Preparation method of heat-resistant fastener material for 700 DEG C ultra-supercritical steam turbine

Abstract

The invention discloses a preparation method of a heat-resistant fastener material for a 700 DEG C ultra-supercritical steam turbine, and belongs to the field of metal materials. Lattice distortion and hysteresis diffusion effect of the high-entropy alloy are used as a core strengthening mechanism, supplementary strengthening is carried out with the assistance of an economic nanoscale L phase, a new alloy component (31-32 wt%) Fe-(33-34 wt%) Ni-(17-18 wt%) Cr-(3-4 wt%) Al-(5-6 wt%) Ti-(8-9 wt%) Nb-(0.2-0.25 wt%) C-(0.03-0.05 wt%) B is designed, Ni is replaced with Fe, precious metal such as Co, W, Mo and rare earth is completely abandoned, and the extreme economical efficiency is achieved from the material source. The alloy is mainly based on lattice distortion + L phase composite strengthening, the tensile strength of the alloy at 700 DEG C is larger than 900 MPa and is superior to that of traditional IN718 alloy and the like, the alloy has excellent creep resistance due to the hysteresis diffusion effect, the alloy is applied to ultra-supercritical steam turbine fasteners at the temperature of about 700 DEG C, such as bolts, nuts and springs, and the service life of the ultra-supercritical steam turbine fasteners is prolonged. And a high-cost-performance high-temperature material solution is provided for high-performance power equipment.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of metal materials, and relates to a preparation method of a 700 DEG C steam turbine heat-resistant fastener material. BACKGROUND

[0002] Under the background of rapid development of ultra-supercritical steam turbine technology, the 700 DEG C high-temperature environment poses unprecedented challenges to heat-resistant fastener (including bolts, nuts, gaskets and springs) materials. At present, traditional heat-resistant steel faces the problems of strength attenuation and insufficient creep resistance in long-term high-temperature operation, and cannot meet the reliability requirements of key components of steam turbines under 700 DEG C ultra-supercritical working conditions. At the same time, although the existing high-strength nickel-based high-temperature alloy can be used at higher temperatures (such as gas turbines and aircraft engines), it has significant performance redundancy in the application at 700 DEG C, resulting in excessive addition of expensive alloy elements such as cobalt and molybdenum, which greatly increases the cost of the material and deviates from the requirement of economy. This imbalance between cost and performance is particularly prominent in the process of energy equipment localization, which restricts the promotion of high-efficiency and clean power generation technology. Therefore, based on the existing nickel-based alloy system, developing a low-cost heat-resistant fastener material optimized for 700 DEG C environment has become a key path to break through the technical bottleneck and upgrade the performance of ultra-supercritical steam turbines. This innovation not only fills the gap between material performance and cost-effectiveness, but also provides core support for the low-carbon transformation of the power industry. SUMMARY

[0003] The alloy composition design of the application breaks the paradigm of traditional nickel-based high-temperature alloys relying on "Ni-Co matrix + high volume fraction γ´ phase", and instead uses the lattice distortion and sluggish diffusion effect of high-entropy alloys as the core strengthening mechanism, supplemented by economical nanoscale L By replacing Ni with Fe, the design completely eliminates Co, W, Mo and rare earth metals, achieving extreme economy at the source of the material. The alloy is applied to 700 DEG C ultra-supercritical steam turbine fasteners such as bolts, nuts and springs, providing a "high performance-price ratio" high-temperature material solution for high-performance power equipment.

[0004] A preparation method of a 700 DEG C ultra-supercritical steam turbine heat-resistant fastener material, the composition of which is (31-32wt%) Fe ~ (33-34wt%) Ni ~ (17-18wt%) Cr ~ (3-4wt%) Al ~ (5-6wt%) Ti ~ (8-9wt%) Nb ~ (0.2-0.25wt%) C ~ (0.03-0.05wt%) B, the main production process of the fastener material includes vacuum induction melting, hot deformation (forging or hot rolling), performance heat treatment, and fastener blank.

[0005] A method for preparing a heat-resistant fastener material for a 700℃ ultra-supercritical steam turbine, the specific manufacturing process and requirements are as follows: (1) Vacuum induction melting alloy: adopt conventional vacuum induction melting furnace according to (31-32wt%) Fe~(33-34wt%) Ni~(17-18wt%) Cr~(3-4wt%) Al~(5-6wt%) Ti~(8-9wt%) Nb~(0.2-0.25wt%) C~(0.03-0.05wt%) B component ratio for smelting, the vacuum degree of smelting is less than 10 -2 Pa; the pouring temperature after smelting is 1500~1550℃; (2) Hot deformation: according to the specific shape of the fastener, hot deformation is carried out, such as forging if the fastener is a bolt, hot rolling into a plate if the fastener is a gasket, and hot rolling into a wire or a rod if the fastener is a spring; before hot deformation, the ingot is first subjected to solid solution treatment at 1180℃ for 12 hours; (3) Performance heat treatment: first, the blank is subjected to solid solution treatment, and then oil cooling after heat preservation to obtain a single-phase supersaturated FCC solid solution; then, aging treatment is carried out to promote the uniform dispersion of nanoscale L Phase to reach peak strength; (4) The heat-treated blank is processed according to the size to obtain the finished fastener.

[0006] Further, the temperature of the hot deformation in step (2) is 1130~1180℃; Further, the solid solution treatment temperature in step (3) is 1140℃~1160℃; Further, the aging treatment temperature in step (3) is 740℃~760℃, and the heat preservation time is 8-12 hours.

[0007] Mechanism of the alloy design of the present application: (1) Mechanism of element selection Based on the concept of high-entropy alloy design, multiple element alloy elements are selected, and a large amount of inexpensive Fe is used as the matrix to replace expensive Ni, which is the most critical measure to reduce cost. Ni and Fe are infinitely soluble and can form a stable FCC structure. Selecting Ni as the second major element can stabilize the FCC structure, ensure that the matrix is face-centered cubic structure, and provide good high-temperature basic plasticity and oxidation resistance potential. Cr element as the core anti-oxidation / corrosion element, forms a dense C protective film, which is higher than that of traditional nickel-based alloys, to compensate for the decrease in oxidation resistance due to the reduction of Al, Ti. Al has a dual function, one is to form L ordered phase (mainly N Al, F Al) to form a precipitate; two is to assist in the formation of A protective film, especially in C Provide repair capacity when damaged. Ti as the core of the precipitation strengthening element, synergistic effect with Al, promote L type ordered phase (such as N Ti, (Ni,Fe Nb has a multi-functional strengthening, one is to produce strong lattice distortion, solid solution strengthening matrix; two is to form a stable MC type carbide (NbC), strengthening the grain boundary. Three can enter the L phase, improve its thermal stability. C as a carbide forming element. In so many groups of the system, can form a variety of small MC and carbide, play the role of grain boundary pinning and secondary strengthening. B as a grain boundary strengthening element, segregation in the grain boundary, greatly improve the grain boundary strength and medium temperature plasticity, prevent creep crack initiation along the grain boundary.

[0008] (2) the mechanism of alloy strengthening

[0009] The main strengthening mechanism of the alloy of the application is FCC solid solution strengthening + nanoscale L ordered phase + grain boundary carbide strengthening.

[0010] Fe, Ni, Cr, Al, Ti, Nb and other elements with different atomic radii are randomly distributed in the FCC lattice, producing a huge stress field, which constitutes an extremely effective obstacle to the movement of dislocations, providing solid solution strengthening effect far beyond traditional alloys. Chaotic lattice makes atomic diffusion need to overcome higher energy barrier, producing a lag diffusion effect, which directly translates into excellent creep resistance, which is crucial for 700℃ application.

[0011] In the aging process, Al and Ti elements will be precipitated with Ni, Fe and other nanoscale L type phase (Ni,Fe (Al,Ti), this phase is coherent with the FCC matrix, and is an ordered structure. When dislocations cut these ordered phases, they need to move in pairs and produce anti-phase boundaries, consuming a lot of energy. This, combined with the background of severe lattice distortion, produces a "1+1>2" strengthening effect.

[0012] Nb and C form thermodynamically stable NbC, which precipitates at the grain boundary and in the grain. They can effectively pin the grain boundary and prevent grain boundary sliding and migration at high temperatures. B element will selectively segregate to the grain boundary, improve the cohesion of the grain boundary, effectively prevent the grain boundary brittleness at medium temperature (~700℃).

[0013] (3) the advantages of the application

[0014] Through component design, "lattice distortion + L The alloy has a tensile strength of greater than 900 MPa at 700 DEG C, is superior to conventional IN718 alloy, and has excellent creep resistance due to the delayed diffusion effect; the high-entropy effect stabilizes the single-phase FCC, and the alloy generally has better as-cast plasticity and processability than the nickel-based alloy containing a large amount of gamma prime phase, so that the alloy of the application can shorten the process and reduce the cost without electroslag remelting. DETAILED DESCRIPTION

[0015] 1. Preparation of 50mm diameter bolt, nut fastener for 700 DEG C steam turbine

[0016] (1) The conventional vacuum induction melting furnace is used to melt according to the component ratio of 31.5wt%Fe-33.5wt%Ni-17.5wt%Cr-3.5wt%Al-5.5wt%Ti-8.25wt%Nb-0.2wt%C-0.05wt%B, and the vacuum degree of melting is less than 10-2Pa. The casting temperature after melting is 1500-1550 DEG C, and the diameter of the ingot is greater than 100mm. -2 Pa. The casting temperature after melting is 1500-1550 DEG C.

[0017] (2) The ingot is forged into a bar, and the hot deformation temperature is 1180 DEG C. The ingot is first subjected to solid solution treatment at 1180 DEG C for 12 hours before hot deformation. The diameter of the forged bar is 55mm.

[0018] (3) The bar is subjected to solid solution treatment at 1160 DEG C, and oil cooling after heat preservation; then, aging treatment is carried out at 760 DEG C for 8 hours.

[0019] (4) The heat-treated blank is processed according to the size to obtain a bolt, nut fastener with a diameter of 50mm.

[0020] 2. Preparation of 1mm thick gasket fastener for 700 DEG C steam turbine

[0021] (1) The conventional vacuum induction melting furnace is used to melt according to the component ratio of 32wt%Fe-33wt%Ni-17.75wt%Cr-3.5wt%Al-5.5wt%Ti-8wt%Nb-0.2wt%C-0.05wt%B, and the vacuum degree of melting is less than 10-2Pa. The casting temperature after melting is 1500-1550 DEG C.

[0022] (2) The ingot is forged into a square bar with a thickness of 10mm, and then hot-rolled into a plate. The thickness of the hot-rolled plate is 1.1mm. The forging and hot-rolling temperature is 1130 DEG C. The ingot is first subjected to solid solution treatment at 1180 DEG C for 12 hours before hot deformation.

[0023] (3) solid solution treatment is performed on the hot-rolled plate, the solid solution treatment temperature is 1150 °C, oil cooling after holding, and a single-phase supersaturated FCC solid solution is obtained; then aging treatment is performed, the aging treatment temperature is 750 °C, and holding is performed for 12 hours; (4) the blank plate with a heat treatment thickness of 1.1 mm is surface polished according to sizes to obtain a finished gasket with a thickness of 1 mm.

Claims

1. A method for preparing heat-resistant fastener material for 700℃ ultra-supercritical steam turbines, characterized in that: The alloy composition is: (31-32wt%) Fe ~ (33-34wt%) Ni ~ (17-18wt%) Cr ~ (3-4wt%) Al ~ (5-6wt%) Ti ~ (8-9wt%) Nb ~ (0.2-0.25wt%) C ~ (0.03-0.05wt%) B. The main manufacturing process of fastener materials includes vacuum induction melting, forging or hot rolling, performance heat treatment, and fastener blanks.

2. The method for preparing heat-resistant fastener material for 700℃ ultra-supercritical steam turbines according to claim 1, characterized in that: The specific manufacturing process and requirements are as follows: (1) Vacuum induction melting of alloys: A conventional vacuum induction melting furnace is used for melting with the following composition ratio: (31-32wt%) Fe ~ (33-34wt%) Ni ~ (17-18wt%) Cr ~ (3-4wt%) Al ~ (5-6wt%) Ti ~ (8-9wt%) Nb ~ (0.2-0.25wt%) C ~ (0.03-0.05wt%) B. The vacuum degree of melting is less than 10. -2 Pa; Casting temperature after smelting is 1500~1550℃; (2) Hot deformation: Hot deformation is carried out according to the specific shape of the fastener. For example, if the fastener is a bolt, it is forged; if the fastener is a gasket, it is hot rolled into a plate; if the fastener is a spring, it is hot rolled into wire or wire rod. Before hot deformation, the ingot is first subjected to solution treatment at 1180℃ for 12 hours. (3) Performance heat treatment: First, the blank is subjected to solution treatment, followed by heat preservation and oil cooling to obtain a single-phase supersaturated FCC solid solution; then, aging treatment is performed to promote the nanoscale L The phase is uniformly dispersed and precipitated, reaching peak intensity; (4) The heat-treated blank is processed according to the dimensions to obtain the finished fastener.

3. The method for preparing heat-resistant fastener material for 700℃ ultra-supercritical steam turbines according to claim 2, characterized in that: The temperature of the heat deformation in step (2) is 1130 to 1180°C.

4. The method for preparing heat-resistant fastener material for 700℃ ultra-supercritical steam turbines according to claim 2, characterized in that: The solution treatment temperature in step (3) is 1140℃~1160℃.

5. The method for preparing heat-resistant fastener material for 700℃ ultra-supercritical steam turbines according to claim 2, characterized in that: The aging treatment in step (3) is carried out at a temperature of 740℃~760℃ for 8-12 hours.

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