JP2002336990A - Welds and welding methods for ferritic stainless steel with excellent high temperature tensile strength, elongation, crack resistance and toughness - Google Patents

Abstract

(57) [abstract] (with revision) [PROBLEMS] To suppress the crystal structure of the weld metal of ferritic stainless steel from growing enormously, and to be caused by repeated cracks and stress (or) thermal stress during welding. Achieve a weld that prevents cracking. SOLUTION: As a welding wire, C: 0.
05% or less, Si: 2.0% or less, Mn: 2.0% or less, Cr: 15 to 25%, Al: 0.01 to 0.20
%, Ti: 0.01 to 0.30% and N: 0.04%
0.2%, O: 0.04% or less, P: 0.0
3% or less, S: 0.01% or less, the balance being an alloy composition substantially consisting of Fe, and Mo and W: 5
%, Nb: 1% or less, Zr: 0.1%, and the particle size of Ti or Al nitride by welding using Ar, O ₂ , CO ₂ and He as shielding gas Is 0.3 μm ² or more and 1.5 × 10 ⁴ pieces /
A weld is obtained that has a density of at least ² mm2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in welding technology for ferritic stainless steel, and more particularly, to a weld having excellent high-temperature tensile strength, elongation, crack resistance, and toughness.
Regarding the welding method that gives it.

[0002]

2. Description of the Related Art When welding a metal base material using a filler metal, it is common practice to match the alloy composition of the filler metal to that of the base metal as much as possible. The same type of steel wire is used for welding ferritic stainless steel,
It is well known that a welded portion obtained by welding has a very large crystal grain size, and therefore is liable to crack during welding. Even if welding cracks could be avoided. When stress (or thermal stress) is repeatedly applied, the weld is liable to crack and has low fatigue properties. Or, it has low toughness and is vulnerable to impact. To cope with this problem, it is necessary to make the crystal grains of the welded portion as fine as possible. The measures proposed so far include:

[0003] One of the methods is to use Ti-based nitride. There is a method in which Ti is added to a welding wire to thereby disperse TiN in a weld metal to make crystal grains finer (JP-A-5-138394). This welding wire is welded using a shielding gas such as 80% Ar-20% CO ₂ .

Another technique is to add an appropriate amount of Al and Mg to the welding wire in a complex manner and to achieve equiaxed crystallization and fine graining by coexistence of these elements (Japanese Patent Laid-Open No. 9-1997).
225680). The welding is performed by TIG, MIG, or the like.

[0005] These known techniques, however, are not yet fully satisfactory and require further improvements. Applicant uses a welding wire mixed with Al, Ti and / or N to form a 5 μm particle size in the weld metal.
250 m or more of oxides and nitrides of Al and Ti
Welding method has been developed to achieve equiaxed crystallization and refinement of the weld metal and to improve ductility and toughness by generating at a density of at least pcs / mm ² , and has already proposed (Japanese Patent Application No. Hei 11-118). 2
No. 55926 and Japanese Patent Application No. 2000-032105).

[0006] As a result of subsequent research, this technology was adopted
When a relatively small amount of nitride is generated densely in the weld metal by reducing the amount of O as much as possible with the addition of a small amount of Al, Ti and N, which are slightly lower than the values, it has been conventionally obtained. In addition to the cracking resistance, the result was that the tensile properties at high temperatures were improved. Therefore, the effects of various additional elements were investigated in detail, and a welding method was developed to obtain a weld having excellent tensile strength at high temperature, elongation, crack resistance and toughness of a weld of ferritic stainless steel.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to solve the problem in welding of ferritic stainless steel, that is, to suppress the crystal structure of the weld metal from growing enormously, to prevent cracking and stress (or) thermal stress during welding. Is to realize a welded portion in which cracks caused by repeated addition of cracks are prevented. It is also an object of the present invention to provide a welding method for providing such a weld.

[0008]

The welded part of ferritic stainless steel of the present invention is a welded part obtained by welding a ferritic stainless steel base material using a ferritic stainless steel welding wire. As alloy compositions, C: 0.05% or less, Si: 2.0% or less, Mn: 2.0% or less, Cr: 15 to 25%, Al:
It contains 0.01 to 0.10%, Ti: 0.01 to 0.15% and N: 0.04% to 0.2%, and O: 0.04%.
% Or less, P: 0.03% or less, S: 0.01% or less, and the balance is substantially the same as Fe.
Alternatively, a ferrite-based alloy in which the grain size of the weld metal is reduced by the presence of Al nitrides having a grain size of 0.3 μm ² or more at a density of 1.5 × 10 ⁴ grains / mm ² or more. Stainless steel welds.

The method for welding ferritic stainless steel of the present invention is a method for welding a ferritic stainless steel base material, wherein a ferrite stainless steel wire having any of the following metal-containing compositions is used as a welding wire. I) C: 0.05% or less, Si: 2.0% or less, Mn:
2.0% or less, Cr: 15 to 25%, Al: 0.01 to
0.20%, Ti: 0.01-0.30% and N:
0.04% to 0.2%, O: 0.04% or less,
P: 0.03% or less, S: 0.01% or less, the balance being substantially Fe II) In addition to the above (I), Mo and / or W:
III) In addition to the above (I) or (II), Nb: 1.0% or less
% Or less, Zr: 0.1% or less, and B: 0.1% or less, a gas having any one of the following compositions (volume%, the same applies hereinafter) as a shielding gas. use performed _{welding, a) Ar + (0~20%} ) O 2 b) Ar + (0~20%) O 2 + (0~50%) CO 2 c) Ar + (0~20%) O 2 + (0 to 50%) CO ₂
+ (0 to 100%) He or Ti
Ferrite to obtain a weld with a fine grain size of the weld metal by having a nitride having a particle size of 0.3 μm ² or more at a density of 1.5 × 10 ⁴ / mm ² or more This is a method for welding stainless steel.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The alloy composition of the welded portion is in addition to the basic alloy composition described above. Mo and / or W: 5.
It may contain 0% or less. Further, in addition to the basic alloy composition, Nb: 1.0% or less, Zr: 0.
1% or less and B: 0.1% or less one, two or three
It may contain seeds.

The reason why the alloy composition of the weld is determined as described above is as follows. It is as follows.

C: 0.05% or less C gives strength to the weld metal. Some presence is required. However, if the content is increased, martensite is generated to increase the hardness of the weld metal and to easily cause weld cracking. Therefore, the content should not exceed 0.05%.

Si: 2.0% or less Si is not only useful as a deoxidizer at the time of melting steel, but is also an element useful for resistance to weld cracking. However, the addition of a large amount impairs the toughness of the weld metal.
Preferably, the addition is limited to 1.5% or less.

Mn: 2.0% or less Mn also serves as a deoxidizing agent, but reduces corrosion resistance, especially oxidation resistance. Very large amounts are not preferred. The addition amount is 2.0% or less, preferably 1: 0% or less.

Cr: 15 to 25% Cr is an important component for increasing the strength of the weld metal and ensuring corrosion resistance. However, in order to obtain a sufficient effect, it is necessary to add 15% or more. However, even if a large amount is added, the effect is saturated and the cost is increased. Therefore, the upper limit is set to 25%.

Al: 0.01 to 0.20% Ti: 0.01 to 0.30% Al and Ti act to form nitrides and refine crystal grains of the weld metal. In order for a nitride of an appropriate size to be present at a sufficient density, it is necessary to add both in an appropriate amount of 0.01% or more. If Al and Ti are excessively present, the nitride aggregates and the density thereof is reduced. Therefore, the addition is made within 0.20% or 0.30%, respectively.

N: 0.04% to 0.2% In order for a nitride of an appropriate size to be present in the weld metal at a sufficient density, the N must be present in an amount of 0.04% or more. On the other hand, since the presence of a large amount of N may impair the integrity of the weld metal, 0: 3% is set as the upper limit of the content.

O: 0.04% or less Excessive presence of O results in that much of Ti and Al become oxides, and it becomes impossible to obtain a nitride effective for crystal grain refinement. Want to be as low as possible. The above values are acceptable limits.

P: 0.03% or less S: 0.01% or less Since both P and S are impurities that easily cause weld cracking and lower the toughness, the content should be as low as possible. The above values are acceptable limits.

The effects of the elements added arbitrarily and the reasons for limiting the composition range are as follows.

Mo and / or W: 5.0% or less Since Mo and W both act to increase the high-temperature strength, it is preferable to add an appropriate amount. If added in a large amount, the impact value and the corrosion resistance are reduced, so the addition of up to 5.0% is appropriate.

Nb: 1.0% or less, Zr: 0.1% or less, B: 0.1% or less 1 type, 2 types or 3 types Nb, Zr and B refine the crystal grains of the weld metal. To act. Since the presence of a large amount is detrimental to the stabilization of the arc and the bead shape, Nb is 1.0%, Z
r is set to 0.1%, and B is set to 0.1% as an upper limit.

The nitride of Ti and AI distributed in the weld metal has an appropriate particle size of 0.3 μm ² or more to help refine the crystal of the weld metal. It is an important opportunity of the present invention to find that effective miniaturization is realized only when the density is × 10 ⁴ / mm ² or more. The close relationship between the number of inclusions and the crystal grain size is apparent in the examples described later.

The method for welding ferritic stainless steel of the present invention is as follows. Another embodiment than the above method is possible,
The welding method uses a ferrite stainless steel wire having any of the following alloy compositions as a welding wire. IV) C: 0.05% or less, Si: 2.0% or less, Mn:
2.0% or less, Cr: 15 to 25%, Al: 0.01 to
Contains 0.20% and Ti: 0.01 to 0.30%, O: 0.04% or less, P: 0.03% or less, S:
0.01% or less, with the balance being substantially Fe V) In addition to (IV) above, Mo and / or W:
5.0% or less VI) In addition to the above (IV) or (V), Nb: 1.0%
Hereafter, Zr: 0.1% or less and B: 0.1% or less 1
Contains two, three or more species. As a shielding gas, one of the following compositions (volume%,
Welding is performed using the gas of the following), d) Ar + (0 to 20)% O ₂ + (2 to 30)% N ₂ e) Ar + (0 to 20)% O ₂ + (0 to 50) )% CO <sub>2
+ (2~30)% N 2 f</sub> ) Ar + (0~20)% O 2 + (0~50)% CO 2
+ (0~100)% He + ( 2~30)% N ₂ weld metal, those particle size 0.3 [mu] m ² or more as a nitride of Ti or Al 5 × 10 ⁴ cells / mm ² or more This is a method of welding a ferritic stainless steel in which a weld portion having a fine grain size of a weld metal is obtained by being present at a high density.

The above-mentioned welding wire groups (I), (I)
I) and (III) are groups (d) of shielding gas,
(E) and (f) can be used in combination, It is included in the welding force method of the present invention.

The above (I), (II) and (III), which are defined as the alloy composition of the welding wire, contain N in their components, which generate nitrides during solidification of the weld metal to refine the crystal grains. Is useful as described above. The generation of nitride is also realized by adding an appropriate amount of N ₂ gas to the shielding gas. The shielding gas compositions (d), (e) and (f) adopt this technique. In this case, since it is not essential that the N component is present in the welding wire, the above-mentioned wire alloy composition (IV),
A welding wire having (V) or (VI) may be used, and the second embodiment of the welding method corresponds to this.

Even if the composition of the shielding scum contains N ₂ gas, the presence of the N component in the welding wire does not matter at all. Welding wire composition (I), (II) or (II)
It goes without saying that I) and the shielding gas composition (d), (e) or (f) may be combined. Such a case is the third mode of the welding method.

The composition of the shielding gas used in the present invention is as follows:
There is no difference in principle from that used for ordinary gas shielded arc welding. For example, the O ₂ component in the shielding gas helps to stabilize the arc, and this effect is obtained in the presence of 1% or more, while if it exceeds 10%, it is liable to oxidize the weld metal, which is unfavorable. A range of 10% is given. C0 ₂ will act to stabilize the arc, because This effect is also observed with the addition of about 1%, there is a disadvantage that lead to deterioration in toughness of the welded portion exceeds 20%,
Select an addition amount less than that. N _2, as described above, a component for generating the nitride in the weld metal, the amount is in the range from 2 to 30%. If less than 2% is added, the amount of nitride generated is insufficient. If more than 30% is added,
It is not preferable because the toughness decreases. As will be readily appreciated, if it contains N alloy cord composition of welding wire, N ₂ shield gas, even selecting the composition which contains, or in relatively small amounts.

[0029] The welded part of the present invention, whether or not the above-described welding method is used, has Ti and Al components, Mo.
And / or W components, and also Nb, Zr and B
All or some of the components of the group can be determined in the base material to be welded. In other words, if the alloy components of the base material are known, it can be expected that some of these components will be replenished therefrom. Needless to say, the weld metal
This is because not only the components of the filler material but also the components of the surrounding base metal mixed and dissolved therein are summed up. However, although there is a slight difference depending on the welding method and conditions, the amount of the base metal component that is melted is not large, so that the amount of the component that can be supplemented from the surroundings is limited.

[0030]

EXAMPLE A ferritic stainless steel having an alloy composition shown in Table 1 was melted and rolled into a plate. Thicknesses of 1.5 mm and 20 mm were produced.

[0031]

Separately, a ferritic stainless steel having an alloy composition shown in Table 2A (Example) and Table 2B (Comparative Example) was melted. A welding wire having a diameter of 1.2 mm was manufactured therefrom.

[0033]

[0034]

The above-mentioned steel sheet and these welding wires are
Combinations were made as shown in Table 3A (Example) and Table 3B (Comparative Example), and overlay welding, butt welding, and T-type (fillet) welding were also performed under the conditions shown in these tables.

[0036]

[0037]

The grain size (Jls) of the weld metal in the butt weld was examined, and the number (per 1 mm ² ) of nitride particles having an area of 0.3 μm ² or more was counted. Example No. 13 and Comparative Example No. For No. 7, the crystal structure of the weld metal was observed with a microscope. FIG. 1 is a photomicrograph of the former, and FIG. 2 is a photomicrograph of the latter (both at a magnification of 100).

The high-temperature tensile test piece was formed along the direction of the weld line.
The test piece was collected as a round bar-shaped test piece so that the entire test piece was made of the deposited metal, and 950 in accordance with JIS G0567.
A high temperature tensile test was performed at ℃.

Cracks were examined on the T-welded ones. What is “T-type welding”? As shown in FIG. 3, the same steel plate (2) is placed in an inverted T-shape on a flat steel plate (1), and fillet welding (3) is performed in advance to restrain both members.
This is an operation to perform fillet welding on the other side again,
The “mold welding crack test” is a test for measuring the degree of welding cracking of the fillet weld (4) performed later.

The butt-welded one was subjected to a bending test. In the bending test, as shown in FIG. 4, one steel plate (5) was restrained and the other steel plate (6) was repeatedly used.
This is a test in which the welded part (7) bends within an angle of 0 degrees and counts how many times the bending endures. Table 4A shows the above test results.
(Example) and Table 4B (Comparative Example).

[0042]

[0043]

Examples and comparative examples will be supplemented below.

In Examples 1 and 7, the amount of heat input was changed, but no difference was obtained. Example 12 is different from Example 14 in that N
, But in this case there was no particular difference. Example 19 is an example in which a base material containing a relatively large amount of Ti was welded and Ti was supplied from the base material. On the other hand, in Comparative Example 7, the wire and the base metal had a small Ti content, so that the amount of nitride generated was insufficient, resulting in poor results. Examples 6 and 20 are examples near the lower or upper limit of the Ti amount, respectively. In Examples 4 and 5, the pulse conditions were changed, but there was no significant difference in the results. Example 8 is an example in which the amount of Al is near the lower limit.

The working sides 13 to 15 include Ti and Al
Is the most appropriate case, and a large amount of nitride is generated. Accordingly, the crystal grain size is fine and the results of the bending test are excellent. Examples 6 and 8 are examples near the upper or lower limit of the Cr content, respectively. In Examples 14 to 17, B, Zr, and Nb were added alone or in combination, and the crystal grain size was fine and the results of the bending test were excellent.

In Comparative Examples 7 to 9, all the components of the welding wire were insufficient. In Comparative Example 7, Ti, Comparative Example 8 were Al, and Comparative Example 9 was N below the lower limit. It is an example. Comparative Example 1 is a case where C is too large, and Comparative Example 3 is a case where both the amounts of CO ₂ in the shielding gas are excessive.

[0048]

The weld metal of ferritic stainless steel according to the present invention is as follows. By having a specific alloy composition and inclusion distribution, as can be seen from the comparison between FIG. 1 and FIG. 2, the crystal structure is not enlarged, and the microstructure is much finer than the weld metal according to the prior art. become. Further, since the size of the precipitate is small, it is possible to obtain a welded part having excellent high-temperature tensile strength, elongation, crack resistance and toughness.

The welding method of the present invention for forming such a weld metal is as follows. This is achieved by properly selecting the metal content of the welding wire, properly selecting the shielding gas composition, and properly selecting both the alloy composition and the shielding gas composition of the welding wire. Further, when the alloy composition of the ferritic stainless steel to be welded can be used, the shortage of the alloy component of the wire can be compensated, so that the implementation is easy.

[Brief description of the drawings]

FIG. 1 is a micrograph of the structure of a weld metal in an example of the present invention.

FIG. 2 is a micrograph of the structure of a weld metal in a comparative example of the present invention.

FIG. 3 is a cross-sectional view including a weld for explaining a “T-type welding crack test” performed in an example of the present invention.

FIG. 4 is a cross-sectional view including a welded portion for explaining a “bending test” performed in an example of the present invention.

[Explanation of symbols]

 1, 2 steel plate 3 fillet welding performed in advance 4 fillet welding performed afterwards 5, 6 steel plate 7 welded portion

Claims (6)

[Claims] 1. A welded portion formed by welding a base material of ferritic stainless steel using a welding wire of ferritic stainless steel, wherein, by weight%, C: 0.05% or less;
i: 2.0% or less, Mn: 2.0% or less, Cr: 15 to
25%, Al: 0.01 to 0.10%, Ti: 0.01
０．１0.15% and N: 0.04% to 0.2%, O: 0.04% or less, P: 0.03% or less, S:
Not more than 0.01%, the balance being an alloy composition substantially composed of Fe, and 1.5 × 10 ⁴ particles / mm of Ti or Al nitride having a particle size of 0.3 μm ² or more
A weld of ferritic stainless steel in which the crystal grain size of the weld metal is reduced by being present at a density of ² or more. 2. A welded part of a ferritic stainless steel having an alloy composition containing Mo and / or W: 5.0% or less in addition to the alloy component according to claim 1. 3. The method according to claim 1, wherein the welding portion is. Nb: 1.0% or less, Zr: 0.1% in addition to the alloy components according to claim 1 or 2.
Or less, B: 0.1% or less of one, two or three
Welded ferritic stainless steel with alloy composition containing seeds. 4. A method for welding a ferritic stainless steel base material, wherein a ferrite stainless steel wire having any of the following metal-containing compositions is used as a welding wire: I) C: 0.05 %, Si: 2.0% or less, Mn:
2.0% or less, Cr: 15 to 25%, Al: 0.01 to
0.20%, Ti: 0.01-0.30% and N:
0.04% to 0.2%, O: 0.04% or less,
P: 0.03% or less, S: 0.01% or less, the balance being substantially Fe II) In addition to the above (I), Mo and / or W:
III) In addition to the above (I) or (II), Nb: 1.0% or less
% Or less, Zr: 0.1% or less, and B: 0.1% or less. Contains one, two or three types as a shielding gas.
Welding is performed using the following gas), a) Ar + (0 to 20%) O ₂ b) Ar + (0 to 20%) O ₂ + (0 to 50%) CO ₂ c) Ar + (0 _{~20%) O 2 + (0~50} %) CO 2
+ (0 to 100%) He 1.5 × 10 ⁴ Ti / Al nitride particles having a particle size of 0.3 μm ² or more are 1.5 × 10 ⁴ / mm on the weld metal portion.
^A method for welding a ferritic stainless steel in which a weld portion having a fine grain size of a weld metal is obtained by being present at a density of ² or more. 5. A method for welding a ferritic stainless steel base material, wherein a ferrite stainless steel wire having any of the following alloy compositions is used as a welding wire: IV) C: 0.05% Below, Si: 2.0% or less, Mn:
2.0% or less, Cr: 15 to 25%, Al: 0.01 to
Contains 0.20% and Ti: 0.01 to 0.30%, O: 0.04% or less, P: 0.03% or less, S:
0.01% or less, with the balance being substantially Fe V) In addition to (IV) above, Mo and / or W:
5.0% or less VI) In addition to the above (IV) or (V), Nb: 1.0%
Hereinafter, Zr: 0.1% or less and B: 0.1% or less 1
Contains two, three or more species. As a shielding gas, one of the following compositions (volume%,
Welding is performed using the gas of the following), d) Ar + (0 to 20)% O ₂ + (2 to 30)% N ₂ e) Ar + (0 to 20)% O ₂ + (0 to 50) )% CO <sub>2
+ (2~30)% N 2 f</sub> ) Ar + (0~20)% O 2 + (0~50)% CO 2
+ (0~100)% He + ( 2~30)% N ₂ weld metal, those particle size 0.3 [mu] m ² or more as a nitride of Ti or Al 5 × 10 ⁴ cells / mm ² or more A method for welding ferritic stainless steel in which a weld portion having a fine grain size of a weld metal is obtained by being present at a high density. 6. A method for welding a ferritic stainless steel base material, wherein a ferrite stainless steel wire having any of the following alloy compositions is used as a welding wire: I) C: 0.05% Below, Si: 2.0% or less, Mn:
2.0% or less, Cr: 15 to 25%, Al: 0.01 to
0.20%, Ti: 0.01-0.30% and N:
0.04% to 0.2%, O: 0.04% or less,
P: 0.03% or less, S: 0.01% or less, the balance being substantially Fe II) In addition to the above (I), Mo and / or W:
III) In addition to the above (I) or (II), Nb: 1.0% or less
% Or less, Zr: 0.1% or less, and B: 0.1% or less.
Welding is performed using the gas of the following), d) Ar + (0 to 20)% O ₂ + (2 to 30)% N ₂ e) Ar + (0 to 20)% O ₂ + (0 to 50) )% CO <sub>2
+ (2~30)% N 2 f</sub> ) Ar + (0~20)% O 2 + (0~50)% CO 2
+ (0 to 100)% He + (2 to 30)% N ₂ 1.5 × 10 ⁴ Ti / Al nitride particles having a particle size of 0.3 μm ² or more are added to the weld metal portion at 1.5 × 10 ⁴ pieces / mm.
^A method for welding a ferritic stainless steel in which a weld portion having a fine grain size of a weld metal is obtained by being present at a density of ² or more.