JP6439579B2 - Method for producing austenitic heat-resistant alloy welded joint and welded joint obtained using the same - Google Patents

Description

  The present invention relates to a method for producing an austenitic heat-resistant alloy welded joint excellent in creep strength used as a high-temperature member such as a main steam pipe and a high-temperature reheat steam pipe of a power generation boiler and crack resistance of a welded part during use, and The present invention relates to a welded joint obtained using

  In recent years, high-temperature and high-pressure operating conditions have been promoted on a global scale in power generation boilers and the like from the viewpoint of reducing environmental impact, and austenitic heat-resistant alloys used as materials for superheater tubes or reheater tubes Therefore, it is required to have superior high-temperature strength and corrosion resistance.

  In addition, application of austenitic heat-resistant alloys is also being studied for various members including thick-walled members such as main steam pipes and reheat steam pipes, which conventionally used ferritic heat-resistant steels.

  Under such technical background, for example, Patent Document 1 discloses a Ni-based alloy product that improves the hot workability by using W to increase the high-temperature strength and defining the effective B amount. Has been. Patent Document 2 discloses an austenitic heat-resistant alloy with improved creep strength by utilizing Cr, Ti, and Zr. Patent Document 3 contains a large amount of W and contains Al and Ti. A Ni-based heat-resistant alloy that has been utilized to enhance the strength by solid solution strengthening and precipitation strengthening by the γ ′ phase is disclosed.

  When these austenitic heat-resistant alloys are used as structures, they are generally assembled by welding. It is known that in a welded joint using an austenitic heat-resistant alloy, various cracks are likely to occur mainly due to metallurgical factors. In particular, there is a problem that so-called stress relaxation cracks occur when used in a high temperature environment for a long time. The stress relaxation crack is a crack generated in the process in which the residual stress generated by welding is relaxed.

  Patent Document 4 uses Mo and W to increase creep strength and regulate the content of impurity elements and Ti and Al, resulting in liquefaction cracking during welding and during long-term use at high temperatures. An austenitic heat-resistant alloy that can prevent cracking is disclosed. According to Patent Literature 4, stress relaxation cracking can be prevented in a butt-welded joint in which the austenitic heat-resistant alloy is used for a member such as a main steam pipe or a high-temperature reheat steam pipe.

  By the way, as shown in Non-Patent Document 1, in austenitic stainless steel or Ni-based alloy, heat treatment after welding is generally not performed. However, in austenitic stainless steel, post-weld heat treatment may be performed in a temperature range of 1000 to 1150 ° C for the purpose of improving corrosion resistance and toughness, and in a temperature range of 800 to 900 ° C for the purpose of removing residual stress. .

  In Non-Patent Document 2, after the 18Cr-12Ni-Nb austenitic stainless steel is heated and held at about 600 ° C. for the purpose of preventing cracks that occur when it is used for a long time at a high temperature. A heat treatment method is disclosed that involves three steps of holding again at 1050 ° C. and finally holding at 900 ° C.

Japanese Patent No. 4631986 International Publication No. 2009/154161 International Publication No. 2010/038826 JP 2010-150593 A

Joint / Welding Technology Q & A 1000 Editorial Committee, “Joint / Welding Technology Q & A 1000”, August 1999, p502-503, 653-654 Torazo Uchiki, Kuki Okabayashi, Munetaka Kuribayashi, Norio Mori Shigeo, “Stress relief annealing cracking of 18Cr-12Ni—Nb steel”, Ishikawajima Harima Technical Report, March 1975, Vol. 15, No. 2, p209-215

  Patent Documents 1 to 3 do not consider the problem of stress relaxation cracks that occur when used at high temperatures for a long time. As described above, Patent Document 4 discloses an austenitic heat-resistant alloy capable of preventing stress relaxation cracking in a butt weld joint used for a member such as a main steam pipe or a high-temperature reheat steam pipe. . However, in actual structures, there are various shapes and sizes of welds. As a result of detailed investigations by the inventors, it has been found that the state of residual stress varies depending on the shape and dimensions of the weld. And it became clear that the effect which prevents a stress relaxation crack may not fully be acquired even if it uses the technique of patent document 4 depending on the shape or dimension of a welding part.

  Further, even if the post-weld heat treatment described in Non-Patent Document 1 or 2 is simply applied to the austenitic heat-resistant alloy targeted by the present invention, the residual stress is relaxed, so that the stress It is possible to prevent relaxation cracking. However, it has been found that the creep strength of the welded joint may be greatly reduced depending on the conditions of the heat treatment after welding.

  The present invention is used as a high-temperature member such as a main steam pipe or a reheat steam pipe of a boiler for thermal power generation, and a method for producing an austenitic heat-resistant alloy welded joint excellent in creep strength and stress relaxation crack resistance, and using the same It aims at providing the welded joint obtained by this.

  In order to solve the above-mentioned problems, the present inventors have conducted detailed studies on austenitic heat-resistant alloy welded joints that have been subjected to post-weld heat treatment. And as a result of performing the creep test about the welded joint which performed the post-heat treatment on various conditions, it turned out that there is a big difference in the degree of the fall of creep strength by the joint.

In order to investigate the cause, the microstructure was observed by using the welded joints before and after the creep test, and the difference in the structure was compared between the case where the decrease in the creep strength was large and the case where it was small. As a result, in the welded joint in which the creep strength was greatly reduced, coarse M ₂₃ C ₆ carbide precipitated loosely before the creep test, and the Cr content constituting the M ₂₃ C ₆ carbide was low. Further, after the creep test, the M ₂₃ C ₆ carbide was remarkably coarsened.

Fine M ₂₃ C ₆ carbide greatly contributes to the improvement of the creep strength by being dispersed in the grains. Therefore, when used for a long time in a high temperature environment, it is considered that the M ₂₃ C ₆ carbides are significantly coarsened, so that the creep strength is greatly reduced. The mechanism by which M ₂₃ C ₆ carbide coarsens can be explained as follows.

  When used for a long time in a high temperature environment, fine carbides precipitate in the grains. As a result, in the welded joint in which coarse carbides originally exist in the grains by post-weld heat treatment, two types of carbides having different sizes are mixed. When the difference in particle size becomes significant, the difference in interfacial energy between particles increases, and the difference in interfacial energy acts as a driving force to eliminate small carbides and grow nearby coarse carbides more. It passes.

In addition, the smaller the difference between the amount of the main constituent element contained in the carbide that is the precipitate and the amount of the element contained in the substrate in the equilibrium state, the carbide is considered to grow more easily. That _is, the low Cr content constituting the _M 23 _{C 6 carbides,} a factor of _M 23 _{C 6} carbides growth promotion.

The present inventors have conducted extensive repeated study, in order to prevent the coarsening of M ₂₃ C ₆ carbides, the heat treatment after welding temperature, M ₂₃ C ₆ carbides are generated from the heat treatment time after welding and after welding heat treatment temperature It was found that it is important to appropriately manage each condition of the rate of temperature decrease to 500 ° C. easily.

  The present invention has been made on the basis of the above findings, and the gist thereof is a manufacturing method of the following austenitic heat-resistant alloy welded joint and a welded joint obtained by using the same.

(1) In mass%,
C: 0.04 to 0.12%,
Si: 1.0% or less,
Mn: 2.0% or less,
P: 0.03% or less,
S: 0.01% or less,
Ni: 42.0-48.0%,
Cr: 20.0-26.0%,
W: 4.0 to 10.0%,
Ti: 0.05 to 0.15%,
Nb: 0.1-0.4%
Al: 0.3% or less,
B: 0.0001 to 0.01%
N: 0.02% or less,
O: 0.01% or less,
Ca: 0 to 0.05%,
Mg: 0 to 0.05%,
REM: 0-0.1%
Co: 0 to 1.0%,
Cu: 0 to 4.0%,
Mo: 0 to 1.0%,
V: 0 to 0.5%
The balance: an alloy base material having a chemical component that is Fe and impurities,
% By mass
C: 0.06-0.18%,
Si: 1.0% or less,
Mn: 2.0% or less,
P: 0.03% or less,
S: 0.01% or less,
Ni: 40.0-60.0%,
Cr: 20.0-26.0%,
Sum of one or both of Mo and W: 6.0 to 13.0%,
Ti: 0.05 to 0.6%,
Al: 1.5% or less,
N: 0.18% or less,
O: 0.01% or less,
Co: 0 to 15.0%,
Nb: 0 to 0.5%,
B: 0 to 0.005%,
The remainder: after welding using a welding material having chemical components that are Fe and impurities,
A method for producing an austenitic heat-resistant alloy welded joint, which is subjected to post-weld heat treatment under conditions satisfying the following expressions (i) to (iii):
800 ≦ T ≦ 1250 (i)
−0.2 × T + 270 ≦ t ≦ −0.6 × T + 810 (ii)
RC ≧ 0.05 × T-10 (iii)
However, the meaning of each symbol in the above formula is as follows.
T: Heat treatment temperature after welding (° C)
t: Heat treatment time after welding (min)
RC: Average cooling rate from T to 500 ° C (° C / h)

(2) The chemical composition of the alloy base material is mass%,
Ca: 0.0001 to 0.05%,
Mg: 0.0001 to 0.05%,
REM: 0.0005 to 0.1%,
Co: 0.01 to 1.0%
Cu: 0.01 to 4.0%,
Mo: 0.01 to 1.0%,
V: 0.01-0.5%
The manufacturing method of the austenitic heat-resistant-alloy weld joint as described in said (1) containing 1 or more types selected from.

(3) The chemical composition of the welding material is mass%,
Co: 0.01 to 15.0%,
Nb: 0.01-0.5%
B: 0.0001 to 0.005%,
The manufacturing method of the austenitic heat-resistant-alloy weld joint as described in said (1) or (2) containing 1 or more types selected from these.

(4) The method for producing an austenitic heat-resistant alloy welded joint according to any one of (1) to (3), wherein the post-weld heat treatment condition further satisfies the following formula (iv):
RH ≧ 40 (iv)
However, the meaning of the symbols in the above formula is as follows.
RH: Average heating rate from 500 ° C. to T (° C./h)

  (5) The method for producing an austenitic heat-resistant alloy welded joint according to any one of (1) to (4), wherein the thickness of the alloy base material exceeds 30 mm.

  (6) An austenitic heat-resistant alloy welded joint obtained by using the production method according to any one of (1) to (5) above.

  According to the production method of the present invention, it is possible to stably obtain an austenitic heat-resistant alloy welded joint capable of achieving both high-temperature creep strength and stress relaxation crack resistance of a welded part during use.

  Hereinafter, each requirement of the present invention will be described in detail. In the following description, “%” for the content means “% by mass”.

1. Chemical composition of alloy base material The reasons for limitation of each element contained in the alloy base material used for manufacturing the austenitic heat-resistant alloy welded joint according to the present invention are as follows.

C: 0.04 to 0.12%
C is an element having the effect of stabilizing austenite, forming fine carbides, and improving the creep strength during use at high temperatures. In order to sufficiently obtain this effect, a C content of 0.04% or more is necessary. However, if the C content is excessive, the carbide becomes coarse and precipitates in a large amount, so that the creep strength is lowered. In particular, when a post-weld heat treatment is performed on a welded joint containing a large amount of C, the growth of carbide is promoted, and the creep strength is significantly reduced. Therefore, the C content is 0.12% or less. The C content is preferably 0.05% or more, and more preferably 0.06% or more. Further, the C content is desirably 0.11% or less, and more desirably 0.08% or less.

Si: 1.0% or less Si is an element that has a deoxidizing action and is effective in improving corrosion resistance and oxidation resistance at high temperatures. However, when Si is contained excessively, the stability of austenite is lowered, leading to a decrease in toughness and creep strength. Therefore, an upper limit is set for the Si content to 1.0% or less. The Si content is desirably 0.8% or less, and more desirably 0.6% or less.

  In addition, it is not necessary to set a lower limit in particular for the Si content, but if it is extremely reduced, the deoxidation effect cannot be sufficiently obtained, the cleanliness of the alloy is deteriorated, and the high temperature corrosion resistance and oxidation resistance are improved. Is difficult to obtain, and the manufacturing cost is greatly increased. Therefore, the Si content is desirably 0.02% or more, and more desirably 0.05% or more.

Mn: 2.0% or less Mn, like Si, is an element having a deoxidizing action. Mn also contributes to stabilization of austenite. However, when the Mn content is excessive, embrittlement is caused, and the toughness and creep ductility are also reduced. Therefore, an upper limit is set for the Mn content to 2.0% or less. The Mn content is desirably 1.8% or less, and more desirably 1.5% or less.

  In addition, it is not necessary to set a lower limit particularly for the Mn content, but if it is extremely reduced, the deoxidation effect cannot be sufficiently obtained, the cleanliness of the alloy is deteriorated, and the austenite stabilizing effect is difficult to obtain. Manufacturing costs also increase significantly. Therefore, the Mn content is desirably 0.02% or more, and more desirably 0.05% or more.

P: 0.03% or less P is contained in the alloy as an impurity, and when it is contained in a large amount, hot workability and weldability are remarkably deteriorated, and creep ductility after long-term use at high temperatures. Also decreases. Therefore, an upper limit is set for the P content to 0.03% or less. The P content is desirably 0.025% or less, and more desirably 0.02% or less.

  Although the P content is preferably reduced as much as possible, the extreme reduction leads to an increase in manufacturing cost. Therefore, the P content is desirably 0.0005% or more, and more desirably 0.0008% or more.

S: 0.01% or less S is contained in the alloy as an impurity as in the case of P, and when it is contained in a large amount, hot workability and weldability are remarkably deteriorated, and further, it is used at a high temperature for a long time. Later creep ductility also decreases. Therefore, an upper limit is set for the S content to 0.01% or less. The S content is desirably 0.008% or less, and more desirably 0.005% or less.

  Although the S content is preferably reduced as much as possible, the extreme reduction leads to an increase in manufacturing cost. Therefore, the S content is desirably 0.0001% or more, and more desirably 0.0002% or more.

Ni: 42.0-48.0%
Ni is an effective element for obtaining austenite, and is an essential element for ensuring the structural stability when used for a long time at a high temperature. In order to obtain a sufficient effect within the range of the Cr content of the present invention, a Ni content of 42.0% or more is necessary. However, Ni is an expensive element, and if it is contained in a large amount, the cost increases. Therefore, an upper limit is provided so that the Ni content is 42.0 to 48.0%. The Ni content is desirably 42.5% or more, and more desirably 43.0% or more. Further, the Ni content is desirably 47.5% or less, and more desirably 47.0% or less.

Cr: 20.0 to 26.0%
Cr is an essential element for securing oxidation resistance and corrosion resistance at high temperatures. Further, Cr contributes to ensuring creep strength by forming fine carbides. In order to obtain the above effects within the range of the Ni content of the present invention, a Cr content of 20.0% or more is necessary. However, if the Cr content exceeds 26.0%, the stability of austenite at high temperatures deteriorates and the creep strength decreases. In particular, in the present invention in which post-weld heat treatment is performed on a welded joint, the growth of carbides is promoted, so that the creep strength is remarkably reduced. Therefore, the content of Cr is set to 20.0 to 26.0%. The Cr content is desirably 20.5% or more, and more desirably 21.0% or more. Further, the Cr content is desirably 25.5% or less, and more desirably 25.0% or less.

W: 4.0 to 10.0%
W is an element that contributes greatly to the improvement of creep strength and tensile strength at high temperatures by forming a solid solution in the matrix or forming a fine intermetallic compound phase. In order to sufficiently obtain this effect, a W content of 4.0% or more is necessary. However, even if W is excessively contained, the effect is saturated, and the creep strength is decreased. Furthermore, since W is an expensive element, if it is excessively contained, the cost increases. Therefore, an upper limit is provided so that the W content is 4.0 to 10.0%. The W content is desirably 4.5% or more, and more desirably 5.0% or more. Further, the W content is desirably 9.5% or less, and more desirably 9.0% or less.

Ti: 0.05 to 0.15%
Ti precipitates in the grains as fine carbonitrides and contributes to the improvement of creep strength and tensile strength at high temperatures. In order to sufficiently obtain the effect, a Ti content of 0.05% or more is necessary. However, when the Ti content is excessive, a large amount of carbonitride precipitates, resulting in a decrease in creep ductility and toughness. Therefore, an upper limit is provided so that the Ti content is 0.05 to 0.15%. The Ti content is desirably 0.06% or more, and more desirably 0.07% or more. Further, the Ti content is desirably 0.14% or less, and more desirably 0.13% or less.

Nb: 0.1 to 0.4%
Nb combines with C or N and precipitates as fine carbides or carbonitrides in the grains, contributing to the improvement of creep strength at high temperatures. In order to sufficiently obtain the effect, an Nb content of 0.1% or more is necessary. However, when the Nb content is excessive, a large amount of carbides and carbonitrides are precipitated, resulting in a decrease in creep ductility and toughness. Therefore, an upper limit is provided so that the Nb content is 0.1 to 0.4%. The Nb content is desirably 0.12% or more, and more desirably 0.15% or more. Further, the Nb content is desirably 0.38% or less, and more desirably 0.35% or less.

Al: 0.3% or less Al is an element having a deoxidizing action. However, when the Al content is excessive, the cleanliness of the alloy is remarkably deteriorated and the hot workability and ductility are lowered. Therefore, an upper limit is set so that the Al content is 0.3% or less. The Al content is desirably 0.2% or less, and more desirably 0.1% or less.

  In addition, it is not necessary to set a lower limit in particular for the Al content, but if it is extremely reduced, the deoxidation effect cannot be sufficiently obtained and the cleanliness of the alloy deteriorates, and the corrosion resistance and oxidation resistance at high temperatures are improved. It becomes difficult to obtain the effect, and the manufacturing cost increases greatly. Therefore, the Al content is desirably 0.0005% or more, and more desirably 0.001% or more.

B: 0.0001 to 0.01%
B is an element effective for improving the creep strength by finely dispersing grain boundary carbides and segregating at the grain boundaries to strengthen the grain boundaries. In order to obtain this effect, the B content needs to be 0.0001% or more. However, if the B content is excessive, a large amount of B is segregated in the heat-affected zone near the melting boundary due to the welding heat cycle during welding, the melting point of the grain boundary is lowered, and the liquefaction cracking sensitivity is increased. Therefore, an upper limit is provided so that the B content is 0.0001 to 0.01%. The B content is desirably 0.0005% or more, and more desirably 0.001% or more. Further, the B content is desirably 0.008% or less, and more desirably 0.006% or less.

N: 0.02% or less N is an element effective for stabilizing austenite. However, if it is excessively contained, a large amount of fine nitride precipitates in the grains during use at high temperatures and creeps. It causes a reduction in ductility and toughness. Therefore, an upper limit is set for the N content to 0.02% or less. The N content is desirably 0.018% or less, and more desirably 0.015% or less.

  Although there is no need to set a lower limit for the N content, if it is extremely reduced, it becomes difficult to obtain the effect of stabilizing austenite, and the manufacturing cost is greatly increased. Therefore, the N content is desirably 0.0005% or more, and more desirably 0.0008% or more.

O: 0.01% or less O (oxygen) is contained as an impurity in the alloy, and when its content is excessive, hot workability is lowered, and further, toughness and ductility are deteriorated. For this reason, an upper limit is set for the O content to 0.01% or less. The O content is desirably 0.008% or less, and more desirably 0.005% or less.

  Although there is no particular need to set a lower limit for the O content, an extreme reduction leads to an increase in manufacturing cost. Therefore, the O content is desirably 0.0005% or more, and more desirably 0.0008% or more.

  The alloy base material used in the manufacture of the austenitic heat-resistant alloy welded joint according to the present invention has a chemical composition containing each of the elements described above, with the balance being Fe and impurities.

  The “impurity” refers to an impurity mixed from ore, scrap, or a production environment as a raw material when the alloy is industrially produced.

  In addition to the above elements, the alloy base material in the present invention may further contain one or more elements selected from Ca, Mg, REM, Co, Cu, Mo and V.

  Below, the effect of said element and the reason for limitation of content are demonstrated.

Ca: 0 to 0.05%
Ca has the effect | action which improves hot workability. For this reason, you may contain Ca. However, when the content of Ca is excessive, it combines with O to remarkably reduce cleanliness, and on the contrary, deteriorate hot workability. Therefore, when Ca is contained, its content is set to 0.05% or less. The Ca content is desirably 0.03% or less.

  In order to obtain the above effect, the Ca content is preferably 0.0001% or more, and more preferably 0.0005% or more.

Mg: 0 to 0.05%
Mg, like Ca, has the effect of improving hot workability. For this reason, you may contain Mg. However, when the Mg content is excessive, it combines with O to significantly reduce cleanliness, and on the contrary, deteriorate hot workability. Therefore, when it contains Mg, the content shall be 0.05% or less. The Mg content is preferably 0.03% or less.

  In order to obtain the above effect, the Mg content is preferably 0.0001% or more, and more preferably 0.0005% or more.

REM: 0 to 0.1%
REM is an element that has a strong affinity for S, has an effect of improving hot workability, and is effective in improving creep ductility during use at high temperatures. For this reason, you may contain REM. However, when the content of REM becomes excessive, it combines with O to remarkably reduce cleanliness, and on the contrary, deteriorate hot workability. Therefore, when it contains REM, the content shall be 0.1% or less. The REM content is desirably 0.06% or less.

  In order to obtain the above effect, the REM content is preferably 0.0005% or more, and more preferably 0.001% or more.

  “REM” is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to the total content of one or more elements of REM. Further, REM is generally contained in misch metal. For this reason, for example, it may be added in the form of misch metal and contained so that the amount of REM falls within the above range.

  Since Ca, Mg, and REM all have an effect of improving hot workability, only one of them can be contained, or two or more of them can be contained in combination. The total amount when these elements are contained in combination may be 0.2%.

Co: 0 to 1.0%
Co, like Ni, is an austenite-forming element and contributes to the improvement of creep strength by increasing phase stability. For this reason, Co may be contained. However, since Co is an extremely expensive element, excessive content of Co causes a significant cost increase. Therefore, when Co is contained, the content is made 1.0% or less. The Co content is desirably 0.8% or less.

  In order to obtain the above effect, the Co content is preferably 0.01% or more, and more preferably 0.03% or more.

Cu: 0 to 4.0%
Cu has the effect of improving the creep strength. That is, Cu is an austenite-forming element like Ni and Co, and contributes to improvement of creep strength by increasing phase stability. For this reason, you may contain Cu. However, when Cu is contained excessively, the hot workability is lowered. Therefore, when it contains Cu, the content shall be 4.0% or less. The Cu content is desirably 3.0% or less.

  In addition, when acquiring said effect, it is desirable to make Cu content 0.01% or more, and it is more desirable to set it as 0.03% or more.

Mo: 0 to 1.0%
Mo has the effect | action which improves creep strength. That is, Mo has a function of improving the creep strength at a high temperature by dissolving in the matrix. For this reason, you may contain Mo. However, when Mo is contained excessively, the stability of austenite is lowered, and instead the creep strength is lowered. Therefore, when it contains Mo, the content shall be 1.0% or less. The Mo content is desirably 0.8% or less.

  In addition, when acquiring said effect, it is desirable to make Mo content into 0.01% or more, and it is more desirable to set it as 0.03% or more.

V: 0 to 0.5%
V has an effect of improving the creep strength. That is, V combines with C or N to form fine carbides or carbonitrides, and has the effect of improving creep strength. For this reason, you may contain V. However, when V is contained excessively, it precipitates in a large amount as a carbide or carbonitride, resulting in a decrease in creep ductility. Therefore, when V is contained, the content is set to 0.5% or less. The V content is desirably 0.4% or less.

  In addition, when acquiring said effect, it is desirable to make content of V into 0.01% or more, and it is more desirable to set it as 0.02% or more.

  Co, Cu, Mo and V all have the effect of improving the creep strength, and therefore, any one of them or a combination of two or more can be contained. The total amount when these elements are contained in combination may be 6.5%.

2. Chemical composition of welding material The reasons for limitation of each element contained in the welding material used for manufacturing the austenitic heat-resistant alloy welded joint according to the present invention are as follows.

C: 0.06 to 0.18%
C is an element that has the effect of stabilizing the austenite in the weld metal after welding, forming fine carbides, and improving the creep strength during use at high temperatures. Furthermore, by forming eutectic carbide with Cr during welding solidification, it contributes to reduction of solidification cracking sensitivity. In order to sufficiently obtain this effect, a C content of 0.06% or more is necessary. However, if the C content is excessive, a large amount of carbide precipitates, so that the creep strength and ductility are reduced. Therefore, the C content is 0.18% or less. The C content is preferably 0.07% or more, and more preferably 0.08% or more. Further, the C content is desirably 0.16% or less, and more desirably 0.14% or less.

Si: 1.0% or less Si is an element that is effective for deoxidation at the time of manufacturing a welding material and is effective for improving the corrosion resistance and oxidation resistance of the weld metal after welding at a high temperature. However, when Si is contained excessively, the stability of austenite is lowered, leading to a decrease in toughness and creep strength. Therefore, an upper limit is set for the Si content to 1.0% or less. The Si content is desirably 0.8% or less, and more desirably 0.6% or less.

  In addition, it is not necessary to set a lower limit in particular for the Si content, but if it is extremely reduced, the deoxidation effect cannot be sufficiently obtained, the cleanliness of the alloy is deteriorated, and the high temperature corrosion resistance and oxidation resistance are improved. Is difficult to obtain, and the manufacturing cost is greatly increased. Therefore, the Si content is desirably 0.02% or more, and more desirably 0.05% or more.

Mn: 2.0% or less Mn, like Si, is an element effective for deoxidation during the production of a welding material. Mn also contributes to stabilization of austenite in the weld metal after welding. However, when the Mn content is excessive, embrittlement is caused, and the toughness and creep ductility are also reduced. Therefore, an upper limit is set for the Mn content to 2.0% or less. The Mn content is desirably 1.8% or less, and more desirably 1.5% or less.

  In addition, it is not necessary to set a lower limit particularly for the Mn content, but if it is extremely reduced, the deoxidation effect cannot be sufficiently obtained, the cleanliness of the alloy is deteriorated, and the austenite stabilizing effect is difficult to obtain. Manufacturing costs also increase significantly. Therefore, the Mn content is desirably 0.02% or more, and more desirably 0.05% or more.

P: 0.03% or less P is an element that is contained in the welding material as an impurity and increases the susceptibility to solidification cracking during welding. Furthermore, the creep ductility of the weld metal after long time use at high temperature is reduced. Therefore, an upper limit is set for the P content to 0.03% or less. The P content is desirably 0.025% or less, and more desirably 0.02% or less.

  Although the P content is preferably reduced as much as possible, the extreme reduction leads to an increase in manufacturing cost. Therefore, the P content is desirably 0.0005% or more, and more desirably 0.0008% or more.

S: 0.01% or less S is contained in the welding material as an impurity as in the case of P, and when it is contained in a large amount, the hot workability and weldability are remarkably deteriorated. Further, S is long at a high temperature. When used for a long time, the weld metal segregates at the columnar grain boundaries, leading to embrittlement and increasing the stress relaxation cracking sensitivity. Therefore, an upper limit is set for the S content to 0.01% or less. The S content is desirably 0.008% or less, and more desirably 0.005% or less.

  Although the S content is preferably reduced as much as possible, the extreme reduction leads to an increase in manufacturing cost. Therefore, the S content is desirably 0.0001% or more, and more desirably 0.0002% or more.

Ni: 40.0-60.0%
Ni is an element effective for stabilizing austenite in the weld metal after welding, and is an essential element for ensuring the structural stability when used for a long time at a high temperature. In order to obtain the effect, the Ni content of the welding material needs to be 40.0% or more. However, Ni is an expensive element, and even in a welding material manufactured in a small scale, if a large amount is contained, the cost increases. Therefore, an upper limit is provided so that the Ni content is 40.0 to 60.0%. The Ni content is desirably 40.5% or more, and more desirably 41.0% or more. Further, the Ni content is desirably 59.5% or less, and more desirably 59.0% or less.

Cr: 20.0 to 26.0%
Cr is an effective element for ensuring oxidation resistance and corrosion resistance at high temperatures of the weld metal after welding. Further, Cr contributes to ensuring creep strength by forming fine carbides. Furthermore, C and eutectic carbides are formed during weld solidification, which contributes to a reduction in solidification crack sensitivity. In order to obtain these effects, a Cr content of 20.0% or more is necessary. However, if the Cr content exceeds 26.0%, the stability of austenite at high temperatures deteriorates and the creep strength decreases. Therefore, the Cr content of the welding material is set to 20.0 to 26.0%. The Cr content is desirably 20.5% or more, and more desirably 21.0% or more. Further, the Cr content is desirably 25.5% or less, and more desirably 25.0% or less.

Total of one or both of Mo and W: 6.0 to 13.0%
Mo and W are elements that make a solid solution in the matrix in the weld metal or form a fine intermetallic compound phase and greatly contribute to the improvement of the creep strength and tensile strength at high temperatures. In order to obtain this effect sufficiently, it is necessary to contain one or both of Mo and W in total of 6.0% or more. However, even if these elements are contained excessively, the effect is saturated, and on the contrary, the creep strength is lowered. Furthermore, since Mo and W are expensive elements, an excessive amount causes an increase in cost. Therefore, an upper limit is provided and the total content of one or both of Mo and W is set to 6.0 to 13.0%. The total content is desirably 6.5% or more, and more desirably 7.0% or more. Further, the total content is desirably 12.5% or less, and more desirably 12.0% or less.

Ti: 0.05-0.6%
Ti precipitates in the grains as fine carbonitrides in the weld metal and contributes to the improvement of creep strength and tensile strength at high temperatures. In order to obtain the effect sufficiently, the Ti content needs to be 0.05% or more. However, when the Ti content is excessive, a large amount of carbonitride precipitates, resulting in a decrease in creep ductility and toughness. Therefore, an upper limit is provided so that the Ti content of the welding material is 0.05 to 0.6%. The Ti content is desirably 0.06% or more, and more desirably 0.07% or more. Further, the Ti content is desirably 0.58% or less, and more desirably 0.55% or less.

Al: 1.5% or less Al is an element effective for deoxidation at the time of manufacturing a welding material. Moreover, a fine intermetallic compound phase is formed in the weld metal, which contributes to the improvement of creep strength. However, when the Al content is excessive, the cleanliness of the alloy is remarkably deteriorated, and the hot workability and ductility of the welding material are lowered, so that the productivity is deteriorated. In addition, a large amount of intermetallic phase is formed in the weld metal, and the stress relaxation cracking susceptibility when used at a high temperature for a long time is remarkably increased. Therefore, an upper limit is set so that the Al content of the welding material is 1.5% or less. The Al content is desirably 1.4% or less, and more desirably 1.3% or less.

  In addition, it is not necessary to set a lower limit in particular for the Al content, but if it is extremely reduced, the deoxidation effect cannot be sufficiently obtained and the cleanliness of the alloy deteriorates, and the corrosion resistance and oxidation resistance at high temperatures are improved. It becomes difficult to obtain the effect, and the manufacturing cost increases greatly. Therefore, the Al content is desirably 0.0005% or more, and more desirably 0.001% or more.

N: 0.18% or less N is an element that stabilizes austenite in the weld metal, improves creep strength, and contributes to securing tensile strength by solid solution. However, if it is contained excessively, a large amount of fine nitride precipitates in the grains during use at high temperatures, leading to a decrease in creep ductility and toughness. Therefore, an upper limit is set for the N content of the welding material to 0.18% or less. The N content is desirably 0.16% or less, and more desirably 0.14% or less.

  Although there is no need to set a lower limit for the N content, if it is extremely reduced, it becomes difficult to obtain the effect of stabilizing austenite, and the manufacturing cost is greatly increased. Therefore, the N content is desirably 0.0005% or more, and more desirably 0.0008% or more.

O: 0.01% or less O (oxygen) is contained as an impurity in the welding material, and when its content is excessive, hot workability is deteriorated and productivity is deteriorated. For this reason, an upper limit is set for the O content to 0.01% or less. The O content is desirably 0.008% or less, and more desirably 0.005% or less.

  Although there is no particular need to set a lower limit for the O content, an extreme reduction leads to an increase in manufacturing cost. Therefore, the O content is desirably 0.0005% or more, and more desirably 0.0008% or more.

Co: 0 to 15.0%
Co, like Ni, stabilizes the austenite structure of the weld metal and contributes to the improvement of the creep strength. Therefore, Co may be contained as necessary. However, since Co is an extremely expensive element, even if it is a welding material, excessive content causes a significant cost increase. Therefore, when Co is contained, the content is made 15.0% or less. The Co content is preferably 14.0% or less, and more preferably 13.0% or less. In order to obtain the above effect, the Co content is desirably 0.01% or more, and more desirably 0.03% or more.

Nb: 0 to 0.5%
Nb combines with C or N and precipitates in the grains as fine carbides or carbonitrides, and contributes to the improvement of creep strength at high temperatures. Therefore, Nb may be contained as necessary. However, when the Nb content is excessive, a large amount of carbides and carbonitrides are precipitated, resulting in a decrease in creep ductility and toughness. Therefore, when Nb is contained, the content is set to 0.5% or less. The Nb content is desirably 0.48% or less, and more desirably 0.45% or less. In order to obtain the above effect, the Nb content is preferably 0.01% or more, and more preferably 0.03% or more.

B: 0 to 0.005%
B is effective for improving the creep strength of the weld metal, and is an element effective for segregating at the grain boundary and strengthening the grain boundary. Therefore, B may be contained as necessary. However, if the B content is excessive, the susceptibility to solidification cracking during welding is significantly increased. Therefore, when B is contained, the content is made 0.005% or less. The B content is desirably 0.004% or less, and more desirably 0.003% or less. In order to obtain the above effect, the B content is desirably 0.0001% or more, and more desirably 0.0005% or more.

  The welding material used for manufacturing the austenitic heat-resistant alloy welded joint according to the present invention has a chemical composition containing each of the elements described above, with the balance being Fe and impurities.

3. Post-weld heat treatment conditions The austenitic heat-resistant alloy welded joint of the present invention can be manufactured by performing post-weld heat treatment after welding the alloy base material using the welding material. As described above, in order to achieve both the creep strength and the stress relaxation crack resistance, it is necessary to perform post-weld heat treatment under conditions that satisfy the following expressions (i) to (iii).

Heat treatment temperature after welding T (° C.): 800 ≦ T ≦ 1250 (i)
As described above, in order to reduce the decrease in creep strength when a welded joint obtained by heat treatment after welding is used for a long time in a high temperature environment, coarse M ₂₃ C in the heat treatment process after welding is reduced. ₆ suppressing the formation of carbides, and it is effective to increase the Cr content of the M ₂₃ C ₆ carbide. In order to achieve these, it is necessary to set the heat treatment temperature after welding low. Therefore, an upper limit is set for the heat treatment temperature after welding, and the temperature is set to 1250 ° C or lower.

  On the other hand, if the post-weld heat treatment temperature is too low, the welding residual stress cannot be relaxed sufficiently, resulting in an increase in stress relaxation cracking sensitivity. Therefore, the heat treatment temperature after welding is set to 800 ° C. or higher. The post-weld heat treatment temperature is desirably 850 ° C. or higher, and more desirably 900 ° C. or higher. The post-weld heat treatment temperature is desirably 1150 ° C. or less, and more desirably 1000 ° C. or less.

Heat treatment time after welding t (min): −0.2 × T + 270 ≦ t ≦ −0.6 × T + 810 (ii)
In order to suppress the formation of coarse M ₂₃ C ₆ carbide in the post-weld heat treatment process, it is not enough to define the post-weld heat treatment temperature, and it is necessary to manage the post-weld heat treatment time in relation to the above temperature. There is. In order to reduce the decrease in the creep strength, it is necessary to set the post-weld heat treatment time short, and it is set to [−0.6 × T + 810] (min) or less. On the other hand, if the post-weld heat treatment time is too short, the welding residual stress cannot be sufficiently relaxed, resulting in an increase in stress relaxation crack sensitivity. Therefore, the heat treatment time after welding is set to [−0.2 × T + 270] (min) or more.

Average cooling rate RC from T to 500 ° C. (° C./h): RC ≧ 0.05 × T−10 (iii)
Only by controlling the post-weld heat treatment temperature and the post-weld heat treatment time, generation of coarse M ₂₃ C ₆ carbide in the post-weld heat treatment process cannot be completely suppressed. Since M ₂₃ C ₆ carbide is generated even when the temperature is lowered after heat treatment after welding, it is necessary to manage the lower limit of the average temperature drop rate from that temperature to 500 ° C. according to the heat treatment temperature after welding. Therefore, a lower limit is set for the average temperature drop rate from the heat treatment temperature after welding to 500 ° C., and it is set to [0.05 × T-10] (° C./h) or more.

  In the method for producing an austenitic heat-resistant alloy welded joint according to the present invention, it is desirable that the conditions for the heat treatment after welding further satisfy the following formula (iv).

Average heating rate from 500 ° C. to T (° C./h): RH ≧ 40 (iv)
When the average heating rate RH from 500 ° C. to the post-welding heat treatment temperature T (° C.) is lower than 40 ° C./h in the temperature raising process of the post-weld heat treatment, fine carbides and carbonitriding in the grains occur during the temperature raising process In the case where an object and an intermetallic compound are precipitated and the shape of a welded part is complicated, stress relaxation cracks may occur in the process of heat treatment after welding. Therefore, it is desirable to set a lower limit to the average rate of temperature rise from 500 ° C. to the post-weld heat treatment temperature to 40 (° C./h) or more.

4). Others No particular limitation is imposed on the shape or dimensions of the alloy base material and the welding material used for manufacturing the austenitic heat-resistant alloy welded joint according to the present invention. However, the manufacturing method according to the present invention is particularly effective when an alloy base material having a thickness exceeding 30 mm is used. Therefore, it is desirable that the thickness of the alloy base material exceeds 30 mm.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  An alloy having the chemical composition shown in Table 1 was melted to produce an ingot. After forming by hot forging using the above ingot, solution heat treatment at 1230 ° C. is performed, and alloy plates having a thickness of 15 mm, a width of 50 mm, a length of 100 mm, a thickness of 32 mm, a width of 150 mm, and a length of 200 mm, respectively. Produced. These alloy plates were subjected to a creep rupture test and confirmation of the presence or absence of stress relaxation cracks, respectively.

  Further, an alloy having the chemical composition shown in Table 2 was melted to produce an ingot, and then a welding material having an outer diameter of 1.2 mm was produced by hot forging, hot rolling and machining.

  The creep rupture test was performed according to the following procedure. After processing a V groove with a groove angle of 30 ° and a root thickness of 1 mm in the longitudinal direction of the alloy plate having a thickness of 15 mm, a width of 50 mm, and a length of 100 mm, it is opened by TIG welding using the above welding material. Multi-layer welding was performed in the tip to produce a welded joint. The obtained welded joint was subjected to post-weld heat treatment under the conditions shown in Table 3. Thereafter, a round bar creep rupture test piece was taken from the welded joint so that the weld metal was at the center of the parallel part, and a creep rupture test was performed at 700 ° C. and 147 MPa under which the target fracture time of the base alloy plate was about 1000 hours. Went.

  Moreover, in order to reproduce the severe stress state in a complicated welded part shape, the confirmation of the presence or absence of stress relaxation cracking was performed by the following procedure. After producing a test piece by machining in accordance with the y-type weld crack test method defined in JIS Z 3158 (1993) using the alloy plate having the thickness of 32 mm, the width of 150 mm, and the length of 200 mm, Single-layer welding was performed on the groove by TIG welding using the welding material, and a welded joint was produced. The obtained welded joint was subjected to post-weld heat treatment under the same conditions as those in the creep rupture test, followed by aging heat treatment at 700 ° C. for 500 hours. Specimens were collected from five locations for the heat affected zone of the welded joint after the treatment. Then, the cross section was mirror-polished and corroded with aqua regia, and then observed with an optical microscope at a magnification of 500 times to investigate the presence or absence of cracks.

  The results of the creep rupture test and crack observation are shown in Table 3. As for the creep rupture test results, “◎” indicates that the rupture time exceeds the target rupture time of the base alloy plate, “○” indicates that the rupture time exceeds 85% of the target rupture time of the base alloy plate, and “ × ”. As for the crack observation results, the welded joint in which no cracks were observed in all five test pieces used for the observation was “◯”, and the welded joint in which only one cross section was cracked was “△”. Passed. And the weld joint by which the crack was recognized by the 2 or more test piece was set to "x", and it was judged that it was disqualified.

  As shown in Table 3, Test Nos. 1 to 5, 7 to 20, and 27 to 45 in which the heat treatment conditions after welding satisfy the provisions of the present invention have good creep strength and are excellent even in severe welded part shapes. It is clear that it has high stress relaxation crack resistance. In Test No. 6, since the temperature rise condition in the heat treatment after welding was below the preferable range, the stress weld cracking as applied in the present example was acceptable although stress relaxation cracking occurred in only one section. It can be seen that it has performance.

  On the other hand, in test number 21, since the post-heat treatment temperature was lower than the specified range of the present invention, the residual stress of the weld was not sufficiently removed, and stress relaxation cracks occurred at a high frequency of two or more cross sections. In Test Nos. 22 and 23, since the retention time in the post heat treatment was less than the lower limit time determined from the post heat treatment temperature, the residual stress in the welded portion was similarly not sufficiently removed, and severe welding as applied in this example was performed. In the part shape, stress relaxation cracks occurred at a high frequency of two or more sections due to long-term aging heat treatment.

Furthermore, in Test Nos. 24 and 25, since the retention time in the post-heat treatment exceeded the upper limit time determined from the post-heat treatment temperature, coarse M ₂₃ C ₆ carbide was generated in the post-heat treatment process, and the necessary creep strength could not be obtained. It was. In Test No. 26, the cooling rate in the post-heat treatment was lower than the lower limit determined from the post-heat treatment temperature, so that coarse M ₂₃ C ₆ carbide was generated in the post-heat treatment process, and the necessary creep strength was not obtained.

  According to the production method of the present invention, it is possible to stably obtain an austenitic heat-resistant alloy welded joint capable of achieving both high-temperature creep strength and stress relaxation crack resistance of a welded part during use.

Claims (6)

% By mass
C: 0.04 to 0.12%,
Si: 1.0% or less,
Mn: 2.0% or less,
P: 0.03% or less,
S: 0.01% or less,
Ni: 42.0-48.0%,
Cr: 20.0-26.0%,
W: 4.0 to 10.0%,
Ti: 0.05 to 0.15%,
Nb: 0.1-0.4%
Al: 0.3% or less,
B: 0.0001 to 0.01%
N: 0.02% or less,
O: 0.01% or less,
Ca: 0 to 0.05%,
Mg: 0 to 0.05%,
REM: 0-0.1%
Co: 0 to 1.0%,
Cu: 0 to 4.0%,
Mo: 0 to 1.0%,
V: 0 to 0.5%
The balance: an alloy base material having a chemical component that is Fe and impurities,
% By mass
C: 0.06-0.18%,
Si: 1.0% or less,
Mn: 2.0% or less,
P: 0.03% or less,
S: 0.01% or less,
Ni: 40.0-60.0%,
Cr: 20.0-26.0%,
Sum of one or both of Mo and W: 6.0 to 13.0%,
Ti: 0.05 to 0.6%,
Al: 1.5% or less,
N: 0.18% or less,
O: 0.01% or less,
Co: 0 to 15.0%,
Nb: 0 to 0.5%,
B: 0 to 0.005%,
The remainder: after welding using a welding material having chemical components that are Fe and impurities,
A method for producing an austenitic heat-resistant alloy welded joint, which is subjected to post-weld heat treatment under conditions satisfying the following formulas (i) to (iii):
800 ≦ T ≦ 1250 (i)
−0.2 × T + 270 ≦ t ≦ −0.6 × T + 810 (ii)
RC ≧ 0.05 × T-10 (iii)
However, the meaning of each symbol in the above formula is as follows.
T: Heat treatment temperature after welding (° C)
t: Heat treatment time after welding (min)
RC: Average cooling rate from T to 500 ° C (° C / h) The chemical composition of the alloy base material is mass%,
Ca: 0.0001 to 0.05%,
Mg: 0.0001 to 0.05%,
REM: 0.0005 to 0.1%,
Co: 0.01 to 1.0%
Cu: 0.01 to 4.0%,
Mo: 0.01 to 1.0%,
V: 0.01-0.5%
The manufacturing method of the austenitic heat-resistant-alloy weld joint of Claim 1 containing 1 or more types selected from these. The chemical composition of the welding material is mass%,
Co: 0.01 to 15.0%,
Nb: 0.01-0.5%
B: 0.0001 to 0.005%,
The manufacturing method of the austenitic heat-resistant-alloy weld joint of Claim 1 or Claim 2 containing 1 or more types selected from these. The method for producing an austenitic heat-resistant alloy welded joint according to any one of claims 1 to 3, wherein the post-weld heat treatment condition further satisfies the following formula (iv):
RH ≧ 40 (iv)
However, the meaning of the symbols in the above formula is as follows.
RH: Average heating rate from 500 ° C. to T (° C./h)   The manufacturing method of the austenitic heat-resistant alloy welded joint according to any one of claims 1 to 4, wherein a thickness of the alloy base material exceeds 30 mm.   An austenitic heat-resistant alloy welded joint obtained by using the production method according to any one of claims 1 to 5.