Accident tolerant cladding material and method of making

Abstract

The application relates to an accident-tolerant cladding material and a preparation method thereof, and belongs to the technical field of nuclear reactor fuel element cladding materials. The cladding material comprises a tempered martensite matrix and a dispersion phase distributed in the tempered martensite matrix; wherein the chemical composition of a low-activation steel powder forming the tempered martensite matrix is as follows in terms of mass fraction: C: 0.08%-1.5%, Cr: 8.0%-10.0%, Ti: 0.01%-0.5%, W: 0.01%-2.0%, V: 0.01%-2.0%, Zr: 0.01%-2.0%, Mn: 0.2%-1.0%, and the balance is Fe and inevitable impurities; and the chemical composition of an iron-chromium-aluminum alloy powder forming the dispersion phase is as follows in terms of mass fraction: C: 0.01%-0.05%, Si: 0.01%-1.0%, Mn: 0.01%-0.3%, Cr: 8.0%-13.0%, Al: 5.0%-8.0%, V: 0.01%-2.0%, Ti: 0.01%-2.0%, W: 0.01%-2.0%, Zr: 0.01%-2%, Y: 0.001%-1.0%, and the balance is Fe and inevitable impurities.

Description

Technical Field

[0001] This application relates to the field of nuclear reactor fuel element cladding materials, and in particular to an accident-tolerant cladding material and its preparation method. Background Technology

[0002] The destructive impact of fossil fuels on the human environment is becoming increasingly apparent. New energy sources such as solar, wind, and hydrogen power are limited by factors like weather, seasons, and energy density, making them unable to provide a stable energy output for humanity. Nuclear energy has become a future energy source that humanity can rely on in the new era. Cladding materials are a core bottleneck affecting the development of nuclear energy. The 2011 Fukushima nuclear disaster made accident-tolerant cladding materials a highly anticipated candidate for fourth-generation light water reactor cladding. Such materials need to possess good corrosion resistance and high-temperature strength under normal operating conditions of 300℃–600℃, while also exhibiting good oxidation resistance and mechanical properties under accident conditions (above 1000℃).

[0003] Iron-chromium-aluminum alloys (ICH-Aluminum alloys) possess excellent high-temperature oxidation resistance and good high-temperature mechanical properties, making them a promising candidate material for fourth-generation light water reactor cladding. However, ICH-Aluminum alloys exhibit strong brittleness around 475°C due to their high chromium content (typically Cr ≥ 13 wt.%), a temperature range that falls precisely within the stable service range of light water reactor cladding. Researchers have attempted to suppress the 475°C brittleness of the α′ chromium-rich phase by controlling the chromium content of traditional ICH-Aluminum alloys to below 13 wt.% and the aluminum content to around 5 wt.%. However, experimental results show that controlling the chromium and aluminum content only reduces the precipitation rate of the α′ chromium-rich phase, but cannot completely suppress its precipitation. Furthermore, radiation effects exacerbate the precipitation of this brittle phase. Therefore, 475°C brittleness has become a significant obstacle to the engineering application of ICH-Aluminum cladding materials. Summary of the Invention

[0004] This application provides an accident-tolerant cladding material and its preparation method to solve the following technical problem: how to avoid the 475℃ brittleness of nuclear reactor cladding materials caused by the precipitation of α′ chromium-rich phase. In a first aspect, embodiments of this application provide an accident-tolerant cladding material, comprising a tempered martensite matrix and a dispersed phase distributed in the tempered martensite matrix; The chemical composition of the low-activation steel powder forming the tempered martensitic matrix is ​​as follows (by mass fraction): C: 0.08%–1.5%, Cr: 8.0%–10.0%, Ti: 0.01%–0.5%, W: 0.01%–2.0%, V: 0.01%–2.0%, Zr: 0.01%–2.0%, Mn: 0.2%–1.0%, with the balance being Fe and unavoidable impurities. The chemical composition of the iron-chromium-aluminum alloy powder forming the dispersed phase, by mass fraction, is as follows: C: 0.01%–0.05%, Si: 0.01%–1.0%, Mn: 0.01%–0.3%, Cr: 8.0%–13.0%, Al: 5.0%–8.0%, V: 0.01%–2.0%, Ti: 0.01%–2.0%, W: 0.01%–2.0%, Zr: 0.01%–2.0%, Y: 0.001%–1.0%, with the balance being Fe and unavoidable impurities; The weight of the iron-chromium-aluminum alloy powder accounts for 5% to 15% of the total weight of the cladding material; The cladding material meets the following properties: ductile-brittle transition temperature ≤ -45℃, room temperature tensile strength ≥ 690MPa, and tensile strength at 650℃ ≥ 230MPa.

[0005] Optionally, the chemical composition of the low-activation steel powder forming the tempered martensitic matrix is ​​as follows by mass fraction: C: 0.08%–0.3%, Cr: 8.5%–9.5%, Ti: 0.03%–0.08%, W: 0.5%–1.5%, V: 0.3%–0.8%, Zr: 0.03%–0.1%, Mn: 0.3%–0.8%, with the balance being Fe and unavoidable impurities.

[0006] Optionally, the chemical composition of the iron-chromium-aluminum alloy powder forming the dispersed phase, by mass fraction, is as follows: C: 0.01%–0.03%, Si: 0.1%–0.5%, Mn: 0.1%–0.25%, Cr: 8.0%–10.0%, Al: 6.0%–8.0%, V: 0.01%–0.1%, Ti: 0.01%–0.5%, W: 0.01%–0.15%, Zr: 0.01%–0.5%, Y: 0.001%–0.01%, with the balance being Fe and unavoidable impurities.

[0007] Secondly, this application provides a method for preparing the accident-tolerant cladding material described in the first aspect, comprising the following steps: Low-activation steel powder and iron-chromium-aluminum alloy powder are mixed to obtain a mixed powder; The mixed powder is subjected to hot isostatic pressing to obtain an alloy ingot; The alloy ingot is subjected to hot extrusion to obtain a tube blank; The tube blank is drawn to obtain a cold-formed clad tube. The cold-formed cladding tube is subjected to normalizing and tempering treatments in sequence to obtain an accident-tolerant cladding material.

[0008] Optionally, the temperature of the hot isostatic pressing treatment is 1150℃~1250℃, and the pressure is 130MPa~180MPa.

[0009] Optionally, the normalizing treatment temperature is 950℃~1150℃, and the time is 3h~5h.

[0010] Optionally, the tempering temperature is 600℃~700℃.

[0011] Optionally, during the process of sequentially normalizing and tempering the cold-formed cladding tube, both the cooling method after normalizing and the cooling method after tempering are air cooling.

[0012] Optionally, before mixing the low-activation steel powder and the iron-chromium-aluminum alloy powder, the method further includes: sieving the low-activation steel powder and the iron-chromium-aluminum alloy powder separately; wherein, The sieving process of the low-activation steel powder includes: subjecting the raw material of the low-activation steel powder to gas atomization powdering to obtain low-activation steel coarse powder. The low-activation steel coarse powder is sieved to remove particles with a particle size greater than 800 μm to obtain low-activation steel powder. The sieving process of the iron-chromium-aluminum alloy powder includes: gas atomizing the raw material of the iron-chromium-aluminum alloy powder to obtain coarse iron-chromium-aluminum alloy powder. The coarse iron-chromium-aluminum alloy powder was sieved to obtain iron-chromium-aluminum alloy powder with a particle size of 30μm to 100μm.

[0013] Optionally, the D50 of the low-activation steel powder is 30μm to 80μm.

[0014] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides an accident-tolerant cladding material and its preparation method. The cladding material includes a tempered martensitic matrix and a dispersed phase distributed within the tempered martensitic matrix. The chemical composition of the low-activation steel powder forming the tempered martensitic matrix, by mass fraction, is: C: 0.08%–1.5%, Cr: 8.0%–10.0%, Ti: 0.01%–0.5%, W: 0.01%–2.0%, V: 0.01%–2.0%, Zr: 0.01%–2.0%, Mn: 0.2%–1.0%, balance... The amount is Fe and unavoidable impurities; the chemical composition of the iron-chromium-aluminum alloy powder forming the dispersed phase is as follows by mass fraction: C: 0.01%–0.05%, Si: 0.01%–1.0%, Mn: 0.01%–0.3%, Cr: 8.0%–13.0%, Al: 5.0%–8.0%, V: 0.01%–2.0%, Ti: 0.01%–2.0%, W: 0.01%–2.0%, Zr: 0.01%–2.0%, Y: 0.001%–1.0%, with the balance being Fe and unavoidable impurities. This application mixes low-activation steel powder with iron-chromium-aluminum alloy powder in a specific ratio, and then performs hot isostatic pressing, hot extrusion, drawing, and normalizing + tempering treatments to construct a composite material with a tempered martensitic matrix and dispersed iron-chromium-aluminum particles. The low-activation steel matrix, with its Cr content controlled below the critical value for α′ phase precipitation and its martensitic structure, fundamentally eliminates 475℃ brittleness. Simultaneously, through normalizing to form fine lath martensite and tempering to precipitate TiC / VC / ZrC / WC nanocarbides, a ductile-brittle transition temperature of ≤-45℃ and a room temperature tensile strength of ≥690MPa are obtained. Under accident conditions, the dispersed iron-chromium-aluminum particles rapidly diffuse high-activity Al to the surface to form a continuous and dense α-Al2O3 film, preventing oxygen from diffusing inward, thus achieving a high-temperature strength of over 230MPa at 650℃ and accident-tolerant oxidation resistance. Attached Figure Description The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0015] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a schematic flowchart illustrating a method for preparing an accident-tolerant cladding material according to an embodiment of this application. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0018] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0019] In a first aspect, embodiments of this application provide an accident-tolerant cladding material, comprising a tempered martensite matrix and a dispersed phase distributed in the tempered martensite matrix; The chemical composition of the low-activation steel powder forming the tempered martensitic matrix is ​​as follows (by mass fraction): C: 0.08%–1.5%, Cr: 8.0%–10.0%, Ti: 0.01%–0.5%, W: 0.01%–2.0%, V: 0.01%–2.0%, Zr: 0.01%–2.0%, Mn: 0.2%–1.0%, with the balance being Fe and unavoidable impurities. The chemical composition of the iron-chromium-aluminum alloy powder forming the dispersed phase, by mass fraction, is as follows: C: 0.01%–0.05%, Si: 0.01%–1.0%, Mn: 0.01%–0.3%, Cr: 8.0%–13.0%, Al: 5.0%–8.0%, V: 0.01%–2.0%, Ti: 0.01%–2.0%, W: 0.01%–2.0%, Zr: 0.01%–2.0%, Y: 0.001%–1.0%, with the balance being Fe and unavoidable impurities; The weight of the iron-chromium-aluminum alloy powder accounts for 5% to 15% of the total weight of the cladding material; The cladding material meets the following properties: ductile-brittle transition temperature ≤ -45℃, room temperature tensile strength ≥ 690MPa, and tensile strength at 650℃ ≥ 230MPa.

[0020] Low-activation steel powder is a precursor for forming the continuous matrix phase of accident-tolerant cladding materials. In low-activation steel powder, when the chromium mass fraction is below 8.0%, the hardenability of the martensitic structure is insufficient, and a fully martensitic structure cannot be obtained after normalizing. When the chromium mass fraction is above 10.0%, the low-activation steel powder itself will enter the local miscibility critical line of the Fe-Cr binary alloy, easily precipitating the α′ Cr-rich phase during subsequent heat treatment or service, leading to 475℃ brittleness. Controlling the chromium mass fraction between 8.0% and 10.0% ensures the formation of a tempered martensitic matrix while completely avoiding the precipitation of the α′ Cr-rich phase. The carbon mass fraction of 0.08%–1.5%, titanium mass fraction of 0.01%–0.5%, tungsten mass fraction of 0.01%–2.0%, vanadium mass fraction of 0.01%–2.0%, and zirconium mass fraction of 0.01%–2.0% together ensure the formation of dispersed carbides during the tempering process, resulting in a secondary hardening effect and improving the room temperature and high temperature tensile strength of the accident-tolerant cladding material.

[0021] Iron-chromium-aluminum alloy powder is a precursor for the dispersed phase in accident-tolerant cladding materials. When the aluminum mass fraction in the iron-chromium-aluminum alloy powder is below 5.0%, the alumina film formation rate is insufficient; when the aluminum mass fraction is above 8.0%, the plasticity of the iron-chromium-aluminum alloy decreases significantly, and the iron-chromium-aluminum particles are prone to cracking during hot extrusion and drawing processes. A chromium mass fraction of 8.0%–13.0% works synergistically with aluminum to ensure the rapid formation of a continuous and dense α-Al₂O₃ protective film by the iron-chromium-aluminum particles above 1000℃. The addition of yttrium at a mass fraction of 0.001%–1.0% in minute quantities significantly improves the adhesion of the alumina film and inhibits its peeling during thermal cycling.

[0022] When the weight percentage of iron-chromium-aluminum alloy powder is less than 5%, the number of iron-chromium-aluminum particles in the accident-tolerant cladding material is insufficient, and the alumina film cannot be continuously covered at high temperatures. When the weight percentage of iron-chromium-aluminum alloy powder is greater than 15%, the iron-chromium-aluminum particles come into contact and agglomerate, forming a continuous or semi-continuous iron-chromium-aluminum network structure, which destroys the continuity of the tempered martensitic matrix and reduces mechanical properties.

[0023] The ductile-brittle transition temperature refers to the temperature at which an accident-tolerant cladding material transitions from a ductile fracture mode to a brittle fracture mode. Above this temperature, significant plastic deformation occurs before fracture, and the fracture surface is fibrous. Below this temperature, no significant plastic deformation occurs before fracture, and the fracture surface is crystalline. The lower the ductile-brittle transition temperature, the stronger the material's resistance to brittle fracture at low temperatures. In the embodiments of this application, a ductile-brittle transition temperature ≤ -45℃ means that the accident-tolerant cladding material maintains a ductile fracture mode within a temperature range of -45℃ and above, and will not experience sudden brittle fracture.

[0024] Room temperature tensile strength refers to the maximum engineering stress that a fault-tolerant cladding material can withstand before tensile fracture at room temperature (typically 25℃±5℃), measured in megapascals (MPa). Tensile strength reflects the material's ability to resist tensile failure. In the embodiments of this application, the room temperature tensile strength of the fault-tolerant cladding material is ≥690MPa.

[0025] The tensile strength at 650℃ refers to the maximum engineering stress that the accident-tolerant cladding material can withstand before tensile fracture at 650℃, and is measured in megapascals (MPa). In the embodiments of this application, the tensile strength of the accident-tolerant cladding material at 650℃ is ≥230MPa.

[0026] In some embodiments, the chemical composition of the low-activation steel powder forming the tempered martensitic matrix is ​​as follows by mass fraction: C: 0.08%–0.3%, Cr: 8.5%–9.5%, Ti: 0.03%–0.08%, W: 0.5%–1.5%, V: 0.3%–0.8%, Zr: 0.03%–0.1%, Mn: 0.3%–0.8%, with the balance being Fe and unavoidable impurities.

[0027] In some embodiments, the chemical composition of the iron-chromium-aluminum alloy powder forming the dispersed phase is as follows by mass fraction: C: 0.01%–0.03%, Si: 0.1%–0.5%, Mn: 0.1%–0.25%, Cr: 8.0%–10.0%, Al: 6.0%–8.0%, V: 0.01%–0.1%, Ti: 0.01%–0.5%, W: 0.01%–0.15%, Zr: 0.01%–0.5%, Y: 0.001%–0.01%, with the balance being Fe and unavoidable impurities.

[0028] Figure 1 This is a schematic flowchart illustrating a method for preparing an accident-tolerant cladding material according to an embodiment of this application.

[0029] Please see Figure 1 Secondly, this application provides a method for preparing the accident-tolerant cladding material described in the first aspect, comprising the following steps: S1. Mix low-activation steel powder and iron-chromium-aluminum alloy powder to obtain mixed powder; S2. The mixed powder is subjected to hot isostatic pressing to obtain an alloy ingot; S3. The alloy ingot is subjected to hot extrusion to obtain a tube blank; S4. The tube blank is drawn to obtain a cold-formed clad tube. S5. The cold-state finished cladding tube is subjected to normalizing and tempering treatments in sequence to obtain an accident-tolerant cladding material.

[0030] The core purpose of the mixing operation is to uniformly disperse two powders with different functions, so that the iron-chromium-aluminum alloy powder is distributed as discrete particles in the low-activation steel powder matrix. Mixing is usually carried out by mechanical powder mixing (such as V-type powder mixer or three-dimensional powder mixer). The mixing time must ensure that the iron-chromium-aluminum alloy powder achieves macroscopic uniform distribution in the low-activation steel powder, laying the foundation for obtaining a two-phase uniform composite alloy ingot by subsequent hot isostatic pressing.

[0031] The core function of hot isostatic pressing (HIP) is to induce plastic flow and atomic diffusion between low-activation steel powder and iron-chromium-aluminum alloy powder, achieving metallurgical bonding between powder particles while eliminating internal porosity to obtain a fully dense alloy ingot. During HIP, strong metallurgical bonds are formed at the interfaces between low-activation steel powder particles, between iron-chromium-aluminum alloy powder particles, and between the two different powder types. The HIP temperature must be higher than the austenitizing temperature of the low-activation steel but lower than the solidus lines of the two alloys to ensure sufficient softening without melting of the powder. The pressure must be sufficient to cause plastic collapse of the powder particles to close the pores, but not to cause excessive deformation or fragmentation of the iron-chromium-aluminum alloy particles.

[0032] Hot extrusion involves heating the obtained alloy ingot to the hot working temperature (usually 1000℃~1200℃), and then passing it through the extrusion die of an extruder, causing the alloy ingot to undergo severe plastic deformation under high pressure, and extruding it from the die hole to form a hollow or solid tube blank.

[0033] The drawing process involves passing the tube blank at room temperature through a drawing machine die, causing the tube blank to undergo plastic deformation under the combined action of axial tension and radial pressure, gradually reducing the diameter and wall thickness to achieve the precise dimensions of the final clad tube, resulting in a cold-formed clad tube.

[0034] The cold-formed cladding tubes are sequentially subjected to normalizing and tempering treatments to obtain accident-tolerant cladding materials. The core function of normalizing is to re-crystallize the low-activation steel matrix into uniform and fine austenite, which then transforms into lath martensite during cooling. Normalizing also eliminates work hardening and residual stress introduced by drawing, and allows carbides to fully dissolve and redistribute uniformly. The core functions of tempering are: first, to eliminate internal stresses generated by martensitic transformation, improving the material's plasticity and toughness; second, to precipitate dispersed nano-sized carbides (such as TiC, VC, ZrC, WC) from supersaturated martensite, producing a secondary hardening effect and improving high-temperature strength; third, to transform brittle quenched martensite into tempered martensite with a good strength-toughness balance; and fourth, to maintain the aluminum activity and structural integrity of the iron-chromium-aluminum alloy particles, ensuring its oxidation resistance under accident conditions. After normalizing and tempering, the low-activation steel matrix is ​​transformed into tempered martensite, and the iron-chromium-aluminum alloy particles are uniformly distributed in the tempered martensite matrix in the form of dispersed phases, with a metallurgical bonding interface between the two phases.

[0035] In some embodiments, the temperature of the hot isostatic pressing treatment is 1150°C to 1250°C, and the pressure is 130MPa to 180MPa.

[0036] Hot isostatic pressing (HIP) is a powder metallurgy process in which mixed powders are placed in a metal sheath, vacuum-sealed, and then placed in a hot isostatic pressing furnace. High temperature and isostatic gas pressure are applied simultaneously, causing the powders to undergo plastic flow and diffusion bonding under high temperature and pressure, ultimately resulting in a fully dense alloy ingot. The temperature of HIP refers to the operating temperature inside the hot isostatic pressing furnace, measured in degrees Celsius (°C). The pressure of HIP refers to the isostatic pressure applied to the sheath surface by an inert gas (usually argon), measured in megapascals (MPa). The HIP temperature must be high enough to activate atomic diffusion between powder particles, allowing the low-activation steel powder and the iron-chromium-aluminum alloy powder to form a metallurgical bond at the interface, rather than a simple mechanical bond. When the temperature is below 1150°C, the diffusion rates of iron, chromium, and aluminum atoms decrease significantly, only partial sintering occurs between powder particles, residual pores cannot close, the relative density of the alloy ingot is below 99%, and the interfacial bonding strength is insufficient, making it prone to cracking along the original powder boundaries during subsequent hot extrusion. Aluminum in iron-chromium-aluminum alloy particles is a key element in the formation of an alumina protective film under accident conditions. Hot isostatic pressing temperatures above 1250℃ cause excessive diffusion of aluminum from the iron-chromium-aluminum alloy particles into the low-activation steel matrix, reducing the aluminum content of the particles and causing aluminum solid solution in the low-activation steel matrix. Aluminum diffusion from the surface of the iron-chromium-aluminum particles into the low-activation steel matrix reduces the aluminum mass fraction of the particles, weakening their ability to form a dense alumina film at high temperatures. This causes the high-temperature oxidation resistance of the accident-tolerant cladding material to drop to a level that fails to meet accident tolerance requirements. Controlling the temperature upper limit to 1250℃ restricts the diffusion distance of aluminum to within a few micrometers of the particle surface, maintaining the aluminum reserves within the particles.

[0037] At temperatures between 1150℃ and 1250℃, the yield strength of low-activation steel and iron-chromium-aluminum alloys decreases significantly. The applied isostatic pressing pressure causes plastic flow in the powder particles, compressing and closing the voids between them. When the pressure is below 130 MPa, the contact stress between powder particles is insufficient, making it difficult to completely eliminate residual porosity. In particular, the closed pores formed between larger powder particles cannot close, resulting in a residual porosity exceeding 0.5% in the alloy ingot. These pores may evolve into crack initiations during subsequent hot extrusion and drawing processes, reducing the density and mechanical properties of the cladding tube. At pressures above 180 MPa, the requirements for hot isostatic pressing equipment increase significantly, raising equipment costs and operational risks. Furthermore, excessive pressure may lead to excessive deformation or even rupture of the cladding, and further improvements in density and interfacial bonding are no longer significant.

[0038] In some embodiments, the normalizing treatment is performed at a temperature of 950°C to 1150°C for a duration of 3 to 5 hours.

[0039] Normalizing is a heat treatment process in which the cold-formed cladding tube is heated to above the austenitizing temperature and held at that temperature for an appropriate time, followed by cooling in air. When the normalizing temperature is below 950℃, the low-activation steel matrix cannot be fully austenitized, leaving residual ferrite. After cooling, sufficient martensite cannot be obtained, resulting in insufficient strength and hardness. When the normalizing temperature is above 1150℃, the austenite grains coarsen, and the resulting martensite laths are large, reducing the material's toughness. Simultaneously, aluminum in the iron-chromium-aluminum alloy particles may diffuse and migrate. The normalizing time is 3 to 5 hours. When the time is less than 3 hours, the temperature is not completely uniform, and austenitization is insufficient; when the time is more than 5 hours, grain coarsening and element diffusion problems are exacerbated. This temperature and time range ensures that the low-activation steel matrix obtains a fine lath martensite structure.

[0040] In some embodiments, the tempering temperature is 600°C to 700°C.

[0041] Tempering is a heat treatment process in which the cold-formed cladding tube, after normalizing, is heated to a temperature below the austenitizing temperature, held at that temperature, and then cooled. When the tempering temperature is below 600℃, the internal stress in the martensite is not fully eliminated, and the amount of carbide precipitation is insufficient, resulting in poor material toughness and a high ductile-brittle transition temperature. When the tempering temperature is above 700℃, excessive martensite recovery occurs, the lath morphology disappears, carbides coarsen, and the strength decreases significantly. Within the tempering temperature range of 600℃ to 700℃, martensite transforms into tempered martensite, precipitating a large number of dispersed nanoscale carbides (such as TiC, VC, ZrC, WC, etc.), achieving the optimal balance between strength and toughness.

[0042] In some embodiments, during the process of sequentially normalizing and tempering the cold-state finished cladding tube, the cooling method after normalizing and the cooling method after tempering are both air cooling.

[0043] Air cooling refers to natural cooling in air, with a cooling rate of approximately 0.5℃ / s to 2℃ / s. Air cooling is used after normalizing because the cooling rate is sufficient to completely transform austenite into martensite, resulting in lath martensite. Air cooling is also used after tempering because the cooling rate is moderate, avoiding rapid cooling that could generate new internal stresses, and also avoiding slow cooling that could lead to carbide coarsening.

[0044] In some embodiments, before mixing the low-activation steel powder and the iron-chromium-aluminum alloy powder, the method further includes: sieving the low-activation steel powder and the iron-chromium-aluminum alloy powder separately; wherein, The sieving process of the low-activation steel powder includes: subjecting the raw material of the low-activation steel powder to gas atomization powdering to obtain low-activation steel coarse powder. The low-activation steel coarse powder is sieved to remove particles with a particle size greater than 800 μm to obtain low-activation steel powder. The sieving process of the iron-chromium-aluminum alloy powder includes: gas atomizing the raw material of the iron-chromium-aluminum alloy powder to obtain coarse iron-chromium-aluminum alloy powder. The coarse iron-chromium-aluminum alloy powder was sieved to obtain iron-chromium-aluminum alloy powder with a particle size of 30μm to 100μm.

[0045] Gas atomization powdering involves melting low-activation steel raw materials into a liquid alloy in a vacuum or inert atmosphere, then impinging the liquid alloy stream with a high-speed jet of inert gas (usually argon or nitrogen). This process breaks the liquid alloy into fine droplets, which are then rapidly cooled and solidified, resulting in spherical or near-spherical low-activation steel coarse powder. The core function of this process is to obtain alloy powder with uniform composition, fine microstructure, and good flowability, providing high-quality raw materials for subsequent mixing and hot isostatic pressing.

[0046] Sieving is used to classify the low-activation steel coarse powder obtained by gas atomization, removing coarse particles larger than 800 μm. These coarse particles are difficult to completely densify during subsequent hot isostatic pressing (HIP), potentially leaving porosity and reducing the density and mechanical properties of the cladding material. After removing particles larger than 800 μm, the maximum particle size of the low-activation steel powder is controlled below 800 μm, ensuring the flowability and uniformity of bulk density of the powder during mixing and HIP packaging.

[0047] The gas atomization powdering process for iron-chromium-aluminum alloy raw materials follows the same principle as the preparation of low-activation steel powder, both utilizing high-speed inert gas to break up the alloy liquid flow and obtain spherical powder. After sieving, the particle size of the iron-chromium-aluminum alloy powder is limited to 30μm–100μm. The lower limit of 30μm avoids the risk of surface oxidation from excessively fine powder; the upper limit of 100μm ensures that the iron-chromium-aluminum alloy particles exist as discrete particles in the low-activation steel powder matrix, avoiding stress concentration or damage to the matrix continuity caused by excessively large particles. Simultaneously, the 30μm–100μm particle size range matches the particle size range of the low-activation steel powder, ensuring uniform dispersion of the two powders during mixing, without severe segregation or separation.

[0048] In some embodiments, the D50 of the low-activation steel powder is 30 μm to 80 μm.

[0049] D50 refers to the particle size at which the cumulative volume distribution of powder reaches 50%, also known as the median diameter. When the D50 of low-activation steel powder is less than 30 μm, the powder is too fine, has an excessively large specific surface area, and is prone to adsorbing oxygen and moisture, increasing the risk of powder surface oxidation. Furthermore, fine powder is prone to dust generation and segregation during hot isostatic pressing (HIP) packaging. When the D50 of low-activation steel powder is greater than 80 μm, the powder particles are relatively coarse, resulting in larger gaps between particles during HIP. Higher temperatures and pressures are required to achieve complete densification. Moreover, the alloy ingots obtained from coarse powder have coarser grains, which is not conducive to subsequent hot extrusion and mechanical properties.

[0050] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0051] Example 1 Preparation of low-activation steel powder: Low-activation steel raw materials were prepared according to the following chemical composition (mass fraction): C: 0.1%, Cr: 9.0%, Ti: 0.05%, W: 1.0%, V: 0.5%, Zr: 0.05%, Mn: 0.5%, with the balance being iron (Fe) and unavoidable impurities. The above low-activation steel raw materials were melted in a vacuum induction melting furnace to obtain low-activation steel molten steel. High-purity argon gas (purity ≥99.99%) was used as the atomizing medium to perform gas atomization powdering treatment on the low-activation steel molten steel to obtain low-activation steel coarse powder. The low-activation steel coarse powder was sieved to remove particles with a particle size greater than 800 μm, obtaining low-activation steel powder. The D50 of this low-activation steel powder was determined to be 50 μm by a laser particle size analyzer.

[0052] Preparation of iron-chromium-aluminum alloy powder: Iron-chromium-aluminum alloy raw materials were prepared according to the following chemical composition (mass fraction): C: 0.02%, Si: 0.2%, Mn: 0.2%, Cr: 8.0%, Al: 8.0%, V: 0.05%, Zr: 0.1%, Ti: 0.01%, W: 0.05%, Y: 0.003%, with the balance being iron (Fe) and unavoidable impurities. The above iron-chromium-aluminum alloy raw materials were then melted in a vacuum induction melting furnace to obtain molten iron-chromium-aluminum alloy steel. High-purity argon gas (purity ≥99.99%) was used as the atomizing medium to perform gas atomization powdering treatment on the molten iron-chromium-aluminum alloy steel to obtain coarse iron-chromium-aluminum alloy powder. The coarse iron-chromium-aluminum alloy powder was subjected to multi-stage sieving to obtain iron-chromium-aluminum alloy powder with a particle size of 50μm to 100μm.

[0053] Mixing: The above-mentioned low-activation steel powder and iron-chromium-aluminum alloy powder were placed in a V-type mixer and mixed at 30 rpm for 5 hours under argon protection to obtain a mixed powder. The weight of the iron-chromium-aluminum alloy powder accounted for 5% of the total weight of the mixed powder.

[0054] Hot isostatic pressing (HIP): The mixed powder is placed in a stainless steel sleeve, degassed under vacuum, and then sealed. The sleeve is placed in a hot isostatic pressing (HIP) apparatus for HIP treatment at a temperature of 1230℃ and a pressure of 150MPa. The resulting alloy ingot is obtained after HIP treatment.

[0055] Hot extrusion treatment: The alloy ingot is processed into an extrusion billet, and then hot extruded at 1100℃ to obtain a tube blank.

[0056] Drawing process: The tube blank is subjected to multiple cold drawing processes to obtain the finished clad tube in a cold state. The finished clad tube in a cold state is placed in a vacuum furnace (vacuum degree ≤10). -2 In a vacuum furnace, the temperature is increased to 1050℃ at a rate of 10℃ / min, held for 4 hours, and then removed and air-cooled to complete the normalizing treatment. Then the cladding tube is placed back into the vacuum furnace and heated to 650℃ at a rate of 10℃ / min. After holding for 4 hours, it is air-cooled to complete the tempering treatment, thus obtaining the accident-tolerant cladding material.

[0057] Performance testing According to GB / T 229-2020 standard, the ductile-brittle transition temperature of the accident-tolerant cladding material was determined to be -50℃ using the Charpy V-notch impact test. The room temperature tensile strength was determined to be 700 MPa and the room temperature elongation to be 15% according to GB / T 228.1-2010 standard. The tensile strength at 650℃ was determined to be 250 MPa according to GB / T 228.2-2015 standard. According to GB / T 13303-91 standard, an oxidation kinetic test was conducted at 1000℃ using the static weight gain method for 100 hours, and the calculated oxidation kinetic parabolic coefficient was 0.02158 μg. 2 mm -4 h -1 .

[0058] Example 2 This embodiment provides a method for preparing an accident-tolerant cladding material. The difference between this embodiment and Embodiment 1 is that the chemical composition, particle size, hot isostatic pressing parameters, and heat treatment parameters of the low-activation steel powder and the iron-chromium-aluminum alloy powder are different.

[0059] The chemical composition (mass fraction) of the low-activation steel powder is as follows: C: 0.1%, Cr: 9.0%, Ti: 0.05%, W: 1.0%, V: 0.5%, Zr: 0.08%, Mn: 0.5%, with the balance being iron (Fe) and unavoidable impurities. The preparation method is the same as in Example 1, yielding low-activation steel powder with a D50 of 55 μm.

[0060] The chemical composition (mass fraction) of the iron-chromium-aluminum alloy powder is as follows: C: 0.02%, Si: 0.2%, Mn: 0.2%, Cr: 9.0%, Al: 7.5%, V: 0.01%, Zr: 0.15%, Ti: 0.5%, W: 0.1%, Y: 0.003%, with the balance being iron (Fe) and unavoidable impurities. The preparation method is the same as in Example 1, and after sieving, iron-chromium-aluminum alloy powder with a particle size of 30 μm to 80 μm is obtained.

[0061] The mixing steps are the same as in Example 1; the hot isostatic pressing temperature is 1250℃ and the hot isostatic pressing pressure is 130MPa; the hot extrusion and drawing processes are the same as in Example 1.

[0062] The normalizing temperature is 1060℃, and the holding time is 3.5 hours. The tempering temperature is 630℃, and the holding time is 4 hours.

[0063] Following the same testing method as in Example 1, the ductile-brittle transition temperature of the accident-tolerant cladding material was measured to be -45°C, the room temperature tensile strength to be 690 MPa, the room temperature elongation to be 15%, the tensile strength at 650°C to be 230 MPa, and the oxidation kinetic parabola coefficient at 1000°C to be 0.02467 μg. 2 mm -4 h -1 .

[0064] Example 3 This embodiment provides a method for preparing an accident-tolerant cladding material. The difference between this embodiment and Embodiment 1 lies in the different chemical composition, hot isostatic pressing parameters, and heat treatment parameters of the low-activation steel powder and the iron-chromium-aluminum alloy powder.

[0065] The chemical composition (mass fraction) of the low-activation steel powder is as follows: C: 0.1%, Cr: 9.0%, Ti: 0.03%, W: 0.5%, V: 0.8%, Zr: 0.06%, Mn: 0.5%, with the balance being iron (Fe) and unavoidable impurities. The preparation method is the same as in Example 1, yielding low-activation steel powder with a D50 of 48 μm.

[0066] The chemical composition (mass fraction) of the iron-chromium-aluminum alloy powder is as follows: C: 0.02%, Si: 0.3%, Mn: 0.2%, Cr: 8.5%, Al: 7.0%, V: 0.01%, Zr: 0.10%, Ti: 0.2%, W: 0.1%, Y: 0.01%, with the balance being iron (Fe) and unavoidable impurities. The preparation method is the same as in Example 1, and after sieving, iron-chromium-aluminum alloy powder with a particle size of 50 μm to 100 μm is obtained.

[0067] The mixing steps are the same as in Example 1; the hot isostatic pressing (HIP) temperature is 1250℃, and the HIP pressure is 150MPa; the hot extrusion and drawing processes are the same as in Example 1. The normalizing temperature is 1060℃, and the holding time is 4 hours. The tempering temperature is 670℃, and the holding time is 4 hours.

[0068] Following the same testing method as in Example 1, the ductile-brittle transition temperature of the accident-tolerant cladding material was measured to be -50°C, the room temperature tensile strength to be 710 MPa, the room temperature elongation to be 15%, the tensile strength at 650°C to be 250 MPa, and the oxidation kinetic parabola coefficient at 1000°C to be 0.02684 μg. 2 mm -4 h -1 .

[0069] Comparative Example 1 This comparative example provides a method for preparing a coating material that uses only low-activation steel powder and does not add iron-chromium-aluminum alloy powder.

[0070] The chemical composition (mass fraction) of the low-activation steel powder is as follows: C: 0.1%, Cr: 9.0%, Ti: 0.03%, W: 0.5%, V: 0.8%, Zr: 0.06%, Mn: 0.5%, with the balance being iron (Fe) and unavoidable impurities. The preparation method is the same as in Example 1, yielding low-activation steel powder with a D50 of 50 μm.

[0071] Hot isostatic pressing (HIP): Low-activation steel powder is placed in a stainless steel casing, vacuum degassed, and then sealed. The HIP temperature is 1250℃, and the HIP pressure is 150MPa. The resulting alloy ingot is obtained after HIP. Hot extrusion and drawing processes are performed as in Example 1 to obtain the cold-formed clad tube.

[0072] The normalizing temperature is 1050℃, and the holding time is 4 hours. The tempering temperature is 650℃, and the holding time is 4 hours.

[0073] Following the same testing method as in Example 1, the ductile-brittle transition temperature of the cladding material was measured to be -50℃, the room temperature tensile strength to be 700 MPa, the room temperature elongation to be 15%, the tensile strength at 650℃ to be 210 MPa, and the oxidation kinetic parabola coefficient at 1000℃ to be 0.07843 μg. 2 mm -4 h -1 .

[0074] Furthermore, one or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages: Completely avoids 475℃ brittleness: By using tempered martensitic low-activation steel as the matrix and controlling the mass fraction of chromium in the matrix between 8.0% and 10.0%, the precipitation of α′ chromium-rich phase is eliminated from the material design, reducing the ductile-brittle transition temperature of the cladding material to below -45℃.

[0075] It provides excellent high-temperature oxidation resistance under accident conditions: the iron-chromium-aluminum particles uniformly dispersed in the low-activation steel matrix rapidly diffuse to the surface under accident conditions, forming a continuous and dense α-alumina protective film, which effectively prevents oxygen from diffusing inward and avoids catastrophic oxidation and hydrogen explosion reaction of the cladding material.

[0076] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. An accident-tolerant cladding material, characterized in that, It includes a tempered martensite matrix and a dispersed phase distributed in the tempered martensite matrix; The chemical composition of the low-activation steel powder forming the tempered martensitic matrix is ​​as follows (by mass fraction): C: 0.08%–1.5%, Cr: 8.0%–10.0%, Ti: 0.01%–0.5%, W: 0.01%–2.0%, V: 0.01%–2.0%, Zr: 0.01%–2.0%, Mn: 0.2%–1.0%, with the balance being Fe and unavoidable impurities. The chemical composition of the iron-chromium-aluminum alloy powder forming the dispersed phase, by mass fraction, is as follows: C: 0.01%–0.05%, Si: 0.01%–1.0%, Mn: 0.01%–0.3%, Cr: 8.0%–13.0%, Al: 5.0%–8.0%, V: 0.01%–2.0%, Ti: 0.01%–2.0%, W: 0.01%–2.0%, Zr: 0.01%–2.0%, Y: 0.001%–1.0%, with the balance being Fe and unavoidable impurities; The weight of the iron-chromium-aluminum alloy powder accounts for 5% to 15% of the total weight of the cladding material; The cladding material meets the following properties: ductile-brittle transition temperature ≤ -45℃, room temperature tensile strength ≥ 690MPa, and tensile strength at 650℃ ≥ 230MPa.

2. The accident-tolerant cladding material according to claim 1, characterized in that, The chemical composition of the low-activation steel powder forming the tempered martensitic matrix is ​​as follows by mass fraction: C: 0.08%–0.3%, Cr: 8.5%–9.5%, Ti: 0.03%–0.08%, W: 0.5%–1.5%, V: 0.3%–0.8%, Zr: 0.03%–0.1%, Mn: 0.3%–0.8%, with the balance being Fe and unavoidable impurities.

3. The accident-tolerant cladding material according to claim 1, characterized in that, The chemical composition of the iron-chromium-aluminum alloy powder forming the dispersed phase, by mass fraction, is as follows: C: 0.01%–0.03%, Si: 0.1%–0.5%, Mn: 0.1%–0.25%, Cr: 8.0%–10.0%, Al: 6.0%–8.0%, V: 0.01%–0.1%, Ti: 0.01%–0.5%, W: 0.01%–0.15%, Zr: 0.01%–0.5%, Y: 0.001%–0.01%, with the balance being Fe and unavoidable impurities.

4. A method for preparing an accident-tolerant cladding material according to any one of claims 1 to 3, characterized in that, Includes the following steps: Low-activation steel powder and iron-chromium-aluminum alloy powder are mixed to obtain a mixed powder; The mixed powder is subjected to hot isostatic pressing to obtain an alloy ingot; The alloy ingot is subjected to hot extrusion to obtain a tube blank; The tube blank is drawn to obtain a cold-formed clad tube. The cold-formed cladding tube is subjected to normalizing and tempering treatments in sequence to obtain an accident-tolerant cladding material.

5. The preparation method according to claim 4, characterized in that, The hot isostatic pressing process is performed at a temperature of 1150℃ to 1250℃ and a pressure of 130MPa to 180MPa.

6. The preparation method according to claim 4, characterized in that, The normalizing treatment is performed at a temperature of 950℃ to 1150℃ for 3 to 5 hours.

7. The preparation method according to claim 4, characterized in that, The tempering temperature is 600℃~700℃.

8. The preparation method according to claim 4, characterized in that, During the process of sequentially normalizing and tempering the cold-state finished cladding tube, the cooling method after normalizing and the cooling method after tempering are both air cooling.

9. The preparation method according to claim 4, characterized in that, Before mixing the low-activation steel powder and the iron-chromium-aluminum alloy powder, the method further includes: sieving the low-activation steel powder and the iron-chromium-aluminum alloy powder separately; wherein... The sieving process of the low-activation steel powder includes: subjecting the raw material of the low-activation steel powder to gas atomization powdering to obtain low-activation steel coarse powder. The low-activation steel coarse powder is sieved to remove particles with a particle size greater than 800 μm to obtain low-activation steel powder. The sieving process of the iron-chromium-aluminum alloy powder includes: gas atomizing the raw material of the iron-chromium-aluminum alloy powder to obtain coarse iron-chromium-aluminum alloy powder. The coarse iron-chromium-aluminum alloy powder was sieved to obtain iron-chromium-aluminum alloy powder with a particle size of 30μm to 100μm.

10. The preparation method according to claim 9, characterized in that, The D50 of the low-activation steel powder is 30μm to 80μm.

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