in the case of other welding methods, it is the weight% in the solid wire or steel portion, and the weight% of the alloying element of the base steel is the weight in the base metal. %
Further, the weight% of the alloying elements of the steel of the weld metal represents the weight% in the weld metal. Since the three are stainless steels of similar composition, the fact that, for example, Ni has the same weight% in each of the three, means that when the three having the same weight are taken, each of them contains almost the same amount of Ni. You can see it.

[0027]

DETAILED DESCRIPTION OF THE INVENTION

1. Material to be welded (base material) The base material in the present invention, that is, the material to be welded is, of stainless steels, austenitic stainless steel, for example, SUS.
304, SUS 304L, SUS 304N1 and SUS 316, SUS316L, SU
The target is S316N.

The alloying elements contained in these base material stainless steels are in the range generally known as the range of stainless steels, for example, those in the following ranges.

First, Cr, Mo, N and Ni, that is, Cr 2 ^{O 3} , Mo 2 ^{O 3} , N 2 ^O and Ni 2 ^O, are Cr 2 ^{O 3} : 15-3.
0%, Mo 2 ^O : 0 to 7%, N 2 ^O : 0.002 to 0.3%
And Ni 2 ^{O 3} may be in the range of 3.5 to 45%.

Next, the other elements are C ⁰ : 0.08
% Or less, Si ⁰ : 0.1 to 1.5%, Mn ⁰ : 0.1
2.5 %, P ⁰ : 0.03% or less, S ⁰ : 0.03% or less, Cu ⁰ : 0 to 3%, Nb ⁰ : 0 to 0.5%, T
i ⁰ : 0 to 0.5%, Al ⁰ : ^{0 to} 0.2%.

The reason why the base material in [Invention 2] is stainless steel containing these alloying elements in the above range is as follows.

Cr ^O : If Cr is less than 15%, the corrosion resistance of the base material itself is not sufficient, and if it exceeds 30%, the workability deteriorates, so the content is made 15 to 30%.

Mo ^O : Mo may not be added to the base material stainless steel. However, since Mo coexists with Cr to improve the corrosion resistance of the base material, Mo is added when exposed to a more severe environment or when dealing with the environment of microorganisms having a strong degree of corrosion promotion. However, if it exceeds 7%, the surface quality deteriorates, so even if it is included, it should be 7% or less. 0.5-for more beautiful surface and corrosion resistance
It is desirable to set it to 4.5%.

N ^O : N may not be intentionally added. The lower limit of N contained in steel when not intentionally added is about 0.002%. However, naturally adding more than the amount contained in steel increases the yield strength, so that C can be lowered and it is effective in improving corrosion resistance. Further, N itself also has the effect of enhancing pitting corrosion resistance, so N is added when it is used in a more severe environment. However, if it exceeds 0.3%, coarse nitrides that cause surface defects are deposited and surface defects occur frequently, so even if it is contained, it is made 0.3% or less. 0.0 for good corrosion resistance and surface quality
It is desirable to be 2 to 0.25%.

Ni 2 ^{O 3} : Ni is necessary to bring the structure into the austenite phase. When Ni is less than 3.5%, austenitization is not sufficient, and when it exceeds 45%, an oxide scale having extremely strong adhesive force is generated to cause indentation flaws and pickling time becomes long. 5-45%
And

C ^O : C tends to form Cr carbide in the HAZ, and in that case, corrosion resistance is deteriorated, so it is desirable to reduce it. However, lowering C raises the steelmaking cost, so the content is made 0.08% or less, which is the allowable range. Further, in order to further improve the corrosion resistance, it is desirable to set it to 0.06% or less.

Si 2 ^{O 5} : Si is necessary for deoxidation of steel,
It also increases corrosion resistance. If it is less than 0.1%, these effects cannot be sufficiently expected, while if it exceeds 1.5%, the workability deteriorates, so the content is made 0.1 to 1.5%.

Mn 2 ^{O 3} : Mn has an effect of deoxidizing steel and plays an auxiliary role of Ni as an austenite former, but if it is less than 0.1%, it is insufficient. On the other hand, 2.
If it exceeds 5%, the corrosion resistance will deteriorate, so 0.1-2.5%
And

Cu ^O : Cu does not have to be added. Since the effect of increasing yield strength and corrosion resistance can be expected if added, it is added in the case of increasing yield strength. But 3
%, The workability deteriorates, so even if it is contained, it should be 3% or less. In order to achieve both good workability and high yield strength, 0.5 to 2.5% is desirable.

Nb 2 ^{O 3} : Nb may not be added. Nb
Fixes C as Nb carbide in steel and can keep corrosion resistance stable and high. Therefore, it is added when it is used at a site that may be exposed to high temperature temporarily or periodically. However, if it exceeds 0.5%, the surface quality deteriorates. Therefore, even if it is contained, the content should be 0.5% or less. To secure both good surface properties and stable corrosion resistance, 0.1 to 0.
It is desirable to set it to 35%.

Ti ^O : 0 to 0.5% Ti may not be added. Ti, like Nb, forms stable carbides, is effective in improving the corrosion resistance, and does not deteriorate the surface properties. Therefore, Ti is added to improve these performances. However, if it exceeds 0.5%, the workability and toughness are deteriorated, so even if it is contained, it is 0.
5% or less. In order to secure good corrosion resistance, workability and toughness, it is desirable that the content be 0.05 to 0.35%.

Al 2 ^O : 0 to 0.2% Al may not be added. Al is an element having a strong deoxidizing power, and is used for continuous casting to prevent the generation of internal defects. Even in that case, the oxides bound to oxygen are floated up and eliminated as much as possible, but Al remaining after the action of deoxidation may remain. However, if the content of Al exceeds 0.2%, coarse oxides are mixed in during casting. Therefore, even if Al is deoxidized and Al is left, the content of Al in steel is 0.2
% Or less. The deoxidation without adding Al is S
i, Ti, etc.

Of the unavoidable impurity elements, P is preferably 0.03% or less in order to prevent deterioration of corrosion resistance, and S is preferably 0.03% or less in order to secure workability.

Further, it is preferable that the stainless steel containing these alloy elements is subjected to solution treatment so that Cr carbides and the like are not precipitated at grain boundaries.

2. Weld Metal Next, the chemical composition of the weld metal in [Invention 1] will be described. This weld metal is welded by the welding method (SMAW, GT
AW, GMAW, SAW, etc.) is applied to all welded welded metal. The range of the content rate of the following alloying elements in the weld metal can be realized by using only the welding material described below, or by using the welding material and the flux in combination, and It can also be realized by combining flux with out of range.

1) Cr ^J (%) − Cr ^O (%) ≧ 2.35 ..... Cr is indispensable for forming a passive film. It is an element. If the difference between the Cr of the weld metal and the Cr of the base metal is less than 2.35%, it is impossible to give a passivation film having microbial corrosion resistance equivalent to that of the base metal in the weld metal having a solidified structure. The value on the left side (hereinafter, “on the left side”) is set to 2.35% or more.

2) Mo ^J (%) − Mo 2 ^O (%) ≧ 0.27 ... Mo is an element effective for improving pitting corrosion resistance. is there. This is Mo
This is because the self-repairing function of the passive film is increased and the film stability is improved. If the difference between the concentration of the weld metal of Mo and that of the base metal is less than 0.27% by weight, the passive film in the weld metal does not have the same corrosion resistance as the base metal, so the value of the formula is 0.27%. That is all.

3) {Cr ^J (%) + 1.1Mo ^J (%) + 12N ^J (%) + 0.1Ni ^J (%)}-{Cr ^O (%) + 1.1Mo ^O (%) + 12N ^O (%) + 0.1Ni 2 ^{O 3} (%)} ≧ 2.85 ... This formula represents the condition for maintaining the corrosion resistance of the weld metal in consideration of deterioration of the corrosion resistance performance due to solidification segregation. {Cr ^J (%) + 1.1Mo in weld metal
^J (%) + 12N ^J (%) + 0.1Ni ^J (%)} is {Cr 2 ^O (%) + 1.1Mo 2 ^O (%) + 12N 2 ^O (%) + 0.1Ni in the base metal.
^If it is less than 2.85% than ^O (%)}, the welded pipe or structure cannot secure sufficient microbial corrosion resistance at the welded portion, so the value of the formula is 2.85%.
That is all. Cr among the individual alloy elements included in the formula
The reason why Mo and Mo are contained in the weld metal more than in the base metal is as described above. The details of Ni and N are as follows.

4) Ni ^J : N of the weld metal compared to the base metal
The reason for increasing i is that M in the solidification structure of the weld metal
This is because the solidification segregation of o into the δ ferrite phase is reduced, and because Ni is a so-called austenite former that stabilizes the austenite phase, it is because the precipitation of the δ ferrite phase in the weld metal is reduced. Although the content of Ni alone in the weld metal is not particularly limited, it is preferably 1.7% or more in order to sufficiently obtain such effects, and the upper limit is 46 in order to prevent deterioration of the molten metal flow. % Is preferable.

5) N ^J : N is effective for improving pitting corrosion resistance and, like Ni, is an austenite former, and therefore has the effect of reducing the precipitation of δ ferrite in the solidification structure of the weld metal. There is. Therefore, in the above equation, the content of weld metal is made higher than that of the base metal together with Cr, Mo and Ni. It is not necessary to take out only N in the welding material to specifically limit the content, but in order to sufficiently obtain these effects, the content of N in the weld metal is preferably 0.007% or more. Further, if N is excessive, blow holes frequently occur and the strength decreases, so
It is preferably 0.35% or less.

6) Other elements of weld metal: The alloying elements in the weld metal are the above-mentioned Cr, Mo, N, Ni, Si and C.
Elements other than Mn, that is, Nb, Ti, and the like are equivalent to those of the base material. Also, Al and oxygen are in the form of oxides,
It may be contained in a larger amount than the base material. Inevitable P
And S are equivalent to the base material.

7) Microstructure δ ferrite has a volume ratio of 7% or less. If it exceeds 7%, segregation of elements such as Cr occurs during solidification as described above, and these elements in austenite are deficient, resulting in selective corrosion of austenite. It is easy to realize the volume ratio of this δ-ferrite as-welded under normal welding heat input conditions described later.

In the weld metal, most of the austenite phase, δ ferrite having a volume ratio of 7% or less, and precipitates such as carbides and nitrides may be contained. The “substantial” refers to a structure (austenite and δ ferrite) containing precipitates such as carbides and nitrides. Carbides and the like may precipitate during cooling after welding, but if coarse Cr carbides and the like do not precipitate along grain boundaries, the degree of deterioration of corrosion resistance is small in the mild environment targeted by the present invention. . However, it is preferable that the volume ratio of the carbide is small. Therefore, post heat treatment or the like for increasing the volume fraction of carbide is not preferable.

3. Welding Material The welding material described below refers to the solid wire, the welded steel rod or the core steel rod of the SMAW welded rod in [Invention 2]. Of the alloying elements of these welding materials, Cr, M
The reasons for limiting the ranges of o, N, Ni, Si, C, and Mn are shown below.

1) Cr ^W (%) − Cr ^O (%) ≧ 2.5 ... Cr is essential for forming a passive film. Element, and the difference between the Cr of the welding material and the Cr of the base metal is 2.5% or more by weight%, the weld metal, which is a solidified structure, has the same microbial corrosion resistance as the base metal. It is possible to give a passivated film which has been provided. When such a welding wire is used when performing GTAW generally used for welding stainless steel, the difference in Cr between the weld metal and the base metal is usually 2.35.
% Or more.

2) Mo ^W (%) − Mo 2 ^O (%) ≧ 0.3 ... Mo is an element effective for improving pitting corrosion resistance. is there. This is Mo
This is because the self-repairing function of the passive film is increased and the film stability is improved. As for Cr, as in the case of Cr, by setting the difference between the concentration in the welding material and that in the base metal to 0.3% or more by weight, the weld metal also has corrosion resistance equivalent to that of the base metal. It is possible to give a passivated film. When GTAW welding is performed using such a welding material, the difference in concentration between the weld metal and the base metal is 0.27% or more.

[0057] ^{3) {Cr W (%)} + 1.1Mo W (%) + 12N W (%) + 0.1Ni W (%)} - {Cr O (%) + 1.1Mo O (%) + 12N O (%) + 0.1Ni 2 ^O 3 (%)} ≧ 3 This equation represents the condition for maintaining the corrosion resistance by compensating the deterioration of the corrosion resistance due to solidification segregation in the weld metal. . {Cr ^W (%) ＋ 1.1Mo in welding material
^W (%) + 12N ^W (%) + 0.1Ni ^W (%)} is {Cr 2 ^O (%) + 1.1Mo 2 ^O (%) + 12N 2 ^O (%) + 0.1Ni in the base metal.
^If it is less than 3% higher than ^O (%)}, the welded pipe or structure cannot secure sufficient microbial corrosion resistance in the welded part. When welding is performed by GTAW, for example, using a welding material having a value of 3% or more, the value of the expression of the weld metal is 2.85% or more and the weld metal is higher than the base metal. A higher value than the base metal of the weld metal improves the corrosion resistance of the weld metal and makes it equivalent to the base metal.

Among the individual alloy elements included in the formula, Cr
As for Mo and Mo, the reason why the amount of these elements of the weld metal is made larger than that of the base metal is as described above. The contents of Ni and N alone are as follows.

4) Ni ^W : N of the welding material compared to the base metal
The reason for increasing i is that M in the solidification structure of the weld metal
This is because the solidification segregation of o into the δ ferrite phase is reduced, and because Ni is a so-called austenite former that stabilizes the austenite phase, it is because the precipitation of the δ ferrite phase in the weld metal is reduced. The content of Ni in the welding material is not particularly limited, but Ni ^W is preferably 2% or more in order to sufficiently obtain such effects, and the upper limit is to prevent deterioration of the molten metal flow. Four
It is desirable to set it to about 7%.

5) N ^W : N is effective for improving the pitting corrosion resistance and, like Ni, is an austenite former and therefore reduces the precipitation of δ ferrite phase in the solidification structure of the weld metal. effective. Therefore, in the above formula, the weight% in the welding material is made higher than that of the base metal together with Cr, Mo and Ni. It is not necessary to extract the content of N in the welding material and limit it in particular, but in order to sufficiently obtain these effects, the weight% of N in the welding material is preferably 0.01% or more. . Further, if N is excessive, the hardness increases, the workability deteriorates, and it causes blowholes in the welded portion.
The following is preferable.

6) 1.34 Cr ^w (eq) −Ni ^w (eq) ≦ 14 .. However, Cr ^w (eq) = Cr ^w (%) + Mo ^w (%) + 1 .5
Si ^w (%) Ni ^w (eq) = Ni ^w (%) + 30 C ^w (%) + 30 N ^w (%) +
0.5 Mn ^w (%) The above formula is a condition that the welding material must satisfy to make the volume ratio of δ ferrite in the solidified structure of the weld metal 7% or less. If the δ ferrite in the weld metal exceeds 7%, elements effective for improving the corrosion resistance such as Mo and Cr are unevenly distributed in the δ ferrite, and the reduction of these elements in the austenite becomes large, so that the resistance to selective dissolution of austenite is increased. This will cause a significant decrease in microbial corrosiveness. If the value of the formula in the welding material exceeds 14%, the δ ferrite of the weld metal exceeds 7%, and Mo in austenite,
Since the decrease of Cr, etc. cannot be ignored, the value of the formula is 14
% Or less.

7) Other elements of welding material: Other elements of welding material, that is, elements such as Ti, Nb,
Although it is the same as the base metal, it is preferable that the Al content be slightly higher than that of the base metal, for example, 0.01 to 0.3% in consideration of the amount of deoxidation and oxidation during welding. In addition, the impurity elements P and S are equivalent to those of the base material.

4. As a welding method for assembling a welded structure in the welding method [Invention 1], a usual welding method such as SMAW, GTAW, GMAW, or SAW is used. However, if the welding heat input is excessively large, the cooling rate becomes slow, and carbides may be precipitated to impair the corrosion resistance. Therefore, the welding heat input is preferably 50 kJ / cm or less. When welding stainless steel,
Since the heat input is usually 10 kJ / cm or less, [Invention 2]
The weld metal of [Invention 1] can be realized without problems by using only the welding material of [1]. In the case of SAW, a flux is used, but by combining with the welding material (solid wire) of [Invention 2], and although the adjustment range is limited, the range of [Invention 2] is adjusted by adjusting the composition of the flux. The weld metal of [Invention 1] can be realized by using a solid wire having a composition slightly deviating from the above.

The same applies to the composite wire. The composition of the flux contained in the composite wire and the composition of the outer steel portion can be adjusted to realize the weld metal of [Invention 1]. This composite wire is mainly used as a welding material for GMAW or GTAW, but it may also be used as a welding wire for SAW or the like.

In SAW, when carbon steel or low alloy steel is welded, high heat input welding of more than 50 kJ / cm may be performed, but in welding stainless steel, it is usually 3
It is set to 0 kJ / cm or less.

5. The welded structure in the welded structure [Invention 1] refers to welded austenitic stainless steel and weld metal.
The welding method is SMAW, GTAW, GMAW, S
It may be welded by any welding method such as AW. Therefore, all welded stainless steels such as welded stainless steel pipes, water pumps, heat exchangers, machine parts, etc. are applicable.

[0067]

EXAMPLES Next, the effects of the present invention will be described with reference to examples.

In general, fresh water such as rivers and lakes, which is an environment where microbial corrosion is a problem, was adopted as a target of corrosive environment. Widely used as the stainless steel for the material to be welded
SUS 304 steel and SUS 316L steel were used.

Table 1 is a table showing the chemical composition of stainless steel (SUS 304 and SUS 316L) which is the material to be welded (base material).

[0070]

[Table 1]

Cr, Mo, N and N of these materials to be welded
With respect to i, C of the welding material so that the chemical composition satisfies the conditions (formula, formula, formula and formula) of [Invention 2].
2.4 mmφ GTAW by adjusting and melting r, Mo, N, Ni, Si, C and Mn, and forging and drawing
Welding material (solid wire) was prototyped. In addition, as part of the comparative example, G of commercially available SUS Y308 and SUS Y316L is used.
The welding material for TAW was used for the experiment.

Table 2 is a list showing the chemical composition of these welding materials. The commercially available welding materials (SUS Y308 and
SUS Y316L) is also included.

[0073]

[Table 2]

Table 3 shows the formulas of the alloying elements of the welding material, the formulas,
It is a list showing a formula and a value of a formula.

[0075]

[Table 3]

Table 4 shows the conditions of GTAW welding performed on the above base material using the above welding material. The heat input was 5 kJ / cm on average, and the conditions were used for ordinary GTAW.

[0077]

[Table 4]

A test piece was prepared from the test piece after welding to prepare a test piece for corrosion test.

FIG. 1 is a view showing a test piece for corrosion test including a welded portion.

The corrosion test was carried out by immersing the river water in a test tank (controlled at 30 ° C.) in which water was continuously pumped up by a pump. For the test piece whose base material is SUS 304 steel, the test was conducted by circulating the water collected in the relatively upstream region of the river. Further, a test piece having a base material of SUS 316L steel was subjected to an immersion test using water collected at a point about 2 km from the river mouth. After one year of immersion, the test piece was taken out and examined for the occurrence of pitting corrosion in the weld, especially in the weld metal.

Table 5 is a list of the results of the corrosion test together with the formula of the weld metal, the formula and the value of the formula, and the volume ratio of δ ferrite. The test code in the table is
It is the same as the code used for the welding material.

[0082]

[Table 5]

As is clear from these results, no microbial corrosion was observed in the weld metal in any of the test pieces of test codes A to H of the present invention. On the other hand, the value of the formula is low and therefore the value of the formula is low,
The test code I of the comparative example having a high δ ferrite volume ratio, or the test code J having a low Ni (eq) value due to the increase of Cr and Mo, or the test code J having a large amount of δ ferrite, or the test having a low value of the formula even if the δ ferrite is small. In all of the reference symbols K, microbial corrosion, which is selective corrosion of the austenite phase, occurred in the weld metal.

Comparative example using commercially available welding rod (SUS Y304
And SUS Y316L), similar selective corrosion of austenite phase occurred.

The effect of inhibiting microbial corrosion in the examples of the present invention is clear.

[0086]

EFFECTS OF THE INVENTION According to the present invention, in a welded structure of stainless steel, even if it is not an expensive high-alloy weld metal, a weld metal having an appropriately adjusted chemical composition is used. A welded structure excellent in corrosiveness can be obtained, and a manufacturing method for constructing the welded structure, particularly a welding material is also provided, which has a great effect on the related industry.

[Brief description of drawings]

FIG. 1 is a drawing showing a test piece for a microbial corrosion resistance test containing weld metal.

[Explanation of symbols]

1 ... Base material 2 ... Weld metal

Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00 302 C22C 38/44

Claims (2)

(57) [Claims] 1. A Ri Do from the weld metal of stainless steel base material and stainless steel, a <br/> welded structure to be used in environments in contact with the natural environment water, Cr of the weld metal, Mo, N And the content of Ni satisfy the following formulas, formulas and formulas with respect to the contents of Cr, Mo, N and Ni of the base material, and the volume fraction of δ ferrite in the microstructure is 7% or less Is a weld metal consisting essentially of austenite. Here,% of the alloy element is taken as% by weight, and the contents of Cr, Mo, N and Ni of the base material are C
r ⁰ , Mo ⁰ , N ⁰ and Ni ⁰ , and the weld metal C
The contents of r, Mo, N, and Ni are calculated as Cr ^J , Mo ^J ,
As N ^J and Ni ^J , Cr ^J (%) − Cr ⁰ (%) ≧ 2.35
・ ・ ・ ・ ・ ・ ・ ・ ・ Mo ^J (%)-Mo ⁰ (%) ≧ 0.27
・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ {Cr ^J (%) + 1.1Mo
^J (%) + 12N ^J (%) + 0.1Ni ^J (%)}-{Cr ⁰ (%)
+ 1.1Mo ⁰ (%) + 12N ⁰ (%) + 0.1Ni ⁰ (%)}
≧ 2.85 ... Wherein Ri Do from the weld metal of the weld material and the stainless steel stainless steel, be used in an environment in contact with the natural environment water
In the production of welded structures that, as a material to be welded, by weight%
And C: 0.08% or less, Si: 0.1 to 1.5%, M
n: 0.1 to 2.5%, Ni: 3.5 to 45%, Cr:
15-30%, Mo: 0-7%, Cu: 0-3%, N:
0.002-0.3%, Nb: 0-0.5%, Ti: 0
.About.0.5% and Al: 0 to 0.2% or less of stainless steel containing Cr, Mo, N as a welding material.
And Ni satisfy the following formulas, formulas and formulas, and Cr, Mo, N, Ni, Si, C, and Mn also satisfy the formulas, welding is performed using a stainless steel. 1. The method for manufacturing a welded structure according to 1. Here, the contents of Cr, Mo, N and Ni of the material to be welded are Cr ⁰ , Mo ⁰ , N ⁰ and Ni ^0, and C of the welding material is
The content ratio of r, Mo, N, Ni, Si, C and Mn is C
r ^W , Mo ^W , N ^W , Ni ^W , Si ^W , C ^W and Mn
^{As W} , Cr ^W (%) − Cr ⁰ (%) ≧ 2.5 ...
・ ・ ・ ・ ・ ・ ・ ・ ・ Mo ^W (%)-Mo ⁰ (%) ≧ 0.3
・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ {Cr ^W (%) + 1.1Mo
^W (%) + 12N ^W (%) + 0.1Ni ^W (%)}-{Cr ⁰ (%)
+ 1.1Mo ⁰ (%) + 12N ⁰ (%) + 0.1Ni ⁰ (%)}
≧ 3 ··· 1.34Cr ^W (eq) −Ni ^W (eq) ≦
14 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ However, Cr
^W (eq) = Cr ^W (%) + Mo ^W (%) + 1.5Si ^W (%) Ni ^W (eq) = Ni ^W (%) + 30C ^W (%) + 30N
^W (%) + 0.5 Mn ^W (%)