Manufacturing method of turbine blade blank

Abstract

To provide a manufacturing method of a turbine blade blank capable of much more uniformly fining crystal grains of a turbine blade blank and improving mechanical characteristics by optimizing manufacturing conditions on and after hot forging in a manufacturing method of a turbine blade blank made of an Fe-based alloy.SOLUTION: A manufacturing method of a turbine blade blank includes: a hot forging process for molding a preform made of an Fe-based alloy having the composition of precipitation hardening stainless steel or martensitic stainless steel into a turbine blade shape having a root and a vane by means of a one-blow hot forging to produce a hot forging material; a first heat treatment process for cooling the hot forging material after heating and holding it over a temperature range of 450 to 900°C; and a third heat treatment process for cooling the first heat treatment material after the first heat treatment process after heating and holding it over a temperature range of 980 to 1080°C, after a second heat treatment cooling the first heat treatment material after the first heat treatment process for cooling the first heat treatment material after heating and holding it over a temperature range of 500 to 600°C. The cooling of the second heat treatment process is performed by cladding the vane with an adiabatic a 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 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 material made of an Fe-based alloy formed into a desired shape by hot forging is solution heat-treated (the solution heat treatment may be referred to as "quenching" and will hereinafter be referred to as "quenching" in the present invention) and tempered to obtain a material for a turbine blade, although the required characteristics are satisfied, there are problems such as, for example, a slightly large crystal grain size or a tendency for the balance between strength and toughness to be slightly deteriorated. The invention of the material for a turbine blade made of the above-described Fe-based alloy aims to control the quenching and heat treatment conditions after quenching to adjust to the characteristics and shape required for the turbine blade, and most of the conventionally proposed ones focus on this quenching and heat treatment after quenching. However, in order to solve the above problems, it is important to optimize the manufacturing conditions including those before quenching. However, at present, the optimization of the manufacturing conditions before quenching, that is, after hot forging, has been insufficiently studied. An object of the present invention is to provide a method for manufacturing a material for a turbine blade made of an Fe-based alloy, which can further uniformly refine the crystal grains of the material for a turbine blade and improve the mechanical properties by optimizing the manufacturing conditions after hot forging.

Means for Solving the Problems

[0005] The present invention has been made in view of the above-described problems. That is, the present invention includes a hot forging step of forming a blank made of an Fe-based alloy having a composition of precipitation hardening stainless steel or martensitic stainless steel into a turbine blade shape having a root portion and a blade portion by one-blow hot forging to obtain a hot-forged material, a first heat treatment step of heating and holding the hot-forged material in a temperature range of 450 to 900°C and then cooling, and after 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, a third heat treatment step of heating and holding in a temperature range of 500 to 600°C and then cooling, wherein the cooling in the second heat treatment step is a method for manufacturing a material for a turbine blade in which the blade portion is covered with a heat insulating material. In the present invention, it is preferable to repeat the third heat treatment step two or more times.

Advantages of the Invention

[0006] According to the present invention, by optimizing the manufacturing conditions after hot forging, it is possible to further uniformly refine the crystal grains of the material for turbine blades, and improve the mechanical properties.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0008] First, the terms defined in the present invention will be explained. The "Fe-based alloy" targeted by the present invention refers to an alloy that contains the most Fe among the components contained, and is typically an alloy having the composition of precipitation hardening stainless steel or martensitic stainless steel. In addition, the "hot forging material" referred to in the present invention is a material formed into a predetermined shape by hot forging and used as a material for the first heat treatment step. For example, a material whose shape is adjusted such as trimming after hot forging is also a hot forging material. Further, the "material for turbine blades" refers to a material after the third heat treatment step defined in the present invention. Hereinafter, the present invention will be described in the order of the manufacturing process.

[0009] <Hot Forging Process> First, prepare a blank made of Fe-based alloy formed into a predetermined shape. It is preferable to coat the surface of the prepared blank with a glass lubricant. Coating with a glass lubricant has advantages such as improving the heat preservation effect of the material for hot forging, enhancing lubricity by reducing the friction coefficient between the forging material and the mold, and suppressing 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 blank. Then, heat it to a predetermined temperature. The range of the hot forging temperature may vary slightly depending on the composition of the Fe-based alloy. In the case of precipitation hardening stainless steel or martensitic stainless steel, it may be in the range of approximately 950 to 1100 °C. Place the heated blank (forging material) on the lower die of the hot forging apparatus, and form it into a turbine blade shape using the upper die and the lower die to obtain a hot forged material. The hot forging apparatus to be used is not particularly limited. However, for a material for a turbine blade with a total length of 40 inches or more, a hydraulic hot forging apparatus may be sufficient. If it is a hydraulic hot forging apparatus, it is possible to form it into a turbine blade shape in one blow (one pressing). When forming by one-blow hot forging, the forging conditions are stable, so more stable tensile strength and ductility can be obtained compared to multi-heat formed products. Also, since it is not necessary to perform heating multiple times, coarsening of grain growth can be suppressed, and the crystal grains can be made into a more uniform and fine martensite structure, which is effective in improving the crystal grain size regulation and mechanical properties strictly required for turbine blades. In addition, one-blow forming can improve productivity and reduce the environmental load compared to multi-heat forming, so one-blow hot forging is applied.

[0010] <First heat treatment step> Next, in the present invention, a first heat treatment step is performed in which the hot forging material is heated and held in a temperature range of 450 to 900 °C and then cooled. In the hot forging material formed into the shape of a turbine blade, martensite transformation occurs, resulting in a state of high strength and low toughness. If left as it is after hot forging, there is a possibility of burn cracking. Therefore, after the completion of hot forging, the first heat treatment step is applied to the hot forging material whose surface temperature has reached 50 to 150 °C. If the temperature of this first heat treatment step is less than 450 °C, burn cracking cannot be prevented. Also, in the temperature range exceeding 900 °C, the effect of preventing forging cracks saturates. Further, in the temperature range exceeding 900 °C, grain growth may occur in the first heat treatment step, 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 step is set to 450 to 900 °C. In the present invention, the holding time of the first heat treatment step is not particularly defined, but it may be approximately 3 to 6 hours. Also, for the cooling in the post-forging heat treatment step, it is preferable to cool at an air-cooling rate or a cooling rate slower than air-cooling. This is because if cooling is performed at a rate faster than air-cooling, the cooling rate becomes non-uniform and deformation may occur in the hot forging material. Preferably, furnace cooling with a cooling rate slower than air-cooling is applied. With furnace cooling, it is possible to more reliably prevent cracking of the hot forging material and also suppress deformation of the hot forging material. Note that the intermediate material after this first heat treatment step is referred to as the "first heat treated material".

[0011] If a temperature range exceeding 700 °C and less than or equal to 900 °C is selected within the temperature range of this first heat treatment step, in addition to the effect of preventing cracking of the above-mentioned hot forging material, the non-uniform forging strain generated during hot forging can be reduced, and the crystal grains obtained after the subsequent second heat treatment step and later can be made more uniformly fine, and it becomes possible to improve the mechanical properties more uniformly. The first heat treatment step performed in this temperature range of 700 to 900 °C is referred to as the "high-temperature side first heat treatment step". The lower limit of the preferable temperature of this high-temperature side first heat treatment step is 750 °C, and more preferably 765 °C. The upper limit of the more preferable temperature is 810 °C, and more preferably 795 °C. When applying this high-temperature side first heat treatment step, 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 carried out in the range of 450 to 700 °C is referred to as the "low-temperature side first heat treatment step". The low-temperature side first heat treatment step mainly aims to prevent the above-mentioned burning cracks. When mainly aiming to prevent burning cracks, the lower limit of the preferable temperature is 620 °C, and the upper limit of the preferable temperature is 680 °C. When applying this low-temperature side first heat treatment step, the cooling rate can be air cooling or furnace cooling slower than air cooling. The selection of the above-mentioned high-temperature side first heat treatment step and low-temperature side first heat treatment step may be appropriately selected according to the purpose. For example, when the cooling of the first heat treatment step is furnace cooling, the heating furnace will be occupied for a long time. In particular, when the high-temperature side first heat treatment step is selected, the occupation time of the heating furnace becomes long. Therefore, in the mass production process, it is realistic to select the treatment temperature of the first heat treatment step in consideration of the occupation time of the heating furnace. 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 Step (Quenching)> In the present invention, a second heat treatment step is performed on the first heat treatment material after the first heat treatment step, which is heated and held in the temperature range of 980 to 1080 °C and then cooled. Since this step is the same treatment as the above-mentioned "quenching", it will be hereinafter referred to as "quenching". In the present invention, the quenching temperature is set to 980 to 1080 °C because when the quenching temperature is less than 980 °C, it is a temperature range where carbides do not dissolve sufficiently, and when it exceeds 1080 °C, there is a risk of grain coarsening and deterioration of mechanical properties due to holding at high temperatures. 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 preferred quenching temperature is 1010 °C, and the upper limit of the preferred quenching temperature is 1050 °C. In this quenching, the heating pattern during temperature rise may be a multi-stage of two to four stages. Since the first heat treatment material has a turbine blade shape, the thin blade part and the thick root part are integrated. In order to raise the temperature of the first heat treatment material in a state close to uniform, it is preferable to raise the temperature in multiple stages. For example, if it is a turbine blade of 40 inches or more, it is better to use three or more stages, and the heat pattern may be appropriately changed according to the size of the first heat treatment material. The holding time of quenching is not particularly specified, but it may be approximately 0.5 to 1.5 hours. Here, the heating and holding time refers to the time at the highest temperature (quenching temperature), and the temperature holding time during the temperature rise to the quenching temperature is not included.

[0013] Also, in the cooling of this quenching process (second heat treatment process), since the first heat treatment material has a turbine blade shape, the thin blade part and the thick root part are integrated. Therefore, when cooling the first heat treatment material during quenching, in order to avoid non-uniform cooling rates between the blade part and the root part and to achieve uniformization of the cooling rate until cooling is completed, it is good to cover the thin blade part with a heat insulating material having heat insulating properties, approximate the cooling rate of the blade part to that of the root part, and perform cooling to reduce the temperature difference between the root part and the blade part. The heat insulating material used here is preferably flexible inorganic fiber. The "inorganic fiber" referred to in the present invention includes glass fiber, ceramic fiber, etc., and it is preferable to select ceramic fiber with excellent heat insulating properties. Among ceramic fibers, for example, Kaowool (registered trademark) etc. are particularly preferable because they are easily available, inexpensive, and it is easy to adjust the covering thickness according to the thickness of the blade part to be cooled. Regarding the cooling during quenching, as described above, since cooling is performed to make the cooling rate uniform from the high-temperature range of 980 to 1080°C, the root of the first heat treatment material coated with the heat insulating material is cooled by air cooling or at a cooling rate faster than air cooling, and it is preferable to adjust the cooling rate of the wing portion so as to approach the cooling rate of the root portion. This is to suppress variations in mechanical properties and, in particular, to adjust the 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 treatment material".

[0014] <Third heat treatment process (tempering)> In the present invention, a third heat treatment process is performed on the quenched quenched material. This is sometimes referred to as "tempering", and hereinafter will be referred to as "tempering". In the present invention, the tempering temperature is set to 500 to 600°C. By heating and holding in this temperature range, it is possible to promote the martensitic transformation of the retained austenite remaining after quenching, and to precipitate the carbon supersaturated in solid solution as carbides to obtain 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 martensitic transformation of the retained 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 lost. Therefore, in the present invention, tempering is performed by heating and holding in the temperature range of 500 to 600°C. The lower limit of the preferable tempering temperature is 540°C, and the upper limit of the preferable tempering temperature is 570°C. Regarding the temperature increase during this tempering, similarly to the quenching described above, it is preferable to adopt a multi-stage heat pattern. Although the holding time of tempering is not particularly defined, it may be approximately 2 to 5 hours. Here, the heating and holding time refers to the time when the highest temperature (tempering temperature) is reached, and the temperature holding time during the temperature increase to the tempering temperature is not included. Also, for the cooling in this tempering process, it is preferable to perform air cooling or cooling at a cooling rate slower than air cooling. This is for adjusting the balance between strength and ductility. Preferably, air cooling is performed. Note that 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 during the cooling of tempering as in the case of the cooling during quenching.

[0015] Also, in the present invention, this tempering can be repeated two or more times. By repeating it two or more times, the martensitic transformation of the retained austenite remaining after quenching is surely promoted, and this is for making the retained austenite as close to zero as possible. Therefore, in tempering, it is preferable to repeat it two or more times. Note that the upper limit of the number of times of performing tempering may be at most three times. Even if tempering is performed more than three 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. Note that 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 making it into 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 an Fe-based alloy of 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 above-mentioned forging material was placed on the lower die of a hydraulic hot forging device and formed into a 40-inch turbine blade shape by one-blow hot forging with the upper die and the lower die to obtain the hot forging material (No.1) of the present invention. Also, as a conventional example, using a blank made of an Fe-based alloy having the same composition as above, hot die forging and reheating were repeated a plurality of times to form it into a 40-inch turbine blade shape to obtain the hot forging material (No.11) of the conventional example.

[0017] Regarding the above-mentioned hot forging material, heat treatment was performed under the conditions shown in Table 1 below to obtain a material for turbine blades. The first heat treatment step of the present invention is a low-temperature side first heat treatment step. After hot forging is completed, it is confirmed that the surface temperature of the hot forging material is in the range of 50 to 150°C, and it is put into a heating furnace for the first heat treatment step. The heat pattern during the heating-up of quenching is a three-stage multi-stage treatment. The first stage is held at 700 to 800°C for 0.5 to 2 hours, the second stage is held at 1000°C for 15 to 30 minutes, and then heated to 1030°C in the third stage (quenching temperature). The heating rate from the first stage to the third stage is 100 to 150°C / hour. Also, during the cooling of quenching, while covering the blade part with a heat insulating material (kaowool) having heat insulation properties and adjusting its thickness, the cooling rate with the root part was made uniform. What is shown in Table 1 is the quenching temperature. Also, the heat pattern during the heating-up of tempering is also a three-stage multi-stage treatment. The first stage is held at 350 to 450°C for 0.5 to 2 hours, the second stage is held at 510 to 540°C for 0.5 to 2 hours, and then heated to 545°C (560°C for the second time) in the third stage (tempering temperature). The heating rate from the first stage to the third stage is 50 to 100°C / hour. Also, during the cooling of quenching, it was air-cooled as it was. What is shown in Table 1 is the tempering temperature. Note that "AC" in Table 1 is air-cooling and "FC" is furnace-cooling.

[0018]

Table 1

[0019] Tensile test specimens and Charpy impact test specimens were taken from the above-described material for turbine blades, and mechanical tests were conducted in accordance with ASTM-A370. The sampling positions of the test specimens were as follows: for the tensile test specimens, the centers of the product thicknesses at the root and the blade section; for the Charpy impact test specimens, the center of the product thickness at the root. The test specimens were taken from those positions. The results of the mechanical tests are shown in Table 2. As shown in Table 2, the 0.2% proof stress, tensile strength, elongation, and reduction of area of the material for turbine blades of the present invention were almost the same as those of the conventional example. The impact value at the root of the material for turbine blades was significantly improved to 40 J or more.

[0020] The cross-sectional metal microstructures of the material for turbine blades (No. 1) of the present invention and the material for turbine blades (No. 11) of the conventional example are shown in FIGS. 1 and 2, respectively. Since both the present invention and the conventional example exhibit a martensite microstructure, the crystal grain size number was read at the prior austenite grain boundaries. The crystal grain size number indicates that the larger the numerical value, the smaller the crystal grains, and it was measured in accordance with the provisions of ASTM-E112. As shown in FIG. 1, the average crystal grain size number of the material for turbine blades of the present invention was 6.0 to 7.0, whereas the average crystal grain size number of the material for turbine blades of the conventional example shown in FIG. 2 was 4.5 to 5.0. Also, the maximum crystal grain size number was 4.0 to 5.0 for the material for turbine blades of the present invention, whereas it was 3.0 for the material for turbine blades of the conventional example. From these results, it was confirmed that the material for turbine blades of the present invention had finer average and maximum crystal grain size numbers. The sampling positions of the test specimens for metal microstructure observation were the centers of the product thicknesses at the root and the blade section of the material for turbine blades. Since the present invention is formed into a turbine blade shape by one-blow hot forging, it is possible to suppress the growth of crystal grains in the hot forging process as compared with the conventional example in which die forging and reheating are repeated a plurality of times to form into a turbine blade shape. And, by performing appropriate first heat treatment step, second heat treatment step, and third heat treatment step, the average crystal grain size number of the material for turbine blades becomes finer than 5.0, and it is possible to suppress coarsening to a range below 3.5 even in the maximum crystal grain size, and to obtain a finer crystal grain diameter. Because of these, since the crystal grains were uniform and fine without coarsening, it is considered that the impact value of the material for turbine blades was significantly improved.

[0021]

Table 2

[0022] As described above, according to the present invention, in the method for manufacturing a material for turbine blades made of an Fe-based alloy, by optimizing the manufacturing conditions after hot forging, it can be seen that the crystal grains of the material for turbine blades can be made more uniform and finer, and the mechanical properties can be improved.

Claims

1. a hot forging step of forming a raw material made of an Fe-based alloy having a composition of precipitation hardening stainless steel or martensitic stainless steel into a turbine blade shape having a root portion and a blade portion 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; The first heat-treated material after the first heat treatment step is heated and held in a temperature range of 980 to 1080 ° C. and then cooled in a second heat treatment step, and then heated and held in a temperature range of 500 to 600 ° C. and then cooled in a third heat treatment step, A manufacturing method of a turbine blade material, wherein the cooling in the second heat treatment process includes covering the airfoil portion with a heat insulating material.

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

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