KR20180012813A - Austenitic heat-resistant alloys and welded structures - Google Patents

Abstract

An austenitic heat-resistant alloy capable of stably obtaining excellent crack resistance and high temperature strength. The austenitic heat-resistant alloy according to claim 1, wherein the chemical composition is 0.04 to 0.14% of C, 0.05 to 1% of Si, 0.5 to 2.5% of Mn, 0.03% or less of P, V: 0.1 to 0.6%, N: 0.1 to 0.3%, B: 0.0005 to 0.01%, Sn: 0.001 0 to 4%, 0 to 4% of Mo, 0 to 4% of Ca, 0 to 0.02% of Ca, 0 to 2% of Cr, 0 to 4% 0 to 0.02% of Mg, 0 to 0.2% of REM, and the remainder of Fe and impurities. The crystal grain size is not less than 2.0 and not more than 7.0 in terms of the grain size number specified in ASTM E112.

Description

Austenitic heat-resistant alloys and welded structures

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

In recent years, from the viewpoint of reducing the environmental load, the high temperature and high-pressure operation of the operating conditions of the power generation boiler and the like are progressing on a global scale. Materials used for superheating or reheating engines are required to have better high temperature strength and corrosion resistance.

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

For example, Japanese Patent Application Laid-Open No. 2004-250783 discloses an austenitic stainless steel excellent in high-temperature strength and corrosion resistance, in which N is 0.1 to 0.35%, Cr is more than 22% and less than 30% Steel has been proposed.

Japanese Patent Application Laid-Open No. 2009-084606 proposes an austenitic stainless steel excellent in high temperature strength and high corrosion resistance, in which N is 0.1 to 0.35%, Cr is more than 22% and less than 30%, and impurity elements are defined .

Japanese Unexamined Patent Publication (KOKAI) No. 12-1749 discloses an austenitic heat-resistant steel containing 0.09 to 0.30% of N and having a high temperature strength and excellent hot workability in which Mo and W are added in a large amount.

WO 2009/044796 discloses high strength austenitic stainless steels comprising 0.03-0.35% N and one or more of Nb, V and Ti.

These austenitic heat-resistant alloys are generally provided for use at high temperatures after being assembled by welding. However, when a welded structure using a high-N content austenitic heat-resistant alloy is used at a high temperature for a long time, cracks called SIPH (Strain Induced Precipitation Hardening) cracks sometimes occur in the weld heat affected zone .

The above-mentioned International Publication No. 2009/044796 discloses that it is possible to prevent cracks occurring at the time of use for a long period of time by defining the element for brittle the grain boundary and the element for strengthening the grain within a predetermined range. Certainly under certain conditions, cracking can be prevented by such a material. However, recently, austenitic heat-resistant alloys have been used in which a large amount of W, Mo or the like is added to further improve the performance such as high temperature strength. Such an austenitic heat-resistant alloy may not be able to prevent cracks stably depending on the welding conditions, the shape and dimensions of the structure, and the like. Specifically, when the heat input to the welding is increased, the thickness of the plate is increased, or the temperature is higher than 650 DEG C, cracking can not be stably prevented in some cases.

An object of the present invention is to provide an austenitic heat-resistant alloy which can stably obtain excellent crack resistance and high temperature strength.

The austenitic heat-resistant alloy according to one embodiment of the present invention has a chemical composition of 0.04 to 0.14% of C, 0.05 to 1% of Si, 0.5 to 2.5% of Mn, 0.03% or less of P, S: less than 0.001%, Ni: 23 to 32%, Cr: 20 to 25%, W: 1 to 5%, Nb: 0.1 to 0.6%, V: 0.1 to 0.6% 0 to 4% of Co, 0 to 4% of Mo, 0 to 4% of Cr, 0 to 4% of Cr, 0 to 4% of Cr, 0 to 4% of Cr, %, Ca: 0 to 0.02%, Mg: 0 to 0.02%, REM: 0 to 0.2%, balance: Fe and impurities and having a crystal grain size of not less than 2.0 times and not more than 7.0 times the crystal grain size specified in ASTM E112 I have.

According to the present invention, an austenitic heat-resistant alloy capable of stably obtaining excellent crack resistance and high temperature strength can be obtained.

1 is a cross-sectional view showing a shape of an improvement (opening) of a plate manufactured in the embodiment.

The present inventors conducted a detailed investigation to solve the above problems. As a result, we found the following knowledge.

In the welded joint using a high-N content austenitic heat-resistant alloy, the SIPH crack generated during use was examined in detail. As a result, (1) the cracks occurred at the grain boundaries of the weld heat affected zone of the coarse grains in the vicinity of the melt wire, and (2) a clear thickening of S was detected from the crack wavefront. (3) In the mouth near the crack, a large amount of nitride or carbonitride was precipitated. Particularly, it was remarkable when Nb was contained in a large amount. In addition, it was found that the larger the initial crystal grain size of the austenite heat-resistant alloy used (4), the larger the grain size of the weld heat affected zone and the easier the cracks to occur.

From these results, it can be considered that the SIPH crack is attributed to the fact that a large amount of nitride or carbonitride is precipitated in the mouth during use at a high temperature, and that the creep strain is concentrated on the grain boundaries there was. S is segregated at the grain boundaries during welding or during use, and the bonding force of the grain boundaries is lowered. In addition, the larger the crystal grain size, the smaller the grain boundary area per unit volume. The grain boundaries function as nucleation sites of nitrides and carbonitrides. Therefore, when the grain boundary is decreased, nitride and carbonitride are liable to precipitate in a larger amount in the mouth. In addition, creep deformation caused by an external force applied during use, for example, welding residual stress, etc., is more likely to be concentrated at a specific grain boundary surface. Therefore, it was thought that the larger the initial crystal grain size of the base material, the easier cracks to occur. Particularly, at a high temperature exceeding 650 占 폚, precipitates are precipitated in a short period of time, and since grain boundary segregation occurs early, it is considered that the problem becomes more susceptible to surface appearance.

In order to prevent this cracking, it is effective to reduce the elements that increase the deformation resistance in the mouth by precipitation strengthening or solid solution strengthening. However, these elements are indispensable elements in terms of securing creep strength at high temperatures. Therefore, there is a trade-off relationship between the prevention of cracking and the securing of creep strength at a high temperature, and it is difficult to make them compatible.

As a result of repeated investigations, it has been found that the alloy containing 0.04 to 0.14% of C, 0.05 to 1% of Si, 0.5 to 2.5% of Mn, 0.03% or less of P, 23 to 32% of Ni, 20 to 25% of Cr, In order to prevent SIPH cracks in the austenitic heat-resistant alloys containing at least one of Nb and at least one element selected from the group consisting of Nb and at least one element selected from the group consisting of Nb, S content is controlled to be 0.1 to 0.6% and less than 0.001%, respectively, and the initial particle size of the base material is controlled to 2.0 or more as determined by the American Society for Testing and Material (ASTM) It is clear that it is effective to do. However, if the crystal grain size is made finer than necessary and the Nb content is restricted, the creep strength of the base material does not satisfy the predetermined value. Therefore, it was found that the crystal grain size needs to be less than 7.0 as the crystal grain size number. In addition, it has been found that the inclusion of 0.1 to 0.6% of V having a lower precipitation strengthening ability than Nb is necessary to satisfy a predetermined creep strength without damaging the SIPH cracking property.

It has been confirmed that SIPH crack can be surely prevented by such measures, but it has been found that there is a possibility that another problem may arise while continuing the examination.

As described above, austenitic heat-resistant alloys are often assembled by welding. When these are welded, usually a consumable material is used. However, even in the case of small-sized thin-walled parts and thick-walled parts, gas-shielded arc welding may be carried out without using a consumable material in the case of super-layer welding or seamless welding. At this time, if the penetration depth is insufficient, the abutting surface of unmelted remains as a defect, and the required strength in the welded joint is not obtained. S has the effect of lowering the SIPH cracking resistance on the one hand and increasing the penetration depth. Therefore, from the viewpoint of the SIPH cracking resistance, if the S content is strictly controlled to be less than 0.001%, it is found that the problem of insufficient penetration tends to surface.

In order to prevent insufficient penetration, it is sufficient to increase the heat input of the welding simply. However, if the heat input to the welding is increased, coarsening of the weld heat affected zone is promoted, and even if the initial particle size of the base material is 2.0 or more as the crystal grain size number, it is impossible to prevent SIPH cracking.

As a result of the investigation, it has been found that it is effective to contain Sn in a range of 0.001 to 0.02% in order to stably prevent penetration failure. This could be attributed to Sn being easily evaporated from the surface of the melting paper during welding and being ionized in the arc, thereby contributing to the formation of a current carrying path and increasing the current density of the arc.

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

[Chemical Composition]

The austenitic heat-resistant alloy according to the present embodiment has the chemical composition described below. In the following description, "%" of the content of element means% by mass.

C: 0.04 to 0.14%

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 excessively contained, a large amount of carbide precipitates, and the SIPH cracking resistance is deteriorated. Therefore, the upper limit is set to 0.14%. 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 effective for deoxidizing and 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, if Si is contained excessively, the stability of the structure is deteriorated and the toughness and the creep strength are lowered. Therefore, the upper limit is set to 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 0.6%, more preferably 0.5%.

Mn: 0.5 to 2.5%

Manganese (Mn), like Si, has a deoxidizing action. Mn also contributes to the stabilization of the austenite structure. In order to sufficiently obtain this effect, it is necessary to contain not less than 0.5%. However, if Mn is excessively contained, it causes brittleness of the alloy, and creep ductility also deteriorates. Therefore, the upper limit is 2.5%. The lower limit of the Mn content is preferably 0.6%, more preferably 0.7%. The upper limit of the Mn content is preferably 2%, more preferably 1.5%.

P: not more than 0.03%

Phosphorus (P) is contained in the alloy as an impurity and is segregated at the crystal grain boundaries of the weld heat affected zone during welding to enhance liquefaction crack susceptibility. P also lowers the creep ductility after prolonged use. Therefore, the upper limit of the P content is set to 0.03% or less. The upper limit of the P content is preferably 0.028%, more preferably 0.025%. The P content is preferably reduced as much as possible, but extreme reduction results in an increase in steelmaking cost. Therefore, the lower limit of the P content is preferably 0.0005%, and more preferably 0.0008%.

S: less than 0.001%

Sulfur (S) is contained in the alloy as an impurity in the same manner as P, and is segregated at the grain boundaries of the weld heat affected zone during welding, thereby enhancing the susceptibility to cracking in liquefaction. S is also an element that segregates at grain boundaries during prolonged use to cause embrittlement and greatly deteriorates SIPH cracking resistance. In order to prevent these in the range of the chemical composition of the present embodiment, the S content should be less than 0.001%. The upper limit of the S content is preferably 0.0008%, and more preferably 0.0005%. The S content is preferably reduced as much as possible, but extreme reduction results in an increase in steelmaking cost. Therefore, the lower limit of the S content is preferably 0.0001%, and more preferably 0.0002%.

Ni: 23 to 32%

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 in the range of Cr and W contents in the present embodiment, it is necessary to contain Ni at 23% or more. However, Ni is an expensive element, and a large amount of Ni causes an increase in cost. Therefore, the upper limit is set to 32%. The lower limit of the Ni content is preferably 25%, more preferably 25.5%. The upper limit of the Ni content is preferably 31.5%, more preferably 31%.

Cr: 20 to 25%

Chromium (Cr) is an essential element for ensuring oxidation resistance and corrosion resistance at high temperatures. Cr also forms fine carbides and contributes to ensuring creep strength. In order to sufficiently obtain this effect in the Ni content range of the present embodiment, it is necessary to contain Cr of 20% or more. However, when Cr is excessively contained, the stability of the austenite phase at high temperature is deteriorated and the creep strength is lowered. Therefore, the upper limit is set to 25%. The lower limit of the Cr content is preferably 20.5%, more preferably 21%. The upper limit of the Cr content is preferably 24.5%, more preferably 24%.

W: 1-5%

Tungsten (W) is dissolved in a matrix or forms a fine intermetallic compound, which contributes greatly to improvement of creep strength and tensile strength at high temperatures. In order to obtain this effect sufficiently, it is necessary to contain 1% or more. However, if W is contained excessively, deformation resistance in the mouth is increased, and the SIPH cracking resistance is lowered, and the creep strength is sometimes lowered. Further, W is an expensive element, and a large amount of W causes an increase in cost. Therefore, the upper limit is set to 5%. The lower limit of the W content is preferably 1.2%, more preferably 1.5%. The upper limit of the W content is preferably 4.5%, more preferably 4%.

Nb: 0.1 to 0.6%

Niobium (Nb) precipitates in the form of Z phase (CrNbN) in addition to being precipitated as fine MX type carbonitrides, and contributes greatly to 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, if Nb is excessively contained, the ability of the precipitates to strengthen it is so large that the SIPH cracking resistance is lowered, and creep ductility and toughness are lowered. Therefore, the upper limit is 0.6%. The lower limit of the Nb content is preferably 0.12%, more preferably 0.15%. The upper limit of the Nb content is preferably 0.55%, more preferably 0.5%.

V: 0.1 to 0.6%

Vanadium (V) precipitates in the mouth as fine MX-type carbonitrides, and contributes to 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 V is excessively contained, carbonitrides are precipitated in a large amount, and the SIPH cracking property is lowered, and creep ductility and toughness are lowered. Therefore, the upper limit is 0.6%. The lower limit of the V content is preferably 0.12%, more preferably 0.15%. The upper limit of the V content is preferably 0.55%, more preferably 0.5%.

N: 0.1 to 0.3%

Nitrogen (N) stabilizes the austenite structure and is dissolved in the matrix or precipitated as a nitride, thereby contributing to improvement of the high temperature strength. In order to obtain this effect sufficiently, it is necessary to contain 0.1% or more. However, if N is excessively contained, a large amount of fine nitrides are precipitated in the mouth during long-time use due to solidification during short-time use, thereby increasing intragranular deformation resistance and degrading SIPH cracking resistance. Also, creep ductility and toughness are lowered. Therefore, the upper limit is set to 0.3%. The lower limit of the N content is preferably 0.12%, more preferably 0.14%. The upper limit of the N content is preferably 0.28%, more preferably 0.26%.

B: 0.0005 to 0.01%

Boron (B) improves the creep strength by finely dispersing the intergranular 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 excessively contained, B is segregated to a large extent in the weld heat affected zone in the vicinity of the melting boundary due to the welding heat cycle during welding, so that the melting point of the grain boundary is lowered and the liquefaction crack susceptibility is enhanced. Therefore, the upper limit is set to 0.01%. The lower limit of the B content is preferably 0.0008, more preferably 0.001%. The upper limit of the B content is preferably 0.008%, more preferably 0.006%.

Sn: 0.001 to 0.02%

Tin (Sn) has the effect of increasing the depth of penetration during welding by evaporating from the melting paper to increase the current density of the arc. In order to obtain this effect sufficiently, it is necessary to contain 0.001% or more. However, when Sn is excessively contained, the susceptibility to liquefied cracks in the weld heat affected zone during welding and the susceptibility to cracking during SIPH during use increase. Therefore, the upper limit is set to 0.02%. The lower limit of the Sn content is preferably 0.0015%, more preferably 0.002%. The upper limit of the Sn content is preferably 0.018%, more preferably 0.015%.

Al: 0.03% or less

Aluminum (Al) has a deoxidizing action. However, if Al is contained excessively, the cleanliness of the alloy deteriorates and the hot workability deteriorates. Therefore, the upper limit is set to 0.03%. The upper limit of the Al content is preferably 0.025%, more preferably 0.02%. The lower limit does not need to be specially set, but an extreme reduction results in 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 an acid soluble Al (sol. Al).

O: 0.02% or less

Oxygen (O) is contained in the alloy as an impurity and has an effect of increasing the depth of penetration during welding. However, when O is excessively contained, the hot workability is deteriorated and the toughness and ductility are deteriorated. Therefore, the upper limit is set to 0.02%. The upper limit of the O content is preferably 0.018%, more preferably 0.015%. The lower limit does not need to be specially set, but an extreme reduction results in an increase in steelmaking costs. Therefore, the lower limit of the O content is preferably 0.0005%, more preferably 0.0008%.

The balance of the chemical composition of the austenitic heat-resistant alloy according to the present embodiment is Fe and impurities. The impurities referred to herein means an element incorporated from an ore used as a raw material, an element incorporated from scrap, or an environment of a manufacturing process when industrially producing a heat resistant alloy.

The chemical composition of the austenitic heat-resistant alloy according to the present embodiment may contain at least one element selected from the group consisting of the first to third groups described below instead of the above-mentioned part of Fe. The following elements are all optional elements. That is, the following elements are not necessarily contained in the austenitic heat-resistant alloy according to the present embodiment. In addition, only a part of them may be contained.

More specifically, for example, only one group among the groups from the first group to the third group may be selected, and at least one element may be selected from the group. In this case, it is not necessary to select all the elements belonging to the selected group. In the first group, a plurality of groups may be selected from the third group, and at least one element may be selected from each group. In this case, it is not necessary to select all the elements belonging to the selected group.

Group 1 Ti: 0 to 0.5%

The element belonging to the first group is Ti. Ti improves the creep strength of the alloy by precipitation strengthening.

Ti: 0 to 0.5%

Titanium (Ti), like Nb and V, bonds with carbon or nitrogen to form a fine carbide or carbonitride, thereby contributing to improvement of creep strength. This effect is obtained when Ti is contained in a small amount. However, if Ti is contained excessively, the amount of the precipitate becomes large, and the SIPH property and the creep ductility are deteriorated. 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%.

Group 2 Co: 0 to 2%, Cu: 0 to 4%, Mo: 0 to 4%

The elements belonging to the second group are Co, Cu, and Mo. These elements improve the creep strength of the alloy.

Co: 0 to 2%

Cobalt (Co), like Ni, is an austenite generating element, which enhances the stability of the austenite structure and contributes to improvement of the creep strength. When Co is contained in a small amount, this effect is obtained. However, Co is an extremely expensive element, and a large amount of Co 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%.

Cu: 0 to 4%

Copper (Cu), like Ni and Co, stabilizes the austenite structure and precipitates finely during use, contributing to improvement of creep strength. If Cu is contained in a small amount, this effect is obtained. However, if Cu is contained excessively, the hot workability is lowered. Therefore, the upper limit is 4%. The lower limit of the Cu content is preferably 0.01%, more preferably 0.03%. The upper limit of the Cu content is preferably 3.8%, more preferably 3.5%.

Mo: 0 to 4%

Molybdenum (Mo), like W, is dissolved in the matrix and contributes to improvement of creep strength and tensile strength at high temperature. If Mo is contained in a small amount, this effect is obtained. However, if Mo is contained excessively, deformation resistance in the mouth is increased, so that the SIPH cracking property is lowered, and the creep strength is lowered in some cases. Further, Mo is an expensive element, and a large amount of Mo causes an increase in cost. Therefore, the upper limit is 4%. The lower limit of the Mo content is preferably 0.01%, more preferably 0.03%. The upper limit of the Mo content is preferably 3.8%, more preferably 3.5%.

Group 3 Ca: 0 to 0.02%, Mg: 0 to 0.02%, REM: 0 to 0.2%

The 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 in a small amount, this effect is obtained. However, when Ca is excessively contained, the cleanliness of the alloy is remarkably lowered by bonding with oxygen, and the hot workability is lowered. Therefore, the upper limit is set to 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. If Mg is contained in a small amount, this effect is obtained. However, when Mg is excessively contained, the cleanliness of the alloy is remarkably lowered by bonding with oxygen, and the hot workability is rather lowered. Therefore, the upper limit is set to 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 the hot workability at the time of production. If the REM is contained in a small amount, this effect is obtained. However, if REM is contained excessively, the cleanliness of the alloy is remarkably lowered by bonding with oxygen, and the hot workability is lowered. Therefore, the upper limit is set to 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 term for the total of 17 elements of Sc, Y and lanthanoids, and the content of REM refers to the total content of one or more elements of REM. In addition, REM is generally contained in misch metal. For this reason, for example, Mishimetal may be added to the alloy so that the REM content falls within the above range.

Among the " REM ", it is more preferable to use Nd because Nd has a strong affinity with S and P, forms a sulfide or phosphite, and has an effect of lowering weld liquefied crack susceptibility.

[group]

Crystal size number: 2.0 or more and less than 7.0

The austenitic heat-resistant alloy according to the present embodiment has a crystal grain size of not less than 2.0 times and not more than 7.0 times as a grain size number specified in ASTM E112.

In order to impart sufficient SIPH cracking resistance to the weld heat affected zone of the welded structure using the austenitic heat-resistant alloy according to the present embodiment, even if a heat cycle by welding is applied, the crystal grain size of the weld heat affected zone is excessively large It is necessary to make the grain size of the structure before welding not less than 2.0 times as large as the grain size number specified in ASTM E112. However, when the crystal grain size becomes 7.0 or more, the required creep strength can not be obtained. Therefore, the crystal grain size should be 2.0 or more and less than 7.0.

The above-mentioned structure having a crystal grain size is obtained by heat-treating an alloy having the chemical composition described above under appropriate conditions. For example, the structure may be formed by forming an alloy of the above chemical composition into a predetermined shape by hot working or cold working, holding it at a temperature of 900 to 1250 캜 for 3 to 60 minutes, . The higher the holding temperature of the solid solution heat treatment and the longer the holding time, the larger the grain size (the grain size number becomes smaller). It is more preferable that the solid solution heat treatment is carried out at a temperature of 1120 to 1220 캜 for 3 to 45 minutes and then water-cooled, more preferably for 3 to 30 minutes at a temperature of 1140 to 1210 캜 and then water-cooled.

The austenitic heat-resistant alloys according to one embodiment of the present invention have been described above. According to this embodiment, it is possible to obtain an austenitic heat-resistant alloy which can stably obtain excellent crack resistance and high temperature strength.

<Examples>

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

Ingredients A to J having the chemical compositions shown in Table 1 were dissolved in a laboratory and injected into the ingot was hot-forged and hot-rolled in a temperature range of 1000 to 1150 캜 to obtain a plate having a thickness of 20 mm. The plate was cold rolled again to a thickness of 16 mm. This plate was held at a temperature of 1200 캜 for a predetermined time and then subjected to a water-cooling treatment. After the heat treatment for solidification, it was molded into a plate having a thickness of 14 mm, a width of 50 mm and a length of 100 mm by machining. Separately from this plate, a sample for observation of the structure was taken from the plate subjected to the heat treatment for solubilization, and the crystal grain size of the structure was measured according to ASTM E112. With respect to the material A, the holding time of the solid solution heat treatment was varied in a range of 3 to 30 minutes to produce a material having a different crystal grain size.

[Welding workability]

The improvement process shown in Fig. 1 was performed along the longitudinal direction of the plate produced above. Welded joints were produced by butt welding two joints per each mark by the gas tungsten arc welding method while facing the plates subjected to the improvement processing. Welding did not use a consumable material and the heat input was 5 kJ / cm.

Among the obtained weld seams, all of the two seams were called &quot; passed &quot; in which the weld bead was formed to have a backside bead having a width of 2 mm or more over the entire length of the weld line. It was judged as "not possible" that there was a portion where no bead was formed even if some of the two joints had poor weldability.

[Cracking of the contents]

The above weld joints welded only in the superstructure layer are welded on a commercially available steel plate (thickness 30 mm, width 200 mm, length 200 mm) corresponding to SM400B specified in JIS G 3106 (2008) The ENi6625 was used for constrained welding. Thereafter, laminate welding was carried out in an improvement by TIG welding at a heat input of 10 to 15 kJ / cm using a teig wire corresponding to SNi6625 specified in JIS Z 3334 (2011), and weld joints were produced by two seams per each mark .

One side of the weld seam of each mark was subjected to aging heat treatment at 700 DEG C x 500 hours. Samples were taken from each of the weld seams as welded and welded joints subjected to aging heat treatment such that the observation plane was the cross section of the joint (perpendicular to the weld bead). The specimens thus obtained were mirror-polished and corroded and then examined by an optical microscope to determine whether or not there was cracks in the weld heat affected zone. The welding seam, in which cracks were not observed in all of the five samples, was judged as "positive" and the welding seam in which cracks were observed in one sample as "a". It was judged that the welding seams in which cracks were observed in two or more samples were "impossible".

[Creep rupture strength]

From the welded seam welded as passed in the welded joint crack test, the welded creep rupture test specimen was taken so that the weld metal was located at the center of the parallel portion. The creep rupture test was performed under the conditions of 700 ° C and 167 MPa at which the target breaking time of the base material was about 1000 hours. And that the breaking time of the base material is 90% or more (that is, 900 hours or more) of the breaking time of the base material.

[Performance evaluation result]

Table 2 shows the performance evaluation results. Table 2 also shows the grain size numbers of the austenitic heat-resistant alloys of the respective marks.

Weld joints based on the austenitic heat-resistant alloys of Marks A-1 to A-4, B to D, and I are appropriate in chemical composition and the initial grain size of the base material is 2.0 to 7.0 . Such welded joints have a good weldability because the backside bead is formed over the entire length in the super-layer welding. In addition, even though the thickness of the base material was relatively small, 14 mm, even when the aging heat treatment was performed, cracks did not occur in the weld heat affected zone, and excellent crack resistance was exhibited. Also, the creep rupture strength at high temperature was sufficient.

A weld joint using the austenitic heat-resistant alloy of Mark A-5 as a base material had cracks which were considered to be SIPH cracks after the aging heat treatment. It can be considered that this is because the crystal grain size of the austenitic heat-resistant alloy of the mark A-5 was too much to assemble.

Weld joints based on the austenitic heat-resistant alloy of Mark A-6 had excellent crack resistance, but the creep rupture time was lower than the target. It can be considered that this is because the crystal grain size of the austenitic heat-resistant alloy of the mark A-6 was too fine.

In the weld joint using the austenitic heat-resistant alloy of Mark E as a base material, beads were not formed in a part of the super-layer welding. It can be considered that this is because the Sn content of the austenitic heat-resistant alloy of Mark E was too small.

The weld joint using the austenitic heat-resistant alloy of Mark F as a base material did not contain Sn, but contained a large amount of S, so that the backside bead was sufficiently formed. However, after the aging heat treatment, a crack was considered to be a SIPH crack.

Weld joints based on the austenitic heat-resistant alloys of Mark G had cracks which were considered to be liquefied cracks and SIPH cracks, respectively, as they were welded and after the aging heat treatment. This can be attributed to the fact that the Sn content of the austenitic heat-resistant alloy of Mark G was too large.

The weld seam having the base material of the austenitic heat-resistant alloy of Mark H was satisfactory in the weldability and the welded fracture toughness but did not satisfy the required creep strength. This is presumably because the Ni content of the austenitic heat-resistant alloy of Mark H was too small and the phase stability became unstable.

Weld joints based on the austenitic heat-resistant alloy of Mark J did not satisfy the required creep strength. It can be considered that this is because the amount of V contained in the austenitic heat-resistant alloy of Mark J is lower than the lower limit.

&Lt; Industrial Availability >

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

Claims (3)

Chemical composition, in% by mass,
C: 0.04 to 0.14%,
Si: 0.05 to 1%
Mn: 0.5 to 2.5%
P: 0.03% or less,
S: less than 0.001%
Ni: 23 to 32%
Cr: 20 to 25%
W: 1 to 5%,
0.1 to 0.6% of Nb,
V: 0.1 to 0.6%,
N: 0.1 to 0.3%,
B: 0.0005 to 0.01%
Sn: 0.001 to 0.02%,
Al: 0.03% or less,
O: 0.02% or less,
Ti: 0 to 0.5%,
Co: 0 to 2%,
Cu: 0 to 4%,
Mo: 0 to 4%,
Ca: 0 to 0.02%,
Mg: 0 to 0.02%,
REM: 0 to 0.2%,
Balance: Fe and impurities,
An austenitic heat-resistant alloy having a crystal grain size of not less than 2.0 and not more than 7.0 in terms of the grain size number specified in ASTM E112. The method according to claim 1,
The austenitic heat-resistant alloy according to any one of claims 1 to 3, wherein the chemical composition comprises, by mass%, at least one element selected from the group consisting of the following first to third groups.
Group 1: Ti: 0.01-0.5%
Group 2: Co: 0.01 to 2%, Cu: 0.01 to 4%, Mo: 0.01 to 4%
Group 3: 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 or 2.

Images