Preparation method of steam cylinder bolt material for 700 DEG C ultra-supercritical steam turbine

Abstract

The invention discloses a preparation method of a 700 DEG C ultra-supercritical steam turbine cylinder bolt material, and belongs to the field of nickel-based alloys. The alloy comprises Ni-(29-31 wt%) Co-(14-16 wt%) Fe-(5-6 wt%) Al-(5-6 wt%) Nb-(5-6 wt%) Ti, the content of added Al, Ti and Nb is the same, a powder metallurgy method is adopted for preparation, by controlling the heat treatment temperature, about 33% of Ni40Co33Al10Ti8Fe3Nb6 phase (belonging to a gamma phase) and 33% of Ni53Co19Al14.5 Ti6.6 Fe4.5 Nb2. 4 (belonging to an ETA phase) can be formed in a nickel-based alloy, the two phases can stably exist within the range of 700-850 DEG C, dislocation motion is effectively hindered, and the alloy can be used as a high-temperature-resistant alloy material. And meanwhile, the matrix is a Ni and Fe solid solution strengthened Co matrix, and compared with a conventional nickel-based high-temperature alloy nickel matrix, the nickel-based high-temperature alloy nickel matrix has higher high-temperature strength. Through the synergistic effect of the two high-temperature-resistant strengthening phases and the high-temperature-resistant matrix, the alloy shows excellent performance at the temperature of 800 DEG C, the yield strength exceeds 850 MPa and is about 40% higher than that of a conventional mark such as Waspaloy at the temperature of 800 DEG C, and when the alloy is used as a steam cylinder bolt at the temperature of 700 DEG C, the safety of the steam cylinder bolt is greatly improved, and the service life of the steam cylinder bolt is greatly prolonged.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of nickel-based alloys, and relates to a preparation method of a material for 700℃ turbine cylinder bolt. BACKGROUND

[0002] In order to further improve the thermal efficiency of combustion power plants, reduce the emissions of CO2, SO X and NO X , 700℃ A-USC power station technology has become an important direction for developing "high efficiency and low consumption" coal power units in some developed countries such as Europe, the United States, Japan and the like in the world. Under the working condition of 700℃ A-USC, the cylinder bolt needs to bear a steam pressure as high as 25-30MPa and thermal stress generated by instantaneous start and stop. The traditional alloy steel will significantly creep and deform at a high temperature for a long time, resulting in relaxation of pre-tightening force. In the design of 700℃ A-USC turbine, the selection of the cylinder bolt material is directly related to the safety and economy of the unit. The nickel-based high-temperature alloy has become an irreplaceable material for the cylinder bolt of 700℃ A-USC turbine due to its excellent high-temperature strength, creep resistance, oxidation corrosion resistance and thermal expansion matching. Although the initial cost is high, the economic benefits in the whole life cycle are significant, which can support the development of the unit to a higher parameter (800℃), and is a key technical choice for ensuring the safe and efficient operation of A-USC unit. However, there is no successful development and application of related key components such as blades and fasteners of 700℃ A-USC turbine in the world. The traditional nickel-based high-temperature alloy based on carbide strengthening and γ´ phase strengthening has the problem of strength reduction caused by carbide aggregation and growth at 700℃ and higher temperature for a long time, and it is urgent to develop new materials to meet the technical requirements of A-USC turbine. SUMMARY

[0003] Based on theoretical calculation and test, the present application designs a material for manufacturing 700℃ A-USC turbine cylinder bolt. The material is prepared by adding five strengthening elements of Co, Nb, Fe, Ti and Al in Ni based on the powder metallurgy process, and finally the size of γ´ phase and ETA phase is small. The two phases are different from the composition of the conventional γ´ phase and ETA phase, and can stably exist in the range of 700℃ to 850℃, which significantly enhances the high-temperature strength of the nickel-based high-temperature alloy, and the performance is better than the high-temperature performance of the conventional alloy such as Waspaloy, GH738 and the like.

[0004] A method for preparing cylinder bolt material for 700℃ ultra-supercritical steam turbine, the composition of which is: Ni~(29-31wt%)Co~(14-16wt%)Fe~(5-6wt%)Al~(5-6wt%)Nb~(5-6wt%)Ti, wherein the contents of Al, Ti and Nb are the same. The main manufacturing process of the bolt material includes vacuum atomization powder preparation, hot isostatic pressing sintering, performance heat treatment and rough machining.

[0005] The preparation method for the cylinder bolt material used in 700℃ ultra-supercritical steam turbines, as described above, includes the following specific manufacturing steps: (1) Vacuum atomization powder preparation: conventional vacuum atomization equipment is used to atomize powder with the following composition: (29-31wt%)Co, (14-16wt%)Fe, (5-6wt%)Al, (5-6wt%)Nb, (5-6wt%)Ti, and the balance being Ni. The alloy composition of Al, Ti and Nb is required to be the same. When preparing the powder, the median particle size of the powder is controlled to be 10-15 micrometers by controlling the gas flow rate and pressure, and the oxygen content is less than 100ppm. (2) Hot isostatic pressing sintering: The sleeve is made according to the actual bolt size and the machining allowance is considered. Then, the conventional hot isostatic pressing sintering furnace is used for densification. After sintering, the bolt is cooled to below 100°C and then taken out of the furnace. (3) The sintered ingot with the sheath is subjected to performance heat treatment. First, it is subjected to solution heat treatment, and then it is cooled to below 100°C by oil quenching. Then it is subjected to aging treatment with a heating rate of 20°C / min. After holding at the temperature, it is cooled to below 100°C and taken out of the furnace with a cooling rate of 15°C / min. (4) The heat-treated ingot is rough machined to remove the sleeve and obtain a bolt blank. Then, it is fine machined according to the thread size to obtain the finished product.

[0006] Furthermore, the sintering temperature in step (2) is 1180℃ to 1200℃, and the holding time is 90-120 minutes.

[0007] Further, the solution temperature in step (3) is 1150-1180℃ and the holding time is 60-90 minutes; the aging temperature is 800-820℃ and the holding time is 90-120 minutes.

[0008] The design mechanism of this invention's alloy: Conventional nickel-based superalloys mainly use carbides and γ' intermetallic compound phases for strengthening. However, carbides become unstable above a certain temperature, such as 700℃, and tend to coarsen and grow, leading to a decrease in the high-temperature performance of nickel-based alloys and affecting their service life. This invention, through compositional design, avoids carbide strengthening. By adding five elements—Co, Nb, Fe, Ti, and Al—to Ni, while controlling the content of Al, Ti, and Nb to be equal, and combining solid solution and aging treatments, approximately 33% Ni can be formed in the alloy. 40 Co 33 Al 10 Ti8Fe3Nb6 phase (belonging to the γ´ phase) and 33% Ni 53 Co 19 Al 14.5 Ti 6.6 Fe 4.5 Nb 2.4 (Belonging to the ETA phase), both phases can exist stably in the range of 700 to 850℃, effectively hindering dislocation movement and avoiding the phenomenon of conventional carbide-strengthened phases failing due to coarsening at high temperatures. Of the two phases, the ETA phase precipitates during alloy solidification. To reduce its size, it is prepared using powder metallurgy, controlling its size to 1-1.5 micrometers by controlling the powder particle size. The γ´ phase precipitates through aging treatment; by controlling the aging temperature, its size can be controlled to 80-100 nm, and it remains coherent with the matrix. The synergistic effect of the two phases at high temperatures effectively hinders dislocation movement. Regarding the alloy matrix, although the original composition has the highest Ni content, a large amount of Ni forms the γ´ and ETA phases with other alloying elements, resulting in a higher Co and Fe content in the matrix than Ni. This forms a Co matrix strengthened by Ni and Fe solid solution, which differs from conventional nickel-based alloys where nickel is the matrix. The Co matrix exhibits higher high-temperature strength than the Ni matrix. In summary, through the synergistic effect of two high-temperature resistant reinforcing phases and a high-temperature resistant matrix, the alloy exhibits excellent performance at 800℃, with a yield strength exceeding 850MPa, which is about 40% higher than that of conventional grades such as Waspaloy at 800℃. Using it as a cylinder bolt at 700℃ will significantly improve its safety and service life.

[0009] The advantages of this invention are as follows: First, the alloy element design is simple, without expensive elements such as W, Mo, and rare earth elements, resulting in low cost and convenient smelting of the master alloy; second, the powder metallurgy process can control the size of the reinforcing phase by controlling the powder particle size, and can also achieve near-net-shape forming of bolts without the need for forging, electroslag remelting, and other processes, which shortens the process and greatly improves the material utilization rate, resulting in higher cost performance. Detailed Implementation

[0010] Since steam turbines require bolts of various diameters, but the bolt manufacturing process is the same, this implementation method uses two typical diameters, 50mm and 80mm, for illustration. This involves manufacturing bolt blanks with diameters of 50mm and lengths of 400mm and 80mm respectively. Specific steps: The first step is to perform vacuum atomization powder preparation: conventional vacuum atomization equipment is used to atomize and prepare powder with the following composition: Ni ~ 30wt% Co ~ 15wt% Fe ~ 5wt% Al ~ 5wt% Nb ~ 5wt% Ti. The content of Al, Ti and Nb is the same. By controlling the gas flow rate and pressure, the median particle size of the powder is controlled at 10 ~ 15 micrometers and the oxygen content is less than 100ppm.

[0011] The second step is hot isostatic pressing sintering: the sleeve is made according to the actual bolt size and the machining allowance. Considering the sintering shrinkage, the sleeves for 50mm and 80mm bolts are 60mm and 90mm respectively, and the length of each is 450mm. Then, conventional hot isostatic pressing sintering furnace is used for densification. The sintering temperature is 1180℃ and the holding time is 90 minutes. After sintering, the bolts are cooled to below 100℃ in the furnace before being removed from the furnace.

[0012] The third step is to perform performance heat treatment on the sintered ingot with the sheath. First, a solution heat treatment is performed at a temperature of 1150℃ for 60 minutes. Then, the ingot is cooled to below 100℃ by oil quenching. Next, an aging treatment is performed at a heating rate of 20℃ / min, an aging temperature of 800℃, and a holding time of 120 minutes. After the holding time is completed, the ingot is cooled to below 100℃ and removed from the furnace at a cooling rate of 15℃ / min.

[0013] The fourth step is to perform rough machining on the heat-treated ingot to remove the cladding, resulting in bolt blanks with diameters of 50mm and 80mm and a length of 400mm. Then, the blanks are finished according to the thread dimensions to obtain the finished product.

Claims

1. A method for preparing cylinder bolt material for a 700℃ ultra-supercritical steam turbine, characterized in that: The composition is: Ni ~ (29-31wt%) Co ~ (14-16wt%) Fe ~ (5-6wt%) Al ~ (5-6wt%) Nb ~ (5-6wt%) Ti, wherein the contents of Al, Ti and Nb are the same. The main manufacturing process of the bolt material includes vacuum atomization powdering, hot isostatic pressing sintering, performance heat treatment and rough machining.

2. A method for preparing cylinder bolt material for a 700℃ ultra-supercritical steam turbine, characterized in that: The specific manufacturing steps are as follows: (1) Vacuum atomization powder preparation: conventional vacuum atomization equipment is used to atomize powder with the following composition: (29-31wt%) Co, (14-16wt%) Fe, (5-6wt%) Al, (5-6wt%) Nb, (5-6wt%) Ti, and the balance being Ni. The alloy composition of Al, Ti and Nb is required to be the same. When preparing the powder, the median particle size of the powder is controlled to be 10-15 micrometers by controlling the gas flow rate and pressure, and the oxygen content is less than 100ppm. (2) Hot isostatic pressing sintering: The sleeve is made according to the actual bolt size and the machining allowance is considered. Then, the conventional hot isostatic pressing sintering furnace is used for densification. After sintering, the bolt is cooled to below 100°C and then taken out of the furnace. (3) The sintered ingot with the sheath is subjected to performance heat treatment. First, it is subjected to solution heat treatment, and then it is cooled to below 100°C by oil quenching. Then it is subjected to aging treatment with a heating rate of 20°C / min. After holding at the temperature, it is cooled to below 100°C and taken out of the furnace with a cooling rate of 15°C / min. (4) The heat-treated ingot is rough machined to remove the sleeve and obtain a bolt blank. Then, it is fine machined according to the thread size to obtain the finished product.

3. The method for preparing cylinder bolt material for 700℃ ultra-supercritical steam turbines according to claim 2, characterized in that: The sintering temperature in step (2) is 1180℃ to 1200℃, and the holding time is 90-120 minutes.

4. The method for preparing cylinder bolt material for 700℃ ultra-supercritical steam turbines according to claim 2, characterized in that: The solution temperature in step (3) is 1150-1180℃ and the holding time is 60-90 minutes; the aging temperature is 800-820℃ and the holding time is 90-120 minutes.