JP3022746B2 - Welding material for high corrosion resistance and high toughness duplex stainless steel welding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding material for welding high corrosion resistant and high toughness duplex stainless steel used in oil country tubular goods, line pipes, and chemical industry plants.

[0002]

2. Description of the Related Art At present, austenitic stainless steels (SUS304, SUS316, etc.) and ferritic stainless steels (SUS430, etc.) are widely used as characteristic materials. In recent years, it has both of these features,
Ferrite-austenite duplex stainless steels (SUS329 series) that complement each other's weaknesses are attracting attention, and the fields of practical application are expanding.

[0003] Duplex stainless steel has the following features.

[0004] Stress corrosion cracking compared to austenitic steels
(SCC) Excellent resistance.

[0005] The toughness is superior to that of ferrite.

[0006] Since it is made of a fine two-phase mixed structure, it has high strength.

[0007] Despite being inexpensive due to low content of Ni, excellent pitting corrosion resistance can be obtained because it contains a large amount of Cr and Mo.

[0008] However, the duplex stainless steel has a drawback that hot workability and weldability are inferior to austenitic stainless steel and ferritic stainless steel. As a welding material for welding duplex stainless steel, a common metal material having the same component as the base metal is generally used. However, in the case of the duplex stainless steel co-metal material, there was a phenomenon that a hot crack was generated when welding was performed with a large heat input. To improve this, for example, JP-A-59-218295 discloses that the nickel balance (Ni-bal.) Is reduced to -10 to -6% by weight in order to reduce the ferrite content of the weld metal to 10 to 20% by volume. Tailored welding materials have been proposed. Japanese Patent Application Laid-Open No. 58-93593 proposes a welding material having a nickel balance (Ni-bal.) Of -20 to -10 from the viewpoints of mechanical properties and pitting corrosion resistance of the weld metal.

In recent years, the problem of depletion of petroleum resources has been raised, and a new duplex stainless steel, a so-called super duplex stainless steel (JIS 329J4L), has been developed as a material suitable for deepening wells and deteriorating the environment of oil wells. (See JP-A-62-55656 and JP-A-5-132741).

As a welding material for welding these super duplex stainless steels, a material having the same components as the base metal is used. Further, when the welding material proposed in the above-mentioned JP-A-59-218295 is used for welding super duplex stainless steel, both the pitting corrosion resistance and the toughness of the weld metal cannot be improved.

In order to make the pitting corrosion resistance of the weld metal portion equal to that of the weld heat affected zone of the base metal, the Cr + 3.3
There is a proposal to make an index calculated by Mo + 16N (a pitting corrosion resistance index PRE not considering W) be 42% by weight or more. However, when the content of Cr and Mo in the weld metal portion is increased, precipitation of the intermetallic compound (σ phase) is increased, and when the content of N is increased, precipitation of nitride is remarkable, and both cause reduction in toughness. Therefore, it has been extremely difficult to simultaneously satisfy the pitting resistance and the toughness.

[0012]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a weld metal having the same pitting corrosion resistance and toughness as those of the base metal,
An object of the present invention is to provide a welding material for welding super duplex stainless steel (for example, JIS 329J4L) excellent in hot workability to a wire.

[0013]

Means for Solving the Problems The present inventor has conducted research on the pitting corrosion resistance and toughness of a welded metal part, and in order to increase the pitting corrosion resistance without reducing the toughness, it is necessary to use Cr as a welding material.
Mo content is limited, W (tungsten) is contained,
It has been found that it is better to use a welding material having a pitting resistance index PREW of 42.0 or more and a ferrite capacity index Ph of 0.25 to 0.35%.

The present invention has been completed based on this finding, and the gist of the invention lies in the following welding materials.

C: 0.03% or less by weight, Si: 1.0% or less, Mn: 1.5% or less, P: 0.04% or less, S: 0.003% or less,
Sol.Al: 0.040% or less, Ni: 2.0-8.0%, Cr: 24.0-26.0
%, Co: 0 to 6.0% or less, Mo: 2.0 to 3.3%, W: 1.5 to 5.0
%, N: 0.24-0.35% and O: 0.007% or less,
The balance consists of Fe and unavoidable impurities, and the following (1)
A welding material for high corrosion resistance and high toughness duplex stainless steel welding having a pitting corrosion index PREW of at least 42.0 shown in the equation and a ferrite capacity index Ph of 0.25 to 0.35 shown in equation (2).

PREW = Cr + 3.3 (Mo + 0.5W) + 16N (1) Ph = {Ni + Co + 30 (C + N) -0.6 (Cr + 1.5Si + Mo + 0.4W) +5.6} {Cr + 1.5Si + Mo + 0.4W- 6｝ ··················································································· (2) where the symbol of the element indicates the content (% by weight) of the element.

The above-mentioned Co: 0 to 6.0% is added for the same purpose as the addition of Ni, and is added as necessary.

When Ni is more than 8.0% and not more than 10.0%,
In addition to the above components except Ni, further Ca: 0.001 to 0.01
%, B: contains at least one of 0.001 to 0.01%, and
A welding material for high corrosion resistance and high toughness duplex stainless steel welding having a pitting corrosion index PREW of 42.0 or more as shown in equation (1) and a ferrite capacity index Ph of 0.25 to 0.35 as shown in equation (2).

PREW = Cr + 3.3 (Mo + 0.5W) + 16N (1) Ph = {Ni + Co + 30 (C + N) -0.6 (Cr + 1.5Si + Mo + 0.4W) +5.6} {Cr + 1.5Si + Mo + 0.4W- 6｝ ··················································································· (2) where the symbol of the element indicates the content (% by weight) of the element.

In addition to the above or components,
The welding material for high corrosion resistance and high toughness duplex stainless steel welding according to or containing at least one of u: 0.2 to 2.0% and V: 0.05 to 1.50%.

[0021]

According to the present invention, in addition to the chemical composition of the welding material, the pitting resistance index PREW is used to enhance the pitting corrosion resistance and toughness of the weld metal.
And the ferrite capacity index Ph.

Hereinafter, the effects of the respective components of the welding material of the present invention and the reasons for limiting the content will be described. All the percentages of the component contents mean% by weight.

C: C is an element that stabilizes the austenite phase, like N described later. However, when the content exceeds 0.03%, carbides are liable to precipitate, and the pitting corrosion resistance of the weld metal part is reduced. Therefore, C content is 0.03%
It was as follows.

Si: Si is used as a deoxidizing element when melting a welding material. However, during welding, the intermetallic compound (σ
Phase) and reduce the toughness of the weld metal. Therefore, its content is set to 1.0% or less. Preferred is 0.5% or less.

Mn: Mn is used as a deoxidizing element at the time of melting a welding material, and combines with S to improve hot workability.
It is an element that further increases the solubility of N. But Mn
If the content exceeds 1.5%, the pitting corrosion resistance of the weld metal part is reduced. Therefore, its content is set to 1.5% or less.

P: P is inevitable during melting of the welding material
It is an impurity element mixed at 0.025% or more, and the smaller the better, the better. If its content exceeds 0.040%, the pitting corrosion resistance and the toughness of the weld metal are significantly reduced. Therefore,
Its content is set to 0.040% or less.

S: Like P, S is an element inevitably mixed during the melting of the welding material. S
Reduces the hot workability of steel, making it difficult to roll the wire, and also forms sulfides, which are the starting points of pitting corrosion and impair the pitting corrosion resistance of the weld metal. Therefore, its content is set to 0.003% or less. It is preferably at most 0.001%.

Sol. Al: Al is added in an amount required for deoxidation during melting of the welding material. However, when combined with N, AlN
(Aluminum nitride) is precipitated and the Sol.Al content is 0.0
If it exceeds 40%, the toughness and pitting corrosion resistance of the weld metal are deteriorated. Therefore, the content of Sol. Al is set to 0.040% or less. In addition, in the welding material of the present invention, it is preferable to employ vacuum melting or the like because it is desired to reduce oxygen.

Ni: Ni stabilizes the austenite phase,
Adjust the ferrite capacity of the weld metal to improve pitting resistance and toughness. However, if the content is 2.0% or less, these effects cannot be obtained. In addition, the ferrite phase of the weld metal has a smaller concentration of Cr, Mo, and W than the base metal,
The pitting resistance decreases. Therefore, in order to improve the pitting corrosion resistance of the weld metal part, it is necessary to reduce the amount of ferrite.
As described above, it is necessary to determine the lower limit of Ni. However, when the Ni content exceeds 10.0%, precipitation of the σ phase is promoted during welding, and the toughness is reduced. In addition, Ni content is 8.0%
If it exceeds 300, the hot workability will decrease. Therefore, when Ca and B are not added, Ni is limited to 8.0%, and when it exceeds 8.0, Ca or B is added.

Co: Co, like Ni, stabilizes the austenite phase, adjusts the ferrite capacity of the weld metal, and improves pitting resistance and toughness. Since the degree of precipitation of the σ phase during welding is smaller than that of Ni, a part of Ni is replaced with Co if necessary. However, Co is more expensive than Ni, so the upper limit was set to 6.0%.

Cr: Cr is an element that enhances pitting resistance.
If the content is less than 24.0%, the pitting corrosion resistance of the weld metal cannot be as high as that of the heat affected zone of the base metal. However, Cr remarkably precipitates intermetallic compounds (such as the σ phase) and lowers the hot workability during wire processing. Further, a σ phase precipitates in a weld metal part at the time of welding and lowers toughness. Therefore,
Its content was 24.0-26.0%.

Mo: Mo, like Cr, is an element that improves the pitting resistance of the weld metal. In particular, in order to enhance pitting corrosion resistance and crevice corrosion resistance, the content is set to 2.0% or more.
On the other hand, if the Mo content exceeds 3.3%, the effect of facilitating the precipitation of the intermetallic compound is strong as in the case of Cr, which causes the material to be embrittled during the production and makes the wire processing difficult. Further, at the time of welding, precipitation of the σ phase is promoted in the weld metal portion, and the joint portion is embrittled. Therefore, its content was made 2.0-3.3%.

W: Like Mo, W is an element that improves the corrosion resistance of the weld metal, especially the resistance to pitting and crevice corrosion, and is a stable oxide that does not reduce the pitting corrosion resistance even in a low pH environment. To form Further, like Cr, Mo, and N, it is an element that improves pitting corrosion resistance.
Do not precipitate phases. That is, the pitting resistance index PREW can be increased by reducing the contents of Cr and Mo. But 1.5
%, The effect is small. Even if the content exceeds 5.0%, the effect is saturated. Therefore, its content is 1.5 ~
5.0%. Preferred is 2.0-5.0%.

N (nitrogen): N forms austenite and improves pitting resistance. When the ferrite-forming elements Cr and Mo are contained in a large amount as in the welding material of the present invention, the balance between the ferrite phase and the austenite phase in the weld metal portion is appropriately adjusted (Ph is set to 0.1 as a means for that purpose).
25 to 0.35) to reduce the N content to 0.
Must be at least 24%. Further, N is a pitting resistance index.
Contributes to PREW and improves pitting resistance, like Cr, Mo and W. However, when it exceeds 0.35%, Cr becomes 24.0-26.0%
In the contained duplex stainless steel, blowhole defects occur in the weld metal part. Moreover, the toughness and corrosion resistance of the weld metal part are degraded due to the formation of nitrides due to the heat effect during welding.

The contents of Cr, Mo, W and N must be adjusted so that the pitting resistance index PREW becomes 42.0 or more.

Ca (calcium), B (boron): Ca and B improve the hot workability of the duplex stainless steel.
In the present invention, the ferrite capacity is adjusted to 25 to 45% in order to secure the pitting corrosion resistance of the weld metal portion. For this reason, welding materials
If the Ni content exceeds 8.0%, the σ phase is precipitated during the rolling of the wire and the hot workability is reduced. In order to improve this, Ca or B is contained. However, if the contents of both Ca and B are less than 0.001%, the above effect cannot be obtained. If the content exceeds 0.01%, the pitting corrosion resistance of the weld metal portion is impaired. Therefore, the content of Ca or B is preferably set to 0.001 to 0.01%.

O (oxygen): O (oxygen) embrittles grain boundaries, impairs hot workability, and makes wire processing difficult. Therefore, the smaller the O content, the better. However, up to 0.007% obtained in a vacuum melting furnace is acceptable. Further, in the welding material of the present invention, since it is desired to reduce Al and Si, it is preferable to employ a vacuum melting method or the like in order to reduce the O content.

Cu, V: Cu and V improve the corrosion resistance of the weld metal of the duplex stainless steel, particularly the acid resistance to acids such as sulfuric acid.

Cu improves the corrosion resistance in a reducing pH environment such as H ₂ SO ₄ or hydrogen sulfide. But,
If the Cu content is less than 0.2%, the effect is not obtained, and if it exceeds 2.0%, the hot workability is reduced. Therefore, its content is
0.2 to 2.0%.

V, when combined with W, improves crevice corrosion resistance. However, if it is less than 0.05%, the effect is not obtained, and if it exceeds 1.50%, the amount of ferrite increases, and the toughness and corrosion resistance of the weld metal part decrease. Therefore, the content was set to 0.05 to 1.50%.

The welding material of the present invention has a pitting corrosion resistance index and a ferrite capacity index in order to enhance the pitting corrosion resistance and toughness of the weld metal part, in addition to the above-mentioned composition limits.

The pitting resistance index PREW is 42.0 or more: The pitting resistance index has been proposed as an index for judging the quality of the pitting corrosion resistance of the duplex stainless steel. In the present invention, the pitting resistance index PREW = Cr + 3.3 (Mo + 0.5W) + 16N (1) in which W is added as an element for improving pitting corrosion resistance is adopted. To 42.0
As described above, the pitting corrosion resistance was improved. Further, by increasing the amount of W, precipitation of the σ phase can be suppressed, and pitting corrosion resistance and toughness can be improved.

Ferrite capacity index Ph: 0.25 to 0.35: The ferrite phase and austenite phase of the duplex stainless steel have different concentrations of the constituent elements, and each element is distributed at a certain ratio (partition coefficient). However, the distribution coefficient of the weld metal is smaller than that of the base metal, and the amounts of Cr and Mo in the ferrite phase are lower than those of the base metal. In particular, elements (Cr, Mo) that increase the corrosion resistance of the weld metal in the ferrite phase
In order to increase the ratio, it is necessary to reduce the proportion of the ferrite phase. However, when the Ph value is less than 0.25, the effect is not obtained. When the Ph value exceeds 0.35, solidification segregation of Cr and Mo during welding becomes remarkable, and precipitation of the σ phase in the weld metal portion is promoted. Therefore, it is necessary to adjust the ferrite capacity index Ph to 0.25 to 0.35 in the welding material.

The ferrite capacity index Ph of the welding material is as follows: Ph = {Ni + Co + 30 (C + N) -0.6 (Cr + 1.5Si + Mo + 0.4W) +5.6} {Cr + 1.5Si + Mo + 0.4W-6} (2) Can be obtained by Here, the element symbols in the formulas (1) and (2) indicate the content (% by weight) of the element.

However, the ferrite capacity index is 0.25 to 0.35
In this case, the hot workability of the material is reduced, and it is difficult to process the wire as a welding material. In particular, when 8.0% or more of Ni is contained, it is preferable to reduce O (oxygen) and S (S) and to contain Ca (calcium) and B (boron).

[0046]

EXAMPLES The hot workability of the welding material of the present invention to a wire, the toughness of a weld metal part and the pitting resistance will be described below by way of examples.

A. Workability into wire: Duplex stainless steel (except No. 14) having the chemical composition shown in Table 1 was cast into a 500 kg ingot by vacuum melting. No.14 dissolved in air. Heat this to 1250 ° C, forging ratio 16 and diameter 50mm
Forged into billets. At this time, in order to evaluate the hot workability, the presence or absence of cracks in the cross section of the obtained steel slab was investigated by using the penetrant flaw detection method. Table 2 shows the results.

[0048]

[Table 1]

[0049]

[Table 2]

No cracks were detected in Nos. 1 to 7 of the invention examples and Nos. 8 to 12 of the comparative examples having the component compositions within the range specified in the present invention, and good forging was performed. However, the comparative example No. 13
No. 14 has a high O content of 0.0088%, because the Ni content is 9.13% and does not contain Ca or B.
Since the Mo content was as high as 4.12%, cracks were detected in each case.

Thus, when the Ni content exceeds 8.0%
It can be seen that hot workability is improved by adding Ca or B or by limiting the oxygen content to a small value.

A steel slab (N
o.1 to No.12, outer diameter 50mm), hot-rolled to an outer diameter 10mm, processed into a wire 2mm in diameter by repeated cold rolling and softening treatment at 1100 ° C to obtain welding material .

B. Weld metal toughness and pitting corrosion resistance: Fig. 1
FIG. 2 is a diagram showing a cross section of a joint obtained by welding a duplex stainless steel plate by TIG (tungsten inert gas). The base material of the welded joint is a super duplex stainless steel 5 of 25% Cr-7% Ni-3% Mo-0.3% N (SUS329J4L, thickness 15mm, width 100mm,
The wire having a diameter of 2 mm was used as a welding material. First, as shown in FIG. 1, a buttering welded portion 2 is formed on a sloped portion 3 of a groove of stainless steel 5 and a surface 4 of a backing material 6 by a TIG welding method in order to eliminate the influence of a base metal component on a welded metal portion. After forming, heat input 2
The welded joint 7 was produced by welding at 0 KJ / cm.

FIG. 2 is a diagram showing the positions where the Charpy impact test specimen (FIG. (A)) and the pitting corrosion test specimen (FIG. (B)) are collected from the joint. Charpy impact test piece 8 (JISZ2202-4)
The pitting test piece 9 was taken from the weld metal as shown in FIG. (B) so that the notch was located at the center of the weld metal as shown in FIG. (A). The Charpy impact test was performed at a temperature of −30 ° C., and the pitting corrosion test was performed by immersing in a 6% FeCl ₃ aqueous solution at 50 ° C. for 24 hours. In the pitting corrosion test, the occurrence of pitting corrosion was visually observed. Table 2 shows the results.

The welding materials Nos. 1 to 7 satisfying the component range specified in the present invention have a Charpy impact value of 108 (kJ / cm ² ) or more, and no pitting corrosion is observed in the pitting test, and the toughness and the toughness are improved. Both pitting resistance was excellent.

On the other hand, the welding materials 8, 11 and 12 of the comparative examples have ferrite capacity Ph values of 0.21, 0.10 and 0.2, respectively.
Since it was as low as 19, pitting corrosion was observed in the pitting test.

Since the welding material No. 9 had a large Ni content of 10.42%, the impact value was as low as 38 kJ / cm ² and the toughness was reduced.

The welding material No. 10 has a pitting resistance index PREW of 3
Since it was as low as 9.6, pitting corrosion was observed in the pitting corrosion test.

[0059]

According to the present invention, the welding material of the present invention has excellent hot workability to a wire rod by specifying a pitting resistance index PREW and a ferrite capacity index Ph in addition to chemical components, and has a toughness of a weld metal part. Increases pitting resistance. Therefore, when this welding material is used for welding super duplex stainless steel, the toughness and pitting corrosion resistance equivalent to the base metal can be imparted to the weld metal portion.

[Brief description of the drawings]

Fig. 1 Duplex stainless steel plate is TIG (tungsten
It is a figure which shows the cross section of the joint which carried out the (inert gas) welding.

FIG. 2 shows a Charpy impact test specimen from a welded joint (Fig. (A)
) And a position at which a pitting corrosion test piece (FIG. (B)) is collected.

[Explanation of symbols]

1. Weld metal part 2. Buttering weld
3. 3. Inclined slope 4. Backing material surface Duplex stainless steel plate
6. Backing material 7. 7. Welded joint Charpy impact test specimen
9. Pitting test specimen

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 B23K 9/23 B23K 35/30 320

Claims (4)

(57) [Claims] (1) C: 0.03% or less by weight, Si: 1.0% or less,
Mn: 1.5% or less, P: 0.04% or less, S: 0.003% or less, Sol.
Al: 0.040% or less, Ni: 2.0-8.0%, Cr: 24.0-26.0%,
Co: 0 to 6.0%, Mo: 2.0 to 3.3%, W: 1.5 to 5.0%, N:
Contains 0.24 to 0.35% and O: 0.007% or less, with the balance being Fe
For high-corrosion-resistant high-toughness duplex stainless steel welding consisting of unavoidable impurities and having a pitting resistance index PREW of 42.0 or more as shown in the following equation (1) and a ferrite capacity index Ph of 0.25 to 0.35 as shown in the following equation (2): Welding material. PREW = Cr + 3.3 (Mo + 0.5W) + 16N ... (1) Ph = {Ni + Co + 30 (C + N) -0.6 (Cr + 1.5Si + Mo + 0.4W) +5.6} {Cr + 1.5Si + Mo + 0.4W-6} (2) However, the symbol of an element in the equations (1) and (2) indicates the content (% by weight) of the element. 2. C: 0.03% or less by weight, Si: 1.0% or less,
Mn: 1.5% or less, P: 0.04% or less, S: 0.003% or less, Sol.
Al: 0.040% or less, Ni: more than 8.0% to 10.0% or less, Cr: 2
4.0 to 26.0%, Co: 0 to 6.0%, Mo: 2.0 to 3.3%, W: 1.5
-5.0%, N: 0.24-0.35% and O: 0.007% or less, Ca: 0.001-0.01%, B: 0.001-0.01%
Or more, and the balance consists of Fe and inevitable impurities, and the pitting resistance index PREW shown in the following equation (1) is 42.0 or more, and the ferrite capacity index Ph shown in the equation (2) is 0.25 to 0.35. Welding material for corrosion resistant high toughness duplex stainless steel welding. PREW = Cr + 3.3 (Mo + 0.5W) + 16N ... (1) Ph = {Ni + Co + 30 (C + N) -0.6 (Cr + 1.5Si + Mo + 0.4W) +5.6} {Cr + 1.5Si + Mo + 0.4W-6} (2) However, the symbol of an element in the equations (1) and (2) indicates the content (% by weight) of the element. (3) C: 0.03% or less by weight, Si: 1.0% or less,
Mn: 1.5% or less, P: 0.04% or less, S: 0.003% or less, Sol.
Al: 0.040% or less, Ni: 2.0-8.0%, Cr: 24.0-26.0%,
Co: 0 to 6.0%, Mo: 2.0 to 3.3%, W: 1.5 to 5.0%, N:
0.24 ~ 0.35% and O: 0.007% or less
u: 0.2 to 2.0%, V: 0.05 to 1.50%, at least one of which contains Fe and inevitable impurities, and
A welding material for high corrosion resistance and high toughness duplex stainless steel welding having a pitting corrosion index PREW of 42.0 or more as shown in equation (1) and a ferrite capacity index Ph of 0.25 to 0.35 as shown in equation (2). PREW = Cr + 3.3 (Mo + 0.5W) + 16N ... (1) Ph = {Ni + Co + 30 (C + N) -0.6 (Cr + 1.5Si + Mo + 0.4W) +5.6} {Cr + 1.5Si + Mo + 0.4W-6} (2) However, the symbol of an element in the equations (1) and (2) indicates the content (% by weight) of the element. (4) C: 0.03% or less by weight, Si: 1.0% or less,
Mn: 1.5% or less, P: 0.04% or less, S: 0.003% or less, Sol.
Al: 0.040% or less, Ni: more than 8.0% to 10.0% or less, Cr: 2
4.0 to 26.0%, Co: 0 to 6.0%, Mo: 2.0 to 3.3%, W: 1.5
~ 5.0%, N: 0.24 ~ 0.35% and O: 0.007% or less, Cu: 0.2 ~ 2.0%, V: 0.05 ~ 1.50%, Ca: 0.001 ~ 0.01%, B: 0.001% ~ 0.01% of 1
And the balance consists of Fe and unavoidable impurities, and the pitting resistance index PREW shown in the following formula (1) satisfies 42.0 or more, and the ferrite capacity index Ph shown in the formula (2) satisfies 0.25 to 0.35. Welding material of high corrosion resistance and toughness for duplex stainless steel consisting of composition. PREW = Cr + 3.3 (Mo + 0.5W) + 16N ... (1) Ph = {Ni + Co + 30 (C + N) -0.6 (Cr + 1.5Si + Mo + 0.4W) +5.6} {Cr + 1.5Si + Mo + 0.4W-6} (2) However, the symbol of an element in the equations (1) and (2) indicates the content (% by weight) of the element.