A process for improving the corrosion resistance of ductile iron surfaces

Abstract

This application discloses a process for improving the corrosion resistance of ductile iron surfaces, relating to the field of metal material surface processing technology. First, the surface of the ductile iron casting is pretreated to remove impurities. Then, a first laser cladding process is performed using alloy powder with high iron content to enhance the bonding between the coating and the substrate. A second laser cladding process is then performed using alloy powder with medium iron content and small amounts of chromium, molybdenum, and vanadium powders to increase the coating hardness. The casting after the second laser cladding is then polished, and pure titanium foil is used to cover the casting. A third laser cladding process is then performed using stainless steel powder, titanium powder, and a small amount of auxiliary powders to form a uniformly distributed titanium-rich layer. Finally, plasma nitriding is performed to form a uniform titanium nitride layer. This application's solution can effectively improve the bonding strength between the coating and the casting, and significantly enhance the wear and corrosion resistance of the casting, making it suitable for surface strengthening of mechanical components under complex operating conditions.

Description

Technical Field

[0001] This application relates to the field of metal surface processing technology, specifically to a process for improving the corrosion resistance of ductile iron surfaces. Background Technology

[0002] Ductile iron, due to its high strength, high toughness, excellent casting performance, and cost-effectiveness, is widely used in machinery manufacturing, automotive industry, construction machinery, and mining machinery, such as key components like engine blocks, crankshafts, gears, and wear-resistant parts for mining machinery. However, in actual service, these components often face complex conditions such as wear, corrosion, and impact, leading to surface failures like wear, rust, and peeling, severely impacting component lifespan and equipment operational stability. Therefore, improving the surface hardness, wear resistance, and corrosion resistance of ductile iron through surface strengthening technology while retaining the high toughness of its matrix has become a core technological requirement for extending the service life of critical components and reducing maintenance costs. Laser cladding technology utilizes a high-energy-density laser beam to melt synchronously or pre-placed metal or ceramic powder on the surface of a metal substrate, then rapidly solidifies to form a metallurgically bonded, high-performance coating. However, due to the extremely high melting point of ceramic materials, directly cladding ceramic materials onto the substrate significantly increases energy consumption. Therefore, optimizing the laser cladding process and reducing energy costs has become a key technical challenge. Chinese invention patent CN103882324B discloses an anti-corrosion and wear-resistant coating and its coating method. The method involves laser-cladding alloy powder onto a substrate surface, followed by plasma nitriding, to obtain a coating with high surface hardness and good wear and corrosion resistance. By laser-cladding the alloy and then plasma nitriding, nitride ceramics are directly polymerized in situ on the coating, reducing the energy consumption of directly cladding ceramic coatings. Simultaneously, the resulting nitride grains are small and have stable performance.

[0003] However, existing laser cladding coatings still suffer from poor adhesion to the substrate and uneven distribution of corrosion-resistant components. Therefore, there is an urgent need to provide a surface coating preparation process to solve these problems. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the primary objective of this application is to provide a process for improving the corrosion resistance of ductile iron surfaces. This is achieved by employing laser cladding technology to construct a composite coating on the surface of ductile iron castings, thereby enhancing their corrosion resistance and wear resistance. This application utilizes a multi-stage laser cladding process combined with plasma nitriding. By designing the composition of each cladding layer, a good bond between the coating and the substrate is achieved, mitigating the problem of poor interfacial bonding between the hard, corrosion-resistant, and wear-resistant layer and the relatively soft iron substrate. Furthermore, by uniformly coating the casting with pure titanium foil, a uniform titanium nitride layer is formed on the casting surface, improving the casting's resistance to pitting corrosion.

[0005] To achieve the above objectives, this application provides the following technical solution:

[0006] This application provides a process for improving the corrosion resistance of ductile iron surfaces, including the following steps:

[0007] Surface pretreatment of ductile iron castings is performed to obtain pretreated ductile iron castings;

[0008] Using the first cladding alloy powder, under argon protection, the pretreated ductile iron casting was subjected to the first laser cladding to obtain a first-treated casting;

[0009] Using a second cladding alloy powder, under argon protection, a second laser cladding is performed on the first-treated casting to obtain a second-treated casting;

[0010] The surface of the secondary-treated casting is ground smooth, and the surface dust is blown away with compressed air to obtain the secondary-treated casting after grinding.

[0011] Pure titanium foil is uniformly coated on the surface of the polished secondary-treated casting. Then, a third laser cladding is performed using a third cladding alloy powder under argon protection to obtain a three-stage treated casting.

[0012] Plasma nitriding was performed on the ternary treated castings to obtain ductile iron castings with improved surface corrosion resistance.

[0013] It should be noted that the first cladding alloy powder, the second cladding alloy powder, and the third cladding alloy powder are prepared according to their component content and then thoroughly mixed in a ball mill to obtain the final product.

[0014] Preferably, the process further includes: after each laser cladding is completed, the casting needs to be allowed to cool naturally to room temperature before proceeding to the next step.

[0015] It should be noted that the high temperature generated during laser cladding will create thermal stress inside the casting. Therefore, it is necessary to cool to room temperature before proceeding with the next step of laser cladding to avoid cracking of the casting due to thermal stress.

[0016] Preferably, the surface pretreatment method is as follows:

[0017] First, grind the surface of the ductile iron casting. Then, use acetone or ethanol to ultrasonically clean the ground ductile iron casting at a power of 200~350W for 10~20 minutes. After cleaning, dry it at a temperature of 80~100℃ for 20~30 minutes.

[0018] It should be noted that surface treatment removes the oxide layer and impurities from the surface of the ductile iron casting, preventing impurities from damaging the metallurgical bonding effect during laser cladding and enhancing the bonding strength between the coating and the casting.

[0019] Preferably, the first cladding alloy powder comprises, by mass percentage: 80-90% iron powder, 5-10% silicon powder, and 3-12% nickel powder; the parameters of the first laser cladding are: laser power 1800-2200W, scanning speed 3-5mm / s, powder feed rate 8-12g / min, spot diameter 3-4mm, and argon flow rate 15-20L / min.

[0020] It should be noted that the high iron content alloy powder has a high degree of matching with the composition of the ductile iron matrix. Under the high temperature of the laser, the matrix surface and the alloy powder melt to form a metallurgical bonding interface, which enhances the interfacial bonding force between the coating and the substrate. At the same time, the addition of silicon powder and nickel powder can reduce the cladding temperature, improve the fluidity of the molten pool, and reduce the generation of porosity and cracks. Ni element can also improve the toughness of the cladding layer.

[0021] Preferably, the second cladding alloy powder comprises, by mass percentage: 65-80% iron powder, 10-20% chromium powder, 4-12% molybdenum powder, and 2-5% vanadium powder; the parameters of the second laser cladding are: laser power 2000-2400W, scanning speed 2.5-4mm / s, powder feed rate 10-14g / min, spot diameter 3-4mm, and argon flow rate 15-20L / min.

[0022] It should be noted that the intermediate layer is a transition layer, its function being to connect the underlying layer and the surface layer. This avoids the risk of peeling and delamination that can easily occur due to the significant differences in properties between the underlying and surface layers. Therefore, the cladding alloy powder in the intermediate layer reduces the iron powder content and increases the content of chromium powder, molybdenum powder, and vanadium powder. These elements combine with nitrogen to form high-hardness nitrides, allowing the coating performance to smoothly transition from the high toughness of the underlying layer to the high hardness and high corrosion resistance of the surface layer. This reduces interfacial stress concentration caused by abrupt performance changes and improves the overall structural stability of the coating. Appropriately increasing the laser power and reducing the scanning speed ensures that the intermediate layer and the underlying layer are fully fused, forming a continuous metallurgical bond and improving the density of the cladding layer.

[0023] Preferably, the thickness of the pure titanium foil is 0.1~0.2mm.

[0024] It should be noted that before coating with titanium foil, the surface of the secondary-processed casting is polished and impurities are removed to ensure that the titanium foil is tightly and evenly coated on the casting surface without wrinkles or gaps. In the third laser cladding, using titanium foil to coat the casting can improve the uniformity of titanium distribution on the casting surface, laying the foundation for the formation of a uniformly distributed titanium nitride layer in the future.

[0025] Preferably, the third cladding alloy powder comprises, by mass percentage: 70-80% stainless steel powder, 15-30% titanium powder, and 0.3-0.8% auxiliary powder; the parameters of the third laser cladding are: laser power 1700-2000W, scanning speed 3.5-4.5mm / s, powder feed rate 6-9g / min, spot diameter 3-4mm, and argon flow rate 18-22L / min.

[0026] It should be noted that the surface of the coating needs to have good corrosion and wear resistance. Therefore, stainless steel powder and a small amount of titanium powder are used. The main element of stainless steel powder is iron, which can ensure a good metallurgical bond with the intermediate layer and also has a certain degree of corrosion resistance. Titanium powder replenishes the titanium lost during the cladding process and helps the titanium foil form a uniform titanium enrichment layer.

[0027] Preferably, the auxiliary powder includes either yttrium oxide powder or cerium oxide powder.

[0028] It should be noted that the main function of yttrium oxide or cerium oxide is to refine the grains of the alloy, which can reduce oxide inclusions in the cladding layer, reduce grain size, and improve the density and mechanical properties of the surface layer. At the same time, rare earth elements can improve the activity of the surface layer and promote the diffusion and bonding of nitrogen atoms during the subsequent nitriding process.

[0029] Preferably, the specific steps of the plasma nitriding treatment are as follows: the tertiary-treated casting is placed in a plasma nitriding furnace, a vacuum is drawn to a pressure ≤10Pa, a mixture of N2 and H2 gas is introduced into the furnace, the furnace pressure is adjusted to 200~300Pa, the voltage is set to 800~1000V, the power is turned on, and then the temperature is raised to 300~500℃. After nitriding for 4~10 hours, heating is stopped, and the workpiece is allowed to cool naturally to below 200℃, and then air-cooled to room temperature.

[0030] It should be noted that through nitriding, nitrogen reacts with titanium, chromium, molybdenum and vanadium in the coating to form nitrides, which improves the hardness and corrosion resistance of the coating. By combining laser cladding with plasma nitriding, a titanium nitride ceramic layer is generated in situ on the surface of the casting, which reduces the energy consumption of directly cladding the titanium nitride coating on the surface of the casting, while also ensuring the bonding strength between the coating and the substrate.

[0031] Preferably, the volume ratio of N2 to H2 in the mixed gas is 1:(3~5).

[0032] The beneficial effects of this application are:

[0033] This application involves uniformly coating a layer of pure titanium foil onto the surface of a casting before the third laser cladding, melting it under laser heating to form a uniformly distributed titanium-rich layer on the casting surface. Then, during plasma nitriding, the titanium-rich layer combines with nitrogen atoms to form a titanium nitride layer. Because titanium nitride has excellent corrosion resistance and high hardness, the wear and corrosion resistance of the coating is greatly improved. At the same time, using pure titanium foil can greatly improve the uniformity of the titanium-rich layer distribution on the casting surface, thereby forming a uniformly distributed wear-resistant and corrosion-resistant coating on the casting surface, which greatly improves the pitting corrosion resistance of the casting.

[0034] To avoid coating detachment caused by significant performance differences between the titanium nitride layer and the casting surface, this application effectively improves the poor interfacial bonding between the coating and the casting by designing the composition of each layer in the laser cladding process. First, a large amount of iron powder is added to the bottom alloy powder, enabling it to form a metallurgical bond with the casting during laser cladding, thus enhancing the bonding strength between the coating and the casting. Then, in the intermediate alloy powder, the iron powder content is reduced, while chromium powder, molybdenum powder, and vanadium powder are increased. This intermediate layer serves as a transition layer, where the iron powder forms a metallurgical bond with the bottom layer, while the chromium powder, molybdenum powder, and vanadium powder, after melting, disperse in the intermediate layer and combine with nitrogen atoms to form nitrides, increasing the hardness of the intermediate layer. This creates a hardness buffer zone between the relatively tough iron-based metal layer at the bottom and the high-hardness titanium nitride layer on the surface, preventing abrupt performance changes that could lead to poor bonding between layers and thus improving the overall structural stability of the coating. Attached Figure Description

[0035] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 A schematic diagram illustrating a process for improving the corrosion resistance of ductile iron surfaces, as provided in this application.

[0037] Figure 2 A flowchart of a process for improving the corrosion resistance of ductile iron surfaces provided in this application. Detailed Implementation

[0038] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the specific embodiments of this application will be described in detail below with reference to the accompanying drawings.

[0039] Many specific details are set forth in the following description in order to provide a full understanding of this application. However, this application may also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0040] The following specific embodiments further illustrate this point:

[0041] Example 1

[0042] like Figure 1 and Figure 2 As shown, this embodiment provides a process for improving the corrosion resistance of ductile iron surfaces:

[0043] 1. Grind the surface of the ductile iron casting, and use ethanol to ultrasonically clean the ground ductile iron casting at a power of 300W for 20 minutes. After cleaning, dry it at 80℃ for 20 minutes to obtain the pretreated ductile iron casting.

[0044] 2. The pretreated ductile iron casting is placed on a laser cladding stage for the first laser cladding. Argon is used as the protective gas with a flow rate of 15L / min. Under a laser with a power of 1800W and a spot diameter of 3mm, the alloy powder, which consists of 80% iron powder, 8% silicon powder and 12% nickel powder by mass percentage, is clad onto the surface of the casting to obtain a first-treatment casting.

[0045] 3. After the first-processed casting has completely cooled to room temperature, it is placed on a laser cladding stage for a second laser cladding. Argon is used as the protective gas with a flow rate of 15L / min. Under a laser with a power of 2000W and a spot diameter of 3mm, at a scanning speed of 2.5mm / s and a powder feeding rate of 10g / min, an alloy powder consisting of 70% iron powder, 20% chromium powder, 5% molybdenum powder, and 5% vanadium powder by mass percentage is clad onto the surface of the casting to obtain a second-processed casting.

[0046] 4. After the secondary-treated casting has completely cooled to room temperature, grind its surface smooth, and then use compressed air to blow away the surface dust to obtain the polished secondary-treated casting.

[0047] 5. A 0.1mm thick pure titanium foil is evenly and smoothly wrapped around the surface of the secondary-treated casting after polishing. Then, a third laser cladding is performed. Argon is used as the protective gas with a flow rate of 18L / min. Under a laser with a power of 1700W and a spot diameter of 3mm, a scanning speed of 4mm / s and a powder feeding rate of 6g / min, an alloy powder consisting of 69% stainless steel powder, 30% titanium powder and 1% yttrium oxide powder by mass percentage is clad onto the surface of the casting to obtain a three-stage treated casting.

[0048] 6. After the castings from the three treatments have completely cooled to room temperature, place them in a plasma nitriding furnace, evacuate the furnace to a pressure ≤10Pa, introduce a N2 and H2 mixture with a volume ratio of 1:4 into the furnace, adjust the furnace pressure to 200Pa, set the voltage to 800V, start the power supply, and then heat to 300℃. After nitriding for 8 hours, stop heating and allow the workpiece to cool naturally to below 200℃, and then air cool to room temperature to obtain the ductile iron castings with improved surface corrosion resistance as described in Example 1.

[0049] Example 2

[0050] like Figure 1 and Figure 2 As shown, this embodiment provides a process for improving the corrosion resistance of ductile iron surfaces:

[0051] 1. Grind the surface of the ductile iron casting, and use acetone to ultrasonically clean the ground ductile iron casting at a power of 200W for 15 minutes. After cleaning, dry it at 90℃ for 30 minutes to obtain the pretreated ductile iron casting.

[0052] 2. The pretreated ductile iron casting is placed on a laser cladding stage for the first laser cladding. Argon is used as the protective gas with a flow rate of 18L / min. Under a laser with a power of 2000W and a spot diameter of 3.5mm, the alloy powder, which consists of 85% iron powder, 5% silicon powder, and 10% nickel powder by mass percentage, is clad onto the surface of the casting to obtain a first-treatment casting.

[0053] 3. After the first-processed casting has completely cooled to room temperature, it is placed on a laser cladding stage for a second laser cladding. Argon is used as the protective gas with a flow rate of 18L / min. Under a laser with a power of 2200W and a spot diameter of 3.5mm, at a scanning speed of 3mm / s and a powder feeding rate of 12g / min, an alloy powder consisting of 65% iron powder, 18% chromium powder, 12% molybdenum powder, and 5% vanadium powder by mass percentage is clad onto the surface of the casting to obtain a second-processed casting.

[0054] 4. After the secondary-treated casting has completely cooled to room temperature, grind its surface smooth, and then use compressed air to blow away the surface dust to obtain the polished secondary-treated casting.

[0055] 5. A 0.15mm thick pure titanium foil is evenly and smoothly wrapped around the surface of the secondary-treated casting after polishing. Then, a third laser cladding is performed. Argon is used as the protective gas at a flow rate of 20L / min. Under a laser with a power of 1800W and a spot diameter of 3.5mm, a scanning speed of 3.5mm / s, and a powder feeding rate of 8g / min, an alloy powder consisting of 75% stainless steel powder, 24.5% titanium powder, and 0.5% cerium oxide powder by mass percentage is clad onto the surface of the casting to obtain a three-stage treated casting.

[0056] 6. After the castings from the three treatments have completely cooled to room temperature, place them in a plasma nitriding furnace, evacuate the furnace to a pressure ≤10Pa, introduce a N2 and H2 mixture with a volume ratio of 1:3 into the furnace, adjust the furnace pressure to 250Pa, set the voltage to 900V, start the power supply, and then heat to 400℃. After nitriding for 4 hours, stop heating and allow the workpiece to cool naturally to below 200℃, and then air cool to room temperature to obtain the ductile iron castings with improved surface corrosion resistance as described in Example 2.

[0057] Example 3

[0058] like Figure 1 and Figure 2 As shown, this embodiment provides a process for improving the corrosion resistance of ductile iron surfaces:

[0059] 1. Grind the surface of the ductile iron casting, and use acetone to ultrasonically clean the ground ductile iron casting at a power of 350W for 10 minutes. After cleaning, dry it at 100℃ for 25 minutes to obtain the pretreated ductile iron casting.

[0060] 2. The pretreated ductile iron casting is placed on a laser cladding stage for the first laser cladding. Argon is used as the protective gas with a flow rate of 20L / min. Under a laser with a power of 2200W and a spot diameter of 4mm, the alloy powder, which consists of 90% iron powder, 7% silicon powder and 3% nickel powder by mass percentage, is clad onto the surface of the casting to obtain a first-treatment casting.

[0061] 3. After the first-processed casting has completely cooled to room temperature, it is placed on a laser cladding stage for a second laser cladding. Argon is used as the protective gas with a flow rate of 20L / min. Under a laser with a power of 2400W and a spot diameter of 4mm, at a scanning speed of 4mm / s and a powder feeding rate of 14g / min, an alloy powder consisting of 80% iron powder, 10% chromium powder, 8% molybdenum powder, and 2% vanadium powder by mass percentage is clad onto the surface of the casting to obtain a second-processed casting.

[0062] 4. After the secondary-treated casting has completely cooled to room temperature, grind its surface smooth, and then use compressed air to blow away the surface dust to obtain the polished secondary-treated casting.

[0063] 5. A 0.2mm thick pure titanium foil is evenly and smoothly wrapped around the surface of the secondary-treated casting after grinding. Then, a third laser cladding is performed. Argon is used as the protective gas with a flow rate of 22L / min. Under a laser with a power of 2000W and a spot diameter of 4mm, a scanning speed of 4.5mm / s and a powder feeding rate of 9g / min, an alloy powder consisting of 80% stainless steel powder, 19.7% titanium powder and 0.3% yttrium oxide powder by mass percentage is clad onto the surface of the casting to obtain a three-stage treated casting.

[0064] 6. After the castings from the third treatment have completely cooled to room temperature, place them in a plasma nitriding furnace, evacuate the furnace to a pressure ≤10Pa, introduce a N2 and H2 mixture with a volume ratio of 1:5 into the furnace, adjust the furnace pressure to 300Pa, set the voltage to 1000V, start the power supply, and then heat to 500℃. After nitriding for 10 hours, stop heating and allow the workpiece to cool naturally to below 200℃, and then air cool to room temperature to obtain the ductile iron casting with improved surface corrosion resistance as described in Example 3.

[0065] Example 4

[0066] like Figure 1 and Figure 2 As shown, this embodiment provides a process for improving the corrosion resistance of ductile iron surfaces:

[0067] 1. Grind the surface of the ductile iron casting, and use ethanol to ultrasonically clean the ground ductile iron casting at a power of 300W for 20 minutes. After cleaning, dry it at 80℃ for 20 minutes to obtain the pretreated ductile iron casting.

[0068] 2. The pretreated ductile iron casting is placed on a laser cladding worktable for the first laser cladding. Argon is used as the protective gas with a flow rate of 15L / min. Under a laser with a power of 1800W and a spot diameter of 3mm, the alloy powder, which consists of 80% iron powder, 10% silicon powder, and 10% nickel powder by mass percentage, is clad onto the surface of the casting to obtain a first-treatment casting.

[0069] 3. After the first-processed casting has completely cooled to room temperature, it is placed on a laser cladding stage for a second laser cladding. Argon is used as the protective gas at a flow rate of 15L / min. Under a laser with a power of 2000W and a spot diameter of 3mm, at a scanning speed of 2.5mm / s and a powder feeding rate of 10g / min, an alloy powder consisting of 72% iron powder, 20% chromium powder, 4% molybdenum powder, and 4% vanadium powder by mass percentage is clad onto the surface of the casting to obtain a second-processed casting.

[0070] 4. After the secondary-treated casting has completely cooled to room temperature, grind its surface smooth, and then use compressed air to blow away the surface dust to obtain the polished secondary-treated casting.

[0071] 5. A 0.1mm thick pure titanium foil is evenly and smoothly wrapped around the surface of the secondary-treated casting after polishing. Then, a third laser cladding is performed. Argon is used as the protective gas with a flow rate of 18L / min. Under a laser with a power of 1700W and a spot diameter of 3mm, a scanning speed of 4mm / s and a powder feeding rate of 6g / min, an alloy powder consisting of 70% stainless steel powder, 19% titanium powder and 1% cerium oxide powder by mass percentage is clad onto the surface of the casting to obtain a three-stage treated casting.

[0072] 6. After the castings from the three treatments have completely cooled to room temperature, place them in a plasma nitriding furnace, evacuate the furnace to a pressure ≤10Pa, introduce a N2 and H2 mixture with a volume ratio of 1:4 into the furnace, adjust the furnace pressure to 200Pa, set the voltage to 800V, start the power supply, and then heat to 300℃. After nitriding for 8 hours, stop heating and allow the workpiece to cool naturally to below 200℃, and then air cool to room temperature to obtain the ductile iron casting with improved surface corrosion resistance as described in Example 4.

[0073] Comparative Example 1

[0074] Comparative Example 1 provides a process for improving the corrosion resistance of ductile iron surface. Compared with Example 1, the difference is that no pretreatment is performed on the surface of the casting. The remaining steps are the same as in Example 1 and will not be repeated here.

[0075] Comparative Example 2

[0076] Comparative Example 2 provides a process for improving the corrosion resistance of ductile iron surface. Compared with Example 1, the difference is that the first two laser cladding steps are not performed. Instead, pure titanium foil is directly wrapped on the surface of the casting for a third laser cladding. The remaining steps are the same as in Example 1 and will not be described again here.

[0077] Comparative Example 3

[0078] Comparative Example 3 provides a process for improving the corrosion resistance of ductile iron surfaces. The difference from Example 1 is that plasma nitriding treatment was not performed. The remaining steps are the same as in Example 1 and will not be repeated here.

[0079] To demonstrate the performance differences between Examples 1-4 and Comparative Examples 1-3, the following test methods were used to test the ductile iron castings with improved surface corrosion resistance prepared in Examples 1-4 and Comparative Examples 1-3.

[0080] Wear resistance test: Prepare a sample and simulate the friction scenario in actual use by making the sample and standard abrasive reciprocate relative motion under a certain pressure. The wear resistance is evaluated by calculating the wear amount. The test results are shown in Table 1.

[0081] Coating bond strength test: Samples were prepared and the coating bond strength of each sample was tested using a universal tensile tester. The test results are shown in Table 1.

[0082] Corrosion resistance test: Samples were prepared and neutral salt spray was continuously applied to the surface of each group of samples. After a certain period of time, the substrate of the counterweight of each group of samples was observed to see if rust appeared. Based on the rust situation, the corrosion resistance of the coating was evaluated. The test results are shown in Table 1.

[0083] Table 1. Performance comparison of samples prepared in Examples 1-4 and Comparative Examples 1-3

[0084]

[0085] As shown in Table 1, the samples prepared in Examples 1-4 all performed excellently in all tests. Comparative Example 1 showed little difference in performance compared to the examples in wear resistance and corrosion resistance tests, but its coating bonding strength decreased. This was because the casting surface was not pretreated in the process of Comparative Example 1, allowing impurities on the casting surface to enter the interface layer during laser cladding, resulting in a decrease in metallurgical bonding strength. The interface bonding strength of Comparative Example 2 also showed a significant decrease. This was because the first two laser cladding steps were not performed in the process of Comparative Example 2, leading to a decrease in the bonding strength between the hard coating and the substrate. The direct contact between the casting and the substrate leads to a significant performance abrupt change, resulting in weak interfacial bonding and a decrease in interfacial bonding strength. Comparative Example 3 shows a decrease in both surface wear resistance and corrosion resistance because the process in Comparative Example 2 did not involve plasma nitriding, preventing the formation of a titanium nitride film on the casting surface and thus reducing its wear and corrosion resistance. In summary, the process for improving the surface corrosion resistance of ductile iron provided in this application can effectively enhance the surface wear and corrosion resistance of ductile iron castings. Furthermore, the coating structure is stable and has high bonding strength, which can significantly extend the service life of ductile iron castings.

[0086] It should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the spirit and scope of the technical solutions of this application, and all such modifications and substitutions should be covered within the scope of the claims of this application.

Claims

1. A process for improving the corrosion resistance of ductile iron surfaces, characterized in that, Includes the following steps: Surface pretreatment of ductile iron castings is performed to obtain pretreated ductile iron castings; Using the first cladding alloy powder, under argon protection, the pretreated ductile iron casting was subjected to the first laser cladding to obtain a first-treated casting; Using a second cladding alloy powder, under argon protection, a second laser cladding is performed on the first-treated casting to obtain a second-treated casting; The surface of the secondary-treated casting is ground smooth, and the surface dust is blown away with compressed air to obtain the secondary-treated casting after grinding. Pure titanium foil is uniformly coated on the surface of the polished secondary-treated casting. Then, a third laser cladding is performed using a third cladding alloy powder under argon protection to obtain a three-stage treated casting. Plasma nitriding was performed on the ternary treated castings to obtain ductile iron castings with improved surface corrosion resistance.

2. The process for improving the surface corrosion resistance of ductile iron according to claim 1, characterized in that, The process also includes: after each laser cladding is completed, the casting needs to be allowed to cool naturally to room temperature before proceeding to the next step.

3. The process for improving the surface corrosion resistance of ductile iron according to claim 1, characterized in that, The surface pretreatment method is as follows: First, grind the surface of the ductile iron casting. Then, use acetone or ethanol to ultrasonically clean the ground ductile iron casting at a power of 200~350W for 10~20 minutes. After cleaning, dry it at a temperature of 80~100℃ for 20~30 minutes.

4. The process for improving the surface corrosion resistance of ductile iron according to claim 1, characterized in that, The first cladding alloy powder comprises, by mass percentage: 80-90% iron powder, 5-10% silicon powder, and 3-12% nickel powder; the parameters of the first laser cladding are: laser power 1800-2200W, scanning speed 3-5mm / s, powder feed rate 8-12g / min, spot diameter 3-4mm, and argon flow rate 15-20L / min.

5. The process for improving the surface corrosion resistance of ductile iron according to claim 1, characterized in that, The second cladding alloy powder, by mass percentage, comprises: 65-80% iron powder, 10-20% chromium powder, 4-12% molybdenum powder, and 2-5% vanadium powder; the parameters of the second laser cladding are: laser power 2000-2400W, scanning speed 2.5-4mm / s, powder feed rate 10-14g / min, spot diameter 3-4mm, and argon flow rate 15-20L / min.

6. The process for improving the surface corrosion resistance of ductile iron according to claim 1, characterized in that, The thickness of the pure titanium foil is 0.1~0.2mm.

7. The process for improving the surface corrosion resistance of ductile iron according to claim 1, characterized in that, The third cladding alloy powder, by mass percentage, comprises: 70-80% stainless steel powder, 15-30% titanium powder, and 0.3-0.8% auxiliary powder; the parameters of the third laser cladding are: laser power 1700-2000W, scanning speed 3.5-4.5mm / s, powder feed rate 6-9g / min, spot diameter 3-4mm, and argon flow rate 18-22L / min.

8. The process for improving the surface corrosion resistance of ductile iron according to claim 7, characterized in that, The auxiliary powder includes either yttrium oxide powder or cerium oxide powder.

9. The process for improving the surface corrosion resistance of ductile iron according to claim 1, characterized in that, The specific steps of the plasma nitriding treatment are as follows: the three-stage treated casting is placed in a plasma nitriding furnace, the vacuum is drawn to a pressure ≤10Pa, a mixture of N2 and H2 gas is introduced into the furnace, the furnace pressure is adjusted to 200~300Pa, the voltage is set to 800~1000V, the power is turned on, and then the temperature is raised to 300~500℃. After nitriding for 4~10 hours, the heating is stopped, and the workpiece is allowed to cool naturally to below 200℃, and then air-cooled to room temperature.

10. The process for improving the surface corrosion resistance of ductile iron according to claim 9, characterized in that, The volume ratio of N2 to H2 in the mixed gas is 1:(3~5).

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