A composite surface treatment method for improving corrosion and wear resistance of metal workpieces

Abstract

The application discloses a composite surface treatment method for improving corrosion resistance and wear resistance of metal workpieces, and relates to the technical field of mechanical manufacturing and metal surface. The method comprises the following steps: pretreating a non-nitrogen-permeation stainless steel base; performing a multi-stage gas nitriding process on the pretreated base, sequentially carrying out strong nitriding at a first temperature and a first nitrogen potential decomposition rate, diffusion at a second temperature and a second nitrogen potential decomposition rate, and secondary strong nitriding at a third temperature and a third nitrogen potential decomposition rate, so as to build a continuous and dense nitride compound layer; and then coating a composite coating containing hard wear-resistant particles and thermosetting resin on the surface of the nitriding layer and performing solidification to form an encapsulation coating. The application also relates to a metal workpiece with a three-layer structure of a composite coating, a nitriding layer and a stainless steel base prepared by the method. The method is particularly suitable for non-nitrogen-permeation steels such as 2605 stainless steel, and can significantly improve the service life and comprehensive performance of the non-nitrogen-permeation steels under extreme working conditions of strong corrosion and high wear.

Description

Technical Field

[0001] This invention relates to the fields of mechanical manufacturing and metal surface engineering technology, specifically to a composite surface treatment method for improving the corrosion and wear resistance of metal workpieces. Background Technology

[0002] In flue gas desulfurization systems of industries such as power, steel, and chemicals, flow components such as slurry circulation pumps, agitators, nozzles, and wear-resistant plates operate in extremely harsh environments. Especially during system startup, commissioning, or periods of unstable operation, the circulating slurry is highly acidic, with a pH value as low as 2–3 and an abnormally high chloride ion concentration, often exceeding 28,000 ppm. Simultaneously, the slurry carries a large number of solid particles, such as unreacted limestone, crystalline gypsum, and even precipitated sulfur particles. These particles, driven by the high-speed fluid, continuously scour the surface of the components, leading to exacerbated corrosion and wear, posing a serious challenge to the integrity of the material surface.

[0003] Currently, these components are mostly made of 2605 or 2205 duplex stainless steel or 316L austenitic stainless steel. Although these materials have a certain degree of corrosion resistance, they are still prone to localized pitting corrosion, crevice corrosion, and erosion wear in the coupled environment of "strong acid + high chloride ion + solid particle erosion," leading to premature component failure and a service life of only 3 to 6 months. Frequent replacement not only incurs high maintenance costs but may also cause unplanned system downtime, affecting production stability and environmental compliance.

[0004] To extend component lifespan, various surface protection methods have been employed in existing technologies, but all have significant limitations: organic coatings such as epoxy resin and polyurethane coatings are easy to apply and offer acceptable corrosion resistance, but they suffer from low hardness, poor wear resistance, and limited adhesion to the metal substrate, making them prone to blistering and peeling under thermal shock, mechanical impact, and long-term fluid erosion. Conventional gas nitriding can form a high-hardness nitride layer on the steel surface, significantly improving wear resistance. However, for austenitic "non-nitriding steels" like 2605, nitrogen atoms diffuse slowly in their crystal lattice, making it difficult for traditional nitriding processes to form a sufficiently thick, typically <0.2μm, continuous, and dense compound layer. This thin layer contains micropores, which can easily become corrosion initiation points in highly corrosive media, resulting in insufficient corrosion protection. Electroplating / chemical plating, such as hard chrome and nickel-phosphorus alloys, offers high hardness and good wear resistance. However, these platings generally contain micropores and their adhesion to the substrate is mechanical rather than metallurgical. Under alternating loads, microcracks are prone to develop and propagate, leading to plating peeling. Furthermore, electroplating processes pose environmental pollution problems.

[0005] In summary, existing technologies struggle to form a composite protective layer on a non-nitrided stainless steel substrate that possesses excellent adhesion, high hardness and wear resistance, extremely dense corrosion resistance, and adaptability to complex geometries. Summary of the Invention

[0006] The purpose of this invention is to provide a composite surface treatment method to improve the corrosion and wear resistance of metal workpieces, so as to solve the technical problems of short life and insufficient performance of metal workpieces under extreme corrosion and wear environments.

[0007] To achieve the above objectives, in a first aspect, the present invention provides a composite surface treatment method for improving the corrosion and wear resistance of metal workpieces, comprising the following steps:

[0008] S1. Substrate Pretreatment: The non-nitrided stainless steel substrate is subjected to machining and surface pretreatment. The machining includes at least roughing, finishing and hole machining processes, and non-destructive testing is performed before or after the finishing process. The surface pretreatment includes at least cleaning and washing of the substrate.

[0009] S2. Gas Nitriding Strengthening: The non-nitrided stainless steel substrate treated in step S1 is placed in a nitriding furnace, and a multi-stage gas nitriding process is performed to construct a continuous and dense nitride compound layer on the surface of the non-nitrided stainless steel substrate; wherein the process sequentially includes three nitriding stages with different process conditions:

[0010] S21, First nitriding stage: Nitriding is carried out in a first temperature range and a first nitrogen potential decomposition rate, wherein the first nitrogen potential decomposition rate is set to: form nitride nuclei on the surface of the non-nitrided stainless steel substrate, while inhibiting the diffusion of nitrogen atoms into the depth of the substrate.

[0011] S22, Second nitriding stage: Nitriding is carried out in a second temperature range and at a second nitrogen potential decomposition rate, wherein the second nitrogen potential decomposition rate is set to promote the diffusion of nitrogen atoms into the depth of the non-nitrided stainless steel matrix and to construct a continuous and dense nitride compound layer.

[0012] S23, Third nitriding stage: Nitriding is carried out in the third temperature range and at the third nitrogen potential decomposition rate, wherein the third nitrogen potential decomposition rate is set to reduce the nitrogen potential of the nitriding atmosphere, thereby promoting the redistribution of nitrogen atoms and the healing of micropores within the formed nitride compound layer.

[0013] S3. Composite Coating Encapsulation: A composite coating containing hard wear-resistant particles and thermosetting resin is applied to the surface of the non-nitrided stainless steel substrate after the treatment in step S2, and then cured to form an encapsulation coating on the nitride compound layer.

[0014] Furthermore, in step S2, the second temperature range is higher than the first temperature range, and the second nitrogen potential decomposition rate is higher than the first nitrogen potential decomposition rate.

[0015] Furthermore, the third temperature range is between the first temperature range and the second temperature range, and the third nitrogen potential decomposition rate is lower than the second nitrogen potential decomposition rate.

[0016] Furthermore, the first temperature range is 510°C to 520°C, and the first nitrogen potential decomposition rate is less than 30%; the second temperature range is 560°C to 570°C, and the second nitrogen potential decomposition rate is 50% to 65%; the third temperature range is 520°C to 530°C, and the third nitrogen potential decomposition rate is 25% to 40%.

[0017] Furthermore, the heat preservation time for the first nitriding stage is 18-22 hours, the heat preservation time for the second nitriding stage is 38-42 hours, and the heat preservation time for the third nitriding stage is 18-22 hours.

[0018] Furthermore, in step S1, before the machining, a heat treatment step is included for the casting blank of the non-nitrided stainless steel matrix, the heat treatment including solution treatment and aging treatment.

[0019] Furthermore, the solution treatment temperature is 1060℃ to 1100℃, and the solution is held for 5-7 hours before being cooled in oil; the aging treatment temperature is 180℃ to 220℃, and the solution is held for 3-5 hours before being cooled in air.

[0020] Furthermore, the composite coating is composed of the following components by mass percentage: 55-65% silicon carbide particles, 30-40% bisphenol F epoxy resin, 1.5-2.0% photoinitiator, 0.8-1.2% photosensitizer, and the balance being leveling agent.

[0021] In a second aspect, the present invention provides a corrosion-resistant and wear-resistant metal workpiece, which is prepared by the method according to any one of claims 1 to 8, and has a three-layer structure of a composite coating, a nitriding layer and a stainless steel substrate stacked sequentially.

[0022] Furthermore, the nitrided layer comprises γ−Fe4N and ε−Fe2N phases.

[0023] Compared with existing technologies, the beneficial effects of this invention are as follows: A dual composite protection system with complementary functions is constructed through the layered design of a gas nitriding reinforcement layer and an organic-inorganic composite encapsulation coating. The inner nitriding layer provides strong support with high hardness and metallurgical bonding, effectively resisting plastic deformation and abrasive embedding; the outer composite coating physically seals the micropores of the underlying layer, isolating corrosive media, and its hard particles directly withstand erosion and wear. The synergy of these two technologies fundamentally overcomes the limitations of single technologies, such as a nitriding layer alone being insufficiently corrosion-resistant or an organic coating alone being insufficiently wear-resistant, achieving long-term protection in highly corrosive and highly wear-coupled environments.

[0024] To address the challenge of slow nitrogen diffusion in austenitic stainless steels such as 2605, an innovative three-stage controllable gas nitriding process—"strong nitriding-diffusion-secondary strong nitriding"—was employed. By precisely controlling the temperature and nitrogen potential decomposition rate at each stage, uniform formation of nitride nuclei, continuous and dense growth of the compound layer, and healing of microscopic defects were achieved. Ultimately, a nitride compound layer of suitable thickness, dominated by γ-Fe4N and ε-Fe2N, and metallurgically bonded to the matrix, was obtained on a substrate traditionally difficult to nitrid, significantly improving the intrinsic hardness and wear resistance of the substrate surface.

[0025] The nitriding layer and the substrate are metallurgically bonded, with a bonding strength far exceeding that of mechanically bonded electroplated layers. The composite coating forms a strong chemical and physical bond with the nitriding layer surface through a curing reaction. Simultaneously, the third nitriding stage effectively heals the micropores of the compound layer through the redistribution of nitrogen atoms. Combined with the flow-filling effect of the composite coating, the final protective system is extremely dense, effectively blocking the penetration of acidic high-chloride ion media and preventing pitting and crevice corrosion. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the overall workflow of the present invention;

[0027] Figure 2 This is a schematic diagram of the matrix pretreatment workflow of the present invention;

[0028] Figure 3 This is a schematic diagram of the gas nitriding enhancement process of the present invention;

[0029] Figure 4 This is a schematic diagram of the composite coating encapsulation process of the present invention;

[0030] Figure 5 This is a schematic diagram of the three-layer composite structure of the metal workpiece of the present invention;

[0031] In the diagram: 1. Stainless steel substrate, 2. Nitriding layer, 3. Composite coating. Detailed Implementation

[0032] The embodiments of this application are described in detail below. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0033] Those skilled in the art will understand that, unless specifically stated otherwise, the singular forms “a,” “an,” and “the” used herein may also include the plural forms. It should be further understood that the term “comprising” as used in the specification of this application means the presence of features, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. The term “and / or” as used herein includes all or any units and all combinations of one or more associated listed items.

[0034] To enable those skilled in the art to more clearly understand the present invention, the following description explains the key terms used in this application:

[0035] Non-nitrided stainless steel: In this article, it refers to a class of stainless steel materials on which conventional gas nitriding processes struggle to form a continuous, dense, and sufficiently thick nitride compound layer on their surface. Examples include austenitic duplex stainless steels, such as grades 2605 and 2205, or austenitic stainless steels such as 316L. These materials have a low nitrogen atom diffusion coefficient, which is the target of this invention.

[0036] Nitrogen potential decomposition rate: In controlled gas nitriding processes, this parameter characterizes the ability of the nitriding atmosphere to supply active nitrogen atoms to the workpiece surface. In the context of this invention, it is directly related to and can be characterized and controlled by the decomposition rate of ammonia (NH3) in the nitriding furnace. A lower nitrogen potential decomposition rate corresponds to a lower ammonia decomposition rate, indicating a lower atmospheric nitrogen potential and a relatively gradual supply of active nitrogen atoms; a higher nitrogen potential decomposition rate corresponds to a higher ammonia decomposition rate, indicating a higher atmospheric nitrogen potential and a more intense supply of active nitrogen atoms. The percentage of nitrogen potential decomposition rate mentioned in each stage refers to the process state with corresponding nitrogen supply capacity achieved by controlling the ammonia decomposition rate.

[0037] γ-Fe4N and ε-Fe2N: These are the two main iron-nitrogen compound phases present in the nitriding layer and nitride compound layer described in this invention. γ-Fe4N is a face-centered cubic interstitial phase, typically exhibiting high hardness and a certain degree of toughness. ε-Fe2N is a close-packed hexagonal interstitial phase, with nitrogen content that can fluctuate within a certain range, and is typically extremely hard and dense. Together, they constitute the continuous and dense compound layer, providing high hardness and a basic corrosion-resistant barrier.

[0038] Composite coating encapsulation: In this invention, after gas nitriding strengthening, a composite material consisting of hard, wear-resistant particles and a thermosetting resin matrix is ​​coated onto the surface of the formed nitrided layer, and then cured to form a firmly bonded protective layer. The main function of this coating is to physically seal the micropores of the underlying nitrided layer, isolate corrosive media, and directly resist wear using the hard particles.

[0039] The composite surface treatment method for improving the corrosion and wear resistance of metal workpieces provided by this invention, targeting non-nitrided stainless steel substrates, includes the following three steps in sequence: substrate pretreatment, gas nitriding strengthening, and composite coating encapsulation. Through this synergistic process, a dual composite protection system consisting of a continuous and dense nitride compound layer and an external organic-inorganic composite encapsulation coating is constructed on the substrate surface.

[0040] Step 1: Matrix pretreatment.

[0041] This step provides a structurally intact, clean, and stable substrate for subsequent processing, and includes at least machining and surface pretreatment.

[0042] Machining: Through roughing, finishing, and hole machining processes, the workpiece achieves the specified geometric dimensions, shape accuracy, and surface roughness. Non-destructive testing, such as ultrasonic testing, is performed before or after finishing to identify internal defects such as cracks and inclusions, ensuring that subsequent surface treatment is carried out on a structurally intact substrate and preventing defects from becoming corrosion initiation points or stress concentration sources.

[0043] Surface pretreatment: The machined surface is cleaned and washed using physical or chemical methods to remove cutting fluid, grease, oxide layer, and adhering particles. A clean surface facilitates the uniform adsorption and diffusion of nitrogen atoms during the subsequent nitriding process and provides a substrate with good surface energy and micro-roughness for the coating, thereby affecting the uniformity of the nitrided layer and the bonding strength of the coating.

[0044] Optional heat treatment: To further optimize matrix properties, the blank can be heat-treated before machining, typically including solution treatment and aging treatment. Solution treatment fully dissolves alloying elements to obtain a homogeneous microstructure; aging treatment is used to precipitate strengthening phases and eliminate internal stresses. This process improves the microstructural stability, mechanical properties, and dimensional stability of the matrix, providing a superior substrate for subsequent high-temperature nitriding processes and service load-bearing capacity.

[0045] Step 2: Gas nitriding enhancement.

[0046] This step involves a three-stage controlled gas nitriding process to form a continuous and dense nitride compound layer on the surface of non-nitrided stainless steel.

[0047] The first nitriding stage is conducted within a predetermined temperature range and at a relatively low initial nitrogen potential decomposition rate. The purpose and conditions of this stage are related as follows: a suitable temperature provides the activation energy required for nitrogen atom infiltration; a low nitrogen potential decomposition rate corresponds to a low ammonia decomposition rate, maintaining the concentration of active nitrogen atoms in the atmosphere at a moderate level. These conditions facilitate the adsorption of nitrogen atoms on the substrate surface and the attainment of a critical concentration, thereby promoting the preferential formation of nitride nuclei, primarily γ-Fe4N, on the surface. Simultaneously, the lower nitrogen potential reduces the nitrogen chemical potential gradient between the surface and the core, effectively limiting excessive diffusion of nitrogen atoms into the substrate during the initial stage. This concentrates nitrogen supply primarily in the surface region, creating conditions for the formation of fine, uniform nitride nuclei and preventing the formation of a loose, porous, brittle layer due to excessively high and rapid surface nitrogen concentration.

[0048] Second nitriding stage:

[0049] The process is carried out at a second temperature range higher than the first temperature range and at a second nitrogen potential decomposition rate higher than the first nitrogen potential decomposition rate. The relationship between the objective and the conditions of this stage is as follows: the increased temperature significantly increases the diffusion coefficient of nitrogen atoms in the crystal lattice; the increased nitrogen potential decomposition rate greatly increases the supply of active nitrogen atoms in the atmosphere. The combined effect of these conditions is twofold: on the one hand, it promotes the diffusion of nitrogen atoms from the surface layer into the depth of the substrate, forming a diffusion layer of a certain thickness as support; on the other hand, the sufficient nitrogen source drives the continuous growth of the nitride nuclei formed in the first stage. The nuclei expand laterally and eventually connect with each other, thereby constructing a continuous and dense nitride compound layer on the substrate surface. This layer is mainly composed of γ−Fe4N and ε−Fe2N phases, and its hardness is significantly higher than that of the substrate.

[0050] Third nitriding stage:

[0051] The process is carried out in a third temperature range between the first and second temperature ranges, and at a third nitrogen potential decomposition rate lower than the second nitrogen potential decomposition rate. The purpose and conditions of this stage are as follows: the maintained temperature ensures sufficient atomic migration ability; the reduced nitrogen potential lowers the surface nitrogen chemical potential, resulting in a nitrogen concentration gradient within the formed compound layer. Driven by this force, nitrogen atoms undergo short-range diffusion and redistribution within the compound layer. The direct technical effect of this process is that it can fill or reduce micropores or component segregation at grain boundaries and phase boundaries that may occur due to rapid growth in the first two stages, thereby improving the overall density and compositional uniformity of the compound layer and enhancing its intrinsic performance as a corrosion barrier.

[0052] In a specific process setup, the first temperature range can be 510℃ to 520℃, with a first nitrogen potential decomposition rate of less than 30%; the second temperature range can be 560℃ to 570℃, with a second nitrogen potential decomposition rate of 50% to 65%; and the third temperature range can be 520℃ to 530℃, with a third nitrogen potential decomposition rate of 25% to 40%. The holding time for each stage can be set to 18-22 hours, 38-42 hours, and 18-22 hours, respectively, to ensure the full progress of the physical metallurgical process at each stage.

[0053] A nitride-based compound layer is formed on the surface of non-nitrided stainless steel, metallurgically bonded to the substrate. This layer has a higher hardness than the substrate, which improves the surface's resistance to plastic deformation and abrasive wear; its continuous and dense structure can delay the direct contact of corrosive media with the substrate.

[0054] Step 3: Composite coating encapsulation.

[0055] This step applies a functional encapsulating coating to the reinforced nitrided layer surface to achieve synergistic protection. A composite coating containing hard, abrasion-resistant particles and a thermosetting resin is applied to the nitrided layer surface and then cured.

[0056] Coating Composition and Function: The hard, wear-resistant particles can be made of silicon carbide, alumina, etc., with a mass fraction of, for example, 55%-65%. Their main function is to directly resist the micro-cutting and impact wear of external abrasive particles. The thermosetting resin can be made of bisphenol F epoxy resin, phenolic resin, etc., with a mass fraction of, for example, 30%-40%. As a continuous matrix, it serves to bind the hard particles, impart toughness to the coating, and provide adhesion to the nitrided layer. The coating also contains necessary additives, such as initiating systems for photocuring or thermocuring, such as photoinitiators, photosensitizers, and leveling agents.

[0057] Coating and curing: The coating is uniformly applied to the surface of the nitrided layer. The resin is cross-linked through the curing reaction to form a three-dimensional network structure, and the cured coating is bonded to the substrate.

[0058] The coating covers the surface of the nitrided layer, filling and sealing any microscopic pores and defects that may exist on its surface, blocking the path of corrosive media to penetrate through the pores. Furthermore, the coating acts as a continuous and dense barrier, physically isolating corrosive liquids and gases from contact with the underlying metal. The hard particles in the coating can directly withstand and disperse the impact and micro-cutting action of external abrasive particles, reducing the transfer of wear energy to the nitrided layer and the substrate. The resin matrix possesses a certain degree of elasticity and toughness, absorbing some mechanical impact and thermal stress, reducing interfacial stress concentration.

[0059] The nitrided layer provides high hardness and metallurgical bonding support, while the composite coating provides dense isolation and wear resistance. Combined, the workpiece surface possesses both the high hardness and resistance to plastic deformation provided by the nitrided layer, and the media isolation, abrasive buffering, and defect sealing functions provided by the composite coating. This structure can extend the service life of the workpiece in corrosion-wear coupled environments.

[0060] The following embodiments will illustrate in detail the specific implementations of the technical solutions in the claims. For clarity, each embodiment will focus on demonstrating the application of different combinations of technical features.

[0061] Example: The following uses the 2605 stainless steel wear-resistant plate of the slurry circulation pump in a flue gas desulfurization system as a typical treatment object to illustrate the specific implementation process of the present invention. This component operates under extreme corrosion-wear conditions with a pH value of 2-3, a chloride ion concentration of approximately 28,000 ppm, and high-speed scouring by solid particles such as limestone.

[0062] S1, Matrix Pretreatment

[0063] S1a. Blank Casting and Initial Inspection: First, 2605N stainless steel base material is selected and cast using a medium-frequency induction furnace to obtain wear-resistant plate blanks. Sufficient machining allowance must be left in the blank dimensions. After casting, a preliminary inspection is performed on the blank to ensure there are no casting defects larger than 0.5mm in diameter, such as porosity, shrinkage cavities, etc.

[0064] S1b. Blank Heat Treatment: The cast blank is placed in a controlled atmosphere box-type multi-purpose furnace for heat treatment to optimize the matrix structure and properties. The heat treatment process includes:

[0065] Solution treatment: Raise the furnace temperature to 1080℃ at a rate of ≤100℃ / h and hold for 6 hours to ensure complete dissolution of alloying elements and homogenization of the austenite structure. After holding, quickly transfer the workpiece to No. 1 machine oil at 40℃ with a kinematic viscosity of 20-30 mm² / s for cooling. Stir the oil during the cooling process to obtain an appropriate cooling rate and inhibit the precipitation of harmful second phases.

[0066] Aging treatment: After solution treatment, the workpiece is immediately transferred to an aging furnace and held at 200°C for 4 hours, then cooled to room temperature with the furnace. This is to eliminate internal stress and stabilize the microstructure.

[0067] S1c, Machining: The heat-treated blank is machined according to the following procedures: Rough Machining: Rough turning is performed using a CNC vertical lathe to remove most of the excess material. Non-destructive Testing: Ultrasonic testing is performed on the rough-machined workpiece to confirm that there are no internal defects such as cracks or inclusions. Finish Machining: Finish machining is performed using a high-precision CNC vertical lathe according to the final drawing dimensions to ensure the form and position tolerances and surface roughness of the flow surface. Hole Machining: HM12 threaded holes as required by the drawing are drilled and tapped using a vertical machining center. Cleaning: All sharp edges and corners are rounded using a handheld pneumatic tool and sandpaper, and all metal shavings and dust on the surface are blown away with dry, clean compressed air.

[0068] S1d. Cleaning: Place the workpiece in an ultrasonic cleaner and clean it with an alkaline metal cleaning agent to remove oil stains. Then rinse with deionized water. After rinsing, wipe with anhydrous ethanol to remove moisture, and finally dry in an oven for 30 minutes. The treated workpiece surface should pass a continuous water film test.

[0069] S2, Gas nitriding enhancement

[0070] S2a. Furnace Preparation: Load the cleaned workpieces into the gas nitriding furnace at a furnace temperature below 150℃. Support the workpieces with high-temperature alloy tooling, maintaining a minimum spacing of 50mm between them, ensuring the nitrided surface faces the gas flow direction. After loading, purify the furnace chamber to reduce the residual oxygen content to below 0.1%.

[0071] S2b, Three-stage nitriding process: Ammonia gas with a purity of 99.99% is introduced into the furnace at a flow rate controlled at 5-8 m³ / h, maintaining a furnace pressure of 0.02-0.03 MPa. The process is executed in the following three stages:

[0072] First stage of intense permeation: The furnace temperature is raised to 515℃ at a heating rate of 50℃ / h, and the ammonia decomposition rate is controlled within the range of 25-28%, corresponding to a nitrogen potential decomposition rate of less than 30%. This temperature is maintained for 20 hours.

[0073] Second stage diffusion: Continue to raise the furnace temperature to 565℃ at a heating rate of 50℃ / h, and adjust the ammonia decomposition rate to the range of 55-60%, corresponding to a nitrogen potential decomposition rate of 50%-65%. Hold at this temperature for 40 hours.

[0074] The third stage, secondary intensive infiltration: heating is stopped, and the furnace temperature is allowed to drop naturally to 525℃. The ammonia decomposition rate is adjusted to the range of 30-35%, corresponding to a nitrogen potential decomposition rate of 25%-40%. This temperature is maintained for 20 hours.

[0075] S2c, Cooling: After the third stage is completed, the ammonia supply is turned off, and the workpiece is cooled to room temperature with the furnace. The cooling rate shall not exceed 30℃ / h.

[0076] S2d Nitriding Layer Inspection: After treatment, the nitriding layer on the workpiece surface is inspected. X-ray diffraction (XRD) analysis confirms that the compound layer mainly consists of γ-Fe4N and ε-Fe2N. The thickness of the compound layer is measured using metallographic methods.

[0077] S3, Composite Coating Encapsulation

[0078] S3a. Surface pretreatment after nitriding: The nitrided workpiece undergoes a secondary cleaning to remove surface contaminants. This includes: ultrasonic cleaning with a solution containing metal cleaning agent, rinsing with deionized water, wiping with anhydrous ethanol to remove moisture, and then drying in an oven for 30 minutes.

[0079] S3b, Composite Coating Preparation: Weigh the following components by mass percentage: 60% 120-mesh silicon carbide particles, 35% bisphenol F epoxy resin, 1.75% DPI.PF6 photoinitiator, 1% NVK photosensitizer, and 2.25% polyol transfer agent.

[0080] First, thoroughly mix the silicon carbide particles with the surface wetting agent, then add the remaining components and stir until the mixture is homogeneous.

[0081] S3c, Coating and Curing:

[0082] First coat: Apply the prepared coating evenly to the workpiece's flow surface, controlling the wet film thickness. Immediately after coating, place the workpiece in a vacuum chamber for 30 minutes to eliminate air bubbles within the coating. After removal, use a scraper to smooth any drips on the surface.

[0083] First curing: Place the workpiece under a high-pressure mercury lamp for light curing. The irradiance is 6.6 mW / cm², and the irradiation time is 10 minutes. After curing, allow it to air dry naturally for 24 hours.

[0084] For the second coating and curing, the silicon carbide in the coating is replaced with 80-mesh particles, while the remaining components and proportions remain unchanged. The above coating, vacuum degassing, UV curing, and post-curing process is repeated. The wet film thickness for the second coat is controlled to be approximately 100 μm.

[0085] Ultimately, a dense, smooth, and silicon carbide-rich composite encapsulation coating is formed on the workpiece surface.

[0086] The wear-resistant plate treated in this embodiment was installed and put into operation. After more than 6 months of cumulative operation, including approximately 2 months of commissioning under extreme conditions, the composite coating surface was inspected and found to be intact, with no blistering, peeling, or obvious corrosion marks, only showing a slight polished appearance in high-flow-rate areas. This result confirms the effective protection of the method of the present invention for 2605 stainless steel workpieces under extreme corrosive and abrasive environments.

[0087] Although embodiments of the present invention have been shown and described, it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, it will be understood by those skilled in the art that all other embodiments obtained by making various changes, modifications, substitutions and variations to these embodiments without departing from the principles and spirit of the present invention and without creative effort are within the scope of protection of the present invention.

Claims

1. A composite surface treatment method for improving the corrosion and wear resistance of metal workpieces, characterized in that, Includes the following steps: S1. Substrate Pretreatment: The non-nitrided stainless steel substrate is subjected to machining and surface pretreatment. The machining includes at least roughing, finishing and hole machining processes, and non-destructive testing is performed before or after the finishing process. The surface pretreatment includes at least cleaning and washing of the substrate. S2. Gas Nitriding Strengthening: The non-nitrided stainless steel substrate treated in step S1 is placed in a nitriding furnace, and a multi-stage gas nitriding process is performed to construct a continuous and dense nitride compound layer on the surface of the non-nitrided stainless steel substrate; wherein the process sequentially includes three nitriding stages with different process conditions: S21, First nitriding stage: Nitriding is carried out in a first temperature range and a first nitrogen potential decomposition rate, wherein the first nitrogen potential decomposition rate is set to: form nitride nuclei on the surface of the non-nitrided stainless steel substrate, while inhibiting the diffusion of nitrogen atoms into the depth of the substrate. S22, Second nitriding stage: Nitriding is carried out in a second temperature range and at a second nitrogen potential decomposition rate, wherein the second nitrogen potential decomposition rate is set to promote the diffusion of nitrogen atoms into the depth of the non-nitrided stainless steel matrix and to construct a continuous and dense nitride compound layer. S23, Third nitriding stage: Nitriding is carried out in the third temperature range and at the third nitrogen potential decomposition rate, wherein the third nitrogen potential decomposition rate is set to reduce the nitrogen potential of the nitriding atmosphere, thereby promoting the redistribution of nitrogen atoms and the healing of micropores within the formed nitride compound layer. S3. Composite Coating Encapsulation: A composite coating containing hard wear-resistant particles and thermosetting resin is applied to the surface of the non-nitrided stainless steel substrate after the treatment in step S2, and then cured to form an encapsulation coating on the nitride compound layer.

2. The composite surface treatment method for improving the corrosion and wear resistance of metal workpieces according to claim 1, characterized in that, In step S2, the second temperature range is higher than the first temperature range, and the second nitrogen potential decomposition rate is higher than the first nitrogen potential decomposition rate.

3. The composite surface treatment method for improving the corrosion and wear resistance of metal workpieces according to claim 2, characterized in that, The third temperature range is between the first temperature range and the second temperature range, and the third nitrogen potential decomposition rate is lower than the second nitrogen potential decomposition rate.

4. The composite surface treatment method for improving the corrosion and wear resistance of metal workpieces according to claim 3, characterized in that, The first temperature range is 510°C to 520°C, and the first nitrogen potential decomposition rate is less than 30%. The second temperature range is 560°C to 570°C, and the second nitrogen potential decomposition rate is 50% to 65%. The third temperature range is 520°C to 530°C, and the third nitrogen potential decomposition rate is 25% to 40%.

5. The composite surface treatment method for improving the corrosion and wear resistance of metal workpieces according to claim 4, characterized in that, The heat preservation time for the first nitriding stage is 18-22 hours, the heat preservation time for the second nitriding stage is 38-42 hours, and the heat preservation time for the third nitriding stage is 18-22 hours.

6. The composite surface treatment method for improving the corrosion and wear resistance of metal workpieces according to claim 1, characterized in that, In step S1, prior to the machining, a heat treatment step is included for the casting blank of the non-nitrided stainless steel matrix, wherein the heat treatment includes solution treatment and aging treatment.

7. The composite surface treatment method for improving the corrosion and wear resistance of metal workpieces according to claim 1, characterized in that, The solution treatment temperature is 1060℃ to 1100℃, and the solution is held for 5-7 hours before being cooled in oil; the aging treatment temperature is 180℃ to 220℃, and the solution is held for 3-5 hours before being cooled in air.

8. The composite surface treatment method for improving the corrosion and wear resistance of metal workpieces according to claim 1, characterized in that, The composite coating, by mass percentage, consists of the following components: 55-65% silicon carbide particles, 30-40% bisphenol F epoxy resin, 1.5-2.0% photoinitiator, 0.8-1.2% photosensitizer, and the balance being leveling agent.

9. A corrosion-resistant and wear-resistant metal workpiece, characterized in that, It is prepared by any one of claims 1 to 8 and has a three-layer structure consisting of a composite coating, a nitriding layer and a stainless steel substrate stacked sequentially.

10. The metal workpiece according to claim 9, characterized in that, The nitrided layer contains γ−Fe4N and ε−Fe2N phases.

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