Manufacturing method for turbine blade materials

Abstract

To provide a manufacturing method for a raw material for a turbine blade made of Fe-based alloy, which can uniformly miniaturize crystal grain of the raw material for a turbine blade furthermore to improve mechanical characteristics, by making a manufacturing condition after hot forging and thereafter more proper.SOLUTION: A manufacturing method for a raw material for a turbine blade includes: a hot-forging step of forming a rough material made of Fe-based alloy into a turbine blade-shape by one blow of hot forging to make a hot forged material; a first thermal processing step of heating and holding the hot forged material in a temperature range of 450-900°C and then cooling the material; a second thermal processing step of heating and holding a first thermal-processed material in a temperature range of 980-1080°C and then cooling the material, after the first thermal processing step; and a third thermal processing step of heating and holding the thermal processed material in a temperature range of 500-600°C and then cooling the material.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a material for a turbine blade. [Background technology]

[0002] Turbine blades used in high-temperature environments are formed into a predetermined shape by hot forging and are subjected to various heat treatments to satisfy the required characteristics. Materials used for turbine blades include Ni-based alloys, Ti-based alloys, and Fe-based alloys, and the appropriate heat treatment conditions are different for each. Among these, Fe-based alloys such as stainless steel also require various heat treatments, and many proposals have been made. For example, Japanese Patent Application Laid-Open No. 2016-166409 (Patent Document 1) discloses that a forged preform (hot forged material) is subjected to a solution treatment at about 2000 to 2100°F (about 1093 to 1149°C) and tempered at about 600°F (about 315°C). In addition, Japanese Patent Application Laid-Open No. 2015-74822 (Patent Document 2) discloses an invention in which a stainless steel member (hot forged 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] JP 2016-166409 A [Patent Document 2] JP 2015-74822 A Summary of the Invention [Problem to be solved by the invention]

[0004] When hot forged material made of an Fe-based alloy formed into a desired shape by hot forging is subjected to solution treatment (solution treatment is sometimes called "quenching", and hereinafter, in this invention, will be referred to as "quenching") and tempering to produce a material for turbine blades, although the required characteristics are satisfied, there are problems such as, for example, a tendency for the crystal grain size to be somewhat large and for the balance between strength and toughness to be somewhat poor. The invention of the Fe-based alloy turbine blade material described above aims to control the conditions of quenching and subsequent heat treatments to adjust the properties and shape required of the turbine blade, and most of the proposals made so far have focused on quenching and subsequent heat treatments. However, in order to solve the above problems, it is important to optimize the manufacturing conditions including those before quenching. However, at present, there has been insufficient research into optimizing the manufacturing conditions before quenching, i.e., after hot forging. An object of the present invention is to provide a method for producing a turbine blade material made of an Fe-based alloy, which can make the crystal grains of the turbine blade material more uniform and fine by optimizing the manufacturing conditions after hot forging, thereby improving the mechanical properties. [Means for solving the problem]

[0005] The present invention has been made in view of the above-mentioned problems. That is, the present invention is a method for manufacturing a material for a turbine blade, the method including: a hot forging step in which a rough Fe-based alloy is formed into a turbine blade shape by one-blow hot forging to produce a hot forged material; a first heat treatment step in which the hot forged material is heated and held at a temperature range of 450 to 900°C and then cooled; a second heat treatment step in which the first heat-treated material after the first heat treatment step is heated and held at a temperature range of 980 to 1080°C and then cooled; and a third heat treatment step in which the first heat-treated material is heated and held at a temperature range of 500 to 600°C and then cooled. In the present invention, it is preferable to repeat the third heat treatment step two or more times. Effect of the Invention

[0006] According to the present invention, by optimizing the manufacturing conditions after hot forging, it is possible to make the crystal grains of the material for turbine blades even finer and more uniform, thereby improving the mechanical properties. [Brief description of the drawings]

[0007] [Figure 1] 1 is a cross-sectional metal structure of a turbine blade material (No. 1) of the present invention. [Diagram 2] 1 shows a cross-sectional metal structure of a conventional turbine blade material (No. 11). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] First, the terms defined in the present invention will be explained. The "Fe-based alloy" of the present invention refers to an alloy that contains the most Fe among its components, and typical examples of such alloys are alloys having the composition of precipitation hardening stainless steel or martensitic stainless steel. The "hot forged material" of the present invention refers to a material that is formed into a predetermined shape by hot forging and is used as a material for the first heat treatment process. For example, a material that has been subjected to shape adjustment such as deburring after hot forging is also a hot forged material. The "material for turbine blades" refers to a material that has been subjected to the third heat treatment process defined in the present invention. The present invention will be described below in the order of the manufacturing steps.

[0009] <Hot forging process> First, a rough surface made of Fe-based alloy formed into a predetermined shape is prepared. The surface of the prepared rough surface is preferably coated with a glass lubricant. Coating with a glass lubricant has advantages such as a heat retention effect of the hot forging material, improved lubricity by lowering 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 to be coated may be about 300 to 450 μm, and it is preferable to coat the entire rough surface. Then, the rough surface is heated to a predetermined temperature. The range of the hot forging temperature may vary slightly depending on the composition of the Fe-based alloy, but in the case of precipitation hardened stainless steel or martensitic stainless steel, it is sufficient to use a range of approximately 950 to 1100 ° C. The heated rough material (forging material) is placed on the lower die of a hot forging device, and is formed into a turbine blade shape by the upper and lower dies to produce a hot forged material. The hot forging device used is not particularly limited, but a hydraulic hot forging device is sufficient to produce a material for a turbine blade with a total length of 40 inches or more. If the hydraulic hot forging device is used, it is possible to form the material into a turbine blade shape with one blow (one pressing). Forming by one-blow hot forging stabilizes the forging conditions, thereby obtaining more stable tensile strength and ductility compared to multi-heat formed products. In addition, since there is no need to perform multiple heating, coarse grain growth is suppressed, and a martensite structure with more uniform and fine grains can be obtained, which is effective in improving the grain size regulations and mechanical properties that are strictly required for turbine blades. In addition, one-blow hot forging is applied because one-blow forming improves productivity and reduces environmental load compared to multi-heat forming.

[0010] <First heat treatment process> Next, in the present invention, the hot forged material is subjected to a first heat treatment process in which the material is heated and held at a temperature range of 450 to 900°C and then cooled. The hot forged material formed into a turbine blade shape undergoes martensitic transformation, resulting in a high strength and low toughness state, and if the material is left as hot forged, quench cracks may occur. Therefore, after hot forging is completed, the first heat treatment process is applied to the hot forged material whose surface temperature is 50 to 150°C. If the temperature of this first heat treatment process is less than 450°C, quench cracks cannot be prevented. In addition, in a temperature range exceeding 900°C, the effect of preventing forging cracks is saturated. In addition, in a temperature range exceeding 900°C, grain growth may occur in the first heat treatment process, and the coarse grains may remain even after the second heat treatment, and the desired mechanical properties may not be obtained. Therefore, the temperature range of the first heat treatment process is set to 450 to 900°C. In the present invention, the heating holding time of the first heat treatment step is not particularly specified, but it may be about 3 to 6 hours. In addition, the cooling of the post-forging heat treatment step is preferably air-cooled or cooled at a cooling rate slower than air-cooled. This is because cooling at a rate faster than air-cooled may result in uneven cooling rates and deformation of the hot forged material. It is preferable to apply furnace cooling with a cooling rate slower than air-cooled. With furnace cooling, it is possible to more reliably prevent cracks in the hot forged material while suppressing deformation of the hot forged material. The intermediate material after this first heat treatment step is referred to as the "first heat-treated material".

[0011] Among the temperature ranges of this first heat treatment step, if a temperature range exceeding 700 ° C and not exceeding 900 ° C is selected, in addition to the effect of preventing cracking of the hot forged material described above, the non-uniform forging strain caused by hot forging can be reduced, and the crystal grains obtained after the second heat treatment step described later can be made even more uniform and fine, making it possible to improve the mechanical properties more uniformly. This first heat treatment step performed in the temperature range of 700 to 900 ° C is referred to as the "high temperature side first heat treatment step". The preferred lower limit of the temperature of this high temperature side first heat treatment step is 750 ° C, more preferably 765 ° C. The more preferred upper limit of the temperature is 810 ° C, more preferably 795 ° C. When this high temperature side first heat treatment step is applied, it is preferable to apply furnace cooling with a cooling rate slower than air cooling. Among the temperature ranges of the first heat treatment step, the one performed in the range of 450 to 700°C is referred to as the "low-temperature side first heat treatment step." The main purpose of the low-temperature side first heat treatment step is to prevent the above-mentioned quench cracking. When the main purpose is to prevent quench cracking, the preferable lower limit of the temperature is 620°C, and the preferable upper limit of the temperature is 680°C. When this low-temperature side first heat treatment step is applied, the cooling rate may be air cooling or furnace cooling, which is slower than air cooling. The above-mentioned high-temperature side first heat treatment step and low-temperature side first heat treatment step may be selected appropriately depending on the purpose, but for example, if the cooling of the first heat treatment step is performed by furnace cooling, the heating furnace will be occupied for a long time. In particular, if the high-temperature side first heat treatment step is selected, the heating furnace will be occupied for a long time, so in a mass production process, it is practical to select the processing temperature of the first heat treatment step taking into consideration the heating furnace occupation time. In addition, in the first heat treatment step, for example, the high-temperature side first heat treatment step may be combined after the low-temperature side first heat treatment step.

[0012] <Second heat treatment process (quenching)> In the present invention, the first heat-treated material after the first heat treatment step is subjected to a second heat treatment step in which the material is heated and held at a temperature in the range of 980 to 1080° C. and then cooled. This step is the same as the above-mentioned “quenching”, and therefore will be referred to as “quenching” hereinafter. In the present invention, the quenching temperature is set to 980 to 1080 ° C., because if the quenching temperature is less than 980 ° C., the carbide does not fully dissolve in the solid solution, and if the quenching temperature exceeds 1080 ° C., the grains may become coarse and the mechanical properties may deteriorate due to high temperature holding. Therefore, in the present invention, quenching is performed by heating and holding in the temperature range of 980 to 1080 ° C. The lower limit of the quenching temperature is preferably 1010 ° C., and the upper limit of the quenching temperature is preferably 1050 ° C. In this quenching, the heat pattern during heating may be multi-staged, i.e., two to four stages. Since the first heat-treated material is in the shape of a turbine blade, the thin blade portion and the thick root portion are integrated. In order to heat the first heat-treated material in a nearly uniform state, it is preferable to heat it in multiple stages. For example, if the first heat-treated material is to be a turbine blade of 40 inches or more, it is preferable to heat it in three or more stages, and the heat pattern may be appropriately changed depending on the size of the first heat-treated material. The holding time for quenching is not particularly specified, but it should be about 0.5 to 1.5 hours. The heating holding time here refers to the time when the temperature is at the highest temperature (quenching temperature), and does not include the time for holding the temperature during the temperature rise to the quenching temperature.

[0013] In addition, in the cooling of this quenching process (second heat treatment process), since the first heat-treated material is in the shape of a turbine blade, the thin blade portion and the thick root portion are integrated. Therefore, when cooling the first heat-treated material during quenching, in order to avoid uneven cooling rates between the blade portion and the root portion and to achieve uniform cooling rates until the completion of cooling, it is preferable to cover the thin blade portion with a heat insulating material having heat insulating properties, bring the cooling rate of the blade portion closer to the cooling rate of the root portion, and perform cooling to reduce the temperature difference between the root portion and the blade portion. The heat insulating material used here is preferably a flexible inorganic fiber. Note that the "inorganic fiber" referred to in the present invention includes glass fiber, ceramic fiber, etc., and it is preferable to select a ceramic fiber with excellent heat insulating properties. Among ceramic fibers, for example, Kaowool (registered trademark) is particularly preferable because it is easy to obtain, inexpensive, and easy to adjust the thickness of the coating according to the thickness of the blade portion to be cooled. As for the cooling during quenching, as mentioned above, the cooling rate is made uniform from the high temperature region of 980 to 1080°C, so it is preferable to cool the root part of the first heat-treated material covered with a heat insulating material by air cooling or at a cooling rate faster than air cooling, and adjust the cooling rate of the wing part so that it approaches the cooling rate of the root part. This is to suppress the variation in mechanical properties, and in particular to achieve a good balance between strength and ductility. Hereinafter, the intermediate material after the quenching process (after the second heat treatment process) will be referred to as the "quenched material" or the "second heat-treated material."

[0014] <Third heat treatment process (tempering)> In the present invention, the quenched material is subjected to a third heat treatment step, which is sometimes called "tempering", and will be referred to as "tempering" hereinafter. The reason why the tempering temperature is set to 500 to 600 ° C in the present invention is that by heating and holding in this temperature range, the untransformed austenite remaining after quenching is promoted to be transformed into martensite, and the carbon dissolved in the supersaturated state is precipitated as carbide, thereby obtaining a forged material with a good balance between strength and ductility. If the tempering temperature is less than 500 ° C, there is a problem that the untransformed austenite is not promoted to be transformed into martensite. On the other hand, if the tempering temperature exceeds 600 ° C, there is a problem that the balance between strength and ductility is lost. Therefore, in the present invention, the tempering is performed by heating and holding in the temperature range of 500 to 600 ° C. The preferable lower limit of the tempering temperature is 540 ° C, and the preferable upper limit of the tempering temperature is 570 ° C. It is preferable that the temperature rise in this tempering is also a multi-stage heat pattern, as in the above-mentioned quenching. The holding time for tempering is not particularly specified, but it may be about 2 to 5 hours. The heating and holding time referred to here is the time when the temperature is at the highest temperature (tempering temperature), and does not include the time during which the temperature is held during the temperature rise to the tempering temperature. In addition, the cooling in the tempering process is preferably air-cooled or cooled at a cooling rate slower than air-cooled. This is to balance strength and ductility. Air-cooling is preferable. Since the tempering temperature is lower than the quenching temperature described above, it is not necessarily required to cover the thin blade portion with a heat insulating material having heat insulating properties during the cooling in tempering, as in the cooling in the quenching described above.

[0015] In the present invention, the tempering can be repeated two or more times. By repeating the tempering two or more times, the untransformed austenite remaining after quenching is reliably promoted to be transformed into martensite, and the amount of untransformed austenite is reduced to as close to zero as possible. Therefore, it is preferable to repeat the tempering two or more times. The upper limit of the number of times tempering can be performed is a maximum of three times. Even if tempering is performed more than three times, it is not expected that the effect of the repeated tempering will be further enhanced. Two times is preferable. According to the present invention, by optimizing the manufacturing conditions after hot forging, it is possible to make the crystal grains of the turbine blade material more uniform and fine, and improve the mechanical properties. The manufacturing method of the turbine blade material of the present invention is effective for those having a martensitic stainless steel composition. The above-mentioned "martensitic stainless steel" is a steel containing 10.5% or more Cr that can be made into a martensitic structure by the above-mentioned heat treatment and can be hardened by making it into a martensitic structure. EXAMPLES

[0016] The present invention will be described in detail below with reference to examples. As the Fe-based alloy rough material, a Fe-based alloy rough material of martensitic stainless steel (improved steel of JIS standard SUS403) was prepared. The rough material was formed into a predetermined shape by hot forging. The rough material was heated to 80 to 90°C, and the entire surface of the rough material was coated with a glass lubricant to a thickness of about 300 to 450 μm. The rough material was heated and held at 950 to 1100°C to prepare a forging material. The forging material was placed on the lower die of a hydraulic hot forging machine, and the upper and lower dies performed one-blow hot forging to form a 40-inch turbine blade shape to produce the hot forged material (No. 1) of the present invention. As a conventional example, a raw material made of an Fe-based alloy having the same composition as above was subjected to hot die forging and reheating multiple times to be formed into a 40-inch turbine blade shape to produce a conventional hot forged material (No. 11).

[0017] The hot forged material was heat-treated under the conditions shown in Table 1 below to obtain a material for a turbine blade. The first heat treatment step of the present invention was a low-temperature first heat treatment step, and after hot forging, it was confirmed that the surface temperature of the hot forged material was in the range of 50 to 150 ° C., and then the material was placed in a heating furnace for the first heat treatment step. The heat pattern during the temperature rise for quenching was a multi-stage process with three stages, the first stage being held at 700 to 800 ° C. for 0.5 to 2 hours, the second stage being held at 1000 ° C. for 15 to 30 minutes, and then the third stage (quenching temperature) was raised to 1030 ° C. The temperature rise rate from the first stage to the third stage was 100 to 150 ° C. / hour. In addition, during cooling for quenching, the blade portion was covered with a heat insulating material (Kawool) having heat insulating properties, and the thickness was adjusted to make the cooling rate with the root portion uniform. Table 1 shows the quenching temperature. The heat pattern for tempering was a three-stage process, with the first stage held at 350-450°C for 0.5-2 hours, the second stage held at 510-540°C for 0.5-2 hours, and then the third stage (tempering temperature) was raised to 545°C (560°C for the second stage). The heating rate from the first stage to the third stage was 50-100°C / hour. The steel was air-cooled after quenching. The tempering temperatures are shown in Table 1. In Table 1, "AC" stands for air cooling, and "FC" stands for furnace cooling.

[0018] [Table 1]

[0019] Tensile test pieces and Charpy impact test pieces were taken from the above turbine blade materials and mechanical tests were performed in accordance with ASTM-A370. The test pieces were taken from the center of the product thickness of the root and airfoil parts for the tensile test pieces, and the Charpy impact test pieces were taken from the center of the product thickness of the root part. The mechanical test results 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 the conventional example. The impact value of the root of the turbine blade material was greatly improved to 40J or more.

[0020] The cross-sectional metal structures of the turbine blade material of the present invention (No. 1) and the turbine blade material of the conventional example (No. 11) are shown in Fig. 1 and Fig. 2, respectively. Since the metal structures of both the present invention and the conventional example are martensite structures, the grain size number was read at the prior austenite grain boundary. The grain size number, which indicates that the larger the numerical value, the smaller the grains, was measured according to the provisions of ASTM-E112. As shown in Fig. 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 turbine blade material of the conventional example shown in Fig. 2 was 4.5 to 5.0. In addition, the maximum grain size number of the turbine blade material of the present invention was 4.0 to 5.0, while that of the turbine blade material of the conventional example was 3.0. From these results, it was confirmed that both the average grain size number and the maximum grain size number of the turbine blade material of the present invention were finer. The test pieces for observing the metal structure were taken from the center of the product thickness of the root and blade parts of the turbine blade material. In the present invention, the turbine blade shape is formed by one-blow hot forging, so that the grain growth during the hot forging process can be suppressed compared to the conventional example in which the turbine blade shape is formed by repeating die forging and reheating multiple times. By carrying out the 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 from coarsening to a range below 3.5, resulting in a finer grain size. As a result, the grains are uniformly fine without coarsening, and it is believed that the impact value of the turbine blade material has been significantly improved.

[0021] [Table 2]

[0022] As described above, according to the present invention, in the method for producing a Fe-based alloy material for turbine blades, it is found that by optimizing the manufacturing conditions after hot forging, it is possible to make the crystal grains of the material for turbine blades even finer and more uniform, and to improve the mechanical properties.

Claims

1. A hot forging process in which a raw material made of an Fe-based alloy having a composition of precipitation hardened stainless steel or martensitic stainless steel is formed into a turbine blade shape by one-blow hot forging to obtain a hot forged material; 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 the same; A second heat treatment step of heating and holding the first heat-treated material after the first heat treatment step in a temperature range of 980 to 1080 ° C. and then cooling the material, and a third heat treatment step of heating and holding the material in a temperature range of 500 to 600 ° C. and then cooling the material; A method for producing a material for a turbine blade, comprising the steps of:

2. The method for manufacturing a turbine blade blank according to claim 1, wherein the third heat treatment step is repeated two or more times.

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