JP2021011610A - Austenitic heat-resistant alloy weld joint - Google Patents

Abstract

To provide an austenitic heat-resistant alloy weld joint that is excellent in both of creep rupture strength and reheat cracking resistance.SOLUTION: The present invention provides an austenitic heat-resistant alloy weld joint in which a base material has a chemical composition, comprising C≤0.009%, Si≤2.0%, Mn≤3.0%, P≤0.040%, S: 0.0001-0.0100%, O≤0.01%, N≤0.020%, Cr: 25.0-38.0%, Ni: 40.0-60.0%, W: 3.0-10.0%, Ti: 0.01-1.20%, Al≤0.30%, B: 0.0001-0.010%, Zr: 0.0001-0.50%, Co: 0-1.0%, Cu: 0-1.0%, Mo: 0-1.0%, V: 0-0.5%, Nb: 0-0.5%, with a balance being Fe and impurities; the number density of Ti carbosulfides/sulfides of ≤φ50 nm is 50-500/μm3; a weld metal has a chemical composition, comprising C≤0.15%, Si≤2.0%, Mn≤3.0%, P≤0.040%, S≤0.0100%, O≤0.010%, N≤0.020%, Cr: 18.0-38.0%, Co: 0.5-25.0%, Ti: 0.10-2.50%, Al≤2.0%, Fe≤15.0%, B: 0.0001-0.010%, Zr: 0.0001-0.50%, W: 0.01-10.0%, Cu: 0-1.0%, Mo: 0-12.0%, V: 0-0.5%, Nb: 0-2.5%, with a balance being Ni and impurities.SELECTED DRAWING: None

Description

The present invention relates to an austenitic heat-resistant alloy welded joint, and more particularly to an austenitic heat-resistant alloy welded joint having excellent creep rupture strength and reheat cracking resistance.

In recent years, from the viewpoint of reducing the environmental load, the operating conditions of boilers for power generation have been increased to higher temperatures and pressures on a global scale, and austenitic heat-resistant alloy members used as materials for superheater tubes and reheater tubes have been used. Is required to have better creep rupture strength.

Against this technical background, various techniques related to austenitic heat-resistant alloys have been proposed. For example, in Patent Document 1, after surface processing is performed to form a plastic work-hardened layer having a temperature of 330 HV or more on the surface, sufficient recrystallization is generated on the cured surface portion and the inside of the recrystallized grains is formed. Alternatively, an austenitic alloy structure having improved intergranular corrosion resistance and stress corrosion cracking resistance by performing a local heat treatment for dispersing and precipitating Cr carbide at the grain boundaries and a method for producing the same are disclosed. ing.

Further, Patent Document 2 discloses a high Ni and high Cr stainless steel having improved hot workability by refining the crystal grains and suppressing S precipitated at the crystal grain boundaries. There is. Patent Document 3 proposes a Ni-based alloy product. This Ni-based alloy product uses W to increase high-temperature strength and controls the amount of effective B to improve hot workability and prevent weld cracking. This austenitic heat resistance is particularly suitable for large products. It is an alloy product.

Further, Patent Document 4 proposes an austenitic heat-resistant alloy in which the creep strength is increased by using the α-Cr phase as a strengthening phase by utilizing Cr, Ti and Zr, a heat-resistant pressure-resistant member made of the alloy, and a method for producing the same. ing. Patent Document 5 proposes a Ni-based heat-resistant alloy in which a large amount of W is contained and the strength is increased by strengthening the solid solution and strengthening the precipitation of the γ'phase by utilizing Al and Ti.

Further, Patent Documents 6 and 7 propose austenitic heat-resistant alloy members that are excellent in crackability during hot working and can be suitably used as thick-walled, large-sized high-temperature members. Patent Document 8 describes austenitic stainless steel, which can prevent both liquefaction cracking of HAZ and embrittlement cracking of HAZ, can prevent defects caused by welding workability that occur during welding work, and also have excellent creep strength at high temperatures. Austenitic heat resistant alloys have been proposed.

When these austenitic heat-resistant alloys are used as structures, they are generally assembled by welding. AWS A5.14-2005 ER NiCrCoMo-1 is known as a welding material for austenitic heat-resistant alloys used when assembling by welding.

Further, Patent Documents 9 to 11 propose various welding materials for austenitic heat-resistant alloys.

Patent Document 9 describes a welding material used for welding a high-strength oxide dispersion-strengthened alloy and a heat-resistant alloy, and by positively containing a solid-soluble reinforcing element such as Mo or Nb. Welding materials for oxide dispersion-strengthened alloys with improved strength have been proposed.

Patent Document 10 proposes a welding material for austenitic heat-resistant alloys, which has increased strength by utilizing solid solution strengthening with Mo and W and precipitation strengthening hardening with Al and Ti. Patent Document 11 proposes a welding material for austenitic heat-resistant alloys, which contains Nb and W to achieve both solidification cracking and creep strength during welding.

In Patent Document 12, a welding material for a Ni-based heat-resistant alloy having excellent high-temperature cracking resistance during welding, high-temperature cracking resistance during welding using the same, and stress-relaxing cracking resistance during long-term use at high temperature Welded joints made of a weld metal having good creep strength and a base material of a Ni-based heat-resistant alloy having excellent high-temperature strength are provided.

Japanese Unexamined Patent Publication No. 2000-265249 JP-A-2002-80942 Japanese Unexamined Patent Publication No. 2011-63838 International Publication No. 2009/154161 International Publication No. 2010/0388826 Japanese Unexamined Patent Publication No. 2014-34725 Japanese Unexamined Patent Publication No. 2014-145109 JP-A-2010-150593 Japanese Unexamined Patent Publication No. 10-193174 International Publication No. 2010/013565 Japanese Unexamined Patent Publication No. 2008-207242 Japanese Unexamined Patent Publication No. 2013-94827

Austenitic heat-resistant alloy welded joints used as materials for superheater tubes and reheater tubes have better creep breaking strength and are excellent in avoiding reheat cracking, which is a problem during post-weld heat treatment or use. It is also required to have reheat crack resistance. In general, it is difficult to obtain both better creep rupture strength and reheat crack resistance, and none of Patent Documents 1 to 12 has solved the above-mentioned problems, and there is room for improvement. ing.

An object of the present invention is to solve the above problems and to provide an austenitic heat-resistant alloy welded joint having excellent both creep rupture strength and reheat cracking resistance.

The present invention has been made to solve the above problems, and the following austenitic heat-resistant alloy welded joints are the gist of the present invention.

(1) A welded joint containing a base metal and a weld metal.
The chemical composition of the base material is mass%.
C: 0.009% or less,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.040% or less,
S: 0.0001 to 0.0100%,
O: 0.010% or less,
N: 0.020% or less,
Cr: 25.0 to 38.0%,
Ni: 40.0 to 60.0%,
W: 3.0 to 10.0%,
Ti: 0.01 to 1.20%,
Al: 0.30% or less,
B: 0.0001 to 0.010%,
Zr: 0.0001 to 0.50%,
Co: 0-1.0%,
Cu: 0-1.0%,
Mo: 0-1.0%,
V: 0-0.5%,
Nb: 0-0.5%,
Remaining: Fe and impurities,
The total number density of Ti carbon sulfides and Ti sulfides having a particle size of 50 nm or less contained in the base material is 50 to 500 / μm ³ .
The chemical composition of the weld metal is mass%.
C: 0.15% or less,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.040% or less,
S: 0.0100% or less,
O: 0.010% or less,
N: 0.020% or less,
Cr: 18.0 to 38.0%,
Co: 0.5-25.0%,
Ti: 0.10-2.50%,
Al: 2.0% or less,
Fe: 15.0% or less,
B: 0.0001 to 0.010%,
Zr: 0.0001 to 0.50%,
W: 0.01 to 10.0%,
Cu: 0-1.0%,
Mo: 0 to 12.0%,
V: 0-0.5%,
Nb: 0-2.5%,
Remaining: Ni and impurities,
Austenitic heat resistant alloy welded joint.

(2) The chemical composition of the base material is mass%.
Co: 0.01-1.0%,
Cu: 0.01-1.0%,
Mo: 0.01-1.0%,
V: 0.01-0.5%, and Nb: 0.01-0.5%,
Contains one or more selected from,
The austenitic heat-resistant alloy welded joint according to (1) above.

(3) The chemical composition of the base material is mass%.
C: 0.0001 to 0.009%,
Contains,
The austenitic heat-resistant alloy welded joint according to (1) or (2) above.

The austenitic heat-resistant alloy welded joint of the present invention is excellent in both reheat crack resistance and long-term creep rupture strength.

As a result of detailed investigation of the reheat cracking resistance and creep rupture characteristics of the austenitic heat-resistant alloy in order to solve the above-mentioned problems, the present inventors have obtained the following findings.

Generally, in order to obtain excellent creep rupture strength, it is considered necessary to strengthen the precipitation of grain boundaries by containing a predetermined amount or more of C. However, when a large amount of C is contained, grain boundary weakening occurs due to strengthening of precipitation in the grains due to carbides, which causes reheat cracking. That is, there is a so-called trade-off relationship between the reheat crack resistance and the creep rupture characteristics.

Therefore, as a result of studies by the present inventors to solve the above problems, it is possible to secure excellent creep rupture strength even if the C content is reduced by utilizing a precipitate such as the α-Cr phase. I found it possible. Then, by reducing the C content, the relative weakening of the grain boundaries caused by the strengthening of the precipitation of carbides in the grains is suppressed, and the reheat cracking resistance can be improved.

Furthermore, by precipitating extremely fine Ti carbon sulfide and / or Ti sulfide in a predetermined amount or more in the base material in advance, the α-Cr phase can be finely precipitated in the grains in the use environment. I found it. The α-Cr phase finely precipitated in the grains has a high effect of improving the creep rupture strength as compared with the case where the α-Cr phase is finely precipitated in the grain boundaries.

In addition, with welding, Ti charcoal sulfide and / or Ti sulfide is dissolved in the weld heat affected zone among the base materials. However, the elements contained in the Ti charcoal sulfide and / or the Ti sulfide do not completely diffuse, resulting in a state in which the concentration becomes non-uniform. Therefore, it is presumed that fine Ti carbon sulfide and / or Ti sulfide is likely to be reprecipitated in the usage environment, and as a result, the α-Cr phase can be finely precipitated in the grains.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

1. 1. Chemical composition of base material The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%".

C: 0.009% or less C is generally known to be an element that stabilizes austenite, forms fine carbides at grain boundaries, and improves creep rupture strength at high temperatures. However, in the present invention, if the C content becomes excessive, the reheat cracking resistance is lowered. Therefore, the C content is set to 0.009% or less. The C content is preferably 0.008% or less, and more preferably 0.005% or less.

It is not necessary to set a lower limit for the C content, but an extreme reduction causes an increase in manufacturing cost. Therefore, the C content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0008% or more.

Si: 2.0% or less Si is an element that has a deoxidizing effect and is effective in improving corrosion resistance and oxidation resistance at high temperatures. However, when Si is excessively contained, the stability of austenite is lowered, resulting in a decrease in toughness and creep rupture strength. Therefore, the Si content is set to 2.0% or less. The Si content is preferably 1.5% or less, more preferably 1.0% or less, and even more preferably 0.5% or less.

It is not necessary to set a lower limit for the Si content. However, if the Si content is extremely reduced, the deoxidizing effect cannot be sufficiently obtained, the cleanliness of the alloy becomes large, and the cleanliness deteriorates. In addition, it becomes difficult to obtain the effect of improving corrosion resistance and oxidation resistance at high temperatures, and the manufacturing cost is greatly increased. Therefore, the Si content is preferably 0.02% or more, and more preferably 0.05% or more.

Mn: 3.0% or less Mn is an element that not only has a deoxidizing effect but also contributes to the stabilization of austenite, like Si. However, an excessive Mn content causes embrittlement and further reduces toughness and creep ductility. Therefore, the Mn content is set to 3.0% or less. The Mn content is preferably 2.8% or less, more preferably 2.5% or less.

It is not necessary to set a lower limit for the Mn content. However, if the Mn content is extremely reduced, the deoxidizing effect cannot be sufficiently obtained and the cleanliness of the alloy is deteriorated. In addition, not only the hot workability is deteriorated, but also the austenite stabilizing effect becomes difficult to obtain, and the manufacturing cost is greatly increased. Therefore, the Mn content is preferably 0.005% or more, and more preferably 0.010% or more.

P: 0.040% or less P is contained in the alloy as an impurity, and when it is contained in a large amount, the hot workability and weldability are remarkably lowered, and the creep ductility after long-term use is also lowered. .. Therefore, the P content is set to 0.040% or less. The P content is preferably 0.030% or less, and more preferably 0.020% or less.

It is preferable to reduce the P content as much as possible, but the extreme reduction causes an increase in manufacturing cost. Therefore, the P content is preferably 0.0005% or more, and more preferably 0.0008% or more.

S: 0.0001 to 0.0100%
S is an element necessary for forming Ti charcoal sulfide and / or Ti sulfide which is a precipitation nucleus of the α-Cr phase which is a strengthening phase. In order to obtain this effect sufficiently, the S content needs to be 0.0001% or more. However, when a large amount of S is contained, the hot workability and weldability are remarkably lowered, and the creep ductility after long-term use is also lowered. Therefore, the S content is set to 0.0001 to 0.0100%. The S content is preferably 0.0003% or more, and more preferably 0.0005% or more. The S content is preferably 0.0090% or less, more preferably 0.0080% or less.

O: 0.010% or less O (oxygen) is contained in the alloy as an impurity, and if the content is excessive, the hot workability is lowered, and the toughness and ductility are further deteriorated. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.008% or less, and more preferably 0.005% or less.

It is not necessary to set a lower limit for the O content, but an extreme reduction causes an increase in manufacturing cost. Therefore, the O content is preferably 0.0005% or more, and more preferably 0.0008% or more.

N: 0.020% or less N is an element effective for stabilizing austenite, but if it is contained in excess, a large amount of fine nitrides are precipitated in the grains during use at high temperature and creep. It causes a decrease in ductility and toughness. Therefore, the N content is set to 0.020% or less. The N content is preferably 0.018% or less, more preferably 0.015% or less.

It is not necessary to set a lower limit for the N content. However, if the N content is extremely reduced, not only is it difficult to obtain the effect of stabilizing austenite, but also the manufacturing cost is greatly increased. Therefore, the N content is preferably 0.0005% or more, and more preferably 0.0008% or more.

Cr: 25.0 to 38.0%
Cr is an essential element for ensuring oxidation resistance and corrosion resistance at high temperatures. It also precipitates as an α-Cr phase and contributes to the improvement of creep rupture strength. In order to obtain the above effect, the Cr content needs to be 25.0% or more. However, if the Cr content exceeds 38.0%, the stability of austenite at high temperatures deteriorates, leading to a decrease in creep rupture strength. Therefore, the Cr content is set to 25.0 to 38.0%. The Cr content is preferably 25.5% or more, more preferably 26.0% or more. Further, the Cr content is preferably 37.5% or less, and more preferably 37.0% or less.

Ni: 40.0 to 60.0%
Ni is an effective element for obtaining austenite and is an essential element for ensuring tissue stability during long-term use. It also precipitates as Ni ₃ Ti and contributes to the improvement of creep rupture strength. In the above-mentioned Cr content range, in order to sufficiently obtain the above-mentioned effect of Ni, the Ni content needs to be 40.0% or more. However, Ni is an expensive element, and if it is contained in a large amount, the cost will increase. Therefore, the Ni content is set to 40.0 to 60.0%. The Ni content is preferably 41.0% or more, and more preferably 42.0% or more. The Ni content is preferably 58.0% or less, more preferably 56.0% or less.

W: 3.0 to 10.0%
W is an element that dissolves in the matrix and greatly contributes to the improvement of creep rupture strength at high temperatures. In order to fully exert the effect, the W content needs to be 3.0% or more. However, even if W is excessively contained, the effect is saturated and the creep rupture strength is rather lowered. Further, since W is an expensive element, if it is contained in an excessive amount, the cost will increase. Therefore, the W content is set to 3.0 to 10.0%. The W content is preferably 3.5% or more, and more preferably 4.0% or more. Further, the W content is preferably 9.5% or less, and more preferably 9.0% or less.

Ti: 0.01 to 1.20%
Ti is an element necessary for forming Ti charcoal sulfide and / or Ti sulfide which is a precipitation nucleus of the α-Cr phase which is a strengthening phase, and in addition, for forming Ni ₃ Ti which is a strengthening phase. Is also a necessary element. In order to obtain these effects, the Ti content needs to be 0.01% or more. However, if the Ti content is excessive, a large amount of carbonitride is precipitated, resulting in a decrease in creep ductility and toughness. Therefore, the Ti content is set to 0.01 to 1.20%. The Ti content is preferably 0.05% or more, and more preferably 0.10% or more. The Ti content is preferably 1.10% or less, and more preferably 1.00% or less.

Al: 0.30% or less Al is an element having a deoxidizing action. However, if the Al content is excessive, the cleanliness of the alloy is significantly deteriorated, and the hot workability and ductility are lowered. Therefore, the Al content is set to 0.30% or less. The Al content is preferably 0.20% or less, more preferably 0.10% or less.

It is not necessary to set a lower limit for the Al content. However, if the Al content is extremely reduced, the deoxidizing effect cannot be sufficiently obtained, the cleanliness of the alloy is adversely deteriorated, and the manufacturing cost is increased. Therefore, the Al content is preferably 0.0005% or more. It is more preferable that the Al content is 0.001% or more in order to stably obtain the deoxidizing effect of Al and to ensure good cleanliness.

B: 0.0001 to 0.010%
B is an element necessary for improving creep rupture strength by segregating at grain boundaries during use at high temperatures to strengthen the grain boundaries and finely dispersing carbides at the grain boundaries. In addition, it also contributes to the improvement of reheat crack resistance. In order to obtain these effects, the B content needs to be 0.0001% or more. However, if the B content is excessive, the weldability is deteriorated and the hot workability is deteriorated. Therefore, the B content is set to 0.0001 to 0.010%. The B content is preferably 0.0005% or more, and more preferably 0.001% or more. The B content is preferably 0.008% or less, and more preferably 0.006% or less.

Zr: 0.0001 to 0.50%
Zr has an action of improving creep rupture strength. That is, Zr is a grain boundary strengthening element, which contributes to the improvement of creep rupture strength at high temperature and further contributes to the improvement of creep ductility. In order to obtain this effect, the Zr content needs to be 0.0001% or more. However, if the Zr content exceeds 0.50%, the hot workability deteriorates. Therefore, the Zr content is set to 0.0001 to 0.50%. The Zr content is preferably 0.30% or less.

In the chemical composition of the austenitic heat-resistant alloy of the present invention, the balance is Fe and impurities. Since Fe is an inexpensive raw material, it is preferably contained in an amount of 0.1% to 20%. Further, here, the "impurity" is a component mixed by various factors of raw materials such as ore and scrap, and various factors in the manufacturing process when an alloy is industrially manufactured, and is within a range that does not adversely affect the present invention. Means what is acceptable.

The austenitic heat-resistant alloy of the present invention may further contain one or more elements selected from Co, Cu, Mo, V and Nb.

Co: 0-1.0%
Co has an action of improving creep rupture strength. That is, Co is an austenite-forming element like Ni, which enhances phase stability and contributes to improvement in creep rupture strength. Therefore, Co may be contained. However, since Co is an extremely expensive element, excessive inclusion of Co causes a significant cost increase. Therefore, the Co content is 1.0% or less. The Co content is preferably 0.8% or less. On the other hand, when the above effect is desired, the Co content is preferably 0.01% or more.

Cu: 0-1.0%
Cu has an action of improving creep rupture strength. That is, Cu is an austenite-forming element like Ni and Co, and contributes to the improvement of phase stability and the creep rupture strength. Therefore, Cu may be contained. However, if Cu is excessively contained, the hot workability is deteriorated. Therefore, the Cu content is 1.0% or less. The Cu content is preferably 0.8% or less. On the other hand, when the above effect is desired, the Cu content is preferably 0.01% or more.

Mo: 0-1.0%
Mo has an action of improving creep rupture strength. That is, Mo has an action of dissolving in a matrix to improve creep rupture strength at high temperature. Therefore, Mo may be contained. However, when Mo is excessively contained, the stability of austenite is lowered, and the creep rupture strength is rather lowered. Therefore, the Mo content is 1.0% or less. The Mo content is preferably 0.8% or less. On the other hand, when the above effect is desired, the Mo content is preferably 0.01% or more.

V: 0-0.5%
V has an action of improving creep rupture strength. That is, V has an action of combining with C or N to form fine carbides or carbonitrides and improving creep rupture strength. Therefore, V may be contained. However, when V is excessively contained, a large amount of V is precipitated as a carbide or a carbonitride, which causes a decrease in creep ductility. Therefore, the V content is set to 0.5% or less. The V content is preferably 0.4% or less. On the other hand, when the above effect is desired, the V content is preferably 0.01% or more.

Nb: 0-0.5%
Like V, Nb combines with C or N and precipitates in the grains as fine carbides or carbonitrides, which contributes to the improvement of creep rupture strength at high temperatures. Therefore, Nb may be contained. However, if the Nb content is excessive, a large amount of carbide or carbonitride is precipitated, resulting in a decrease in creep ductility and toughness. Therefore, the Nb content is set to 0.5% or less. The Nb content is preferably 0.4% or less. On the other hand, when the above effect is desired, the Nb content is preferably 0.01% or more.

The above-mentioned Co, Cu, Mo, V and Nb can be contained in only one of them or in combination of two or more. The total amount when these elements are compounded and contained may be 4.0%.

2. 2. Ti charcoal sulfide and Ti sulfide in the base material As described above, in the present invention, in order to obtain the long-term creep rupture strength of the austenitic heat-resistant alloy welded joint, it acts as a precipitation nucleus of the α-Cr phase. , It is necessary to appropriately control the total amount of fine Ti carbon sulfide and the number density of Ti sulfide. Specifically, it is necessary to control the total number density of Ti carbon sulfides of 50 nm or less and Ti sulfides contained in the base metal in the range of 50 to 500 pieces / μm ³ .

When the total number density of Ti charcoal sulfide and Ti sulfide in the base metal is less than 50 pieces / μm ³ , a sufficient α-Cr phase cannot be obtained and excellent creep rupture strength cannot be obtained. On the other hand, when the total number density of Ti charcoal sulfide and Ti sulfide in the base metal exceeds 500 pieces / μm ³ , the coarsening of the α-Cr phase is accelerated, and the creep rupture strength is rather lowered, and the creep rupture resistance is reduced. Not only thermal cracking, but also creep ductility and toughness deteriorate. From the viewpoint of improving the creep rupture strength, the total number density of Ti charcoal sulfide and Ti sulfide in the base metal is preferably 100 pieces / μm ³ or more.

In the present invention, the total number density of Ti carbon sulfides having a size of 50 nm or less and Ti sulfides contained in the base material is measured by a transmission electron microscope (TEM). Specifically, a thin film having a thickness of 100 nm is prepared from the sample and observed by TEM. The magnification at this time is 100,000 times. Then, the area of the Ti charcoal sulfide or the one specified as Ti sulfide is measured by image processing, and the total number of Ti charcoal sulfides and Ti sulfides having a circle equivalent diameter of 50 nm or less is measured. Then, the total number density is obtained by dividing the measured total number by the volume of the visual field. In the present invention, the “base material” refers to a portion excluding the welding heat affected zone.

3. 3. Chemical composition of weld metal In the welded joint of the present invention, the weld metal is
By mass%
C: 0.15% or less,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.040% or less,
S: 0.0100% or less,
O: 0.010% or less,
N: 0.020% or less,
Cr: 18.0 to 38.0%,
Co: 0.5-25.0%,
Ti: 0.10-2.50%,
Al: 2.0% or less,
Fe: 15.0% or less,
B: 0.0001 to 0.010%,
Zr: 0.0001 to 0.50%,
W: 0.01 to 10.0%,
Cu: 0-1.0%,
Mo: 0 to 12.0%,
V: 0-0.5%,
Nb: 0-2.5%,
Remaining: Has a chemical composition of Ni and impurities.

In the present invention, the chemical composition of the weld metal refers to the chemical composition of the first layer portion of the welded joint.

Among the above, the C content is preferably 0.01% or more, and preferably 0.14% or less. The Si content is preferably 0.01% or more, and preferably 1.8% or less. The Mn content is preferably 0.01% or more, and preferably 2.8% or less. It is preferable that the P content is 0.030% or less, the S content is 0.008% or less, and the O content is 0.008% or less.

The N content is preferably 0.0005% or more, and preferably 0.018% or less. The Cr content is preferably 18.5% or more, and preferably 37.5% or less. The Co content is preferably 1.0% or more, and preferably 24.0% or less. The Ti content is preferably 0.15% or more, and preferably 2.40% or less.

Further, the Al content is preferably 0.25% or more, and preferably 1.8% or less. The Fe content is preferably 0.01% or more, and preferably 10.0% or less.

The B content is preferably 0.0002% or more, and preferably 0.0090% or less. The Zr content is preferably 0.0010% or more, and preferably 0.40% or less. The W content is preferably 0.10% or more, and preferably 9.0% or less.

The Cu content is preferably 0.01% or more, and preferably 0.8% or less. The Mo content is preferably 0.01% or more, and preferably 9.5% or less. The V content is preferably 0.01% or more, and preferably 0.4% or less. The Nb content is preferably 0.01% or more, and preferably 2.3% or less.

The chemical composition of the weld metal is determined by the inflow ratio of the base metal and the welding material at the time of welding. The suitable chemical composition of the welding material used for manufacturing the welded joint according to the present invention will be described below.

4. Chemical composition of welding material The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%".

C: 0.06 to 0.15%
C is an element that has the effect of enhancing the phase stability in the weld metal after welding, and also has the effect of forming fine carbides and improving the creep strength during high-temperature use. Furthermore, by forming eutectic carbides with Cr during welding solidification, it also contributes to the reduction of solidification crack sensitivity. However, if the C content is excessive, a large amount of carbides are precipitated, which may rather reduce the creep strength and ductility. Therefore, the C content is preferably 0.06 to 0.15%. The C content is more preferably 0.07% or more, and further preferably 0.08% or more. Further, the C content is more preferably 0.14% or less, and further preferably 0.12% or less.

Si: 1.0% or less Si is an element that is effective for deoxidation during the production of welding materials and is also effective for improving the corrosion resistance and oxidation resistance of the weld metal after welding at high temperatures. However, when Si is excessively contained, the phase stability is lowered, which may lead to a decrease in toughness and creep strength. Therefore, the Si content is preferably 1.0% or less. The Si content is more preferably 0.8% or less, and further preferably 0.6% or less.

It is not necessary to set a lower limit for the Si content, but if it is extremely reduced, the deoxidizing effect cannot be sufficiently obtained, the cleanliness of the alloy deteriorates, and the effect of improving corrosion resistance and oxidation resistance at high temperatures is improved. Will be difficult to obtain, and the manufacturing cost will increase significantly. Therefore, the Si content is preferably 0.01% or more, and more preferably 0.03% or more.

Mn: 2.0% or less Mn, like Si, is an element effective for deoxidation during the production of welding materials. Mn also contributes to the improvement of phase stability in the weld metal after welding. However, if the Mn content is excessive, embrittlement may occur, and further, toughness and creep ductility may decrease. Therefore, the Mn content is preferably 2.0% or less. The Mn content is more preferably 1.8% or less, and even more preferably 1.5% or less.

It is not necessary to set a lower limit for the Mn content, but if it is extremely reduced, the deoxidizing effect cannot be sufficiently obtained, the cleanliness of the alloy deteriorates, and the effect of improving the phase stability becomes difficult to obtain. In addition, the manufacturing cost will increase significantly. Therefore, the Mn content is preferably 0.02% or more, and more preferably 0.05% or more.

P: 0.030% or less P is an element that is contained in the welding material as an impurity and enhances the susceptibility to solidification and cracking during welding. Further, it reduces the creep ductility of the weld metal after long-term use at high temperature. Therefore, the P content is preferably 0.030% or less. The content of P is more preferably 0.025% or less, and further preferably 0.020% or less.

It is preferable to reduce the P content as much as possible, but the extreme reduction causes an increase in manufacturing cost. Therefore, the P content is preferably 0.0005% or more, and more preferably 0.0008% or more.

S: 0.0100% or less S is an element that is contained in the welding material as an impurity like P and enhances the susceptibility to solidification and cracking during welding. Further, S segregates at columnar grain boundaries during long-term use in the weld metal, causing embrittlement and increasing the sensitivity to reheat cracking. Therefore, the S content is preferably 0.0100% or less. The S content is more preferably 0.0080% or less, and further preferably 0.0050% or less.

It is preferable to reduce the S content as much as possible, but the extreme reduction causes an increase in manufacturing cost. Therefore, the S content is preferably 0.0001% or more, and more preferably 0.0002% or more.

O: 0.010% or less O (oxygen) is contained in the welding material as an impurity, and if the content is excessive, the hot workability is lowered and the manufacturability may be deteriorated. Therefore, the O content is preferably 0.010% or less. The O content is more preferably 0.008% or less, and further preferably 0.005% or less.

It is not necessary to set a lower limit for the O content, but an extreme reduction causes an increase in manufacturing cost. Therefore, the content of O is preferably 0.0005% or more, and more preferably 0.0008% or more.

N: 0.020% or less N is an element that stabilizes the structure in the weld metal, improves the creep strength, and dissolves in solid solution to contribute to ensuring the tensile strength. However, if it is excessively contained, a large amount of fine nitrides may be precipitated in the grains during use at a high temperature, resulting in deterioration of creep ductility and toughness. Therefore, the N content is preferably 0.020% or less. The N content is more preferably 0.018% or less, and further preferably 0.015% or less.

It is not necessary to set a lower limit for the N content, but if it is extremely reduced, it becomes difficult to obtain the effect of improving the phase stability, and the manufacturing cost also increases significantly. Therefore, the content of N is preferably 0.0005% or more, and more preferably 0.0008% or more.

Cr: 18.0-27.0%
Cr is an element effective for ensuring the high temperature oxidation resistance and corrosion resistance of the weld metal after welding. In addition, Cr forms fine carbides and contributes to ensuring creep strength. Furthermore, by forming eutectic carbides with C during welding, it also contributes to the reduction of solidification crack sensitivity. However, if the Cr content exceeds 27.0%, the phase stability at high temperature may deteriorate and the creep strength may decrease. Therefore, the Cr content is preferably 18.0 to 27.0%. The Cr content is more preferably 18.5% or more, and even more preferably 19.0% or more. Further, the Cr content is more preferably 26.5% or less, and further preferably 26.0% or less.

Co: 8.0-25.0%
Like Ni, Co enhances the stability of the structure and contributes to the improvement of creep strength. However, since Co is an extremely expensive element, excessive content in the welding material causes a significant increase in cost. Therefore, the Co content is set to 8.0 to 25.0%. The Co content is preferably 8.5% or more, and more preferably 9.0% or more. The Co content is preferably 23.5% or less, and more preferably 22.0% or less.

Ti: 0.1 to 2.5%
Ti is an element that precipitates as a fine intermetallic compound phase and contributes to the improvement of creep strength and tensile strength at high temperatures. However, if the Ti content is excessive, a large amount of the intermetallic compound phase may be precipitated, which may lead to a decrease in creep ductility and toughness. Therefore, the Ti content is preferably 0.1 to 2.5%. The Ti content is more preferably 0.15% or more, and even more preferably 0.2% or more. Further, the Ti content is more preferably 2.4% or less, and further preferably 2.3% or less.

Al: 0.2-2.0%
Like Ti, Al is an element that precipitates as a fine intermetallic compound phase and contributes to the improvement of creep strength and tensile strength at high temperatures. However, if the Al content is excessive, a large amount of the intermetallic compound phase may be precipitated, which may lead to a decrease in creep ductility and toughness. Therefore, the Al content is preferably 0.2 to 2.0%. The Al content is more preferably 0.25% or more, and further preferably 0.3% or more. Further, the Al content is more preferably 1.8% or less, and further preferably 1.6% or less.

Fe: 0 to 15.0%
Since Fe is an element having an effect of improving the hot workability when it is contained in a Ni-based alloy even in a small amount, it may be contained in a welding material and the effect may be utilized. However, when the Fe content becomes excessive, the coefficient of thermal expansion of the weld metal increases and the steam oxidation resistance also deteriorates. Therefore, the Fe content is preferably 15.0% or less. The Fe content is preferably 10.0% or less, more preferably 8.0% or less. When the above effect is to be obtained, the Fe content is preferably 0.01% or more, and more preferably 0.02% or more.

B: 0 to 0.005%
Since B is an element effective for improving the creep strength of the weld metal, it may be contained. However, when the B content becomes excessive, the susceptibility to solidification cracking during welding becomes significantly high. Therefore, the B content is preferably 0.005% or less. The B content is more preferably 0.004% or less, and further preferably 0.003% or less. When the above effect is desired, the content of B is preferably 0.0001% or more, and more preferably 0.0005% or more.

W: 0 to 10.0%
W may be contained because it is an element that dissolves in a matrix and has an effect of improving creep strength and tensile strength at high temperatures. However, even if W is excessively contained, the effect is saturated and the creep strength may be lowered. Moreover, since it is an expensive element, if it is contained in an excessive amount, the cost will increase. Therefore, the W content is preferably 10.0% or less. The W content is more preferably 9.0% or less, and even more preferably 8.0% or less. When the above effect is to be obtained, the W content is preferably 0.01% or more, and more preferably 0.03% or more.

Mo: 0 to 12.0%
Like W, Mo is an element that dissolves in a matrix of weld metals and has an effect of improving creep strength and tensile strength at high temperatures, and therefore may be contained. However, even if Mo is contained in an excessive amount, the effect is saturated and the creep strength may be lowered. Furthermore, since it is an expensive element, its excessive content causes an increase in cost. Therefore, the Mo content is preferably 12.0% or less. The Mo content is more preferably 11.0% or less, and further preferably 10.0% or less. When the above effect is desired, the Mo content is preferably 0.01% or more, more preferably 0.03% or more.

Nb: 0-0.5%
Like Ti, Nb may be contained because it combines with C or N and precipitates in the grains as fine carbides or carbonitrides, which contributes to the improvement of creep strength at high temperatures. However, if the Nb content is excessive, a large amount of carbide or carbonitride may be precipitated, resulting in a decrease in creep ductility and toughness. Therefore, the Nb content is preferably 0.5% or less. The Nb content is more preferably 0.48% or less, and further preferably 0.45% or less. When the above effect is to be obtained, the Nb content is preferably 0.01% or more, and more preferably 0.03% or more.

In the chemical composition of the welding material, the balance is Ni and impurities. Here, the "impurity" is a component mixed with raw materials such as ore and scrap and various factors in the manufacturing process when an alloy is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something.

5. Manufacturing Method There is no particular limitation on the manufacturing method of the base metal used for manufacturing the austenitic heat-resistant alloy welded joint of the present invention, but for example, hot working is performed on a steel ingot or slab having the above-mentioned chemical composition. Can be manufactured by. Further, after the hot working, if necessary, hot working by a different method such as hot extrusion may be further performed.

Further, after the above steps, in order to precipitate fine Ti carbon sulfide and / or Ti sulfide, after heating and holding in a temperature range of 1100 to 1250 ° C., the average cooling rate to 300 ° C. is 0. Cool to room temperature under conditions of 1-5 ° C / s.

Alternatively, in order to increase the amount of fine Ti charcoal sulfide and / or Ti sulfide precipitated, it is preferable to perform cooling in two steps described below after the above steps.

First, cooling is performed from a temperature range of 1100 to 1250 ° C. to a temperature range of 500 to 1000 ° C. at an average cooling rate of 0.01 to 1 ° C./s (first cooling step). Then, it is held in the temperature range for 1 to 10 hours (holding step). Subsequently, the product is cooled to room temperature under the condition that the average cooling rate from the temperature range of 500 to 1000 ° C. to 300 ° C. is 0.01 to 1 ° C./s (second cooling step).

Further, there is no particular limitation on the manufacturing method of the welded joint. Manufactured by welding the above base metal. The welding method is not particularly limited, and for example, gas tungsten arc welding, gas metal arc welding, shielded metal arc welding, and the like can be used.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

Austenitic heat-resistant alloys 1 to 16 having the chemical compositions shown in Table 1 were melted in a laboratory to prepare an ingot. Then, the ingot was formed by hot forging and rolling, and then subjected to final heat treatment under the conditions shown in Table 2 to obtain an austenitic heat-resistant alloy plate.

The heating temperature in the final heat treatment is the same as the "cooling start temperature" in the "first cooling step" shown in Table 2. When performing two-stage cooling (two-stage cooling), the average cooling rate in the first cooling step means the average cooling rate between the cooling start temperature and the holding temperature, and the average cooling in the second cooling step. Rate means the average cooling rate between the holding temperature and 300 ° C. Further, in the case of performing one-step cooling (one-step cooling), the average cooling rate in the first cooling step means the average cooling rate between the cooling start temperature and 300 ° C.

Then, a TEM observation test piece was cut out from each alloy plate, and the total number density of Ti charcoal sulfide and Ti sulfide was measured. Specifically, a thin film having a thickness of 100 nm was prepared from each test material and observed by TEM. The magnification at this time was 100,000 times. Then, the area of the Ti charcoal sulfide or the one identified as Ti sulfide was measured by image processing, and the total number of Ti charcoal sulfides and Ti sulfides having a circle equivalent diameter of 50 nm or less was measured. Then, the total number density was obtained by dividing the measured total number by the volume of the visual field. The results are also shown in Table 2.

Next, two test materials were obtained from each alloy plate. One of them had a thickness of 15 mm, a width of 50 mm and a length of 100 mm, and one had a thickness of 32 mm, a width of 150 mm and a length of 200 m.

Further, a welding material having an outer diameter of 1.2 mm is obtained by hot forging, rolling and machining from an ingot in which alloys having the chemical compositions shown in Table 3 (welding materials W to Z) are melted and cast in a laboratory. (Welding wire) was produced.

The test material having a thickness of 15 mm is machined with a V groove having an angle of 30 ° in the longitudinal direction and a root thickness of 1 mm, and then multi-layer welding is performed in the groove by TIG welding using the welding material described above. A welded joint was manufactured.

Then, in the above-mentioned welded joint, the chemical composition of the first layer portion of the weld metal was measured. Table 4 shows the chemical composition of the weld metal of each welded joint.

Then, a round bar creep rupture test piece having a diameter of 6 mm and a gauge point distance of 30 mm described in JIS Z 2241 (2011) was collected from each weld joint so that the weld metal was in the center of the parallel portion, and the temperature was 750 ° C. and 110 MPa. A creep rupture test was conducted under the conditions of. The test was carried out in accordance with JIS Z 2271 (2010). Those having a creep rupture time of 1000 or more were regarded as acceptable (◯), and those having a creep rupture time of less than 1000 hours were evaluated as rejected (x).

On the other hand, for an alloy plate for a weld base material having a thickness of 32 mm, in order to reproduce a severe stress state in a complicated welded portion shape, a test piece conforming to the y-type weld crack test piece described in JIS Z 3158 (1993) is used. It was manufactured by machining, and single-layer welding was performed on the groove by TIG welding to prepare a welded joint.

The obtained welded joint was subjected to an aging heat treatment at 700 ° C. for 500 hours and subjected to the next test. The cross section of the sample collected from each of the five welded joints was mirror-polished and corroded, and then examined with an optical microscope to investigate the presence or absence of cracks in the weld heat-affected zone. Then, the welded joint without cracks in all five samples was marked with "◯", and when cracks were found in at least one of the five samples, it was marked with "x".

The results are summarized in Table 5.

As shown in Table 5, the total number density of Ti carbon sulfides having a diameter of 50 nm or less and Ti sulfides is within the specified range of the present invention. Nos. 1 to 19 showed good results in both creep rupture strength and reheat crack resistance. On the other hand, the total number density of Ti carbon sulfide and Ti sulfide is less than the specified value of the present invention. No. 20 did not obtain sufficient creep rupture strength. In addition, Test No. in which the total number density of Ti charcoal sulfide and Ti sulfide exceeds the specified value of the present invention. No. 21 did not have sufficient creep rupture strength and reheat cracking resistance.

The austenitic heat-resistant alloy welded joint of the present invention is excellent in both reheat crack resistance and long-term creep rupture strength. Therefore, the austenite-based heat-resistant alloy welded joint of the present invention is not only used as a superheater pipe, a reheater pipe, etc. of a boiler for power generation, but also a large-diameter, thick-walled high-temperature member such as a main steam pipe and a reheated steam pipe. Suitable for use as.

Claims (3)

A welded joint containing a base metal and a weld metal.
The chemical composition of the base material is mass%.
C: 0.009% or less,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.040% or less,
S: 0.0001 to 0.0100%,
O: 0.010% or less,
N: 0.020% or less,
Cr: 25.0 to 38.0%,
Ni: 40.0 to 60.0%,
W: 3.0 to 10.0%,
Ti: 0.01 to 1.20%,
Al: 0.30% or less,
B: 0.0001 to 0.010%,
Zr: 0.0001 to 0.50%,
Co: 0-1.0%,
Cu: 0-1.0%,
Mo: 0-1.0%,
V: 0-0.5%,
Nb: 0-0.5%,
Remaining: Fe and impurities,
The total number density of Ti carbon sulfides and Ti sulfides having a particle size of 50 nm or less contained in the base material is 50 to 500 / μm ³ .
The chemical composition of the weld metal is mass%.
C: 0.15% or less,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.040% or less,
S: 0.0100% or less,
O: 0.010% or less,
N: 0.020% or less,
Cr: 18.0 to 38.0%,
Co: 0.5-25.0%,
Ti: 0.10-2.50%,
Al: 2.0% or less,
Fe: 15.0% or less,
B: 0.0001 to 0.010%,
Zr: 0.0001 to 0.50%,
W: 0.01 to 10.0%,
Cu: 0-1.0%,
Mo: 0 to 12.0%,
V: 0-0.5%,
Nb: 0-2.5%,
Remaining: Ni and impurities,
Austenitic heat resistant alloy welded joint. The chemical composition of the base material is mass%.
Co: 0.01-1.0%,
Cu: 0.01-1.0%,
Mo: 0.01-1.0%,
V: 0.01-0.5%, and Nb: 0.01-0.5%,
Contains one or more selected from,
The austenitic heat-resistant alloy welded joint according to claim 1. The chemical composition of the base material is mass%.
C: 0.0001 to 0.009%,
Contains,
The austenitic heat-resistant alloy welded joint according to claim 1 or 2.