METHOD FOR PRODUCING NI-BASED ALLOY

Abstract

 A method for producing a Ni-based alloy which is capable of maintaining sufficient hot workability of a Ni-based alloy material even when a temperature of the material is reduced during hot forging or before hot forging is started. In the production method of the Ni-based alloy in which hot forging is performed on the Ni-based alloy material having a composition such as Waspaloy, the hot forging is performed at a time when the temperature of the Ni-based alloy material is at least 900°C while the Ni-based alloy material heated to a preheating temperature of T₁ to T₂°C is reduced in temperature, and a temperature decrease rate of the Ni-based alloy material during the temperature reduction from the preheating temperature to 900°C is 2.0°C/second or less, provided that T₁ is a temperature at which all y' phases are in solid solution in a matrix phase of the material, and T₂ is a temperature at which the matrix phase of the material starts to melt.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for producing a Ni-based alloy.

BACKGROUND ART

[0002] Ni-based alloys having an intermetallic compound called the y' (gamma prime) phase on the matrix phase have excellent high-temperature strength and are widely used for jet aircraft engine components. It is known that the Ni-based alloys have excellent high-temperature strength when this reinforcing phase, the y' phase, is present in a fine state. On the other hand, however, when the y' phase has a fine microstructure, the high high-temperature strength of Ni-based alloys causes hot forgeability of the Ni-based alloy material to deteriorate. In response to such a problem, a method may be used in which the Ni-based alloy material is heated to an elevated temperature at which the y' phase is in solid solution in the matrix phase, and hot forging is performed in a state in which the y' phase does not exist. In addition, Patent Documents 1 and 2 show a method for suppressing strengthening by the y' phase and increasing hot forgeability of the Ni-based alloy material by subjecting the Ni-based alloy material to a long aging treatment after precipitation of the y' phase so as to coarsen the y' phase.

REFERENCE DOCUMENT LIST

PATENT DOCUMENTS

[0003] 

Patent Document 1:	JP 05-508194 A
Patent Document 2:	JP 2006-225756 A
Patent Document 3:	WO 2021/182606 A

SUMMARY OF THE INVENTION

PROBLEM TO BE SOLVED BY THE INVENTION

[0004] Among the above-described conventional technologies, in the method of performing hot forging after forming a y' phase solid solution in the matrix phase, the reprecipitation of the y' phase when the Ni-based alloy material is cooled from the y' phase solid solution temperature is a problem. Therefore, Patent Document 3 examines coating the Ni-based alloy material with a heat insulating material, etc., as a method for suppressing this cooling, but when the Ni-based alloy material heated to the y' phase solid solution temperature is transported to forging equipment while maintaining this temperature, it may cool due to various factors such as contact with the forging equipment. As a result, the temperature of the Ni-based alloy material before forging is started or during forging is reduced below the y' phase solid solution temperature, causing the y' phase, that was once in solid solution to reprecipitate. Furthermore, this reprecipitated y' phase is not expected to grow during the forging, so it is finely dispersed, and the hot forgeability is greatly impaired. On the other hand, the method of coarsening the y' phase by a prolonged aging treatment may cause problems in terms of operability due to the complication of the processes and the extension of the heat treatment time.

[0005] An object of the present invention is to provide a method for producing a Ni-based alloy which can maintain sufficient hot forgeability of the material even when the temperature of the Ni-based alloy material is reduced, either during hot forging or before hot forging is started, using a simple method.

MEANS FOR SOLVING THE PROBLEM

[0006] That is, the present invention is a method for producing a Ni-based alloy, the method including hot forging a Ni-based alloy material having a composition including, by mass, 0.02 to 0.10% of C, 0.15% or less of Si, 0.1% or less of Mn, 0.015% or less of P, 0.015% or less of S, 18 to 21% of Cr, 3.5 to 5.0% of Mo, 12 to 15% of Co, 0.1% or less of Cu, 1.2 to 1.6% of Al, 2.75 to 3.25% of Ti, 2% or less of Fe, 0.003 to 0.01% of B, and 0.02 to 0.08% of Zr, the balance of Ni with impurities,

wherein the hot forging is performed at a time when a temperature of the Ni-based alloy material is at least 900°C while the Ni-based alloy material heated to a preheating temperature of T₁ to T₂°C is reduced in temperature,

wherein a temperature decrease rate of the Ni-based alloy material during the temperature decrease from the preheating temperature to 900°C is 2.0°C/second or less.

[0007] Here T₁ is a temperature at which all y' phases are in solid solution in a matrix phase of the material, and T₂ is a temperature at which the matrix phase of the material starts to melt.

[0008] In addition, the hot forging is preferably started when the temperature of the Ni-based alloy material is 1000°C or more. In addition, the hot forging is preferably completed when the temperature of the Ni-based alloy material is 700°C or more.

[0009] The Ni-based alloy material may further include, by mass, 0.01% or less of Mg.

EFFECTS OF THE INVENTION

[0010] According to the present invention, a method for producing a Ni-based alloy that can maintain sufficient hot workability of the material even when the temperature of the Ni-based alloy material is reduced during hot forging or before hot forging is started can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] 

FIG. 1 is a figure showing an example of a production method of Ni-based alloys of Examples of the present invention and Comparative Example.

FIG. 2 is a figure showing an example of the effect of improving hot forgeability by the production method of the Ni-based alloys of Examples of the present invention and Comparative Example.

FIG. 3 is a figure showing examples of microstructures of forged materials produced by the production method of the Ni-based alloys of Examples of the present invention and Comparative Example.

FIG. 4 is a figure showing an example of the effect of improving hot forgeability by the production method of the Ni-based alloys of Examples of the present invention.

MODE FOR CARRYING OUT THE INVENTION

[0012] A feature of the present invention is to have found a method that can impart sufficient hot workability to a Ni-based alloy material during hot forging or before hot forging is started, even if the temperature of the material is reduced. Hereinafter, details of the present invention will be described.

[0013] 

(1) A method for producing a Ni-based alloy of the present invention is one in which "hot forging is performed on a Ni-based alloy material having a composition including, by mass, 0.02 to 0.10% of C, 0.15% or less of Si, 0.1% or less of Mn, 0.015% or less of P, 0.015% or less of S, 18 to 21% of Cr, 3.5 to 5.0% of Mo, 12 to 15% of Co, 0.1% or less of Cu, 1.2 to 1.6% of Al, 2.75 to 3.25% of Ti, 2% or less of Fe, 0.003 to 0.01% of B, and 0.02 to 0.08% of Zr, the balance of Ni with impurities".

C: 0.02 to 0.10% by mass (hereinafter simply referred to as "%")

[0014] C is an element that forms carbides with Cr and Ti to improve strength and ductility at room temperature and at high temperature in a balanced manner by refining crystal grains. It is also an element that forms compounds with S and has the effect of increasing grain boundary strength. However, if too little is added, the amount of MC-type carbides produced will be small and the effect will not be sufficient. On the other hand, if too much is added, coarse MC-type carbides will be produced, reducing ductility and the amount of Ti required for age-hardening in service. Therefore, C is set at 0.02 to 0.10%. An upper limit for C is preferably 0.05%.

Si: 0.15% or less

Mn: 0.1% or less

[0015] Si and Mn are elements that can be included as deoxidizing elements. However, excessive addition may reduce high-temperature strength. Therefore, even if these elements are included, Si and Mn are limited to 0.15% or less and 0.1% or less, respectively. Preferably Si and Mn are 0.05% or less and 0.05% or less, respectively.

P: 0.015% or less

S: 0.015% or less

[0016] P and S are impurity elements, and less is better and can be 0% each. P and S may be mixed in from raw materials, etc., even if they are not actively added. In the case of impurities, P and S are set at 0.015% or less because P and S do not adversely affect the properties of the Ni-based alloy of the present invention when they are 0.015% or less. P is preferably 0.005% or less. S is preferably 0.005% or less, and more preferably 0.001% or less.

Cr: 18 to 21%.

[0017] Cr is a necessary element to maintain the oxidation resistance of Ni-based alloys. If too little is added, the oxidation resistance required for the Ni-based alloys cannot be achieved. On the other hand, too much Cr makes the FCC phase, which is the matrix phase of the Ni-based alloys, unstable and produces harmful embrittlement phases such as the sigma (σ) phase with prolonged use, which reduces the strength and ductility of the Ni-based alloys. For this reason, the Cr content is set at 18 to 21%.

Mo: 3.5 to 5.0%

[0018] Mo is an element effective in increasing room temperature strength and high temperature strength through solid solution strengthening by forming a solid solution in the FCC phase, the matrix phase of the Ni-based alloys. It is a necessary and important element because its interaction with dislocations during service at elevated temperatures provides a deformation-suppressing effect at elevated temperatures. Too little Mo is not effective in improving high-temperature strength, whereas too much Mo can cause the formation of brittle phases such as M6C-type carbides and Laves phases. Therefore, the Mo content is set at 3.5 to 5.0%. A desirable lower limit for Mo is 4.0%.

Co: 12 to 15%

[0019] Co is an effective element not only for increasing strength by solid solution strengthening in the FCC phase, the matrix phase of the Ni-based alloys, but also for indirectly promoting solid solution strengthening and age hardening by increasing solid solution formation of Mo, Al, Ti, etc. Too little Co tends to make the above effects inadequate. Too much Co, on the other hand, increases work hardening and reduces cold formability, and also tends to cause the formation of an embrittled phase when used at elevated temperatures. Therefore, the Co content is set at 12 to 15%. Preferably it is 14% or less.

Cu: 0.1% or less

[0020] Cu is an element that improves corrosion resistance in non-oxidizing environments. On the other hand, too much Cu cause embrittlement due to segregation at the grain boundaries of the FCC phase. Therefore, even when present, Cu is limited to 0.1% or less.

Al: 1.2 to 1.6%

[0021] Al, together with Ti, is one of the constituents of the y' phase that is an intermetallic compound age precipitating during aging or service and is necessary to increase high-temperature strength in service. Too little Al will not provide sufficient strength in the service temperature range, while too much Al will increase the formation of the y' phase and reduce hot workability. Therefore, Al is set at 1.2 to 1.6%.

Ti: 2.75 to 3.25%.

[0022] Ti, together with Al, is one of the constituents of the y' phase that is an intermetallic compound age precipitating during aging or service and is an effective element for increasing high-temperature strength in service. Ti also forms MC-type carbides together with C and is effective in suppressing the growth of crystal grains of the FCC phase, the matrix phase of the Ni-based alloys, and in maintaining an appropriate crystal grain size. In addition, since MC-type carbides containing Ti can form solid solution with S, they are effective in effectively trapping S, which tends to segregate at the grain boundaries of the FCC phase, to improve cleanliness and increase high-temperature strength. However, if too much Ti is added, the Ti concentration in the y' phase may increase, and there is a concern that the solid solution temperature of the γ' phase may increase and the eutectic γ' phase may increase. Therefore, Ti is set at 2.75 to 3.25%.

Fe: 2% or less

[0023] Fe has the effect of improving the hot workability and cold workability of the Ni-based alloys. However, since too much Fe can reduce high-temperature strength and deteriorate oxidation resistance, Fe, even if present, is limited to 2% or less. To ensure the above effect of Fe, it is preferable that the lower limit of Fe be 0.3%.

B: 0.003 to 0.01%

[0024] B is an effective element for increasing strength and ductility at elevated temperatures by strengthening the grain boundary by incorporating a small amount of B. However, if too little is added, the above effect is not sufficient because the amount of segregation to the grain boundary is small. On the other hand, if too much is added, the solidus temperature during heating will be reduced and hot workability will be reduced. Therefore, B is set at 0.003 to 0.01%.

Zr: 0.02 to 0.08%

[0025] Zr must be included to strengthen the grain boundary. Since its atomic size is significantly smaller than that of Ni, which is an atom constituting the matrix, it segregates at the grain boundary and has the effect of suppressing grain boundary sliding at elevated temperatures. In particular, it has the effect of greatly relaxing notch rupture susceptibility. This has the effect of improving the creep rupture strength and creep rupture ductility. However, excessive addition of Zr degrades oxidation resistance. Therefore, Zr is set at 0.02 to 0.08%.

[0026] The Ni-based alloy of the present invention can be composed of Ni and impurities, as the remainder other than the above elemental species.

[0027] In this case, Mg is an impurity element that can be included as a deoxidizer. On the other hand, Mg is also an element that combines with S segregated at grain boundaries to form, for example, MgS, etc., to fix S. Since S is an intergranular embrittlement element, fixing S and suppressing grain boundary segregation contribute to improving hot workability. Therefore, Mg is an element that can be added as needed, for example, up to 0.01%. In order to achieve the above effects, it is preferable that Mg be 0.003% or more.

[0028] For example, Waspaloy (UNS N07001; Waspaloy is a registered trademark of United Technologies) is a representative Ni-based alloy having the above composition.

[0029] (2) The method for producing the Ni-based alloy of the present invention is one in which the hot forging of (1) is "while the above Ni-base alloy material heated to a preheating temperature of T₁ to T₂°C is reduced in temperature, the hot forging is performed at a time when a temperature of the Ni-based alloy material is at least 900°C".

[0030] First, the above "T₁" and "T₂" are explained. T₁ is "the temperature at which all γ' phases are in solid solution in a matrix phase" when the Ni-based alloy is heated, the so-called "solvus temperature (y' phase solid solution temperature)". This temperature can be calculated from the composition of the Ni-based alloy using thermodynamic calculations. For example, in the case of the Ni-based alloy having a specific composition, by mass, of C = 0.03%, Si = 0.03%, Mn = 0.008%, Cr = 19%, Mo = 4%, Co = 13%, Cu = 0.005%, Al = 1.35%, Ti = 3%, Fe = 0.4%, B = 0.005%, Zr = 0.06%, and the balance of Ni (equivalent to Waspaloy), T₁ is about 1,010°C (0.015% or less of P, 0.015% or less of S, and impurities can be disregarded).

[0031] Then, T₂ is "the temperature at which the base material starts to melt" when the Ni-based alloy is heated, which is "the solidus temperature of the matrix phase". This temperature can also be calculated from the composition of the FCC phase, which is the matrix phase of the Ni-based alloy, using thermodynamic calculations. In addition, for example, if it is the Ni-based alloy (equivalent to Waspaloy) having the above specific composition, T₂ can be calculated to be about 1,200°C.

[0032] Therefore, when the composition of the Ni-based alloy is specifically the one above , the preheating temperature with respect to the present invention can be set in the range "1,010°C to 1,200°C". Even when the composition of the Ni-based alloy is in the range of (1), it is acceptable to set the range of the preheating temperature with respect to the present invention at, for example, "1,010°C to 1,200°C".

[0033]  When hot forging the Ni-based alloy material, by setting the preheating temperature at this time (hot forging start temperature) in the above temperature range, the y' phase can be in solid solution in the matrix without melting the matrix phase, which gives the material hot workability. Ideally, a series of hot forging should be completed while maintaining the temperature of the Ni-based alloy material at the solvus temperature or more. However, in practice, as described above, the temperature of material is reduced during forging (or even during transportation before forging is started). In addition, if the temperature of material is reduced below the solvus temperature to 900°C during forging or before forging is started, the γ' phase precipitates and the hot workability of the material deteriorates.

[0034] Therefore, the present invention provides a method for producing the Ni-based alloy in which subsequent hot forging can be easily performed, by limiting the forging conditions to those in which the Ni-based alloy material heated to a preheating temperature at the solvus temperature or more is reduced in temperature and continues hot forging even when the temperature reaches 900°C (or hot forging itself has not yet been started).

[0035] (3) The method for producing the Ni-based alloy of the present invention is one in which "the temperature decrease rate in lowering the temperature of the Ni-based alloy material during the temperature decrease from the above preheating temperature to 900°C is 2.0°C/second or less" in the hot forging of (1).

[0036] In the above, it is described that when the temperature of the Ni-based alloy material is reduced to 900°C during forging (or even before forging is started), the y' phase precipitates and the hot forgeability of the material deteriorates. However, the inventor has found that the deterioration of hot forgeability due to this temperature decrease is significantly affected not only by the temperature itself, but also by the "temperature decrease rate" during the temperature decrease. In other words, the inventor has found that if the temperature decrease rate when the material is reduced in temperature is large (fast), the precipitated y' phase becomes finer, and in particular, reduction in area and elongation deteriorate more than expected, and defects such as fractures and cracks are likely to occur in the material during forging.

[0037] Therefore, with respect to the above temperature decrease, if the temperature decrease rate at which the material is reduced in temperature during the temperature decrease from the preheating temperature to 900°C is made "small (slow)", the precipitated y' phase will grow coarse, and deterioration of hot workability of material can be suppressed. In addition, it has been found that if the temperature decrease rate at this time is slowed down to "2.0°C/second or less", the precipitated y' phase has enough time to grow, and coarsening can be promoted, which is effective in maintaining sufficient hot forgeability of the Ni-based alloy material. The above temperature decrease rate is preferably 1.5°C/second or less, more preferably 1.0°C/second or less, and even more preferably 0.5°C/second or less.

[0038] In determining the above "temperature decrease rate", the temperature of Ni-based alloy material can be evaluated by its "surface temperature". The temperature decrease rate can be calculated using the formula "(Preheating temperature-900°C)/Time required to reach 900°C from the preheating temperature".

[0039] A process for holding the Ni-based alloy material at a certain temperature while the temperature of the Ni-based alloy material reaches 900°C from the preheating temperature may be included. However, in view of the fact that the effect of the present invention is achieved by a simple method, it is not necessary to prepare special heat-holding work (equipment) for the above process. In addition, as long as the temperature decrease rate with respect to the present invention is satisfied, the temperature should be continuously reduced, as sufficient time can be ensured for the y' phase to grow. If forging has already started before the temperature of the Ni-based alloy material reaches 900°C, it is also expected that the Ni-based alloy material will be reheated during forging and the temperature of the Ni-based alloy material will also increase (the temperature decrease rate described above will become even smaller).

[0040] In order to achieve the effects of the present invention, no special lower limit is required for the above temperature decrease rate. However, if the temperature decrease rate is too slow, which results in a time-consuming process and interferes with the forging schedule, etc., a lower limit of the temperature decrease rate can be set as appropriate. For example, the lower limit can be adjusted to 0.05°C/second or 0.1°C/second.

[0041] As a method for adjusting the above temperature decrease rate to "2.0°C/second or less", for example, a method for adjusting the size (specific surface area) of the Ni-based alloy itself can be used, in addition to a method for covering the surface of the Ni-based alloy material with a glass coating or an insulating material such as inorganic fiber composed of ceramics, etc., or for slowly cooling from the y' phase reprecipitation to near the forging temperature by furnace cooling. The temperature decrease rate at the surface can be adjusted by adjusting the size of the Ni-based alloy itself.

[0042] If the temperature behavior of the Ni-based alloy material when such a method is applied is determined in advance by using test materials prepared separately from the production material or by various simulations such as a finite element method, the method of achieving the temperature decrease rate of "2.0°C/second or less" with respect to the present invention with the Ni-based alloy material for the production material can be determined in advance. Therefore, when hot forging the Ni-based alloy material for production, by applying the above method, the temperature decrease rate of "2.0°C/second or less" with respect to the present invention can be reproducibly achieved without measuring the temperature of the Ni-based alloy material in parallel with the hot forging process.

[0043] (4) The method for producing the Ni-based alloys of the present invention is preferably one in which the hot forging of (1) "starts hot forging when the temperature of the Ni-based alloy material is 1,000°C or more".

[0044] In the present invention, even if the temperature of the Ni-based alloy material heated to the preheating temperature is reduced, so that the temperature of the material at the start of hot forging (hot forging start temperature) is 900°C or less, hot forging is possible because the material at that time has been given hot workability by the above temperature decrease rate. However, as described above, the Ni-based alloys have excellent hot forgeability when the y' phase, which is the strengthening phase, is not present or when the amount of y' phase formed is small. Therefore, it is no better than being able to start hot forging at a temperature close to the solvus temperature or at the solvus temperature or more. In addition, in the method for producing the Ni-based alloys of the present invention, it is preferable that the hot forging be started when the temperature of the Ni-based alloy material is 1,000°C or more. More preferably, it is 1,010°C or more (i.e., the above preheating temperature).

[0045] (5) The method for producing the Ni-based alloys of the present invention is preferably one in which the hot forging of (1) "completes the hot forging when the temperature of the Ni-based alloy material is 700°C or more".

[0046] In the present invention, even if the temperature of the material at the completion of hot forging (hot forging completion temperature) is 900°C or less, hot forging is still possible because the material at that time is still given hot workability by the above temperature decrease rate. Considering the possibility that the y' phase may grow slightly during the elapsed time until the temperature of the material is reduced, the strengthening by the y' phase may be weakened and the reduction in area and elongation may be slightly improved. However, when the temperature of the material is reduced to about 700°C, the tensile strength increases with the reduction in material temperature, which increases the load required for forging and may make hot forging difficult. Therefore, according to the present invention, after the temperature of the material is reduced to 900°C or less, it is no better than being able to complete the hot forging before the temperature is reduced completely. In addition, in the method for producing the Ni-based alloy of the present invention, it is preferable that this hot forging be completed when the temperature of the Ni-based alloy material is 700°C or more, and more preferably, when it is 800°C or more.

EXAMPLE 1

[0047] Billets of compositions 1 and 2 with a diameter of 360 mm, corresponding to Waspaloy in Table 1, were produced by cogging ingots of Ni-based alloys. Then, four round bar tensile test specimens (the parallel part diameter: 8 mm, the parallel part length: 24 mm) were taken from different positions of the billet (i.e., positions with different amounts of strain during the cogging forging) as test specimens simulating actual Ni-based alloy materials. The specimens were taken from the periphery of the billet and from positions of 3D/8 and D/4 from the periphery toward the center of the billet in the radial direction (D is the diameter of the billet). Preparing specimens with different amounts of strain also enables evaluation of the effects on the tensile properties (i.e., hot forgeability) of the specimens when subjected to the tensile test, described later.

Table 1

(mass%)
Composition 1	C	Si	Mn	P	S	Cr	Mo	Co
	0.032	0.03	0.008	0.001	0.0002	19.09	4.18	12.91
	Cu	Al	Ti	Fe	B	Zr	Mg	Ni
	0.005	1.35	2.97	0.37	0.0048	0.06	0.0006	Bal.
Composition 2	C	Si	Mn	P	S	Cr	Mo	Co
	0.03	0.02	0.02	0.003	0.0003	19.34	4.22	13.32
	Cu	Al	Ti	Fe	B	Zr	Mg	Ni
	0.01	1.34	3.08	1.22	0.005	0.05	0.001	Bal.

[0048] Then, to simulate the actual hot forging with respect to the present invention, the above specimens were heated to a preheating temperature of 1,050°C, and they were subjected to tensile tests until fracture, to evaluate the tensile properties. Details of the tensile tests are shown in FIG. 1. The temperature increasing rate to the preheating temperature was 5°C/second, and the holding time at the preheating temperature was 10 minutes. The specimens were then reduced to 900°C at the temperature decrease rates Nos. 1 to 3 (Examples of the present invention) and 10 (Comparative Example) in Table 2, and then tensile tests were conducted at this test temperature at a strain rate of 0.1/second, followed by air cooling. The specimens that had been reduced to 900°C were held for 5 seconds before the tensile test to equalize the heat in the parallel part.

Table 2

No.	Temperature decrease rate (°C/second)
1	1.0
2	0.5
3	0.1
10	10

[0049] FIG. 2 shows the results of the tensile test organized by temperature decrease rates Nos. 1 to 3 and 10. It was confirmed that the reduction in area increased as the temperature decrease rate decreased. The increase in the reduction in area was more clearly observed in the range of temperature decrease rates of 2.0°C/second or less. Regarding the effect on tensile properties due to the different positions from which the specimens were taken from the billet, a slight difference in the reduction in area was observed due to the behavior of the crystal grain size caused by differences in the amount of strain. However, the differences were as small as about 5% at each of the temperature decrease rates, and did not affect the effect of the present invention.

[0050] FIG. 3 shows, with respect to the specimen taken from the 3D/8 position of the billet, SE images (secondary electron images) obtained by electron microscopy of the microstructure of the specimen after the tensile test, in cases that were subjected to the temperature decrease rates Nos. 1 and 3. To clarify the y' phase, the microstructure was electrolytically corroded before observation.

[0051] From FIG. 3, the microstructure of the specimen (i.e., equivalent to forged material) after the tensile test that was subjected to the temperature decrease rate No. 3, which was smaller (slower) than the temperature decrease rate No. 1, had large y' phases and a change in shape. In other words, the y' phase was spherical and its grain size was about 50 nm in the specimen with the temperature decrease rate No. 1. On the other hand, the y' phase was large and long deformed by tensile test (equivalent to hot forging) in the specimen that was with the temperature decrease rate No. 3. The longer diameter of the y' phase was about 400 nm, but to evaluate using a spherical particle diameter before deformation, when converted to a circular equivalent diameter using image analysis software ImageJ (provided by the US National Institutes of Health (NIH)), the diameter was about 100 nm, so that the y' phase was confirmed to have grown coarser than that in the case with the temperature decrease rate No. 1.

[0052] From the above results, by lowering (slowing down) the temperature decrease rate to "2.0°C/second or less" in relation to the above, the reduction in area of the Ni-based alloy material increased, and the hot workability was significantly improved. In addition, by further lowering the temperature decrease rate, the y' phase became coarser and the strengthening mechanism decreased to the extent that the y' phase itself was deformed by hot working, and the hot workability of the Ni-based alloy material was further improved.

EXAMPLE 2

[0053] A 360 mm diameter billet of composition 3, corresponding to Waspaloy in Table 3, was produced by cogging an ingot of a Ni-based alloy. Then, two round bar tensile test specimens (the parallel part diameter: 8 mm, the parallel part length: 24 mm) were taken from a position of D/4 from the periphery to the center of the billet in the radial direction (D is the diameter of the billet) as test specimens simulating an actual Ni-based alloy material. It was confirmed that the difference in the location at which a specimen was taken had little effect on reduction in area, as shown in Example 1. Therefore, the specimens were taken only from the D/4 position described above.

Table 3

(mass%)
Composition 3	C	Si	Mn	P	S	Cr	Mo	Co
	0.032	0.01	0.01	0.003	0.0002	18.89	4.18	12.77
	Cu	Al	Ti	Fe	B	Zr	Mg	Ni
	0.01	1.45	3.04	0.51	0.005	0.06	0.0036	Bal.

[0054] Then, to simulate the actual hot forging with respect to the present invention, the above specimens were heated to a preheating temperature of 1,050°C, and tensile tests were conducted on them until fracture to evaluate the tensile properties. The details of the tensile tests were the same as in Example 1 (FIG. 1). Then, the specimens, after being held at the preheating temperature, were then reduced to 900°C at the temperature decrease rates Nos. 1 and 3 (Examples of the present invention) in Table 4, and then the tensile tests were conducted at this test temperature at a strain rate of 0.1/second, followed by air cooling. The specimens that had been reduced to 900°C were held for 5 seconds before the tensile test to equalize the heat in the parallel part.

Table 4

No.	Temperature decrease rate (°C/second)
1	1.0
3	0.1

[0055] FIG. 4 shows the results of the tensile tests, together with the results of Example 1, organized by the temperature decrease rates Nos. 1 to 3 and 10. In the evaluation of composition 3, it was confirmed that even if the composition contained Mg, the reduction in area increased with a decrease in the temperature decrease rate, as in the case of compositions 1 and 2. Even at the same temperature decrease rate as compositions 1 and 2, the reduction in area for composition 3 was about 5% higher when the temperature decrease rate was 1.0°C/second and about 7% higher when the temperature decrease rate was 0.1°C/second.

Claims

1. A method for producing a Ni-based alloy, the method comprising hot forging a Ni-based alloy material having a composition comprising, by mass, 0.02 to 0.10% of C, 0.15% or less of Si, 0.1% or less of Mn, 0.015% or less of P, 0.015% or less of S, 18 to 21% of Cr, 3.5 to 5.0% of Mo, 12 to 15% of Co, 0.1% or less of Cu, 1.2 to 1.6% of Al, 2.75 to 3.25% of Ti, 2% or less of Fe, 0.003 to 0.01% of B, and 0.02 to 0.08% of Zr, the balance of Ni with impurities,

wherein the hot forging is performed at a time when a temperature of the Ni-based alloy material is at least 900°C while the Ni-based alloy material heated to a preheating temperature of T₁ to T₂°C is reduced in temperature,

wherein a temperature decrease rate of the Ni-based alloy material during the temperature reduction from the preheating temperature to 900°C is 2.0°C/second or less, and

wherein T₁ is a temperature at which all y' phases are in solid solution in a matrix phase of the material, and T₂ is a temperature at which the matrix phase of the material starts to melt.

2. The method for producing the Ni-based alloy according to claim 1, wherein the hot forging is started when the temperature of the Ni-based alloy material is 1,000°C or more.

3. The method for producing the Ni-based alloy according to claim 1, wherein the hot forging is completed when the temperature of the Ni-based alloy material is 700°C or more.

4. The method for producing the Ni-based alloy according to claim 1, wherein the Ni-based alloy material further comprises, by mass, 0.01% or less of Mg.

Drawing