KR102048479B1 - Austenitic Heat-resistant Alloys and Welded Structures - Google Patents

Abstract

Provided is an austenitic heat resistant alloy in which excellent crack resistance and high temperature strength are stably obtained. As for the austenitic heat-resistant alloy, the chemical composition is 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%, Nb: 0.1 to 0.6%, V: 0.1 to 0.6%, N: 0.1 to 0.3%, B: 0.0005 to 0.01%, Sn: 0.001 ~ 0.02%, Al: 0.03% or less, O: 0.02% or less, Ti: 0-0.5%, Co: 0-2%, Cu: 0-4%, Mo: 0-4%, Ca: 0-0.02% , Mg: 0% to 0.02%, REM: 0% to 0.2%, remainder: Fe and impurities, and have a grain size of 2.0 or more and less than 7.0 times with a grain size number specified in ASTM E112.

Description

Austenitic Heat-resistant Alloys and Welded Structures

The present invention relates to a welding structure comprising an austenitic heat resistant alloy and an alloy thereof.

In recent years, from the viewpoint of reducing the environmental load, the high temperature and high pressure of the operating conditions of the power generation boiler and the like have been advanced on a global scale. Materials used for superheat engines and reheat engines are required to have better high temperature strength and corrosion resistance.

As a material satisfying such a requirement, 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 having excellent high temperature strength and corrosion resistance, in which N is 0.1 to 0.35%, Cr is more than 22% and less than 30%, and the metal structure is defined. A river is proposed.

Japanese Unexamined Patent Publication No. 2009-084606 proposes an austenitic stainless steel having excellent high temperature strength corrosion resistance, in which N is 0.1 to 0.35%, Cr is more than 22% and less than 30%, and an impurity element is specified. .

Japanese Patent Application Laid-Open No. 2012-1749 discloses an austenitic heat resistant steel which contains 0.09% to 0.30% of N and has a high temperature strength and hot workability in which a large amount of Mo and W are added in combination.

International Publication No. 2009/044796 discloses high-strength austenitic stainless steels containing N of 0.03 to 0.35% and one or two or more of Nb, V, and Ti.

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

In International Publication No. 2009/044796 described above, it is described that cracks generated during long time use can be prevented by defining an element embrittling grain boundaries and an element for strengthening the particle size in a predetermined range. Certainly, under certain conditions, such materials can prevent cracking. However, in recent years, austenitic heat resistant alloys in which a large amount of W, Mo and the like are added to further improve the performance such as high temperature strength have been used. In such an austenitic heat-resistant alloy, cracks may not be prevented stably depending on welding conditions, shapes of structures, dimensions, and the like. Specifically, cracks may not be stably prevented when welding heat input is increased, plate thickness is increased, or when used at a high temperature exceeding 650 ° C.

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

As for the austenitic heat-resistant alloy which concerns on one Embodiment of this invention, chemical composition is mass%, C: 0.04-0.14%, Si: 0.05-1%, Mn: 0.5-2.5%, P: 0.03% or less, S: Less than 0.001%, Ni: 23-32%, Cr: 20-25%, W: 1-5%, Nb: 0.1-0.6%, V: 0.1-0.6%, N: 0.1-0.3%, 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%, remainder: Fe and impurities, and have a grain size of 2.0 or more and less than 7.0 times with the grain size specified in ASTM E112. Have

According to the present invention, an austenitic heat resistant alloy in which excellent crack resistance and high temperature strength are stably obtained is obtained.

1: is sectional drawing which shows the shape of the improvement of the board | substrate produced in the Example.

The present inventors made detailed investigation in order to solve said subject. As a result, the following findings were found.

In the weld joint using the austenitic heat-resistant alloy containing high N, SIPH cracks generated during use were examined in detail. As a result, (1) a crack generate | occur | produces in the grain boundary of the weld heat influence part of the granulation vicinity near a melting line, and (2) the clear thickening of S was detected from the crack fracture surface. (3) In the mouth near the crack, a large amount of nitride and carbonitride were deposited. In particular, it was remarkable when it contained a large amount of Nb. In addition, it was found that the larger the initial grain size of the austenitic heat-resistant alloy used (4), the larger the grain size of the weld heat-affected zone was, and cracks were more likely to occur.

From these, SIPH cracks are attributed to the precipitation of a large amount of nitrides and carbonitrides in the mouth during use at high temperatures, and it is thought that the creep deformation reached an opening as a result of the concentration of the creep being concentrated at the grain boundary due to the difficulty of deformation of the mouth. there was. S segregates in the grain boundary during welding or in use, and reduces the bonding force of the grain boundary. The larger the grain size, the smaller the area of grain boundaries per unit volume. The grain boundary functions as a nucleation site of nitride or carbonitride. Therefore, when the grain boundary decreases, nitrides and carbonitrides are more easily precipitated in the mouth. In addition, creep deformation caused by external force received during use, for example, weld residual stress, etc., tends to be more concentrated at a specific grain boundary. Therefore, it could be considered that the larger the crystal grain size of the initial stage of the base material, the easier the cracking occurs. In particular, at a high temperature exceeding 650 ° C, in addition to the precipitates being precipitated in a short time, the grain boundary segregation also occurs early, it was thought that problems tend to surface.

In order to prevent this crack, it is effective to reduce the element which raises strain resistance in a particle | grain by precipitation strengthening or solid solution strengthening. However, these elements are essential elements from the viewpoint of ensuring creep strength at high temperature. Therefore, the prevention of cracking and securing creep strength at high temperature are in a trade-off relationship, and it is difficult to make them compatible.

As a result of examination, C: 0.04 to 0.14%, Si: 0.05 to 1%, Mn: 0.5 to 2.5%, P: 0.03% or less, Ni: 23 to 32%, Cr: 20 to 25%, and W: 1 In the austenitic heat-resistant alloy containing -5%, N: 0.1-0.3%, B: 0.0005-0.01%, Al: 0.03% or less, and O: 0.02% or less, in order to prevent SIPH cracking, Nb and The S content is strictly controlled to 0.1 to 0.6% and less than 0.001%, respectively, and the initial grain size of the base material is set to 2.0 or more with the grain size specified in the American Society for Testing and Material (ASTM). Made it clear that it is valid to do. However, when the grain size is made finer than necessary and the Nb content is limited, the creep strength of the base material does not satisfy a predetermined value. Therefore, it turned out that it is necessary to set grain size less than 7.0 by the grain size number. In addition, it has been found that containing 0.1 to 0.6% of V having a lower precipitation strengthening capacity than Nb is necessary to satisfy a predetermined creep strength without impairing SIPH crack resistance.

By the way, although it was confirmed that SIPH cracking can be prevented by these measures, it turned out that another problem may arise while continuing review.

As described above, the austenitic heat resistant alloy is often assembled by welding. In the case of welding these, a filler material is usually used. However, even in the case of a small thin part or a thick part, in a first layer welding or a temporary welding, gas shielded arc welding may be performed without using a filler material. At this time, when the penetration depth is insufficient, the butt face of the unmelted melt remains as a defect, and the strength required for the welded joint is not obtained. S has the effect of reducing the SIPH crack resistance and increasing the penetration depth. Therefore, from the viewpoint of SIPH crack resistance, if the amount of S is strictly controlled to be less than 0.001%, it has been found that the problem of infiltration is likely to surface.

In order to prevent a penetration shortage, simply welding heat input may be large. However, when the welding heat input is increased, the coarsening of the weld heat affecting portion is encouraged, and SIPH cracking cannot be prevented even when the initial particle size of the base material is 2.0 or more by the crystal grain size number.

As a result of examination, it was found that it is effective to contain Sn in the range of 0.001% to 0.02% in order to stably prevent the penetration failure. It is thought that this is because Sn easily evaporates from the surface of the molten pool during welding and is ionized in the arc, thereby contributing to the formation of the energization path and increasing the current density of the arc.

Based on the above knowledge, this invention was completed. EMBODIMENT OF THE INVENTION Hereinafter, the austenitic heat-resistant alloy which concerns on one Embodiment of this invention is explained in full detail.

[Chemical composition]

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

C: 0.04 to 0.14%

Carbon (C) stabilizes austenite structure, forms fine carbides, and improves creep strength during high temperature use. In order to fully acquire this effect, it is necessary to contain 0.04% or more. However, when C is excessively contained, carbides are precipitated in a large amount, which lowers the SIPH crack resistance. Therefore, an upper limit is made into 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 in improving the corrosion resistance and oxidation resistance at high temperatures while having a deoxidizing action. In order to fully acquire this effect, it is necessary to contain 0.05% or more. However, when Si is excessively contained, the stability of the structure is lowered, leading to a decrease in toughness and creep strength. Therefore, an upper limit shall be 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 ~ 2.5%

Manganese (Mn) has a deoxidation effect similarly to Si. Mn also contributes to stabilization of the austenite structure. In order to fully acquire this effect, it is necessary to contain 0.5% or more. However, excessively containing Mn causes embrittlement of the alloy, and creep ductility is lowered. Therefore, an upper limit is made into 2.5%. The minimum of Mn content becomes like this. Preferably it is 0.6%, More preferably, it is 0.7%. The upper limit of the Mn content is preferably 2%, more preferably 1.5%.

P: 0.03% or less

Phosphorus (P) is contained in the alloy as impurities and segregates at the grain boundaries of the weld heat affected zone during welding to increase the liquefied cracking susceptibility. P also lowers creep ductility after long time use. Therefore, the upper limit is set to P content, and it is made into 0.03% or less. The upper limit of the P content is preferably 0.028%, more preferably 0.025%. It is preferable to reduce P content as much as possible, but the extreme reduction causes an increase in steelmaking cost. Therefore, the lower limit of the P content is preferably 0.0005%, more preferably 0.0008%.

S ： Less than 0.001%

Sulfur (S), like P, is contained in the alloy as impurities, segregates at the grain boundaries of the weld heat affected zone during welding, and increases the liquefied cracking susceptibility. S is also an element that segregates at grain boundaries during long time use, causes embrittlement, and greatly reduces the SIPH crack resistance. In order to prevent these in the chemical composition of this embodiment, it is necessary to make S content into less than 0.001%. The upper limit of the S content is preferably 0.0008%, more preferably 0.0005%. It is preferable to reduce S content as much as possible, but the extreme reduction causes an increase in steelmaking cost. Therefore, the minimum of S content becomes like this. Preferably it is 0.0001%, More preferably, it is 0.0002%.

Ni: 23-32%

Nickel (Ni) is an essential element in order to ensure the stability of the austenite phase when used for a long time. In order to fully acquire this effect in the range of Cr and W content of this embodiment, it is necessary to contain Ni 23% or more. However, Ni is an expensive element, and a large amount of content leads to an increase in cost. Therefore, an upper limit is made into 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: 20-25%

Chromium (Cr) is an essential element for securing oxidation resistance and corrosion resistance at high temperatures. Cr also forms fine carbides and contributes to securing creep strength. In order to fully acquire this effect in the Ni content range of this embodiment, it is necessary to contain Cr 20% or more. However, when Cr is excessively contained, the structure stability of the austenite phase at high temperature is deteriorated, and the creep strength is lowered. For this reason, the upper limit is made 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, and contributes greatly to the improvement of creep strength and tensile strength at high temperature. In order to fully acquire this effect, it is necessary to contain 1% or more. However, when W is excessively contained, the strain resistance in the mouth increases, the SIPH crack resistance decreases, and the creep strength may decrease. W is an expensive element, and a large amount of content leads to an increase in cost. Therefore, an upper limit is made into 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: 0.1-0.6%

Niobium (Nb) is precipitated in the mouth as Z phase (CrNbN) in addition to being precipitated as fine MX-type carbonitride, which greatly contributes to the improvement of creep strength and tensile strength at high temperature. In order to fully acquire this effect, it is necessary to contain 0.1% or more. However, when Nb is excessively contained, the reinforcing ability of these precipitates is too large, resulting in a decrease in the SIPH crack resistance and a decrease in creep ductility and toughness. Therefore, an upper limit shall be 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) is precipitated in the mouth as fine MX-type carbonitride, and contributes to the improvement of creep strength and tensile strength at high temperature. In order to fully acquire this effect, it is necessary to contain 0.1% or more. However, when V is excessively contained, a large amount of carbonitride is precipitated to reduce the SIPH cracking resistance, leading to a decrease in creep ductility and toughness. Therefore, an upper limit shall be 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: 0.1-0.3%

Nitrogen (N) stabilizes austenite structure, is dissolved in a matrix, or precipitates as a nitride, thereby contributing to the improvement of high temperature strength. In order to fully acquire this effect, it is necessary to contain 0.1% or more. However, when N is excessively contained, a large amount of fine nitride precipitates in the mouth during long-term use and by solid solution during long-term use, thereby increasing the intraoral strain resistance and lowering the SIPH crack resistance. In addition, creep ductility and toughness decrease. Therefore, an upper limit is made into 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 ~ 0.01%

Boron (B) improves creep strength by finely dispersing grain boundary carbides and segregates at grain boundaries to strengthen grain boundaries. In order to fully acquire this effect, it is necessary to contain 0.0005% or more. However, when B is excessively contained, a large amount of B segregates in the weld heat affected zone near the melting boundary due to the welding heat cycle during welding, thereby lowering the melting point of the grain boundary and increasing the liquefaction crack susceptibility. Therefore, an upper limit is made into 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: 0.001-0.02%

Tin (Sn) has the effect of increasing the penetration depth at the time of welding by evaporating from a molten paper and increasing the current density of an arc. In order to fully acquire this effect, it is necessary to contain 0.001% or more. However, when Sn is excessively contained, the liquefied cracking susceptibility of the weld heat affected zone during welding and the SIPH cracking susceptibility during use are increased. Therefore, an upper limit is made into 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 deoxidation effect. However, when Al is contained excessively, the cleanliness of an alloy will deteriorate and hot workability will fall. Therefore, an upper limit is made into 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 set in particular, but the extreme reduction causes an increase in steelmaking cost. Therefore, the minimum of Al content becomes like this. Preferably it is 0.0005%, More preferably, it is 0.001%. In the present invention, Al means acid soluble Al (sol. Al).

O: 0.02% or less

Oxygen (O) is contained in the alloy as impurities and has the effect of increasing the penetration depth during welding. However, when O is excessively contained, the hot workability is lowered and the toughness and the ductility deteriorate. Therefore, an upper limit is made into 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 set in particular, but the extreme reduction causes an increase in steelmaking cost. Therefore, the minimum of O content becomes like this. Preferably it is 0.0005%, More preferably, it is 0.0008%.

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

The chemical composition of the austenitic heat-resistant alloy according to the present embodiment may contain at least one element selected from any one of the following groups from the first group to the third group, instead of a part of the above Fe. The following elements are all optional elements. That is, all of the following elements do not need to 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 group 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 the elements belonging to the selected group. Moreover, you may select some group from 1st group from 3rd group, and may select 1 or more types of elements from each group. Also in this case, it is not necessary to select all the elements belonging to the selected group.

1st group 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, combines with carbon or nitrogen to form fine carbides or carbonitrides, contributing to the improvement of creep strength. If Ti is contained at least, this effect is obtained. However, when Ti is excessively contained, a large amount of precipitates will lower the SIPH resistance and creep ductility. Therefore, an upper limit is made into 0.5%. The lower limit of the Ti content is preferably 0.01%, more preferably 0.03%. Preferably the upper limit of Ti content is 0.45%, More preferably, it is 0.4%.

2nd group Co: 0-2%, Cu: 0-4%, Mo: 0-4%

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

Co: 0-2%

Cobalt (Co) is an austenite generating element similar to Ni, and contributes to the improvement of creep strength by increasing the stability of the austenite structure. If any amount of Co is contained, this effect is obtained. However, Co is an extremely expensive element, and a large amount of content leads to an increase in cost. Therefore, an upper limit is made into 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 ~ 4%

Like Ni and Co, copper (Cu) stabilizes austenite structure and finely precipitates during use, contributing to improvement of creep strength. If any amount of Cu is contained, this effect is obtained. However, when Cu is excessively contained, the hot workability is deteriorated. Therefore, an upper limit is made into 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 ~ 4%

Molybdenum (Mo), like W, is dissolved in a matrix and contributes to the improvement of creep strength and tensile strength at high temperature. If Mo is contained at all, this effect is obtained. However, when Mo is excessively contained, the strain resistance in the mouth is increased, the SIPH crack resistance is lowered, and the creep strength may be lowered in some cases. In addition, Mo is an expensive element, and a large amount of content leads to an increase in cost. Therefore, an upper limit is made into 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%

Elements belonging to the third group are Ca, Mg, and REM. These elements improve the hot workability of the alloy.

Ca ： 0 ~ 0.02%

Calcium (Ca) improves hot workability at the time of manufacture. If any amount of Ca is contained, this effect is obtained. However, when Ca is excessively contained, the cleanliness of the alloy is remarkably lowered by bonding with oxygen, and rather the hot workability is lowered. Therefore, an upper limit is made into 0.02%. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.001%. The upper limit of Ca content is preferably 0.01%, more preferably 0.005%.

Mg: 0% to 0.02%

Magnesium (Mg), like Ca, improves the hot workability at the time of manufacture. If any amount of Mg is contained, this effect is obtained. However, when Mg is excessively contained, the cleanliness of the alloy is remarkably lowered by bonding with oxygen, and rather the hot workability is lowered. Therefore, an upper limit is made into 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: 0-0.2%

The rare earth element (REM), like Ca and Mg, improves hot workability at the time of manufacture. If any amount of REM is contained, this effect is obtained. However, when the REM is excessively contained, the cleanliness of the alloy is significantly reduced by bonding with oxygen, and the hot workability is lowered rather. Therefore, an upper limit is made into 0.2%. The lower limit of the REM content is preferably 0.0005%, more preferably 0.001%. The upper limit of REM content becomes like this. Preferably it is 0.15%, More preferably, it is 0.1%.

"REM" is a generic term for 17 elements in total of Sc, Y and lanthanoid, and the content of REM refers to the total content of one or two or more elements in the REM. In addition, REM is generally contained in mismetal. For this reason, for example, a misch metal may be added to the alloy so that the content of REM falls within the above range.

In addition, among "REM", Nd has affinity with S and P, has the effect of forming a sulfide or a phosphide, and reducing weld liquefaction cracking sensitivity, and it is more preferable to utilize this.

[group]

Crystal grain size: It is less than 7.0 times more than 2.0 times

The austenitic heat-resistant alloy according to the present embodiment has a structure in which the grain size is 2.0 or more and 7.0 or less by the grain size number specified in ASTM E112.

In the weld structure using the austenitic heat-resistant alloy according to the present embodiment, in order to provide sufficient SIPH crack resistance to the weld heat affected zone, the crystal grain diameter of the weld heat affected zone is excessively adjusted even when subjected to heat cycle by welding. It is necessary to make the grain size of the structure before welding into the grain size 2.0 or more by the grain size number prescribed | regulated to ASTM E112 so that it may not become bad. However, if the grain size becomes fine grains of 7.0 or more, the required creep strength cannot be obtained. Therefore, the grain size is made into 2.0 times or more and less than 7.0 times.

The structure which has said crystal grain size is obtained by heat-processing the alloy of said chemical composition on appropriate conditions. This structure is formed by, for example, forming an alloy of the above chemical composition into a predetermined shape by hot working or cold working, and then maintaining a solution at a temperature of 900 to 1250 ° C. for 3 to 60 minutes to perform a solid solution heat treatment for cooling water. Is achieved. The higher the holding temperature of the solid solution heat treatment and the longer the holding time, the larger the grain size (the smaller the grain size number). It is more preferable to carry out water cooling after maintaining solid solution heat processing at the temperature of 1120-1220 degreeC for 3 to 45 minutes, and it is more preferable to carry out water cooling after maintaining for 3 to 30 minutes at the temperature of 1140-1210 degreeC.

In the above, the austenitic heat-resistant alloy which concerns on one Embodiment of this invention was demonstrated. According to this embodiment, the austenitic heat-resistant alloy in which excellent crack resistance and high temperature strength are stably obtained is obtained.

<Example>

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

The ingot which melt | dissolved and inject | poured the material of A-J which has the chemical composition shown in Table 1 by the hot forging and hot rolling in the temperature range of 1000-1150 degreeC was made into the board of thickness 20mm. The plate was further cold rolled to a thickness of 16 mm. The plate was held at 1200 ° C. for a predetermined time and then subjected to a solid solution heat treatment for cooling with water. After solid solution heat treatment, it shape | molded into the board of thickness 14mm, width 50mm, and length 100mm by machining. In addition to this plate, a sample for texture observation was taken from a plate subjected to a solid solution heat treatment, and the grain size of the structure was measured in accordance with ASTM E112. In addition, about the material A, the holding time of the solid solution heat treatment was changed in the range of 3-30 minutes, and the material from which a crystal grain size differs was produced.

[Welding workability]

The improvement process shown in FIG. 1 was implemented along the longitudinal direction of the plate produced above. The plates subjected to the improvement process were faced to each other, and the butt welding was performed by two joints per mark by the gas tungsten arc welding method to produce a welded joint. Welding did not use a filler material, and the heat input amount was 5 kJ / cm.

Among the obtained welded joints, the two joints were formed with a backside bead having a width of 2 mm or more over the entire length of the weld line, and were referred to as "passed" because the weldability was good. It was judged that there was a part in which a back side bead was not formed even in some of 2 joints, and was "not impossible" as having poor weldability.

[Content cracking]

The above-mentioned welding seam welded only to the first layer is coated arc welding rod specified in JIS Z 3224 (2010) on a commercially available steel plate (thickness 30 mm, width 200 mm, length 200 mm) equivalent to SM400B specified in JIS G 3106 (2008). Constrained welds were made around all sides using ENi6625. Thereafter, using a Tig wire corresponding to SNi6625 specified in JIS Z 3334 (2011), lamination welding was performed within the furnace by TIG welding at a heat input of 10 to 15 kJ / cm to produce a welded joint by two seams per mark. .

On one side of the welded joint of each mark, the aging heat treatment of 700 degreeC x 500 hours was performed. Samples were taken from each of the five welded joints as they were welded and the welded joints subjected to aging heat treatment so that the observation plane was a cross section (cross section perpendicular to the weld bead) of the joint. After the sample was mirror polished and corroded, it was inspected by an optical microscope to investigate the presence or absence of cracks in the weld heat affected zone. It was judged as "pass" for the weld joint in which the crack was not observed in all five samples as "amount", and the weld joint in which the crack was observed in one sample as "a". It was judged that the welded joint in which the crack was observed in two or more samples was "not possible".

[Creep Breaking Strength]

Round bar creep rupture test pieces were taken from the welded weld seam that passed the weld cracking test so that the weld metal was at the center of the parallel portion. The creep rupture test was done on 700 degreeC and 167 Mpa which the target breaking time of a base material will be about 1000 hours. The breaking of the base material and the breaking time of 90% or more (that is, 900 hours or more) of the breaking time of the base material were referred to as "passing".

[Performance evaluation result]

Table 2 shows the results of the performance evaluation. In Table 2, the grain size of the austenitic heat-resistant alloy of each mark is shown together.

Welding joints based on austenitic heat-resistant alloys of marks A-1 to A-4, B to D, and I have an appropriate chemical composition, and the initial particle diameter of the base material is from 2.0 to less than 7.0 times as the grain size. It was. Such weld joint had the back surface bead formed over the full length in superlayer welding, and had favorable weldability. In addition, even though the thickness of the base material was relatively thick (14 mm), even when the aging treatment was performed, cracks did not occur in the weld heat affected zone and had excellent crack resistance. Moreover, the high temperature creep rupture strength was also enough.

The weld joint which uses the austenitic heat-resistant alloy of mark A-5 as a base material generate | occur | produced the crack which is considered to be SIPH crack after an aging heat treatment. This can be considered to be because the crystal grain size of the austenitic heat-resistant alloy of mark A-5 was too granulated.

Although the welded joint made from the austenitic heat-resistant alloy of mark A-6 had excellent crack resistance, creep rupture time fell short of the target. This may be because the grain size of the austenitic heat-resistant alloy of mark A-6 was too fine.

As for the welding joint which uses the austenitic heat-resistant alloy of mark E as a base material, some back surface beads were not formed in superlayer welding. This may be considered to be because Sn content of the austenitic heat-resistant alloy of mark E was too small.

Since the welding joint which uses the austenitic heat-resistant alloy of mark F as a base material did not contain Sn but contained a large amount of S, the back surface bead was fully formed. However, after the aging heat treatment, a crack, which is thought to be a SIPH crack, occurred.

As for the welding joint which uses the austenitic heat-resistant alloy of mark G as a base material, the crack considered to be a liquefaction crack and SIPH crack generate | occur | produced as it was welded and after aging heat treatment, respectively. This may be considered to be because Sn content of the austenitic heat-resistant alloy of mark G was too large.

Although the weld joint which used the austenite type heat-resistant alloy of mark H as a base material had favorable weldability and weld cracking resistance, it did not satisfy the required creep strength. This is considered to be because the phase stability became unstable because the Ni content of the austenitic heat-resistant alloy of the mark H was too small.

The weld seam based on the austenitic heat-resistant alloy of mark J also did not satisfy the required creep strength. This is considered to be because the amount of V contained in the austenitic heat-resistant alloy of mark J was below the lower limit.

Industrial availability

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

Claims (3)

In chemical composition,
C: 0.04 to 0.14%,
Si: 0.05% to 1%,
Mn: 0.5-2.5%,
P: 0.03% or less,
S: less than 0.001%,
Ni: 23-32%,
Cr: 20-25%,
W: 2.7-5%,
Nb: 0.1-0.6%,
V: 0.1-0.6%,
N: 0.1-0.3%,
B: 0.0005% to 0.01%,
Sn: 0.001-0.02%,
Al: 0.03% or less
O: 0.02% or less,
Ti: 0 to 0.5%
Co: 0-2%,
Cu: 0-4%,
Mo: 0-4%,
Ca: 0% to 0.02%
Mg: 0% to 0.02%
REM: 0-0.2%,
Remainder: Fe and impurities
An austenitic heat resistant alloy having a grain size of 2.0 or more and less than 7.0 as the grain size number specified in ASTM E112. The method according to claim 1,
The austenitic heat-resistant alloy in which the said chemical composition contains 1 or more types of elements chosen from any one group from the following 1st group to 3rd group by mass%.
1st group Ti: 0.01 to 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% The welding structure provided with the austenitic heat-resistant alloy of Claim 1 or 2.

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