JPH11277292A - Welding metal and welding joint for high temp. high strength steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welding metal and a welding joint for austenite steel capable of preventing micro crack in multi-layer welding and excellent in high temp. strength. SOLUTION: A steel contains, by weight, 0.03-0.13% C, 0.2-4% Mn, 1-5% Cu, 15-25% Ni, 15-25% Cr, 0.5-3% W, 0.15-1.5% Nb, 0.1-0.35% N, 0.001-0.01% B, <=0.01% Al, <=0.01% O, 0-1.5% Mo, 0-0.01% Ca, 0-0.01% Mg, 0-0.02% Zr, 0-0.02% Hf and 0.02% Ta. In this case, Si and Nb satisfy the formula, Si <=0.2Nb+0.25, and also Ni, Cu and Cr satisfy the formula, 0.7<=(Ni+Cu)/Cr<=1.4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding material and a welding joint used for welding austenitic steel excellent in high-temperature strength, which is a main member of a high-temperature device such as a power boiler.

[0002]

2. Description of the Related Art Conventionally, 18Cr-8Ni-based austenitic stainless steel has been mainly used for power generation boilers, high-temperature devices and the like. However, as operating conditions in boilers and the like have become more severe in recent years, materials that are more excellent in high-temperature strength and corrosion resistance have been required. From such demands, many new materials have been developed in which the high-temperature strength is increased by adding various elements. For example, Japanese Patent Application Laid-Open No. 8-13102 proposes an austenitic steel in which the high-temperature strength is improved by adding Cu, W, N, Nb, and B.

[0003] Such welding materials for austenitic steel include a co-metallic welding material using a base material as it is as a welding material and a welding material for high Ni alloys (for example,
It is conceivable to use YNiCr-3) in JISZ3324.
However, after the base material is melted, the structure is adjusted by rolling and heat treatment to ensure high-temperature strength, while the weld metal is used in most cases with the solidified structure,
It is essentially not easy to increase the high-temperature strength of the co-metallic welding material compared to the base metal. For this reason, many co-metallic welding materials have been proposed (Japanese Patent Application Laid-Open Nos. 63-309392 and 5-2).
No. 20594, JP-A-6-142980, JP-A-7-60481, JP-A-8-71784, etc.), these co-metal welding materials are proposed in JP-A-8-13102. If used for certain materials, the high temperature strength cannot be satisfied.

[0004] Of these, the co-metallic welding material disclosed in Japanese Patent Application Laid-Open No. H8-71784 is satisfactory in high-temperature tensile strength and short-time creep strength, but Ni is less than Cr. And the stability of the organization is insufficient. As a result, when used at a high temperature for a long time, the high-temperature strength may suddenly decrease or the impact performance may deteriorate. Further, it has been found that there is a problem that extremely minute cracks occur in the weld metal during multi-layer welding.

On the other hand, welding materials for high Ni alloys are expensive and are not preferred from the viewpoint of economy.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a welding material for austenitic steel capable of preventing microcracks during multi-layer welding and having excellent high-temperature strength, and a welding joint using the welding material.

[0007]

Means for Solving the Problems The present inventors conducted the above-mentioned various tests and were able to confirm the following items.

(A) Various steels were subjected to a long-term creep test, and a detailed investigation was carried out on those steels which showed a sharp decrease in strength at high temperature and for a long time. As a result, it became clear that the cause of the decrease in creep strength was instability of the structure. In other words, the weld metal remains substantially solid as it is solidified.
Despite being a single phase of austenite, due to elemental segregation during solidification during high-temperature, long-term testing,
An intermetallic compound phase containing fragile Mo, W, etc. is generated, and the strength is reduced. The stability of this austenitic phase is Cu, N
It can be ensured by limiting i and Cr to predetermined ranges.

(B) According to the findings of the above (a), although the stability of the structure at high temperature and long time could be ensured, when Cu, Ni, and Cr were restricted to a predetermined range, very fine Cracks are likely to occur. From observation of the fracture surface morphology, this crack was found to be along the grain boundary. When various analytical techniques were used, significant enrichment of Si and C was recognized at the grain boundaries where reheat cracking occurred during multi-layer welding. Therefore, in the reheat cracking, Si and C segregated at the crystal grain boundaries generate Fe and a eutectic compound having a low melting point during the next heat cycle, and are liquefied and opened. In the following description, this minute crack is referred to as a reheat crack in principle. The present inventors have found that the addition of Nb is effective for preventing reheat cracking. Nb fixed C as NbC, removed the cause of reheat cracking, and confirmed that cracking could be prevented. Therefore, the relationship between the amount of Nb required for preventing reheat cracking and Si was investigated.

FIG. 1 shows the range of Nb and Si where reheat cracking does not occur. We have found that Zr, Hf,
A test was also performed on Ta and Ta to confirm that these elements were effective in preventing reheat cracking.

The present invention has been completed through many welding tests based on the above matters, and has the following welding materials and welded joints as its gist.

(1) By weight%, C: 0.03 to 0.13%, Mn: 0.
2-4%, Cu: 1-5%, Ni: 15-25%, Cr: 15-25%, W: 0.5
~ 3%, Nb: 0.15 ~ 1.5%, N: 0.1 ~ 0.35%, B: 0.001 ~ 0.0
1%, Al: 0.01% or less, O (oxygen): 0.01% or less, Mo: 0 to 1.5
%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, Zr: 0 to 0.02%, Hf: 0 to
0.02% and Ta: 0-0.02%, and as impurities
A welding material that is a steel in which P and S satisfy P: 0.01% or less and S: 0.005% or less, respectively, and Si and Nb satisfy the relationship of the formula, and Ni, Cu, and Cr satisfy the relationship of the formula.

Si ≦ 0.2 · Nb + 0.25: 0.7 ≦ (Ni + Cu) /Cr≦1.4 The elemental symbols in the expressions and the expressions represent the content (% by weight) of the elements.

(2) A welded joint comprising a base material and a weld metal, wherein the base material is Cr: 15 to 30% and Ni: 1 by weight%.
Austenitic steel containing 5-30%, the weld metal is C:
0.03 to 0.13%, Mn: 0.2 to 4%, Cu: 1 to 5%, Ni: 15 to 25%,
Cr: 15-25%, W: 0.5-3%, Nb: 0.1-1.5%, N: 0.1-0.
35%, B: 0.001 to 0.01%, Al: 0.05% or less, O (oxygen): 0.08
% Or less, Mo: 0 to 1.5%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, Z
r: 0 to 0.02%, Hf: 0 to 0.02%, and Ta: 0 to 0.02%, and P and S as impurities are respectively P: 0.02% or less, S: 0.0
A welded joint that satisfies 1% or less, furthermore, steel in which Si and Nb satisfy the relationship of the above formula, and Ni, Cu and Cr satisfy the above formula.

The welding material of the present invention can be applied to ordinary welding methods. For example, gas shielded arc welding (GMAW:
Gas Metal Arc Welding, GTAW: Gas Tungsten Arc Weldi
ng) wire, core wire of coated arc welding material coated with flux (SMAW: Shielded Metal Arc Welding), wire for sub-merged arc welding (SAW) in flux, etc. The welding material of the method may be used. In short, the chemical composition of the steel portion of the welding material may be within the scope of the present invention. As for the welded joint, it may be welded by any welding method within the range of the above (2). The base material described in (2) is Cr and
Any shape may be used as long as Ni falls within the range of the base material, and may be a tube, a plate, a rod, or the like.

[0016]

Next, the reasons for limiting the chemical composition of the welding material and the welding metal of the present invention will be described. Al, O (oxygen), Nb, other elements other than P as an impurity element,
Since the base metal is similar to the chemical composition of the welding material, the composition of the welding metal is almost the same as that of the welding material. Therefore, unless otherwise required, the following description will be made without distinguishing between the welding material and the welding metal. Further, in the following description, “%” of the alloy element means “% by weight”.

1. Welding material and weld metal C: 0.03 to 0.13% C contributes to improvement of high-temperature strength. For that, 0.03% or more is required. However, if contained excessively, it precipitates as a large amount of carbonitride, causing a decrease in strength and segregating at grain boundaries.
Reacts with Fe and Fe to produce a low melting point compound and increases the reheat cracking susceptibility. A more desirable range is from 0.05 to 0.11%.

Mn: 0.2 to 4% Mn is added as a deoxidizing agent, but has an effect of fixing S, thereby contributing to a reduction in susceptibility to solidification cracking during welding. Further, by reducing the activity of N in the weld metal, the scattering of N from the arc atmosphere is suppressed, which contributes to the improvement in high-temperature strength. For that, 0.2% or more is necessary. But,
The excess content is set to 4% or less because embrittlement is caused. A preferred upper limit is 3%.

Cu: 1-5% Cu has the effect of stabilizing the austenite phase of the weld metal. Further, it precipitates at a high temperature and contributes to improvement in high-temperature strength. To obtain the effect of improving the strength, 1% or more is necessary, and in order to stabilize the structure, it is necessary to satisfy a relational expression between Ni and Cr described below. However, when included in excess, ε
-The upper limit is 5% because excessive precipitation of the Cu phase causes brittleness and increases the susceptibility to solidification cracking during welding.
And A more preferred range is 1.5-4.5%.

Ni: 15 to 25% Ni is an essential element for stabilizing the austenite phase, and contributes to improvement in high-temperature strength. However, in order to ensure the stability of the structure at a high temperature for a long time, it is necessary that the content is 15% or more, and it is necessary to satisfy a relational expression between Cu and Cr described later. However, since Ni is an expensive element, a large amount of addition causes an increase in cost and a remarkable increase in reheat cracking susceptibility. Therefore, the upper limit is set to 25%.

Cr: 15 to 25% Cr is an essential element for ensuring high-temperature strength, oxidation resistance and corrosion resistance. In order to satisfy sufficient oxidation resistance and corrosion resistance, 15% or more is required, and from the viewpoint of ensuring the stability of the structure, it is necessary to satisfy a relational expression with Ni and Cr described below. However, excessive addition degrades the stability of the structure at high temperatures, lowers the strength, and degrades the hot workability, so the upper limit is 25%.

W: 0.5-3% W is an element that forms a solid solution in the matrix and contributes to improvement in high-temperature strength. In order to exhibit this effect, a content of 0.5% or more is required. However, the effect is saturated when contained in excess, and the cost is increased because the element is an expensive element. Therefore, the upper limit is set to 3%. A more desirable range is 0.8-2.5%.

Nb: welding material 0.15 to 1.5%, welding metal 0.
1 to 1.5% Nb contributes to the improvement of high-temperature strength by precipitating finely as intra-granular and grain boundaries as carbonitride. Further, C is fixed to prevent reheat cracking during multi-layer welding. In order to exert the effect, the content of the weld metal needs to be 0.1% or more, and further, it is necessary to satisfy a relationship with Si described later. However, when it is contained excessively, it causes the coarsening of NbC and the deterioration of toughness. Therefore, the upper limit is 1.5%. Desirably, it is 0.15 to 1.2%. In addition, even in consideration of dilution with a base material that does not contain Nb or has a low Nb content in the welding wire, it is 0.
If 15% or more is included, it is considered that the above lower limit in the weld metal is satisfied, so 0.15% or more, and the upper limit is the same as the weld metal.
1.5%.

N: 0.1 to 0.35% N is an essential element for ensuring high-temperature strength. That is, N solid-dissolves in the matrix of the solidified structure and strengthens it, and at the same time, partly precipitates as nitride to strengthen precipitation. However, if it is contained excessively, a large amount of carbonitride precipitates during use at high temperatures, causing embrittlement. Furthermore, it causes blowhole formation during welding. Therefore, it is set to 0.1 to 0.35%. A desirable upper limit is 0.3%.

B: 0.001 to 0.01% B segregates at the grain boundaries and contributes to the improvement of creep strength. For that purpose, 0.001% or more is required. However, if contained excessively, solidification cracking during welding is promoted, so the upper limit is made 0.01%.

Al: welding material 0.01% or less, welding metal 0.0
5% or less Al is added as a deoxidizer, but a large amount added to the welding wire promotes slag formation, degrades the flow of the weld metal and the uniformity of the weld bead, and significantly reduces weldability. Let it. Furthermore, the Al content in the welding wire needs to be 0.01% or less in order to narrow the welding condition region in which a backwash is formed. However, in the weld metal, coating arc welding or SAW
During welding, Al is mixed in from the flux component, and the Al content increases. In this case, if 0.05% or more of Al is contained in the weld metal, the cleanliness deteriorates remarkably, resulting in embrittlement. Therefore, the Al content in the weld metal is set to 0.05% or less. The lower limit is not specified, but is 0.00
It is desirable to be at least about 05%.

O: welding material 0.01% or less, welding metal 0.08
% Or less O is excessively added to the welding material, which promotes slag generation, degrades the flow of the molten metal and reduces the welding workability, so the O in the welding material needs to be 0.01% or less. However, O is mixed into the weld metal from a shield gas or a flux component, and the content increases. In particular, the content in the weld metal
If it is 0.08% or more, the cleanliness is remarkably deteriorated and embrittlement is caused. Therefore, the O content in the weld metal must be 0.08% or less. There is no particular lower limit, but it is desirable to set the lower limit to about 0.0005% in both the welding material and the welding metal.

Mo: 0-1.5% Mo forms a solid solution in the matrix and has the effect of increasing the high-temperature strength. Therefore, if high-temperature strength is required, it may be included. However, the effect is not only saturated at about 1.5%, but also causes deterioration of corrosion resistance. Therefore, the upper limit is set to 1.5%.

Ca: 0 to 0.01% Ca may not be contained. Since it is effective for improving hot workability during wire working, it may be added when it is necessary to increase hot workability. However, if it is contained excessively, inclusions in the weld metal increase and the cleanliness deteriorates, so the content is made 0.01% or less.

Mg: 0 to 0.01% Mg may not be contained. Mg, like Ca, is effective for improving hot workability during wire processing, and may be added. However, if it is contained excessively, inclusions in the weld metal increase and the cleanliness deteriorates, so the content is made 0.01% or less.

Zr: 0 to 0.02% Hf: 0 to 0.02% Ta: 0 to 0.02% Zr, Hf and Ta may not be contained. However, these elements, like Nb, fix C and are effective in reducing the susceptibility to reheat cracking. It can be used when there is a high risk of reheat cracking. If the content is less than 0.0005%, the above effects of these elements are not sufficient. Therefore, if they are contained, the content of each element needs to be 0.0005% or more. However, if the content is 0.02% or more, the carbide becomes coarse and embrittlement is caused. Therefore, the content is set to 0.02% or less.

P: welding material 0.01% or less, welding metal 0.02
% Or less P forms a low melting point eutectic during solidification of the weld metal during welding and causes hot cracking. Therefore, in the weld metal
P must be 0.02% or less. Also, P in the welding material
If 0.01% or less is considered even if the dilution with the base material is considered, since the above-mentioned range of the P content in the weld metal can be satisfied, 0.01%
The following is assumed.

S: welding material 0.005% or less, welding metal 0.0
1% or less S, like P, forms a low melting point eutectic during solidification of the weld metal during welding, causing hot cracking. Therefore, S in the weld metal must be 0.01% or less. Also, if S in the welding material exceeds 0.005%, the arc becomes unstable,
The welding workability deteriorates. Further, if the content is set to 0.005% or less even in consideration of the dilution with the base material, the S content in the weld metal described above can be satisfied, so the content is set to 0.005% or less.

Si: ≦ 0.2 · Nb + 0.25 Si is added as a deoxidizing agent, but segregates at the grain boundary during solidification of the weld metal, reacts with C and Fe, generates a compound having a low melting point, and performs multi-layer welding. It causes reheat cracking at the time. As described above, Nb fixes C and reduces the susceptibility to reheat cracking. However, in order to ensure sufficient reheat cracking resistance, Si must be replaced with (0.2 · Nb +
0.25)% or less. However, an excessive reduction leads to an increase in manufacturing cost, and thus no particular lower limit is set, but it is preferably 0.01% or more.

0.7 ≦ (Ni + Cu) /Cr≦1.4 Ni and Cu stabilize the austenite phase at high temperatures,
Is unstable. If these elements have a (Ni + Cu) / Cr of 0.
When it is 7 or more, the austenite phase is stable even at high temperatures, and high-temperature strength is secured. However, (Ni + Cu) / Cr is
If it is larger than 1.4, not only the susceptibility to solidification cracking during welding increases, but also the susceptibility to occurrence of reheat cracking increases. Therefore, it must be 1.4 or less.

2. Base material Base material is N to ensure high temperature strength and other performance
i: 15-30%, and Cr: 15-30%. If Ni is less than 15%, the high temperature strength and hot workability are not sufficiently ensured, while if it exceeds 30%, the surface properties deteriorate, so the content is set to 30% or less. Cr is 1
If it is less than 5%, high-temperature corrosion resistance is not sufficient. On the other hand, if it exceeds 30%, hot working may be difficult, so the content is made 30% or less.

The alloying elements of the other base materials are desirably within the following ranges, for example, from the viewpoints of high-temperature strength, high-temperature corrosion resistance, and reheat crack resistance. C: 0.02-0.16%, Si: 0.5%
Hereinafter, Cu: 1 to 5%, W: 0.5 to 3%, Nb: 0.1 to 1.5%, N: 0.05 to
0.4%, B: 0.001 to 0.02%, the balance substantially desirably Fe or the like.

[0038]

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

Table 1 shows the chemical composition of the base material used in the experiment.

[0040]

[Table 1]

[0041] 10 ⁴ hours creep strength at the base material is 700 ℃ 14
It is a high-strength austenitic steel sheet of kgf / mm ² (sheet thickness: 12 mm).

Table 2 shows the 16 types of welding materials used.

[0043]

[Table 2]

[0044] These welding materials were prepared by melting
It was processed into a 1.2mm wire rod. For A11, metal carbonate, metal fluoride, S
It is a coated arc welding rod (SMAW rod) coated with a coating material composed of an i-compound, a Ti compound and a metal powder. In the joint performance test, the base material subjected to the groove processing shown in FIG. 2 was subjected to constraint welding as shown in FIG.
Multi-layer welding was performed by AW (TIG welding), MAG welding, or SMAW.

Table 3 shows the chemical compositions of the weld metals obtained by these weldings.

[0046]

[Table 3]

During welding, cracks are easily generated due to thermal stress caused by welding because of being restrained. After welding, a micro test piece, a side bending test piece, and a creep test piece having a weld metal portion at the center were sampled and subjected to each test. FIG. 4 shows a side bending test piece, and FIG. 5 shows a creep test piece.

For the micro test piece, after buffing, the entire weld metal portion was observed with an optical microscope at a magnification of 400 times, and the occurrence of reheat cracking was observed. In the side bending test, a 180 ° bending was performed with a bending radius twice as large as the plate thickness, and the presence or absence of solidification cracks in the weld metal was examined. A creep test was performed only on joints having no defects in the reheat cracking and bending tests. The creep test was performed at a temperature of 700 when the fracture time of the base material was 3000 hours.
A test was performed under the conditions of a temperature of 17 ° C. and a stress of 17 kgf / mm ² , and the fracture time of the weld metal was investigated. Then, 80% of the rupture time of the base material was used as a criterion for judging the quality, and those that did not reach the criterion were judged as insufficient creep strength.

Table 4 shows the evaluation results of the reheat cracking resistance and the joint performance.

[0050]

[Table 4]

As is clear from Table 4, when the welding material B1 of the comparative example was used, the (Cu + Ni) / Cr of the welding material was 0.66.
Therefore, the weld metal (Cu + N)
i) Since / Cr was out of the range of 0.69, the structure at the time of the creep test was unstable, and the breaking time was 2203 hours, which did not satisfy 80% of the base material. The welding material B2 contains 0.45% Si.
And did not satisfy 0.42 (= 0.2 · Nb + 0.25) or less. Therefore, in the welded joints WB2 and WB3, the Si of the weld metal was 0.43% and 0.42%, respectively, which did not satisfy the formula. As a result, many fine reheat cracks occurred in the weld metal.
In addition, when the welding material B3 was used, since the (Cu + Ni) / Cr of the weld metal of the welded joint WB4 exceeded 1.4, Si and Nb satisfied the formula, but the reheat cracking sensitivity increased, and reheat cracking occurred. . When using welding material B4, the weld joint WB
5. Although the (Cu + Ni) / Cr of the weld metal of WB6 is slightly lower than 1.4, reheating cracks occur because Si is contained excessively in the formula, and it expanded during the side bending test. It was cracked. When the welding material B1 is used, the welding joint WB8 has the same welding material (Cu +
Since Ni) / Cr was less than the predetermined range of 0.69, the structure during the creep test became unstable, the rupture time was 2310 h, and did not satisfy 80% of the base material.

On the other hand, the welding materials A1 to A11 of the present invention
Further, the welded joints WA1 to WA13 in which the chemical components of the weld metal were within the limited range of the present invention satisfied all of the excellent reheat crack resistance, bending resistance, and high-temperature strength.

[0053]

The welding material and the welded joint of the present invention have excellent resistance to reheat cracking and high temperature under a wide range of welding conditions.
Extensive application to high temperature equipment such as boilers is expected.

[Brief description of the drawings]

FIG. 1 shows the ranges of the contents of Si and Nb that do not cause reheat cracking.

FIG. 2 shows a groove shape in a reheat cracking test.

FIG. 3 shows a test method of a reheat cracking test. (A) is a top view, (b) is a front view.

FIG. 4 shows the shape of a side bending test piece.

FIG. 5 shows a creep test piece shape.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Base material of reheat cracking test 2 ... Restraint plate of reheat cracking test 3 ... Constraint bead of reheat cracking test 4 ... Test bead (weld metal) of reheat cracking test 5 ... Weld metal

Claims (2)

[Claims] (1) C: 0.03 to 0.13%, Mn: 0.2 to 4% by weight
%, Cu: 1 to 5%, Ni: 15 to 25%, Cr: 15 to 25%, W: 0.5 to 3
%, Nb: 0.15 to 1.5%, N: 0.1 to 0.35%, B: 0.001 to 0.01
%, Al: 0.01% or less, O (oxygen): 0.01% or less, Mo: 0 to 1.5
%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, Zr: 0 to 0.02%, Hf: 0 to
0.02% and Ta: 0-0.02%, and as impurities
P and S each satisfy P: 0.01% or less, S: 0.005% or less, and furthermore, Si and Nb satisfy the relationship of the formula, and Ni, Cu and Cr are steels satisfying the relationship of the formula. Material for high temperature high strength steel. : Si ≦ 0.2 · Nb + 0.25: 0.7 ≦ (Ni + Cu) /Cr≦1.4 The elemental symbols in the formulas and the formulas represent the contents (% by weight) of the elements. 2. A welded joint comprising a base metal and a weld metal, wherein, by weight%, the base material is Cr: 15-30% and Ni: 15-30%.
Austenitic steel containing, the weld metal is C: 0.03 ~
0.13%, Mn: 0.2-4%, Cu: 1-5%, Ni: 15-25%, Cr: 15
~ 25%, W: 0.5 ~ 3%, Nb: 0.1 ~ 1.5%, N: 0.1 ~ 0.35%,
B: 0.001 to 0.01%, Al: 0.05% or less, O (oxygen): 0.08% or less, Mo: 0 to 1.5%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, Zr: 0
0.02%, Hf: 0 to 0.02%, and Ta: 0 to 0.02%, and P and S as impurities are respectively P: 0.02% or less, S: 0.01%
A welded joint that satisfies the following, furthermore, a steel in which Si and Nb satisfy a relationship of the following formula, and Ni, Cu, and Cr satisfy the following formula. : Si ≦ 0.2 · Nb + 0.25: 0.7 ≦ (Ni + Cu) /Cr≦1.4 The elemental symbols in the formulas and the formulas represent the contents (% by weight) of the elements.