EP2979774A1

EP2979774A1 - Method for manufacturing annular molded article

Info

Publication number: EP2979774A1
Application number: EP14775622.5A
Authority: EP
Inventors: Hiroaki Kikuchi; Hideo Takizawa; Yuji Ishiwari; Jun Ohsone
Original assignee: Hitachi Metals Ltd; Hitachi Metals MMC Superalloy Ltd
Current assignee: Proterial Ltd
Priority date: 2013-03-28
Filing date: 2014-03-28
Publication date: 2016-02-03
Anticipated expiration: 2034-03-28
Also published as: ES2932530T3; WO2014157662A1; MX2015013639A; CN105228771A; JP2014188580A; JP6292761B2; RU2015146287A; EP2979774A4; RU2631221C2; EP2979774B1

Abstract

The present invention is directed to a method for manufacturing an annular formed body comprising a forging step of forging an alloy piece to provide a forged body having a disc shape, and a ring rolling step of ring-rolling an annular intermediate body prepared by forming a through-hole in the forged body to provide an annular formed body, characterized in that the forging step comprises at least two hot forging steps, each of the hot forging steps being carried out under the conditions that a strain rate is at most 0.5 s^-1, an absolute value εθ1 of strain to the forged body in its circumferential direction is at least 0.3, an absolute value εh of strain to the forged body in its height direction is at least 0.3, and a ratio εh/εθ1 between the absolute values of strain is within a range from 0.4 to 2.5.

Description

TECHNICAL FIELD

The present invention relates to a method for manufacturing an annular formed body, which is used as a stock in producing an annular product such as a turbine disk for aircraft engines, for example.
The present application clams priority from Japanese Patent Application No. 2013-069205 filed on March 28, 2013 , which is incorporated herein by reference in its entirety.

BACKGROUND ART

The turbine disk is an annular member having a through-hole and is configured to rotate together with plural turbine blades, which are arranged on the outer circumferential side thereof.
The outer circumferential portion of the turbine disk is exposed to combustion gas and reaches a high temperature of appropriately 600 to 700°C, whereas the inner circumferential portion is maintained at a relatively low temperature, and thus, thermal stress is repeatedly generated therein as the engine is started and stopped. Accordingly, it is desired that turbine disks have superior low cycle fatigue characteristics. In addition, it is necessary for turbine disks to have high creep strength characteristics because centrifugal force is applied to the outer circumferential portion due to the high-speed rotation around the axis under high temperatures. Furthermore, it is required that turbine disks have a high tensile strength and a high yield strength.
In order to secure mechanical strengths high enough to respond to various demands described above, annular formed bodies to be used for turbine disks are produced and output by forging a material with a high heat resistance constituted by a Ni-based superalloy and cutting the obtained annular forged body, as discussed in Patent Documents 1 and 2, for example. More specifically, strain is imparted to an annular formed body and crystal grains of the material are refined by forging, and thereby the tensile strength, the fatigue strength, and the like are improved. For equipment for applying forging, it is preferable to use a hydraulic-control forging press capable of strictly controlling the forging speed, and application of entire-surface forging for simultaneously molding the entire material has been recognized to be preferable in order to obtain the structure (crystal grains) of an annular formed body that are circumferentially uniform.
Meanwhile, in recent years, it has been desired to increase the size of turbine disks as higher output of engines for aircraft is desired. In increasing the size of an annular formed body as the size of turbine disks is increased, a large hydraulic-control forging press with a capacity of several tens of thousands of tons becomes necessary (e.g., see Non-Patent Document 1).
However, the above-described large hydraulic-control forging presses are very expensive, and few such presses are available worldwide; accordingly, if such a large hydraulic-control forging press is used, the capacity of supplying annular formed bodies may be limited and the costs of the products to be produced may remain high. The trend toward larger turbine disks in recent years has achieved so high a level that closed forging may be difficult even if a large hydraulic-control forging press is used, which may cause problems such that preferable mechanical characteristics are difficult to achieve for some regions of the annular formed bodies to be forged, and that it becomes difficult to ensure uniformity of the structure of the product.
On the other hand, instead of molding an annular formed body by using a forging press, a method of forming an annular formed body by ring rolling may be used. In this case, the cost of equipment can be reduced, and it becomes easy to responsively produce large annular formed bodies. However, in general, the anisotropy of mechanical characteristics (strength characteristics) more easily occur in ring-rolled products than in press-forged products, and thus, ring rolling is not suitable for production of products that require the isotropy of mechanical characteristics such as turbine disks.
A method in which an annular formed body is molded by a combination of a forging press and ring rolling may be used; however, if this method is used, a problem may arise such that it becomes necessary to further carry out final forging after the ring rolling to obtain a desired uniform and fine structure, that the production processes may thus become complex, and that the production costs may become high.
In order to solve this problem, in Patent Document 3, a method is presented in which a forging process and a ring rolling process are used in combination, and in the forging process, hot forging is carried out a plurality of times in which strain for a forged body in the circumferential direction εθ1, strain for the forged body in the height direction εh, and a strain ratio between these values εh/εθ1 are controlled to appropriate values, which thereby enables production of an annular formed body having a fine crystal structure with an excellent uniformity being secured at low costs.

CITATION LIST

PATENT DOCUMENT

[Patent Document 1] JP 07-138719 A
[Patent Document 2] JP 62-211333 A
[Patent Document 3] JP 2011-255409 A

NON-PATENT DOCUMENT

[Non-Patent Document 1] "Year 2002 Research Report - Report Regarding Development of Innovative Members Using Ultra-Large Forging Press Machine", (New Energy and Industrial Technology Development Organization, March 2003, pp. 10-11 and pp. 37-41)

SUMMARY OF INVENTION

TECHNICAL PROBLEM

Meanwhile, recently, production of high output aircraft engines has been increasing, and thus, the demand for a large annular formed body has also increased. Therefore, a production method is desired which is capable of stably producing annular formed bodies with a uniform structure by volume production.
The inventor of the present invention has studied the method for manufacturing annular formed bodies discussed in Patent Document 3, and as a result, it has been concluded that indeed, an annular formed body with fine crystal grains with a uniform grain size can be obtained by carrying out hot forging in which strain for a forged body in the circumferential direction εθ1, strain for the forged body in the height direction εh, and a strain ratio between these values εh/εθ1 are controlled to appropriate values; however, in producing large, thick annular formed bodies, for example, the grain sizes of the annular formed bodies are not uniform in some cases due to uneven operation conditions and the like.
The present invention has been made in consideration of these circumstances, and an object of the present invention is to provide a method for manufacturing an annular formed body capable of producing annular formed bodies having excellently high mechanical strengths while ensuring the uniformity of their structure stably and at low cost.

SOLUTION TO PROBLEM

In order to solve these problems and achieve the object mentioned above, according to an aspect of the present invention, a method for manufacturing an annular formed body includes a forging step of forging an alloy piece to provide a forged body having a disc shape, and a ring rolling step of ring-rolling an annular intermediate body prepared by forming a through-hole in the forged body to provide an annular formed body and is characterized in that the forging step comprises at least two of hot forging steps, each of the hot forging steps being carried out under conditions that a strain rate is at most 0.5 s^-1, an absolute value εθ1 of strain to the forged body in its circumferential direction is at least 0.3, an absolute value εh of strain to the forged body in its height direction is at least 0.3, and a ratio εh/εθ1 between the absolute values of strain is within a range from 0.4 to 2.5.
In the method for manufacturing an annular formed body according to the present invention, the strain rate in the forging step is at most 0.5 s^-1. When the strain rate exceeds 0.5 s^-1, the temperature inside the forged body is excessively increased due to the processing heat (i.e., a phenomenon known as "heat build-up") which coarsens the crystal grains inside the forged body. In ring rolling after forging, the crystal grains inside the forged body cannot be refined, because sufficient strain cannot be imparted to the inside of the forged body. According to the present invention, the strain rate is controlled to be within a range of 0.5 s^-1 or less, and thus, the difference between the temperature on the surface of the forged body and the temperature in the inside thereof during the forging can be smaller, allowing the structure to be more uniform. In order to more securely obtain the effects mentioned above, the strain rate in the forging step is preferably at most 0.15 s^-1.
The strain rate is defined by the following expression:
$Strain rate (s^{- 1}) = \frac{\sqrt{2 / 3 ({ε_{h}}^{2} + {ε_{θ}}^{2} + {(- ε_{h} - ε_{θ})}^{2})}}{forging time (s)}$
In the forging step, the absolute value εθ1 of the strain in the circumferential direction is set at a large value of at least 0.3, and thus, the amount of strain in the circumferential direction to be imparted to the annular intermediate body in the ring rolling step can be relatively reduced. Furthermore, because the absolute value εh of the strain to be imparted in the height direction is set at a large value of 0.3 or more, the strain to be imparted in the height direction, which is difficult to impart by ring rolling, can be securely imparted by a sufficient amount. Thus, the working ratio in the ring rolling step can be lowered, the anisotropy of the strength properties of the annular formed body is suppressed while the isotropy is increased, and as a result, a fine crystal structure can be obtained in which sufficient uniformity is secured.
The ratio εh/εθ1 denotes the balance among the directions of the strain to be imparted, and is an index for controlling the variation of relative positions in the material before and after the process. In the subsequent ring rolling step, the corresponding numerical value necessarily becomes "0" or close to "0" due to the production method, and thus, it is essential to appropriately set the ratio of strain to be imparted in the height direction in the forging step in order to suppress the anisotropy; however, if the ratio εh/εθ1 is lower than 0.4, the effect thereof may be insufficient. On the other hand, if the ratio εh/εθ1 exceeds 2.5, the distribution of the strain to be imparted in the height direction may become excessive, the plastic flow may thus become instable, and as a result, the axial symmetry, which is essential in imparting uniformity, may degrade.
According to the present invention, the ratio εh/εθ1 between the absolute values of strain is controlled to be within a range from 0.4 to 2.5, and thereby stabilizing the plastic flow and securing the axial symmetry to make the structure uniform.
In the method for manufacturing an annular formed body according to the present invention, in the ring rolling step, hot rolling may be carried out so that an absolute value εθ2 of strain to the annular formed body in its circumferential direction of at least 0.5 can be imparted to the annular formed body, and thus, the grain size in a product region of the annular formed body can be an ASTM grain size number of at least 8.
In this case, in the ring rolling step, by performing hot rolling in which the strain in the annular formed body in the circumferential direction by the absolute value εθ2 of 0.5 or more is imparted, the crystal grains in the product region of the annular formed body to be processed and machined into a product is securely refined to an ASTM grain size number of at least 8. Accordingly, the mechanical strengths of the product obtained from the annular formed body can be securely increased.
It is understood that the ASTM grain size number is determined in conformity to the standards defined by ASTM E122 by the American Society for Testing and Materials (ASTM).
Also, in the method for manufacturing an annular formed body according to the present invention, a difference among grain sizes in a product region of a cross section of the annular formed body along a direction including an axis of the annular formed body is within a range of ±2 by ASTM grain size numbers.
In this case, because the difference in the grain sizes in the product region in the cross section of the annular formed body is within ±2 by ASTM grain size numbers, sufficient uniformity of the grain size of the annular formed body in the radial direction and in the height direction is ensured.
In addition, in the method for manufacturing an annular formed body according to the present invention, a grain size of the forged body in the forging step may be controlled to an ASTM grain size number of at least 7.
In this case, because the large amount of strain is imparted in the forging step as described above, the grain size of the forged body can be refined to an ASTM grain size number of at least 7. With this case, the structure of the annular formed body can be refined while the amount of the strain to be imparted in the subsequent ring rolling step is reduced.
Furthermore, in the method for manufacturing an annular formed body according to the present invention, the annular intermediate body may be formed so that a ratio T/H between a thickness T of the annular intermediate body in its radial direction and a height H of the annular intermediate body in its axial direction is controlled to be within a range from 0.6 to 2.3, and then the annular intermediate body may be ring-rolled so that a difference between grain sizes at plural equivalent positions of the annular formed body uniformly arranged along its circumferential direction is within ±1.5 by ASTM grain size numbers.
In this case, by molding the annular intermediate bodies so that a ratio T/H between a thickness T of the annular intermediate body in a radial direction and a height H of the annular intermediate body in the axial direction is controlled to be from 0.6 to 2.3 and then by ring-rolling the annular intermediate body, the difference between the sizes of the crystal grains at a plurality of mutually equivalent positions of the annular intermediate bodies set uniformly in the circumferential direction can be restricted to be within ±1.5 by ASTM grain size numbers. More specifically, for the annular formed body obtained by molding the annular intermediate body, uniformity of the grain size in the circumferential direction can be ensured. Specifically, it has been known that in ring rolling, which is a local process, the processes are sequentially performed differently from general partial forging, and thus, the axial symmetry of the structure after molding is high, and therefore, the deviation of the material characteristics of the annular formed body in the circumferential direction is small. In the present invention, by setting the above-described ratio T/H for the annular intermediate body before the ring rolling within the above-described range as in the present embodiment, the shape (circularity) of the molded annular formed body can be further improved and the axial symmetry of the structure of the molded annular formed body can be further increased.
In other words, because the above-described ratio T/H is controlled to be within the range of 0.6 to 2.3, stability of rolling can be obtained, which is essential to impart uniformity. Specifically, in a region in which the ratio T/H is below 0.6, the area of contact among both rolls used in the rolling (a main roll and a mandrel roll) and the material becomes large, and thus, the degree of influence from heat release relatively increases, and as a result, it becomes difficult to obtain the circumferential uniformity. In contrast, as the ratio T/H increases, it becomes easier for buckling to occur. Specifically, in a region in which the ratio T/H is greater than 2.3, the above-described tendency becomes greater, and it thereby becomes difficult to obtain the circumferential uniformity.
In the method for manufacturing an annular formed body according to the present invention, the alloy piece may be made of a Ni-based alloy. In this case, the forging step is preferably carried out at a temperature from 950°C to 1,075°C, or the ring rolling step is preferably carried out at a temperature from 900°C to 1,050°C.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a method can be provided for manufacturing an annular formed body capable of producing annular formed bodies having superior high mechanical strengths while ensuring uniformity of their structure stably and at low costs.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 is a top plan view showing an embodiment of an annular formed body according to the present invention.
[FIG. 2] FIG. 2 is a cross sectional view of the body as viewed in the direction of arrow line X-X of FIG. 1.
[FIG. 3] FIG. 3 is a flow chart showing an embodiment of a method for manufacturing an annular formed body and a turbine disk according to the present invention.
[FIG. 4] FIG. 4 is a cross sectional view showing an annular intermediate body used in the manufacturing method shown in FIG. 3.
[FIG. 5] FIG. 5 is a perspective view illustrating a ring rolling performed in the manufacturing method shown in FIG. 3.
[FIG. 6] FIG. 6 is an explanatory view illustrating a ring rolling step with the use of a mandrel roll and a main roll.
[FIG. 7] FIG. 7 is an explanatory view illustrating a ring rolling step with the use of a mandrel roll and a main roll.
[FIG. 8] FIG. 8 is a graph showing the correlation between the tensile strength and the reduction of the examples of an annular formed body according to the present invention.
[FIG. 9] FIG. 9 is a graph showing the correlation between the yield strength and the reduction of the examples of an annular formed body according to the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments according to the present invention will be described with reference to the accompanying drawings. An annular formed body 10 according to the present embodiment is used as a stock for molding turbine disks of engines of aircraft.
As shown in FIGs. 1 and 2, the annular formed body 10 has through-holes and has an annular shape around an axis O, and is provided with a main body 11, an inner protrusion 12 that protrudes from the main body 11 toward a radial inward direction, and an outer protrusion 13 that protrudes from the main body 11 toward a radial outward direction.
The annular formed body 10 is constituted by a Ni-based superalloy having excellent heat resistance, and in the present embodiment, the annular formed body 10 is constituted by a Ni-based alloy "Alloy718".
The Ni-based alloy Alloy718 has an alloy composition including 50.00 to 55.00% by mass of Ni, 17.0 to 21.0% by mass of Cr, 4.75 to 5.60% by mass of Nb, 2.8 to 3.3% by mass of Mo, 0.65 to 1.15% by mass of Ti, 0.20 to 0.80% by mass of Al, and 0.01 to 0.08% by mass of C, and the balance of Fe with inevitable impurities.
In the annular formed body 10, the grain size of the structure in a desired region (not shown) to be machined into a turbine disk (i.e., product) (hereinbelow, this region will be referred to as a "product region") is an ASTM grain size number of at least 8. Also, virtual planes VS1, VS2 illustrated in FIG. 2 are cross sections of the annular formed body 10 along a direction including an axis O of the annular formed body 10, i.e., the virtual planes VS1, VS2 are set at mutually equivalent positions determined by evenly dividing the annular formed body 10 along the circumferential direction. The annular formed body 10 ensures the uniformity because the difference between the grain sizes in the structure of a product region in the cross section of the virtual plane VS1 (or VS2) is within ±2 by the difference in the ASTM grain size number,. The difference in the sizes of the crystal grain at the mutually equivalent positions of the annular formed body 10 in the circumferential direction, i.e., the difference between the grain size in the virtual plane VS1 and the grain size in the virtual plane VS2, is within ±1.5 by the difference by ASTM grain size numbers.
Next, a method for manufacturing the annular formed body 10 and a method for manufacturing the turbine disk will be described with reference to FIGs. 3 to 7.

Melting and casting step S1

First, molten metal of the Ni-based alloy Alloy718 is prepared by smelting. In this step, a melted raw material is prepared so that its components are within the above-described ranges of the components of the Ni-based alloy Alloy718, and an ingot is produced by performing vacuum induction melting (VIM). Next, this ingot is remelted by electro slag remelting (ESR) to produce an ingot again. Furthermore, this ingot is subjected to vacuum arc remelting (VAR), and then hot forging is performed to produce a cylindrical billet (alloy piece).
The billet is formed so as to have a diameter of 7 to 12 inches (more specifically, 165 to 315 mm), for example. The structure of the produced billet is of ASTM No. 6, approximately, by ASTM grain size numbers. By performing the above-described melting three times (triple melting), an extremely clean billet is produced, in which solidification segregation is small, the solidification structure is controlled, and only an extremely small amount of inclusions is included.

Forging step S2

Next, the billet is forged so that the billet is pressed in the direction of the axis of the billet to prepare a forged body having a disc-like shape.
In this forging step S2, hot forging is performed at least twice so that an absolute value εθ1 of strain to the forged body in the circumferential direction is 0.3 or higher, an absolute value εh of strain to the forged body in the height direction is 0.3 or higher, and a ratio εh/εθ1 between the absolute values of the strain is within a range of 0.4 to 2.5 in a state in which the billet has been heated to a temperature ranging from 950 to 1,075 °C, for example.
The strain rate in the hot forging in the forging step S2 is set to 0.5 s^-1 or less.
In the present embodiment, the hot forging in the forging step S2 is implemented by using a hydraulic-control forging press apparatus. The hydraulic-control forging press apparatus is capable of adjusting the strain rate by hydraulic control during forging within the above-described ranges with a high accuracy. Note that in the present embodiment, the strain rate in the hot forging in the forging step S2 is set at 0.01 s^-1 or greater.
Furthermore, in the present embodiment, the absolute value εθ1 of the amount of strain imparted in the circumferential direction is set at 0.3 or more. The absolute value εh of the amount of strain imparted in the height direction along the axis direction of the forging is set to be greater than 0.3.
The height of the forged body is adjusted by the forging step S2 to approximately 60 mm to 500 mm, for example. By performing the forging step described above, sufficient strain is imparted to the forged body and the grain size of the forged body is refined to ASTM No. 7 or more by ASTM grain size numbers.

Perforation and intermediate ring rolling step S3

Subsequently, a through-hole having a circular cross section is formed in the center of the obtained forged body by using a water cutter. Furthermore, intermediate ring rolling is performed after forming the through-hole where necessary. By performing the perforation and intermediate ring rolling step S3, an annular intermediate body 20 is produced.
In the present embodiment, the annular intermediate body 20 has a circumferentially perpendicular cross section with a substantially polygonal shape as illustrated in FIG. 4, and includes a base portion 21 having a circumferentially perpendicular cross section with a substantially polygonal shape and an upper surface and a lower surface extending in a direction substantially perpendicular to an axis O; an inner protrusion 22 radially and inwardly protruding from the base portion 21; and an outer protrusion 23 radially and outwardly protruding from the base portion 21.
Specifically, a height H of the annular intermediate body 20 (the base portion 21) in the direction of the axis O is set within a range of H = 60 mm to 500 mm. Furthermore, the annular intermediate body 20 is molded so that a ratio T/H between a radial thickness T perpendicular to the axis O and the above-described height H is within a range of 0.6 to 2.3.

Ring rolling step S4

Next, the annular intermediate body 20 is ring-rolled. The ring rolling is implemented by hot rolling performed at a temperature in a range of 900 °C to 1,050 °C, for example.
As shown in FIG. 5, in the present embodiment, a ring rolling apparatus 30 includes a main roll 40 arranged on an outer circumferential side of the annular intermediate body 20; a mandrel roll 50 arranged on an inner circumferential side of the annular intermediate body 20; and a pair of axial rolls 31, 32 that contact end surfaces (in the present embodiment, the upper surface and the lower surface of the base portion 21) of the annular intermediate body 20 in the direction of the axis O.
The main roll 40 and the mandrel roll 50 are arranged so that the rotation axes thereof are in parallel to each other and configured to nip and press the annular intermediate body 20 from the inner circumferential side and the outer circumferential side thereof and roll the annular intermediate body 20 while circumferentially turning the annular intermediate body 20.
The pair of axial rolls 31, 32 is configured to nip and press the annular intermediate body 20 in the direction of the axis O and control the dimension of the annular intermediate body 20 in the height direction.
As shown in FIG. 6, an accommodation recess 41 in which a part of the annular intermediate body 20 can be accommodated is arranged on an outer circumferential portion of the main roll 40, and in the present embodiment, the accommodation recess 41 has a depth sufficient to accommodate the outer protrusion 23, the base portion 21, and an outer circumferential portion of the inner protrusion 22 of the annular intermediate body 20. Furthermore, on a bottom 41A of this accommodation the recessed portion 41, a first molding groove 42 for molding the outer protrusion 13 of the annular formed body 10 is formed so as to be grooved in a radially inward direction in relation to the main roll 40 (rightward in FIG. 6). The first molding groove 42 has a depth equivalent to the height of protrusion of the outer protrusion 13 to be molded.
On the other hand, an engagement portion 51, which can engage with the main roll 40 in an inside of the accommodation recess 41, is arranged in the outer circumferential portion of the mandrel roll 50, while on an outer peripheral surface of the engagement portion 51, a second molding groove 52 for molding the inner protrusion 12 of the annular formed body 10 is formed so as to be grooved in a radially inward direction in relation to the mandrel roll 50 (leftward in FIG. 6). The second molding groove 52 has a depth equivalent to the height of protrusion of the inner protrusion 12 to be molded.
The main roll 40 and the mandrel roll 50 configured as described above operate so as to come close to each other, and thereby the annular intermediate body 20 is nipped and pressed between the main roll 40 and the mandrel roll 50. Specifically, the main roll 40 and the mandrel roll 50 come close to each other while the main roll 40 is turned around the rotation axis of the main roll 40, and thereby, the annular intermediate body 20 is turned around the axis O due to frictional resistance generated between the annular intermediate body 20 and the main roll 40.
On the other hand, the mandrel roll 50 is configured rotatable around the rotation axis of the mandrel roll 50, and is drivenly rotated due to frictional resistance generated between the annular intermediate body 20 and the mandrel roll 50. The annular intermediate body 20 plastically deforms as it is filled into the insides of the accommodation recess 41 and the first molding groove 42 of the main roll 40 and the inside of the second molding groove 52 of the mandrel roll 50, and thereby the annular formed body 10 is molded. In this deformation, the inner protrusion 12 of the annular formed body 10 is plastically deformed along the shape of the second molding groove 52. In addition, the outer protrusion 13 is plastically deformed along the shape of the first molding groove 42.
By performing the ring rolling in the above-described manner, the annular intermediate body 20 is plastically deformed so as to extend in the circumferential direction and the inner diameter and the outer diameter thereof are enlarged, and thereby the annular formed body 10 illustrated in FIG. 7 is produced.
In this ring rolling step S4, an absolute value εθ2 of the strain in the annular formed body 10 in the circumferential direction of 0.5 or more is imparted. Specifically, the hot rolling is performed at least once or more to set the total value of strain absolute values εθ2 to be within a range of 0.5 to 1.3.

Heat treatment step S5/Machining step S6

The annular formed body 10 produced in the above-described manner is subjected to adjustment of properties by heat treatment, molded into a final shape by cutting, to be formed into a turbine disk for engines for aircraft.
According to the present embodiment of a method for manufacturing an annular formed body described above, in the forging step S2 for producing the forged body by forging the billet, the strain rate is 0.5 s^-1 or less, and thereby, excessive rise of temperature inside the forged body (a phenomenon known as "heat build-up", which may occur due to processing heat, can be prevented. Accordingly, the difference between the temperature on the surface of the forged body and that in the inside thereof during the forging can be controlled to be small, and thereby the structure of the forged body can be unified. In order to reliably obtain these effects, it is preferable that the strain rate in the forging step S2 be set at 0.15 s^-1 or less.
In addition, because a high value of 0.3 or more is set as the absolute value εθ1 of the strain in the circumferential direction in the forging step S2, the ratio of the amount of the strain in the circumferential direction to be imparted to the annular intermediate body 20 in the ring rolling step S4 can be reduced. Furthermore, because a high value of 0.3 or more is set as the absolute value εh of the strain in the height direction, the amount of the strain in the height direction, which is otherwise difficult to impart, can be sufficiently imparted in the ring rolling step S4. Thus, the working ratio in the ring rolling step S4 can be reduced, the anisotropy of the strength properties of the annular formed body 10 is suppressed while the isotropy is increased, and as a result, a fine crystal structure can be obtained in which sufficient uniformity is ensured.
Furthermore, because a high value of 0.4 or more is set as the ratio εh/εθ1 between the absolute value εθ1 of the strain in the circumferential direction and the absolute value εh of the strain in the height direction, a sufficient ratio of the strain to be imparted in the height direction can be ensured, and as a result, the uniformity of the structure can be ensured even if sufficient strain cannot be imparted in the height direction in the subsequent ring rolling step S4. In addition, because the ratio εh/εθ1 is 2.5 or less, the distribution in the height direction would not be excessive, and thus, the plastic flow becomes stable, and thereby the axial symmetry of the plastic flow essential for imparting uniformity can be ensured. It is preferable that the ratio εh/εθ1 between the absolute values of the strain be 0.6 to 2.1. Accordingly, the axial symmetry of the plastic flow can be increased, and thus, the uniformity of the structure can be more reliably ensured.
Furthermore, in the present embodiment, because the absolute value εθ1 of the amount of strain to be imparted in the circumferential direction is set at 0.3 or higher and the absolute value εh of the amount of strain to be imparted in the height direction along the axis of the forged body is set at 0.3 or higher in the forging step S2, coarsening of the crystal grains, which may occur due to rise of temperature inside the forged body because of the processing heat, can be prevented.
Furthermore, because in the ring rolling step, hot rolling is performed in which the strain in the annular formed body 10 in the circumferential direction by the absolute value εθ2 of 0.5 or more is imparted, the crystal grains in the product region of the annular formed body 10 is securely refined to an ASTM grain size number of at least 8. Accordingly, the mechanical strengths of the product obtained from the annular formed body 10 are reliably increased. Note that it is preferable that εθ2 be 1.3 or less. In addition, the grain size of the annular formed body 10 is preferably an ASTM grain size number of 8 to 13. Thus, the mechanical strengths of the product obtained from the annular formed body 10 can be reliably increased.
Because the difference in the grain sizes in the product region in the cross section along a direction including the axis O of the annular formed body 10 is within ±2 by ASTM grain size numbers, sufficient uniformity of the grain size of the annular formed body 10 in the radial direction and in the height direction is ensured.
Moreover, because a large amount of strain is imparted in the forging step as described above, the grain size of the forged body can be refined to an ASTM grain size number of at least 7. Thus, the structure of the annular formed body 10 can be refined while the amount of the strain to be imparted in the subsequent ring rolling step is reduced.
In addition, because the ring rolling is performed after molding the annular intermediate body 20 so that the ratio T/H between the radial thickness and the height H of the annular intermediate body 20 is within a range from 0.6 to 2.3, the difference between the grain sizes at the mutually equivalent positions on the annular formed body 10 along the circumferential direction can be reduced within ±1.5 by ASTM grain size numbers. In other words, for the annular formed body 10 obtained by molding the annular intermediate body 20, sufficiently high uniformity of the grain sizes in the circumferential direction can be ensured.
Specifically, it has been known that in ring rolling, which is a local process, the processes are sequentially performed differently from general partial forging, and thus, the axial symmetry of the structure after molding is high, and therefore, the deviation of the material characteristics of the annular formed body 10 in the circumferential direction is small. Accordingly, by setting the above-described ratio T/H for the annular intermediate body 20 before the ring rolling within the above-described range as in the present embodiment, the shape (circularity) of the molded annular formed body 10 can be further improved and the axial symmetry of the structure of the molded annular formed body 10 can be further increased.
In other words, because the above-described ratio T/H is controlled to be within the range of 0.6 to 2.3, stability of rolling can be obtained, which is essential to impart uniformity. Specifically, in a region in which the ratio T/H is below 0.6, the area of contact among both rolls used in the rolling (the main roll 40 and the mandrel roll 50) and the material becomes large, and thus, the degree of influence from heat release relatively increases, and as a result, it becomes difficult to obtain the circumferential uniformity. In contrast, as the ratio T/H becomes higher, it becomes easier for buckling to occur. Specifically, in a region in which the ratio T/H is higher than 2.3, the above-described tendency becomes higher, and it thereby becomes difficult to obtain the circumferential uniformity.
As described above, according to the method for manufacturing an annular formed body which is the present embodiment, it is made possible to produce annular formed bodies, in which the uniformity of the structure is ensured and the mechanical strengths are sufficiently high, in a reliable manner and at low costs.
The present invention is not limited to the embodiment described above, and can also be implemented by various modifications and alterations within a scope not exceeding the gist of the present invention.
For example, the shape of the annular formed body 10 and the annular intermediate body 20 is not limited to the present embodiment, and the design can be modified as necessary in consideration of the shape of the annular product such as a turbine disk to be produced.
The annular formed body 10 and the annular intermediate body 20 are constituted by Ni-base alloy Alloy718 as described above; however, the material of the annular formed body 10 and the annular intermediate body 20 is not limited to this, and the annular formed body 10 and the annular intermediate body 20 may be constituted of other materials (e.g., Waspaloy (registered trademark) (United Technology Inc.), Alloy720, Co-based alloys, and Fe-based alloys).
In the above-described embodiment, molten metal for the Ni-based alloy, Alloy718, is smelted and the billet is produced by casting, but the present invention is not limited to this. Alternatively, a billet may be produced by powder molding and the produced billet is then subjected to the forging step and the ring rolling step.
Further alternatively, the billet may be produced by double melting (VIM + ESR or VIM + VAR) instead of producing the billet by the above-described triple melting.
The present embodiment includes the perforation step for forming the through-hole in the center of the disc-like forged body by using the water cutter, but the present invention is not limited to this. Alternatively, the through-hole may be formed by means other than the water cutter. Further alternatively, the through-hole may be formed at the time of the forging, and thus, the perforation step may be omitted. Still further alternatively, the perforation may be performed in the course of the forging step by using the water cutter or the like.
Also, after the annular formed body 10 is molded by the ring rolling step S4 illustrated in FIG. 3 and before the heat treatment step S5 illustrated in FIG. 3 is performed, additional processes such as partial forging may be performed in order to impart a shape to the annular formed body 10 or to adjust its dimensions.
In the present embodiment, the mutually equivalent positions (the virtual planes VS1, VS2) on the annular formed body 10 determined by evenly dividing the annular formed body 10 into halves in the circumferential direction are used as the reference positions for controlling the difference between the grain size on the virtual plane VS1 and the grain size on the virtual plane VS2 within ±1.5 by ASTM grain size numbers; however, the number of the virtual planes used for the comparison is not limited to two. In other words, because the equivalence of the annular formed body 10 is ensured entirely in the circumferential direction, the difference between the grain sizes at the mutually equivalent positions determined by evenly dividing the annular formed body 10 into threes along the circumferential direction may be controlled to be within ±1.5 by ASTM grain size numbers instead of determining the difference between the grain sizes at the mutually equivalent positions determined by evenly dividing the annular formed body 10 into twos. The circumferential positions in the annular formed body 10 at which the mutually equivalent positions are to be set are not limited to those described above in the present embodiment.

EXAMPLES

The present invention will be described below in more detail with reference to Examples. However, the present invention is not limited to the following Examples.

Preparation of samples

First, molten metal for the Ni-based alloy, Alloy718, was smelted. Specifically, the melting raw material was prepared so that it would satisfy the condition for the range of components of the Ni-based alloy, Alloy718, mentioned in the embodiment described above. Then triplex melting was carried out on this molten metal. Specifically, vacuum induction melting (VIM), electro slag remelting (ESR), and vacuum arc remelting (VAR) were carried out to produce a cylindrical billet with a diameter of 254 mm.
Subsequently, the billet was subjected to the forging step to prepare a disc-like forged body. For the forging, hot forging was performed twice in which the billet was heated to 1,000 °C.
The forging step was carried out under the following conditions illustrated in Table 1 with respect to the absolute value εθ1 of strain to the forged body in the circumferential direction, the absolute value εh of strain to the forged body in the height direction, the ratio εh/εθ1 between the absolute values of the strain, and the strain rate.
Subsequently, the through-hole was formed by a water cutter in the center of the forged body to prepare an annular intermediate body 20. The annular intermediate body 20 was molded so that the ratio T/H between its thickness T and its height H would be the value illustrated in Table 1.
Then the annular intermediate body 20 was subjected to ring rolling. For the ring rolling, hot rolling was carried out twice in which the annular intermediate body 20 was heated to 1,000 °C. The ring rolling was performed so that the total sum of the absolute value εθ2 of the circumferential strain in the annular formed body 10 would satisfy the following conditions illustrated in Table 1 by performing the hot forging twice.

Then the annular formed body 10 was subjected to heat treatment. As a direct ageing material, a material was prepared that had undergone an aging treatment under conditions of 718 °C/8 hours + 621 °C/8 hours + air cooling (A.C.). As a solution aging material, a material was prepared that had undergone an aging treatment under conditions of 718 °C/8 hours + air cooling (A.C.) after solution treatment under conditions of 970 °C/1 hour + water quenching (W.Q.) performed after the ring rolling.

[Table 1]

		Forging step								Ring rolling step		Heat treatment
		First time				Second time				Annular Intermediate body T/H	ε_θ2
		ε_θ1	ε_h	ε_h/ε_θ1	Strain rate	ε_θ1	ε_h	ε_h/ε_θ1	Strain rate	Annular Intermediate body T/H	ε_θ2
Examples	1	0.3	0.6	2.0	0.03	1.0	0.7	0.7	0.07	1.4	0.7	Water cooling after rolling + ageing treatment
	2	0.3	0.6	2.0	0.06	1.0	0.7	0.7	0.15	1.4	0.7	Water cooling after rolling + ageing treatment
	3	0.3	0.6	2.0	0.48	1.0	0.7	0.7	0.28	1.4	0.7	Water cooling after rolling + ageing treatment
	4	0.3	0.6	2.0	0.04	0.7	0.4	0.6	0.13	0.8	0.5	970°C solution treatment + ageing treatment
Comparative Example	1	0.3	0.6	2.0	0.82	1.0	0.7	0.7	0.75	1.4	0.6	Water cooling after rolling + ageing treatment
	2	0.6	1.2	2.0	0.57	0.6	0.0	0.0	0.57	0.8	0.3	Water cooling after rolling + ageing treatment

Measurement of grain size

By using the prepared annular formed body 10, maximum crystal grains in the product regions within the cross sections including the virtual planes VS1, VS2 and average grain sizes around the maximum crystal grain were measured and compared. For the average grain size around the maximum crystal grain, an average grain size in a maximum crystal grain-observed portion (excluding the maximum crystal grain) was used. The results are shown in Table 2. [Table 2]

Maximum grain size Average grain size around maximum crystal grain

Examples 1 7.5 11.0

2 6.5 10.5

3 5.5 10.0

4 7.0 9.0

Comparative examples 1 1.5 9.0

2 2.0 7.5

High-temperature tensile characteristics determination test

Among the annular formed bodies 10 prepared in the above-described manner, with respect to those prepared for Example 1 of the present invention and Comparative Example 3, tensile test pieces were sampled from locations in the circumferential direction, in the height direction, and in the radial direction from the mutually equivalent positions including the virtual planes VS1, VS2 illustrated in FIG. 1, and high-temperature tensile tests were carried out at 650 °C, respectively. The tests were carried out by using ASTM E8 small size test pieces with the parallel portion diameter of 6.35 mm and in conformity with ASTM E21, and the tensile strength, the yield strength (0.2% yield strength), and the reduction were measured. To examine the deviations of the circumferential, heightwise, and radial measurement values, heightwise and radial ratios were calculated with the circumferential measurement value being set as a reference value "1" (100%). FIG. 8 illustrates the correlation between the tensile strength and the reduction, and FIG. 9 illustrates the correlation between the yield strength and the reduction.
In Comparative Examples 1 and 2, in which the strain rate in the forging step exceeds 0.5 s^-1, the difference between the maximum grain size and the average grain size around the maximum grain size was large, and it was observed that the structure was not uniform. This was estimated to have occurred because local coarsening of the crystal grains inside the forged body occurred due to excessive increase of the temperature inside the forged body, which had been brought about because of the processing heat (i.e., because a phenomenon known as "heat build-up" had occurred).
In contrast, in Examples 1 to 4 of the present invention, in which the rates of strain in the forging step was 0.5 s^-1 or less, the difference between the maximum grain size and the average grain size around the maximum grain size was small, and it was observed that the structures were sufficiently unified. It was estimated that the sufficiently unified structures were obtained due to the small difference between the temperature on the surface and the temperature in the inside of the forged body during the forging, which was achieved by controlling the strain rate being in the range of 0.5 s^-1 or less. Note that in Examples 1, 2, and 4 of the present invention in which the strain rates were controlled to be within the range of 0.15 s^-1 or less, the structures were better unified.
As shown in FIGs. 8 and 9, as a result of the high-temperature tensile strength determination tests, it was observed that Example 1 of the present invention was better than Comparative Example 2 in terms of all of the tensile strength, the 0.2% yield strength, and the reduction.
More specifically, it was observed that in Example 1 of the present invention, the isotropy of the strength characteristics increased, and Example 1 of the present invention had a fine crystal grain structure in which sufficient structural uniformity was ensured.

INDUSTRIAL APPLICABILITY

According to the method for manufacturing an annular formed body of the present invention, annular formed bodies in which the uniformity of the structure is ensured and the mechanical strengths are sufficiently high can be produced stably and at low costs. Accordingly, the method for manufacturing an annular formed body of the present invention can be suitably used in production of turbine disks and the like of engines for aircraft.

LIST OF REFERENCE SYMBOLS

10: Annular formed body
20: Annular intermediate body
H: Height of the annular intermediate body in the axial direction
O: Axis
S2: Forging step
S4: Ring rolling step
T: Thickness of the annular intermediate body in the radial direction
VS1: Virtual plane (equivalent position)
VS2: Virtual plane (equivalent position)

Claims

A method for manufacturing an annular formed body comprising a forging step of forging an alloy piece to provide a forged body having a disc shape, and a ring rolling step of ring-rolling an annular intermediate body prepared by forming a through-hole in the forged body to provide an annular formed body,
characterized in that the forging step comprises at least two hot forging steps, each of the hot forging steps being carried out under the conditions that a strain rate is at most 0.5 s^-1, an absolute value εθ1 of strain to the forged body in its circumferential direction is at least 0.3, an absolute value εh of strain to the forged body in its height direction is at least 0.3, and a ratio εh/εθ1 between the absolute values of strain is within a range from 0.4 to 2.5.
The method for manufacturing an annular formed body according to claim 1, characterized in that a difference among grain sizes in a product region of a cross section of the annular formed body along a direction including an axis of the annular formed body is within a range of ±2 by ASTM grain size numbers.
The method for manufacturing an annular formed body according to claim 1 or 2, characterized in that in the forging step, a grain size of the forged body is an ASTM grain size number of at least 7.
The method for manufacturing an annular formed body according to any one of claims 1 to 3, characterized in that the annular intermediate body is formed so that a ratio T/H between a thickness T of the annular intermediate body in its radial direction and a height H of the annular intermediate body in its axial direction is controlled to be within a range from 0.6 to 2.3, and then the annular intermediate body is ring-rolled so that a difference between grain sizes at plural equivalent positions of the annular formed body uniformly arranged along its circumferential direction is within ±1.5 by ASTM grain size numbers.
The method for manufacturing an annular formed body according to any one of claims 1 to 4, characterized in that in the ring rolling step, hot rolling is carried out so that an absolute value εθ2 of strain to the annular formed body in its circumferential direction of from 0.5 to 1.3 is imparted to the annular formed body.
The method for manufacturing an annular formed body according to any one of claims 1 to 5, characterized in that the alloy piece is made of a Ni-based alloy.
The method for manufacturing an annular formed body according to claim 6, characterized in that the forging step is carried out at a temperature from 950 °C to 1,075 °C.
The method for manufacturing an annular formed body according to claim 6 or 7, characterized in that the ring rolling step is carried out at a temperature from 900 °C to 1,050 °C.