Ni-based alloy, ni-based alloy for gas turbine combustor, member for gas turbine combustor, liner member, transition piece member, liner, and transition piece

Abstract

A Ni-based alloy comprises nitrides, of which an estimated largest size is an area-equivalent diameter of 12 m to 25 m, the estimated largest size of the nitrides being determined by calculating an area-equivalent diameter D which is defined as D=A.sup.1/2 in relation to an area A of a nitride with a largest size among nitrides present in a measurement field of view area S.sub.0 of an observation of the Ni-based alloy, repeatedly performing this operation for n times corresponding to a measurement field of view number n to acquire n pieces of data of the area-equivalent diameter D, arranging the pieces of data of area-equivalent diameter D in ascending order into D.sub.1, D.sub.2, . . . D.sub.n to calculate a standardized variable y.sub.j, plotting the area-equivalent diameter D and the standardized variable y.sub.j on X and Y axes of an X-Y coordinate system, respectively, to obtain a regression line y.sub.j=aD+b (wherein a and b are constants) to calculating y.sub.j where a cross-sectional area to be predicted S is 100 mm.sup.2, and substituting the obtained value of y.sub.j into the regression line to obtain the estimated largest size of the nitrides.

Claims

1. A Ni-based alloy having a composition comprising, by mass, 20.0% to 26.0% Cr, 4.7% to 9.4% Co, 5.0% to 16.0% Mo, 0.5% to 4.0% W, 0.3% to 1.5% Al, 0.1% to 1.0% Ti, 0.001% to 0.15% C, and at most 5% Fe and the balance of Ni with inevitable impurities, wherein the Ni-based alloy comprises titanium nitrides, of which an estimated largest size is an area-equivalent diameter of 12 m to 25 m, the estimated largest size of the titanium nitrides being determined by calculating an area-equivalent diameter D which is defined as D=A.sup.1/2 in relation to an area A of a titanium nitride with a largest size among titanium nitrides present in a measurement field of view area S.sub.0 of an observation of the Ni-based alloy, repeatedly performing this operation for n times corresponding to a measurement field of view number n to acquire n pieces of data of the area-equivalent diameter D, arranging the pieces of data of area-equivalent diameter D in ascending order into D.sub.1, D.sub.2, . . . D.sub.n to calculate a standardized variable y.sub.j which is defined by the following expression:
y.sub.j=ln [ln {j/(n+1)}][Expression 1] wherein j denotes the number of the arranged ascending order of the data of the area-equivalent diameter D, plotting the area-equivalent diameter D and the standardized variable y.sub.j on X and Y axes of an X-Y coordinate system, respectively, to obtain a regression line y.sub.j=aD+b (wherein a and b are constants), calculating y.sub.j from the following expression: $\begin{array}{cc}{y}_j=-\ln \left(-\ln \frac{S}{S_0+S}\right)& \left[\mathrm{Expression}2\right]\end{array}$ where a cross-sectional area to be predicted S is 100 mm.sup.2, and substituting the obtained value of y.sub.j into the regression line to obtain the estimated largest size of the titanium nitrides wherein S.sub.0 equals 0.306 mm.sup.2, n is greater than or equal to 30, and the observation of the Ni-based alloy is at an observation magnification of 400 to 1000 times or 1000 to 3000 times.

2. The Ni-based alloy according to claim 1, wherein a scrap material is used as a raw material for the Ni-based alloy.

3. A Ni-based alloy for a gas turbine combustor, which is used for a gas turbine combustor, the Ni-based alloy consisting of the Ni-based alloy according to claim 1.

4. A member for a gas turbine combustor, comprising the Ni-based alloy for gas turbine combustor according to claim 3.

5. A liner member for a gas turbine combustor, comprising the Ni-based alloy for gas turbine combustor according to claim 3.

6. A transition piece member for a gas turbine combustor, comprising the Ni-based alloy for gas turbine combustor according to claim 3.

7. A liner of a gas turbine combustor, comprising the Ni-based alloy for gas turbine combustor according to claim 3.

8. A transition piece of a gas turbine combustor, comprising the Ni-based alloy for gas turbine combustor according to claim 3.

9. A hot-rolled plate comprising the Ni-based alloy according to claim 1, wherein a content of Fe is 0.01% to 5% by mass.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an explanatory view showing a process of selecting a nitride with the largest size within the field of view of microscopic observation in an embodiment of a Ni-based alloy according to the present invention.

(2) FIG. 2 is a graph showing a result of plotting area-equivalent diameters and standardized variables of nitrides on an X-Y coordinate system in an embodiment of a Ni-based alloy according to the present invention.

(3) FIG. 3 is a graph showing a result of plotting area-equivalent diameters and standardized variables of nitride on an X-Y coordinate system in the examples.

DESCRIPTION OF EMBODIMENTS

(4) An embodiment of a Ni-based alloy according to the present invention will be described below. The embodiment of a Ni-based alloy according to the present invention is used as a raw material for a member for gas turbine combustor, a liner member or transition piece member for gas turbine combustor, and a liner or transition piece for gas turbine combustor.

(5) The Ni-based alloy according to the present embodiment has a composition consisting of, by mass, 20.0% to 26.0% Cr, 4.7% to 9.4% Co, 5.0% to 16.0% Mo, 0.5% to 4.0% W, 0.3% to 1.5% Al, 0.1% to 1.0% Ti, 0.001% to 0.15% C, at most 5% Fe, and the balance of Ni with inevitable impurities.

(6) The reasons for the content ranges of the alloy elements mentioned above will be described below.

(7) Cr

(8) Cr is an element which has an effect for improving the high-temperature corrosion resistance such as a high-temperature oxidation or sulfurization resistance by forming an excellent protective film.

(9) If the content of Cr is less than 20% by mass, sufficient high-temperature corrosion resistance cannot be secured. In contrast, if the content of Cr is more than 26% by mass, harmful phases such as a phase and phase may be precipitated, and thus, the high-temperature corrosion resistance may adversely reduce. To prevent this, the content of Cr is set within a range of 20.0% to 26.0% by mass.

(10) Co

(11) Co is an element which has an effect for improving the high-temperature strength properties such as the creep properties by solid solution strengthening with the matrix.

(12) If the content of Co is less than 4.7% by mass, sufficient high-temperature strength properties cannot be imparted. In contrast, if the content of Co is more than 9.4% by mass, the hot workability may reduce and the high-temperature ductility during the use of the combustor may reduce. To prevent this, the content of Co is set within a range of 4.7% to 9.4% by mass.

(13) Mo

(14) Mo is an element which has an effect for improving the high-temperature strength properties such as the high-temperature tensile properties, the creep properties and the creep fatigue properties by solid solution strengthening with the matrix. The above-described effect is exhibited as a combined effect particularly in the presence of W.

(15) If the content of Mo is less than 5.0% by mass, sufficient high-temperature ductility and creep fatigue properties cannot be imparted. In contrast, if the content of Mo is more than 16.0% by mass, the hot workability may reduce and harmful phases such as a phase may be precipitated, which may result in developing brittleness. To prevent this, the content of Mo is set within a range of 5.0% to 16.0% by mass.

(16) W

(17) W is an element which has an effect for improving the high-temperature strength properties such as the high-temperature tensile properties, the creep properties, and the creep fatigue properties by solid solution strengthening with the matrix. The above-described effect is exhibited as a combined effect particularly in the presence of Mo.

(18) If the content of W is less than 0.5% by mass, sufficient high-temperature ductility and creep fatigue properties cannot be imparted. In contrast, if the content of W is more than 4.0% by mass, the hot workability may reduce and the ductility may also reduce, which is not preferable. To prevent this, the content of W is set within a range of 0.5% to 4.0% by mass.

(19) Al

(20) Al is an element which strengthens a solid solution with the matrix and forms a phase (Ni.sub.3Al) during the use of the combustor, and has an effect for improving the high-temperature strength properties such as the high-temperature tensile properties, the creep properties, and the creep fatigue properties. In a Ni-based alloy having such a phase, nitride forms a harmful phase.

(21) If the content of Al is less than 0.3% by mass, the degrees of the solid solution effect with the matrix and the precipitation of the phase during the use may become insufficient, and thus, the desired high-temperature strength cannot be secured.

(22) In contrast, if the content of Al is more than 1.5% by mass, the hot workability may reduce and the cold workability may also reduce, which is not preferable. To prevent this, the content of Al is set within a range of 0.3% to 1.5% by mass.

(23) Ti

(24) Ti is an element which strengthens a solid solution with the matrix and the phase and has an effect for improving the high-temperature strength properties such as the high-temperature tensile properties, the creep properties, and the creep fatigue properties. Ti also has an effect for improving the grain boundary strength by forming a carbide, i.e., primarily an MC-type carbide, and an effect for preventing the growth of crystal grains, which may occur due to heating during hot working and solution heat treatment.

(25) If the content of Ti is less than 0.1% by mass, the degrees of the solid solution effect with the matrix and the precipitation of the phase during the use may become insufficient, and thus, the desired high-temperature strength cannot be secured; thus, the amount of carbide to be formed may be insufficient, and the desired effect for preventing the growth of crystal grains cannot be obtained. In contrast, if the content of Ti is more than 1.0% by mass, the hot workability may reduce and the tendency of generating coarse nitrides with titanium nitride and the carbide functioning as the core may increase, which is not preferable. To prevent this, the content of Ti is set within a range of 0.1% to 1.0% by mass.

(26) C

(27) C is an element which has an effect for improving the grain boundary strength by forming M.sub.6C-type and MC-type carbides with Ti and Mo and an effect for preventing the growth of crystal grains which may occur due to heating during hot working and solution heat treatment.

(28) If the content of C is less than 0.001% by mass, because the ratio of precipitation of the M.sub.6C-type and MC-type carbides may become insufficient, a sufficient grain boundary strengthening function and a sufficient grain boundary pinning effect cannot be obtained. In contrast, if the content of C is more than 0.15% by mass, the carbides may be formed by an excessive constitutional quantity, and the hot workability, the weldability, and the ductility, and the like may reduce, and it may become easy for a coarse nitride to be formed with the MC-type carbide generated in the solidification process after the melting being the nucleation site of nitride, which is not preferable. To prevent this, the content of C is set within a range of 0.001% to 0.15% by mass.

(29) Fe

(30) Fe is an element which is easily entrained in a Ni-based alloy as an impurity element. If the content of Fe is more than 5% by mass, the high-temperature strength may dramatically reduce, which is not preferable. To prevent this, it is necessary to limit the content of Fe to 5% by mass or less.

(31) Because Fe is inexpensive and economical and has an effect for improving the hot workability, Fe can also be added within a range of 0.01% to 5% by mass, optionally.

(32) In addition to the above-described elements, the composition of the present embodiment of Ni-based alloy may further contain one or more elements selected from the group consisting of, by mass, 0.0005% to 0.05% Ca, 0.0005% to 0.05% Mg, 0.001% to 0.15% a rare earth element, 0.01% to 1.0% Nb, 0.01% to 1.0% Ta, 0.01% to 1.0% V, 0.002% to 0.01% B, and 0.001% to 0.05% Zr, optionally.

(33) Ca and Mg are elements that have an effect for improving the hot workability and the cold workability.

(34) The rare earth elements such as Y, Ce, and La are elements that have an effect for improving the oxidation resistance and the hot workability.

(35) Nb, Ta, and V are elements that have an effect for forming carbides and preventing the growth of crystal grains which may occur due to heating during hot working and solution heat treatment.

(36) B is an element which has an effect for improving the creep strength by forming a boride and strengthening the grain boundary.

(37) Zr is an element which has an effect for improving the ductility of the grain boundary by segregating on the grain boundary.

(38) To obtain such an effect, it is preferable to add the respective types of elements within the above-described ranges.

(39) Furthermore, the composition of the present embodiment of Ni-based alloy may contain 1% by mass or less of Mn, 1% by mass or less of Si, 0.015% by mass or less of P, 0.015% by mass or less of S, or 0.5% by mass or less of Cu.

(40) The above-described various characteristics can also be maintained when these elements are contained within the above-described ranges.

(41) In the present embodiment of Ni-based alloy, an estimated largest size of nitrides is an area-equivalent diameter of 12 m to 25 m. The estimated largest size of nitrides is determined by calculating an area-equivalent diameter D which is defined as D=A.sup.1/2 in relation to an area A of a nitride with a largest size among nitrides present in a measurement field of view area S.sub.0 of an observation of the Ni-based alloy, repeatedly performing this operation for n times corresponding to a measurement field of view number n to acquire n pieces of data of the area-equivalent diameter D, arranging the pieces of data of area-equivalent diameter D in ascending order into D.sub.1, D.sub.2, . . . D.sub.n to calculate a standardized variable y.sub.j which is defined by the following expression:
y.sub.j=ln [ln {j/(n+1)}][Expression 3]

(42) wherein j denotes the number of the arranged ascending order of the data of the area-equivalent diameter D,

(43) plotting the area-equivalent diameter D and the standardized variable y.sub.j on X and Y axes of an X-Y coordinate system, respectively, to obtain a regression line y.sub.j=aD+b (wherein a and b are constants), calculating y.sub.j from the following expression:

(44) $\begin{array}{cc}{y}_j=-\ln \left(-\ln \frac{S}{S_0+S}\right)& \left[\mathrm{Expression}4\right]\end{array}$

(45) where a cross-sectional area to be predicted S is 100 mm.sup.2, and substituting the obtained value of y.sub.j into the regression line to obtain the estimated largest size of the nitrides.

(46) In the present embodiment, the nitrides are mostly titanium nitrides.

(47) The method of estimating the estimated largest size of nitrides described above will be described below with reference to FIGS. 1 and 2.

(48) First, the measurement field of view area S.sub.0 for microscopy is set and the nitrides in this measurement field of view area S.sub.0 are observed. In this operation, the observation magnification is preferably 400 to 1,000 times. Then, as shown in FIG. 1, a nitride with maximum size among nitrides observed in the measurement field of view area S.sub.0 are selected. In order to measure the size at a high accuracy, the elected nitride is magnified, the area A of the selected nitride is measured, and the area-equivalent diameter D=A.sup.1/2 is calculated. In this operation, the observation magnification is preferably 1,000 to 3,000 times.

(49) The observation of the nitrides is carried out at the magnification of 400 to 1,000 times for preferably 30 or more measurement fields of view, more preferably 50 or more, as the measurement field of view number n. For the measurement of the area of the nitrides, it is preferable to carry out the measurement by a method in which the luminance distribution is acquired by using image processing, a threshold for the luminance is determined, and the nitrides, the matrix phase, the carbides, and the like are separated. In this measurement, color difference (RGB) may be used in substitution for the luminance. Particularly if the carbides recited in Patent Document 3 exist, they may be hardly distinguished from nitrides by using the luminance only, and it is thus preferable to separate between them by color difference (RGB). Test pieces used for the observation were observed with a scanning electron microscope and analyzed by using an energy dispersive X-ray analyzer (EDS) equipped to the scanning electron microscope, and thus, it was verified that the nitrides were titanium nitride.

(50) This operation is repeatedly carried out for n times corresponding to the measurement field of view number n to obtain n pieces of data of the area-equivalent diameter D. Then the n pieces of the area-equivalent diameter D are arranged in ascending order of the area-equivalent diameter to obtain data D.sub.1, D.sub.2, . . . D.sub.n.

(51) Then the standardized variable y.sub.j defined by the following expression is calculated by using the data D.sub.1, D.sub.2, . . . D.sub.n:
y.sub.j=ln [ln {j/(n+1)}][Expression 5]

(52) (where j denotes the number of the arranged ascending order of the data of the area-equivalent diameter D).

(53) Then, as shown in FIG. 2, the data D.sub.1, D.sub.2, . . . D.sub.n of the n area-equivalent diameters and the values of the standardized variables y.sub.1, y.sub.2, . . . y.sub.n corresponding to these pieces of data are plotted on an X-axis and a Y-axis of an X-Y coordinate system, respectively.

(54) A regression line y.sub.j=aD+b (a, b are constants) is obtained by the plotted graph.

(55) Then, the solution to y.sub.j is calculated by the following expression. In this case, a cross-sectional area to be predicted S is 100 mm.sup.2.

(56) $\begin{array}{cc}{y}_j=-\ln \left(-\ln \frac{S}{S_0+S}\right)& \left[\mathrm{Expression}6\right]\end{array}$

(57) Specifically, in the graph shown in FIG. 2, the value of D.sub.j of the regression line for the value of y.sub.j corresponding to the cross-sectional area to be predicted S (a straight line H in FIG. 2) is the estimated largest size of nitrides, and the estimated largest size of nitrides is set within a range of 12 m to 25 m.

(58) An embodiment of a method for producing a Ni-based alloy according to the present invention will be described below.

(59) First, the melting raw materials are mixed and the mixture of the raw materials is pickled, and the melting is carried out in a vacuum melting furnace. A variety of scrap materials are used as the melting raw materials. In this stage, a reactive metal such as Al and Ti is preferably added so that the resulting concentration thereof is lower than the objective concentration thereof.

(60) In the present embodiment, the scrap materials include a material prepared for other purposes than the raw material, a part made of the material, and other materials or parts generated during a process of producing the part, which have a variety of forms such as bulk, chip, and powder. Different scrap materials can be appropriately used in combination, and thus, a scrap material including a different concentration of an alloy element from the objective concentration thereof may be used, or an integrated scrap material including such scrap materials having the different concentrations, e.g., by welding, may be used.

(61) In addition, a composition ratio of scrap materials is preferably 5% by mass or more because the contributions of scraps to stabilities of production, supply and price of materials becomes greater as the composition ratio of scraps becomes higher. When the composition ratio is further higher, the energy required for melting materials can be reduced and the melting time can be shortened, but scraps may contain unexpected component factors, and thus, the composition ratio is preferably 40% to 99% by mass.

(62) Before starting the melting, the atmosphere inside the furnace is substituted with high-purity argon for 3 times or more, evacuation is carried out, and then the temperature of the inside of the furnace is raised. Then, after the molten metal is held for a predetermined time, the reactive metals Ti and Al are added into the molten metal. The resulting mixture is held for a predetermined time and then is discharged into a mold to obtain an ingot. From a point of view of preventing coarsening of the nitrides, the addition of Ti is preferably carried out as soon as possible before the discharge of the mixture.

(63) This ingot is then subjected to hot forging to produce a hot forging not including a cast structure. Further, hot rolling is carried out to shape the hot forging into a hot rolled plate, and then solution heat treatment is performed on the hot rolled plate. The Ni-based alloy according to the present embodiment is produced by the above-described processes.

(64) In the Ni-based alloy produced by the above-described production method, the concentration of the nitrogen in the Ni-based alloy is low, and the time of holding the reactive element of Ti at high-temperatures is short, and thus, generation and growth of titanium nitride can be prevented. With this configuration, as described above, the estimated largest size of the nitride (titanium nitride) becomes 12 m to 25 m when the cross-sectional area to be predicted S satisfies the expression: S=100 mm.sup.2.

(65) According to the Ni-based alloy of the present embodiment having the configuration described above, the estimated largest size of the nitride when the cross-sectional area to be predicted S is 100 mm.sup.2 is 25 m or less by the area-equivalent diameter D.sub.j, and thus, no large-size nitride exists in the inside of the Ni-based alloy, which thereby enables improvement of mechanical properties of the Ni-based alloy.

(66) In addition, because the estimated largest size of the nitride when the cross-sectional area to be predicted S is 100 mm.sup.2 is 12 m or more by the area-equivalent diameter D.sub.j, great increase in the production costs for the Ni alloy according to the present embodiment can be avoided, which enables industrial production of the Ni-based alloy.

(67) In particular, in the present embodiment, the reactive element of Ti is contained and the nitride is titanium nitride. Because titanium nitride has a polygonal shape in a cross-sectional view, even if the particle size of titanium nitride is small, the mechanical properties may be greatly affected. Accordingly, the mechanical properties of the Ni-based alloy can be securely improved by evaluating the largest size of titanium nitride in the Ni-based alloy at a high accuracy by using the above-described method.

(68) In addition, the Ni-based alloy according to the present embodiment has a composition including, by mass, 20.0% to 26.0% Cr, 4.7% to 9.4% Co, 5.0% to 16.0% Mo, 0.5% to 4.0% W, 0.3% to 1.5% Al, 0.1% to 1.0% Ti, 0.001% to 0.15% C, and at most 5% Fe, and thus has excellent high-temperature corrosion resistance, excellent high-temperature strength properties such as creep properties and creep fatigue, and excellent workability, and thus, the Ni-based alloy according to the present embodiment is suitable for use as a raw material for a variety of members of gas turbine combustors.

(69) Further, in the Ni-based alloy according to the present embodiment, scraps are used as the melting raw materials, and thus raw materials such as minor metals can be stably secured. In addition, by appropriately selecting the shapes and the like of the scraps, melting can be sufficiently promoted and the energy required for the melting can be reduced. If scraps are used, evaluation of the nitrides can be performed at a high accuracy as described above, and thus degradation of mechanical properties, machinability, and the like can be prevented.

(70) The Ni-based alloy according to the present embodiment is as described above; however, the present invention is not limited to this, and can be appropriately modified or altered within a scope not deviating from the technical concept of the present invention.

(71) In addition, the production method of this Ni-based alloy is not limited to the examples described above with reference to the embodiment and may be produced by other production methods. For example, the Ni-based alloy can be produced by performing the melting in a vacuum atmosphere and continuous casting. To produce the Ni-based alloy of the present invention, the results of evaluation of the nitrides by the above-described method may show that the estimated largest size of the nitride when the cross-sectional area to be predicted S is 100 mm.sup.2 is within a range of 12 m or more and 25 m or less by the area-equivalent diameter.

(72) For example, a method may be employed in which a high-purity Ar gas is bubbled into the molten metal melted inside a vacuum melting furnace to reduce the concentration of nitrogen in the molten metal, and then reactive elements such as Ti are added.

(73) Alternatively, another method may be employed in which the pressure in the inside of a chamber of a vacuum melting furnace is reduce, then a high-purity Ar gas is introduced into the chamber, and thus, the pressure in the inside of the chamber is controlled to be positive to prevent outside air from entering the chamber, and in this state, reactive elements such as Ti are added before the melting.

(74) As described above, scraps are used as the melting raw material; however, the melting raw material is not limited thereto.

EXAMPLES

(75) The results of the verification tests performed to verify the effects of the present invention will be described below.

Examples 1 to 12 of the Present Invention

(76) Alloys of Examples 1 to 11 shown in Table 1 were melted by vacuum melting in an induction melting furnace to produce ingots with a diameter of 100 mm and a height of 150 mm by casting. An alloy of Example 12 of the present invention was melted by atmospheric melting in the induction melting furnace to produce an ingot with the same size as the above-mentioned examples by casting. These ingots were hot-forged to produce hot-forgings with the dimensions having a thickness of 50 mm, a width of 120 mm, and a length of 200 mm. The hot-forgings were further hot-rolled to produce hot-rolled plates having a thickness of 5 mm, which were subjected to a solution heat treatment, in which the plates were held at the temperature of 1,180 C. for 15 minutes and then were water-cooled.

(77) The alloys having the composition ratio of scrap was 35% by mass or less, as shown in Table 1, were melted in the following manners.

(78) Virgin raw materials such as Ni, Cr, Co, Mo and the like, except Al and Ti, and scrap materials having the average composition satisfying the chemical composition range defined in claim 1 were pickled and charged into an MgO crucible at the composition ratios shown in Table 1. After the raw materials were charged, the atmosphere of the inside of the furnace was evacuated before starting the melting, substitution with argon in which up to 0.5 atm of high-purity argon was introduced was repeated 3 times or more, then evacuation was performed, the temperature of the inside of the furnace was raised, and the melting at 1,450 C. was performed. After 10 minutes had passed since melt-down, the reactive elements of Ti and Al were added.

(79) The alloys having the composition ratio of scrap was 40% by mass or more, as shown in Table 1, were melted in the following manners.

(80) Virgin raw materials such as Ni, Cr, Co, Mo and the like, except Al and Ti, and scrap materials including Al by the concentration less than 0.3% and Ti by the concentration less than 0.1% were pickled and charged into an MgO crucible at the composition ratios shown in Table 1. After the raw materials were charged, the atmosphere of the inside of the furnace was evacuated before starting the melting, substitution with argon in which up to 0.5 atm of high-purity argon was introduced was repeated 3 times or more, then evacuation was performed, the temperature of the inside of the furnace was raised, and the melting at 1,450 C. was performed. After 10 minutes had passed since melt-down, the reactive elements of Ti and Al were added.

(81) For Example 12 of the present invention, scrap materials with the desired chemical composition range were serially charged in the furnace, the temperature of the inside of the furnace was raised, and casting was performed when the temperature inside the furnace had reached 1,450 C.

Comparative Examples 1 and 2

(82) The alloys shown in Table 1 were melted by atmospheric melting in the induction melting furnace to produce ingots with a diameter of 100 mm and a height of 150 mm by casting. These ingots were hot-forged to produce hot-forgings with the dimensions having a thickness of 50 mm, a width of 120 mm, and a length of 200 mm. The hot-forgings were further hot-rolled to produce hot-rolled plates having a thickness of 5 mm, which were subjected to a solution heat treatment, in which the plates were held at the temperature of 1,180 C. for 15 minutes and then were water-cooled.

(83) The alloys were melted in the following manners. Raw materials such as Ni, Cr, Co, Mo, Ti, Al and the like were charged into an MgO crucible without being pickled and then were melted. In this process, after the melt-down, the resultant was held at 1,500 C. for 10 minutes and then at 1,450 C. for 10 minutes.

(84) Estimation of the Largest Size of Nitrides

(85) By using the hot-rolled plates of Examples 1 to 12 of the present invention and the hot-rolled plates of Comparative Examples 1 and 2 obtained in the above-described manner, the estimation of the largest size of nitrides was performed by the following process.

(86) Sample pieces for observation of the microstructure were cut out from the obtained plates, were polished, and then were observed by microscope. The estimated largest size of nitrides was determined by the above-described process, when the cross-sectional area to be predicted S satisfied the expression: S=100 mm.sup.2. In the present embodiment, the measurement field of view area S.sub.0 satisfied the expression: S.sub.0=0.306 mm.sup.2. The selection of the nitrides with the largest size within the measurement field of view area S.sub.0 was carried out by observation at the magnification of 450 times, and the measurement of the area of the selected nitride was carried out by observation at the magnification of 1,000 times. The measurement field of view number n satisfied the expression: n=50. The estimated largest size of nitride is shown in Table 2. The regression lines obtained by plotting on the X-Y coordinate system are shown in FIG. 3.

(87) Cutting Test

(88) Cutting tests were carried out on the rolled surface of the obtained hot-rolled plates by using a ball-end mill constituted by hard metal under a solvent cutting oil environment and under the following conditions: revolution: 20,000 rpm, feeding rate: 1,400 mm/min, cutting rate: 188 mm/min, and axial cutting depth: 0.3 mm, and the cutting length obtained until the timing at which a chipping occurred on the cutting edge was measured. The results are shown in Table 2.

(89) Low-Cycle Fatigue Test

(90) Plate-like test pieces with the dimensions having a parallel portion width of 6.4 mm, a parallel portion thickness of 3 mm, and a parallel portion length of 16 mm were sampled from the obtained billets, and these test pieces were heated up to 700 C., tensile/compression total strain of 0.7% was repeatedly imparted, and thereby low-cycle fatigue tests were performed and the number of cycles performed up to a timing at which the peak stress on the tension side had reduced to of the maximum value or a timing at which the test piece were broken. The results are shown in Table 2.

(91) TABLE-US-00001 TABLE 1 Composition ratio Chemical composition (% by mass) of scrap Cr Co Mo W Al Ti C Ni (% by mass) Examples of 1 21.9 8.1 8.9 3.1 0.9 0.3 0.07 Balance 60 the present 2 20.0 4.7 16.0 0.5 0.3 0.6 0.03 Balance 5 invention 3 24.0 6.5 5.0 2.1 1.5 0.2 0.08 Balance 50 4 25.9 9.4 8.1 4.0 1.1 0.2 0.06 Balance 70 5 20.1 7.8 9.9 2.7 0.8 0.3 0.08 Balance 99 6 22.1 7.9 9.4 2.8 1.2 0.1 0.09 Balance 80 7 21.5 8.1 8.7 3.1 0.8 0.6 0.02 Balance 90 8 20.9 7.0 8.3 2.1 0.6 1.0 0.01 Balance 25 9 22.2 8.2 9.2 3.3 0.5 0.8 0.001 Balance 40 10 22.4 8.0 8.8 2.9 1.3 0.1 0.15 Balance 55 11 20.1 8.1 8.4 3.0 0.4 0.1 0.04 Balance 0 12 22.1 7.9 8.5 2.8 0.5 0.3 0.06 Balance 100 Comparative 1 23.1 7.1 7.0 2.3 0.7 0.05 0.13 Balance 0 Examples 2 20.9 9.1 11.7 3.7 1.3 0.8 0.10 Balance 0

(92) TABLE-US-00002 TABLE 2 Cutting length Number of cycles Estimated largest obtained in low-cycle size of nitrides by cutting fatigue test (m) test (m) (cycles) Examples of 1 17 34 1,717 the present 2 21 30 1,007 invention 3 24 29 1,018 4 19 30 1,031 5 22 28 1,669 6 19 31 2,273 7 19 33 1,521 8 20 32 1,880 9 16 36 4,167 10 12 37 3,127 11 23 28 1,124 12 24 27 1,050 Comparative 1 29 20 461 Examples 2 28 22 430

(93) In Comparative Examples 1 and 2 in which the estimated largest size of the nitrides when the cross-sectional area to be predicted S was 100 mm.sup.2 exceeded 25 m by the area-equivalent diameter, the cutting length reached before the timing at which chippings had occurred on the cutting edge in the cutting test was as short as 20 m and 22 m, and thus, it was observed that the machinability was low. In addition, in the low-cycle fatigue test, the number of cycles performed up to a timing at which the test pieces were broken was as small as 461 times and 430 times, and it was thus observed that the fatigue strength was low.

(94) In contrast, in Examples 1 to 12 of the present invention in which the estimated largest size of the nitrides when the cross-sectional area to be predicted S was 100 mm.sup.2 was a range of 12 m to 25 m, the cutting length reached before the timing at which chippings had occurred on the cutting edge in the cutting test was 27 m or longer, which was relatively long, and thus, it was observed that the machinability was high.

(95) In addition, in the low-cycle fatigue test, the number of cycles performed up to a timing at which the test pieces were broken was as many as 1,007 times or more, and it was thus observed that the fatigue strength had been greatly improved. The same effects as those of Examples 1 to 10 of the present invention were observed also in Example 11 of the present invention in which the ratio of scrap was 0% and Example 12 in which atmospheric melting was performed.

(96) As described above, according to the present invention, there can be provided a Ni-based alloy having excellent high-temperature strength properties and high-temperature corrosion resistance, of which the nitrides present in an inside can be appropriately and accurately evaluated.