JP6299344B2 - Method for forging disc-shaped products - Google Patents

Description

  The present invention relates to a method for forging a disk-shaped product, and more particularly to a method for forging a disk-shaped product by free sequential forging.

Heat-resistant alloys containing Ni as a main component are widely used as power generation turbine disks that require high tensile strength and fatigue strength at high temperatures.
Three types of strengthening mechanisms, known as solid solution strengthening, carbide precipitation strengthening, and γ '(gamma prime) precipitation strengthening, are known as the strengthening mechanism of this kind of Ni-containing heat-resistant alloy, especially Ni ₃ (Al, Ti). The γ 'strengthening mechanism using strengthening by precipitation of γ' phase of intermetallic compounds is widely used.
Nimonic Alloy 901 (hereinafter referred to as Alloy 901; Nimonic is a trademark of Special Metals) is an example. This Ni-containing heat-resistant alloy undergoes complex strengthening by precipitation strengthening of carbide and precipitation strengthening of γ 'phase. ing.

Mechanical properties such as high-temperature tensile strength, impact properties, and fatigue properties of members made of Ni-containing heat-resistant alloys depend on the grain size of the Ni-based heat-resistant alloys.
Therefore, in Ni-containing heat-resistant alloys, the crystal grains are refined by recrystallizing the crystals by hot forging at a temperature higher than the recrystallization temperature and suppressing the growth of the recrystallized grains. Yes.

The mechanical properties such as high-temperature tensile strength, impact properties, and fatigue properties of Ni-base heat-resistant alloy members increase as the crystal grains become finer. Therefore, in general, hot forging is performed so as to make the crystal grains as fine as possible.
For the tensile strength and fatigue strength of Ni-base heat-resistant alloy members, the grain boundary works as a barrier for dislocation movement, so the grain boundary strength is larger than the intragranular strength, and therefore the grain can be refined. It is said that it is effective for improving the strength such as the tensile strength.

In the process of refining crystal grains by hot forging, the crystal grains are deformed by applying strain by forging, and recrystallized grains are generated along the grain boundaries of the deformed crystal grains.
The generated recrystallized grains then grow and gradually increase.However, if the initial crystal grains are coarse, crystal growth due to recrystallization does not reach the core of the crystal grains, and the core is not formed. It tends to remain as a recrystallized part.

In particular, in die forging in which a material is confined in a die-carved mold, the problem is likely to occur because one forging is generally performed.
In this die forging, the strain applied to the workpiece tends to be non-uniform, and the crystal grains tend to be non-uniform due to the non-uniform strain.

  On the other hand, the material (work material) is struck with an anvil and compressed between two planes (the material is not constrained in two directions perpendicular to the compression direction) and is usually struck several times at the same location. In addition, in the free sequential forging that gives strain, recrystallization from the grain boundaries of the recrystallized grains is repeated, so that the recrystallized area expands and the crystal grains are spread over the entire workpiece until reaching the core. It is easy to effectively equalize and refine the grain size, and to control the grain size by adjusting the degree of processing.

Therefore, when forming a turbine disk or the like in a power generation gas turbine by forging, it is desirable to form by such free sequential forging.
In a turbine disk or the like in this kind of power generation facility, the crystal grain size of each part is required to be as uniform as possible, and it is required to have a fine crystal grain size of ASTM # 4 or more until reaching the central core part. According to free sequential forging, such a structure is easily obtained.

By the way, in such a free sequential forging, the recrystallization range is expanded by repeatedly striking the same part, and the forging step and reheating step in which striking is usually applied because of expanding to the core part. Will be repeated.
In this case, in a forging process using a carbide such as Ti or Mo or an alloy 901 equivalent material having a precipitation phase of γ ′ as a strengthening phase, the carbide phase or γ ′ phase may precipitate during forging depending on the processing conditions. Contrary to this, it is feared that the solid solution is dissolved, and that the growth behavior of the crystal grains is greatly affected by this.
Thus, when the effect of destabilization of grain growth occurs due to the precipitation or solid solution behavior of the carbide phase and γ 'phase, which become the strengthening phase, fine crystal grains of ASTM # 4 or higher are produced in the finish forging process. It becomes difficult to obtain stably.

  As a prior art to the present invention, the following Patent Document 1 discloses an invention about a “Ni-base alloy forging method”, in which, as a method of manufacturing a billet from a Ni-base alloy ingot, The amount of compression by one stroke in the direction perpendicular to the feed direction is reduced so that minute deformation is accumulated, the first strike position and the next strike position are shifted, and the crystal in the forge and stretch feed direction cross section It is disclosed that the particle size is made uniform.

Further, the following Patent Document 2 discloses an invention relating to a “method for producing a superalloy material”, in which a primary forging is performed by heating to a precipitate solid solution temperature or higher, followed by rapid cooling at a cooling rate of water cooling or higher. It is disclosed that the forging strain energy generated by the forging is left inside the material, the precipitation aging treatment is performed, and the secondary forging is performed at a temperature at which the precipitate does not dissolve.
However, those described in Patent Documents 1 and 2 are different from the present invention in the composition of the target material to be processed, and are specific about the combination of the total reduction ratio and reheat conditions for obtaining the target crystal grain size. There is no disclosure, which is different from the present invention.

JP 2003-251429 A JP-A-8-31626

  The present invention is based on the above circumstances, and when a disk-shaped product is forged using an alloy 901 equivalent material, fine grain size of # 4 or more, which is the target until the core is reached in finish forging. The object of the present invention is to provide a method for forging a disk-shaped product by free sequential forging, in which a structure can be obtained stably and easily.

Thus, the forging method according to claim 1 is C: 0.02 to 0.06%, Ni: 40 to 45%, Cr: 12 to 14%, Mo: 6 to 7%, Ti: 2.5 to 3.5% in mass%. Al: 0.12 to 0.30%, B: 0.010 to 0.020%, the work piece made of Ni- base heat-resistant alloy having the composition of the balance Fe and unavoidable impurities, in finish forging, under the pinning action by carbide precipitation The disc is subjected to free sequential forging in which a forging step for reducing at a reduction rate of 10% or more and a reheating step for holding at 950 to 1060 ° C. for 1 hour or more are repeated one or more times, and at least two times of impact are applied to the same location It is forged into a shape to obtain a disk product having a structure of ASTM grain size # 4 or more.

  The present inventors use alloy 901 equivalent material having the above composition, and as a disk-shaped product structure, a structure of ASTM grain size # 4 or more capable of realizing excellent high-temperature tensile strength, impact strength, and fatigue strength is subjected to free sequential forging. In order to obtain by forging by the following, the following preliminary test was performed, and first, the grain growth behavior when forging an alloy 901 equivalent material was examined.

  As shown in FIG. 1 (A), a cylindrical test piece 10 having a diameter of 15 mm and a height of 22.5 mm having a structure of ASTM grain size # 4 obtained by rough forging before finish forging is prepared. As shown in FIG. 1 (B), the temperature is first heated to 1100 ° C. and held for 60 seconds, then the temperature is lowered, and at various test temperatures, the height is 19.80 mm (rolling rate 12%) and 16.88 mm (rolling). (Rate 25%) Upset forging, then held at each test temperature for a predetermined time and then water-cooled, and the grain growth of the crystal grains was examined for each test temperature and each holding time.

FIG. 1 (C) shows a photograph of the test piece 10 at the initial grain size, FIG. 2 shows a structure photograph for each holding temperature and holding time when the rolling reduction is 12%, and FIG. Structure photographs for each holding temperature and holding time when the rate is 25% are shown.
Further, FIG. 4 shows the relationship between the length of the holding time and the crystal grain size, that is, the behavior of grain size change (grain growth) for each test temperature.
2 and FIG. 3 are optical micrographs.

As a result of the above preliminary test, the following was found.
1) That is, at the stage of the initial grain size ASTM # 4, Ti and Mo carbides are already precipitated on the old grain boundaries in the structure (shown in black dots in the photograph of FIG. 1C). This carbide is produced by air cooling treatment at the end of billet forging as rough forging, that is, air cooling treatment at the end of rough forging is carbide precipitation treatment).
2) The precipitation phase of γ 'is not observed on the initial grain size photograph and the structure photograph after holding at various temperatures for a predetermined time after forging, that is, the γ' phase is not changed during the forging step and the subsequent heating and holding. Not depositing,
3) Reduction ratio = 12% (strain = 0.128), recrystallization occurs at a holding temperature of 980 ° C. or higher,
4) Under a rolling reduction of 25% (strain = 0.288), the crystal grains are rapidly coarsened at a holding temperature of 1100 ° C. over 1800 s (30 minutes), and from the structure photograph at this time, While it is recognized that the carbide is rapidly decreased, the carbide is not significantly decreased within the temperature range of 980 ° C. to 1060 ° C., and the grain growth of the crystal grains is suppressed, and finally a constant crystal A tendency to converge to particle size was observed.

  This means that under a rolling reduction of 25% and holding at 1100 ° C., if the holding time is 1800 s or more, the carbide dissolves and the crystal pinning effect by the carbide is lost, and the crystal grains grow rapidly, Within the range of 980 ° C. to 1060 ° C., it means that the crystal grains only converge to a certain crystal grain size and do not grow excessively even if kept for a long time.

  2 and 3 show the change in the grain size of the crystal grains due to heating and holding at 980 ° C. or higher. If the temperature is 950 ° C. or higher, the crystal grains are recrystallized and the pinning effect due to the presence of carbides. It has been confirmed that grain growth is suppressed.

The present invention has been made based on the above knowledge, and when forging a disk-shaped product by free sequential forging using an alloy 901 equivalent material, in finish forging, under the pinning action by carbide precipitation, Repeat the forging step at a total rolling reduction of 10% or more (preferably 12% or more and 45% or less) and the reheating step of holding at 950 to 1060 ° C. for 1 hour or more at least once, and at least twice or more at the same location. To obtain a disk-shaped product of a heat-resistant alloy containing Ni as a main component, and according to the present invention, the mechanical properties such as tensile strength, impact strength, and fatigue strength are obtained. A structure having an excellent ASTM crystal grain size # 4 or more can be obtained stably and easily.
In the present invention, the reduction ratio is the ratio of the total amount of reduction for each impact to the dimension before reduction when the reduction is applied several times at the same location between reheats (reduction amount). Is a value obtained by dividing 100 by the dimension before rolling and multiplying by 100).

  As is clear from the above, the present invention maintains the carbide precipitation state, and performs hot forging, specifically, forging step and reheating step, thereby recrystallizing crystal grains until reaching the core. It suppresses the excessive growth of crystal grains (grain growth suppression) by exerting the pinning effect by carbides, and according to the present invention, the reheat time is increased in actual operation. Even if the reheat time may slightly vary with respect to the set time during actual operation, it is possible to stably obtain a desired crystal grain size, that is, a crystal grain size of ASTM # 4 or higher.

  When high temperature creep strength is required, it is desirable to set the final crystal grain size to ASTM # 7 or less, that is, to set a forging pass so as to obtain such a crystal grain size.

A turbine disk or the like in a power generation gas turbine may be subject to creep deformation due to centrifugal force accompanying high-speed rotation at a high temperature, and it may be required to increase the strength against such creep deformation.
With respect to such high temperature creep strength, it is known that if the crystal grains are excessively fine, the characteristics deteriorate.
This is because when the strain rate is low, such as creep deformation at high temperature, grain boundary slip occurs and breaks at the grain boundary. At that time, if the crystal grains are excessively fine, the grain boundary area is large. This is considered to be due to the fact that breakage at the grain boundaries is likely to occur.
Therefore, when high temperature creep strength is particularly required, it is better not to make the crystal grains excessively fine. In such a case, it is desirable to set the crystal grain size to ASTM # 7 or less.

In the present invention, following the final forging step, the disk molded product can be heat-treated, and a grain growth step can be performed in which the crystal grains of the molded product are grown to the crystal grain size (Claim 2).
By doing so, it is easier to obtain a structure having a desired crystal grain size.

  In that case, it is desirable that the grain growth treatment be performed in a temperature range of 970 ° C. to 1010 ° C. under a condition of holding time of 1 hour or more.

  The present invention is particularly useful as a method for forging a turbine disk in a power generation gas turbine.

In the present invention, alloy 901 equivalent material is used as a heat-resistant alloy containing Ni as a main component.
The chemical components in the present invention are described below.
C: 0.02 to 0.06%
The C element works as a solid solution strengthening element that penetrates between crystal lattices and dissolves, and generates strain to strengthen the matrix. Also, carbides such as TiC and MoC are precipitated. For its function, it is contained in the range of 0.02 to 0.06%.

Ni: 40-45%
Ni element is a parent phase alloy element along with Fe, and forms Ti, Al and Ni ₃ X phase (γ ′ phase) as a strengthening phase. Because of its function, the present invention contains Ni at 40 to 45%.

Cr: 12-14%
Cr element is a substitutional form with Ni and dissolves in the matrix to improve high temperature corrosion resistance. For its function, it is contained at 12-14%.

Mo: 6-7%
Mo element is substituted for Ni and is dissolved in the matrix to improve the high temperature strength. For its function, it is contained at 6-7%.

Ti: 2.5-3.5%
Ti element is a substitutional form of Ni and dissolves in the matrix, and coherently precipitates mainly as Ni ₃ Ti phase (γ 'phase), greatly improving the high temperature strength. For its function, it is contained at 2.5 to 3.5%.

Al: 0.12 to 0.30%
Al element is a substitutional solution with Ni and dissolves in the matrix, mainly coherently precipitated as the Ni ₃ Al phase (γ 'phase), greatly improving the high-temperature strength. For its function, it is contained at 0.12 to 0.30%.

B: 0.010-0.020%
Element B precipitates very finely at the grain boundary and has the function of improving the grain boundary strength. For its function, it is contained at 0.010 to 0.020%.

  According to the present invention as described above, when forging a disk-shaped product, a structure having ASTM crystal grain size # 4 or more excellent in mechanical properties such as tensile strength, impact strength, fatigue strength and the like can be obtained stably and easily. be able to.

It is explanatory drawing of the preliminary test method of a grain growth behavior investigation. It is a structure | tissue photograph after heating holding when a rolling reduction is 12%. It is a structure | tissue photograph after a heating holding | maintenance when the reduction rate is 25%. It is the figure which showed the relationship between retention time and crystal grain size. It is the figure which showed the workpiece shape at the time of completion | finish of billet forging in the Example of this invention. It is explanatory drawing of the cross forging method and the radiation forging method in the Example. It is the figure which showed the change of the crystal grain size for every process in the disk forge of the Example.

Next, an embodiment when the present invention is applied to forging of a turbine disk will be described below.
An alloy containing Ni having a chemical composition shown in Table 1 as a main component (alloy 901 equivalent material) is melted in a vacuum induction furnace (VIF), and further remelted in a vacuum arc furnace (VAR) to 2.5 tons. Got the ingot.
Thereafter, billet forging as pre-forging (rough forging) is performed to obtain a columnar processed article, ie, billet (workpiece) 12 having an octagonal cross section shown in FIG. 5 having a cross-sectional dimension of 450 mm and an axial dimension of 1170 mm. It was.
In this billet forging, the initial cast structure (grain size is ASTM # -3) is ASTM # 2 at the billet 12 stage.
However, this crystal grain size is the crystal grain size after heating (1060 ° C. × 4 hr) before upsetting (1) in the process of disc forging as the subsequent finish forging.

Using the billet 12 of FIG. 5 obtained by billet forging as a starting material, disk forging as finish forging was performed according to the process shown in Table 2.
As shown in Table 2, in this disk forging process, after the initial heating step, the billet 12 having a grain size of ASTM # 2 is first placed using an anvil in the step (1). And the axial dimension was compressed from 1170 mm to 800 mm. At this time, the radial dimension is changed to 544 mm.
The rolling reduction in the upsetting (1) process is 31.6% as shown in Table 2.

Subsequently, after performing the reheat (1) step under the condition of 1060 ° C. × 4 hr, the upsetting (2) step is performed using an anvil, the axial dimension is further compressed to 500 mm, and the radial dimension is 688 mm. It was.
As shown in Table 2, the rolling reduction in this upsetting (2) step is 37.5%.
Following this, the reheat (2) step was again performed under the condition of 1060 ° C. × 4 hr.
After that, the cross forging step was executed.

Here, the cross forging is applied to the intermediate workpiece 14 as shown in FIG. 6 by hitting an anvil 16 having a width smaller than the radial dimension against one surface in the axial direction, Thereafter, the position of the anvil 16 relative to the workpiece 14 is changed relative to each other, and the workpiece 14 is hit again. Thereafter, the anvil 16 makes one turn in the rotation direction relative to the workpiece 14 in the same procedure. It is a processing method that applies a blow until
Actually, the workpiece 14 is placed on the turntable 18 and the workpiece 14 is rotated little by little by the turntable 18 so that the striking position of the anvil 16 against the workpiece 14 is varied. .

In this example, the height of the processed product 14 was first compressed from 500 to 460 mm before processing by the step of cross forging (1) as described above.
Subsequently, the cross forging (2) step was performed, and the workpiece 14 was similarly hit with the anvil 16 until the height (thickness) became 460 to 420 mm.
In the cross forging (1), (2), (3), (4), the flat part was processed using an anvil with a width of 200 mm.

After completing the steps of cross forging (1) and (2) above, the processed product 14 is then turned upside down so that the front surface is the back surface, and the back surface is the front surface. As described above, the workpiece 14 was inverted on the turntable 18, and thereafter, the cross forging (3) step and the cross forging (4) step were executed by the same method as described above.
Here, in the cross forging (3) step, the height dimension was set to 420 to 385 mm, and in the cross forging (4) step, the height dimension was further set to 385 to 350 mm.
Table 2 shows the reduction amount and the total reduction ratio of the cross forgings (1), (2), (3), and (4).
After performing the above cross forging process, the third reheat (3) step was performed under the condition of 1060 ° C. × 4 hr.

Next, following the step of reheat (3), each step of radiation forging (1), (2), (3), (4) was performed.
FIG. 6B shows a method of radiation forging.
As shown in the figure, here, the anvil 20 is positioned such that one end side of the anvil 20 is located near the center of the workpiece 14 and the other end protrudes radially outward from the workpiece 14 with respect to the workpiece 14. The workpiece 14 is hit with the anvil, and then the anvil 20 is slightly changed in the rotational direction (circumferential direction) relative to the workpiece 14, and the same hammer is applied to the workpiece 14 with the anvil 20. After that, the same striking is applied one after another while changing the position of the anvil 20 in the circumferential direction of the workpiece 14 at the same pitch until the anvil 20 reaches a position that makes one round with respect to the workpiece 14, thereby reducing the workpiece 14. .

Here, the radial forging (1) is a process until the anvil 20 is rotated once with respect to the workpiece 14, and the radial forging (2) is performed while hitting the same surface of the workpiece 14 again with the anvil 20 after that. In this process, the anvil 20 is moved to a position that makes one turn with respect to the processed product 14, and hits are sequentially applied during that time.
The radial forging (3) performs the same processing by reversing the direction of the processed product 14, and the radial forging (4) further executes the same processing as the second processing.

In the above radial forging (1), the height was set to 300 mm, and in the radial forging (2), the thickness was further set to 260 mm.
And when thickness became this dimension, the radial forging (3) was performed by making the processed goods 14 into the reverse direction, and height was set to 225 mm. Then, a disk-shaped product having a final size and shape of 195 mm in height and 1102 mm in the radial direction was obtained by radiation forging (4).
In the above radial forging (1), (2), (3), (4), the flat portion was processed using an anvil with a width of 100 mm.
The radial forging (1) has a reduction amount of 50 mm, the radial forging (2) has a reduction amount of 40 mm, the radial forging (3) has a reduction amount of 35 mm, the radial forging (4) has a reduction amount of 30 mm, and the radial forging ( As shown in Table 2, the total reduction ratio of 1), (2), (3), and (4) is 44.3%.

After performing the above forging and reheating processes, the disk molded product was subjected to grain growth treatment under conditions of 1010 ° C. × 4 hr following air cooling (AC).
This grain growth process is a process in which crystals are recrystallized based on the strain applied to the disk molded product in the radial forging process and grown to a target crystal grain size.
Note that this grain growth process may also be performed as an ST process (solution heat treatment).
Incidentally, the change in crystal grain size for each process of finish forging as described above is shown in FIG.

From FIG. 7, the initial crystal grain size ASTM # 2 is refined by executing the forging step, grown in the subsequent reheat step, and alternately repeated so that the final crystal grain size is ASTM #. A crystal grain size of 6 or more was obtained, and a fine crystal grain size larger than the required crystal grain size ASTM # 4 could be achieved.
The crystal grain size was measured by taking the structure after each step with an optical microscope and determining the crystal grain size from the number of crystal grains within a certain area in the photograph (average of 6 fields of view).

12 Billets 14 Processed products 16, 20 Anvil 18 Turntable

Claims (3)

By mass% C: 0.02 to 0.06%
Ni: 40-45%
Cr: 12-14%
Mo: 6-7%
Ti: 2.5-3.5%
Al: 0.12 to 0.30%
B: 0.010-0.020%
A forging step in which a work piece made of a Ni- base heat-resistant alloy having a composition of the balance Fe and inevitable impurities is subjected to reduction at a total reduction ratio of 10% or more in finish forging under a pinning action by carbide precipitation; The reheating step held at 1060 ° C. for 1 hour or more is repeated at least once each time, subjected to free sequential forging that strikes at least twice at the same location, forged into a disk shape, and has a structure of ASTM grain size # 4 or more A disc-shaped forging method characterized by obtaining a disc product.
  2. The disk shape according to claim 1, wherein after the final forging step, a disk-shaped molded product is heat-treated, and a grain growth step is performed to grow crystal grains of the molded product to the crystal grain size. Product forging process.   The method for forging a disk-shaped product according to claim 1, wherein the disk-shaped product is a turbine disk in a power generation gas turbine.