JP2021167439A - Austenitic heat-resistant alloy weld joint - Google Patents

Abstract

To provide an austenitic heat-resistant alloy weld joint having excellent creep rupture strength.SOLUTION: An austenitic heat-resistant alloy weld joint has a base material having a chemical composition consisting of C: 0.01-0.15%, 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 the balance being Fe and impurities. In the base material and a heat affected zone, the number density of Ti carbosulfides/sulfides of ≤φ50 nm is 50-500/μm3. The weld metal has a chemical composition consisting of C≤0.18%, Si≤2.0%, Mn≤3.0%, P≤0.040%, S≤0.0100%, O≤0.020%, Cr: 20.0-38.0%, Ni: 40.0-60.0%, Mo+W: 3.0-13.0%, Ti: 0.01-1.50%, N≤0.20%, Al≤1.5%, B: 0.0001-0.01%, Zr: 0.0001-0.50%, Co: 0-1.0%, Cu: 0-1.0%, V: 0-0.5%, Nb: 0-0.5%, with the balance being Fe 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 breaking strength.

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 austenite-based alloy structure having improved intergranular corrosion resistance and stress corrosion cracking resistance by performing a local heat treatment for dispersing and precipitating Cr carbides 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 welding cracks.Austenitic heat resistance, which 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 which 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, prevent defects caused by welding workability that occur during welding work, and 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 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 an austenitic heat-resistant alloy, 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 , And a welded metal having good creep strength, and a welded joint made of a base material of a Ni-based heat-resistant alloy having excellent high-temperature strength.

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 Japanese Unexamined Patent Publication No. 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 or reheater tubes are required to have better creep rupture strength. However, as a result of verification by the present inventors, it has become clear that there is room for improvement in any of Patent Documents 1 to 12.

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

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 material and a weld metal, and a welding heat-affected zone is formed on the base material side of the boundary between the base material and the weld metal.
The chemical composition of the base material is mass%.
C: 0.01-0.15%,
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 charcoal sulfides and Ti sulfides having a particle size of 50 nm or less contained in the base metal and the heat-affected zone of welding is 50 to 500 / μm ³ .
The chemical composition of the weld metal is mass%.
C: 0.18% or less,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.040% or less,
S: 0.0100% or less,
O: 0.020% or less,
Cr: 20.0 to 38.0%,
Ni: 40.0 to 60.0%,
Total of one or more selected from Mo and W: 3.0 to 13.0%,
Ti: 0.01 to 1.50%,
N: 0.20% or less,
Al: 1.5% or less,
B: 0.0001 to 0.01%,
Zr: 0.0001 to 0.50%,
Co: 0-1.0%,
Cu: 0-1.0%,
V: 0-0.5%,
Nb: 0-0.5%,
Remaining: Fe 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.

The austenitic heat-resistant alloy welded joint of the present invention has excellent long-term creep rupture strength.

As a result of detailed investigation of the 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.

It has been found that the α-Cr phase can be finely precipitated in the grains in the usage environment by precipitating extremely fine Ti carbon sulfide and / or Ti sulfide in a predetermined amount or more in the base material. .. 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, a part or all of Ti carbon sulfide and / or Ti sulfide is dissolved in the welding heat-affected zone. However, by controlling the number density of Ti charcoal sulfide and / or Ti sulfide to an appropriate size, it remains in a state where it is not completely dissolved. Therefore, fine Ti charcoal sulfide and / or Ti sulfide can be redeposited by performing post-welding heat treatment under predetermined conditions.

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: 0.01 to 0.15%
C stabilizes austenite and forms fine carbides at the grain boundaries to improve creep rupture strength at high temperatures. It also contributes to the formation of Ti charcoal sulfide. In order to obtain these effects sufficiently, the C content needs to be 0.01% or more. However, when C is excessively contained, carbides or Ti carbon sulfides become coarse and precipitate in a large amount, so that the ductility of grain boundaries is lowered, and the toughness and creep rupture strength are also lowered. .. Therefore, the C content is set to 0.01 to 0.15%. The C content is preferably 0.03% or more, and more preferably 0.05% or more. The C content is preferably 0.12% or less, more preferably 0.10% or less.

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 amount of Mn 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, more preferably 0.020% or less.

It is preferable to reduce the P content as much as possible, but an extreme reduction leads to 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, 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 production 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, and more preferably 26.0% or more. The Cr content is preferably 37.5% or less, more preferably 37.0% or less.

Ni: 40.0 to 60.0%
Ni is an element effective 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, it is necessary to set the Ni content to 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, it is necessary to set the W content to 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. The W content is preferably 9.5% or less, 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 core 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, more preferably 0.10% or more. The Ti content is preferably 1.10% or less, more preferably 1.00% or less.

Al: 0.30% or less Al is an element having a deoxidizing action. However, when the Al content becomes 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, and 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. In order to stably obtain the deoxidizing effect of Al and ensure good cleanliness, the Al content is more preferably 0.001% or more.

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 temperature to strengthen the grain boundaries and finely dispersing carbides at the grain boundaries. In order to obtain these effects, the B content needs to be 0.0001% or more. However, when the B content becomes excessive, in addition to the deterioration of weldability, 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, 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 of creep rupture strength. Therefore, Co may be contained. However, since Co is an extremely expensive element, excessive inclusion of Co causes a significant increase in cost. 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 amount of Nb 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 contain only one of them or a combination of two or more of them. The total amount when these elements are compounded and contained may be 4.0%.

2. Ti charcoal sulfide and Ti sulfide in the base metal and the weld heat-affected zone The welded joint according to the present invention contains the base metal and the weld metal, and on the base metal side of the boundary between the base metal and the weld metal, Welding heat affected zone is formed. 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, the number of fine Ti charcoal sulfides and Ti sulfides that act as precipitation nuclei of the α-Cr phase. The total amount of density needs to be controlled appropriately. Specifically, it is necessary to control the total number density of Ti carbon sulfides having a diameter of 50 nm or less and Ti sulfides contained in the base metal and the heat-affected zone of welding in ^{the range of 50 to 500 pieces / μm 3.}

By setting the total number density of Ti carbon sulfide and Ti sulfide in the base metal and the heat-affected zone of welding to 50 pieces / μm ³ or more, a sufficient amount of α-Cr phase is generated and excellent creep rupture occurs. Strength can 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} , it does not function effectively as a precipitated nucleus of the α—Cr phase, and the creep rupture strength is rather lowered. 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 diameter of 50 nm or less and Ti sulfides contained in the base metal and the heat-affected zone of welding is measured by a transmission electron microscope (TEM). Specifically, a region having an average thickness of about 100 nm or less in the photographing field of view is observed by TEM from each of the base material and the heat-affected zone of welding. The resolution at this time is 0.5 nm or less. When the image is taken at a resolution of 1024 pixels × 1024 pixels, the magnification corresponds to 100,000 times or more.

In the image obtained here, even extremely minute precipitates can be identified, but the substantially lower limit of the circle-equivalent diameter of the identifiable precipitates is 1 nm. 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. The number density is measured from a region where the total volume of the imaging field of view is 1 μm ^{3 or more.} 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. As for the heat-affected zone of welding, a thin film for measurement is cut out at a position 1 mm from the welding fusion line, which is the boundary between the base metal and the weld metal, to the base metal side.

3. 3. Chemical composition of weld metal In the welded joint of the present invention, the weld metal is
By mass%
C: 0.18% or less,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.040% or less,
S: 0.0100% or less,
O: 0.020% or less,
Cr: 20.0 to 38.0%,
Ni: 40.0 to 60.0%,
Total of one or more selected from Mo and W: 3.0 to 13.0%,
Ti: 0.01 to 1.50%,
N: 0.20% or less,
Al: 1.5% or less,
B: 0.0001 to 0.01%,
Zr: 0.0001 to 0.50%,
Co: 0-1.0%,
Cu: 0-1.0%,
V: 0-0.5%,
Nb: 0-0.5%,
Remaining: Has a chemical composition of Fe 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 0.15% or less. The Si content is preferably 0.02% or more, and preferably 1.0% or less. The Mn content is preferably 0.02% or more, and preferably 1.8% 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 Cr content is preferably 20.5% or more, and preferably 32.5% or less. The Ni content is preferably 40.5% or more, and preferably 59.5% or less. The total content of one or more selected from Mo and W is preferably 6.5% or more, and preferably 12.5% or less. The Ti content is preferably 0.06% or more, and preferably 1.30% or less.

Further, the N content is preferably 0.0005% or more, and preferably 0.16% or less. The Al content is preferably 0.0005% or more, and preferably 1.4% or less. The B content is preferably 0.0002% or more, and preferably 0.009% or less. The Zr content is preferably 0.001% or more, and preferably 0.40% or less.

The Co content is preferably 0.01% or more, and preferably 0.8% or less. The Cu content is preferably 0.01% or more, and preferably 0.8% 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 0.4% 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 0.18%
C is an element that has the effect of stabilizing austenite 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 cracking susceptibility. 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 0.18%. The C content is more preferably 0.07% or more, and further preferably 0.08% or more. Further, the C content is more preferably 0.16% or less, and further preferably 0.14% 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 stability of austenite 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 0.02% or more, and more preferably 0.05% 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 stabilization of austenite 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 austenite stabilizing effect becomes difficult to obtain. Manufacturing costs will also rise significantly. Therefore, the Mn content is preferably 0.02% or more, and more preferably 0.05% or more.

P: 0.040% 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. In addition, it reduces the creep ductility of the weld metal after long-term use at high temperatures. Therefore, the P content is preferably 0.040% or less. The content of P is more preferably 0.030% or less, and further preferably 0.020% or less.

It is preferable to reduce the P content as much as possible, but an extreme reduction leads to 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 the columnar grain boundaries during long-term use in the weld metal, causing embrittlement and increasing the susceptibility 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.

The S content is preferably reduced as much as possible, but an extreme reduction leads to 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.

Ni: 40.0 to 60.0%
Ni is an element effective for stabilizing austenite in the weld metal after welding, and is an element for ensuring creep strength during long-term use. However, Ni is an expensive element, and even in a welding material manufactured on a small scale, if it is contained in a large amount, the cost will increase. Therefore, the Ni content is preferably 40.0 to 60.0%. The Ni content is more preferably 40.5% or more, further preferably 41.0% or more. Further, the Ni content is more preferably 59.5% or less, and further preferably 59.0% or less.

Cr: 20.0 to 33.0%
Cr is an element effective for ensuring the oxidation resistance and corrosion resistance of the weld metal after welding at high temperatures. In addition, Cr forms a bcc phase enriched with fine carbides or Cr, which also contributes to ensuring creep strength. Furthermore, by forming eutectic carbides with C during welding, it also contributes to the reduction of solidification crack susceptibility. However, if the Cr content exceeds 33.0%, the stabilization of austenite at high temperatures deteriorates in the above Ni content range, which may lead to a decrease in creep strength. Therefore, the Cr content is preferably 20.0 to 33.0%. The Cr content is more preferably 20.5% or more, and even more preferably 21.0% or more. Further, the Cr content is more preferably 32.5% or less, and further preferably 32.0% or less.

Total of one or more types of Mo and W: 6.0 to 13.0%
Mo and W are elements that dissolve in a matrix in a weld metal or form fine intermetallic compounds, which greatly contribute to the improvement of creep strength and tensile strength at high temperatures. However, even if these elements are excessively contained, the effect is saturated and the creep strength may be lowered. Further, since Mo and W are expensive elements, if they are contained in an excessive amount, the cost will increase. Therefore, the total content of one or more selected from Mo and W is preferably 6.0 to 13.0%. The total content is more preferably 6.5% or more, and even more preferably 7.0% or more. Further, the total content is more preferably 12.5% or less, and further preferably 12.0% or less.

Ti: 0.05 to 1.50%
Ti is an element that is precipitated in the grain as a fine carbonitride in the weld metal and further as an intermetallic compound with Ni, and contributes to the improvement of creep strength and tensile strength at high temperature. However, if the Ti content is excessive, a large amount of carbonitride may be precipitated, which may lead to a decrease in creep ductility and toughness. Therefore, the Ti content is preferably 0.05 to 1.50%. The Ti content is more preferably 0.06% or more, and further preferably 0.07% or more. Further, the Ti content is more preferably 1.30% or less, and further preferably 1.10% or less.

N: 0.18% or less N is an element that stabilizes austenite in the weld metal, improves creep strength, and dissolves in solid solution to contribute to ensuring tensile strength. However, if it is contained in an excessive amount, a large amount of fine nitrides may be precipitated in the grains during use at a high temperature, resulting in a decrease in creep ductility and toughness. Therefore, the N content is preferably 0.18% or less. The N content is more preferably 0.16% or less, and further preferably 0.14% 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 stabilizing austenite, 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.

Al: 1.5% or less Al is an element effective for deoxidation during the production of welding materials. In addition, it forms a fine intermetallic compound phase in the weld metal and contributes to the improvement of creep strength. However, if the Al content is excessive, the cleanliness of the alloy is significantly deteriorated, and the hot workability and ductility of the welding material are lowered, so that the manufacturability may be lowered. In addition, a large amount of intermetallic compound phase may be formed in the weld metal, which may significantly increase the susceptibility to reheat cracking when used at a high temperature for a long time. Therefore, the Al content is preferably 1.5% or less. The Al content is more preferably 1.4% or less, and further preferably 1.3% or less.

It is not necessary to set a lower limit for the Al content, but if it is extremely reduced, the deoxidizing effect cannot be sufficiently obtained, the cleanliness of the alloy is deteriorated, and the manufacturing cost is greatly increased. Therefore, the Al content is preferably 0.0005% or more, and more preferably 0.001% 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.

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 desired, 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 Fe 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 charcoal sulfide and / or Ti sulfide, after heating and holding in a temperature range of 1100 to 1250 ° C., the average cooling rate up to 300 ° C. is 0. Cool to room temperature under conditions of 1 to 5.0 ° C./s. Dislocations imparted by hot working become one of the typical precipitation sites, and Ti charcoal sulfide and / or Ti sulfide becomes finely dispersed.

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 1.0 ° 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 1.0 ° C./s (second cooling step).

Further, there is no particular limitation on the method of manufacturing 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.

As described above, with welding, a part or all of Ti charcoal sulfide and / or Ti sulfide is dissolved in the weld heat affected zone. Here, if the precipitate remains without being completely dissolved, it is possible to reprecipitate fine Ti charcoal sulfide and / or Ti sulfide by performing post-welding heat treatment under predetermined conditions. Become. Therefore, for the purpose of reprecipitating Ti charcoal sulfide and / or Ti sulfide, post-weld heat treatment is performed on the welded portion including the weld heat affected zone. If all of the precipitate is dissolved, it is difficult to reprecipitate it thereafter. This is because the dislocations that become precipitation sites disappear by welding.

Specifically, the heat treatment is performed under the conditions that satisfy the following equations (i) and (ii).
850 ≤ T ≤ 1200 ... (i)
−0.1 × T + 140 ≦ t ≦ −0.6 × T + 780 ・ ・ ・ (ii)
However, in the above formula, T means the holding temperature (° C.) and t means the holding time (min).

By setting the holding temperature in the post-weld heat treatment in the range of 850 to 1200 ° C., it is possible to reprecipitate Ti charcoal sulfide and / or Ti sulfide in the welding heat affected zone. Further, if the holding temperature is 1200 ° C. or lower, it is possible to carry out post-heat treatment on various welded portions using existing equipment. The holding temperature is preferably 1100 ° C. or higher, more preferably 1150 ° C. or higher.

Further, by satisfying the equation (ii) in relation to the holding time in relation to the holding temperature, Ti charcoal sulfide and / or Ti sulfide can be reprecipitated in the weld heat affected zone. Further, by setting the holding time to [−0.6 × T + 780] (min) or less, it is possible to prevent a decrease in toughness due to the growth of crystal grains.

From the viewpoint of further promoting the reprecipitation of Ti carbon sulfide and / or Ti sulfide, the average cooling rate up to 300 ° C. is 0.1 to 5.0 ° C./s after the post-welding heat treatment. It is preferable to cool to room temperature under the conditions.

In the above description, post-weld heat treatment is performed only on the welded zone including the weld heat affected zone. However, if it is difficult to perform the post-weld heat treatment only on the welded portion, the post-weld heat treatment may be performed on the welded portion together with the base metal. Even in that case, the post-welding heat treatment is performed under the conditions satisfying the above equations (i) and (ii).

Further, when the base metal is also heat-treated after welding, the Ti carbon sulfide and / or the Ti sulfide precipitated in the base metal may be dissolved. Therefore, from the viewpoint of precipitating Ti charcoal sulfide and / or Ti sulfide again, the room temperature is such that the average cooling rate up to 300 ° C. is 0.1 to 5.0 ° C./s after the heat treatment after welding. Cool to.

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 6 having the chemical compositions shown in Table 1 were melted in a laboratory to prepare an ingot. Then, after hot forging and rolling to form the ingot, the final heat treatment was performed 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 90 to 110 nm was prepared from each test material and observed by TEM. The magnification at this time was 115,000 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, test materials having a thickness of 15 mm, a width of 50 mm, and a length of 100 mm were obtained from each alloy plate.

Further, a welding material (welding wire) having an outer diameter of 1.2 mm was produced from an ingot in which an alloy having the chemical composition shown in Table 3 was melted and cast in a laboratory by hot forging, rolling and machining. ..

A V-groove having an angle of 30 ° in the longitudinal direction and a root thickness of 1 mm was machined on the test material, and then multi-layer welding was performed in the groove by TIG welding using the above-mentioned welding material to prepare a welded joint. ..

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, the welded portion including the weld heat affected zone was subjected to post-weld heat treatment under the conditions shown in Table 5. Then, a TEM observation test piece was cut out from a position 1 mm on the base metal side from the welding melt line of the welding heat-affected zone, and the total number density of Ti carbon sulfide and Ti sulfide was measured. The measuring method is the same as that for the base metal.

Subsequently, 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 welded joint so that the weld metal was in the center of the parallel portion, and the temperature was 700 ° C. A creep rupture test was conducted under the condition of 170 MPa. The test was performed in accordance with JIS Z 2271 (2010). Those having a creep rupture time of 2000 h or more were regarded as acceptable (◯), and those having a creep rupture time of less than 2000 h were evaluated as rejected (x).

The results are also shown 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 contained in the weld heat-affected zone is within the specified range of the present invention. Nos. 1, 3 to 6 and 8 to 16 showed good results in creep rupture strength.

On the other hand, the total number density of Ti charcoal sulfide and Ti sulfide in the weld heat affected zone is less than the specified value of the present invention. Test Nos. 2 and 17-29, and test numbers exceeding the specified values. At No. 7, sufficient creep rupture strength could not be obtained.

Further, for 5 types of the joints shown in Table 4, the welded portions together with the base metal were subjected to post-weld heat treatment under the conditions shown in Table 6. Then, the total number density of Ti carbon sulfides having a diameter of 50 nm or less and Ti sulfides contained in the weld heat-affected zone and the base metal was evaluated by the above method. In addition, the creep rupture strength was evaluated by the above method.

The results are also shown in Table 6.

As shown in Table 6, the total number density of Ti carbon sulfides having a diameter of 50 nm or less and Ti sulfides contained in both the heat-affected zone and the base metal is within the specified range of the present invention. 30 to 34 showed good results in creep rupture strength.

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

Claims (2)

A welded joint containing a base metal and a welding metal, and a welding heat-affected zone is formed on the base metal side of the boundary between the base metal and the welding metal.
The chemical composition of the base material is mass%.
C: 0.01-0.15%,
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 charcoal sulfides and Ti sulfides having a particle size of 50 nm or less contained in the base metal and the heat-affected zone of welding is 50 to 500 / μm ³ .
The chemical composition of the weld metal is mass%.
C: 0.18% or less,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.040% or less,
S: 0.0100% or less,
O: 0.020% or less,
Cr: 20.0 to 38.0%,
Ni: 40.0 to 60.0%,
Total of one or more selected from Mo and W: 3.0 to 13.0%,
Ti: 0.01 to 1.50%,
N: 0.20% or less,
Al: 1.5% or less,
B: 0.0001 to 0.01%,
Zr: 0.0001 to 0.50%,
Co: 0-1.0%,
Cu: 0-1.0%,
V: 0-0.5%,
Nb: 0-0.5%,
Remaining: Fe 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.