Ni-base alloy

Abstract

In a Ni-base alloy, an area-equivalent diameter D is calculated. D is defined by D=A.sup.1/2 from an area A of a largest nitride in a field of view when an observation area S.sub.0 is observed. This process is repeated in n fields of view for measurement, where n is the number of the fields of view for measurement, so as to acquire n pieces of data on D, and the pieces are arranged in ascending order D.sub.1, D.sub.2, . . . , D.sub.n to obtain a reduced variate y.sub.j. The obtained values are plotted on X-Y axis coordinates, where an X axis corresponds to D and a Y axis corresponds to y.sub.j. In a regression line y.sub.j=a×D+b, y.sub.j is obtained when a target cross-sectional area S is set to 100 mm.sup.2. When the obtained y.sub.j is substituted into the regression line, the estimated nitride maximum size is ≦25 μm in diameter.

Claims

1. A cast and plastically-worked Ni-base alloy comprising: 13 mass % to 30 mass % of Cr; 0.01 mass % to 6 mass % of Ti; 8 mass % or less of Al; and 25 mass % or less of Fe, wherein nitrides are present, and an area-equivalent diameter D is calculated, and the area-equivalent diameter D is defined by D=A.sup.1/2 from an area A of a largest nitride in a field of view when observation is performed at a magnification of 400 times to 3000 times for an observation area S.sub.0 for measurement, this process is repeated in n fields of view for measurement, where n is the number of the fields of view for measurement, so as to acquire n pieces of data on the area-equivalent diameter D, and the pieces of data on the area-equivalent diameter D are arranged in ascending order of D.sub.1, D.sub.2, . . . , D.sub.n to obtain a reduced variate y.sub.j which is defined by the following Expression (1):
[Formula 1]
y.sub.j=−ln [−ln {j/(n+1)}]  (1) (in the Expression (1), j is a rank number when the pieces of data on the area-equivalent diameter D are arranged in ascending order), the obtained values are plotted on X-Y axis coordinates, where an X axis corresponds to the area-equivalent diameter D and a Y axis corresponds to the reduced variate y.sub.j, a regression line y.sub.j=a×D+b (a and b are constants) is obtained, and when a target cross-sectional area S for prediction is set to 100 mm.sup.2, y.sub.j is obtained through the following Expression (2): $\begin{array}{cc}\left[\mathrm{Formula}2\right]& \\ y_j=-\ln \left(-\ln \frac{S}{S_0+S}\right),& \left(2\right)\end{array}$  and when the obtained value of y.sub.j is substituted into the regression line to calculate an estimated nitride maximum size, the estimated nitride maximum size is equal to or less than 25 μm in terms of area-equivalent diameter.

2. The cast and plastically-worked Ni-base alloy according to claim 1, wherein the nitride is a titanium nitride.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein like designations denote like elements in the various views, and wherein:

(2) FIG. 1 is a diagram illustrating a procedure for extracting a nitride having a maximum size from a field of view for microscopic observation in a Ni-base alloy according to an embodiment.

(3) FIG. 2 is a graph showing results of plotting of area-equivalent diameters of nitrides and reduced variates on X-Y coordinates in the Ni-base alloy according to the embodiment.

(4) FIG. 3 is a graph showing results of plotting of area-equivalent diameters of nitrides and reduced variates on X-Y coordinates in example.

DETAILED DESCRIPTION OF THE INVENTION

(5) Hereinafter, a Ni-base alloy according to an embodiment of the invention will be described.

(6) The Ni-base alloy according to this embodiment contains Cr: 13 mass % to 30 mass %, Fe: 25 mass % or less, and Ti: 0.01 mass % to 6 mass %, with the balance being Ni and unavoidable impurities.

(7) In the Ni-base alloy according to this embodiment, an area-equivalent diameter D is calculated, and the area-equivalent diameter D is defined by D=A.sup.1/2 from an area A of a largest nitride in a field of view when observation is performed for an observation area S.sub.0 for measurement. This process is repeated in n fields of view for measurement, where n is the number of the fields of view for measurement, so as to acquire n pieces of data on the area-equivalent diameter D. These pieces of data on the area-equivalent diameter D are arranged in ascending order of D.sub.1, D.sub.2, . . . , D.sub.n to obtain a reduced variate y.sub.j which is defined by the following Expression (1).
[Formula 3]
y.sub.j=−ln [−ln {j/(n+1)}]  (1)

(8) (In the Expression (1), j is a rank number when the pieces of data on the area-equivalent diameter D are arranged in ascending order)

(9) The obtained values are plotted on X-Y axis coordinates, where an X axis corresponds to the area-equivalent diameter D and a Y axis corresponds to the reduced variate y.sub.j, and a regression line y.sub.j=a×D+b (a and b are constants) is obtained. When a target cross-sectional area S for prediction is set to 100 mm.sup.2, y.sub.j is obtained through the following Expression (2).

(10) $\begin{array}{cc}\left[\mathrm{Formula}4\right]& \\ y_j=-\ln \left(-\ln \frac{S}{S_0+S}\right)& \left(2\right)\end{array}$

(11) When the obtained value of y.sub.j is substituted into the regression line to calculate an estimated nitride maximum size, the estimated nitride maximum size is equal to or less than 25 μm in terms of area-equivalent diameter.

(12) In this embodiment, the nitride is mainly a titanium nitride.

(13) Here, the above-described method of estimating the estimated nitride maximum size will be described with reference to FIGS. 1 and 2.

(14) First, an observation area S.sub.0 for measurement is set for observation with a microscope, and nitrides in the observation area S.sub.0 for measurement are observed. At this time, the observation magnification is preferably set to be in a range of 400 times to 1,000 times. As shown in FIG. 1, a nitride having a maximum size is selected among the nitrides observed in the observation area S.sub.0 for measurement. In order to measure the size with high precision, the selected nitride is observed at a higher magnification and an area A thereof is measured to calculate an area-equivalent diameter D=A.sup.1/2. At this time, the observation magnification is preferably set to be in a range of 1,000 times to 3,000 times.

(15) In the nitride observation, the magnification is preferably set to be in a range of 400 times to 1,000 times, and the number n of fields of view for measurement is preferably equal to or more than 30, and more preferably equal to or more than 50. In addition, in the measurement of the nitride area, it is preferable that first, a luminance distribution be acquired using image processing, a luminance boundary be determined to separate a nitride, a matrix phase, a carbide, and the like, and then an area of the nitride be measured. At this time, a color difference (RGB) may be used in place of the luminance. Particularly, in the case where a carbide such as the carbide shown in Japanese Unexamined Patent Application, First Publication No. S61-139633 is present, it may be difficult to be distinguished from the nitride only with the luminance. Therefore, the separation is more preferably performed with a color difference (RGB). In addition, the test piece provided for observation is observed with a scanning electron microscope, and analysis is performed using an energy dispersive X-ray analyzer (EDS) mounted on the scanning electron microscope. As a result, it is confirmed that the nitride is a titanium nitride.

(16) This process is repeated in n fields of view for measurement, where n is the number of fields of view for measurement, so as to acquire n pieces of data on the area-equivalent diameter D. The n area-equivalent diameters D are arranged in ascending order to obtain data of D.sub.1, D.sub.2, . . . , D.sub.n.

(17) Using the data of D.sub.1, D.sub.2, . . . , D.sub.n, a reduced variate yj which is defined by the following Expression (1) is obtained.
[Formula 5]
y.sub.j=−ln [−ln {j/(n+1)}]  (1)

(18) In the Expression (1), j is a rank number when the pieces of data on the area-equivalent diameter D are arranged in ascending order.

(19) Next, as shown in FIG. 2, the pieces of data are plotted on X-Y coordinates, where an X axis corresponds to the data of the n area-equivalent diameters D.sub.1, D.sub.2, . . . , D.sub.n, and a Y axis corresponds to values of reduced variates y.sub.1, y.sub.2, . . . , y.sub.n corresponding to the data.

(20) A regression line y.sub.j=a×D.sub.j+b (a and b are constants) is obtained by the plotting.

(21) Next, an answer of y.sub.j is calculated through the following Expression (2). At this time, a target cross-sectional area S for prediction is set to 100 mm.sup.2. That is, the value of y.sub.j corresponding to the target cross-sectional area S for prediction (=100 mm.sup.2) is calculated from the Expression (2).

(22) $\begin{array}{cc}\left[\mathrm{Formula}6\right]& \\ y_j=-\ln \left(-\ln \frac{S}{S_0+S}\right)& \left(2\right)\end{array}$

(23) In the graph shown in FIG. 2, the value of D.sub.j of the regression line at the value of y.sub.j corresponding to the target cross-sectional area S for prediction (the straight line H in FIG. 2) becomes an estimated nitride maximum size. In this embodiment, the estimated maximum size is equal to or less than 25 μm.

(24) Hereinafter, an example of a method of manufacturing a Ni-base alloy according to this embodiment will be described.

(25) Raw materials including elements other than Ti and Al are mixed and melted in a vacuum melting furnace. At this time, high-purity raw materials having a small nitrogen content are used as the raw materials of Ni, Cr, Fe, or the like.

(26) Before the melting is started, the atmosphere in the furnace is repeatedly replaced three or more times with high-purity argon. Thereafter, vacuuming is performed, and the temperature in the furnace is raised. The molten metal is held for predetermined hours, and then Ti and Al which are active metals are added thereto, and the molten metal is held for predetermined hours. The molten metal is poured into a mold to obtain an ingot. From the viewpoint of preventing coarsening of nitrides, Ti is desirably added as immediately before pouring the molten metal into the mold as possible. The ingot is subjected to plastic working to manufacture a billet having no casting structure.

(27) The Ni-base alloy manufactured through such a manufacturing method has a low nitrogen content. In addition, the time during Ti, which is an active element, is held at high temperature is short. Therefore, generation and growth of a titanium nitride can be suppressed. Accordingly, as described above, the estimated nitride (titanium nitride) maximum size when the target cross-sectional area S for prediction is set to 100 mm.sup.2 is equal to or less than 25 μm.

(28) According to the Ni-base alloy of this embodiment having the above-described properties, the estimated nitride maximum size when the target cross-sectional area S for prediction is set to 100 mm.sup.2 is equal to or less than 25 μm in terms of area-equivalent diameter D.sub.j. Therefore, nitrides having a large size are not present in the Ni-base alloy; and thereby, the mechanical properties of the Ni-base alloy can be improved.

(29) Particularly, in this embodiment, Ti which is an active element is contained and the nitride is a titanium nitride. The titanium nitride has a polygonal cross-section. Therefore, it has a great influence on mechanical properties even when its size is small. Accordingly, by evaluating the maximum size of the titanium nitride in the Ni-base alloy with high precision using the above-described method, the mechanical properties of the Ni-base alloy can be securely improved.

(30) Although the Ni-base alloy according to the embodiment of the invention has been described as above, the invention is not limited thereto, and appropriate modifications can be made without departing from the features of the invention.

(31) For example, the Ni-base alloy has been described which has a composition including Cr: 13 mass % to 30 mass %, Fe: 25 mass % or less, and Ti: 0.01 mass % to 6 mass %, with the balance being Ni and unavoidable impurities; however, the invention is not limited thereto, Ni-base alloy having other compositions may be provided. For example, Al may be contained.

(32) In addition, the Ni-base alloy manufacturing method is not limited to the method exemplified in this embodiment, and other manufacturing methods may be applied. As a result of the evaluation of the nitrides using the above-described method, the estimated nitride maximum size should be equal to or less than 25 μm in terms of area-equivalent diameter when the target cross sectional area S for prediction is set to 100 mm.sup.2.

(33) For example, a method may be employed which includes: bubbling the molten metal in the vacuum melting furnace with high-purity Ar gas so as to reduce the nitrogen content in the molten metal; and then adding an active element such as Ti.

(34) In addition, a method may be employed which includes: reducing the pressure in the chamber of the vacuum melting furnace; introducing high-purity Ar gas into the chamber so as to make the chamber pressure positive to thus prevent incorporation of air; and in this state, adding and melting an active element such as Ti.

EXAMPLES

(35) Hereinafter, results of a confirmation test performed to confirm the effects of the invention will be described.

Invention Examples A to E

(36) 10 kg of an alloy shown in Table 1 was melted in a vacuum melting furnace. First, acid-pickled raw materials such as Ni, Cr, Fe, Nb, Mo, and Co were charged in a crucible and subjected to high-frequency induction melting. At this time, the melting temperature was set to 1450° C. and a crucible made of high-purity MgO was used. The raw materials such as Ni, Cr, Fe, Nb, Mo, and Co were charged, and then before the melting was started, the atmosphere in the furnace was repeatedly replaced three or more times with high-purity argon. Thereafter, vacuuming was performed, and the temperature was raised in the furnace.

(37) The addition of Ti and Al which were active elements was performed in the following two ways (i) and (ii).

(38) (i) One half of the addition amount of Ti and Al, which were active elements, was charged in a crucible simultaneously with the raw materials such as Ni, Cr, Fe, Nb, Mo, and Co. In addition, the remaining half was added after 10 minutes passed from melt-down.

(39) (ii) The total amount of Ti and Al was added after 10 minutes passed from melt-down of the raw materials.

(40) The molten metal in which the component adjustment had been conducted was held for 3 minutes, and then the molten metal was poured into a cast-iron mold (φ80×250 H) to manufacture an ingot. This ingot was subjected to billet forging to provide plastic strain of 1.5 by cogging; and thereby, a billet having no casting structure was manufactured. In this case, the nitrogen content in the ingot was in a range of 50 ppm to 300 ppm.

Comparative Examples F and G

(41) 10 kg of an alloy shown in Table 1 was subjected to air melting in a high-frequency induction melting furnace. First, raw materials such as Ni, Cr, Fe, Nb, Mo, Co, Ti, and Al, which were not subjected to acid pickling, were charged in a crucible and melted. At this time, after the melting, the molten metal was held for 10 minutes at 1500° C., and then the molten metal was held for 10 minutes at 1450° C. A crucible made of high-purity MgO was used. Then, the molten metal was poured into a cast-iron mold (φ80×250 H) to manufacture an ingot. This ingot was subjected to billet forging to provide plastic strain of 1.5 by cogging; and thereby, a billet having no casting structure was manufactured. In this case, the nitrogen content in the ingot was in a range of 300 ppm to 500 ppm.

(42) A sample for structure observation was cut out of the obtained billet, and the sample was polished and subjected to microscopic observation. An estimated nitride maximum size when a target cross-sectional area S for prediction was set to 100 mm.sup.2 was calculated according to the above-described procedure. In this example, an observation area S.sub.0 for measurement was set to 0.306 mm.sup.2. The selection of the nitride having the maximum size in the observation area S.sub.0 for measurement was performed by observation at a 450-fold magnification, and the area of the selected nitride was measured by observation at a 1,000-fold magnification. The number n of fields of view for measurement was 50.

(43) FIG. 3 shows regression lines obtained by plotting the data on the X-Y coordinates. Here, a reduced variate y.sub.j is 5.78 when a target cross-sectional area S for prediction is set to 100 mm.sup.2 and an observation area S.sub.0 for measurement is set to 0.306 mm.sup.2. A value (area-equivalent diameter D.sub.j) of the X-coordinate of an intersection between the straight line in which y.sub.j is 5.78 and a regression line is an estimated nitride maximum size. It is confirmed that in the invention examples A to E, the estimated nitride maximum sizes (area-equivalent diameters D.sub.j) are equal to or less than 25 μm. In contrast, it is confirmed that in the comparative examples F and G, the estimated nitride maximum sizes (area-equivalent diameters Dj) are greater than 25 μm.

(44) Next, a sample for measurement was cut out of the obtained billet, and a nitrogen content in the Ni-base alloy was measured. The sample was melted in inert gas, and the nitrogen content was measured through a heat conduction method. Since TiN was difficult to decompose, the measurement was performed by raising the temperature to 3,000° C.

(45) In addition, a test piece was prepared from the obtained billet to evaluate fatigue strength through low-cycle fatigue test. The low-cycle fatigue test was performed according to ASTM E606 under conditions where the atmosphere temperature was 600° C., the maximum strain was 0.94%, the stress ratio (minimum stress/maximum stress) was 0, and the frequency was 0.5 Hz to measure the number of times of failure (the number of repetitions of the testing cycle up to the failure). The fatigue strength was evaluated from the number of times of failure. The surface of the test piece was subjected to machining, and then polished to be finished. The evaluation results are shown in Table 1.

(46) TABLE-US-00001 TABLE 1 Estimated Nitride Number of Times Alloy Nominal Component Maximum Size of Failure Type Composition Method of Adding Ti and Al (μm) (times) Invention UNS No. Ni-19 wt % Cr-18 wt % The total amount was added after 10 16 5.1 × 10.sup.4 Example A 7718 Fe-5.1 wt % Nb-3 wt % minutes passed from melt-down. Mo-0.9 wt % Ti-0.5 wt % Al Invention UNS No. Ni-20 wt % Cr-14 wt % The total amount was added after 10 17 1.0 × 10.sup.4 Example B 7001 Co-4 wt % Mo-3 wt % minutes passed from melt-down. Ti-1 wt % Al Invention UNS No. Ni-19 wt % Cr-18 wt % One half of the total amount was added 21 3.2 × 10.sup.4 Example C 7718 Fe-5.1 wt % Nb-3 wt % when raw materials were charged before Mo-0.9 wt % Ti-0.5 wt % Al melting. The remaining half was added after 10 minutes passed from melt-down. Invention UNS No. Ni-19 wt % Cr-18 wt % The total amount was added after 10 24 2.4 × 10.sup.4 Example D 7718 Fe-5.1 wt % Nb-3 wt % minutes passed from melt-down. Mo-0.9 wt % Ti-0.5 wt % Al Invention UNS No. Ni-19 wt % Cr-18 wt % One half of the total amount was added 25 3.1 × 10.sup.4 Example E 7718 Fe-5.1 wt % Nb-3 wt % when raw materials were charged before Mo-0.9 wt % Ti-0.5 wt % Al melting. The remaining half was added after 10 minutes passed from melt-down. Comparative UNS No. Ni-19 wt % Cr-18 wt % The total amount was added when raw 28 5.6 × 10.sup.3 Example F 7718 Fe-5.1 wt % Nb-3 wt % materials were charged before melting. Mo-0.9 wt % Ti-0.5 wt % Al Comparative UNS No. Ni-20 wt % Cr-14 wt % The total amount was added when raw 29 3.8 × 10.sup.3 Example G 7001 Co-4 wt % Mo-3 wt % materials were charged before melting. Ti-1 wt % Al

(47) In the comparative examples F and G in which the estimated nitride maximum size when the target cross-sectional area S for prediction was set to 100 mm.sup.2 was greater than 25 μm in terms of area-equivalent diameter, the number of times of failure was small; and therefore, the fatigue strength was confirmed to be low.

(48) In contrast, in the invention examples A to E in which the estimated nitride maximum size when the target cross-sectional area S for prediction was set to 100 mm.sup.2 was 25 μm or less in terms of area-equivalent diameter, the fatigue strength was confirmed to be significantly improved.

INDUSTRIAL APPLICABILITY

(49) A Ni-base alloy according to an aspect of the invention is excellent in mechanical properties, especially, fatigue strength. Therefore, the Ni-base alloy according to an aspect of the invention is suitable as a material of parts such as blades, vanes, disks, cases, combustors, and the like of aircrafts and gas turbines.