Isothermal quenching process for preparing corrosion and oxidation resistant diffusion layer for pipelines

Abstract

This invention relates to the field of heat treatment technology for metallic materials, and discloses an isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines. The method involves first pre-treating the metal workpiece by degreasing and derusting to obtain a clean surface, then sequentially coating it with an inner layer of rare-earth modified aluminoborosilicate glass, a middle layer of core-shell structured composite powder and nano-reinforcing phase, and an outer layer of aluminum powder and glass frit, forming a gradient composite coating. After pre-sintering and controlled-temperature austenitization, the workpiece is rapidly transferred to a nitrate bath for isothermal quenching. Finally, residual coatings are removed by sandblasting or mechanical grinding, resulting in a workpiece with a dense, oxidation-resistant infiltration layer. This invention forms an oxidation-resistant composite coating on the workpiece surface through a three-layer gradient synergy; combined with pre-sintering to reduce thermal stress and a controlled-temperature process, it effectively inhibits oxidation and decarburization while achieving uniform bainitic transformation, improving the surface hardness, salt water corrosion resistance, and infiltration layer density of the workpiece. It is suitable for efficient oxidation protection during isothermal quenching of metallic workpieces.

Description

Technical Field

[0001] This application relates to the field of heat treatment technology for metallic materials, specifically to an isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant diffusion layer for pipelines. Background Technology

[0002] Isothermal quenching is an important heat treatment process for obtaining high-strength, high-toughness, and low-deformation metal components. It enables workpieces to achieve high strength and hardness while maintaining excellent toughness and minimal dimensional deformation. It is widely used in the production of precision structural parts, molds, gears, and other key components, covering multiple high-end manufacturing fields such as aerospace, automotive manufacturing, machinery equipment, and precision instruments. Especially in the manufacture of pipeline metal components (such as oil and gas pipelines, chemical media pipelines, and high-pressure fluid pipelines), the workpieces not only need a good balance of strength and toughness to withstand internal pressure and external loads, but also must resist corrosion and oxidation from the transported media (such as sulfur-containing oil and gas, acidic solutions, and high-temperature steam) during service. This places extremely stringent requirements on surface quality, dimensional accuracy, and corrosion and oxidation resistance.

[0003] During isothermal quenching, the workpiece must undergo two key stages: high-temperature austenitization and medium-temperature isothermal transformation. The high-temperature austenitization stage requires heating the workpiece to a temperature above the end of the heating transformation temperature (Ac3 point) to completely transform the metal structure into austenite. Subsequently, it is rapidly cooled to the bainite transformation temperature range and held at this temperature for a certain period of time to transform the austenite structure into bainite, thereby optimizing the mechanical properties and dimensional stability of the workpiece. However, during this process, the surface of the workpiece in the high-temperature environment is highly susceptible to chemical reactions with the oxygen, water vapor, carbon dioxide, and other media in the furnace, leading to oxidation and decarburization. Oxidation forms a loose, easily detachable oxide scale on the workpiece surface, increasing the workload of subsequent grinding and polishing processes. Uneven oxide scale removal can also cause dimensional deviations in the workpiece. For precision components used in pipelines, these dimensional deviations can directly affect the pipeline's sealing performance and connection reliability. Decarburization reduces the carbon content on the workpiece surface, disrupting the uniformity of the surface structure and significantly reducing the workpiece's surface hardness, wear resistance, and fatigue life. In severe cases, it can even cause fatal defects such as surface cracks, making the pipeline highly susceptible to early corrosion or cracking failure under complex stress conditions. These surface defects not only affect the dimensional accuracy and surface quality of the workpiece but also significantly reduce the product yield, increase raw material losses, and increase production costs such as rework, becoming a core bottleneck restricting the development of isothermal quenching processes towards high-end and precision applications.

[0004] To address these issues, traditional protective methods in the industry are mainly divided into three categories: First, protective atmosphere protection, which involves introducing inert gases or controlled atmospheres into the heat treatment furnace to isolate air and reduce oxidation and decarburization on the workpiece surface. However, this method requires complex atmosphere preparation, purification, and control systems, resulting in high equipment investment costs, high energy consumption, and extremely high requirements for atmosphere purity. Even trace amounts of impurities can cause protection failure, making it unsuitable for small- to medium-scale production scenarios. Second, single inorganic coating protection, which often uses high-temperature ceramic coatings or glass coatings. Although these coatings possess certain high-temperature resistance, their adhesion to the workpiece substrate is weak, making them prone to cracking and peeling under the thermal shock of high-temperature heating and rapid cooling, thus failing to achieve stable protection throughout the entire process. Third, organic coating protection, which relies on organic components to form an isolation film. However, its high-temperature resistance is limited, and it is prone to decomposition and carbonization during the austenitization high-temperature stage. This not only results in the loss of protective function but also the potential for decomposition products to contaminate the workpiece surface and the furnace environment, further exacerbating workpiece defects.

[0005] It is evident that existing protective methods generally suffer from common technical challenges such as poor adaptability across a wide temperature range and insufficient coating adhesion. These limitations make it difficult to provide continuous, stable, and removable antioxidant protection throughout the entire process window, from high-temperature austenitization (850-950℃) to intermediate-temperature isothermal transformation (300-450℃). Therefore, there is an urgent need to develop a protective technology with strong adhesion to address the shortcomings of existing methods, improve the surface quality and mechanical properties of isothermally quenched workpieces, increase product yield, reduce production costs, and promote the technological upgrading and industrial application of isothermal quenching processes in high-end pipeline components. Summary of the Invention

[0006] To address the aforementioned issues, this application provides an isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant diffusion layer for pipelines. This invention involves sequentially coating the surface of a metal workpiece with a rare-earth modified aluminum borosilicate inner layer, a middle layer containing core-shell structure powder and nano-reinforcing phases, and a second glass frit outer layer containing aluminum powder. After pre-sintering, austenitization, and isothermal quenching in a nitrate bath, the coating is mechanically removed. This forms a dense, oxidation-resistant barrier compatible with nitrate baths over a wide temperature range, effectively inhibiting oxidation and decarburization and improving the surface quality and mechanical properties of the workpiece.

[0007] To achieve the above objectives, this application adopts the following technical solution:

[0008] This application provides an isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant diffusion layer for pipelines, comprising the following:

[0009] S1. The surface of the metal workpiece is degreased and derusted to obtain a clean metal workpiece A;

[0010] S2. An inner layer, a middle layer, and an outer layer are sequentially coated on the surface of metal workpiece A to form a gradient composite coating, resulting in metal workpiece B. The inner layer is a rare-earth oxide-modified aluminum borosilicate-based glass, obtained by reacting rare-earth oxides with aluminum borosilicate-based glass at high temperature. The middle layer comprises a core-shell structured composite powder, a first glass material, and a nano-reinforcing phase. The outer layer comprises a second glass material and aluminum powder. The core-shell structured composite powder consists of an aluminum core and a nickel or copper shell covering the surface of the aluminum core. The first glass material is high borosilicate glass or aluminum borosilicate glass. The nano-reinforcing phase includes any one of nano-sized Al2O3, SiC, and ZrO2. The second glass material is zinc borosilicate glass or barium aluminum silicate glass.

[0011] S3. Place the metal workpiece B in a heating furnace, perform pre-sintering treatment first, then control the heating rate to raise the temperature to the austenitizing temperature and hold it at the temperature to austenitize the metal workpiece, and obtain the austenitized metal workpiece C.

[0012] S4. The metal workpiece C is transferred to an isothermal quenching medium for isothermal treatment to obtain the metal workpiece D that has completed the isothermal transformation;

[0013] S5. Remove the metal workpiece D from the isothermal quenching medium, cool it to room temperature, and then remove the residual composite coating on the surface by mechanical grinding or sandblasting to obtain a clean metal workpiece E.

[0014] The S1 process described in this application achieves surface cleaning of the metal workpiece through a two-step synergistic treatment of degreasing and rust removal, laying the foundation for the effective adhesion and function of the subsequent gradient composite coating. During degreasing, the alkaline degreasing agent converts the grease on the workpiece surface into water-soluble soaps through a saponification reaction. Simultaneously, the surfactant emulsifies and disperses the grease, breaking the adhesion between the grease and the metal surface, causing the grease to detach from the workpiece surface and disperse in the degreasing system, thoroughly removing surface oil. During rust removal, the acidic solution undergoes an acid-base neutralization reaction with the oxide scale (Fe2O3, FeO, etc.) and rust products on the workpiece surface, converting insoluble metal oxides into water-soluble salts. This also inhibits excessive corrosion of the metal substrate. After cleaning, a clean metal surface free of oil, rust, and oxide scale is obtained, ensuring close contact between the subsequent inner layer of rare earth modified aluminoborosilicate glass and the metal substrate. This prevents oil and rust products from hindering the interaction between the coating and the substrate, guaranteeing the quality of the subsequent gradient composite coating formation.

[0015] The S2 layer constructs a multi-layered synergistic protection system consisting of an inner, middle, and outer layer by sequentially applying a gradient composite coating to the surface of a clean metal workpiece. The inner layer is a rare-earth oxide-modified aluminum borosilicate-based glass frit. The rare-earth oxides enhance the wettability and adhesion of the glass frit to the metal substrate at high temperatures by regulating the glass network structure and reducing melt viscosity, providing an interface basis for the metallurgical bonding between the coating and the substrate. The middle layer comprises a core-shell structured composite powder, a first glass frit, and a nano-reinforcing phase. In the core-shell structured composite powder, the aluminum core is encapsulated by a nickel or copper shell, undergoing local diffusion or exothermic reactions at high temperatures, releasing micro-heat to promote the uniform and dense sintering of the middle layer glass frit. The nano-reinforcing phase inhibits crack propagation through a pinning effect, improving the coating's thermal shock resistance and mechanical strength. The outer layer comprises a composite system of a second glass frit and aluminum powder. The second glass frit softens and melts at high temperatures to form a continuous glass film, while the aluminum powder oxidizes to form a dense Al2O3 protective layer. Together, they block oxygen diffusion. Each layer is applied sequentially by brushing, spraying, or dipping to form a gradient composite coating with appropriate total thickness and reasonable layer thickness ratio, providing structural protection for oxidation resistance during subsequent high-temperature processing.

[0016] S3 is a key step in the austenitization of the metal workpiece and the in-situ densification of the gradient composite coating. Its physicochemical mechanism revolves around the microstructure transformation under temperature control and the synergistic effect of the coating. During the pre-sintering process, a lower temperature is first maintained to allow moisture and solvents to evaporate, preventing the coating from cracking due to subsequent rapid heating. Then, pre-sintering is carried out at a medium temperature, during which the inner rare earth modified glass frit begins to soften and wet the matrix, the middle core-shell particles remain stable and the nano-reinforcing phase is uniformly distributed, and the outer aluminum powder undergoes slight oxidation, resulting in initial densification of the coating. Subsequently, the heating rate is controlled to reach the austenitization temperature. During the heating process, a crystal structure transformation occurs inside the metal workpiece, with pearlite and ferrite gradually transforming into austenite, achieving uniform austenitization. Meanwhile, the high-temperature environment triggers the synergistic protection of the gradient composite coating: the outer second glass frit completely melts, forming a dense physical barrier layer together with the Al2O3 film generated by the oxidation of aluminum powder, effectively inhibiting oxygen from contacting the substrate; in the middle core-shell structure powder, the aluminum core and nickel / copper shell undergo a local exothermic reaction, promoting the dense sintering of high borosilicate or aluminoborosilicate glass frit, and the nano-reinforcing phase absorbs thermal stress and prevents coating cracking through the pinning effect; the inner rare earth modified aluminoborosilicate glass frit undergoes a solid-liquid interface reaction with the steel substrate, with rare earth elements enriched at the interface, promoting the diffusion of Si, B, and Al into the substrate micro-regions, while Fe in the substrate migrates into the melt layer, generating composite oxide interface phases such as Fe-Si-O and Fe-BO, forming a metallurgical bonding transition layer of chemical bonding and element interdiffusion. Through the gradient synergy of inner bonding, middle reinforcement, and outer oxygen barrier, a dense anti-oxidation coating that is metallurgically bonded to the substrate is formed in situ on the workpiece surface, achieving full-process protection of the metal workpiece during austenitization.

[0017] The S4 process achieves a stable transformation of the internal structure of the metal workpiece through isothermal quenching, while ensuring the integrity of the surface gradient composite coating. Its core physicochemical mechanism lies in the bainite transformation under constant temperature and the maintenance of compatibility between the coating and the nitrate bath. The metal workpiece C is rapidly transferred from the heating furnace to the isothermal quenching medium in the nitrate bath, rapidly cooled to the isothermal temperature and maintained constant, avoiding deformation and cracking caused by sudden temperature changes. Simultaneously, it inhibits the transformation of austenite to martensite, providing a stable temperature environment for the austenite-bainite transformation. During the isothermal process, austenite gradually decomposes into bainite, which has a uniform and dense microstructure, significantly improving the strength, toughness, and overall mechanical properties of the metal workpiece. During this process, the gradient composite coating on the workpiece surface, with its pre-sintered dense structure and outer aluminum powder oxide film, maintains good chemical compatibility with the nitrate bath (no violent reaction), while continuously providing physical barrier properties to prevent oxygen or the nitrate bath medium from contacting the metal substrate during the isothermal process, thus avoiding secondary oxidation, decarburization, or corrosion on the workpiece surface. The volume expansion caused by the bainitic phase transformation applies compressive stress to the coating, further enhancing the mechanical interlock and interfacial bonding strength between the coating and the substrate, thus achieving a synergistic effect of protection and structural strengthening.

[0018] The S5 process, through cooling and mechanical removal, ultimately yields a clean metal workpiece with a uniform internal structure. Its physicochemical mechanism lies in coating removal and surface quality assurance. After being removed from the isothermal quenching medium, the metal workpiece D is cooled to room temperature. During this cooling process, the bainitic structure inside the workpiece further stabilizes and solidifies. Since the gradient composite coating has formed a metallurgical bond with the substrate at high temperatures, and after isothermal quenching, the coating is dense and complete, making it impossible to remove with conventional cleaning. Therefore, mechanical grinding or sandblasting (using appropriate abrasives and pressure) is used to completely peel off the residual coating, obtaining a clean metal surface free of oxidation, decarburization, and coating residue, without damaging the substrate structure. The final result is a metal workpiece E with no oxide scale or decarburized layer on the surface and a uniform bainitic structure inside, achieving a dual improvement in oxidation protection and mechanical properties.

[0019] Preferably, in S1, the metal workpiece includes any one of bearing steel (GCr15), high-speed steel (W6Mo5Cr4V2), and hot work die steel (H13); the degreasing is performed using an alkaline degreasing agent, the degreasing temperature is 50-80℃, and the degreasing time is 10-30 minutes; the rust removal is performed using pickling, the pickling temperature is 20-40℃, and the pickling time is 5-15 minutes.

[0020] Preferably, in S1, the alkaline degreasing agent includes any one of sodium hydroxide degreasing agent, sodium carbonate degreasing agent, and sodium silicate degreasing agent; the pickling includes any one of hydrochloric acid pickling, sulfuric acid pickling, and phosphoric acid pickling.

[0021] Preferably, in S2, the rare earth oxide includes any one of yttrium oxide, cerium oxide, and lanthanum oxide.

[0022] Preferably, in S2, the aluminum borosilicate-based glass material is aluminum borosilicate or calcium aluminum borosilicate.

[0023] Preferably, in S2, the mass ratio of rare earth oxides to aluminoborosilicate glass frit is (1-5):100.

[0024] Preferably, in S2, the temperature of the high-temperature reaction is 1200-1500℃.

[0025] Preferably, in S2, the particle size of the core-shell composite powder is 1-20 μm, and the mass ratio of the aluminum core to the nickel or copper shell is 1:(4-9).

[0026] Preferably, in S2, the mass ratio of the core-shell composite powder, the first glass frit, and the nano-reinforcing phase is (50-70):(20-40):(5-15).

[0027] Preferably, in S2, the mass ratio of the second glass material to the aluminum powder is (3-7):1.

[0028] Preferably, in S2, the coating method is any one of brushing, spraying, and dipping, and the total thickness of the gradient composite coating is 0.1-0.5 mm, wherein the thickness ratio of the inner layer, middle layer, and outer layer is approximately 1:(1.5-2):(0.8-1.2).

[0029] Preferably, in S2, the preparation method of each layer of slurry in the gradient composite coating is as follows: the required powder raw materials for each layer are mixed with binder, solvent and additives, and then ball-milled to obtain a uniform slurry; the binder includes any one of silica sol, water glass and carboxymethyl cellulose; the additives include dispersant and thickener, the dispersant is polyethylene glycol and the thickener is bentonite.

[0030] Preferably, the mass ratio of the powder raw materials, binder, solvent, dispersant and thickener required for each layer is 100: (10-30): (20-50): (1-5): (0.5-3).

[0031] Preferably, in S3, the pre-sintering temperature is 550-650℃ and the pre-sintering time is 0.5-1h; the heating rate is 5-15℃ / min; the austenitizing temperature is 850-950℃ and the holding time is 0.5-2h.

[0032] Preferably, in S4, the isothermal quenching medium is a nitrate bath, the isothermal treatment temperature is 300-450℃, and the isothermal treatment time is 1-4h.

[0033] Preferably, in S5, the mechanical grinding or sandblasting uses brown corundum or quartz sand with a particle size of 60-120 mesh, the sandblasting pressure is 0.4-0.6 MPa, and the sandblasting time is 3-5 min.

[0034] Compared with the prior art, the beneficial effects of this application are as follows:

[0035] This application provides an isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant diffusion layer for pipelines. In this application, after pretreatment, the metal workpiece is sequentially coated with an inner layer, a middle layer, and an outer layer gradient composite coating: the inner layer is a rare earth oxide modified aluminoborosilicate glass material, which utilizes rare earth elements to regulate the glass network structure, reduce melt viscosity, and enhance wetting and bonding to the substrate; the middle layer contains aluminum core@nickel / copper shell core-shell structure powder, high borosilicate or aluminoborosilicate glass, and nano-Al2O3 / SiC / ZrO2 reinforcing phase. The core-shell particles undergo local diffusion or exothermic reactions at high temperatures, promoting dense sintering of the coating, and the nano-phase inhibits crack propagation through pinning effect; the outer layer is a zinc / barium aluminosilicate glass and aluminum powder composite system, in which the aluminum powder oxidizes at high temperatures to form a dense Al2O3 film, which works synergistically with the glass melt to block oxygen.

[0036] After pre-sintering and austenitizing heating, a solid-liquid interface reaction occurs between the inner glass layer and the steel substrate: rare earth elements accumulate at the interface, promoting the diffusion of Si, B, and Al into the substrate micro-regions, while Fe from the substrate migrates into the melt layer, generating composite oxide interface phases such as Fe-Si-O and Fe-BO, forming a metallurgical bonding transition layer of chemical bonding and element interdiffusion. The exothermic effect of the middle core-shell structure and the nano-reinforcing phase jointly enhance the coating's thermal shock resistance, while the outer aluminum powder oxide film ensures good compatibility with the nitrate bath. Subsequently, the coating is transferred to the nitrate bath for isothermal quenching, and rapid cooling preserves the interface structure. The volume expansion caused by the bainitic phase transformation of the substrate applies compressive stress to the coating, further enhancing the mechanical interlocking and interfacial bonding strength. After quenching, residual coatings are thoroughly removed by mechanical grinding or sandblasting, resulting in a workpiece with no oxidation or decarburization, a clean surface, and a bainitic toughening structure. This process achieves a deep synergistic effect of high-temperature protection and matrix toughening by forming a stable and removable antioxidant barrier over a wide temperature range through gradient multi-layer synergy (inner layer bonding, middle layer reinforcement, and outer layer oxygen barrier) and coupling with isothermal quenching phase transformation stress. Attached Figure Description

[0037] Figure 1 A simplified flow chart of the isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines. Detailed Implementation

[0038] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the application will be further described in detail below with reference to embodiments. However, this should not be construed as limiting the scope of this application to the following examples. All other embodiments obtained by those skilled in the art without creative effort without departing from the above-described methodological spirit of this application are within the scope of protection of this application.

[0039] In this application, the terminology used is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0040] The singular forms “for,” “or,” “a,” “any,” and “the” used in this application are intended to include the plural forms unless the context clearly indicates otherwise.

[0041] The following will describe in detail, with reference to different embodiments, an isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines provided in this application.

[0042] Example 1

[0043] like Figure 1 As shown, this embodiment provides an isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant diffusion layer for pipelines, including the following steps:

[0044] S1. The surface of bearing steel (GCr15) is first degreased with a 5wt% sodium hydroxide degreaser at 50℃ for 10 minutes, and then pickled with a 5wt% hydrochloric acid at 20℃ for 5 minutes to remove rust, resulting in a clean metal workpiece A.

[0045] S2. Yttrium oxide and aluminum borosilicate are mixed at a mass ratio of 1:100, melted at 1200℃, quenched in water, and pulverized to obtain rare earth modified aluminum borosilicate-based glass powder. Using this powder as the inner layer powder raw material, the inner layer powder raw material, silica sol, deionized water solvent, polyethylene glycol and bentonite were weighed in a mass ratio of 100:10:20:1:0.5. After mixing the components, they were ball-milled to obtain a uniform inner layer slurry. The inner layer slurry was then coated onto the surface of the metal workpiece A by brushing. The core-shell structured composite powder, high borosilicate glass and nano Al2O3 were mixed in a mass ratio of 50:20:5, wherein the mass ratio of aluminum core to nickel shell was 1:4, to obtain the middle layer powder raw material. The powder raw material, silica sol, deionized water solvent, polyethylene glycol and bentonite were weighed in a mass ratio of 100:10:20:1:0.5. After mixing the components, they were ball-milled to obtain a uniform middle layer slurry. The middle layer slurry is coated onto the inner layer surface using a brush coating method. Zinc borosilicate glass and aluminum powder are mixed at a mass ratio of 3:1 to obtain the outer layer powder raw material. Based on this outer layer powder raw material, the powder raw material, silica sol, deionized water solvent, polyethylene glycol, and bentonite are weighed at a mass ratio of 100:10:20:1:0.5. The components are mixed and ball-milled to obtain a uniform outer layer slurry. The outer layer slurry is then coated onto the middle layer surface using a brush coating method. Finally, a metal workpiece B with a gradient composite coating thickness of 0.1 mm and an inner layer:middle layer:outer layer thickness ratio of approximately 1:1.5:0.8 is obtained.

[0046] S3. Place the metal workpiece B in a heating furnace and perform a pre-sintering treatment at a temperature of 550℃ for 0.5h. Then, control the heating rate to 5℃ / min and heat to the austenitizing temperature of 850℃ and hold for 0.5h to austenitize the metal workpiece, thus obtaining the austenitized metal workpiece C.

[0047] S4. Quickly transfer the metal workpiece C to a nitrate bath and perform isothermal treatment at 300°C for 1 hour. The quick transfer means that the time for transferring the metal workpiece C from the heating furnace to the nitrate bath does not exceed 10 seconds, thus obtaining the metal workpiece D that has completed the isothermal transformation.

[0048] S5. Remove the metal workpiece D from the nitrate bath, cool it to room temperature, and then remove the residual composite coating on the surface by mechanical grinding: use 60-mesh brown corundum as the abrasive, 0.4 MPa as the blasting pressure, and 5 min as the blasting time to obtain a clean metal workpiece E.

[0049] Example 2

[0050] like Figure 1 As shown, this embodiment provides an isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant diffusion layer for pipelines, including the following steps:

[0051] S1. The surface of high-speed steel (W6Mo5Cr4V2) is first degreased with a 12wt% sodium carbonate degreaser at 60℃ for 15 minutes, and then pickled with a 12wt% sulfuric acid at 27℃ for 8 minutes to remove rust, resulting in a clean metal workpiece A.

[0052] S2. Cerium oxide and calcium aluminum borosilicate are mixed at a mass ratio of 3:100, melted at 1300℃, water-quenched, and pulverized to obtain rare earth modified aluminum borosilicate-based glass powder. Using this powder as the inner layer powder raw material, the powder raw material, water glass, deionized water solvent, polyethylene glycol, and bentonite are weighed at a mass ratio of 100:20:30:3:2. After mixing the components, the mixture is ball-milled to obtain a uniform inner layer slurry. The inner layer slurry is coated onto the surface of metal workpiece A by spraying. A core-shell structured composite powder, aluminum borosilicate glass, and nano-SiC are mixed at a mass ratio of 60:30:10, wherein the mass ratio of aluminum core to nickel shell is 1:6, to obtain the middle layer powder raw material. The powder raw material, water glass, deionized water solvent, polyethylene glycol, and bentonite are weighed at a mass ratio of 100:20:30:3:2. After mixing the components, the mixture is ball-milled to obtain the middle layer slurry. The middle layer slurry is coated onto the inner layer surface using a spraying method. Barium aluminum silicate glass and aluminum powder are mixed at a mass ratio of 5:1 to obtain the outer layer powder raw material. Based on this outer layer powder raw material, the powder raw material, water glass, deionized water solvent, polyethylene glycol, and bentonite are weighed at a mass ratio of 100:20:30:3:2. The components are mixed and ball-milled to obtain the outer layer slurry. The outer layer slurry is then coated onto the middle layer surface using a spraying method. Finally, a metal workpiece B with a gradient composite coating total thickness of 0.2 mm and an inner layer:middle layer:outer layer thickness ratio of approximately 1:1.8:1 is obtained.

[0053] S3. Place the metal workpiece B in a heating furnace and perform a pre-sintering treatment at a temperature of 600℃ for 0.5h. Then, control the heating rate to 10℃ / min, raise the temperature to the austenitizing temperature of 880℃ and hold for 1h to austenitize the metal workpiece, thus obtaining the austenitized metal workpiece C.

[0054] S4. Quickly transfer the metal workpiece C to a nitrate bath for isothermal treatment at a temperature of 380℃ for 2 hours. The time for transferring the metal workpiece C from the heating furnace to the nitrate bath shall not exceed 10 seconds, thus obtaining the metal workpiece D that has completed the isothermal transformation.

[0055] S5. Remove the metal workpiece D from the nitrate bath and cool it to room temperature. Then, use mechanical grinding to remove the residual composite coating on the surface: use 90-mesh quartz sand as the abrasive, 0.5 MPa as the blasting pressure, and 4 min as the blasting time to obtain a clean metal workpiece E.

[0056] Example 3

[0057] like Figure 1 As shown, this embodiment provides an isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant diffusion layer for pipelines, including the following steps:

[0058] S1. The surface of hot work die steel (H13) is first degreased with a 20wt% sodium silicate degreaser at 80℃ for 30 minutes, and then pickled with a 25wt% phosphoric acid at 40℃ for 15 minutes to remove rust, resulting in a clean metal workpiece A.

[0059] S2. Lanthanum oxide and calcium aluminum borosilicate are mixed at a mass ratio of 5:100, melted at 1500℃, water-quenched, and pulverized to obtain rare earth modified aluminum borosilicate-based glass powder. Using this powder as the inner layer powder raw material, the powder raw material, carboxymethyl cellulose, deionized water solvent, polyethylene glycol, and bentonite are weighed at a mass ratio of 100:30:50:5:3. After mixing the components, the mixture is ball-milled to obtain a uniform inner layer slurry. The inner layer slurry is coated onto the surface of metal workpiece A by dip coating. A core-shell structured composite powder, high borosilicate glass, and nano ZrO2 are mixed at a mass ratio of 70:40:15, wherein the mass ratio of aluminum core to nickel shell is 1:9, to obtain the middle layer powder raw material. The powder raw material, carboxymethyl cellulose, deionized water solvent, polyethylene glycol, and bentonite are weighed at a mass ratio of 100:30:50:5:3. After mixing the components, the mixture is ball-milled to obtain a uniform middle layer slurry. The middle layer slurry is coated onto the inner layer surface using a dip-coating method. Barium aluminum silicate glass and aluminum powder are mixed at a mass ratio of 7:1 to obtain the outer layer powder raw material. Based on this outer layer powder raw material, the powder raw material, carboxymethyl cellulose, deionized water solvent, polyethylene glycol, and bentonite are weighed at a mass ratio of 100:30:50:5:3. The components are mixed and ball-milled to obtain a uniform outer layer slurry. The outer layer slurry is then coated onto the middle layer surface using a dip-coating method. Finally, a metal workpiece B with a total gradient composite coating thickness of 0.5 mm and an inner layer:middle layer:outer layer thickness ratio of approximately 1:2:1.2 is obtained.

[0060] S3. Place the metal workpiece B in a heating furnace and perform a pre-sintering treatment at a temperature of 650℃ for 1 hour. Then, control the heating rate to 15℃ / min, raise the temperature to the austenitizing temperature of 950℃ and hold for 2 hours to austenitize the metal workpiece, thus obtaining the austenitized metal workpiece C.

[0061] S4. Quickly transfer the metal workpiece C to a nitrate bath for isothermal treatment at a temperature of 450℃ for 4 hours. The time for transferring the metal workpiece C from the heating furnace to the nitrate bath shall not exceed 10 seconds, thus obtaining the metal workpiece D that has completed the isothermal transformation.

[0062] S5. Remove the metal workpiece D from the nitrate bath, cool it to room temperature, and then remove the residual composite coating on the surface by sandblasting: the sandblasting abrasive is 120-mesh quartz sand, the sandblasting pressure is 0.6MPa, and the sandblasting time is 3min, to obtain a clean metal workpiece E.

[0063] Comparative Example 1

[0064] A method for preparing an isothermal quenching process for a corrosion-resistant and oxidation-resistant infiltration layer for pipelines differs from Example 1 in that step S2 is omitted, while other operation steps and process parameters are exactly the same as in Example 1.

[0065] Comparative Example 2

[0066] A method for preparing an isothermal quenching process for a corrosion-resistant and oxidation-resistant infiltration layer for pipelines differs from Example 1 in that only the inner layer (rare earth modified aluminoborosilicate glass) is coated in S2, and the middle and outer layers are not coated. Other operation steps and process parameters are exactly the same as in Example 1.

[0067] Comparative Example 3

[0068] A method for preparing an isothermal quenching process for a corrosion-resistant and oxidation-resistant infiltration layer for pipelines differs from Example 1 in that the core-shell structured composite powder in the middle layer of S2 is replaced with pure aluminum powder of the same particle size (without nickel shell), while the other operation steps and process parameters are exactly the same as in Example 1.

[0069] Comparative Example 4

[0070] A method for preparing an isothermal quenching process for a corrosion-resistant and oxidation-resistant infiltration layer for pipelines, which differs from Example 1 in that the outer layer in S2 does not contain aluminum powder, while the other operation steps and process parameters are exactly the same as in Example 1.

[0071] Performance testing:

[0072] 1. Surface hardness test: Using a Rockwell hardness tester, five different test points were selected on the surface of the metal workpiece E obtained in Examples 1-3 and Comparative Examples 1-4. The hardness values ​​were measured and the average value was taken to evaluate the mechanical properties of the workpiece and the surface decarburization.

[0073] 2. Saltwater immersion corrosion resistance test: The metal workpieces E obtained in Examples 1-3 and Comparative Examples 1-4 were immersed in a 5 wt% sodium chloride solution for 24 hours. After removal, the surface of the workpiece was visually observed to see if rust appeared, and the density and protective stability of the anti-oxidation penetration layer were evaluated.

[0074] 3. Copper sulfate spot test: Copper sulfate solution (10g CuSO4·5H2O dissolved in 90mL distilled water) was dropped onto the surface of metal workpiece E obtained in Examples 1-3 and Comparative Examples 1-4. After standing for 3 minutes, it was observed whether red copper spots were precipitated to evaluate the density and porosity of the surface diffusion layer.

[0075] The performance test data analysis is as follows:

[0076] Table 1. Basic performance test data of metal workpiece E obtained from Examples 1-3 and Comparative Examples 1-4.

[0077]

[0078] Table 1 shows that the synergistic process scheme of Examples 1-3, which involves pretreatment of the workpiece surface for degreasing and rust removal, layer-by-layer spraying of a gradient composite coating (rare earth modified inner layer, core-shell structure middle layer, and aluminum-containing outer layer), pre-sintering and controlled-temperature austenitization, rapid transfer isothermal quenching, and sandblasting to remove the coating, resulted in excellent surface hardness and corrosion resistance of the workpieces. The surface hardness of Examples 1-3 were 60.2 HRC, 58.5 HRC, and 56.1 HRC, respectively, all significantly higher than those of the comparative examples. After soaking in salt water for 24 hours, no rust was found on the surface, and no red copper spots were observed in the copper sulfate spot test, indicating that a dense, non-porous anti-oxidation layer was formed on the surface, effectively blocking oxygen erosion during the high-temperature austenitization process.

[0079] First, the workpiece undergoes precise temperature-controlled degreasing and pickling pretreatment to ensure surface cleanliness, laying the foundation for uniform adhesion of the gradient coating. Second, the inner layer of rare-earth modified aluminoborosilicate glass softens and melts at high temperatures, forming a dense glassy oxygen barrier layer. Simultaneously, rare-earth elements promote interfacial reactions and element diffusion, enhancing the bond with the substrate. The middle layer uses a core-shell structure (aluminum@nickel) composite powder. The nickel shell delays the oxidation release of the aluminum core, allowing it to be released and oxidized to Al2O3 in the later stages of austenitization, achieving self-repair and densification of the coating. Combined with nano-Al2O3, this further enhances the coating's mechanical strength and barrier properties. The outer layer contains aluminum powder and a second glass frit, which further consumes residual oxygen and promotes coating flow and spreading at high temperatures, forming a complete outer protective layer. This three-layer gradient design works synergistically to ensure the workpiece is protected from oxidation and decarburization throughout the austenitization process. Simultaneously, pre-sintering treatment (550-650℃) reduces internal stress and volatile matter in the coating, while controlled heating and rapid transfer isothermal quenching ensure uniform transformation of the substrate structure. Finally, after sandblasting to remove residual coating, a dense, antioxidant diffusion layer composed of diffused rare earth elements, Si, B, and Al has formed on the substrate surface. The synergistic effect of each process achieves simultaneous improvement in efficient oxidation resistance and mechanical properties of the metal workpiece during isothermal quenching.

[0080] Compared with Example 3, Comparative Example 1 omitted step S2 (without any protective coating). The workpiece was directly exposed to a high-temperature oxygen-containing environment during the austenitization process, resulting in severe oxidation and decarburization on the surface, causing the hardness to drop sharply to 34.6 HRC. After soaking in salt water, large-area corrosion appeared (covering more than 50% of the surface). The copper sulfate spot test showed a clear red copper film (continuous copper spots), indicating that its surface was loose and porous and had no protective ability at all.

[0081] Compared to Example 3, Comparative Example 2 only had an inner layer (rare-earth modified aluminum borosilicate glass) coated, without the middle and outer layers. While the single inner layer provides some oxygen barrier capability, it is prone to microcracks at high temperatures. The lack of the core-shell structure of the aluminum powder in the middle layer and the auxiliary densification of the outer layer resulted in through-pores in the coating. The final hardness decreased to 54.8 HRC, two tiny rust spots appeared after salt water immersion, and three red copper spots appeared in the copper sulfate spot test, indicating localized defects in the infiltration layer. The protective effect was significantly inferior to Example 3.

[0082] Compared to Example 3, Comparative Example 3 replaced the core-shell composite powder in the middle layer with pure aluminum powder of the same particle size (without a nickel shell). Pure aluminum powder oxidizes prematurely in the early stages of heating, failing to exert its self-healing effect in the later stages, and the volume expansion caused by premature oxidation may exacerbate coating cracking. The final hardness was 57.3 HRC, slightly lower than Example 3. Three tiny rust spots appeared after salt water immersion, and two red copper spots appeared in the copper sulfate spot test, indicating decreased coating density and the presence of localized micropores.

[0083] Compared to Example 3, Comparative Example 4 did not contain aluminum powder in the outer layer. The lack of aluminum powder in the outer layer, which aids in densification and reduces residual oxygen consumption, resulted in insufficient overall coating density. The final hardness was 56.9 HRC (slightly lower than Example 3). After immersion in salt water, a small rust spot appeared, and a red copper spot appeared in the copper sulfate spot test, indicating the presence of a small number of penetrating pores in the infiltrated layer. Therefore, the protective effect was still inferior to Example 3.

[0084] In summary, Examples 1-3 effectively suppressed the oxidation and decarburization behavior of metal workpieces during isothermal quenching by coupling multiple processes, including surface cleaning pretreatment, gradient composite coating construction, pre-sintering, controlled heating austenitization, rapid transfer isothermal quenching, and sandblasting to remove the coating, while simultaneously improving surface hardness and the density and protective properties of the infiltrated layer.

[0085] Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this application. Various changes and modifications can be made to this application without departing from the spirit and scope thereof, and all such changes and modifications fall within the scope of this application as claimed. The scope of protection of this application is defined by the equivalents of the appended claims.

Claims

1. An isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines, characterized in that, Including the following: S1. The surface of the metal workpiece is degreased and derusted to obtain a clean metal workpiece A; S2. An inner layer, a middle layer, and an outer layer are sequentially coated on the surface of metal workpiece A to form a gradient composite coating, resulting in metal workpiece B. The inner layer is a rare-earth oxide-modified aluminum borosilicate-based glass, obtained by reacting rare-earth oxides with aluminum borosilicate-based glass at high temperature. The middle layer comprises a core-shell structured composite powder, a first glass material, and a nano-reinforcing phase. The outer layer comprises a second glass material and aluminum powder. The core-shell structured composite powder consists of an aluminum core and a nickel or copper shell covering the surface of the aluminum core. The first glass material is high borosilicate glass or aluminum borosilicate glass. The nano-reinforcing phase includes any one of nano-sized Al2O3, SiC, and ZrO2. The second glass material is zinc borosilicate glass or barium aluminum silicate glass. S3. Place the metal workpiece B in a heating furnace, perform pre-sintering treatment first, then control the heating rate to raise the temperature to the austenitizing temperature and hold it at the temperature to austenitize the metal workpiece, and obtain the austenitized metal workpiece C. S4. The metal workpiece C is transferred to an isothermal quenching medium for isothermal treatment to obtain the metal workpiece D that has completed the isothermal transformation; S5. Remove the metal workpiece D from the isothermal quenching medium, cool it to room temperature, and then remove the residual composite coating on the surface by mechanical grinding or sandblasting to obtain a clean metal workpiece E.

2. The isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines according to claim 1, characterized in that, In S1, the metal workpiece includes any one of bearing steel, high-speed steel, and hot work die steel; the degreasing is performed using an alkaline degreasing agent, the degreasing temperature is 50-80℃, and the degreasing time is 10-30 minutes; the rust removal is performed using pickling, the pickling temperature is 20-40℃, and the pickling time is 5-15 minutes.

3. The isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines according to claim 1, characterized in that, In S2, the rare earth oxide includes any one of yttrium oxide, cerium oxide, and lanthanum oxide.

4. The isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines according to claim 1, characterized in that, In S2, the particle size of the core-shell composite powder is 1-20 μm, and the mass ratio of the aluminum core to the nickel or copper shell is 1:(4-9).

5. The isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines according to claim 1, characterized in that, In S2, the mass ratio of the core-shell composite powder, the first glass material, and the nano-reinforcing phase is (50-70):(20-40):(5-15).

6. The isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines according to claim 1, characterized in that, In S2, the mass ratio of the second glass material to aluminum powder is (3-7):

1.

7. The isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines according to claim 1, characterized in that, In S2, the coating method is any one of brushing, spraying, and dipping, and the total thickness of the gradient composite coating is 0.1-0.5 mm, wherein the thickness ratio of the inner layer, middle layer, and outer layer is 1:(1.5-2):(0.8-1.2).

8. The isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines according to claim 1, characterized in that, In S2, the preparation method of each layer of slurry in the gradient composite coating is as follows: the required powder raw materials for each layer are mixed with binder, solvent and additives, and then ball-milled to obtain a uniform slurry; the binder includes any one of silica sol, water glass and carboxymethyl cellulose; the additives include dispersant and thickener, the dispersant is polyethylene glycol and the thickener is bentonite.

9. The isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines according to claim 1, characterized in that, In S3, the pre-sintering temperature is 550-650℃ and the pre-sintering time is 0.5-1h; the heating rate is 5-15℃ / min; the austenitizing temperature is 850-950℃ and the holding time is 0.5-2h.

10. The isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines according to claim 1, characterized in that, In S4, the isothermal quenching medium is a nitrate bath, the isothermal treatment temperature is 300-450℃, and the isothermal treatment time is 1-4h.

11. The isothermal quenching process for preparing a corrosion-resistant and oxidation-resistant infiltration layer for pipelines according to claim 1, characterized in that, In S5, the mechanical grinding or sandblasting uses brown corundum or quartz sand with a particle size of 60-120 mesh, and the sandblasting pressure is 0.4-0.6 MPa.

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