JP7540368B2 - Manufacturing method for turbine blade materials - Google Patents

Description

The present invention relates to a method for manufacturing a material for a turbine blade in which the blade portion and the root portion are integrally molded.

The turbine blade material, in which the blade portion and the root portion are integrally molded, is formed into a predetermined shape by hot working such as hot forging. This turbine blade material has a complex twisted three-dimensional shape, and is made into a hot-formed material by hot working such as hot forging, and then is formed into a turbine blade by heat treatment and machining.
The above-mentioned turbine blade material has different thicknesses at the root and the blade, and it is difficult to adjust the cooling rate to a uniform rate when performing a predetermined heat treatment. In addition, if the material has a martensitic stainless steel composition, for example, when the hot-formed material is quenched, it passes through the transformation point from austenite to martensite. In this case, the turbine blade material, whose thickness varies from part to part, has a timing at which the transformation occurs that varies partially within the integral body. For this reason, cooling methods have been studied to reduce the temperature difference within the hot-formed material during heat treatment.
For example, JP 2015-74822 A (Patent Document 1) discloses two cooling methods. These methods divide the hot-formed material that will become the raw material for turbine blades into a "large surface area portion" and a "small surface area portion" and cool them. The first method adjusts the amount of air for cooling with a fan. The second method is to cover the thin-walled portion of the blade that will become the large surface area portion with a coating material in advance. This reduces the temperature difference between the large surface area portion and the small surface area portion during cooling.

JP 2015-74822 A

The method disclosed in the above-mentioned Patent Document 1 is to reduce the temperature difference in the hot-formed material near the phase transformation region in the so-called quenching cooling process. However, as shown in Fig. 7 of Patent Document 1, when passing through the phase transformation region and entering the low temperature side, the temperature difference widens again. Furthermore, if a large-area portion is to be covered with a metallic covering material that is attached in advance as in Patent Document 1, machining is required to match the blade shape of the turbine blade material, and further, it becomes difficult to know whether the portion covered with the covering material has been heated to the specified temperature.
Meanwhile, high 0.02% proof stress is required for turbine blades used at high temperatures. The invention of the above-mentioned Patent Document 1 does not take into consideration the improvement of the 0.02% proof stress. Moreover, the cooling method shown in Patent Document 1, which reduces the temperature difference only in the transformation point region, is insufficient in improving the 0.02% proof stress.
The object of the present invention is to provide a method for manufacturing coarse turbine blade material, which can reduce the temperature difference within the hot-formed material during the hardening process of the hot-formed material formed into the shape of the turbine blade material, and can improve the 0.02% yield strength of the turbine blade material after heat treatment.

The inventors conducted extensive research into the relationship between the cooling conditions during quenching and the 0.02% yield strength of hot-formed martensitic stainless steel materials with a composition that passes through the martensite-austenite transformation point. As a result, they discovered that in order to improve the 0.02% yield strength, it is necessary to reduce the difference in surface temperature between the wing and root in the temperature range below the Ms point, and then, when the surface temperatures of both the wing and root reach a specific temperature, to start heating to the tempering temperature in the next process, thereby achieving the present invention.

That is, the present invention is a method for manufacturing a turbine blade material, which includes a quenching step in which a hot-formed material having a martensitic stainless steel composition, in which an airfoil portion and a root portion are integrally molded, is heated to a quenching temperature, held thereat, and then cooled to obtain a quenched material, and a tempering step in which the quenched material is heated to a tempering temperature, held thereat, and then cooled to obtain a tempered material, wherein during the cooling step of the quenching step, the difference in surface temperature between the airfoil portion and the root portion is kept within 100°C within a temperature range below the Ms point, and heating to the tempering temperature is initiated when the surface temperatures of both the airfoil portion and the root portion reach a temperature between room temperature and 100°C.
Preferably, in this method for manufacturing a turbine blade material, the cooling for quenching is performed in two or more stages, the surface temperature difference between the airfoil portion and the root portion is kept within 100°C within a temperature range not exceeding the Ms point, and when the surface temperatures of both the airfoil portion and the root portion reach a temperature between room temperature and 100°C, heating to a tempering temperature is started in final cooling for quenching.
More preferably, the first cooling step in the quenching process is oil cooling, in which the hot-formed material is immersed in an oil bath and then pulled out, and then the blade portion of the hot-formed material pulled out of the oil bath is covered with inorganic fiber for final cooling in this manufacturing method for turbine blade materials.

According to the present invention, even in the case of a turbine blade material in which the root and blade portions have different thicknesses, it is possible to reduce the temperature difference within the hot-formed material in the temperature range below the Ms point, and by starting the next tempering heating process in an appropriate temperature range, it is possible to improve the 0.02% yield strength.

FIG. 4 is a diagram showing the relationship between the temperature at which a hot-formed material is removed from the quenching temperature and the cooling time.

A typical turbine blade material targeted by the present invention is a hot forged material in which the root and the blade are integrally formed. The material has a martensitic stainless steel composition. Note that the "martensitic stainless steel" referred to in the present invention is a steel containing 10.5% or more Cr that can be made into a martensitic structure by heat treatment and can be hardened by making it into a martensitic structure.
In addition, in the present invention, the term "hot-formed material" refers to a material in which the blade portion and the root portion are integrally molded by hot processing such as hot forging, and the quenching is completed, and the term "turbine blade material" refers to a material that has been quenched and tempered.
The manufacturing method of the present invention will be described below in the order of steps.

<Quenching process>
In the present invention, a hot-formed material having the above composition is heated to and held at a quenching temperature, and then cooled to obtain a quenched material.
The heating and holding temperature for hardening (hardening temperature) may be 980 to 1080°C. If the hardening temperature is less than 980°C, the carbides may not be sufficiently dissolved, and if it exceeds 1080°C, the high temperature holding may cause grain coarsening and deterioration of mechanical properties. Therefore, it is preferable to heat and hold the material in the hardening process at a temperature range of 980 to 1080°C. The lower limit of the hardening temperature is preferably 1010°C, and the upper limit of the hardening temperature is preferably 1050°C.
In this quenching, the heat pattern during heating may be multi-staged, i.e., two to four stages. In order to heat the hot-formed material, which is a thin blade and a thick root, in a nearly uniform state, it is preferable to heat it in multiple stages. For example, if a turbine blade is to be made 40 inches or more, it is preferable to use three or more stages. The heat pattern may be changed appropriately depending on the size of the hot-formed material. The holding time for quenching is not particularly specified, but it may be approximately 0.5 to 2 hours. The heating holding time referred to here is the time when the highest temperature (quenching temperature) is reached, and does not include the time for holding the temperature during the temperature rise to the quenching temperature.

In addition, the cooling during quenching may be performed by directly cooling to a predetermined temperature, or by performing multi-stage cooling of two or more stages in which the cooling conditions such as the cooling medium are changed during the quenching. The cooling method is not particularly limited, and may be air cooling, water cooling, oil cooling, gas, air blast, mist, etc., but the surface temperature difference between both the wing portion and the root portion is within 100 ° C. in the temperature range below the Ms point during quenching cooling. In the present invention, the temperature range below the Ms point is set to reduce the temperature difference between the wing portion and the root portion in order to uniformize the timing at which transformation expansion due to martensitic transformation occurs in the wing portion and the root portion and to relieve the transformation stress acting inside the material, thereby suppressing the deformation of the material after quenching, and to uniformize the timing of heating to the tempering temperature described later. In the preferred temperature range below the Ms point, the surface temperature difference between both the wing portion and the root portion is within 50 ° C., more preferably within 30 ° C., and even more preferably within 20 ° C.

For example, when air-cooling from the quenching temperature to a predetermined temperature below the Ms point, if the blade is left as is, the surface temperature difference between the thin blade and the thick root will be large. In that case, it is preferable to make the cooling rate of the blade close to that of the root to reduce the surface temperature difference between the two. To achieve this, when the surface temperature of the blade, which has a faster cooling rate, reaches at least the Ms point to Ms point + 400°C, the thin blade is coated with a coating material having heat insulating properties to make the cooling rate with the root uniform. The timing of coating with this coating material varies depending on the cooling method. For example, in the case of air-cooling, the thin blade is coated with a coating material having heat insulating properties when the surface temperature reaches 100 to 400°C higher than the Ms point. It is preferable to coat with the coating material from a high temperature region of 200°C or more higher than the Ms point, and more preferably to coat with the coating material from a high temperature region of 300°C or more higher than the Ms point.

Also, for example, when performing multi-stage cooling in two or more stages from the quenching temperature to a predetermined temperature below the Ms point, it is advisable to perform the first (initial stage) cooling from the quenching temperature with a fast cooling rate such as oil cooling, mist, gas, or air blast, and then, when the blade portion reaches the Ms point to Ms point + 50° C., to cover it with the above-mentioned coating material and allow it to cool naturally in the next stage. This is to shorten the cooling time, increase productivity, and exert the effect of reducing the temperature difference between the blade portion and the root portion, which is obtained when covering with the above-mentioned coating material.
For example, when the present invention is applied to multi-stage cooling in two stages, the cooling by covering with the above-mentioned covering material is the final (last stage) cooling, and the cooling before the final cooling may be performed by a known method capable of increasing the cooling rate, such as oil cooling, mist, gas, air blast, mist, etc. It is preferable to cool to a temperature between the Ms point and about Ms point + 50°C by these known cooling methods, and then apply cooling by covering with a covering material.
In the present invention, it is preferable to apply oil cooling among the above-mentioned known cooling methods. In the case of oil cooling, the cooling rate is slower than, for example, water cooling, and therefore excessive deformation of the hot-formed material due to thermal stress caused by the difference in cooling rate between the inside and outside of the material during cooling can be prevented. In addition, even if the material has a complex shape such as a turbine blade material, the cooling medium (oil) contacts the entire surface of the material, making it possible to uniformly cool the entire surface of the material. Since cooling can be performed in one go to a predetermined temperature for coating with the above-mentioned coating material, it is preferable to select oil cooling in the present invention. When oil cooling is performed, the hot-formed material heated and held at the quenching temperature is immersed in an oil tank and pulled up, and then the wing portion of the hot-formed material pulled up from the oil tank is covered with inorganic fibers for final cooling. In other words, the process is composed of a "first stage cooling" in which the hot-formed material heated and held at the quenching temperature is immersed in an oil tank, and then a "next or final stage cooling" in which the wing portion of the hot-formed material pulled up from the oil tank after the first stage cooling is covered with inorganic fibers for final cooling.

The covering material is preferably a flexible inorganic fiber. The term "inorganic fiber" as used herein includes glass fiber, ceramic fiber, etc., and it is preferable to select ceramic fiber, which has excellent heat insulating properties. Ceramic fiber has the effect of slowing down the cooling rate of the blade during cooling, thereby keeping the blade warm, and the cooling rate of the blade can be made to approximate that of the thick root.
Among the above-mentioned ceramic fibers, for example, KAOWOOL (registered trademark) is particularly preferred because it is easy to obtain, inexpensive, and the thickness of the coating can be easily adjusted according to the thickness of the blade to be cooled. In addition, unlike the metallic coating material that is attached in advance to the large area portion as in Patent Document 1, no machining is required to match the blade blade shape. If the coating material is applied in advance, it is difficult to know whether the coated part has been heated to a predetermined temperature, but if the inorganic fiber is applied during cooling, these concerns are eliminated. In addition, the location of the inorganic fiber coating should be arranged so that it covers the entire surface of one side of the blade when the hot-formed material is placed horizontally during quenching and cooling. This is because covering the entire surface of one side of the blade can reduce the temperature difference within the blade, rather than covering only a part such as the thin part (large surface area part) of the blade with the coating material as in Patent Document 1. If the thickness of the blade near the root side and the tip of the blade is significantly different, the number of layers of inorganic fiber to be coated can be adjusted.

During the above-mentioned final cooling, when the surface temperatures of both the blade and root parts reach room temperature to 100°C, the quenching is terminated to obtain a quenched material, and heating to the tempering temperature is immediately started. This heating from the final cooling to the tempering temperature is performed when the surface temperatures of both the blade and root parts reach room temperature to 100°C. This heating to the tempering temperature does not need to be completed until the Mf point. In other words, it is preferable to perform this at a timing when a few percent of austenite remains, and according to the inventor's study, it is sufficient to perform this at a timing when the martensite is about 90 to 95%, and the preferred lower limit of the martensite amount is 92%. The preferred upper limit is 94%. The reason why heating to the tempering temperature is started at a timing when the martensite is about 90 to 95% is to improve the 0.02% proof stress, which has a positive correlation with the amount of martensite after tempering, by allowing an appropriate amount of martensite to remain after tempering. If heating to the tempering temperature is performed at a timing other than the above, the amount of martensite after tempering is likely to be less than 97%, which may lead to a decrease in the 0.02% yield strength.
In addition, when determining the Ms point or the Mf point, there is a method of taking a test piece for measuring martensite from the hardened material and measuring the amount of retained austenite using an X-ray analyzer to measure the amount of martensite, or a method of using, for example, commercially available calculation software JMatPro capable of calculating metal physical property values. There is no large discrepancy between the actual measured value using the test piece and the calculated value using the calculation software JMatPro, so it is sufficient to select either method.

<Tempering process>
In the present invention, the quenched material is tempered. As described above, tempering is performed following (continuously with) the cooling step of quenching to promote the transformation of untransformed austenite remaining after quenching into martensite.
The tempering temperature in the present invention is preferably 500 to 600°C. By heating and holding in this temperature range, it is possible to promote the transformation of untransformed austenite remaining after quenching into martensite, and to precipitate supersaturated solid-solubilized carbon as carbide, thereby obtaining a tempered material with a good balance between strength and ductility. If the tempering temperature is less than 500°C, the transformation of untransformed austenite into martensite may not be promoted. On the other hand, if the tempering temperature exceeds 600°C, the balance between strength and ductility may be lost. Therefore, in the present invention, it is preferable to perform tempering 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 tempering holding time is not particularly specified, but it is sufficient if it is approximately 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 achieve a good balance between 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 covering material during the cooling in tempering, as in the cooling in the quenching described above.

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.
In the tempered material (turbine blade material) after tempering, if the amount of martensite in the quenched material is more than approximately 95%, the martensite after quenching will reverse transform to austenite during the tempering process, and the amount of martensite in the tempered material will be less than 97%, which may lead to a decrease in the 0.02% proof stress value. Also, if the amount of martensite after quenching is less than approximately 90%, untransformed austenite will remain after quenching, and the amount of martensite after tempering will be less than 97%, which may lead to a decrease in the 0.02% proof stress value. In light of the above, it is important to consider the amount of martensite in the quenched material to be approximately 90 to 95%, and to perform appropriate tempering at an appropriate time.
According to the manufacturing method for turbine blade material described above, even in the case of a turbine blade material having different thicknesses at the root and blade portions, it is possible to reduce the temperature difference within the hot-formed material in the temperature range below the Ms point, and by starting the next tempering heating process in an appropriate temperature range, it is possible to improve the 0.02% yield strength.

Example 1
First, an experiment was conducted to confirm the effect of reducing the temperature difference within the hot-formed material in the temperature range below the Ms point, and of starting heating to the tempering temperature at an appropriate timing, which is the greatest feature of the present invention.
A 40-inch hot-formed material having a martensitic stainless steel composition equivalent to SUS420J1 was prepared. The hot-formed material was formed by integrally forming the wing and root parts by hot die forging. The martensitic stainless steel used as the hot-formed material had an Ms point of about 210°C and an Mf point below room temperature.
The hot-formed material was placed in a heating furnace for quenching. The heat pattern for quenching was a three-stage multi-stage process, with the first stage being held at 700-800°C for 0.5-2 hours, the second stage being held at 1000°C for 15-30 minutes, and then the third stage being heated to 1030°C (quenching temperature) and held for 1 hour. The heating rate from the first stage to the third stage was 100-150°C/hour.

Cooling from the quenching temperature was divided into three parts:
As an example 1 of the present invention, a hot-formed material at the quenching temperature was removed from the heating furnace, and then the blade portion was covered with inorganic fiber (KAOWOOL (registered trademark)) and allowed to cool in the air.
In Reference Example 1, the hot-formed material at the quenching temperature was taken out of the heating furnace and air-cooled in the air.
As Comparative Example 1, the hot-formed material at the quenching temperature was removed from the heating furnace and was cooled in oil.
The relationship between temperature and time during the above three cooling steps is shown in Figure 1. The temperatures shown in Figure 1 were measured using a radiation thermometer.

The effect of the present invention will be described with reference to FIG. 1. As described above, the uniform cooling rate of the present invention is adjusted so that the cooling rate of the blade side coincides with the cooling rate of the root side. From FIG. 1, it can be seen that the cooling pattern of the air cooling of Reference Example 1 and the cooling pattern of the present invention 1 cool with almost the same behavior. In the case of the normal air cooling of Reference Example 1, when the root is at about 600°C, the temperature of the blade drops to 300°C, whereas in the case of the present invention 1, it can be seen that the temperature difference between the root and the blade is within 10°C from around 600°C to around room temperature. In the case of the air cooling of Reference Example 1, the temperature difference between the root and the blade exceeds 100°C near the Ms point. In addition, it can be seen that the cooling rate of Comparative Example 1, which was subjected to oil cooling, is fast, but the temperature difference between the blade and the root is large.
The amount of martensite was approximately 95% at around 50° C. The amount of martensite was measured by calculating the Ms point and the Mf point using commercially available calculation software JMatPro capable of calculating metal physical property values, and the amount of martensite of 95% was calculated by applying a coefficient obtained from the calculation result to a theoretical formula.

In the cooling shown in Figure 1, tempering heating was started when the temperature of Inventive Example 1 reached approximately 50-60°C. The temperature difference between the blade and root at the start of heating to the tempering temperature was at a level of 5°C or less, meaning there was almost no temperature difference. For Comparative Example 1, tempering heating was started when the blade was at 20-30°C and the root was at 60-70°C.
Tempering was carried out twice in total. The first tempering was carried out by holding the material at a tempering temperature of 545°C for 2 to 4 hours, and the second tempering was carried out by holding the material at a tempering temperature of 560°C for 2 to 4 hours. The temperature increase rate during the tempering process was 50 to 100°C/hour. When cooling after tempering, the material was air-cooled or slowly cooled to obtain the tempered material (raw material for turbine blades).
Tensile test pieces were taken from the blade of the quenched and tempered turbine blade material. The pieces were taken from the root and the center of the blade, and a tensile test was performed according to the tensile test method of ASTM A 370. The results of the tensile test are shown in Table 1.
As shown in Table 1, both Example 1 of the present invention and Comparative Example 1 had excellent results of tensile strength of 1300 MPa or more. On the other hand, 0.02% of Example 1 of the present invention had excellent results of 980 MPa or more, while Comparative Example 1 had a tensile strength of less than 980 MPa. The average grain size number of the root and wing portions of Example 1 of the present invention was 4.0 by ASTM, while the average grain size number of Comparative Example 1 was 3.5. The amount of martensite of Example 1 of the present invention was 97% or more, while the amount of martensite of Comparative Example 1 was about 96%. The amount of martensite was measured using an X-ray analyzer.

Example 2
As Example 2 of the present invention, a hot-formed material was prepared having the same size, material and composition as those of Example 1. The hot forming was carried out by hot die forging to integrally form the wing portion and the root portion.
The hot-formed material was placed in a heating furnace for quenching. The heat pattern for quenching was a three-stage multi-stage process, with the first stage being held at 600-700°C for 0.5-2 hours, the second stage being held at 700-800°C for 0.5-2 hours, and the third stage being held at 1020°C for 15-30 minutes, after which the third stage was heated to 1050°C (quenching temperature) and held for approximately 1 hour. The heating rate from the first stage to the third stage was 100-150°C/hour.
The cooling from the quenching temperature was performed in two stages, with the first cooling being oil cooling, and the final cooling being done by covering the blades with inorganic fiber (KAOWOOL (registered trademark) and cooling in the air. The stopping temperature for oil cooling was set between 200 and 250°C, and the blades of the hot-formed material pulled out of the oil tank were covered with KAOWOOL (registered trademark), and tempering heating was started when the blade temperature dropped to 50°C. When the blades reached 50°C, the temperature of the root was approximately 60 to 80°C, and the temperature difference between the blades and the root was 10 to 30°C.

Tempering was carried out twice in total. The first tempering was carried out by holding the material at a tempering temperature of 550°C for 2 to 4 hours, and the second tempering was carried out by holding the material at a tempering temperature of 560°C for 2 to 4 hours. The temperature rise rate during the tempering process was 50 to 100°C/hour. When cooling after tempering, the material was air-cooled or slowly cooled to obtain the tempered material (raw material for turbine blades).
Tensile test pieces were taken from the blade of the quenched and tempered turbine blade material. The pieces were taken from the root and the center of the blade, and a tensile test was performed according to the tensile test method of ASTM A 370. The results of the tensile test are shown in Table 1.
As shown in Table 2, inventive example 2 the tensile strength was 1300 MPa or more, and in the case of 0.02% it was 980 MPa or more, which was an excellent result.

According to the manufacturing method for turbine blade materials described above, even if the thickness of the root and blade parts of the turbine blade material is different, it is possible to reduce the temperature difference within the hot-formed material in the temperature range below the Ms point, and by starting the next tempering heating process in an appropriate temperature range, it is possible to improve the 0.02% yield strength.

Claims (2)

a quenching process in which the hot-formed material having a martensitic stainless steel composition and in which the wing portion and the root portion are integrally formed is heated to a quenching temperature, maintained at that temperature, and then cooled to obtain a quenched material;
The tempering process includes heating and holding the quenched material at a tempering temperature, and then cooling the quenched material to obtain a tempered material.
The cooling for quenching is performed in two or more stages, and during the cooling for quenching, the difference in surface temperature between the airfoil portion and the root portion is kept within 100°C within a temperature range below the Ms point, and heating to a tempering temperature is initiated when the surface temperatures of both the airfoil portion and the root portion reach a temperature between room temperature and 100°C. 2. A method for manufacturing a turbine blade material as described in claim 1, wherein the first cooling in the quenching process is oil cooling, in which the hot-formed material is immersed in an oil bath and pulled out, and then the blade portion of the hot-formed material pulled out of the oil bath is covered with inorganic fiber and final cooling is performed.