JP2013095949A - Austenitic heat resistant alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic heat resistant alloy which contains a lot of amount of Co and is excellent in weldability, among austenitic heat resistant alloys excellent in creep strength.SOLUTION: The austenitic heat resistant alloy has a chemical composition containing C:0.03-0.15%, Si≤1%, Mn≤2%, Ni:40-80%, Cr:15-40% Mo:4-15%, Co:15-25%, Ti:≤3%, Al:≤2%, B:0.0005-0.01%, N:≤0.03%, O:≤0.03%, and the remainder of Fe and impurities, wherein P and S in the impurities are P:≤0.03%, S:≤0.03%, respectively. In the alloy, an average crystal particle size d (μm) satisfies [d≤-3300×(10×B+2×P+S)+250]. The austenitic heat resistant alloy may contain one or more of Ca:≤0.02%, Mg:≤0.02% and REM:≤0.08% in place of the portion of Fe.

Description

  The present invention relates to an austenitic heat-resistant alloy. More specifically, the present invention relates to an austenitic heat-resistant alloy used in high-temperature equipment such as a power generation boiler, petroleum refining, and heating furnace tube of a petrochemical industry plant. More specifically, the present invention relates to an austenitic heat-resistant alloy containing a large amount of Co, which is excellent in weldability among the austenitic heat-resistant alloys having excellent creep strength, in particular, liquid cracking resistance in HAZ.

  In recent years, new super-critical pressure boilers with higher steam temperature and pressure have been developed all over the world for higher efficiency.

Specifically, it is also planned to increase the steam temperature, which has been around 600 ° C. until now, to 650 ° C. or higher, and further to 700 ° C. or higher. This is based on the fact that energy conservation, effective utilization of resources, and reduction of CO ₂ gas emissions for environmental conservation are one of the challenges for solving energy problems and are important industrial policies. In the case of a power generation boiler for burning fossil fuel, a reaction furnace for chemical industry, and the like, a highly efficient ultra super critical pressure boiler and reaction furnace are advantageous.

  The high temperature and high pressure of steam raises the temperature at the time of actual operation of high-temperature equipment consisting of boiler superheater tubes and chemical industry reaction furnace tubes, thick plates and forged products as heat and pressure resistant members, to over 700 ° C. Let Therefore, a material that is used for a long time in such a harsh environment is required to have not only high-temperature strength and high-temperature corrosion resistance but also good long-term metal structure stability and creep characteristics.

  Therefore, Patent Documents 1 to 3 disclose heat-resistant alloys that increase the Cr and Ni contents and further include one or more of Mo and W to improve the creep strength as high-temperature strength. .

  Furthermore, in response to increasingly demanding requirements for high-temperature strength characteristics, particularly creep strength, Patent Documents 4 to 7 contain 28% to 38% Cr and 35% to 60% Ni in mass%. Further, a heat-resistant alloy that further improves the creep strength by utilizing precipitation of an α-Cr phase having a body-centered cubic structure mainly composed of Cr is disclosed.

On the other hand, Patent Documents 8 to 11 include Mo and / or W to enhance solid solution strengthening, and Al and Ti are included in a γ ′ phase that is an intermetallic compound, specifically, Ni ₃ ( A Ni-based alloy that is used in the severe environment described above by utilizing precipitation strengthening of Al, Ti) is disclosed.

  Further, Patent Document 12 proposes a high Ni austenitic heat-resistant alloy in which the creep strength is improved by adjusting the range of Al and Ti contents and precipitating a γ 'phase.

  By the way, austenitic heat-resistant alloys are generally assembled into various structures by welding and used at high temperatures. However, as reported in Non-Patent Document 1, when the amount of alloying elements of the austenitic heat-resistant alloy increases, the weld heat affected zone (hereinafter referred to as “HAZ”), particularly adjacent to the melting boundary during welding construction. There arises a problem that liquefaction cracking occurs in HAZ. As for the cause of liquefaction cracking (hereinafter referred to as “HAZ liquefaction cracking”) in the HAZ adjacent to the melting boundary, various theories such as the grain boundary precipitation phase cause or the grain boundary segregation cause have been proposed. The mechanism is not fully specified.

  In particular, austenitic heat-resistant alloys containing a large amount of Co have been used mainly for special parts such as turbine blades that are not normally welded. For this reason, in the case of an austenitic heat-resistant alloy containing a large amount of Co, the mechanism regarding the liquefaction cracking of the HAZ has not been elucidated.

  Thus, in austenitic heat-resistant alloys, it has long been recognized as a problem that liquefaction cracking of HAZ during welding is a problem, but in particular in the case of austenitic heat-resistant alloys containing a large amount of Co, the mechanism Insufficient elucidation. Therefore, in the case of a heat-resistant alloy containing a large amount of Co, measures against HAZ liquefaction cracking during welding, especially measures from the material side, have not been established.

  On the other hand, as described above, in recent years, the need for heat exchanger tubes excellent in high-temperature strength and corrosion resistance has increased due to the increase in temperature and pressure of boilers for thermal power generation.

  For this reason, it has been studied to further include various alloy elements in the austenitic heat-resistant alloy containing a large amount of Co, which is limited to use in turbine blades that do not require welding.

  In addition, the austenitic heat-resistant alloy containing a large amount of Co is a thick-walled member typified by a heat exchanger pipe and a main steam pipe that often involve welding, and a member having a complicated shape typified by a water wall pipe. It is also being considered for use in places that are mechanically difficult. As a result, in an austenitic heat-resistant alloy containing a large amount of Co, liquefaction cracking of HAZ during welding, which has not been considered in the past, tends to become apparent.

  Therefore, in Patent Document 13, the content of B and the content of P which is an impurity element are controlled in accordance with the content of Cr, and a specific amount of Nd is contained, so that HAZ near the melting boundary is included. An austenitic heat-resistant alloy containing 0.03 to 25% Co that suppresses liquefaction cracking is disclosed.

Japanese Patent Laid-Open No. 60-1000064 JP-A 64-55352 Japanese Patent Laid-Open No. 2-200756 JP 7-216511 A JP-A-7-331390 JP-A-8-127848 JP-A-8-218140 JP-A-51-84726 Japanese Patent Laid-Open No. 51-84727 Japanese Unexamined Patent Publication No. 7-150277 JP 2002-518599 A JP-A-9-157779 International Publication No. 2011/071054

Japan Welding Society: Welding / Joining Handbook 2nd edition (2003, Maruzen), 948-950 pages

  Patent Documents 1 to 12 described above disclose austenitic heat-resistant alloys with improved creep strength, but have not been studied from the viewpoint of “weldability” when assembled as a structural member.

  The austenitic heat-resistant alloy proposed by the present inventors in Patent Document 13 can be suitably used as a material for high-temperature equipment such as a power generation boiler and a chemical industrial plant.

  However, since austenitic heat-resistant alloys are used in various applications, depending on each application, for example, even in the case of alloy pipes, it can be processed into various member forms such as thin-walled small diameter pipes and thick-walled large diameter pipes. Necessary. Therefore, there are many manufacturing methods according to the form of the member. For this reason, even if the austenitic heat-resistant alloy proposed by the present inventors in the above-mentioned Patent Document 13 is used as a material, it may be considered that liquefaction cracking of HAZ at the time of welding cannot always be suppressed depending on the manufacturing method. Things to be left are left.

  An object of the present invention is to provide an austenitic heat-resistant alloy excellent in weldability containing a large amount of Co among austenitic heat-resistant alloys excellent in creep strength.

  Note that “excellent weldability” specifically means that the weldability is excellent when welding, and that HAZ liquefaction cracking can be prevented during welding.

  In order to solve the above-mentioned problems, the present inventors conducted detailed examination and investigation on creep strength and liquefaction cracking of HAZ generated during welding using an austenitic heat-resistant alloy containing 10 to 25% by mass of Co. went.

  As a result, it has been clarified that a sufficient creep strength can be secured by first containing B in an austenitic heat-resistant alloy containing 10 to 25% by mass of Co.

  Next, in order to ensure sufficient creep strength, in an austenitic heat-resistant alloy containing B as an essential element, the content of B and the contents of P and S in the impurity element against liquefaction cracking of HAZ It became clear that the amount and the crystal grain size had a great influence.

  Furthermore, when the above amount of Co is included, a specific range in which the average crystal grain size is related to the B, P, and S contents without reducing the contents of P and S in the impurity element so much. It became clear that the liquefaction cracking of the HAZ can be prevented by restricting to 1.

  In addition, it became clear as a result of detailed investigations about the liquefaction cracking of HAZ that occurs during welding using the austenitic heat-resistant alloy containing B and the above-mentioned amount of Co. Specifically, it is the following items (a) to (e).

  (A) Liquefaction cracks occurred at the HAZ grain boundaries near the melting boundary. That is, the content of Non-Patent Document 1 that liquefaction cracks occur at the HAZ grain boundaries near the melting boundary during welding could be confirmed.

  (B) Melt marks are observed on the fracture surface of the cracks generated in the HAZ. Further, P, B and S are concentrated on the fracture surface. Among the above elements, B concentration is particularly significant, and then P concentration is significant.

  (C) As the amount of P, B and S contained in the base material (alloy) increases, liquefaction cracking of HAZ is more likely to occur.

  (D) The degree of influence of each content of P, B, and S on the liquefaction cracking of HAZ becomes more conspicuous as the average crystal grain size of the base material increases.

  (E) As the average crystal grain size increases, the concentration of P, B and S, particularly the concentration of B, is particularly significant on the fractured surface of the HAZ. It is remarkable.

  As described above, it has been found that P, B and S present at the grain boundaries have a strong influence on the liquefaction cracking of HAZ during welding, and the average crystal grain size is related to the behavior.

  The present inventors estimated that the above phenomenon is caused by the following mechanism.

  That is, P, B, and S are segregated at the grain boundaries of the HAZ near the melting boundary due to the thermal cycle during welding. P, B, and S segregated at the grain boundaries are all elements that lower the melting point of the grain boundaries. For this reason, a grain boundary melts locally during welding, and the melted portion is opened by welding thermal stress, resulting in liquefaction cracking.

  In the austenitic heat-resistant alloy containing a large amount of Co, the grain boundary segregation of B is particularly remarkable, and has the greatest effect on the HAZ liquefaction cracking during the welding of B. Next to B, the grain boundary segregation of P is large. And P also has the largest effect on liquefaction cracking of HAZ after B.

  This is because Co has a strong affinity for S, and if Co is contained in a large amount in the matrix (base material), S is bound to Co, so that grain boundary segregation of S is suppressed. As a result, B and P are likely to segregate at the segregation site where a void has occurred. And since B is an interstitial element, it diffuses easily in the metal lattice. Therefore, B is more likely to segregate at the grain boundaries than P.

  Furthermore, when the crystal grain size is large, the grain interface area per unit volume is small. Accordingly, when the average crystal grain size is large, the amount of grain boundary segregation of B, P and S increases, and the stress applied to the specific grain interface, that is, the external stress (welding thermal stress) which is the cause of crack opening Becomes larger, and liquefaction cracking of HAZ tends to occur.

  From the above, it can be easily imagined that reducing the contents of P, B and S is effective in order to prevent liquefaction cracking of HAZ.

  However, B is an inexpensive element that can ensure sufficient creep strength. For this reason, B should be contained as an essential element.

  On the other hand, the reduction of the contents of P and S in the impurities, particularly the reduction of the P content, causes an extreme increase in production cost.

  Therefore, as a result of repeated detailed studies by the present inventors, as described above, the average crystal grain size can be set to B, P, and P without reducing the contents of P and S in the impurity element so much. It has been found that by restricting to a specific range related to the S content, sufficient creep strength can be secured and liquefaction cracking of HAZ can be prevented.

That is, in an austenitic heat-resistant alloy containing, in mass%, Co: 10 to 25%, B: 0.0005 to 0.01%, P: 0.03% or less and S: 0.03% or less, P, Depending on the content of B and S, the average crystal grain size d (μm) is a specific relational expression, specifically,
d ≦ −3300 × F1 + 250 [1]
However,
F1 = 10 × B + 2 × P + S [2]
If it satisfies, it can prevent the liquefaction crack of HAZ at the time of welding, after ensuring sufficient creep strength.

  In addition, the element symbol in said [2] Formula represents content (mass%) of the element.

  The present invention has been completed based on the above findings, and the gist of the present invention resides in the austenitic heat-resistant alloys shown in the following (1) and (2).

(1) By mass%, C: 0.03 to 0.15%, Si: 1% or less, Mn: 2% or less, Ni: 40 to 80%, Cr: 15 to 40%, Mo: 4 to 15% Co: 10-25%, Ti: 3% or less, Al: 2% or less, B: 0.0005-0.01%, N: 0.03% or less and O: 0.03% or less, The balance consists of Fe and impurities, and P and S in the impurities have chemical compositions of P: 0.03% or less and S: 0.03% or less, respectively, and the average crystal grain size d (μm) is An austenitic heat resistant alloy satisfying the following formula [1] related to the contents of B, P, and S:
d ≦ −3300 × F1 + 250 [1]
However,
F1 = 10 × B + 2 × P + S [2]
The element symbol in the above formula [2] represents the content (% by mass) of the element.

  (2) Instead of a part of Fe, by mass%, it contains one or more elements of Ca: 0.02% or less, Mg: 0.02% or less, and REM: 0.08% or less. The austenitic heat-resistant alloy as described in (1) above.

  The “impurities” in “Fe and impurities” as the balance refer to those mixed from ore, scrap, or production environment as raw materials when industrially producing an austenitic heat-resistant alloy.

  “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.

  The austenitic heat-resistant alloy of the present invention is excellent in weldability, particularly liquefaction cracking resistance in HAZ, even though it contains 10 to 25% Co by mass%. For this reason, the austenitic heat-resistant alloy of this invention can be used suitably as a raw material of high temperature apparatuses, such as a power generation boiler, a petroleum refinery, and a heating furnace tube of a petrochemical industry plant.

It is a figure explaining the shape of groove processing. In addition, the unit of the dimension in a figure is "mm".

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

(A) Chemical composition:
C: 0.03-0.15%
C stabilizes austenite and produces carbides at grain boundaries, improving the creep strength at high temperatures. In order to obtain this effect, a C content of 0.03% or more is necessary. However, if the content of C increases, especially exceeding 0.15%, a large amount of carbide precipitates at the grain boundaries during use at high temperatures, reducing the ductility of the grain boundaries and causing a decrease in creep strength. . Therefore, the content of C is set to 0.03 to 0.15% or less. The preferable lower limit of the C content is 0.05%, and the preferable upper limit is 0.12%.

Si: 1% or less Si is an element that has a deoxidizing action and is effective for improving corrosion resistance and oxidation resistance at high temperatures. However, if the Si content increases and exceeds 1%, the stability of austenite decreases, leading to a decrease in creep strength and toughness. Therefore, the Si content is 1% or less. The Si content is desirably 0.5% or less, and more desirably 0.3% or less.

  Although there is no particular need to set a lower limit for the Si content, the extreme reduction does not provide a sufficient deoxidation effect, which deteriorates the cleanliness of the alloy and improves the corrosion resistance and oxidation resistance at high temperatures. However, the manufacturing cost is also greatly increased. Therefore, the desirable lower limit of the Si content is 0.01%. A desirable lower limit of the Si content when the production cost is important is 0.02%.

Mn: 2% or less Mn, like Si, has a deoxidizing action. Mn also contributes to stabilization of austenite. However, if the Mn content increases, especially exceeding 2%, embrittlement occurs, and creep ductility and toughness decrease. Therefore, the Mn content is 2% or less. The Mn content is desirably 1.5% or less, and more desirably 1.0% or less.

  In addition, it is not necessary to provide a lower limit particularly for the content of Mn. Manufacturing costs also increase significantly. Therefore, the desirable lower limit of the Mn content is 0.01%. A desirable lower limit of the Mn content when manufacturing cost is important is 0.02%.

Ni: 40-80%
Ni is an effective element for obtaining austenite, and is an essential element for ensuring the structural stability during long-time use and obtaining sufficient creep strength. In order to sufficiently obtain the above Ni effect within the Cr content range of 15 to 40% of the present invention, a Ni content of 40% or more is necessary. On the other hand, a large amount of Ni, which is an expensive element, exceeding 80% causes an increase in cost. Therefore, the Ni content is 40 to 80%. A desirable lower limit of the Ni content is 42%, and a desirable upper limit is 75%.

Cr: 15-40%
Cr is an essential element for securing oxidation resistance and corrosion resistance at high temperatures. In order to obtain the above Cr effect within the Ni content range of 40 to 80% of the present invention, a Cr content of 15% or more is necessary. However, if the Cr content becomes excessive, especially exceeding 40%, the stability of austenite at high temperatures deteriorates, leading to a decrease in creep strength. Therefore, the Cr content is 15 to 40%. A desirable lower limit of the Cr content is 17%, and a desirable upper limit is 38%.

Mo: 4-15%
Mo is an element that contributes to the improvement of the creep strength at high temperatures by dissolving in the austenite matrix. In order to acquire this effect, it is necessary to contain 4% or more. However, if the Mo content increases and exceeds 15% in particular, the stability of austenite is lowered and the creep strength is lowered. For this reason, content of Mo is made into 4 to 15%. A desirable lower limit of the Mo content is 5%. The desirable upper limit of the Mo content is 11%, and the more desirable upper limit is 10%.

Co: 10-25%
Co, like Ni, is an austenite-forming element and contributes to the improvement of creep strength by increasing the stability of austenite. The present invention is an austenitic heat-resistant alloy containing a large amount of Co, which is excellent in high-temperature strength and corrosion resistance, which has recently increased in needs, and must be a component that takes into account liquefaction cracking of HAZ. If the Co content exceeds 25%, the HAZ liquefaction cracking susceptibility increases, and the price of the material alloy becomes extremely high. On the other hand, in order to stably obtain the effect of Co in the heat-resistant alloy having excellent high-temperature strength and corrosion resistance, the lower limit of the Co content must be 10%. Therefore, the Co content is 10 to 25%. A desirable lower limit of the Co content is 11%. Further, the upper limit of the Co content is preferably 20%, and more preferably 18%.

Ti: 3% or less Ti is an important element that forms the basis of the present invention together with Al. That is, Ti is an essential element for bonding with Ni and finely precipitating as an intermetallic compound to ensure creep strength at high temperatures. However, when the Ti content is increased, particularly exceeding 3%, the intermetallic compound phase rapidly coarsens during use at high temperatures, resulting in an extreme decrease in creep strength and toughness. It causes a decrease in cleanliness and deteriorates manufacturability. Therefore, the Ti content is 3% or less. The upper limit of Ti content is desirably 2.5%.

  On the other hand, in order to stably obtain the effect of Ti described above, the lower limit of the Ti content is desirably 0.01%.

Al: 2% or less Al is an important element that forms the basis of the present invention together with Ti. That is, Al is an element essential for securing the creep strength at a high temperature by binding to Ni and finely precipitating as an intermetallic compound. However, if the Al content increases, especially exceeding 2%, the intermetallic phase rapidly coarsens during use at high temperatures, resulting in an extreme decrease in creep strength and toughness. Sometimes the cleanliness is lowered and the productivity is deteriorated. Therefore, the Al content is 2% or less. The upper limit of the Al content is desirably 1.5%.

  On the other hand, in order to stably obtain the above-described effect of Al, the lower limit of the Al content is desirably 0.001%.

B: 0.0005 to 0.01%
B segregates at grain boundaries and finely disperses grain boundary carbides, thereby contributing to grain boundary strengthening and improving creep strength as high-temperature strength. In order to obtain this effect, a B content of 0.0005% or more is necessary. However, when the B content increases, the melting point of the grain boundary decreases. In particular, when the content exceeds 0.01%, the decrease in the melting point of the grain boundary increases, and the average grain size becomes the B, P and S content. Even when restricted to a specific range related to, HAZ liquefaction cracks occur during welding. Therefore, the B content is set to 0.0005 to 0.01%. The minimum with preferable B content is 0.0010%, and a preferable upper limit is 0.005%.

  The content of B must satisfy the relational expression with the average crystal grain size described later together with the contents of P and S.

N: 0.03% or less N is an element effective for stabilizing austenite. However, when the N content is excessive and exceeds 0.03%, a nitride of Cr is formed in addition to a nitride of Ti and Al, leading to a decrease in creep ductility and toughness. Therefore, the N content is 0.03% or less.

  Although there is no particular need to set a lower limit for the N content, an extreme reduction makes it difficult to obtain the effect of stabilizing austenite, and the manufacturing cost also greatly increases. Therefore, the desirable lower limit of the N content is 0.0005%.

O: 0.03% or less O (oxygen) is contained in the alloy as an impurity, and when its content increases and exceeds 0.03%, the hot workability decreases, and the toughness and ductility It causes deterioration. Therefore, the content of O is set to 0.03% or less. The content of O is desirably 0.02% or less.

  Although there is no particular need to set a lower limit for the O content, an extreme reduction leads to an increase in manufacturing cost. Therefore, the desirable lower limit of the O content is 0.001%.

  The austenitic heat-resistant alloy of the present invention contains the above-described elements from C to O, the balance is composed of Fe and impurities, and P and S in the impurities are respectively P: 0.03% or less and S: 0.00. The chemical composition is 03% or less, and the average crystal grain size d (μm) satisfies the following formula [1] related to the contents of B, P and S.

  As already stated, the “impurities” in the remaining “Fe and impurities” are those that are mixed from ore, scrap, or the production environment as raw materials when industrially producing austenitic heat-resistant alloys. Point to.

  Hereinafter, the reason why the contents of P and S in the impurities are limited to the above ranges in the present invention will be described.

P: 0.03% or less P is contained in the alloy as an impurity, segregates at the grain boundary due to the welding heat cycle at the time of welding, and lowers the melting point of the grain boundary to increase the liquefaction cracking sensitivity of HAZ. In particular, if the P content exceeds 0.03%, even if the average crystal grain size is restricted to a specific range related to the B, P and S contents, liquefaction cracking of HAZ will occur. Therefore, the content of P in the impurities is set to 0.03% or less. The content of P in the impurities is preferably 0.02% or less.

  Although it is preferable to reduce the P content as much as possible, extreme reduction leads to an increase in steelmaking costs. Therefore, the desirable lower limit of the P content is 0.002%.

  The content of P needs to satisfy the relational expression with the average crystal grain size described later together with the contents of B and S.

S: 0.03% or less S is also contained in the alloy as an impurity, segregates at the grain boundary due to the welding heat cycle at the time of welding, and lowers the melting point of the grain boundary to increase the liquefaction cracking sensitivity of HAZ. In particular, when the S content exceeds 0.03%, HAZ liquefaction cracking occurs even when the average crystal grain size is restricted to a specific range related to the B, P and S content. Therefore, the content of S in the impurities is set to 0.03% or less. The content of S in the impurities is preferably 0.02% or less.

  The S content is preferably reduced as much as possible, but extreme reduction leads to an increase in steelmaking costs. Therefore, the desirable lower limit of the S content is 0.0001%.

  The content of S needs to satisfy the relational expression with the average crystal grain size described later together with the contents of B and P.

  The austenitic heat-resistant alloy of the present invention may contain one or more elements selected from Ca, Mg, and REM, if necessary, instead of part of the Fe.

  Hereinafter, the operational effects of the optional elements Ca, Mg, and REM and the reasons for limiting the content will be described.

Ca: 0.02% or less Ca has an effect of improving hot workability. For this reason, Ca may be contained. However, when the content of Ca is excessive, it combines with O to significantly reduce the cleanliness of the alloy, and on the contrary, deteriorate the hot workability. For this reason, when making it contain, the upper limit is provided in the quantity of Ca, and it is set to 0.02% or less. The content of Ca is desirably 0.01% or less.

  On the other hand, in order to stably obtain the effect of Ca described above, the amount of Ca is preferably 0.0005% or more.

Mg: 0.02% or less Mg, like Ca, has an effect of improving hot workability. For this reason, you may contain Mg. However, if the Mg content is excessive, it combines with O to significantly reduce the cleanliness of the alloy and, on the contrary, deteriorate the hot workability. For this reason, the upper limit is set to the amount of Mg in the case of containing 0.02% or less. The Mg content is desirably 0.01% or less.

  On the other hand, in order to stably obtain the above-described effect of Mg, the amount of Mg is preferably 0.0005% or more.

REM: 0.08% or less REM has an effect of improving hot workability. For this reason, you may contain REM. However, when the content of REM becomes excessive, it combines with O to significantly reduce the cleanliness of the alloy and, on the contrary, deteriorate the hot workability. For this reason, the upper limit is provided to the amount of REM in the case of making it contain to 0.08% or less. The content of REM is desirably 0.07% or less.

  On the other hand, in order to stably obtain the above-described REM effect, the amount of REM is preferably 0.001% or more.

  As already described, “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.

  Note that REM is generally contained in misch metal. For this reason, for example, it may be added in the form of misch metal and contained so that the amount of REM falls within the above range.

  Said Ca, Mg, and REM can be contained only in any one of them, or 2 or more types of composites. The total amount when these elements are contained in combination is preferably 0.1% or less.

(B) Average crystal grain size:
The austenitic heat-resistant alloy of the present invention has an average crystal grain size d (μm),
The following formula [1] related to the contents of B, P and S, that is,
d ≦ −3300 × F1 + 250 [1]
Must be satisfied.
However,
F1 = 10 × B + 2 × P + S [2]
The element symbol in the above formula [2] represents the content (% by mass) of the element.

  P, B, and S are segregated at the grain boundary of the HAZ near the melting boundary during the welding due to the thermal cycle, lowering the melting point and increasing the HAZ liquefaction cracking sensitivity. In the austenitic heat-resistant alloy of the present invention containing a large amount of Co, as described above, B grain boundary segregation is particularly remarkable, and next to B, P grain boundary segregation is large.

  That is, in the austenitic heat-resistant alloy of the present invention, the influence of P, B, and S on the liquefaction cracking of HAZ increases in the order of S, P, and B, and the degree of influence increases as the crystal grain size increases. Become prominent. Therefore, in order to prevent liquefaction cracking of HAZ, it is necessary to manage the average crystal grain size d (μm) according to the amounts of B, P and S contained in the alloy.

  In the alloy of the present invention containing C: 0.03 to 0.15%, Ni: 40 to 80%, Cr: 15 to 40% and Co: 10 to 25%, as described in the section (A), HAZ No noticeable coarsening occurs at. For this reason, what is necessary is just to manage with the average crystal grain diameter of the base material (namely, alloy) before welding.

  Specifically, if the average crystal grain size d (μm) of the base material satisfies the above formula [1], B: 0.0005 to 0.01%, P: 0.03% or less, and S: In the heat-resistant alloy of the present invention containing 0.03% or less, HAZ liquefaction cracking can be prevented.

  The lower limit of the average crystal grain size d (μm) is 20 μm for the reason of ensuring the creep strength.

  Although depending on the chemical composition, for example, the above average crystal grain size d can be expressed by the above formula [1] by holding the solution in a temperature range of 1150 to 1250 ° C. for 0.5 to 5 hours and performing a solution heat treatment. Can be met.

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

  Austenitic alloys A to L having the chemical composition shown in Table 1 were melted, hot forged, and hot rolled by ordinary methods, then the temperature was 1150 to 1280 ° C., and the holding time at the temperature was 0. The solution heat treatment changed in the range of 5 to 5 hours was performed.

  Further, various alloy plates having a plate thickness of 12 mm, a width of 50 mm, and a length of 100 mm were produced by machining.

  For each test number, five test pieces were cut out from each alloy plate having a thickness of 12 mm, a width of 50 mm, and a length of 100 mm so that the test surface had a cross-section, and mirror-polished. Next, after corroding with aqua regia, an optical microscope was used to observe five visual fields for each test piece at a magnification of 100 times, and an average grain section length for each of the five test pieces was measured by a cutting method. The average grain slice length for each test piece was further arithmetically averaged and multiplied by 1.128 to obtain the average crystal grain size d (μm).

  Further, a groove having the shape shown in FIG. 1 is processed in the longitudinal direction of each of the alloy plates having a thickness of 12 mm, a width of 50 mm, and a length of 100 mm, and a welding wire (AWS standard A5.14 ERNiCrCoMo-1) is used. After the initial layer welding by TIG welding at a heat input of 9 kJ / cm, a coated arc welding rod (JIS standard) was applied on a SM400C steel plate (JIS standard G 3106 (2008)) having a thickness of 25 mm, a width of 150 mm, and a length of 150 mm. Z 3224 (2010), “E Ni 6625”) specified in the 4 rounds was restrained and welded.

  Thereafter, using the same welding wire, lamination welding was performed in the groove by TIG welding at a heat input of 9 to 15 kJ / cm, and two joints were produced for each test number and subjected to the test.

  Specifically, for each test number, a total of 10 test pieces were cut out from each welded joint as described above so that the surface to be tested had a cross section, a total of 10 test pieces, mirror-polished, and corroded with aqua regia. After that, the presence or absence of liquefaction cracking of HAZ was examined by inspection with an optical microscope.

  Next, for each test number in which a crack was observed in the HAZ, the fracture surface of the crack was observed with a scanning electron microscope (hereinafter referred to as “SEM”).

  As a result of observation by SEM, melting marks were observed on all fracture surfaces. Therefore, it became clear that all the above-mentioned cracks were “HAZ liquefaction cracks” that occurred during welding.

  Table 2 shows the results of the above tests. Table 2 shows the value of F1 in the formula [2] obtained from the B amount, the P amount and the S amount contained in the alloy, and the right side of the formula [1] using the F1, that is, “−3300 × F1 + 250”. The values are also shown.

  In Table 2, “O” in the “Liquid cracking of HAZ” column indicates that no crack was observed by observation with the optical microscope. On the other hand, “x” indicates that a crack was observed.

  As shown in Table 2, test numbers 1 to 5, 7, 10, 16 to 19, 22 to 23, 25 to 26, and 28 to 29 of examples of the present invention using alloy plates that satisfy the conditions specified in the present invention. 31 to 32 and 34 (in the case of welded joint codes, A1 to A3, B1 to B2, C1, D1, F1 to F3, G1, H1 to H2, I1 to I2, J1 to J2, K1 to K2, and L1) No liquefaction cracks of HAZ were observed in the entire cross section. That is, in the case of each test number of the above-mentioned example of the present invention, it is clear that the liquid crystal is excellent in liquefaction cracking resistance in HAZ despite containing a large amount of Co of 11.9 to 19.9%.

  On the other hand, test numbers 6, 8-9, 11-15, 20-21, 24, 27, 30, 33, and 35-36 of comparative examples using alloy plates that deviate from the conditions specified in the present invention ( In the case of B3, C2 to C3, D2 to D3, E1 to E3, G2 to G3, H3, I3, J3, K3, and L2 to L3), HAZ liquefaction cracks occurred.

That is, in the case of each test number of the comparative example, although the content of each element of the alloy plate satisfies the conditions specified in the present invention, the average crystal grain size d (μm) of the alloy plate is B, P and S. D ≦ −3300 × F1 + 250 ... [1]
Does not satisfy the formula.
However,
F1 = 10 × B + 2 × P + S [2]
It is.

  For this reason, in the case of each test number of the said comparative example, the liquefaction crack of HAZ cannot be prevented.

  The austenitic heat-resistant alloy of the present invention is excellent in weldability, particularly liquefaction cracking resistance in HAZ, even though it contains 10 to 25% Co by mass%. For this reason, the austenitic heat-resistant alloy of this invention can be used suitably as a raw material of high temperature apparatuses, such as a power generation boiler, a petroleum refinery, and a heating furnace tube of a petrochemical industry plant.

Claims (2)

In mass%, C: 0.03-0.15%, Si: 1% or less, Mn: 2% or less, Ni: 40-80%, Cr: 15-40%, Mo: 4-15%, Co: 10-25%, Ti: 3% or less, Al: 2% or less, B: 0.0005-0.01%, N: 0.03% or less and O: 0.03% or less, with the balance being Fe And P and S in the impurity have chemical compositions of P: 0.03% or less and S: 0.03% or less, respectively, and the average crystal grain size d (μm) is B, An austenitic heat-resistant alloy characterized by satisfying the following formula [1] related to the contents of P and S:
d ≦ −3300 × F1 + 250 [1]
However,
F1 = 10 × B + 2 × P + S [2]
The element symbol in the above formula [2] represents the content (% by mass) of the element.   It is characterized by containing one or more elements of Ca: 0.02% or less, Mg: 0.02% or less, and REM: 0.08% or less in mass% instead of part of Fe. The austenitic heat-resistant alloy according to claim 1.

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