Manufacturing method for turbine blade materials
Optimizing manufacturing conditions after hot forging with a series of heat treatments refines crystal grains and improves mechanical properties of Fe-based alloy turbine blades, addressing grain size and strength-toughness balance issues.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- PROTERIAL LTD
- Filing Date
- 2025-03-03
- Publication Date
- 2026-07-07
AI Technical Summary
Existing methods for manufacturing Fe-based alloy turbine blades focus on quenching and subsequent heat treatments, neglecting the optimization of conditions before quenching, leading to issues like large grain size and poor strength-toughness balance.
A method involving hot forging, followed by a series of heat treatments including a first heat treatment at 450 to 900°C, quenching at 980 to 1080°C with insulating material for uniform cooling, and tempering at 500 to 600°C, repeated twice, to refine crystal grains and improve mechanical properties.
The method achieves uniform refinement of crystal grains and enhances mechanical properties of turbine blade materials, improving strength and toughness through optimized post-forging manufacturing conditions.
Smart Images
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Abstract
Description
Technical Field
[0001] The present invention relates to a method for manufacturing a material for a turbine blade.
Background Art
[0002] A turbine blade used in a high-temperature environment is formed into a predetermined shape by hot forging and subjected to various heat treatments so as to satisfy required characteristics. Materials used for turbine blades include Ni-based alloys, Ti-based alloys, Fe-based alloys, etc., and appropriate heat treatment conditions are different for each. Among these, for Fe-based alloys such as stainless steel, various heat treatments are also required, and many proposals have been made. For example, Japanese Unexamined Patent Application Publication No. 2016-166409 (Patent Document 1) discloses that a forging preform (hot forging material) is subjected to a solution treatment at about 2000 to 2100°F (about 1093 to 1149°C) and a tempering at about 600°F (about 315°C). Further, Japanese Unexamined Patent Application Publication No. 2015-74822 (Patent Document 2) discloses an invention in which a stainless steel member (hot forging material) after hot forging is heated to 1000°C or higher to perform a solution treatment, and cooling is performed to reduce the temperature difference of the stainless steel member during cooling.
Prior Art Documents
Patent Documents
[0003]
Patent Document 1
Patent Document 2
Summary of the Invention
Problems to be Solved by the Invention
[0004] When a hot-forged Fe-based alloy material, formed into a desired shape by hot forging, is subjected to solution treatment (solution treatment is sometimes referred to as "quenching," and hereafter referred to as "quenching" in this invention) and tempering to become a turbine blade material, the required properties are satisfied, but there are problems such as the grain size being slightly large or the balance between strength and toughness being somewhat poor. The aforementioned invention of Fe-based alloy materials for turbine blades aims to control the quenching and subsequent heat treatment conditions to adjust the characteristics and shape required for turbine blades, and most of the proposals made in the past have focused on this quenching and subsequent heat treatment. However, in order to solve the above problems, it is important to optimize the manufacturing conditions, including those before quenching, but currently, there has been insufficient consideration given to optimizing the manufacturing conditions before quenching, that is, after hot forging. The object of the present invention is to provide a method for manufacturing a turbine blade material made of Fe-based alloy, which allows for further uniform refinement of the crystal grains of the turbine blade material and improvement of its mechanical properties by optimizing the manufacturing conditions after hot forging. [Means for solving the problem]
[0005] This invention was made in view of the above-mentioned problems. In other words, the present invention is a method for manufacturing turbine blade material comprising: a hot forging step of forming a rough material made of an Fe-based alloy having a precipitation-hardening stainless steel composition or a martensitic stainless steel into a turbine blade shape having a root portion and an wing portion by hot forging in one blow; a first heat treatment step of heating and holding the hot forged material at a temperature range of 450 to 900°C and then cooling it; a second heat treatment step of heating and holding the first heat treated material after the first heat treatment step at a temperature range of 980 to 1080°C and then cooling it; and a third heat treatment step of heating and holding the material at a temperature range of 500 to 600°C and then cooling it, wherein the cooling in the second heat treatment step covers the wing portion with an insulating material. In the present invention, it is preferable to repeat the third heat treatment step two or more times. [Effects of the Invention]
[0006] According to the present invention, by optimizing the manufacturing conditions after hot forging, it is possible to further refine and make the crystal grains of the turbine blade material more uniform, thereby improving its mechanical properties. [Brief explanation of the drawing]
[0007] [Figure 1] This is the cross-sectional metallic structure of the turbine blade material (No. 1) of the present invention. [Figure 2] This is the cross-sectional metallic structure of a conventional turbine blade material (No. 11). [Modes for carrying out the invention]
[0008] First, let me explain the terms used in this invention. The "Fe-based alloy" covered by this invention refers to an alloy containing the most Fe among its components, and typical materials include alloys having the composition of precipitation-hardening stainless steel or martensitic stainless steel. Furthermore, the "hot-forged material" as used in this invention refers to a material formed into a predetermined shape by hot forging and used in the first heat treatment process. For example, a material that has undergone shape adjustment such as deburring after hot forging is also considered a hot-forged material. Furthermore, the "material for turbine blades" refers to a material that has completed the third heat treatment process defined in this invention. The present invention will be described below in the order of the manufacturing process.
[0009] <Hot forging process> First, a rough surface made of Fe-based alloy, formed into a predetermined shape, is prepared. It is preferable to coat the surface of the prepared rough surface with a glass lubricant. Coating with a glass lubricant has advantages such as heat retention for the material to be hot forged, improved lubricity by reducing the coefficient of friction between the forging material and the die, and suppression of scale formation due to heating. The thickness of the glass lubricant coating should be about 300 to 450 μm, and it is preferable to cover the entire rough surface. Then, it is heated to a predetermined temperature. The range of hot forging temperatures may vary slightly depending on the composition of the Fe-based alloy, but for precipitation-hardening stainless steel and martensitic stainless steel, a range of approximately 950 to 1100°C is sufficient. The heated rough material (forging material) is placed on the lower die of a hot forging machine, and the upper and lower dies form it into a turbine blade shape to produce a hot forged material. While there are no specific limitations on the hot forging machine used, a hydraulic hot forging machine is suitable for turbine blades with a total length of 40 inches or more. A hydraulic hot forging machine allows for shaping into a turbine blade in a single blow (single press). Single-blow hot forging stabilizes the forging conditions, resulting in more stable tensile strength and ductility compared to multi-heat molded products. Furthermore, because multiple heating cycles are unnecessary, coarse grain growth is suppressed, resulting in a more uniform and fine martensite structure, which is effective in improving the grain size specifications and mechanical properties required for turbine blades. Additionally, single-blow forming offers advantages in productivity and environmental impact compared to multi-heat forming, hence the application of single-blow hot forging.
[0010] <First heat treatment process> Next, in this invention, the hot-forged material is subjected to a first heat treatment step, in which it is heated and held in a temperature range of 450 to 900°C, followed by cooling. Hot-forged material formed into a turbine blade shape undergoes martensitic transformation, resulting in a state of high strength and low toughness, and if left as is after hot forging, it may develop quench cracks. Therefore, after hot forging is completed, the first heat treatment step is applied to the hot-forged material when its surface temperature reaches 50 to 150°C. If the temperature of this first heat treatment step is below 450°C, quench cracks cannot be prevented. Furthermore, the effect of preventing forging cracks saturates in the temperature range above 900°C. In addition, in the temperature range above 900°C, grain growth may occur in the first heat treatment step, and these coarse grains may remain even after the second heat treatment, potentially preventing the acquisition of the desired mechanical properties. Therefore, the temperature range for the first heat treatment step is set to 450 to 900°C. In this invention, the heating and holding time in the first heat treatment step is not specifically defined, but it is acceptable to have it for approximately 3 to 6 hours. Furthermore, cooling in the post-forging heat treatment step is preferably done by air cooling or at a cooling rate slower than air cooling. This is because cooling at a rate faster than air cooling can result in uneven cooling and potentially cause deformation of the hot-forged material. Preferably, furnace cooling at a cooling rate slower than air cooling is applied. Furnace cooling can more reliably prevent cracking of the hot-forged material while also suppressing deformation of the hot-forged material. The intermediate material after this first heat treatment step will be referred to as the "first heat-treated material".
[0011] By selecting a temperature range between 700°C and 900°C within the temperature range of this first heat treatment process, in addition to the effect of preventing cracking of the hot-forged material as described above, it is possible to reduce the uneven forging distortion generated during hot forging, further uniformly refine the crystal grains obtained in the second heat treatment process and subsequent processes described later, and improve the mechanical properties more uniformly. This first heat treatment process performed in the temperature range of 700 to 900°C is referred to as the "high-temperature first heat treatment process." The preferred lower temperature limit for this high-temperature first heat treatment process is 750°C, and more preferably 765°C. The more preferred upper temperature limit is 810°C, and even more preferably 795°C. When applying this high-temperature first heat treatment process, it is preferable to apply furnace cooling, which is slower than air cooling. Furthermore, the first heat treatment process performed within the temperature range of 450 to 700°C is referred to as the "low-temperature first heat treatment process." The primary purpose of the low-temperature first heat treatment process is to prevent the aforementioned quenching cracks. When the primary purpose is to prevent quenching cracks, the preferred lower temperature limit is 620°C, and the preferred upper temperature limit is 680°C. When applying this low-temperature first heat treatment process, the cooling rate may be air cooling or furnace cooling, which is slower than air cooling. The selection between the high-temperature and low-temperature first heat treatment processes mentioned above can be appropriately chosen according to the purpose. However, if the cooling of the first heat treatment process is performed using furnace cooling, the heating furnace will be occupied for a long time. In particular, if the high-temperature first heat treatment process is selected, the heating furnace will be occupied for a long time. Therefore, in mass production processes, it is practical to select the processing temperature of the first heat treatment process while considering the heating furnace occupancy time. Furthermore, in the first heat treatment process, for example, the high-temperature first heat treatment process may be combined after the low-temperature first heat treatment process.
[0012] <Second heat treatment process (quenching)> In this invention, the first heat-treated material after the first heat treatment step is subjected to a second heat treatment step in which it is heated and held at a temperature range of 980 to 1080°C, and then cooled. This step is the same as the "quenching" process described above, and will therefore be referred to as "quenching" hereafter. In this invention, the quenching temperature is set to 980-1080°C because temperatures below 980°C are in a temperature range where carbides do not sufficiently solid dissolve, and temperatures above 1080°C may cause grain coarsening and a decrease in mechanical properties due to high-temperature holding. Therefore, in this invention, quenching is performed by heating and holding within the temperature range of 980-1080°C. The preferred lower limit of the quenching temperature is 1010°C, and the preferred upper limit is 1050°C. In this quenching, the heating pattern during heating can be multi-stage, ranging from two to four stages. Since the first heat-treated material is turbine blade shaped, it consists of a thin blade section and a thick root section integrated together. To heat the first heat-treated material in a nearly uniform manner, it is preferable to heat it in multiple stages. For example, for turbine blades of 40 inches or larger, it is preferable to use three or more stages, and the heating pattern should be appropriately changed depending on the size of the first heat-treated material. While there are no specific requirements for the holding time during quenching, approximately 0.5 to 1.5 hours is acceptable. The heating and holding time referred to here is the time at the highest temperature (quenching temperature), and does not include the time spent holding the temperature during the heating process to reach the quenching temperature.
[0013] Furthermore, the cooling process during this quenching (second heat treatment) is performed because the first heat-treated material is turbine blade shaped, with a thin blade section and a thick root section integrated together. Therefore, when cooling this first heat-treated material during quenching, in order to avoid uneven cooling rates between the blade section and the root section and to ensure a uniform cooling rate until the cooling is complete, it is preferable to cover the thin blade section with an insulating material to bring the cooling rate of the blade section closer to that of the root section, thereby reducing the temperature difference between the root and the blade section. The insulating material used here is preferably a flexible inorganic fiber. In this invention, "inorganic fiber" includes glass fiber, ceramic fiber, etc., and it is preferable to select ceramic fiber, which has excellent heat insulation properties. Among ceramic fibers, for example, Kaowool (registered trademark) is particularly preferable because it is readily available and inexpensive, and the thickness of the covering can be easily adjusted according to the thickness of the blade section to be cooled. Furthermore, regarding cooling during quenching, as mentioned above, since cooling is performed from a high temperature range of 980-1080°C to bring the cooling rate closer to uniform, it is preferable to cool the root portion of the first heat-treated material, which is covered with insulating material, with air cooling or at a faster cooling rate than air cooling, and to adjust the cooling rate of the wing portion to approach the cooling rate of the root portion. This is to suppress variations in mechanical properties and, in particular, to balance strength and ductility. Hereafter, the intermediate material after the quenching process (after the second heat treatment process) will be referred to as "quenched material" or "second heat-treated material".
[0014] <Third heat treatment process (tempering)> In this invention, a third heat treatment step is performed on the hardened material. This is sometimes called "tempering," and will be referred to as "tempering" hereafter. In this invention, the tempering temperature is set to 500-600°C. The purpose of heating and holding within this temperature range is to promote the martensite transformation of untransformed austenite remaining after quenching, and to precipitate supersaturated dissolved carbon as carbides, thereby obtaining a forged material with a good balance of strength and ductility. If the tempering temperature is below 500°C, there is a problem that the martensite transformation of untransformed austenite is not promoted. On the other hand, if the tempering temperature exceeds 600°C, there is a problem that the balance between strength and ductility is disrupted. Therefore, in this invention, tempering is performed by heating and holding within the temperature range of 500-600°C. The preferred lower limit of the tempering temperature is 540°C, and the preferred upper limit of the tempering temperature is 570°C. It is preferable to use a multi-stage heat pattern for heating during tempering, similar to the quenching process described above. The tempering holding time is not specifically defined, but approximately 2-5 hours is sufficient. Note that the heating and holding time referred to here is the time at the highest temperature (tempering temperature), and does not include the time spent holding the temperature during the heating process to reach the tempering temperature. Also, the cooling in this tempering process is preferably air cooling or cooling at a slower cooling rate than air cooling. This is to adjust the balance between strength and ductility. Preferably, air cooling is performed. Since the tempering temperature is lower than the quenching temperature described above, it is not always necessary to cover the thin blade part with a heat insulating material having heat insulating properties like the cooling during quenching when cooling during tempering.
[0015] Also, in the present invention, this tempering can be repeated two or more times. By repeating two or more times, the martensitic transformation of the retained austenite remaining after quenching is surely promoted, and the retained austenite is made as close to zero as possible. Therefore, in tempering, it is preferable to repeat two or more times. The upper limit of the number of times of tempering may be a maximum of 3 times. Even if tempering is performed more than 3 times, it cannot be expected that the effect of the repeated tempering will be further enhanced. Preferably, two times are sufficient. According to the present invention, by optimizing the manufacturing conditions after hot forging, the crystal grains of the material for turbine blades can be further uniformly refined, and the mechanical properties can be improved. The manufacturing method of the material for turbine blades of the present invention is effective for those having the composition of martensitic stainless steel. The above-mentioned "martensitic stainless steel" is a steel containing 10.5% or more of Cr that can be made into a martensite structure by the above-mentioned heat treatment and can be hardened by forming a martensite structure.
Examples
[0016] Hereinafter, the present invention will be described in detail with examples. As a blank made of an Fe-based alloy, a blank made of a martensitic stainless steel (improved steel of JIS standard SUS403) was prepared. The blank was formed into a predetermined shape by hot forging. The above-mentioned blank was heated to 80 to 90°C, and the entire surface of the blank was coated with a glass lubricant to a thickness of about 300 to 450 μm. This blank was heated and held at 950 to 1100°C to obtain a forging material. The aforementioned forging material was placed on the lower die of a hydraulic hot forging apparatus, and formed into a 40-inch turbine blade shape by hot forging in one blow using the upper and lower dies to obtain the hot forged material (No. 1) of the present invention. Furthermore, as a conventional example, a rough material made of an Fe-based alloy with the same composition as described above was used, and hot die forging and reheating were repeated multiple times to form a 40-inch turbine blade shape, resulting in the conventional hot forged material (No. 11).
[0017] The hot-forged material described above was heat-treated under the conditions shown in Table 1 below to produce a turbine blade material. The first heat treatment step of this invention is a low-temperature first heat treatment step. After the completion of hot forging, the surface temperature of the hot-forged material was confirmed to be in the range of 50 to 150°C, and it was placed in the heating furnace for the first heat treatment step. The heat pattern during the quenching heating was a three-stage multi-stage process. The first stage involved holding the material at 700 to 800°C for 0.5 to 2 hours, the second stage involved holding it at 1000°C for 15 to 30 minutes, and then heating was increased to the third stage (quenching temperature) of 1030°C. The heating rate from the first to the third stage was 100 to 150°C / hour. During the cooling after quenching, the blade section was covered with an insulating material (KaoWool) with insulating properties, and its thickness was adjusted to equalize the cooling rate with the root section. Table 1 shows the quenching temperatures. Furthermore, the heat pattern during tempering involved a three-stage multi-stage process: the first stage involved holding at 350-450°C for 0.5-2 hours, the second stage involved holding at 510-540°C for 0.5-2 hours, and then the temperature was raised to the third stage (tempering temperature) of 545°C (560°C the second time). The heating rate from the first to the third stage was 50-100°C / hour. During the cooling of the quenched material, it was simply air-cooled. Table 1 shows the tempering temperatures. In Table 1, "AC" indicates air-cooled and "FC" indicates furnace-cooled.
[0018] [Table 1]
[0019] Tensile and Charpy impact test specimens were taken from the turbine blade material described above and mechanically tested according to ASTM-A370. For the tensile test specimens, the specimens were taken from the center of the product thickness of the root and blade sections, and for the Charpy impact test specimens, the specimens were taken from the center of the product thickness of the root section. The results of the mechanical tests are shown in Table 2. As shown in Table 2, the 0.2% yield strength, tensile strength, elongation, and reduction of area of the turbine blade material of the present invention were almost the same as those of conventional examples. The impact value at the base of the turbine blade material was significantly improved to over 40 J.
[0020] Figures 1 and 2 show the cross-sectional microstructures of the turbine blade material of the present invention (No. 1) and the conventional turbine blade material (No. 11), respectively. Since both the present invention and the conventional example exhibit a martensite microstructure, the grain size number was read at the prior austenite grain boundary. The larger the grain size number, the smaller the grains, and the measurement was performed according to the ASTM-E112 standard. As shown in Figure 1, the average grain size number of the turbine blade material of the present invention was 6.0 to 7.0, while the average grain size number of the conventional turbine blade material shown in Figure 2 was 4.5 to 5.0. Furthermore, the maximum grain size number of the turbine blade material of the present invention was 4.0 to 5.0, while that of the conventional turbine blade material was 3.0. From these results, it was confirmed that the turbine blade material of the present invention has finer average and maximum grain size numbers. The sampling locations for the microstructure observation test specimens were the center of the product thickness of the root and blade portions of the turbine blade material. Because this invention forms the turbine blade shape by a single blow hot forging, it is possible to suppress grain growth during the hot forging process compared to conventional examples where the turbine blade shape is formed by repeating die forging and reheating multiple times. Furthermore, by performing appropriate first, second, and third heat treatment steps, the average grain size number of the turbine blade material becomes finer than 5.0, and even the maximum grain size can be suppressed to a range below 3.5, resulting in a finer grain size. As a result, the grain remained uniformly fine without coarsening, which is thought to have significantly improved the impact value of the turbine blade material.
[0021] [Table 2]
[0022] As explained above, according to the present invention, by optimizing the manufacturing conditions after hot forging in a method for manufacturing Fe-based alloy materials for turbine blades, it is possible to further uniformly refine the crystal grains of the turbine blade material and improve its mechanical properties.
Claims
1. A hot forging process to form a turbine blade shape having a root and an wing portion from a rough material made of an Fe-based alloy having the composition of precipitation-hardening stainless steel or martensitic stainless steel by hot forging in one blow, The hot-forged material is subjected to a first heat treatment step in which it is heated and held at a temperature range of 450 to 900°C, and then cooled. The first heat-treated material after the first heat treatment step is subjected to a second heat treatment step in which it is heated and held at a temperature range of 980 to 1080°C and then cooled, followed by a third heat treatment step in which it is heated and held at a temperature range of 500 to 600°C and then cooled. A method for manufacturing a turbine blade material, characterized in that the cooling in the second heat treatment step involves covering the blade portion with an insulating material.
2. The method for manufacturing a turbine blade material according to claim 1, characterized in that the third heat treatment step is repeated two or more times.