A high-strength and tough nickel-iron-based heat-resistant alloy for high-temperature gas-cooled reactor and a preparation method thereof

Abstract

A high-strength and tough nickel-iron-based heat-resistant alloy for high-temperature gas-cooled reactor and a preparation method thereof, belonging to the technical field of high-temperature metal structural materials, the chemical composition by weight percentage is C 0.01-0.1%, Ni 52.0-57.0%, Cr 17.0-21.0%, Mo 4.0-8.0%, Mn 1.0-2.0%, W 1.0-4.0%, Zr 0.01-0.3%, Y 0-0.07%, B 0.0005-0.02%, the balance is Fe and inevitable impurity elements; the preparation method comprises: 1) preparing raw materials according to alloy composition and content; 2) vacuum smelting the raw materials and casting them into ingots; 3) performing low temperature pretreatment on the ingots at 1075-1125°C; 4) performing homogenization treatment at 1200-1250°C; 5) forging into bars at 1030-1180°C; 6) performing rolling at 1030-1180°C; 7) performing heat treatment at 1050-1150°C. The alloy of the present invention meets the stringent requirements of high temperature gas-cooled reactors for high temperature strength and plasticity, fatigue performance, creep performance, corrosion resistance and radiation resistance of materials by optimizing the content of key elements, hot working technology and heat treatment system, and can be used for a long time under the condition of not more than 900°C.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-temperature metal structural materials, and specifically relates to a high-strength and tough nickel-iron-based heat-resistant alloy for high-temperature gas-cooled reactors and a preparation method thereof. Background Art

[0002] The fourth-generation nuclear energy system has significant advantages such as high safety, good economy, small amount of nuclear waste, and can effectively prevent nuclear proliferation. It represents the development trend and technological frontier of the current advanced nuclear energy system. Among the several advanced reactor types of the fourth-generation nuclear energy system, the high-temperature gas-cooled reactor has unique advantages such as excellent inherent safety, high power generation efficiency, small single-reactor power, modular design and convenient construction. It is considered an important choice for the next generation of nuclear energy technology and has broad application prospects.

[0003] my country also attaches great importance to the research and development of the fourth-generation nuclear reactor, and has certain advantages in high-temperature gas-cooled reactor technology. At present, China is still facing many difficulties in promoting the application of high-temperature gas-cooled reactor technology in energy demand fields such as petrochemicals, coal gasification, hydrogen production and steelmaking, and space power supply. Among them, the selection and preparation of structural materials has become one of the engineering problems that need to be solved urgently.

[0004] In the huge and sophisticated nuclear reactor system, the safety and reliability of the key components of the reactor body (including reactor vessels, intermediate heat exchangers and other in-core components) cannot be ignored. It is an important basis for ensuring the safe operation of the reactor in the process of providing a stable energy source. The service conditions of these components are extremely harsh. For example, the components of a certain type of high-temperature gas-cooled reactor energy supply system need to withstand high temperatures (reactor outlet temperature can reach 950°C), gas medium corrosion (helium coolant), alternating loads and strong neutron irradiation, and need to operate reliably within 15 to 20 years without inspection or preventive measures. Obviously, traditional austenitic stainless steel (such as 316 series stainless steel) can no longer meet the above requirements. Nickel-iron-based alloys have been widely used in many reactor types of fourth-generation nuclear reactors due to their good comprehensive properties and organizational stability. Compared with austenitic stainless steel, the higher Ni content ensures its high-temperature strength and radiation resistance; at the same time, compared with nickel-based alloys, a certain Fe content makes it lower in cost and better in hot working performance.

[0005] The structural materials used for the key components of the reactor body need to take into account the performance of high temperature resistance, toughness, creep fatigue resistance, corrosion resistance and radiation swelling resistance. At present, although a variety of materials can meet the individual requirements of the high temperature gas-cooled reactor components used in the above-mentioned energy and power systems, no material can meet all the requirements at the same time. Therefore, in order to meet the development needs of my country's energy and power field, it is urgent to develop nickel-iron-based heat-resistant alloys with excellent strength, plasticity, toughness, high temperature resistance, corrosion resistance, radiation resistance and process performance. Summary of the invention

[0006] In order to solve the demand for nickel-iron-based heat-resistant alloys for key components of a certain type of high-temperature gas-cooled reactor in my country, the purpose of the present invention is to provide a high-strength and tough nickel-iron-based heat-resistant alloy for high-temperature gas-cooled reactor pipes and a preparation method thereof. The nickel-iron-based alloy has low cost, good strength, plasticity, toughness, high temperature resistance, corrosion resistance, radiation resistance and process performance.

[0007] In order to achieve the above object, the technical solution of the present invention is as follows:

[0008] A high-strength and tough nickel-iron-based heat-resistant alloy for a high-temperature gas-cooled reactor, wherein the chemical composition ranges by weight percentage as follows: C 0.01-0.1%, Ni 52.0-57.0%, Cr 17.0-21.0%, Mo 4.0-8.0%, Mn 1.0-2.0%, W 1.0-4.0%, Zr 0.01-0.3%, Y 0-0.07%, and the balance is Fe and unavoidable impurity elements.

[0009] At the same time, the weight percentage of the B element is strictly controlled to be 0.0005-0.02%.

[0010] Furthermore, the weight percentages of Ni and Fe elements satisfy: Ni / Fe≥3.3;

[0011] Furthermore, the weight percentages of Mo and W elements satisfy: Mo+W≥7.5%;

[0012] Furthermore, the weight percentages of C, B and Zr elements satisfy: C+B+Zr≥0.055%;

[0013] Furthermore, the above alloy also contains inevitable impurities, and the impurity contents are as follows by weight: O≤0.01%, Pb≤0.001%, Bi≤0.001%, As≤0.005%, Sb≤0.01%, Sn≤0.005%, Co≤0.1%, Al≤0.15%, Ti≤0.15%, Si≤0.3%, N≤0.03%, Cu≤0.1%, P≤0.015%, and S≤0.01%.

[0014] Furthermore, the mechanical properties of the high-strength and tough nickel-iron-based heat-resistant alloy for high-temperature gas-cooled reactors at room temperature meet the following requirements: tensile strength ≥ 600MPa, yield strength ≥ 250MPa, elongation ≥ 40%, cross-sectional shrinkage ≥ 65%, and impact toughness KV2 ≥ 300J; the tensile properties at 850°C meet the following requirements: tensile strength ≥ 230MPa, yield strength ≥ 130MPa, elongation ≥ 50%, and cross-sectional shrinkage ≥ 55%.

[0015] The preparation method of the high-strength and tough nickel-iron-based heat-resistant alloy for high-temperature gas-cooled reactor is as follows: raw materials are prepared according to the above chemical composition ratio, smelted and cast into ingots, and then low-temperature pretreatment, homogenization treatment, heat treatment and solution treatment are carried out in sequence.

[0016] The specific steps are as follows:

[0017] 1) Prepare alloy: the alloy composition meets the following range requirements by weight percentage: C 0.01-0.1%, Ni 52.0-57.0%, Cr 17.0-21.0%, Mo 4.0-8.0%, Mn 1.0-2.0%, W 1.0-4.0%, Zr 0.01-0.3%, Y 0-0.07%, B 0.0005-0.02%, and the balance is Fe;

[0018] 2) Smelting: Smelting the prepared alloy in vacuum to form alloy liquid, and casting the alloy liquid into ingots;

[0019] 3) Low temperature pretreatment: Place the ingot into a heating furnace and heat it to 1075-1125°C with the furnace, and keep it warm for ≥2h;

[0020] 4) Homogenization treatment: Based on step 3), the furnace temperature is raised to 1200-1250°C, and the heat preservation time is ≥15h;

[0021] 5) Forging: Forging the ingot at 1030-1180°C into bars with a forging ratio of 7-9;

[0022] 6) Rolling: The heat-treated bar is rolled at 1030-1180°C to form a rolled bar;

[0023] 7) Solution treatment: keep the rolled bar at 1050-1150℃ for 30-90min, and cool it in air or water.

[0024] In the above step 2), the vacuum smelting condition is a vacuum degree ≤ 1Pa.

[0025] In the above step 5), the grain size is measured according to GB / T 6394-2002 "Method for Determination of Average Grain Size of Metals", and the grain size of the forged bar is required to be 4.0 to 8.0.

[0026] In the above step 6), the rolling process is performed for 5 to 9 passes, and the diameter ratio of the rolled bar to the forged bar is 1:(4 to 5).

[0027] In the above step 6), the grain size is measured according to GB / T 6394-2002 "Method for Determination of Average Grain Size of Metals", and the average grain size of the rolled rod is 10.0 to 14.0.

[0028] In the above step 7), the grain size is measured according to GB / T 6394-2002 "Method for Determination of Average Grain Size of Metals", and the average grain size of the high-strength and tough nickel-iron based alloy is 4.0 to 8.0.

[0029] The design ideas of the present invention are as follows:

[0030] In order to meet the stringent requirements of high temperature gas-cooled reactors on high temperature strength, plasticity, creep, fatigue, corrosion resistance and radiation resistance of materials, the present invention is mainly based on the following alloy design concepts:

[0031] 1) Ingredient design and optimization

[0032] Based on the "multi-element composite strengthening" theory and the "selective strengthening" theory, the alloy composition is comprehensively optimized and designed. The specific reasons for selecting the above main chemical components are as follows (alloy composition is calculated by weight percentage):

[0033] C: C is a strong austenite forming element, which can stabilize and expand the austenite phase area, and can also play a role in interstitial solid solution strengthening. As a carbide forming element, C can form MC, M6C, M 23 Carbides such as C6, which are preferentially precipitated at the grain boundaries, have an important influence on the grain boundary properties of the material. For example, discontinuous granular carbides can not only hinder grain boundary sliding and crack propagation to improve creep strength, but also improve creep plasticity. In addition, an appropriate amount of C can also improve the castability of the alloy. However, when the C content is too high, it is easy to cause a large amount of carbides to form, thereby reducing the content of solid solution strengthening elements in the matrix, and it is also easy to cause medium temperature "sensitization" phenomenon. High C content is also not good for the welding performance of the alloy. Therefore, the C content range is controlled to be 0.01-0.1%.

[0034] Ni: Ni is a matrix (γ austenite) forming element, which makes the alloy have good organizational stability and high strength and plasticity at high temperature, and at the same time ensures that the alloy has good heat resistance, corrosion resistance and radiation swelling resistance. When increasing the content of other elements, it is not advisable to reduce the Ni content as a prerequisite. Therefore, the Ni content in the alloy is strictly controlled to be 52.0-57.0%.

[0035] Cr: Cr is soluble in austenite, improving the oxidation resistance and corrosion resistance of the alloy. In order to ensure that the alloy has good oxidation resistance and corrosion resistance in a high-temperature gas medium environment within 900°C, a larger Cr content is required; and too high Cr will promote the precipitation of harmful topological dense phase (TCP phase) and reduce the structural stability of the alloy. Therefore, the Cr content should not be too high and should be controlled at 17.0-21.0%.

[0036] Mo, W: Mo and W are important solid solution strengthening elements, which can significantly improve the high temperature strength and long life of the alloy. Both can slow down the diffusion rate of other elements to a certain extent, thereby achieving the purpose of improving the stability of the organization. However, excessive Mo and W will promote the precipitation of M6C, TCP, etc., reducing the solid solution strengthening effect. In addition, when the Mo content is too high, it is easy to cause pitting, and when the W content is too high, it is easy to cause segregation during smelting, thereby reducing the hot working performance of the alloy. Therefore, the upper limit content of Mo and W is controlled to 8.0% and 4.0%, respectively.

[0037] Mn: Mn is an austenite stabilizing element. A small amount of Mn added to the alloy melt can remove S and O. However, excessive Mn will reduce the oxidation resistance and grain boundary bonding strength of the alloy, promote the precipitation of harmful phases, and damage the alloy performance. Therefore, the Mn content of the alloy of the present invention is controlled at 1.0-2.0%.

[0038] Zr: Zr is an important grain boundary strengthening element in high temperature alloys. It is beneficial to improve the grain boundary bonding force and reduce the diffusion rate of grain boundaries by segregating at the grain boundaries. It can also promote the formation of MC carbides and pin the grain boundaries. Studies have reported that adding a small amount of Zr (0.03-0.08%) to GH3539 alloy can also effectively improve the medium-temperature brittleness of the alloy. However, when the Zr content is too high, it is easy to form low-melting point harmful brittle phases such as Ni5Zr, which reduces the melting point of the alloy and is not conducive to subsequent hot processing processes such as forging and blanking.

[0039] Y: As a rare earth element, Y plays an important role in improving the performance of high-temperature alloys. A trace amount of Y addition can improve the bonding strength of grain boundaries, strengthen and purify grain boundaries, and also improve oxidation resistance. Too high Y content is easy to form inclusions and pollute the alloy melt. The upper limit of Y content is controlled to 0.07%.

[0040] B: B is a microalloying element. Adding an appropriate amount of B can purify the grain boundary, enhance the grain boundary bonding force, significantly improve the creep fatigue resistance, and improve the high temperature resistance and radiation resistance. However, excessive B will cause the formation of a large number of low melting point phases, which is not conducive to the process performance of the alloy. In addition, B has a large neutron absorption cross section and poses a nuclear risk. Therefore, the content of B in the alloy of the present invention is strictly controlled to be 0.0005-0.02%.

[0041] Fe: Fe is a matrix element that can replace Ni to a certain extent, and can achieve the hot working properties required by the alloy at a lower cost; for example, Ni-Fe based alloys such as GH984G, HT700P, and GH3536 all contain a certain amount of Fe, ranging from 15% to 40%. However, too high Fe content will reduce the structural stability and high temperature strength of the alloy, so the Fe content is used as the balance in this alloy to fully ensure the comprehensive performance of the alloy.

[0042] Al, Ti: Al and Ti are elements that form the γ′ phase. The γ′ phase can provide a good strengthening effect in a short period of time, but it will coarsen and fail during long-term service at high temperatures. This situation may become more unstable in a high-temperature strong neutron irradiation environment, which is not conducive to the performance of the alloy. In addition, Al and Ti have high chemical activity and are easily burned during welding to form brittle and hard compounds, which is not conducive to welding performance. Therefore, it is not suitable to add Al and Ti elements to the alloy of the present invention.

[0043] Co: Co is a solid solution strengthening element that can reduce the stacking fault energy of the alloy and improve the high temperature strength and plasticity. However, since the alloy of the present invention is used in a nuclear reactor and faces a strong neutron irradiation environment, the isotope of Co is radioactive and activated, which is not conducive to the safe operation and maintenance of the reactor. Therefore, Co is not added to the alloy of the present invention, which also reduces the alloy cost.

[0044] In addition, to ensure the performance of the alloy, the five harmful elements and other impurity elements should be controlled as low as possible.

[0045] The present invention optimizes the content of the above alloy elements so that the weakening mechanism is suppressed as much as possible and the strengthening mechanism is maximized. The optimization of the alloy solid solution strengthening effect is achieved by the reasonable addition of Mo and W; the use of C, B, and Zr microalloying can improve the grain boundary bonding force on the one hand, and promote the formation of MC carbides on the other hand, which can pin the grain boundary, refine the grains, and ensure the strong plasticity of the alloy. In addition, MC can capture He bubbles and improve the alloy's resistance to radiation swelling; the content of Cr and Mo is strictly controlled to control the size, quantity and morphology of the alloy grain boundary carbides to ensure the grain boundary strength; the addition of Al, Ti and Co is strictly limited to ensure that the alloy has good organizational stability and resistance to radiation damage during long-term service.

[0046] 2) Micro-organization regulation

[0047] Grain size control: From the perspective of toughness, fatigue and radiation resistance, the smaller the grain size, the better. However, in the main operating temperature range of the alloy of 800-900°C, the permanent creep mechanism is mainly grain boundary sliding. Considering the creep resistance, the grain size should not be too small. Therefore, the grain size of the alloy of the present invention should be controlled within an appropriate range.

[0048] Carbide control: Through the optimized hot deformation process and heat treatment system, a proper amount of MC, M6C and other carbides are randomly distributed in the grains or grain boundaries of the alloy, and a small amount of M6C is precipitated in a discontinuous distribution at the grain boundaries. 23 C6 carbide pins the grain boundaries, improving high temperature strength and stability. On the other hand, it can serve as a site for capturing He bubbles, improving the alloy's resistance to radiation swelling.

[0049] Compared with the prior art, the present invention has the following beneficial technical effects:

[0050] 1. The high-strength and tough nickel-iron-based heat-resistant alloy for high-temperature gas-cooled reactor of the present invention is a new alloy design proposed according to the service environment of high-temperature gas-cooled reactor, which can simultaneously meet the stringent requirements of high-temperature gas-cooled reactor on high-temperature strength and plasticity, creep performance, fatigue performance, corrosion resistance and radiation resistance, and can be used for a long time under the condition of not higher than 900°C, such as manufacturing high-temperature gas-cooled reactor pipes with working temperature below 900°C.

[0051] 2. Compared with the existing process technology, the alloy of the present invention optimizes the content of matrix elements such as Ni and Cr in terms of chemical composition, adds a certain amount of Fe as a matrix element, and reduces the alloy cost; maximizes the role of strong solid solution elements such as Mo and W; appropriately adjusts the content of micro-alloying elements such as C, B, and Zr; and strictly limits the addition of elements such as Al, Ti, and Co. Ultimately, the alloy has good organizational stability and mechanical properties, excellent corrosion resistance, radiation resistance, and processing performance, and can adapt to the service environment of high-temperature gas media and strong neutron radiation.

[0052] 3. The room temperature mechanical properties of the high-strength and tough nickel-iron-based heat-resistant alloy of the present invention meet the following requirements: tensile strength ≥ 600 MPa, yield strength ≥ 250 MPa, elongation ≥ 40%, cross-sectional shrinkage ≥ 65%, and impact toughness KV2 ≥ 300 J.

[0053] 4. The high-strength and tough nickel-iron-based heat-resistant alloy of the present invention has the following mechanical properties at 850°C: tensile strength ≥ 230 MPa, yield strength ≥ 130 MPa, elongation ≥ 50%, and cross-sectional shrinkage ≥ 55%.

[0054] 5. The high-strength and tough nickel-iron-based heat-resistant alloy of the present invention has the advantages of simple preparation method, easy operation, low cost and good deformation performance. It can be prepared into plates, bars and forgings of different specifications for the manufacture of key components of high-temperature gas-cooled reactor bodies.

[0055] 6. The chemical composition of the high-strength and tough nickel-iron-based heat-resistant alloy of the present invention can be extended to the production of high-performance forgings, bars and pipes for other fourth-generation advanced nuclear reactors (such as ultra-high temperature gas-cooled reactors) or high-temperature gas-cooled reactors in other energy supply fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 This is the surface crack defect of the high-strength and toughness nickel-iron-based heat-resistant alloy forging bar prepared in Example 1.

[0057] Figure 2 This is the good macroscopic morphology of the high-strength and tough nickel-iron-based heat-resistant alloy forging rod prepared in Example 2.

[0058] Figure 3 This is the surface crack defect of the high-strength and tough nickel-iron-based heat-resistant alloy rolled bar prepared in Example 4.

[0059] Figure 4 The metallographic structure characteristics of the high-strength and tough nickel-iron-based heat-resistant alloy prepared in Example 4 without optimizing the composition and preparation process.

[0060] Figure 5 This is the good macroscopic morphology of the high-strength and tough nickel-iron-based heat-resistant alloy rolled bar prepared in Example 5.

[0061] Figure 6 (a) is the mixed crystal structure characteristics of the high-strength and toughness nickel-iron-based heat-resistant alloy prepared in Example 5 with improper heat treatment system.

[0062] Figure 6 (b) is the coarse structure characteristics of the high-strength and tough nickel-iron-based heat-resistant alloy prepared in Example 5 with improper heat treatment system.

[0063] Figure 7 The metallographic structure characteristics of the optimized high-strength and toughness nickel-iron based heat-resistant alloy prepared in Example 5.

[0064] Figure 8 The scanning electron microscope microstructure characteristics of the optimized high-strength and toughness nickel-iron based heat-resistant alloy prepared in Example 5.

[0065] Fig. 9 The scanning electron microscope microstructure characteristics of the optimized high-strength and toughness nickel-iron-based heat-resistant alloy prepared in Example 5 after long-term aging at 850°C for 1000 hours. DETAILED DESCRIPTION

[0066] The device used to observe the metallographic structure in the embodiment of the present invention is a ZEISS Axiovert 200MAT inverted metallographic microscope.

[0067] The device used for observing the scanning electron microscope structure in the embodiment of the present invention is a TESCAN MAIA3 field emission scanning electron microscope.

[0068] In the embodiment of the present invention, the high-strength and tough nickel-iron-based heat-resistant alloy is a rod with a diameter of 12 to 18 mm.

[0069] The above vacuum smelting steps are specifically as follows:

[0070] 1) Industrial pure iron, electrolytic nickel (Ni9990), metal chromium (JGr99-A), metal molybdenum (GB / T3462-82), metal tungsten (GB / T3459-82) and pyrolytic graphite (GB / T1426-1978) are loaded into a crucible of a vacuum induction furnace, wherein the film on the surface of industrial pure iron and electrolytic nickel is pre-treated, and nickel-boron alloy (containing B15.634% by weight), sponge zirconium, nickel-yttrium alloy (containing Y19.98% by weight) and metal manganese are loaded into a hopper in order;

[0071] 2) Using an aluminum-magnesium spinel dry pot for vacuum induction melting, heating the metal in the crucible under vacuum conditions (vacuum degree ≤ 1Pa) until it is completely melted, and refining treatment at 1530-1570°C for 10-20min;

[0072] 3) After the above refining is completed, nickel-boron alloy, sponge zirconium, and nickel-yttrium alloy are added in order, and after each alloy is added, the melt is stirred and refined for 5 to 10 minutes;

[0073] 4) After the above refining is completed, argon is filled into the vacuum induction melting furnace, and metallic manganese is added to the refined melt, and stirring is continued for 5 to 15 minutes to complete vacuum induction melting, and cast into an ingot at 1480 to 1520° C. After the ingot is completely solidified, the mold is opened and taken out.

[0074] The impurity content of the alloy of the present invention is less than 1% by weight.

[0075] In the embodiment of the present invention, SISC-IAS image analysis software is used to measure the average grain size, and the standard is GB / T6394 "Method for Determination of Average Grain Size of Metals".

[0076] The standard adopted for measuring the room temperature impact toughness in the embodiment of the present invention is GB / T 229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials".

[0077] The standards adopted for measuring tensile strength, yield strength, elongation and reduction of area in the embodiments of the present invention are GB / T228.1-2021 "Tensile test of metallic materials Part 1: Room temperature test method" and GB / T 228.2-2015 "Tensile test of metallic materials Part 2: High temperature test method".

[0078] The unavoidable impurities in the embodiment of the present invention are as follows by weight percentage: O≤0.01%, Pb≤0.001%, Bi≤0.001%, As≤0.005%, Sb≤0.01%, Sn≤0.005%, Co≤0.1%, Al≤0.15%, Ti≤0.15%, Si≤0.3%, N≤0.03%, Cu≤0.1%, P≤0.015%, and S≤0.01%.

[0079] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0080] Example 1

[0081] The high-strength and tough nickel-iron-based heat-resistant alloy of this embodiment includes, by weight percentage, 0.035% C, 55.0% Ni, 19.1% Cr, 6.1% Mo, 1.4% Mn, 2.5% W, 0.028% Zr, 0.03% Y, and the balance is Fe;

[0082] The preparation method of the high-strength and tough nickel-iron-based heat-resistant alloy comprises the following specific steps:

[0083] 1) Raw material preparation: prepare the raw materials according to the above weight percentage ratio;

[0084] 2) Melting: The prepared raw materials are loaded into the crucible of the vacuum induction melting furnace, smelted in the vacuum induction furnace, and cast into ingots. After the ingots are completely solidified, the mold is opened and taken out;

[0085] 3) Low temperature pretreatment: the ingot is heated to 1120°C with the furnace and kept at this temperature for 2 hours to dissolve the low melting point Ni5Zr phase;

[0086] 4) Homogenization treatment: After step 3), the furnace temperature is raised to 1225°C and kept at this temperature for 24 hours to reduce component and structural segregation;

[0087] 5) Forging: Heat the ingot to 1150℃, keep it warm for 2h, quickly place it on an air hammer forging machine to forge it into a bar, then air cool it to room temperature, the forging ratio is 8.3, and cracks and other defects appear on the surface of the bar, such as Figure 1 As shown, the subsequent hot working and preparation process in this embodiment is stopped immediately. It can be seen that the deformation performance of the new alloy needs to be improved.

[0088] Example 2

[0089] The high-strength and tough nickel-iron-based heat-resistant alloy of this embodiment includes, by weight percentage, 0.034% C, 55.0% Ni, 19.1% Cr, 6.1% Mo, 1.4% Mn, 2.5% W, 0.029% Zr, 0.03% Y, 0.005% B, and the balance is Fe;

[0090] The preparation method of the high-strength and tough nickel-iron-based heat-resistant alloy comprises the following specific steps:

[0091] 1) Raw material preparation: prepare the raw materials according to the above weight percentage ratio;

[0092] 2) Melting: The prepared raw materials are loaded into the crucible of the vacuum induction melting furnace, smelted in the vacuum induction furnace, and cast into ingots. After the ingots are completely solidified, the mold is opened and taken out;

[0093] 3) Low temperature pretreatment: the ingot is heated to 1120°C with the furnace and kept at this temperature for 2 hours to dissolve the low melting point Ni5Zr phase;

[0094] 4) Homogenization treatment: After step 3), the furnace temperature is raised to 1225°C and kept at this temperature for 24 hours to reduce component and structural segregation;

[0095] 5) Forging: Heat the ingot to 1150℃, keep it warm for 2h, quickly place it on an air hammer forging machine to forge it into a bar, then air cool it to room temperature, the forging ratio is 8.5, and there are no cracks or other defects on the surface of the bar, such as Figure 2 As shown, the average grain size of the forged bar is 7.8.

[0096] In Example 2, the alloy composition is adjusted and element B is added. The rest is the same as in Example 1. The obtained alloy has no defects such as cracks during the forging process, which shows that B is beneficial to the deformation performance of the alloy. At the same time, considering the nuclear risk, its content in this alloy is strictly limited.

[0097] Example 3

[0098] The high-strength and tough nickel-iron-based heat-resistant alloy of this embodiment includes, by weight percentage, 0.032% C, 55.0% Ni, 19.0% Cr, 6.0% Mo, 1.5% Mn, 2.5% W, 0.18% Zr, 0.03% Y, 0.005% B, and the balance is Fe;

[0099] The preparation method of the high-strength and tough nickel-iron-based heat-resistant alloy comprises the following specific steps:

[0100] 1) Raw material preparation: prepare the raw materials according to the above weight percentage ratio;

[0101] 2) Melting: The prepared raw materials are loaded into the crucible of the vacuum induction melting furnace, smelted in the vacuum induction furnace, and cast into ingots. After the ingots are completely solidified, the mold is opened and taken out;

[0102] 3) Homogenization treatment: The ingot is directly heated to 1225°C with the furnace temperature rising, and kept warm for 24 hours to reduce the segregation of composition and organization;

[0103] 4) Forging: The ingot is heated to 1170°C, kept warm for 2 hours, and quickly placed on an air hammer forging machine to forge into a rod, and then air-cooled to room temperature. The forging ratio is 8.5. When cracks and other defects appear on the surface of the rod, the subsequent hot processing and preparation process in this embodiment is stopped immediately.

[0104] In Example 3, the alloy composition is adjusted to contain 0.18% Zr by weight, which is higher than the Zr content in Example 2; no low-temperature pretreatment is performed, and homogenization treatment is directly performed; the hot working temperature is increased to be higher than the forging temperature in Example 2, and the rest is the same as Example 2. The obtained alloy has defects such as cracks during the forging process, indicating that the Zr content of the alloy should not be too high, and it is necessary to perform low-temperature pretreatment to eliminate the adverse effects of low-melting point phases (such as Ni5Zr) on the hot working performance; at the same time, it is explained that the thermal deformation window of the alloy is small, and the deformation parameters (such as temperature and strain rate) should be carefully controlled.

[0105] Example 4

[0106] The high-strength and tough nickel-iron-based heat-resistant alloy of this embodiment includes, by weight percentage, 0.036% C, 55.0% Ni, 19.1% Cr, 6.1% Mo, 1.4% Mn, 2.5% W, 0.024% Zr, 0.005% B, 0.03% Y, and the balance is Fe;

[0107] The preparation method of the high-strength and tough nickel-iron-based heat-resistant alloy comprises the following specific steps:

[0108] 1) Raw material preparation: prepare the raw materials according to the above weight percentage ratio;

[0109] 2) Melting: The prepared raw materials are loaded into the crucible of the vacuum induction melting furnace, smelted in the vacuum induction furnace, and cast into ingots. After the ingots are completely solidified, the mold is opened and taken out;

[0110] 3) Low temperature pretreatment: the ingot is heated to 1120°C with the furnace and kept at this temperature for 2 hours to dissolve the low melting point Ni5Zr phase;

[0111] 4) Homogenization treatment: After step 3), the furnace temperature is raised to 1225°C and kept at this temperature for 24 hours to reduce component and structural segregation;

[0112] 5) Forging: Heat the ingot to 1150℃, keep it warm for 2h, quickly place it on an air hammer forging machine to forge it into a bar, and then air cool it to room temperature. The forging ratio is 8.5. There are no cracks or other defects on the surface of the bar. The average grain size of the forged bar is 7.6.

[0113] 6) Rolling: The forged bars are heated to 1130°C, kept at this temperature for 2 hours, and rolled in a horizontal hot rolling mill for 8 passes, and then air-cooled to room temperature. The diameter ratio of the rolled bars to the forged bars is 1:4.3. After rolling, microcracks and other defects appear on the surface of the bars. Figure 3 As shown, the average grain size of the rolled bar is 11.6;

[0114] 7) Solution treatment: The rolled bars are heat treated at 1100°C for 30 minutes and then cooled to room temperature. The average grain size of the bars after solution treatment is 8.3, showing a mixed crystal structure. Figure 4 shown.

[0115] The high-strength and tough nickel-iron-based heat-resistant alloy has a tensile strength of 725MPa, a yield strength of 310MPa, an elongation of 53%, and a cross-sectional reduction rate of 72% at room temperature; the tensile strength at 850°C is 263MPa, the yield strength is 168MPa, the elongation is 76%, and the cross-sectional reduction rate is 86%.

[0116] The results of the above steps 6) and 7) indicate that the rolling process and the C and Zr contents of the alloy need to be optimized.

[0117] Example 5

[0118] The high-strength and tough nickel-iron-based heat-resistant alloy of this embodiment includes, by weight percentage, 0.029% C, 55.0% Ni, 19.0% Cr, 6.1% Mo, 1.6% Mn, 2.6% W, 0.09% Zr, 0.03% Y, 0.004% B, and the balance is Fe;

[0119] The preparation method of the high-strength and tough nickel-iron-based heat-resistant alloy comprises the following specific steps:

[0120] 1) Raw material preparation: prepare the raw materials according to the above weight percentage ratio;

[0121] 2) Melting: The prepared raw materials are loaded into the crucible of the vacuum induction melting furnace, smelted in the vacuum induction furnace, and cast into ingots. After the ingots are completely solidified, the mold is opened and taken out;

[0122] 3) Low temperature pretreatment: the ingot is heated to 1120°C with the furnace and kept at this temperature for 2 hours to dissolve the low melting point Ni5Zr phase;

[0123] 4) Homogenization treatment: After step 3), the furnace temperature is raised to 1225°C and kept at this temperature for 24 hours to reduce component and structural segregation;

[0124] 5) Forging: Heat the ingot to 1150℃, keep it warm for 2h, and quickly place it on an air hammer forging machine to forge it into a bar with a forging ratio of 8.5. Then air cool it to room temperature. The forged bar is well formed and there are no defects on the surface.

[0125] 6) Rolling: The forged bars are heated to 1150℃, kept at this temperature for 2h, rolled in a horizontal hot rolling mill for 8 passes, and then air-cooled to room temperature. The diameter ratio of the rolled bars to the forged bars is 1:4.4. The rolled bars are well formed, with no surface defects. Figure 5 As shown, the average grain size of the rolled bar is 11.8.

[0126] 7) Solution treatment: The rolled bars are subjected to gradient heat treatment to explore the appropriate solution treatment system. If the treatment system is not suitable, mixed crystal structure or coarse structure will appear, such as Figure 6 As shown, considering the uniformity of grain size distribution and grain size comprehensively, the bar was kept at 1100℃ for 30min and water-cooled to room temperature. The average grain size of the bar after solution treatment was 5.8.

[0127] Example 5 The rolling temperature is increased to be higher than that in Example 4. The alloy exhibits good hot working properties and no defects such as cracks occur during the rolling process. At the same time, the alloy composition is adjusted to contain 0.029% C and 0.09% Zr by weight. The C and Zr contents are reasonably controlled to ensure that the alloy is homogeneous in structure under a suitable solution treatment system. The structure of the prepared optimized bar is shown in FIG. Figure 7 and Figure 8 As can be seen from the figure, the grain distribution is homogeneous, the structure is a single-phase austenite structure, massive MC and M6C carbides are dispersed in the matrix, and granular M6C carbides are intermittently distributed on the grain boundaries. 23 C6 carbide.

[0128] In fact, Zr microalloying is also a highlight of the present invention, the purpose of which is to improve the grain boundary bonding strength, thereby improving the alloy's resistance to grain boundary cracking, and improving strength and plasticity. Taking into account the feasibility of the preparation process and the uniformity of the structure, the preferred amount of Zr in the alloy of the present invention is 0.05-0.15%.

[0129] The high-strength and tough nickel-iron-based heat-resistant alloy has a tensile strength of 692MPa, a yield strength of 279MPa, an elongation of 55%, and a cross-sectional reduction rate of 76% at room temperature; a tensile strength of 257MPa, a yield strength of 166MPa, an elongation of 72%, and a cross-sectional reduction rate of 94% at 850°C; and an impact toughness of 378J at room temperature.

[0130] Tissue stability test

[0131] The alloy of the present application has excellent structural stability at high temperature. After long-term aging at 850℃ for 1000h, the structure of the alloy is shown in Fig. 9 No harmful TCP phase precipitation was observed, which shows that the alloy has excellent organizational stability at high temperatures.

[0132] The high-strength and tough nickel-iron-based heat-resistant alloy prepared in Example 5 has a tensile strength of 706 MPa, a yield strength of 280 MPa, an elongation of 50%, and a cross-sectional reduction rate of 65% at room temperature after long-term aging at 850°C for 1000 h. The tensile strength at 850°C is 257 MPa, the yield strength is 152 MPa, the elongation is 70%, and the cross-sectional reduction rate is 93%. It can be seen that the strength and plasticity of the alloy remain basically stable after aging for a long time at high temperature.

[0133] Comparative Example 1

[0134] GH1059 alloy (patent announcement number: CN111647790B) has a tensile strength of 282MPa, a yield strength of 139MPa, an elongation of 80%, and a cross-sectional shrinkage of 84% at 750°C. Due to the insufficient content of Ni, Mo, and W, it can be inferred that its yield strength at 850°C cannot meet the service requirements of high-temperature gas-cooled reactors.

[0135] Comparative Example 2

[0136] The tensile strength of 316 stainless steel at 750°C is 268MPa, the yield strength is 123MPa, and the elongation is 49%. Due to the insufficient content of Ni, Mo, and W, it can be inferred that its yield strength at 850°C cannot meet the service requirements of high-temperature gas-cooled reactors.

[0137] Comparative Example 3

[0138] After standard heat treatment, the GH3536 alloy rods have a tensile strength of 708MPa, a yield strength of 310MPa, an elongation of 48%, and a cross-sectional reduction of 46% at room temperature as specified in the technical standards (source: China High Temperature Alloy Handbook). Due to the lack of micro-alloying elements such as Zr and B, the elongation and cross-sectional reduction are significantly low; in addition, the presence of Co increases costs and poses nuclear risks, and cannot meet the service requirements of high-temperature gas-cooled reactors.

[0139] Comparative Example 4

[0140] The tensile strength of GH1015 alloy at room temperature is 710MPa, the elongation is 48.5%, and the cross-sectional shrinkage is 61.3%. Due to the excessively high W and Nb contents and insufficient Ni content, the room temperature plasticity of the alloy cannot meet the service requirements of high-temperature gas-cooled reactors. In addition, Nb is more susceptible to neutron activation, which is not conducive to the operation and maintenance of the reactor.

[0141] The results of the embodiments show that the novel high-strength and high-toughness nickel-iron-based heat-resistant alloy of the present invention, based on the control of chemical composition and microstructure, combined with optimized thermal processing technology and heat treatment system, simultaneously meets the stringent performance requirements of strength and toughness, creep fatigue resistance, high temperature resistance, corrosion resistance and radiation swelling resistance, and can be used as a structural material for high-temperature gas-cooled reactor pipes.

[0142] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A high-strength and tough nickel-iron-based heat-resistant alloy for high-temperature gas-cooled reactor, characterized in that: The chemical composition is as follows by weight percentage: C 0.01~0.1%, Ni 52.0~57.0%, Cr 17.0~21.0%, Mo 4.0~8.0%, Mn 1.0~2.0%, W 1.0~4.0%, Zr0.01~0.3%, Y 0.03~0.07%, B 0.0005~0.02%, and the balance is Fe and unavoidable impurity elements; the high temperature refers to 850°C; the high-strength and tough nickel-iron-based heat-resistant alloy for high-temperature gas-cooled reactor meets the following mechanical properties at room temperature: tensile strength ≥600MPa, yield strength ≥250MPa, elongation ≥40%, section shrinkage ≥65%, and impact toughness KV2 ≥300J; the tensile properties at 850°C meet the following conditions: tensile strength ≥230MPa, yield strength ≥130MPa, elongation ≥50%, and section shrinkage ≥55%.

2. The high-strength and tough nickel-iron-based heat-resistant alloy for a high-temperature gas-cooled reactor according to claim 1, characterized in that: The weight percentages of Ni and Fe elements satisfy: Ni / Fe≥3.3; the weight percentages of Mo and W elements satisfy: Mo+W≥7.5%; the weight percentages of C, B and Zr elements satisfy: C+B+Zr≥0.055%.

3. The method for preparing the high-strength and tough nickel-iron-based heat-resistant alloy for high-temperature gas-cooled reactor according to any one of claims 1 to 2, characterized in that: The following steps are involved: 1) Alloy preparation: prepare alloy component raw materials according to weight percentage; 2) Smelting: The prepared alloy is subjected to vacuum smelting to be melted into alloy liquid, and the alloy liquid is cast into ingots; 3) Low temperature pretreatment: Place the ingot into a heating furnace and heat it to 1075~1125℃ with the furnace, and keep it warm for ≥2h; 4) Homogenization treatment: Based on step 3), the furnace temperature is raised to 1200-1250°C and the heat preservation time is ≥15 h; 5) Forging: Forging the ingot at 1030-1180 °C into bars with a forging ratio of 7-9; 6) Rolling: The heat-treated bar is rolled at 1030-1180°C to form a rolled bar; 7) Solution treatment: keep the rolled bar at 1050~1150℃ for 30~90 min, and cool it in air or water.

4. The method for preparing a high-strength and tough nickel-iron-based heat-resistant alloy for a high-temperature gas-cooled reactor according to claim 3, characterized in that: In step 2), the vacuum smelting condition is a vacuum degree ≤ 1 Pa.

5. The method for preparing a high-strength and tough nickel-iron-based heat-resistant alloy for a high-temperature gas-cooled reactor according to claim 3, characterized in that: In step 5), the grain size of the forged bar is required to be 4.0~8.

0.

6. The method for preparing a high-strength and tough nickel-iron-based heat-resistant alloy for a high-temperature gas-cooled reactor according to claim 3, characterized in that: In step 6), the rolling process is performed for 5 to 9 passes, and the diameter ratio of the rolled bar to the forged bar is 1:(4 to 5).

7. The method for preparing a high-strength and tough nickel-iron-based heat-resistant alloy for a high-temperature gas-cooled reactor according to claim 3, characterized in that: In step 6), the average grain size of the rolled rod is 10.0 to 14.

0.

8. The method for preparing a high-strength and tough nickel-iron-based heat-resistant alloy for a high-temperature gas-cooled reactor according to claim 3, characterized in that: In step 7), the average grain size of the obtained high-strength and tough nickel-iron based alloy is 4.0 to 8.0.

Images