Cr-volatilization-resistant Mo-doped high-temperature alloy for BOP assembly and preparation method of Cr-volatilization-resistant Mo-doped high-temperature alloy

Abstract

The invention discloses a Cr-volatilization-resistant Mo-doped high-temperature alloy for a BOP assembly and a preparation method of the Cr-volatilization-resistant Mo-doped high-temperature alloy, and relates to the technical field of alloy high-temperature surface protection. The invention aims to solve the problems of Cr volatilization in a high-temperature environment and protection failure of a traditional coating. The alloy comprises the following components in percentage by weight: 1-3% of Mo, 25% of Cr, 20% of Ni and the like, and the balance of Fe. By accurately controlling the composition and the process, Mo-rich Laves phases which are uniformly distributed are formed on an oxide / metal interface, so that the ion diffusion rate is effectively reduced, and the Cr volatilization resistance is improved. The alloy has good processing and welding performance, and is suitable for preparation of solid oxide fuel cell BOP key components in a high-temperature and high-humidity environment.

Description

Technical Field

[0001] This invention relates to the field of high-temperature surface protection technology for alloys, and in particular to a high-temperature alloy with anti-Cr volatilization and Mo doping for BOP components and its preparation method. Background Technology

[0002] Solid oxide fuel cells (SOFCs) can cleanly and efficiently convert chemical energy directly into electrical energy, becoming an important component of future distributed power generation and clean energy systems. A complete SOFC system includes a core power generation unit consisting of multiple individual cells assembled into a stack, and an auxiliary subsystem, namely the Balance of Plant (BOP). BOP components include many parts necessary for stable stack operation, such as fluid transport, thermal management, and gas handling, including air / fuel preheaters, heat exchangers, piping, valves, and exhaust gas recirculation devices. These components play a crucial role in the system, responsible for providing the stack with gases for reaction and efficiently recovering waste heat and unreacted fuel from the exhaust gas, directly affecting the overall power generation efficiency, thermal efficiency, and operating costs of the system.

[0003] However, under the operating conditions of fuel cell stack systems, most BOP (Battery Opening) components (especially those located in the high-temperature exhaust flow path of the stack) must be exposed to a high-temperature and high-humidity environment for extended periods. Chromium-containing stainless steel (such as 310S and 430) is currently a commonly used material for BOP components, possessing significant advantages such as high electrical conductivity, good thermal conductivity, simple processing, ease of forming, and low cost. Furthermore, at high temperatures, a dense Cr2O3 protective oxide layer can form on its surface, providing oxidation protection for the material.

[0004] However, under the service conditions of BOP components, the Cr2O3 layer reacts with O2 or H2O to generate gaseous chromium, which not only causes degradation of the protective layer and continuous oxidation and corrosion of the substrate, but more seriously, the volatilized gaseous chromium species enter the cathode with the air and poison the battery cathode, and undergo deposition reactions on the cathode surface, blocking the active sites of the three-phase interface, significantly reducing the performance and life of the solid oxide fuel cell system, and seriously restricting the commercial application of SOFC.

[0005] Therefore, there is an urgent need to develop a new type of Cr volatilization-resistant alloy material that can effectively suppress high-temperature Cr volatilization, maintain good processing performance, weldability and cost advantages, and eliminate the need for external coating protection, so as to meet the requirements of long-term stable operation of solid oxide fuel cell BOP components under harsh conditions. Summary of the Invention

[0006] In view of this, the present invention provides a Mo-doped high-temperature alloy for BOP (Boiler Plant) components that resists Cr volatilization and its preparation method. By precisely controlling the alloy composition and optimizing the preparation process, the present invention ensures the uniform distribution of Mo in the matrix and the controllable formation of the Laves phase during subsequent oxidation. This provides material assurance for the long-term stable operation of BOP components in solid oxide fuel cells under harsh conditions. It not only effectively solves the problem of high-temperature Cr volatilization in traditional stainless steel but also maintains the material's good mechanical properties, processing performance, and cost advantages, possessing significant application value and broad market prospects.

[0007] The first aspect of the present invention is to provide a Mo-doped high-temperature alloy resistant to Cr volatilization for BOP components, wherein the alloy comprises the following components by mass percentage: Mo 1-3wt%, Cr 25wt%, Ni 20wt%, Mn 1.5wt%, Si 0.4wt%, with the balance being Fe and unavoidable impurities; The alloy is obtained by melting, refining, casting and furnace cooling. During the high-temperature oxidation process, the alloy forms a Mo-rich Laves phase.

[0008] Preferably, the total amount of impurities is ≤0.05%, and the actual content of each element deviates from the designed composition by no more than ±0.3%.

[0009] Preferably, the Laves phase is of type C14.

[0010] Preferably, the content of Mo is 2 wt%.

[0011] A second aspect of the present invention is to provide a method for preparing the above-mentioned Cr-resistant Mo-doped high-temperature alloy for BOP components, comprising the following steps: S1. Place the raw materials into the vacuum melting furnace in order of their melting points from high to low. S2. After evacuating the furnace body, fill it with high-purity argon gas and proceed with melting and refining in sequence. S3. Electromagnetically stir at the refining temperature, then cast into ingots; S4. Cool the furnace to room temperature in a protective atmosphere.

[0012] Preferably, in step S1, the raw materials are in the following order of melting point from high to low: molybdenum, chromium, nickel, iron, manganese, and silicon; the purity of the raw materials is not less than 99.9%; and high-purity titanium sheets are added to the furnace as getter before smelting.

[0013] Preferably, in step S2, the vacuum is ≤1×10⁻⁶. -2Pa, the melting temperature is 1500℃-1550℃, and refining is carried out after all components are melted and cleared; the refining temperature is 1520℃-1570℃, and the refining time is 10-20 min.

[0014] Preferably, in step S3, the electromagnetic stirring lasts for 2-3 minutes each time, followed by standing for 1-2 minutes, and the cycle is repeated 3-5 times.

[0015] Preferably, in step S4, furnace cooling refers to cooling to below 600°C at a cooling rate of no more than 30°C / min under an argon protective atmosphere, followed by natural cooling to room temperature.

[0016] The Cr-resistant Mo-doped high-temperature alloy for BOP components is suitable for heat exchangers, valves, and gas duct structures in solid oxide fuel cell systems that are exposed to high temperature and humidity environments of 700℃-900℃ for extended periods.

[0017] The core principle of this invention is based on the formation of a Mo-rich Laves phase layer at the metal / oxide interface during high-temperature oxidation of Mo, which effectively suppresses the volatilization of Cr. In the BOP (Balance of Plant) component operating environment of a solid oxide fuel cell system, Cr-containing stainless steel experiences severe Cr volatilization under high-temperature conditions. Cr in the Cr2O3 protective layer formed on the surface escapes as gaseous CrO3 or CrO2(OH)2. This not only damages the protective oxide layer, leading to continuous oxidation and corrosion of the substrate, but also causes the volatilized Cr species to deposit on the cathode surface of the fuel cell, resulting in severe performance degradation or even failure. While traditional ferritic stainless steel possesses good electrical conductivity and a low coefficient of thermal expansion, its high-temperature Cr volatilization problem limits its application in fuel cell systems.

[0018] This invention fundamentally alters the high-temperature oxidation behavior of Fe-Cr-Ni based alloys by doping them with an appropriate amount of Mo. Mo has a high melting point and a low diffusion coefficient, existing in a solid solution state within the alloy matrix. When the alloy is used in a high-temperature oxidizing environment, a Cr-rich oxide layer initially forms on the surface. As the oxidation process progresses, Mo accumulates at the interface between the oxide layer and the metal matrix, reacting with Fe, Cr, and other elements in the matrix to form the intermetallic compound Laves phase, with the chemical formula (Fe,Cr)₂Mo and a C14 hexagonal crystal structure. This Laves phase forms a continuous and dense phase band at the interface, exhibiting high thermodynamic stability and an extremely low ion diffusion coefficient.

[0019] Mo-rich Laves phase layers play a crucial barrier role in the anti-Cr volatilization mechanism. On one hand, this phase layer effectively pins the metal / oxide interface, significantly reducing the outward diffusion rate of Cr ions from the metal matrix to the oxide layer, thereby reducing the available Cr sources for volatilization in the oxide layer. On the other hand, the dense Laves phase layer also hinders the inward diffusion of oxygen ions from the oxide layer to the metal matrix, inhibiting the growth rate of new oxide layers and transforming the entire oxidation process from rapid oxidation to slow, steady-state oxidation. Furthermore, Mo itself is not easily volatilized at high temperatures, and its enrichment at the interface can stabilize the oxide layer structure, preventing oxide layer peeling and cracking. Through this bidirectional diffusion barrier effect, the Laves phase layer significantly reduces the Cr volatilization rate, extending the service life of the material under high-temperature environments.

[0020] The amount of Mo doping has a decisive influence on the formation of the Laves phase and its resistance to Cr volatilization. When the Mo content is too low, the amount of Laves phase formed at the interface is insufficient, the phase layer is not continuous and dense enough, and the barrier effect on Cr diffusion is limited. When the Mo content is too high, the excessive Mo will lead to excessive precipitation of the Laves phase, coarsening of phase particles and uneven distribution. This may even cause microcracks or stress concentration at the phase interface, affecting the overall density of the oxide layer and resulting in a slight decrease in the resistance to Cr volatilization. Only when the Mo content is moderate, the Laves phase layer formed is the most continuous, dense and uniformly distributed, with good Cr volatilization inhibition and small oxidation weight gain. This ensures sufficient Laves phase formation while avoiding the negative effects of excessive precipitation.

[0021] Compared with the prior art, the beneficial technical effects of the present invention are as follows: This invention utilizes vacuum melting technology combined with specific process parameter control, and adds high-purity titanium sheets as a getter before melting to further reduce the oxygen content in the furnace, effectively ensuring the high purity and compositional uniformity of the alloy, laying the foundation for the uniform formation of the Laves phase during subsequent high-temperature oxidation.

[0022] The Mo element doped in this invention will accumulate at the oxide / metal interface of the alloy and form a Mo-rich Laves phase. This phase has an extremely low ion diffusion coefficient and high thermodynamic stability, which can effectively pin the interface and form a continuous and dense diffusion barrier layer, significantly reducing the rate of Cr ion diffusion to the outside and oxygen ion diffusion to the inside of the alloy. Furthermore, the optimal balance between the amount and distribution of Laves phase is achieved by precisely controlling the Mo content.

[0023] Compared to traditional surface coating protection, the Mo doping modification method used in this invention has superior performance and reliability. By alloying with Mo, a thermodynamically stable Laves phase is formed in situ within the alloy, fundamentally avoiding the failure problems such as peeling and cracking that may occur due to thermal expansion mismatch and long-term service aging of external coatings. At the same time, eliminating the need for external coatings simplifies the manufacturing process, reduces production costs, and avoids defects that may be introduced during coating application. It is particularly suitable for shell-and-tube heat exchangers, valve interiors, and other BOP components with complex geometries or internal cavity structures. Furthermore, bulk modification eliminates the problem of the coating-substrate interface. Even if local damage or wear occurs on the surface, the internal Laves phase layer can still provide continuous protection, ensuring the stability and reliability of the material during long-term service.

[0024] The Mo-doped alloy of the present invention maintains excellent resistance to Cr volatilization while having a low Mo content, which has little impact on the plasticity of the alloy matrix and still maintains good processing performance. Furthermore, it does not lead to a large amount of brittle phase precipitation during welding, and the weld and heat-affected zone maintain good toughness and strength. It is suitable for BOP components that require complex welding structures.

[0025] The alloy of this invention is suitable for key components such as heat exchangers, valves, gas duct structures, fuel / air preheaters, and exhaust gas recirculation pipelines that are exposed to high temperature and humidity environments of 700-900℃ for a long time, and has broad application prospects in solid oxide fuel cell BOP modules. Attached Figure Description

[0026] The present invention will be further described below with reference to the accompanying drawings.

[0027] Figure 1 This is a process flow diagram of the method for preparing the Cr-resistant Mo-doped high-temperature alloy for BOP components according to the present invention. Figure 2 This is a scanning electron microscope image of the surface of the Mo-doped high-temperature alloy prepared in Example 1 of the present invention; Figure 3 The image shows a scanning electron microscope image and a corresponding chemical element distribution map of the Mo-doped high-temperature alloy prepared in Example 1 of this invention after oxidation weight gain test. Figure 4 The graphs show the changes in Cr volatilization rate over time in Examples 1-3 and Comparative Example 1 of the present invention under a high temperature and high humidity environment of 800℃. Figure 5 The graphs show the changes in oxidative weight gain over time in Examples 1-3 and Comparative Example 1 of the present invention at a high temperature of 800℃. Detailed Implementation

[0028] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] Unless otherwise specified, all experiments were repeated three times, and the results are expressed as averages.

[0030] Example 1: A method for preparing a Mo-doped high-temperature alloy resistant to Cr volatilization for BOP components, comprising the following steps: (1) The alloy composition is 1 wt% Mo, 25 wt% Cr, 20 wt% Ni, 1.5 wt% Mn, 0.4 wt% Si, with the balance being Fe. The purity of all raw materials used is 99.9%. The raw materials are placed in the melting groove of the vacuum melting furnace in order of metal melting point from high to low, from bottom to top. The specific order is: molybdenum granules, chromium blocks, nickel sheets, iron blocks, manganese blocks and silicon blocks. The loading order takes advantage of the high melting point of molybdenum and chromium, placing them at the bottom for direct heating, while nickel and iron are placed at the top to melt first and drip down, promoting the uniformity of overall melting and reducing element burn-off. (2) Before the start of smelting, high-purity titanium sheets are placed in a pre-set groove in the furnace, which is independent of the main smelting groove, as a getter to further reduce the oxygen concentration in the furnace. (3) Evacuate the vacuum melting furnace to ≤1×10 -2 After Pa, high-purity argon gas is introduced to bring the furnace pressure to 0.06 MPa, and melting is carried out at 1550℃; after all components are melted and cleared, the temperature is raised to 1560℃ for refining and held for 15 min. (4) Perform electromagnetic stirring three times at the refining temperature, each stirring lasting 2 minutes, followed by standing for 2 minutes. This process is repeated to ensure that the elements are evenly distributed at both the macroscopic and microscopic scales. After stirring and standing, the mixture is cast into an ingot. (5) The ingot is cooled in the furnace under an argon protective atmosphere and slowly cooled to 600°C at a cooling rate of 20°C / min, and then naturally cooled to room temperature. This slow cooling system helps to reduce the internal stress of the ingot and avoid cracking.

[0031] The Mo-doped high-temperature alloy ingot prepared in this embodiment has a highly uniform composition. Microstructure analysis of the alloy prepared in this embodiment was performed (…). Figure 2 A uniform and dense oxide layer structure can be observed on the alloy surface. This is further supported by scanning electron microscopy images and chemical element distribution maps of the cross-section of the alloy after oxidation. Figure 3It can be clearly observed that a Mo-rich Laves phase layer is formed at the interface between the oxide layer (Cr2O3) and the metal matrix. The elemental distribution map shows that Cr is mainly concentrated in the oxide layer, Fe and Ni are distributed in the metal matrix, while Mo is significantly enriched at the metal / oxide interface, forming a continuous Mo-rich phase band. The O elemental distribution map shows a high oxygen content in the oxide layer region. The formation of this interfacial Laves phase is key to the anti-Cr volatilization mechanism of this invention; this phase effectively pins the interface, hindering the outward diffusion of Cr ions and the inward diffusion of oxygen ions.

[0032] Example 2: A method for preparing a Mo-doped high-temperature alloy resistant to Cr volatilization for BOP components, comprising the following steps: (1) The alloy composition is 2 wt% Mo, 25 wt% Cr, 20 wt% Ni, 1.5 wt% Mn, 0.4 wt% Si, with the balance being Fe. The content of each major element deviates from the design target by less than ±0.3%, and the purity of the raw materials used is not less than 99.9%. The raw materials are placed in the melting groove of the vacuum melting furnace in order of metal melting point from high to low and from bottom to top. The specific order is: molybdenum granules, chromium blocks, nickel sheets, iron blocks, manganese blocks and silicon blocks. (2) Before the start of smelting, high-purity titanium sheets are placed in a pre-set groove in the furnace, which is independent of the main smelting groove, as a getter to further reduce the oxygen concentration in the furnace. (3) Evacuate the vacuum melting furnace to ≤1×10 -2 After Pa, high-purity argon gas is introduced to bring the furnace pressure to 0.06 MPa, and melting is carried out at 1550℃; after all components are melted and cleared, the temperature is raised to 1560℃ for refining and held for 15 min. (4) Perform electromagnetic stirring three times at the refining temperature, each stirring lasting 2 minutes, followed by standing for 2 minutes. This process is repeated to ensure that the elements are evenly distributed at both the macroscopic and microscopic scales. After stirring and standing, the mixture is cast into an ingot. (5) The ingot is cooled in the furnace under an argon protective atmosphere and slowly cooled to 600°C at a cooling rate of 20°C / min, and then naturally cooled to room temperature. This slow cooling system helps to reduce the internal stress of the ingot and avoid cracking.

[0033] The Mo-doped high-temperature alloy ingot prepared in this embodiment has a highly uniform composition.

[0034] Example 3 A method for preparing a Mo-doped high-temperature alloy resistant to Cr volatilization for BOP components, comprising the following steps: (1) The alloy composition is 3 wt% Mo, 25 wt% Cr, 20 wt% Ni, 1.5 wt% Mn, 0.4 wt% Si, with the balance being Fe. The content of each major element deviates from the design target by less than ±0.3%, and the purity of the raw materials used is not less than 99.9%. The raw materials are placed in the melting groove of the vacuum melting furnace in order of metal melting point from high to low and from bottom to top. The specific order is: molybdenum granules, chromium blocks, nickel sheets, iron blocks, manganese blocks and silicon blocks. (2) Before the start of smelting, high-purity titanium sheets are placed in a pre-set groove in the furnace, which is independent of the main smelting groove, as a getter to further reduce the oxygen concentration in the furnace. (3) Evacuate the vacuum melting furnace to ≤1×10 -2 After Pa, high-purity argon gas is introduced to bring the furnace pressure to 0.06 MPa, and melting is carried out at 1550℃; after all components are melted and cleared, the temperature is raised to 1560℃ for refining and held for 15 min. (4) Perform electromagnetic stirring three times at the refining temperature, each stirring lasting 2 minutes, followed by standing for 2 minutes. This process is repeated to ensure that the elements are evenly distributed at both the macroscopic and microscopic scales. After stirring and standing, the mixture is cast into an ingot. (5) The ingot is cooled in the furnace under an argon protective atmosphere and slowly cooled to 600°C at a cooling rate of 20°C / min, and then naturally cooled to room temperature. This slow cooling system helps to reduce the internal stress of the ingot and avoid cracking.

[0035] The Mo-doped high-temperature alloy ingot prepared in this embodiment has a highly uniform composition.

[0036] Comparative Example 1 Same as Example 1, except that: the alloy composition does not contain Mo, and the specific composition is 25 wt% Cr, 20 wt% Ni, 1.5 wt% Mn, 0.4 wt% Si, with the balance being Fe.

[0037] Test Example 1 The Cr volatility of the alloys prepared in Examples 1-3 and Comparative Example 1 was determined under high temperature and high humidity conditions of 800℃. The test results are as follows: Figure 4 As shown.

[0038] Comparative Example 1, serving as the baseline sample without added Mo, exhibited severe Cr volatilization. The highest Cr volatilization occurred after 200 hours of testing, and the volatilization rate continued to increase over time, indicating that without Mo protection, the chromium oxide layer on the alloy surface could not effectively prevent the continuous escape of Cr. In Example 1, the addition of 1 wt% Mo significantly reduced Cr volatilization. This improvement demonstrates that Mo doping can form a certain Laves phase layer at the metal / oxide interface, thus significantly hindering Cr diffusion. Example 2, using a 2 wt% Mo formulation, achieved the best anti-Cr volatilization performance, with the lowest Cr volatilization. This effect stems from the most continuous and dense interfacial Laves phase layer formed at this Mo content, which can most effectively pin the interface and simultaneously hinder the bidirectional transport process of Cr ion outward diffusion and oxygen ion inward diffusion. In Example 3, further increasing the Mo content to 3 wt% resulted in a slight increase in Cr volatilization, but it was still better than Comparative Example 1 and Example 1. This is because excessive Mo content leads to excessive precipitation of the Laves phase, which negatively affects the density and continuity of the oxide layer.

[0039] from Figure 5 The oxidation weight gain curves further validate the above conclusions. Comparative Example 1 showed the largest oxidation weight gain during the 1000-h test, indicating that without Mo protection, the oxidation process proceeded rapidly and the oxide layer growth was uncontrolled. The oxidation weight gain curves of Examples 1, 2, and 3 were all significantly lower than those of Comparative Example 1, with Example 2 showing the smallest oxidation weight gain, further confirming that the use of Mo can form an effective protective oxide layer.

[0040] The above results indicate that the alloy of the present invention can provide long-term and reliable high-temperature oxidation resistance and Cr volatilization resistance for solid oxide fuel cell BOP components in practical applications, and has good application prospects.

[0041] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A Mo-doped high-temperature alloy resistant to Cr volatilization for BOP components, characterized in that, The alloy comprises, by mass percentage, a Mo-rich Laves phase, Mo 1-3 wt%, Cr 25 wt%, Ni 20 wt%, Mn 1.5 wt%, Si 0.4 wt%, with the balance being Fe and unavoidable impurities.

2. The Mo-doped high-temperature alloy for BOP components with resistance to Cr volatilization as described in claim 1, characterized in that, The total amount of impurities is ≤0.05%, and the actual content of each element deviates from the designed composition by no more than ±0.3%.

3. The Mo-doped high-temperature alloy for BOP components with resistance to Cr volatilization as described in claim 1, characterized in that, The Laves phase is of type C14.

4. A Mo-doped high-temperature alloy resistant to Cr volatilization for BOP components according to claim 1, characterized in that, The content of Mo is 2 wt%.

5. A method for preparing the Cr-resistant Mo-doped high-temperature alloy of the BOP component according to any one of claims 1-4, characterized in that, Includes the following steps: S1. Place the raw materials into the vacuum melting furnace in order of their melting points from high to low. S2. After evacuating the furnace body, fill it with high-purity argon gas and proceed with melting and refining in sequence. S3. Electromagnetically stir at the refining temperature, then cast into ingots; S4. Cool the furnace to room temperature in a protective atmosphere.

6. The preparation method according to claim 5, characterized in that, In step S1, high-purity titanium sheets are added to the furnace as a getter before smelting.

7. The preparation method according to claim 5, characterized in that, In step S2, the melting temperature is 1500℃-1550℃; the refining temperature is 1520℃-1570℃, and the refining time is 10-20 min.

8. The preparation method according to claim 5, characterized in that, In step S3, the electromagnetic stirring lasts for 2-3 minutes each time, followed by standing for 1-2 minutes, and the cycle is repeated 3-5 times.

9. The preparation method according to claim 5, characterized in that, In step S4, furnace cooling refers to cooling to below 600°C at a cooling rate of no more than 30°C / min under an argon protective atmosphere, followed by natural cooling to room temperature.

10. The application of the Cr-resistant Mo-doped high-temperature alloy of the BOP component according to any one of claims 1-4 in the preparation of heat exchangers, valves and gas passage structural components in solid oxide fuel cell systems that are exposed to high temperature and high humidity environments of 700℃-900℃ for a long time.

Images