JP6520516B2 - Austenitic heat-resistant alloy members - Google Patents

Description

  The present invention relates to an austenitic heat-resistant alloy member.

  In recent years, high-temperature and high-pressure operating conditions have been promoted on a global scale in boilers for power generation etc. from the viewpoint of reducing environmental impact, and for austenitic heat-resistant alloys used as materials for superheater tubes and reheater tubes It is sought to have better high temperature strength and corrosion resistance. The application of austenitic heat-resistant alloys is also being studied in large-diameter and thick-walled members such as main steam pipes and reheat steam pipes in which ferrite heat-resistant steels have been conventionally used.

  Under such technical background, techniques relating to various austenitic heat-resistant alloys have been proposed.

  For example, Patent Document 1 proposes a Ni-based alloy product. This Ni-based alloy product uses W to enhance high temperature strength and controls the effective B content to improve hot workability and to prevent weld cracking, particularly austenitic heat resistance suitable as a large-sized product It is an alloy product.

  Patent Document 2 proposes an austenitic heat-resistant alloy, a heat-resistant and pressure-resistant member made of the alloy, and a method of manufacturing the same, wherein creep strength is enhanced by using α-Cr phase as a reinforcement phase by utilizing Cr, Ti and Zr.

  Patent Document 3 proposes a Ni-based heat-resistant alloy in which a large amount of W is contained and Al and Ti are utilized to increase the strength by solid solution strengthening and precipitation strengthening of the γ 'phase.

  In order to use these austenitic heat-resistant alloy members as a structure, cold or hot plastic working is performed.

  However, when the present inventors conducted detailed investigations, even when using the austenitic heat-resistant alloys disclosed in Patent Documents 1 to 3, when the member is plastically processed, the outer surface and the inside of the vicinity of the surface are It has become apparent that fine cracks, which have not been confirmed so far, may occur, and that such fine cracks are likely to occur particularly when a thick-walled member is hot-worked.

  In order to solve such a problem, the inventors of Patent Document 4 disclose that the average crystal grain size d (μm) of the member and the maximum hardness HV0.1 (in the region from the outer surface of the member to a depth of 5 mm) We have proposed an austenitic heat-resistant alloy member that specifies the relationship with max).

JP, 2011-63838, A WO 2009/154161 WO 2010/038826 JP 2014-34725 A

  However, when the present inventors further studied, even if the member satisfies the requirements described in Patent Document 4, fine cracks are generated on the surface of the member by subsequent plastic processing, and the fine cracks It has been found that the creep properties may be reduced. Therefore, even if a fine crack (hereinafter, also referred to as a "micro crack" or simply "a crack") is generated on the surface of a member during plastic working, it is possible to suppress the deterioration of the creep characteristics. It is an important issue.

  The present invention has been made to solve the above-mentioned problems, and is excellent in creep characteristics after plastic working such as bending and forging, etc., with respect to a thick-walled material, and is excellent An object of the present invention is to provide an austenitic heat-resistant alloy member suitable for use by hot working into a thick, large-diameter high-temperature member such as a heat steam pipe.

  MEANS TO SOLVE THE PROBLEM As a result of conducting earnest research, in order to solve said subject, the present inventors acquired the knowledge of the following (a)-(c).

  (A) In an austenitic heat-resistant alloy member having a fine-grained metal structure, creep characteristics are degraded. Therefore, the metallographic structure in the central portion of the thickness of the member needs to have a coarse grain structure of ASTM grain size No. 4.0 or less in average grain size.

  (B) However, in the case of an austenitic heat-resistant alloy member having a coarse-grained metal structure, micro cracks may be generated on the surface during hot plastic deformation. This is because, in a member having a coarse-grained metal structure, it is difficult to be deformed during plastic working, and the grain boundaries are weakened. In particular, a member using a thick material having a thickness of 20 mm or more has a higher heating temperature, a longer heating time, and a higher cooling rate in heat history during manufacturing or heat treatment of a pipe than in a thin material. Since it becomes slow, the metal structure inevitably tends to be coarse particles, and micro cracks are easily generated on the surface.

(C) Some of the above-mentioned cracks have an adverse effect on creep properties, while others do not. Therefore, as a result of examining the shape of the crack which adversely affects the creep characteristics in detail, the present inventors have stressed the crack tip in the case of a crack having a depth of 40 μm or more and satisfying the following equation. It has been found that the creep strength and the creep rupture strength are reduced, and the creep rupture time is shortened if the member is ruptured prematurely.
0 <w / d ≦ 0.5
Where d: the maximum depth of the crack (μm), w: the width of the crack at the position of the maximum depth d (μm).

The inventors of the present invention, as described above, have identified a crack that adversely affects the creep characteristics, and conducted further studies on a method of suppressing the deterioration of the creep characteristics due to such cracks. And, even if a micro crack is generated on the surface when plastically working the austenitic heat resistant alloy member hot, by forming a predetermined Cr ₂ O ₃ main oxide scale inside the crack, creep characteristics can be obtained. The inventors have found that the reduction can be suppressed and complete the present invention.

  The present invention is made on the basis of the above-described findings, and the gist of the austenitic heat-resistant alloy member described below.

(1) An austenitic heat-resistant alloy member having a crack on its surface,
The chemical composition is in mass%,
C: 0.01 to 0.15%,
Si: 1.0% or less,
Mn: 2.0% or less,
P: 0.03% or less,
S: 0.01% or less,
Ni: 30.0 to 70.0%,
Cr: 19.0 to 35.0%,
W: 3.0 to 10.0%,
Ti: 0.01 to 3.0%,
Al: 3.0% or less,
B: 0.0001 to 0.01%,
N: 0.02% or less,
O: 0.01% or less,
Ca: 0 to 0.05%,
REM: 0 to 0.1%
Co: 0 to 25.0%,
Cu: 0 to 1.0%,
Mo: 0 to 10.0%,
V: 0 to 0.5%,
Nb: 0 to 3.0%,
Zr: 0 to 0.5%,
Remainder: Fe and impurities,
The metallographic structure consists of grains of which the thickness center is less than ASTM grain size No. 4.0,
The inner surface of Crack generated on the surface to form an oxide scale of _C r 2 _{O 3} mainly
The crack and the oxide scale satisfy the following equations (1) to (4):
Austenitic heat-resistant alloy members with excellent creep characteristics .
40 ≦ d ≦ 500 (1)
0 <w / d ≦ 0.5 (2)
a / d ≧ 0.5 (3)
a = s × {(2d / w) ² +1} ^1/2 (4)
However, the meaning of each symbol in the above formula is as follows.
d: Maximum depth of crack (μm)
w: Width of the crack at the position of the maximum depth d at the surface of the base metal (μm)
a: Depth of oxide scale at the position of maximum depth d (μm)
s: Thickness of oxide scale on member surface (μm)

(2) An austenitic heat-resistant alloy member having a crack on its surface,
The chemical composition is in mass%,
C: 0.01 to 0.15%,
Si: 1.0% or less,
Mn: 2.0% or less,
P: 0.03% or less,
S: 0.01% or less,
Ni: 40.0 to 55.0%,
Cr: 20.0 to 35.0%,
W: 3.0 to 10.0%,
Ti: 0.01 to 1.2%,
Al: 0.3% or less,
B: 0.0001 to 0.01%,
N: 0.02% or less,
O: 0.01% or less,
Ca: 0 to 0.05%,
REM: 0 to 0.1%
Co: 0 to 1.0%,
Cu: 0 to 1.0%,
Mo: 0 to 1.0%,
V: 0 to 0.5%,
Nb: 0 to 0.5%,
Zr: 0 to 0.5%,
Remainder: Fe and impurities,
The metallographic structure consists of grains of which the thickness center is less than ASTM grain size No. 4.0,
The inner surface of Crack generated on the surface to form an oxide scale of _C r 2 _{O 3} mainly
The crack and the oxide scale satisfy the following equations (1) to (4):
Austenitic heat-resistant alloy members with excellent creep characteristics .
40 ≦ d ≦ 500 (1)
0 <w / d ≦ 0.5 (2)
a / d ≧ 0.5 (3)
a = s × {(2d / w) ² +1} ^1/2 (4)
However, the meaning of each symbol in the above formula is as follows.
d: Maximum depth of crack (μm)
w: Width of the crack at the position of the maximum depth d at the surface of the base metal (μm)
a: Depth of oxide scale at the position of maximum depth d (μm)
s: Thickness of oxide scale on member surface (μm)

(3) The austenitic heat-resistant alloy member according to the above (2), wherein the chemical composition contains, in mass%, one or more selected from the elements shown in the following (A) and (B).
(A) Ca: 0.0001 to 0.05% and REM: 0.001 to 0.1%
(B) Co: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Mo: 0.01 to 1.0%, V: 0.01 to 0.5%, Nb: 0 .01 to 0.5% and Zr: 0.01 to 0.5%

  According to the present invention, excellent creep characteristics can be maintained even if micro cracks are formed during plastic working in hot conditions. Therefore, the austenitic heat-resistant alloy member of the present invention is suitable for use as a thick, large-diameter high-temperature member such as a main steam pipe or a reheat steam pipe of a power generation boiler.

Enlarged view in the vicinity of a crack on the surface of an austenitic heat-resistant alloy member on which oxide scale is formed Transverse cross section of parallel part of creep rupture test specimen

1. Chemical composition C: 0.01 to 0.15%
C stabilizes austenite and forms fine carbides at grain boundaries to improve creep strength at high temperatures. In order to sufficiently obtain this effect, a C content of 0.01% or more is required. However, when C is contained in excess, the carbides become coarse and precipitate in a large amount, so that the ductility of grain boundaries is reduced, and furthermore, the toughness and the creep strength are also reduced. Therefore, the upper limit is provided, and the content of C is set to 0.01 to 0.15%. A desirable lower limit of the C content is 0.03%, a more desirable lower limit is 0.04%, and a further desirable lower limit is 0.05%. Moreover, the desirable upper limit of C content is 0.12%, and a more desirable upper limit is 0.10%.

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

  Although it is not necessary to set a lower limit on the content of Si, extreme reduction can not achieve a sufficient deoxidizing effect and degrade the cleanliness of the alloy, as well as improve the corrosion resistance and oxidation resistance at high temperatures. It becomes difficult to obtain, and the manufacturing cost also rises greatly. Therefore, the desirable lower limit of Si content is 0.02%, and the more desirable lower limit is 0.05%.

Mn: 2.0% or less Mn has a deoxidizing action like Si. Mn also contributes to the stabilization of austenite. However, when the content of Mn is excessive, embrittlement occurs, and furthermore, toughness and creep ductility also decrease. Therefore, the upper limit of the content of Mn is set to 2.0% or less. The content of Mn is desirably 1.8% or less, more desirably 1.5% or less.

  Although it is not necessary to set the lower limit of the content of Mn either, an extreme reduction does not sufficiently obtain the deoxidizing effect and degrades the cleanliness of the alloy, and makes it difficult to obtain the austenite stabilization effect, and further manufacture The cost also rises significantly. Therefore, the desirable lower limit of the Mn content is 0.02%, and the more desirable lower limit is 0.05%.

P: 0.03% or less P is contained in the alloy as an impurity, and when it is contained in a large amount, it significantly reduces the hot workability and weldability, and also reduces the creep ductility after long-term use . Therefore, the upper limit of the content of P is set to 0.03% or less. The content of P is desirably 0.025% or less, more desirably 0.02% or less.

  In addition, although it is preferable to reduce content of P as much as possible, extreme reduction causes increase of manufacturing cost. Therefore, the desirable lower limit of P content is 0.0005%, and the more desirable lower limit is 0.0008%.

S: 0.01% or less S is contained in the alloy as an impurity like P, and when it is contained in a large amount, it significantly reduces hot workability and weldability, and furthermore, creep after long time use It also reduces ductility. Therefore, the upper limit of the content of S is set to 0.01% or less. The content of S is desirably 0.008% or less, more desirably 0.005% or less. In addition, it is preferable to reduce content of S as much as possible.

Ni: 30.0 to 70.0%
Ni is an element effective for obtaining austenite, and is an element essential for securing the structural stability during long-term use. In the range of Cr content of the present invention of 20.0 to 35.0% described later, Ni content of 30.0% or more is necessary to sufficiently obtain the above-described effect of Ni. However, Ni is an expensive element, and if it is contained in a large amount, the cost increases. Therefore, the upper limit is provided, and the content of Ni is set to 30.0 to 70.0%. A desirable lower limit of Ni content is 40.0%, a more desirable lower limit is 41.0%, and a more desirable lower limit is 42.0%. The upper limit of the Ni content is preferably 55.0%, more preferably 54.0%, and still more preferably 53.0%.

Cr: 19.0 to 35.0%
Cr is an essential element for securing oxidation resistance and corrosion resistance at high temperatures. In the range of the Ni content of the present invention of 30.0 to 70.0%, a Cr content of 19.0% or more is necessary to obtain the above-described effect of Cr. However, when the content of Cr exceeds 35.0%, the stability of austenite at high temperatures is deteriorated, which causes a decrease in creep strength. Therefore, the content of Cr is set to 19.0 to 35.0%. The desirable lower limit of the Cr content is 20.0%, the more desirable lower limit is 20.5%, and the more desirable lower limit is 21.0%. The desirable upper limit of the Cr content is 34.5%, and the more desirable upper limit is 34.0%.

W: 3.0 to 10.0%
W is an element which forms a solid solution in the matrix and greatly contributes to the improvement of creep strength at high temperature. In order to exert its effect sufficiently, a W content of at least 3.0% is required. However, even if W is excessively contained, the effect is saturated and the creep strength may be reduced. Furthermore, since W is an expensive element, if it is contained excessively, cost will increase. Therefore, the upper limit is provided, and the content of W is set to 3.0 to 10.0%. A desirable lower limit of W content is 3.5%, and a more desirable lower limit is 4.0%. The desirable upper limit of the W content is 9.5%, and the more desirable upper limit is 9.0%.

Ti: 0.01 to 3.0%
Ti precipitates in the grains as fine carbonitrides and contributes to creep strength at high temperatures. In order to acquire the effect, Ti content of 0.01% or more is required. However, when the content of Ti is excessive, a large amount of it precipitates as carbonitrides, leading to a decrease in creep ductility and toughness. For this reason, the upper limit is provided and the content of Ti is made 0.01 to 3.0%. The desirable lower limit of the Ti content is 0.03%, and the more desirable lower limit is 0.05%. The upper limit of the Ti content is preferably 1.2%, more preferably 1.0%, and still more preferably 0.8%.

Al: 3.0% or less Al is an element having a deoxidizing action. However, when the content of Al is excessive, the cleanliness of the alloy is significantly degraded, and the hot workability and the ductility are reduced. Therefore, the upper limit of the content of Al is set to 3.0% or less. The content of Al is desirably 0.3% or less, more desirably 0.2% or less, and further desirably 0.1% or less.

  Although it is not necessary to set a lower limit in particular with respect to the content of Al, the extreme reduction causes deterioration of the cleanliness of the alloy because the deoxidizing effect is not sufficiently obtained, and the production cost is increased. Therefore, the desirable lower limit of the Al content is 0.0005%. In order to stably obtain the deoxidizing effect of Al and to ensure good cleanliness of the alloy, the lower limit of the Al content is more preferably 0.001%.

B: 0.0001 to 0.01%
B is an element necessary to improve creep strength by segregating at grain boundaries to strengthen grain boundaries during use at high temperatures and by finely dispersing grain boundary carbides. In order to obtain this effect, the B content needs to be 0.0001% or more. However, when the content of B is excessive, in addition to the deterioration of the weldability, the hot workability is deteriorated. Therefore, the upper limit is provided, and the content of B is made 0.0001 to 0.01%. The desirable lower limit of the B content is 0.0005%, and the more desirable lower limit is 0.001%. Moreover, the desirable upper limit of B content is 0.008%, and a more desirable upper limit is 0.006%.

N: 0.02% or less Although N is an element effective for stabilizing austenite, when it is contained in excess, a large amount of fine nitrides precipitates in the grains during use at high temperatures, resulting in creep. It causes a decrease in ductility and toughness. Therefore, the upper limit of the content of N is set to 0.02% or less. The content of N is desirably 0.018% or less, more desirably 0.015% or less.

  Although it is not necessary to set a lower limit in particular for the content of N, if it is extremely reduced, it is difficult to obtain the effect of stabilizing austenite, and the manufacturing cost is also greatly increased. Therefore, the desirable lower limit of N content is 0.0005%, and the more desirable lower limit is 0.0008%.

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

  Although it is not necessary to set a lower limit in particular for the content of O, an extreme reduction leads to an increase in manufacturing cost. Therefore, the desirable lower limit of O content is 0.0005%, and the more desirable lower limit is 0.0008%.

Ca: 0 to 0.05%
Ca has the effect of improving the hot workability. Specifically, Ca is an element having an effect of improving hot workability by generating CaS and suppressing grain boundary segregation of S. For this reason, Ca may be contained. However, when the content of Ca is excessive, it combines with O to significantly reduce the cleanliness and rather deteriorate the hot workability. Therefore, when Ca is contained, the content is made 0.05% or less. The upper limit of the Ca content is desirably 0.04%. The above effect is remarkable when the content of Ca is 0.0001% or more.

REM: 0 to 0.1%
REM has the effect of improving hot workability. That is, REM has a strong affinity with S and contributes to the improvement of hot workability. For this reason, REM may be contained. However, when the content of REM is excessive, it combines with O to significantly reduce the cleanliness and rather deteriorate the hot workability. For this reason, when REM is contained, the content is made 0.1% or less. The upper limit of the REM content is desirably 0.08%. The above effect is remarkable when the content of REM is 0.001% or more.

  In addition, "REM" is a general term for 17 elements in total of Sc, Y and lanthanoid, and the content of REM indicates the total content of one or more elements of REM. Further, REM is generally contained in misch metal. Therefore, for example, it may be added in the form of misch metal, and the amount of REM may be contained so as to fall within the above range.

  The above Ca and REM can be contained in any one of them alone or in a combination of two. When these elements are contained in combination, the total amount may be 0.15%.

Co: 0 to 25.0%
Co has the effect of improving creep strength. That is, Co, like Ni, is an austenite-forming element, which enhances the phase stability and contributes to the improvement of the creep strength. Therefore, Co may be contained. However, since Co is a very expensive element, excessive inclusion of Co causes a significant cost increase. For this reason, when Co is contained, the content is made 25.0% or less. The upper limit of the Co content is desirably 1.0%, and more desirably 0.8%. The above effect is remarkable when the content of Co is 0.001% or more.

Cu: 0 to 1.0%
Cu has an effect of improving creep strength. That is, Cu, like Ni and Co, is an austenite-forming element, and contributes to the improvement of creep strength by enhancing the phase stability. Therefore, Cu may be contained. However, when Cu is contained excessively, it causes a decrease in hot workability. Therefore, when Cu is contained, the content is made 1.0% or less. The upper limit of the Cu content is desirably 0.8%. On the other hand, the above-mentioned effect becomes remarkable when the content of Cu is 0.01% or more.

Mo: 0 to 10.0%
Mo has the effect of improving creep strength. That is, Mo has a function of dissolving in the matrix to improve the creep strength at high temperature. Therefore, Mo may be contained. However, when Mo is contained in excess, the stability of austenite decreases, which in turn causes a decrease in creep strength. Therefore, when it contains Mo, the content is made into 10.0% or less. The upper limit of the Mo content is desirably 1.0%, and more desirably 0.8%. On the other hand, the above-mentioned effect becomes remarkable when the content of Mo is 0.01% or more.

V: 0 to 0.5%
V has the effect of improving creep strength. That is, V combines with C or N to form fine carbides or carbonitrides, and has an effect of improving creep strength. Therefore, V may be contained. However, when V is contained in excess, a large amount of carbide or carbonitride precipitates, which leads to a decrease in creep ductility. Therefore, when V is contained, the content is made 0.5% or less. The upper limit of the V content is desirably 0.4%. On the other hand, the above-mentioned effect is remarkable when the content of V is 0.01% or more.

Nb: 0 to 3.0%
Like V, Nb combines with C or N and precipitates in the grains as fine carbides or carbonitrides, and contributes to the improvement of creep strength at high temperatures. Therefore, Nb may be contained. However, when the content of Nb is excessive, a large amount of Nb precipitates as carbides and carbonitrides, resulting in a decrease in creep ductility and toughness. Therefore, when Nb is contained, the content is made 3.0 or less. The upper limit of the Nb content is desirably 0.5%, and more desirably 0.4%. On the other hand, the above-mentioned effect becomes remarkable when the content of Nb is 0.01% or more.

Zr: 0 to 0.5%
Zr has an effect of improving creep strength. That is, Zr is a grain boundary strengthening element, and contributes to the improvement of creep strength at high temperature, and further contributes to the improvement of creep ductility. Therefore, Zr may be contained. However, when the content of Zr exceeds 0.5%, the hot workability may be reduced. Therefore, when Zr is contained, the content is made 0.5% or less. The upper limit of the Zr content is desirably 0.4%. On the other hand, the above-mentioned effect becomes remarkable when the content of Zr is 0.01% or more.

  The above Co, Cu, Mo, V, Nb and Zr can be contained alone or in combination of two or more.

  The austenitic heat-resistant alloy member of the present invention has a chemical composition containing the above-described elements, with the balance being Fe and impurities.

  In addition, when manufacturing an austenitic heat-resistant-alloy member industrially, an "impurity" points out what is mixed from the ore as a raw material, scrap, or a manufacturing environment.

2. Grain Size of Center of Thickness of Member The austenitic heat-resistant alloy member of the present invention has a metal structure in which the center of thickness has an average grain size and is composed of crystal grains of ASTM grain size No. 4.0 or less. Excellent creep properties can be obtained by making the metal structure in the central part of the thickness of the member into a coarse-grained structure with an average grain size of ASTM grain size number 4.0 or less. In the present invention, "a thickness central portion of a member" means a region excluding each surface side which is 25% of the member thickness.

  In order to obtain better creep properties, the average grain size at the center of the thickness of the member is preferably ASTM No. 3.0 or less, more preferably 2.0 or less. On the other hand, if the metallographic structure in the central portion of the thickness of the member is excessively coarse, the creep ductility may deteriorate and the impact value may decrease, so the average grain size in the central portion of the thickness of the member is the ASTM grain size The number is preferably -2.0 or more, more preferably -1.0 or more.

  The average grain size in the central portion of the thickness of the member can be determined by the following procedure. The test piece is cut out so that the cross section of the member is the test surface. After mirror-polishing the test surface of the cut-out test piece, it is corroded with aqua regia and observed with an optical microscope for arbitrary three fields of view in the thickness center of the member at a magnification of 100 times, and the average grain segment length by a cutting method Measure the From the measured average segment length, it is converted to the grain size by the ASTM method to obtain an average grain size.

  The average grain size at the central portion of the thickness of the member can be adjusted by controlling the temperature and time of solution heat treatment of the member. As conditions for solution heat treatment, it is preferable to hold for 0.1 to 5 hours in a temperature range of 1000 to 1280 ° C. The temperature range of heat treatment is more preferably 1100 to 1250 ° C., and the holding time is more preferably 0.2 to 1.5 h.

3. Crack forming Cr ₂ O ₃ main oxide scale 40 ≦ d ≦ 500
When the maximum depth d of the crack generated on the surface of the member is less than 40 μm, the crack is small and does not affect the decrease in creep characteristics. On the other hand, when the maximum depth d of the crack exceeds 500 μm, the heat treatment temperature is high and the heat treatment time is long to fill the crack with the oxide scale generated by the heat treatment, and as a result, the performance of the base material is degraded Become. Furthermore, when the maximum depth d of the crack exceeds 500 μm, as described later, even if the inside of the crack is filled with the oxide scale, the creep characteristics deteriorate.

0 <w / d ≦ 0.5
When the ratio (w / d) of the width w of the crack at the position of the maximum depth d and the maximum depth d of the crack exceeds 0.5, the angle of the crack tip is large and the crack is not notch-like The stress concentration at the crack tip is small, and the crack does not greatly affect the decrease in creep characteristics. However, when the ratio (w / d) is 0.5 or less, the creep properties of the member are deteriorated.

Therefore, in the present invention, it is decided to form an oxide scale based on Cr ₂ O _{3 on} the inner surface of the crack satisfying the following equations (1) and (2).
40 ≦ d ≦ 500 (1)
0 <w / d ≦ 0.5 (2)
However, the meaning of each symbol in the above formula is as follows.
d: Maximum depth of crack (μm)
w: Width of crack at maximum depth d (μm)

4. Conditions of Cr ₂ O ₃ -based oxide scale formed inside cracks of member surface F1: adhesion between base metal and Cr ₂ O ₃ -based oxide scale, F2: force to propagate crack, and Cr Assuming that the adhesion between ₂ O ₃ -based oxide scales is F3, the relationship F1> F2 ≧ F3 holds, that is, the adhesion between the base material and the Cr ₂ O ₃ -based oxide scales is the other two. It is large compared to the force. Then, when the Cr ₂ O ₃ -based oxide scale is filled or coated inside the cracks of the matrix, in the vicinity of the interface between the matrix and the Cr ₂ O ₃ , the matrix and the Cr ₂ O ₃ -based oxide scale are The high adhesive strength acts as a drag to propagate the crack and can suppress the growth of the crack in the base material. Further, adhesion between the oxide scale of the base material and the Cr ₂ O ₃ principal is higher than the adhesion force between the base material and Fe oxides, oxide scale of Cr ₂ O ₃ principal, preform Hard to peel off. From these things, the fall of a creep characteristic can be suppressed by forming the oxide scale of the Cr ₂ O ₃ main body inside a crack.

Furthermore, an austenitic heat-resistant alloy having a high content of Ni and Cr is likely to form a Cr ₂ O _3- based oxide scale having good adhesion to the base material, as compared to a low alloy or ferritic heat-resistant steel. Here, the "Cr ₂ O ₃ -based oxide scale" refers to an oxide scale in which the content of Cr ₂ O ₃ in the oxide scale is 50% by mass or more.

In the austenitic heat-resistant alloy member of the present invention, the oxide scale mainly composed of Cr ₂ O ₃ filled in the inside of the crack satisfies the following equations (3) and (4).
a / d ≧ 0.5 (3)
a = s × {(2d / w) ² +1} ^1/2 (4)
However, the meaning of each symbol in the above formula is as follows.
d: Maximum depth of crack (μm)
w: Width of crack at maximum depth d (μm)
a: Depth of oxide scale at the position of maximum depth d (μm)
s: Thickness of oxide scale on member surface (μm)

The above equation (3) means that the inside of the crack is filled with a Cr ₂ O ₃ -based oxide scale to a depth of half or more of the maximum depth of the crack. Within this range, it is possible to obtain the adhesion between the base metal and the oxide scale mainly composed of Cr ₂ O ₃ larger than the force for developing the crack, thereby suppressing the development of the crack and suppressing the deterioration of the creep characteristics. It is possible. (A / d) is preferably at least 0.75. The larger the (a / d), the better. However, since there is a concern that the Cr concentration in the base material will be reduced and the creep characteristics will be reduced, it is preferably made 1.5 or less. However, when the maximum depth d of the crack exceeds 500 μm, even if the inside of the crack is completely filled with oxide scale, the force to propagate the crack is better than the adhesion between the base material and Cr ₂ O ₃ It becomes large, can not stop the progress of the crack, and the creep property is degraded.

Here, referring to FIG. 1, it is assumed that the shape of the crack is symmetrical and that the thickness s of the oxide scale on the surface of the member is constant, the Cr ₂ O ₃ based oxide scale formed inside the crack. Explain the conditions. As shown in FIG. 1, since both the triangle ABC and the triangle ADE are similar to right-angled triangles, s: b = d: w / 2, and b = (2d / w) × s holds. Further, since triangle ABC is a right triangle, b ² + s ² = a ² and a = (b ² + s ² ) ^1/2 . Substituting b = (2d / w) × s into this gives the above equation (4).

  In FIG. 1, oxide scale is formed on the slopes (inner surface) on both sides of the crack. At this time, an oxide scale is formed on the entire slope (for example, the side AE in FIG. 1) having a length longer than a half (for example, the side DE in FIG. 1) of the width w of the crack at the position of the maximum depth d of the crack It will be done. Therefore, in order to completely fill the inside of the crack with the oxide scale, the oxide scale may be formed up to the central position (point D in FIG. 1) of the width W of the crack at the position of the maximum depth d.

5. Method of forming an oxide scale of a desired depth inside a crack In the austenitic heat-resistant alloy member of the present invention, the oxide scale is formed inside the crack on the surface of the member to a desired depth a that satisfies the equation (3). In order to form it, it is preferable to heat-process within the range prescribed | regulated next. Here, the atmosphere may be used as the atmosphere having the maximum oxygen content (20% by volume).
Atmospheric pressure: atmospheric pressure (1 atm (0.1013 MPa))
Composition range of atmosphere: Mixed gas of oxygen content 0.1 to 20% by volume and balance inert gas Heat treatment temperature: 1000 to 1250 ° C.
Heat treatment time: 0.5 to 10 hours

6. Method of measuring depth a of oxide scale inside crack The depth a of oxide scale inside a crack can be measured by polishing a cross section of a sample taken from a member and observing it directly with a microscope. Also, if the maximum depth d of the crack formed on the surface of the member and the width w of the corresponding crack are known in advance, the member may be observed without forming the oxide scale after the formation of the oxide scale inside the crack. If the thickness s of the oxide scale formed on the surface of the surface is measured by microscopic observation, the values of w, d and s are substituted into the equation (4) to calculate the depth a of the oxide scale inside the crack. Is also possible.

Furthermore, the depth a of the oxide scale inside the crack can be calculated from the above equation (4) also by using the value calculated from the following equation (5) as the thickness s of the oxide scale on the surface of the member.
_{s = (P O2 +0.8) ×} {6 × (10 0.0008 × T-21.2 × t) 1/2} / (5 × 10 -7) ··· (5)
Where s: thickness of oxide scale on the surface of the member (μm), T: heat treatment temperature (° C.), t: heat treatment time (h), P _{O 2} : oxygen fraction (volume%, specifically 0.01) % By volume). However, when P _O2 = 0, s = 0.

  Further, in order to obtain an oxide scale having a desired thickness s, the member may be heat-treated under the condition satisfying the above-mentioned equation (5).

7. Thickness of Preferred Member The austenitic heat-resistant alloy member of the present invention may be applied to a thick material whose metal structure becomes coarse grains by plastic working or heat treatment in hot, particularly to a thick material having a thickness of 20 mm or more preferable. When the metal structure is coarse, the grain boundaries are weakened and micro cracks are easily generated, so that the effect of the present invention for suppressing the deterioration of the creep characteristics due to the micro cracks is more exhibited.

  EXAMPLES Hereinafter, the present invention will be more specifically described by way of examples, but the present invention is not limited to these examples.

  An austenitic heat-resistant alloy having the chemical composition shown in Table 1 was laboratory melted to produce an ingot. The ingot was subjected to hot forging and rolling forming and solution heat treatment under the conditions shown in Table 2 to prepare a plurality of 25 mm thick, 100 mm wide and 500 mm long alloy plates.

  From each alloy plate obtained as described above, a test piece for determining the average grain size was cut out and mirror-polished so that the cross section became the test surface. Thereafter, the mirror-polished surface is corroded with aqua regia, and an optical microscope observation is made with an optical microscope at a magnification of 100 × for any three fields of view in the thickness center of the alloy plate, and the average grain segment length is measured by a cutting method. The grain segment length was multiplied by 1.128 to determine the average grain size. Furthermore, the average grain size was converted to the grain size by the ASTM method, and the average grain size was determined.

  Next, the test piece for using for a tension test was cut out about each above-mentioned alloy board. Test pieces for tensile test were square rod-shaped test pieces of 10 mm square and 130 mm in length parallel to the longitudinal direction of the alloy sheet, and a plurality of pieces were machined from each alloy sheet.

The tensile test was conducted at a low strain rate such as the above processing temperature of 1100 ° C. and a strain rate of 0.0001 s ⁻¹ using the test piece for the above-mentioned tensile test. Then, when the elongation (strain amount) reaches 10%, 15% or 20%, the tensile test is interrupted, the cross section of the test piece after the interruption of the tensile test is polished, and the presence or absence of cracks in the surface of the test piece The shape (depth and width) was investigated. Table 2 shows the maximum depth d of the crack and the width w of the corresponding crack.

  In addition, the V-shaped crack arose according to the applied distortion amount on the surface of a test piece. The width w of the crack at the position of the maximum depth d of the crack was 30 μm, 40 μm and 50 μm for strain amounts of 10%, 15% and 20%, respectively.

  Moreover, the oxide scale production | generation test was done using the test piece for said tension test. The elongation (strain amount) under the above conditions was applied to the test piece to generate a micro crack. It was estimated that each test piece had a crack similar to that observed in the test piece whose cross-section was polished after the above-mentioned tensile test interruption.

  The test pieces cracked by processing were subjected to heat treatment to obtain test pieces having oxide scales of various thicknesses on the surface and inside the cracks. The conditions (oxygen partial pressure, temperature and time) of the post-process heat treatment were as shown in Table 2. For each test piece, the thickness s of the oxide scale on the surface of the member is calculated from the heat treatment condition using the above equation (5), and the maximum depth d of the crack and the maximum depth of the crack determined by the above cross-sectional observation The depth a of the oxide scale inside the crack was calculated by substituting the width w of the crack at the position of d with the equation (4). The calculated oxide scale thickness s and depth a are also shown in Table 2.

  The test piece subjected to heat treatment after processing was machined into a test piece for a round bar-like creep rupture test having a diameter of 6 mm in a parallel portion and a gauge point distance of 30 mm. However, as shown in FIG. 2, in the parallel part of the test piece, it is a surface before machining, and a plane in which a crack is present has a shape with a width of 3 mm and a length of 30 mm.

  The creep rupture test was conducted under the conditions of 750 ° C. and 120 MPa using the test pieces for the above-mentioned creep rupture test. Table 2 shows creep rupture time as a result of the creep rupture test. In the evaluation of the creep rupture test results, the creep rupture time was determined to pass 1000 h or more, and was rejected less than 1000 h.

  As shown in Table 2, test numbers 1 to 4, 8 to 11, 15 to 15 that the average grain size number, the width w and depth d of the crack, and the depth a of the oxide scale inside the crack satisfy the definition of the present invention The creep rupture time of each of 18 and 22 to 25 was 1570 h or more, and had good creep properties.

  On the other hand, in the test numbers 6, 13, 20 and 27, the average grain size number was larger than 4.0 and the crystal grains were too fine. Therefore, the creep rupture time was as short as 937 h or less, resulting in poor creep characteristics.

In test numbers 7, 14, 21 and 28, the a / d is smaller than 0.5, the depth a of the oxide scale inside the crack is small, and the base metal-Cr ₂ O ₃ is larger than the force to propagate the crack. The adhesion strength of the steel can not be obtained, and the progress of the crack can not be suppressed. Therefore, the creep rupture time was as short as 890 h or less, resulting in poor creep characteristics.

  The test Nos. 5, 12, 19 and 26 mentioned as reference examples had w / d exceeding 0.5 and the notch effect of the crack tip was small. Therefore, no decrease in creep characteristics was observed regardless of the magnitude of a / d.

  According to the present invention, excellent creep characteristics can be maintained even if micro cracks are formed during plastic working in hot, including the case of using a thick alloy member having a thickness of 20 mm or more. . Therefore, the austenitic heat-resistant alloy member of the present invention is suitable for use as a thick, large-diameter high-temperature member such as a main steam pipe or a reheat steam pipe of a power generation boiler.

a. Depth of oxide scale at maximum depth of crack d. Maximum depth of crack s. Thickness of oxide scale on member surface w. Crack width w at the maximum depth position of the crack

Claims (3)

An austenitic heat-resistant alloy member having a crack on its surface,
The chemical composition is in mass%,
C: 0.01 to 0.15%,
Si: 1.0% or less,
Mn: 2.0% or less,
P: 0.03% or less,
S: 0.01% or less,
Ni: 30.0 to 70.0%,
Cr: 19.0 to 35.0%,
W: 3.0 to 10.0%,
Ti: 0.01 to 3.0%,
Al: 3.0% or less,
B: 0.0001 to 0.01%,
N: 0.02% or less,
O: 0.01% or less,
Ca: 0 to 0.05%,
REM: 0 to 0.1%
Co: 0 to 25.0%,
Cu: 0 to 1.0%,
Mo: 0 to 10.0%,
V: 0 to 0.5%,
Nb: 0 to 3.0%,
Zr: 0 to 0.5%,
Remainder: Fe and impurities,
The metallographic structure consists of grains of which the thickness center is less than ASTM grain size No. 4.0,
The inner surface of Crack generated on the surface to form an oxide scale of _C r 2 _{O 3} mainly
The crack and the oxide scale satisfy the following equations (1) to (4):
Austenitic heat-resistant alloy members with excellent creep characteristics .
40 ≦ d ≦ 500 (1)
0 <w / d ≦ 0.5 (2)
a / d ≧ 0.5 (3)
a = s × {(2d / w) ² +1} ^1/2 (4)
However, the meaning of each symbol in the above formula is as follows.
d: Maximum depth of crack (μm)
w: Width of the crack at the position of the maximum depth d at the surface of the base metal (μm)
a: Depth of oxide scale at the position of maximum depth d (μm)
s: Thickness of oxide scale on member surface (μm) An austenitic heat-resistant alloy member having a crack on its surface,
The chemical composition is in mass%,
C: 0.01 to 0.15%,
Si: 1.0% or less,
Mn: 2.0% or less,
P: 0.03% or less,
S: 0.01% or less,
Ni: 40.0 to 55.0%,
Cr: 20.0 to 35.0%,
W: 3.0 to 10.0%,
Ti: 0.01 to 1.2%,
Al: 0.3% or less,
B: 0.0001 to 0.01%,
N: 0.02% or less,
O: 0.01% or less,
Ca: 0 to 0.05%,
REM: 0 to 0.1%
Co: 0 to 1.0%,
Cu: 0 to 1.0%,
Mo: 0 to 1.0%,
V: 0 to 0.5%,
Nb: 0 to 0.5%,
Zr: 0 to 0.5%,
Remainder: Fe and impurities,
The metallographic structure consists of grains of which the thickness center is less than ASTM grain size No. 4.0,
The inner surface of Crack generated on the surface to form an oxide scale of _C r 2 _{O 3} mainly
The crack and the oxide scale satisfy the following equations (1) to (4):
Austenitic heat-resistant alloy members with excellent creep characteristics .
40 ≦ d ≦ 500 (1)
0 <w / d ≦ 0.5 (2)
a / d ≧ 0.5 (3)
a = s × {(2d / w) ² +1} ^1/2 (4)
However, the meaning of each symbol in the above formula is as follows.
d: Maximum depth of crack (μm)
w: Width of the crack at the position of the maximum depth d at the surface of the base metal (μm)
a: Depth of oxide scale at the position of maximum depth d (μm)
s: Thickness of oxide scale on member surface (μm) The austenitic heat resistant alloy member according to claim 2, wherein the chemical composition contains, in mass%, at least one selected from the elements shown in the following (A) and (B).
(A) Ca: 0.0001 to 0.05% and REM: 0.001 to 0.1%
(B) Co: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Mo: 0.01 to 1.0%, V: 0.01 to 0.5%, Nb: 0 .01 to 0.5% and Zr: 0.01 to 0.5%

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