JP2017014575A - Austenitic heat resistant alloy and weldment structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic heat resistant alloy excellent in weldment processability, crack resistance of a heat affected zone during welding, and high temperature strength.SOLUTION: An austenitic heat resistant alloy has a chemical composition of, by mass%, C:0.04 to 0.15%, Si:0.05 to 1%, Mn:0.3 to 2.5%, P:0.04% or less, S:0.002% or less, Cu:2 to 4%, Ni:11 to 16%, Cr:16 to 20%, W:2 to 5%, Nb:0.1 to 0.8%, N:0.001 to 0.15%, B:0.0005 to 0.01%, Al:0.03% or less, O:0.02% or less, one or more of Se, Te, Bi, Sn, Zn and Pb of total 0.001 to 0.03%, V:0 to 0.5%, Ti:0 to 0.5%, Co:0 to 2%, Mo:0 to 5%, Ca:0 to 0.02%, Mg:0 to 0.02%, REM:0 to 0.2% and the balance Fe with impurities.SELECTED DRAWING: None

Description

  The present invention relates to an austenitic heat resistant alloy and a welded structure including the alloy.

  In recent years, from the viewpoint of reducing the environmental load, high-temperature and high-pressure operation conditions for power generation boilers and the like have been promoted on a global scale. Materials used for superheater tubes and reheater tubes are required to have better high-temperature strength and corrosion resistance.

  As materials satisfying such requirements, various austenitic heat-resistant alloys containing a large amount of nitrogen have been disclosed.

  For example, Patent Document 1 (Japanese Patent Application Laid-Open No. Sho 62-133048) discloses an austenitic steel excellent in high-temperature strength containing 0.05 to 0.35% N and 0.05 to 1.5% Nb. Has been. Patent Document 2 (Japanese Patent Laid-Open No. 2000-256803) includes 0.05 to 0.3% of N and 0.05 to 0.2% of Nb (%) / Cu (%) to form a solution. An austenitic stainless steel excellent in high-temperature strength and ductility is disclosed in which the amount of undissolved Nb after treatment is 0.04 × Cu (%) to 0.085 × Cu (%).

  Patent Document 3 (Japanese Unexamined Patent Publication No. 2000-328198) includes N of 0.05 to 0.3%, Cu of 2 to 6%, and one or more of Y, La, Ce and Nd. At a high temperature with a specific range of numerical values represented by the relational expressions of Mn, Mg, Ca, Y, La, Ce, Nd, Al, Cu, and S An austenitic stainless steel excellent in strength and hot workability is disclosed.

  In Patent Document 4 (Japanese Patent Laid-Open No. 2003-268503), N is contained in an amount of 0.005 to 0.2%, and the crystal grain size number is set to 7 or more so that high temperature strength and steam oxidation resistance can be achieved. An excellent austenitic stainless steel pipe is disclosed. Patent Document 5 (International Publication No. 2013/073055) includes N in an amount of 0.005 to 0.3%, and the surface layer portion is covered with a high energy density processing layer having an average thickness of 5 to 30 μm. An austenitic stainless steel having excellent steam oxidation resistance is disclosed.

  Patent Document 6 (Japanese Patent Application Laid-Open No. 2013-44013) discloses austenite containing 0.07 to 0.13% of N and having high temperature strength adjusted to austenite balance by Mo, W and other alloy elements and toughness after aging. A heat resistant steel is disclosed. Patent Document 7 (Japanese Patent Laid-Open No. 2014-88593) describes austenitic stainless steel containing N of 0.10 to 0.35% and Ta of 0.25 to 0.8% and excellent in high temperature strength and oxidation resistance. Steel is disclosed.

  Patent Document 8 (International Publication No. 2009/044796) discloses a high-strength austenitic system containing 0.03 to 0.35% N and one or more of Nb, V, and Ti. Stainless steel is disclosed.

JP-A-62-133048 JP 2000-256803 A JP 2000-328198 A JP 2003-268503 A International Publication No. 2013/073055 JP 2013-44013 A JP 2014-88593 A International Publication No. 2009/044796 JP-A-8-120410

  The austenitic heat-resistant alloy as described above is generally assembled by welding. When these are welded, a filler material is usually used. However, gas shield arc welding may be performed without using a filler material in first layer welding or tack welding even for small thin parts or thick parts. At this time, if the penetration depth is insufficient, the unmelted butted surface remains as a defect, and the required strength cannot be obtained in the welded joint.

  In order to prevent poor penetration, welding heat input may be increased. However, in recent years, austenitic heat-resistant alloys have been used in which a large amount of W, Mo or the like is added to measure further improvement in performance such as high-temperature strength. For these austenitic heat-resistant alloys, if the welding heat input is increased, weld cracks such as liquefaction cracks in the weld heat affected zone, undercuts and burn-out, etc. occur and the soundness of the welded joints decreases. Turned out to be.

  The above-mentioned patent documents 1 to 8 do not describe this problem. On the other hand, Patent Document 9 (Japanese Patent Laid-Open No. 8-120410) discloses a stainless steel containing a large amount of Mn and capable of increasing the penetration depth. Further, the patent document describes that S in the material increases the penetration depth. However, in the austenitic heat-resistant alloy used at high temperatures, Mn decreases the creep strength. Furthermore, S significantly increases the sensitivity of cracks called liquefaction cracks in the weld heat affected zone during welding.

  An object of the present invention is to provide an austenitic heat-resistant alloy excellent in welding workability, crack resistance of a weld heat affected zone during welding, and high temperature strength.

  The austenitic heat resistant alloy according to one embodiment of the present invention has a chemical composition of mass%, C: 0.04 to 0.15%, Si: 0.05 to 1%, Mn: 0.3 to 2.5. %, P: 0.04% or less, S: 0.002% or less, Cu: 2 to 4%, Ni: 11 to 16%, Cr: 16 to 20%, W: 2 to 5%, Nb: 0.0. 1 to 0.8%, N: 0.001 to 0.15%, B: 0.0005 to 0.01%, Al: 0.03% or less, O: 0.02% or less, and Se, Te, Total of one or more of Bi, Sn, Zn, and Pb: 0.001 to 0.03%, V: 0 to 0.5%, Ti: 0 to 0.5%, Co: 0 to 2 %, Mo: 0 to 5%, Ca: 0 to 0.02%, Mg: 0 to 0.02%, REM: 0 to 0.2%, balance: Fe and impurities.

  According to the present invention, an austenitic heat-resistant alloy excellent in welding workability, crack resistance of a weld heat affected zone during welding, and high-temperature strength can be obtained.

FIG. 1 is a cross-sectional view showing the shape of a groove of a plate produced in the example.

  The present inventors conducted a detailed investigation in order to solve the above problems. As a result, the following findings became clear.

  That is, C: 0.04 to 0.15%, Si: 0.05 to 1%, Mn: 0.3 to 2.5%, P: 0.04% or less, S: 0.002% or less, Cu : 2-4%, Ni: 11-16%, Cr: 16-20%, W: 2-5%, Nb: 0.1-0.8%, N: 0.001-0.15%, B In order to obtain a sufficient penetration depth in an austenitic heat-resistant alloy containing: 0.0005 to 0.01%, Al: 0.03% or less, and O: 0.02% or less, Se, Te, Bi It has been found that it is necessary to contain at least one of Sn, Zn and Pb.

  The reason was considered as follows. Of these elements, Se, Te, and Bi act as surface activating elements, and have an effect of strengthening inward convection in the molten pool even when contained in a trace amount. Therefore, it becomes easy to transport the heat from the arc in the depth direction, contributing to an increase in the penetration depth. On the other hand, Sn, Zn, and Pb tend to evaporate from the surface of the molten pool being welded, and contribute to the formation of a current-carrying path by ionizing in the arc. Therefore, the current density of the arc is increased, and the effect of increasing the penetration depth is exhibited.

  In order to make use of the effects of these elements and obtain a sufficient penetration depth, the inventors of the present invention are effective to contain at least one of these elements in a total amount of 0.001% or more. I found. On the other hand, in order to increase the cracking susceptibility of the weld heat-affected zone during welding, it has also been clarified that the total content of these elements needs to be 0.03% or less.

  Based on the above findings, the present invention has been completed. Hereinafter, an austenitic heat resistant alloy according to an embodiment of the present invention will be described in detail.

[Chemical composition]
The austenitic heat-resistant alloy according to this embodiment has a chemical composition described below. In the following description, “%” of the element content means mass%.

C: 0.04 to 0.15%
Carbon (C) stabilizes the austenite structure and forms fine carbides to improve the creep strength during high temperature use. In order to obtain this effect sufficiently, it is necessary to contain 0.04% or more. However, when C is contained excessively, the effect is saturated and a large amount of carbides are precipitated, and the creep ductility is lowered. Therefore, the upper limit is made 0.15%. The lower limit of the C content is preferably 0.05%, more preferably 0.06%. The upper limit of the C content is preferably 0.13%, more preferably 0.12%.

Si: 0.05 to 1%
Silicon (Si) is an element that has a deoxidizing action and is effective in improving corrosion resistance and oxidation resistance at high temperatures. In order to obtain this effect sufficiently, it is necessary to contain 0.05% or more. However, when Si is contained excessively, the stability of the structure is lowered, and the toughness and the creep strength are lowered. Therefore, the upper limit is 1%. The lower limit of the Si content is preferably 0.08%, more preferably 0.1%. The upper limit of the Si content is preferably 0.5%, more preferably 0.35%.

Mn: 0.3 to 2.5%
Manganese (Mn), like Si, has a deoxidizing action. Mn is also an element that contributes to the stabilization of the austenite structure, and has the effect of increasing the arc current density and increasing the penetration depth. In order to obtain this effect sufficiently, it is necessary to contain 0.3% or more. However, when Mn is contained excessively, embrittlement of the alloy is caused and creep ductility is further reduced. Therefore, the upper limit is set to 2.5%. The lower limit of the Mn content is preferably 0.4%, more preferably 0.5%. The upper limit of the Mn content is preferably 2%, more preferably 1.5%.

P: 0.04% or less Phosphorus (P) is contained in the alloy as an impurity and segregates at the grain boundary of the heat-affected zone during welding to increase the liquefaction cracking sensitivity. P further reduces the creep ductility after long-term use. Therefore, an upper limit is set for the P content to 0.04% or less. The upper limit of the P content is preferably 0.035%, more preferably 0.03%. Although it is preferable to reduce the P content as much as possible, extreme reduction leads to an increase in steelmaking costs. Therefore, the lower limit of the P content is preferably 0.0005%, and more preferably 0.0008%.

S: 0.002% or less Sulfur (S) is contained in the alloy as an impurity like P. Although S has the effect of increasing the penetration depth during welding, S segregates at the grain boundaries of the weld heat affected zone during welding to increase the liquefaction cracking sensitivity. Therefore, the upper limit is made 0.002%. The upper limit of the S content is preferably 0.0018%, more preferably 0.0015%. The S content is preferably reduced as much as possible, but extreme reduction leads to an increase in steelmaking costs. Therefore, the lower limit of the S content is preferably 0.0001%, more preferably 0.0002%.

Cu: 2 to 4%
Copper (Cu) stabilizes the austenite structure and precipitates finely during use, contributing to the improvement of creep strength. In order to obtain this effect sufficiently, it is necessary to contain 2% or more. However, when Cu is contained excessively, the hot workability is lowered. Therefore, the upper limit is 4%. The lower limit of the Cu content is preferably 2.3%, more preferably 2.5%. The upper limit of the Cu content is preferably 3.8%, more preferably 3.5%.

Ni: 11-16%
Nickel (Ni) is an essential element for ensuring the stability of the austenite phase when used for a long time. In order to sufficiently obtain this effect within the Cr and W content ranges of the present embodiment, it is necessary to contain 11% or more of Ni. However, Ni is an expensive element, and a large amount causes an increase in cost. Therefore, the upper limit is 16%. The lower limit of the Ni content is preferably 11.5%, more preferably 12%. The upper limit of the Ni content is preferably 15.5%, more preferably 15%.

Cr: 16-20%
Chromium (Cr) is an essential element for ensuring oxidation resistance and corrosion resistance at high temperatures. Cr also contributes to ensuring creep strength by forming fine carbides. In order to sufficiently obtain this effect within the range of the Ni content of the present embodiment, it is necessary to contain 16% or more of Cr. However, when Cr is excessively contained, the structural stability of the austenite phase at high temperatures deteriorates and the creep strength decreases. Therefore, the upper limit is 20%. The lower limit of the Cr content is preferably 16.5%, and more preferably 17%. The upper limit of the Cr content is preferably 19.5%, more preferably 19%.

W: 2-5%
Tungsten (W) greatly contributes 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. In order to obtain this effect sufficiently, it is necessary to contain 2% or more. However, even if W is contained excessively, the effect is saturated and the creep strength may be lowered. Furthermore, W is an expensive element, and a large amount causes an increase in cost. Therefore, the upper limit is 5%. The lower limit of the W content is preferably 2.2%, more preferably 2.5%. The upper limit of the W content is preferably 4.8%, more preferably 4.5%.

Nb: 0.1 to 0.8%
Niobium (Nb) precipitates in the grains as fine carbonitrides and contributes to the improvement of creep strength and tensile strength at high temperatures. In order to obtain this effect sufficiently, it is necessary to contain 0.1% or more. However, when Nb is contained excessively, a large amount of carbonitride precipitates, resulting in a decrease in creep ductility and toughness. Therefore, the upper limit is 0.8%. The lower limit of the Nb content is preferably 0.12%, more preferably 0.15%. The upper limit of the Nb content is preferably 0.7%, more preferably 0.65%.

N: 0.001 to 0.15%
Nitrogen (N) stabilizes the austenite structure and dissolves in the matrix or precipitates as a nitride, contributing to the improvement of the high-temperature strength. In order to obtain this effect sufficiently, it is necessary to contain 0.001% or more. However, when N is contained excessively, a large amount of fine nitride precipitates in the grains during use at a high temperature, and creep ductility and toughness deteriorate. Therefore, the upper limit is made 0.15%. The lower limit of the N content is preferably 0.002%, more preferably 0.004%. The upper limit of the N content is preferably 0.14%, more preferably 0.12%.

B: 0.0005 to 0.01%
Boron (B) improves the creep strength by finely dispersing grain boundary carbides and segregates at the grain boundaries to strengthen the grain boundaries. In order to obtain this effect sufficiently, it is necessary to contain 0.0005% or more. However, when B is contained excessively, B is segregated in a large amount 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, the upper limit is made 0.01%. The lower limit of the B content is preferably 0.0008, and more preferably 0.001%. The upper limit of the B content is preferably 0.008%, more preferably 0.006%.

Al: 0.03% or less Aluminum (Al) has a deoxidizing action. However, when Al is contained excessively, the cleanliness of the alloy is deteriorated and the hot workability is lowered. Therefore, the upper limit is made 0.03%. The upper limit of the Al content is preferably 0.025%, more preferably 0.02%. There is no particular need to provide a lower limit, but extreme reduction leads to an increase in steelmaking costs. Therefore, the lower limit of the Al content is preferably 0.0005%, and more preferably 0.001%. In the present invention, Al means acid-soluble Al (sol. Al).

O: 0.02% or less Oxygen (O) is contained as an impurity in the alloy and has the effect of increasing the penetration depth during welding. However, when O is contained excessively, hot workability is deteriorated and toughness and ductility are deteriorated. Therefore, the upper limit is made 0.02%. The upper limit of the O content is preferably 0.018%, more preferably 0.015%. There is no particular need to provide a lower limit, but extreme reduction leads to an increase in steelmaking costs. Therefore, the lower limit of the O content is preferably 0.0005%, more preferably 0.0008%.

Total of one or more of Se, Te, Bi, Sn, Zn, and Pb: 0.001 to 0.03%,
Of these elements, Se, Te, and Bi affect the temperature dependence of the surface tension of the molten metal, and Sn, Zn, and Pb evaporate from the molten pool and increase the current density of the arc. This has the effect of increasing the penetration depth during welding. In order to obtain this effect sufficiently, it is necessary to contain one or more of these elements in total of 0.001% or more. However, when these elements are contained excessively, the liquefaction cracking sensitivity of the weld heat affected zone during welding is increased. Therefore, the upper limit of the total content of these elements is 0.03%. The lower limit of the total content of these elements is preferably 0.0015%, more preferably 0.002%, and still more preferably 0.003%. The upper limit of the total content of these elements is preferably 0.025%, more preferably 0.02%.

  Note that the chemical composition of the austenitic heat-resistant alloy according to the present embodiment does not need to include all elements of Se, Te, Bi, Sn, Zn, and Pb, and may include at least one of these elements. It ’s fine.

  The balance of the chemical composition of the austenitic heat-resistant alloy according to this embodiment is Fe and impurities. An impurity here means the element mixed from the ore and scrap utilized as a raw material, or the element mixed from the environment of a manufacturing process, etc. when manufacturing a heat-resistant alloy industrially.

  The chemical composition of the austenitic heat-resistant alloy according to the present embodiment further contains one or more elements selected from any one of the following first to third groups in place of a part of the above-mentioned Fe. May be. The following elements are all selective elements. That is, none of the following elements may be contained in the austenitic heat-resistant alloy according to the present embodiment. Moreover, only a part may be contained.

  More specifically, for example, only one group may be selected from the groups from the first group to the third group, and one or more elements may be selected from the group. In this case, it is not necessary to select all elements belonging to the selected group. Further, a plurality of groups may be selected from the first group to the third group, and one or more elements may be selected from each group. Also in this case, it is not necessary to select all the elements belonging to the selected group.

First group V: 0 to 0.5%, Ti: 0 to 0.5%
Elements belonging to the first group are V and Ti. V and Ti improve the creep strength of the alloy by precipitation strengthening.

V: 0 to 0.5%
Vanadium (V), like Nb, combines with carbon or nitrogen to form fine carbides or carbonitrides and contributes to the improvement of creep strength. If V is contained even a little, this effect can be obtained. However, when V is contained excessively, the amount of precipitates increases and the creep ductility decreases. Therefore, the upper limit is 0.5%. The lower limit of the V content is preferably 0.01%, more preferably 0.03%. The upper limit of the V content is preferably 0.45%, more preferably 0.4%.

Ti: 0 to 0.5%
Titanium (Ti), like Nb, forms fine carbonitrides and contributes to the improvement of creep strength and tensile strength at high temperatures. This effect can be obtained if Ti is contained even a little. However, when Ti is contained excessively, the amount of precipitates becomes large and creep ductility and toughness are lowered. Therefore, the upper limit is 0.5%. The lower limit of the Ti content is preferably 0.01%, more preferably 0.03%. The upper limit of the Ti content is preferably 0.45%, more preferably 0.4%.

Second group Co: 0-2%, Mo: 0-5%
Elements belonging to the second group are Co and Mo. These elements improve the creep strength of the alloy.

Co: 0 to 2%
Cobalt (Co) is an austenite-forming element like Ni and Cu, and contributes to the improvement of creep strength by increasing the stability of the austenite structure. This effect can be obtained if Co is contained even a little. However, Co is an extremely expensive element, and a large amount causes an increase in cost. Therefore, the upper limit is 2%. The lower limit of the Co content is preferably 0.01%, more preferably 0.03%. The upper limit of the Co content is preferably 1.8%, more preferably 1.5%.

Mo: 0 to 5%
Molybdenum (Mo), like W, contributes to improving the creep strength and tensile strength at high temperatures by dissolving in a matrix. This effect can be obtained if even a small amount of Mo is contained. However, even if Mo is contained excessively, the effect is saturated and the creep strength may be lowered. Furthermore, Mo is an expensive element, and a large amount causes an increase in cost. Therefore, the upper limit is 5%. The lower limit of the Mo content is preferably 0.01%, more preferably 0.03%. The upper limit of the Mo content is preferably 4.8%, more preferably 4.5%.

Third group Ca: 0 to 0.02%, Mg: 0 to 0.02%, REM: 0 to 0.2%
Elements belonging to the third group are Ca, Mg, and REM. These elements improve the hot workability of the alloy.

Ca: 0 to 0.02%
Calcium (Ca) improves hot workability during production. If Ca is contained even a little, this effect is obtained. However, if Ca is contained excessively, it combines with oxygen to significantly reduce the cleanliness of the alloy, and on the contrary, the hot workability is reduced. Therefore, the upper limit is made 0.02%. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.001%. The upper limit of the Ca content is preferably 0.01%, more preferably 0.005%.

Mg: 0 to 0.02%
Magnesium (Mg), like Ca, improves hot workability during production. This effect can be obtained if even a small amount of Mg is contained. However, when Mg is contained excessively, it combines with oxygen to significantly reduce the cleanliness of the alloy, and on the contrary, the hot workability is lowered. Therefore, the upper limit is made 0.02%. The lower limit of the Mg content is preferably 0.0005%, more preferably 0.001%. The upper limit of the Mg content is preferably 0.01%, more preferably 0.005%.

REM: 0 to 0.2%
Rare earth elements (REM), like Ca and Mg, improve hot workability during production. This effect can be obtained if REM is contained even a little. However, when REM is contained excessively, it combines with oxygen and remarkably decreases the cleanliness of the alloy, and on the contrary, the hot workability decreases. Therefore, the upper limit is made 0.2%. The lower limit of the REM content is preferably 0.0005%, more preferably 0.001%. The upper limit of the REM content is preferably 0.15%, more preferably 0.1%.

  “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. REM is generally contained in misch metal. Therefore, for example, misch metal may be added to the alloy so that the REM content falls within the above range.

  In the above, the austenitic heat-resistant alloy by one Embodiment of this invention was demonstrated. According to this embodiment, an austenitic heat resistant alloy excellent in welding workability, crack resistance of the weld heat affected zone during welding, and high temperature strength can be obtained.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited to these examples.

  An ingot in which materials A to H having the chemical composition shown in Table 1 were melted and cast in a laboratory was hot forged and hot rolled in a temperature range of 1000 to 1150 ° C. to obtain a plate having a thickness of 15 mm. “Fn1” in Table 1 indicates the total content of Se, Te, Bi, Sn, Zn, and Pb. This plate was further cold-rolled to a thickness of 12 mm. The plate was held at 1200 ° C. for 10 minutes and then subjected to a solution heat treatment by water cooling. After the solution heat treatment, a plate having a thickness of 10 mm, a width of 50 mm, and a length of 100 mm was formed by machining.

[Welding workability]
The groove processing shown in FIG. 1 was performed along the longitudinal direction of the plate produced above. The plates subjected to the groove processing were butted together, and two joints were butt welded for each symbol by a gas tungsten arc welding method to produce a welded joint. Welding did not use a filler material, and the heat input was 5 kJ / cm.

  Among the obtained welded joints, the two joints in which the back bead having a width of 1 mm or more was formed over the entire length of the weld line were regarded as acceptable as the welding workability was good. Of the accepted joints, those having a width of the back bead of 2 mm or more over the entire length were judged as “good”, and those having a part with a width of less than 2 mm were judged as “good”. Moreover, it was determined as “impossible” when there was a part where the back bead was not formed in some of the two joints and where there was a part where the width of the back bead was less than 1 mm.

[Weld crack resistance]
The above welded joint, in which only the first layer is welded, is defined in JIS Z 3224 (2010) on a commercially available steel plate (thickness 25 mm, width 200 mm, length 200 mm) equivalent to SM400B defined in JIS G 3106 (2008). 4 turns were restrained and welded using a covered arc welding rod ENi6625. After that, using a TIG wire corresponding to SNi6625 defined in JIS Z 3334 (2011), lamination welding was performed in the groove by TIG welding at a heat input of 10 to 15 kJ / cm to prepare a welded joint.

  Samples were collected from each of the five welded joints produced so that the observation surface was a cross section of the joint (cross section perpendicular to the weld bead). The collected sample was mirror-polished and corroded, and then examined with an optical microscope to investigate the presence of cracks in the weld heat affected zone. A weld joint in which no cracks were observed in all five samples was judged as “good”, and a weld joint in which cracks were observed in one sample was judged as “good”. A welded joint in which cracks were observed in two or more samples was judged as “impossible”.

[Creep rupture strength]
A round bar creep rupture test piece was collected from a welded joint that passed the weld cracking resistance test so that the weld metal was in the center of the parallel part. The creep rupture test was performed under the conditions of 700 ° C. and 186 MPa, at which the target rupture time of the base material was about 1000 hours. A material that was broken and the break time was 90% or more of the break time of the base material (that is, 900 hours or more) was determined as “pass”.

[Performance evaluation results]
The performance evaluation results are shown in Table 2.

  The chemical composition of the welded joint using the austenitic heat-resistant alloys of symbols A to E and H as the base material was appropriate. In these welded joints, the back bead was formed over the entire length in the first layer welding and had good weldability. Moreover, the weld heat-affected zone was not cracked and had the required crack resistance. Furthermore, the high temperature creep rupture strength was sufficient.

  In the welded joint using the austenitic heat-resistant alloy of the symbol F as a base material, a part of the back bead did not satisfy the criterion in the first layer welding. This is probably because the total content of Se, Te, Bi, Sn, Zn, and Pb was too small.

  In the welded joint using the austenitic heat-resistant alloy of surrogate G as a base material, cracks considered to be liquefaction cracks occurred in the weld heat affected zone. This is probably because the total content of Se, Te, Bi, Sn, Zn, and Pb was too much.

  The present invention can be suitably used as an austenitic heat-resistant alloy used as a high-temperature member such as a main steam pipe or a high-temperature reheat steam pipe of a power generation boiler.

Claims (3)

Chemical composition is mass%,
C: 0.04 to 0.15%,
Si: 0.05 to 1%
Mn: 0.3 to 2.5%
P: 0.04% or less,
S: 0.002% or less,
Cu: 2 to 4%,
Ni: 11-16%
Cr: 16 to 20%,
W: 2-5%
Nb: 0.1 to 0.8%
N: 0.001 to 0.15%,
B: 0.0005 to 0.01%,
Al: 0.03% or less,
O: 0.02% or less, and the total of one or more of Se, Te, Bi, Sn, Zn, and Pb: 0.001 to 0.03%,
V: 0 to 0.5%
Ti: 0 to 0.5%,
Co: 0 to 2%,
Mo: 0 to 5%,
Ca: 0 to 0.02%,
Mg: 0 to 0.02%,
REM: 0-0.2%
The balance: austenitic heat-resistant alloy which is Fe and impurities. The austenitic heat-resistant alloy according to claim 1,
An austenitic heat-resistant alloy, wherein the chemical composition contains one or more elements selected from the following first group to third group in mass%.
First group V: 0.01 to 0.5%, Ti: 0.01 to 0.5%
Second group Co: 0.01-2%, Mo: 0.01-5%
Third group Ca: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, REM: 0.0005 to 0.2%   A welded structure comprising the austenitic heat-resistant alloy according to claim 1.

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