Corrosion-resistant alloy suitable for high-temperature chloride environment as well as preparation process and application of corrosion-resistant alloy

Abstract

The invention discloses a corrosion-resistant alloy suitable for a high-temperature chloride environment as well as a preparation process and application of the corrosion-resistant alloy. The alloy comprises the following components in percentage by mass: less than or equal to 0.08% of C, 0.10-0.40% of N, 25.0-50.0% of Ni, 20.5-38.0% of Cr, 4.0-16.0% of Mo, 0.05-5.0% of W, 0.1-2.3% of a stabilizing element sigma (xi), less than or equal to 4.2% of Co, 0.015-0.08% of RE and the balance of Fe and impurities. The microstructure is single austenite, Nieq / Creq is larger than or equal to 0.85, and pitting corrosion resistance equivalent PREN is larger than or equal to 40. Through synergistic optimization of components, the alloy has excellent high-temperature chlorine corrosion resistance, chloride pitting corrosion resistance, alkali corrosion resistance and high-temperature structure stability, and is particularly suitable for manufacturing chemical equipment in extreme corrosion environments such as pyridine high-temperature photochlorination and the like.

Description

Technical Field

[0001] This invention belongs to the field of high-performance metallic materials technology, specifically relating to a corrosion-resistant alloy suitable for high-temperature chloride environments, its preparation process, and its applications. This alloy is suitable for manufacturing equipment in extremely corrosive environments, particularly for critical components such as reactors, pressure vessels, heat exchangers, and pipelines under highly corrosive conditions in industries such as pharmaceuticals, chemicals, pesticides, marine engineering, and nuclear power. Background Technology

[0002] High-temperature photochlorination of pyridine compounds is a key fine chemical process in the synthesis of pesticide and pharmaceutical intermediates. Its goal is to selectively introduce chlorine atoms onto the pyridine ring to generate high-value-added products such as 2-chloropyridine and 2,6-dichloropyridine. This process utilizes ultraviolet light to excite chlorine gas into active chlorine atoms at high temperatures of 350–420°C, achieving chlorination of the pyridine ring through a free radical chain reaction. However, this highly efficient process is accompanied by an extremely harsh corrosive environment, posing significant challenges to the equipment and materials used.

[0003] This extremely harsh corrosion condition involves high-temperature chlorine corrosion, high-temperature embrittlement corrosion of alkalis, and pitting corrosion of chlorides, making it one of the most demanding corrosive environments in the entire chemical industry. High-temperature chlorine corrosion: Chlorine concentration in the gas phase ≥99.6% decomposes into highly reactive chlorine atoms at high temperatures, possessing strong oxidizing properties. This can destroy the oxide film on the metal surface, forming loose chlorides and leading to uniform corrosion. Corrosion intensifies when chromium (Cr) content is below 25%, while corrosion resistance increases with increasing Cr content when Cr ≥ 25%. Acidic chloride corrosion: The presence of hydrogen chloride (HCl) and chloride ions in the liquid phase triggers general uniform corrosion, pitting corrosion, crevice corrosion, and stress corrosion cracking. Molybdenum (Mo) is crucial for resistance to pitting and crevice corrosion, but its content needs to be controlled in conjunction with Cr and nitrogen (N). High-temperature alkali corrosion: With an aqueous phase pH of 10–13, intergranular corrosion and stress corrosion cracking (alkali embrittlement) are induced at high temperatures. The risk of alkali embrittlement increases significantly when nickel (Ni) content is below 25%. Temperature effect: High temperatures of 350–420℃ accelerate all corrosion reaction rates, further exacerbating material failure. Limitations of existing materials: Ordinary stainless steel and super austenitic stainless steel (such as 904L, 254SMo, 654SMo): cannot resist the combined corrosion of high-temperature chlorine gas and alkaline environment.

[0004] Super duplex stainless steel: sensitive to high temperatures, insufficient nickel content, and prone to alkali embrittlement.

[0005] Nickel-based alloys (such as B-2, C-276, 625, 690): B-2 is a nickel-based high-molybdenum alloy (Mo: 26-30%) with very low chromium content (Cr≤1.0%), exhibiting excellent resistance to chloride ion corrosion, but poor resistance to high-temperature strong chlorination corrosion. C-276 has high molybdenum content (15-26%) but low chromium content (≤16.5%). The low chromium content leads to insufficient stability of the passivation film, making it susceptible to dissolution by chlorination after corrosion damage in strong chlorination environments. The weld heat-affected zone of B-2 and C-276 alloys is prone to the precipitation of brittle phases such as σ, μ, ρ, and M6C, significantly reducing corrosion resistance in extreme corrosive environments. Alloy 625: High niobium content, insufficient carbon control, and chromium content (20-23%) cannot meet the requirements for extreme corrosion under high-temperature chlorine gas, and the weld heat-affected zone is prone to the precipitation of brittle phases such as γ'' and M6C. Alloy 690: Chromium content ≥27%, but lacks molybdenum, resulting in significantly insufficient resistance to pitting corrosion. Practical application in a high-temperature photochlorination reactor shows that C-276 Hastelloy and Inconel 625 still exhibit significant corrosion in this environment, proving that existing materials cannot simultaneously resist the synergistic effects of high-temperature strong oxidation, chloride ion pitting corrosion, and alkaline corrosion.

[0006] Therefore, it is of great significance to develop a new material with corrosion resistance to meet the requirements of such extremely harsh corrosive conditions. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention optimizes the component ratio and preparation process to provide a corrosion-resistant alloy that combines excellent oxidation resistance, pitting corrosion resistance, alkali corrosion resistance, and high-temperature stability, filling a technological gap in existing materials for use in extreme corrosive environments. This invention also provides a corrosion-resistant alloy suitable for high-temperature chloride environments, its preparation process, and its applications.

[0008] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: In a first aspect, a corrosion-resistant alloy, the chemical composition of which, by mass percentage, comprises: C: ≤0.08%, N: 0.10-0.40%, Ni: 25.0-50.0%, Cr: 20.5-38.0%, Mo: 4.0-16.0%, W: 0.05-5.0%, Cu: ≤5.0%, Si: ≤0.8%, Mn: ≤8.0%, stabilizing element ∑(xi): 0.1-2.3%, wherein the stabilizing element ∑(xi) is the sum of at least two of Ti, Zr, Hf, Ta, Nb, and V; Co: ≤4.2%, RE: 0.015-0.08%, S: ≤0.035%, P: ≤0.040%, with the balance being Fe and unavoidable impurities.

[0009] Nickel plays a triple key role in this alloy. First, as the primary austenite-forming element, it ensures a single, stable austenitic structure in the solid solution state, which is the microscopic basis for achieving high toughness and excellent processing performance. Second, nickel itself is a core element for improving the inherent toughness of the material, effectively inhibiting the formation and damage of brittle phases. Finally, in high-temperature alkaline environments, nickel is fundamental to imparting excellent resistance to alkaline corrosion (anti-alkali embrittlement); when the nickel content is increased to above 25%, the material's resistance to stress corrosion cracking of intergranular morphology is significantly enhanced. Therefore, ensuring sufficient nickel content is a fundamental prerequisite for simultaneously achieving microstructural stability, high toughness, and resistance to alkaline corrosion.

[0010] Chromium is a key element in the corrosion resistance of stainless steel. When dissolved in the alloy, it rapidly forms an extremely thin, dense, and stable layer on the material surface. Passivation film. This passivation film acts as a physical barrier, effectively preventing direct contact between corrosive media and the metal substrate, serving as the first line of defense for establishing the material's corrosion resistance. With increasing chromium content, the stability and self-healing ability of the passivation film are significantly enhanced. In this invention, the chromium content is set at 20.5% or higher to impart a sufficiently thick and stable passivation film to the alloy, specifically designed to withstand the severe corrosion from highly oxidizing media such as chlorine in medium- and high-temperature environments.

[0011] Molybdenum is a key element in enhancing the resistance of alloys to localized corrosion. When its content is ≥3%, the material possesses basic resistance to pitting corrosion; with increasing content, resistance to chloride pitting corrosion, crevice corrosion, and reducing acid corrosion (such as HCl) will be significantly enhanced. The core mechanism of molybdenum's action lies in its ability to accumulate in... The transition region beneath the passivation film is formed by... Or adsorb molybdate ions ( This effectively blocks the active sites for pitting corrosion nucleation and promotes a denser corrosion product layer structure, thereby greatly inhibiting the initiation and expansion of pitting corrosion. However, molybdenum is a strong ferrite-forming element, and excessively high content will strongly promote the precipitation of brittle intermetallic compounds such as σ phase and μ phase at grain boundaries, seriously impairing the toughness and corrosion resistance of the material. Therefore, this invention, while pursuing corrosion resistance, must also consider the stability of the microstructure, scientifically limiting the molybdenum content to between 4% and 16%.

[0012] Nitrogen is a key element in achieving high performance in the corrosion-resistant alloy of this invention, playing a multifaceted and synergistic role. First, as a strong austenite-forming element, nitrogen effectively stabilizes the austenite matrix. Second, nitrogen significantly enhances the material's resistance to pitting corrosion, making it one of the core factors in calculating the pitting corrosion equivalent (PREN). Furthermore, the high-melting-point (>2000℃) nitrides formed by nitrogen with stabilizing elements such as titanium, niobium, and vanadium act as stable, dispersed strengthening phases, significantly improving the material's high-temperature strength and creep resistance. Simultaneously, the addition of nitrogen inhibits carbide precipitation and growth, contributing to microstructure stability. To ensure its comprehensive benefits, this invention sets the nitrogen content to be no less than twice the carbon content (N / C ≥ 2), and the total mass percentage to be no less than 0.10%.

[0013] In the corrosion-resistant alloy design of this invention, manganese plays a crucial role as a highly efficient "nitrogen carrier." Manganese significantly increases the solubility of nitrogen in the austenitic matrix, thereby ensuring the stable solid solution of the high nitrogen content (N≥0.10%) required by this design. By dissolving and stabilizing a large amount of nitrogen, manganese indirectly and powerfully enhances the austenitic stability and pitting corrosion resistance brought about by nitrogen. Although manganese's own austenitic ability is about half that of nickel, and its solid solution strengthening effect is limited, nickel and molybdenum significantly reduce the solubility of nitrogen in this high-alloy system. Therefore, to successfully achieve the design goal of high-nitrogen alloying, the upper limit of manganese content is increased to 8% to balance the composition and ensure that the function of the key element nitrogen is fully utilized.

[0014] Tungsten (W) is a key element in the corrosion-resistant alloy of this invention, possessing both "stabilizing" and "alloying" functions. As a strong carbide-forming element, tungsten effectively suppresses harmful chromium carbides (such as Cr) by capturing free carbon atoms to form stable nanoscale MC-type carbides. 23 C6 precipitates at grain boundaries, significantly reducing the material's susceptibility to intergranular corrosion; this mechanism enables it to play a superior stabilizing role. Simultaneously, tungsten in the matrix generates a strong precipitation strengthening effect through dispersed nano-carbide and tungsten-containing intermetallic compounds (such as the Laves phase), effectively pinning dislocations and hindering grain boundary slip, thereby significantly improving the material's high-temperature strength and creep resistance. Furthermore, tungsten and molybdenum exhibit a significant synergistic effect in corrosion resistance, jointly enhancing the stability of the passivation film and effectively improving the alloy's resistance to pitting and crevice corrosion. In summary, tungsten plays multiple roles in the corrosion-resistant alloy of this invention, acting as a "carbon distribution regulator," a "high-temperature strengthening contributor," and a "corrosion resistance synergist," making it a crucial component for optimizing the material's overall performance.

[0015] Copper (Cu), as a key functional element in this alloy design, primarily imparts excellent seawater corrosion resistance to the material. Its mechanism of action manifests in three aspects: First, copper ions can integrate into the surface passivation film, enhancing its stability in chlorine-containing environments; second, during high-temperature service, copper can precipitate in the form of a nanoscale copper-rich phase, producing a significant precipitation strengthening effect; most importantly, the continuous release of trace amounts of copper ions (Cu) from the material surface... It possesses broad-spectrum antibacterial properties, effectively inhibiting the attachment and growth of marine microbial films, fundamentally solving the corrosion problem induced by microorganisms such as sulfate-reducing bacteria. This characteristic makes the corrosion-resistant alloy of this invention particularly suitable for harsh marine engineering environments with a risk of biofouling.

[0016] The microstructure of the corrosion-resistant alloy is a single austenite, and the ratio of the nickel equivalent (Nieq) to the chromium equivalent (Creq) of the corrosion-resistant alloy satisfies: Nieq / Creq ≥ 0.85; where, The formula for calculating nickel equivalent is: (Nieq) = Ni + Co + 0.5*Mn + 30*C + 25*N + 0.25*Cu; The formula for calculating chromium equivalent is: (Creq) = Cr + Mo + W + 1.5*Si + 0.7*∑(xi), where the symbols of each element represent the mass percentage of that element in the alloy.

[0017] To achieve long-term stability of the material under extreme corrosion, high temperature, and low temperature environments, this invention strictly controls its microstructure to be a single austenite. Residual ferrite poses a threat to material properties in several ways: during welding thermal cycles, carbides easily precipitate within the ferrite, leading to chromium depletion at grain boundaries and significantly increasing the risk of intergranular corrosion; during long-term high-temperature service, ferrite serves as a preferential nucleation site for σ-brittle phases, resulting in material embrittlement; in corrosive environments, ferrite undergoes selective dissolution due to its more reactive electrochemical properties. Therefore, this invention ensures microstructure purity through precise nickel equivalent (Nieq) and chromium equivalent (Creq) balance calculations. To achieve a single austenite microstructure and ensure its stability, Nieq / Creq ≥ 0.85 is controlled. This microstructure control strategy is one of the core advantages of this invention. By employing a high-nitrogen design and fully utilizing the elemental synergies in the nickel equivalent formula, a stable single austenite microstructure can be obtained without relying on extremely high nickel content. This avoids many performance problems associated with ferrite and significantly reduces the cost of using large amounts of expensive nickel, making the material both economical and capable of delivering comprehensive performance.

[0018] The pitting resistance equivalent PREN of the corrosion-resistant alloy is ≥ 40. The PREN is calculated using the formula: PREN = Cr + 3.3*(Mo + 0.5*W) + 16*N, where each element symbol represents the mass percentage of that element in the alloy.

[0019] In the stabilizing element ∑(xi), the mass percentage of V is 0.05-1.0%, and / or the mass percentage of Nb+Ta is 0.05-1.0%.

[0020] In the stabilizing element ∑(xi), the sum of the mass percentages of Ti, Zr, and Hf is ≤0.3%.

[0021] Stabilizing elements are one of the key design considerations for achieving high performance in this alloy. Their core role is to preferentially combine with carbon (C) and nitrogen (N) in the steel to form fine, stable, and dispersed carbides, nitrides, or carbonitridium composite phases. These precipitates effectively fix C and N atoms, preventing them from combining with key corrosion-resistant elements such as chromium (Cr) and molybdenum (Mo) at grain boundaries. This avoids intergranular corrosion sensitivity caused by Cr / Mo depletion at grain boundaries, thus comprehensively improving the material's microstructure stability and corrosion resistance. In high-alloy austenitic systems, excessively high molybdenum content exacerbates the segregation tendency of carbides and nitrides at grain boundaries and promotes the precipitation of brittle intermetallic compounds such as the σ phase. By adding stabilizing elements such as titanium (Ti), zirconium (Zr), hafnium (Hf), niobium (Nb), tantalum (Ta), and vanadium (V), more stable MC and MN-type precipitates can be preferentially formed, effectively preventing the formation and aggregation of harmful phases such as molybdenum carbide (MoC) or molybdenum nitride, thereby improving the overall performance of the material.

[0022] To ensure stabilization and avoid the drawbacks of using a single element, the corrosion-resistant alloy described in this invention employs a multi-element composite addition strategy: Control of Ti, Zr, and Hf: These elements are strong nitride-forming elements. Appropriate addition can form beneficial dispersed phases, but excessive amounts easily form coarse nitride inclusions, severely impairing the processing, welding, and service performance of the material. Therefore, their total content (Ti + Zr + Hf) is strictly controlled within 0.3%.

[0023] Nb, Ta, and V all have strong affinity for carbon and nitrogen. In traditional designs, adding large amounts of Nb alone (e.g., more than 10 times the C content) easily leads to the formation of coarse, blocky NbC between dendrites and also easily generates δ-carbon (NbC). Brittle phases are also detrimental to performance. Therefore, this invention does not rely on a single element, but rather adds Nb, Ta, and V as a group of elements.

[0024] To ensure stabilization and avoid the negative impact of a single element, at least two elements from Ti, Zr, Hf, Ta, Nb, and V are added in combination. Their different dissolution and precipitation kinetics can be utilized to promote finer and more dispersed precipitates, thus achieving better stabilization.

[0025] Cobalt (Co) plays a unique "polymerization" role in the system. It reduces the activity of carbon in the austenitic matrix and promotes the precipitation of carbonitrides formed by stabilizing elements such as Ti, Nb, and V in a finer, more dispersed form, contributing to the formation of highly thermally stable micron / nano-scale mixed carbonitrides (M(CN)). Furthermore, Co prevents the precipitation of the brittle μ-phase (Fe,Ni,Co)7(Mo,W,Cr)6 in high-Mo,W alloys. Therefore, when the molybdenum equivalent (Moeq) in the alloy design is ≥10, the amount of Co added should be 5%-20% of the molybdenum equivalent, and not higher than 4.2%. The molybdenum equivalent (Moeq) is calculated as Mo + W.

[0026] When this invention is used in the nuclear power field, in order to reduce the generation of radioactive isotopes (e.g., cobalt-60) in the material under neutron irradiation, the cobalt (Co) content should be controlled to below 0.05%.

[0027] Rare earth elements (REs) are strong deoxidizers and desulfurizers. They are chemically very active and can react with residual oxygen (O) and sulfur (S) in molten steel to form high-melting-point rare earth oxides and sulfides. This significantly improves the purity of molten steel, purifies grain boundaries, and thus significantly enhances the material's creep resistance and toughness.

[0028] Secondly, a process for preparing a corrosion-resistant alloy includes the following steps: The initial molten steel is obtained by smelting in an electric furnace with the addition of alloying elements such as nickel, chromium, molybdenum, manganese, and tungsten. The initial molten steel is transferred into an AOD furnace for decarburization, dephosphorization and desulfurization treatment to obtain the first molten steel. The first molten steel is transferred into a VOD furnace for vacuum degassing to obtain the second molten steel. The second molten steel is subjected to nitrogen alloying treatment: nitrogen gas is blown into the molten steel through a gas conveying device located at the bottom of the VOD furnace ladle, and argon gas is used for stirring until the nitrogen content in the molten steel reaches 0.10-0.40%; After completing the nitrogen alloying treatment, cobalt is added to the molten steel, and niobium and vanadium are added as needed. The mixture is then stirred with argon gas to obtain a third molten steel. The third molten steel is transferred to the LF refining furnace for final fine-tuning of the alloy composition and deep desulfurization, and titanium, zirconium or hafnium elements are added as needed. Rare earth elements are added for final purification before the molten steel is poured. The molten steel is poured into a casting to obtain an alloy steel billet.

[0029] Thirdly, a metal product made of a corrosion-resistant alloy, said product being a plate, pipe, forging, composite plate, or welding material.

[0030] The composite plate is a composite plate made by explosive bonding or rolling bonding of corrosion-resistant alloy with low alloy steel or chromium-molybdenum steel.

[0031] Fourthly, the application of a corrosion-resistant alloy in the preparation of chemical equipment for harsh corrosive environments.

[0032] The harsh corrosive environment is a high-temperature environment containing chlorine, chlorides and / or alkaline media; the chemical equipment includes reactors, pressure vessels, heat exchangers, furnace tubes or pipelines.

[0033] The advantages of this invention over the prior art are as follows: 1. Comprehensive corrosion resistance Through the synergistic design of high chromium (Cr 20.5-38.0%), high molybdenum (Mo 4.0-16.0%), and high nitrogen (N 0.10-0.40%), the alloy of this invention constructs a multi-layered corrosion defense system. The high chromium content results in an extremely stable and highly self-healing alloy. The passivation film effectively resists corrosion from strong oxidizing media such as high-temperature chlorine gas. The synergistic effect of molybdenum and nitrogen significantly enhances the resistance to chloride pitting corrosion, crevice corrosion, and reducing acid corrosion. Through composition optimization, the alloy of this invention can achieve extremely high pitting resistance equivalent (PREN). In the preferred embodiment, the pitting resistance equivalent (PREN) of the alloy is as high as 70.65, far exceeding existing high-end alloys such as 254SMO and 625, and its resistance to strong oxidizing corrosion far exceeds that of C-276 Hastelloy alloy, enabling it to withstand the extreme combined corrosion environment of "high-temperature chlorine gas (strong oxidizing agent) + chloride ions + high-temperature alkaline solution" in the high-temperature photochlorination process of pyridine.

[0034] 2. Excellent high-temperature performance and tissue stability This invention achieves synergistic performance enhancement through microalloying design using tungsten (W), stabilizing elements, cobalt (Co), and rare earth elements (RE). The addition of tungsten not only further improves corrosion resistance through its synergistic effect with molybdenum, but also significantly enhances the material's high-temperature strength and creep resistance by forming nanoscale carbides and a stable Laves phase. The synergistic effect of the stabilizing elements with cobalt and rare earth elements promotes the formation of fine, dispersed carbonitrides, pinning grain boundaries and purifying molten steel. This ensures that the material maintains a pure austenitic single-phase structure and excellent toughness during long-term service at temperatures of 350–420℃ and even higher (700–1000℃). It also completely avoids the risk of precipitation of brittle phases such as σ, μ, and M6C in the weld heat-affected zone, as well as performance degradation caused by ferrite residue.

[0035] 3. Flexible applicability and broad application prospects This invention allows for compositional optimization and adjustment based on different specific operating conditions (such as ordinary chloride environments, harsh marine environments, high-temperature environments, etc.), resulting in a series of PREN alloys with different grades from ≥40 (such as alloys 1 to 7 in the examples). This enables the materials of this invention not only to perfectly solve the specific problem of high-temperature photochlorination of pyridine, but also to be widely used in the manufacture of equipment in other extremely harsh corrosive environments such as petrochemicals, nuclear power, marine engineering, and biomedicine, covering a wide range of equipment components such as reactors, heat exchangers, and pipelines. Detailed Implementation

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The following description of at least one exemplary embodiment is illustrative in nature and is in no way intended to limit the present invention or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0037] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0038] Unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of components and steps set forth in these embodiments do not limit the scope of the invention. Techniques, methods, and apparatus known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and apparatus should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as exemplary and not as limiting. Therefore, other examples of exemplary embodiments may have different values. Example 1

[0039] To address the issue of equipment materials for the photochlorination reaction of pyridine-based organic compounds with chlorine at 350–420°C, this invention employs a high nickel-chromium-molybdenum content design, along with a comprehensive formulation incorporating nitrogen, stabilizing elements, cobalt, and rare earth elements. Details are as follows: A corrosion-resistant alloy, wherein the chemical composition of the corrosion-resistant alloy comprises, by mass percentage: C: 0.01%, N: 0.25%, Ni: 40.0%, Cr: 32.0%, Mo: 10.0%, W: 1.0%, Si: 0.2%, Mn: 3.0%, Cu: 0.5%, and stabilizing element ∑(xi): the sum of Ti, Zr, Hf, Ta, Nb, and V, wherein: Ti+Zr+Hf=0.10%, V: 0.25%, Nb+Ta=0.25%, Co: 1.2%, RE: 0.015%, S: 0.015%, P: 0.025%, and the balance being Fe and unavoidable impurities 12.1%.

[0040] Based on the actual addition amounts of each element in the above corrosion-resistant alloys, the phase equilibrium and pitting corrosion resistance were calculated, and the results are as follows: 1) Phase equilibrium calculation of nickel equivalent and chromium equivalent: Nickel equivalent (Nieq) = Ni + Co + 0.5 * Mn + 30 * C + 25 * N + 0.25 * Cu Chromium equivalent (Creq) = Cr + Mo + W + 1.5 * Si + 0.7 * ∑(xi), where the symbols of each element represent the mass percentage of that element in the alloy; Substituting the actual amounts added according to the composition design, the calculated ratio of nickel equivalent to chromium equivalent is: Nieq / Creq = 49.375 / 43.72 ≈1.13 In practical engineering applications, it has been found that when the ratio of the nickel equivalent to the chromium equivalent in the solid solution of austenitic stainless steels such as 304 and 316L is ≥0.70, the microstructure can be classified as a single austenitic structure. Since this result is much greater than 0.70, it can be confirmed that the alloy has a single austenitic microstructure. Due to the high molybdenum equivalent (Moeq) in this compositional design, to delay and prevent the precipitation of the brittle μ phase, the ratio of the nickel equivalent to the chromium equivalent is designed to be ≥1.0, and sufficient Co content is added. This ratio reflects excellent balance in its compositional design and exhibits superior structural stability.

[0041] To ensure the structural stability of the new corrosion-resistant alloy material in engineering applications, the ratio of its nickel equivalent (Nieq) to chromium equivalent (Creq) should be ≥0.85.

[0042] 2) Pitting Corrosion Equivalent (PREN) Pitting corrosion equivalent (PREN) is a key indicator of the pitting corrosion resistance of stainless steel. For high-end nitrogen-containing stainless steels, the most commonly used formula is: PREN = Cr + 3.3*(Mo + 0.5*W) + 16*N; where each element symbol represents the mass percentage of that element in the alloy; The calculated result (PREN) after substituting the actual amount added into the formula is 70.65. These calculations demonstrate that the material possesses exceptional resistance to pitting corrosion, far exceeding that of 254SMO super stainless steel and even surpassing the pitting corrosion resistance of nickel-based 625. This indicates that the material can also be designed for extremely harsh corrosive environments, such as high-temperature, high-concentration chloride environments (e.g., seawater, bleach, pulp liquor), exhibiting outstanding resistance to uniform corrosion, pitting corrosion, and crevice corrosion.

[0043] To ensure that the new corrosion-resistant alloy material has excellent corrosion resistance, the minimum threshold of pitting corrosion equivalent (PREN) after determining its composition design should be ≥40.

[0044] Based on phase equilibrium calculations and pitting corrosion equivalent calculations, the corrosion-resistant alloy in this invention not only possesses excellent resistance to high-temperature oxidative corrosion but also exhibits a highly stable austenitic structure and excellent resistance to pitting corrosion. This new material achieves a triple corrosion defense: First line of defense (comprehensive anti-oxidation): 32% high Cr content, resisting the extreme oxidative corrosion of high-temperature chlorine gas; The second line of defense (local anti-pitting corrosion): 10% Mo and high N work together to block local corrosion breakthroughs; Basic guarantee (structure stability): Optimized Ni / N content and stabilizing elements ensure structural stability during long-term use, making it alkali-resistant and less prone to embrittlement.

[0045] In addition, to prevent the precipitation of harmful phases during welding thermal cycling, the addition of 1% W and 1.2% Co, as well as stabilizing elements, can significantly delay the precipitation of σ phase, μ phase and M6C, thereby improving the corrosion resistance of the weld heat-affected zone.

[0046] Based on the ingeniously designed composition range of this invention, it is not limited to the highly corrosive environment of high-temperature photochlorination processes, but can also be extended to other application environments.

[0047] For the preparation process of the corrosion-resistant alloy in this invention, an electric furnace combined with ladle refining should be used for smelting. It is essential to ensure that the steel billet achieves excellent purity and avoid excessive gas residue and low-melting-point inclusions. The ladle refining can employ an AOD (argon-oxygen smelting) + VOD (vacuum smelting) + LF (freezing-heat smelting) method, or an electric furnace smelting combined with electroslag remelting can be used to ensure sufficient purity of the steel billet.

[0048] A process for preparing a corrosion-resistant alloy includes the following steps: The initial molten steel is obtained by smelting in an electric furnace with the addition of alloying elements such as nickel, chromium, molybdenum, manganese, and tungsten. The initial molten steel is transferred into an AOD furnace for decarburization, dephosphorization and desulfurization treatment to obtain the first molten steel. The first molten steel is transferred into a VOD furnace for vacuum degassing to obtain the second molten steel. The second molten steel is subjected to nitrogen alloying treatment: nitrogen gas is blown into the molten steel through a gas conveying device located at the bottom of the VOD furnace ladle, and argon gas is used for stirring until the nitrogen content in the molten steel reaches 0.10-0.40%; After completing the nitrogen alloying treatment, cobalt is added to the molten steel, and niobium and vanadium are added as needed. The mixture is then stirred with argon gas to obtain a third molten steel. The third molten steel is transferred to the LF refining furnace for final fine-tuning of the alloy composition and deep desulfurization, and titanium, zirconium or hafnium elements are added as needed. Rare earth elements are added for final purification before the molten steel is poured. The molten steel is poured into a casting to obtain an alloy steel billet.

[0049] Because the material of this invention has a high nitrogen content, the smelting process should avoid the segregation and aggregation of nitrides and oxides formed by the strong nitrogen oxide elements Ti, Zr, and Hf with oxygen and nitrogen. If the addition of Ti, Zr, and Hf is considered, it should be done in the late-stage LF smelting process.

[0050] The corrosion-resistant alloy obtained by this process has high purity, uniform composition, and high nitrogen recovery rate.

[0051] During the cooling process of the steel billet during smelting, casting, and crystallization in this invention, nitrogen preferentially forms nitrides with elements that have a strong affinity for it. At approximately 900-1070℃, nanoscale composite nitrides containing Ti, Zr, Hf, Al, W, V, Nb, Mo, and Cr preferentially precipitate. Subsequently, below 900℃, nanoscale composite carbides containing Ti, Zr, Hf, Ta, W, V, Nb, Mo, and Cr are formed. Furthermore, under the binding effect of Co, the nitrides and carbides agglomerate together to form a nanoscale mixed precipitate phase -M(CN), pinning C and N within the grains and grain boundaries. This disordered precipitation state is diffusely distributed throughout the material matrix. During welding thermal cycling or high-temperature use, it does not diffuse and precipitates in a network pattern at the grain boundaries, thereby improving the material's resistance to intergranular corrosion and high-temperature creep.

[0052] The steel billet of the super corrosion-resistant alloy is obtained by smelting and casting according to the above smelting method. Subsequently, the steel billet is processed into plates, forgings or pipes by rolling, forging or drawing processes; the plates obtained therefrom (preferably with a thickness of 2-6 mm) can also be combined with low alloy steel or chromium-molybdenum steel by explosive bonding or rolling bonding to make composite plates; the steel billet can also be rolled into steel strips or coils, and then rolled or drawn multiple times to make welding materials of different specifications, such as welding strips, argon arc welding wires, submerged arc welding wires and welding rods.

[0053] The resulting plates, forgings, pipes, composite plates, and welding materials can be used to manufacture and are widely applied in industries such as petrochemicals, biomedicine, pesticides, nuclear power, boilers, and marine engineering, for equipment and components that need to withstand extreme corrosion, high temperatures, or high pressures. These include, but are not limited to, reactors, pressure vessels, atmospheric pressure vessels, heat exchangers, air coolers, furnace tubes, pipes, and various structural components. Example 2

[0054] Based on the composition design range and different applications of the corrosion-resistant alloy according to the present invention, various combinations (%) of chemical composition can be designed:

[0055] Phase equilibrium calculations and pitting resistance equivalent calculations were performed on the materials designed based on the above combinations of chemical compositions:

[0056] In the novel materials designed above, the microstructure after deformation rolling and solution treatment is uniform austenite, exhibiting sufficient structural stability. All materials have a pitting corrosion resistance index exceeding 40, classifying them as corrosion-resistant alloys of this invention.

[0057] Based on their high-alloy properties, alloy 1 can be used in corrosive environments below 200℃ in pyridine chloride projects; alloys 2, 3, 4, 5, and 6 all have pitting resistance indices greater than 60, exhibiting excellent corrosion resistance. Alloys 2, 3, 6, and 7 have higher nickel-to-chromium equivalent ratios, resulting in better high-temperature stability and excellent resistance to high-temperature oxidation, suitable for environments up to 700-1000℃. Alloys 3 and 5 have the highest pitting resistance index and contain 1.5% Cu, providing not only excellent resistance to chloride corrosion but also superior resistance to marine microbial corrosion. When combined with low-alloy steel to form composite plate materials, they can be used in the manufacture of submarine and ship hulls. To prevent the generation of radioactive Co-60 under neutron radiation conditions in the nuclear power field, alloys 6 and 7 have Co content controlled below 0.05%, intended for use in the high-temperature environment of nuclear power plants.

[0058] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A corrosion-resistant alloy suitable for high-temperature chloride environments, characterized in that, The chemical composition of the corrosion-resistant alloy, by mass percentage, includes: C: ≤0.08%, N: 0.10-0.40%, Ni: 25.0-50.0%, Cr: 20.5-38.0%, Mo: 4.0-16.0%, W: 0.05-5.0%, Cu: ≤5.0%, Si: ≤0.8%, Mn: ≤8.0%, Co: ≤4.2%, RE: 0.015-0.08%, S: ≤0.035%, P: ≤0.040%, with the balance being Fe and unavoidable impurities.

2. The corrosion-resistant alloy according to claim 1, characterized in that, It also includes stabilizing elements, ∑(xi): 0.1-2.3%, wherein the stabilizing elements are selected from at least two of Ti, Zr, Hf, Ta, Nb, and V, and ∑(xi) represents the sum of the mass percentages of the selected at least two stabilizing elements.

3. The corrosion-resistant alloy according to claim 2, characterized in that, The microstructure of the corrosion-resistant alloy is a single austenite, and the ratio of the nickel equivalent (Nieq) to the chromium equivalent (Creq) of the corrosion-resistant alloy satisfies: Nieq / Creq ≥ 0.85; where, The formula for calculating nickel equivalent is: (Nieq) = Ni + Co + 0.5*Mn + 30*C + 25*N + 0.25*Cu; The formula for calculating chromium equivalent is: (Creq) = Cr + Mo + W + 1.5*Si + 0.7*∑(xi); where the symbol of each element represents the mass percentage of that element in the alloy.

4. The corrosion-resistant alloy according to claim 1, characterized in that, The corrosion-resistant alloy has a pitting resistance equivalent PREN ≥ 40. The PREN is calculated using the formula: PREN = Cr + 3.3*(Mo + 0.5*W) + 16*N, where each element symbol represents the mass percentage of that element in the alloy.

5. The corrosion-resistant alloy according to any one of claims 1-4, characterized in that, When the molybdenum equivalent (Moeq) in the corrosion-resistant alloy is ≥10, the Co content is 5%-20% of the molybdenum equivalent, and does not exceed 4.2%; wherein, the formula for calculating the molybdenum equivalent is: (Moeq) = Mo + W.

6. The corrosion-resistant alloy according to any one of claims 1-4, characterized in that, The mass percentage of cobalt (Co) is ≤ 0.05%.

7. The corrosion-resistant alloy according to claim 2, characterized in that, In the stabilizing element ∑(xi), the mass percentage of V is 0.05-1.0%, and / or the mass percentage of Nb+Ta is 0.05-1.0%.

8. The corrosion-resistant alloy according to claim 2, characterized in that, In the stabilizing element ∑(xi), the sum of the mass percentages of Ti, Zr, and Hf is ≤0.3%.

9. A preparation process for the corrosion-resistant alloy according to any one of claims 1-8, characterized in that, Includes the following steps: The initial molten steel is obtained by smelting in an electric furnace with the addition of alloying elements such as nickel, chromium, molybdenum, manganese, and tungsten. The initial molten steel is transferred into an AOD furnace for decarburization, dephosphorization and desulfurization treatment to obtain the first molten steel. The first molten steel is transferred into a VOD furnace for vacuum degassing to obtain the second molten steel. The second molten steel is subjected to nitrogen alloying treatment: nitrogen gas is blown into the molten steel through a gas conveying device located at the bottom of the VOD furnace ladle, and argon gas is used for stirring until the nitrogen content in the molten steel reaches 0.10-0.40%; After completing the nitrogen alloying treatment, cobalt is added to the molten steel, and niobium and vanadium are added as needed. The mixture is then stirred with argon gas to obtain a third molten steel. The third molten steel is transferred to the LF refining furnace for final fine-tuning of the alloy composition and deep desulfurization, and titanium, zirconium or hafnium elements are added as needed. Rare earth elements are added for final purification before the molten steel is poured. The molten steel is poured into a casting to obtain an alloy steel billet.

10. A metal product made of the corrosion-resistant alloy according to any one of claims 1-8, characterized in that, The products are plates, pipes, forgings, composite plates, or welding materials.

11. The metal product according to claim 10, characterized in that, The metal product is a composite plate, which is a composite plate made by explosive bonding or rolling bonding of the corrosion-resistant alloy with low alloy steel or chromium-molybdenum steel.

12. The use of the corrosion-resistant alloy according to any one of claims 1-8 in the preparation of chemical equipment for harsh corrosive environments.

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