A method for preparing a ni-al gradient coating capable of improving hardness of low carbon steel

Abstract

The application discloses a preparation method of a Ni-Al gradient coating capable of improving the hardness of low-carbon steel, and the method comprises the following steps: performing surface oil removal and rust removal pretreatment on a low-carbon steel sample; performing nickel electroplating on the surface of the low-carbon steel substrate; and performing solid powder embedding aluminizing on the surface of the sample after the nickel plating. The application combines the nickel electroplating process and the solid powder embedding aluminizing process well by improving the coating raw material formula and the preparation process, and then a Ni-Al functional gradient coating with good combination, high hardness, no surface defects and not easy to fall off is prepared on the surface of the low-carbon steel, so as to meet the application requirement of high hardness of the low-carbon steel surface.

Description

Technical Field

[0001] This invention relates to the field of coating preparation technology, specifically a method for preparing a Ni-Al gradient coating that can improve the hardness of low-carbon steel. Background Technology

[0002] Low-carbon steel is carbon steel with a carbon content of less than 0.25%. Its notable characteristics are low strength, low hardness, and softness. It typically requires further heat treatment for applications demanding higher hardness. Besides heat treatment, applying a protective coating to the surface of low-carbon steel is another technique to improve its hardness.

[0003] Functionally graded materials (FJCTs) are a class of heterogeneous composite materials that achieve gradual changes in properties through continuous or quasi-continuous variations in structure and composition. Among the various types of FJCTs, functionally graded coatings hold unique value. Solid powder embedding and aluminizing are common processes for steel materials, while nickel electroplating is another method for improving the surface hardness of steel. Therefore, it is essential to effectively combine the aluminizing process with the hardness-enhancing process of nickel electroplating to form a structurally continuous and high-hardness Ni-Al gradient coating on the surface of low-carbon steel, thereby improving the surface hardness and wear resistance of the low-carbon steel. Summary of the Invention

[0004] The technical objective of this invention is to improve the coating raw material formulation and preparation process, so as to combine the electroplating nickel process and the solid powder embedding aluminizing process, thereby preparing a Ni-Al functional gradient coating with good adhesion, high hardness, no surface defects, and not easy to fall off on the surface of low carbon steel, so as to meet the application requirements of high hardness of low carbon steel surface.

[0005] To solve the above-mentioned technical problems, the present invention provides a method for preparing a Ni-Al gradient coating that can improve the hardness of low-carbon steel. The method includes a pretreatment step of surface degreasing and rust removal on a low-carbon steel sample; a step of electroplating nickel on the surface of the low-carbon steel substrate; and a step of solid powder embedding and aluminizing on the surface of the nickel-plated sample. In the nickel plating step, the electroplating solution used consists of nickel sulfate, nickel chloride, boric acid, and deionized water, and the pH of the electroplating solution is 4.0, and the current density is 2~3 A / dm³. 2 The electroplating time is 2-3 hours. After nickel plating, the sample needs to be cleaned and dried. In the solid powder embedding aluminizing step, the aluminizing agent used consists of 5-10 wt% aluminum powder, 3-8 wt% catalyst AlCl3, and the remaining filler Al2O3 powder. During embedding aluminizing, the nickel-plated sample and the aluminizing agent should be placed together in an alumina crucible, and the alumina crucible should be placed in a vacuum tube furnace and heated to 550-700°C. oC, hold at the temperature for 15~25 h, cool to room temperature with the furnace, remove and clean to obtain a low-carbon steel product with a Ni-Al gradient coating, consisting of an outer Ni2Al3 intermetallic compound coating, a middle interdiffusion transition layer, and an inner low-carbon steel substrate.

[0006] Preferably, the specific operation method of the degreasing and rust removal pretreatment step is as follows: first, the sample surface is polished with metallographic sandpaper, then polished on a polishing machine, then ultrasonically cleaned with distilled water, alcohol and acetone in sequence, and finally the sample is dried.

[0007] Preferably, in the nickel electroplating step, the sample should be placed upright in the electroplating solution.

[0008] Preferably, in the nickel electroplating step, the concentration of NiSO4•6H2O in the electroplating solution is 240 g / L, the concentration of NiCl2•6H2O is 20 g / L, and the concentration of H3BO3 is 30 g / L, and the electroplating solution needs to be continuously stirred during nickel electroplating.

[0009] Preferably, in the nickel electroplating step, the temperature of the electroplating solution is 40-50°C. oC .

[0010] Preferably, in the solid powder embedding and aluminizing step, the Al2O3 powder used as the filler needs to be placed at 1050 °C before use. o High-temperature calcination was carried out at temperature C for 2 hours.

[0011] Preferably, after the Al2O3 filler powder is calcined at high temperature, it is cooled to room temperature and then taken out and mixed with aluminum powder. After that, the mixed powder is placed in a drying oven for drying for 2 hours. After cooling to room temperature again, it is ground and mixed with the pre-weighed and ground AlCl3 filler in a glass mortar within 1-5 minutes until uniform.

[0012] Preferably, in the solid powder embedding aluminizing step, the nickel-plated sample should be placed in the middle of the aluminizing agent powder in the alumina crucible. After the nickel-plated sample and the aluminizing agent are placed in the alumina crucible, the alumina crucible needs to be sealed with an adhesive and placed in a drying oven for drying for 2 hours. After that, it is placed in a vacuum tube furnace, and the vacuum tube furnace is continuously purged with argon three times before the heating program is started.

[0013] Preferably, the binder is aluminum dihydrogen phosphate.

[0014] Preferably, in the step of embedding and aluminizing solid powder, the alumina crucible is first heated to 150°C. o Hold at temperature C for 2 hours, then raise the temperature to 550~700. oKeep warm at C for 15-25 hours.

[0015] Beneficial effects:

[0016] 1. This invention discloses a method for preparing a Ni-Al gradient coating that can improve the hardness of low-carbon steel. The method involves first plating nickel onto the surface of a low-carbon steel substrate, followed by a process of embedding and aluminizing a solid powder layer onto the nickel surface. This ultimately forms a Ni-Al gradient coating on the surface of the low-carbon steel product. This method is simple and reliable, achieving metallurgical bonding between the coating and the substrate, thereby enhancing the connection strength between them. This makes the finished Ni-Al gradient coating less prone to peeling off after molding, thus extending the service life of the low-carbon steel product.

[0017] 2. The Ni-Al gradient coating prepared by this invention, through XRD and SEM analysis of its composition and morphology, shows that the surface phase of the coating is composed of aluminum-rich Ni2Al3. The cross-sectional morphology shows that the Ni2Al3 coating is tightly bonded to the low-carbon steel substrate, without defects such as cracks, gaps, or pores. There is an interdiffusion transition layer under the aluminum-rich Ni2Al3 on the coating surface. This interdiffusion transition layer contains Al, Ni, and Fe elements and has a continuous structural variation characteristic. Nanoindentation microhardness testing of the Ni-Al gradient coating prepared by this invention revealed that the nanoindentation hardness of the coating is as high as 15 GPa, which is about 60 times the hardness of the low-carbon steel substrate. That is, the Ni-Al gradient coating prepared by this invention significantly improves the surface hardness of low-carbon steel, enabling it to meet the hardness requirements of low-carbon steel in the machinery and chemical industries.

[0018] 3. The electroplating nickel formulation used in this invention is readily available, its composition is easy to control, and the electroplating process is simple. The DC electroplating process of this invention can quickly prepare a dense nickel plating layer on a low-carbon steel substrate. The solid powder embedding aluminizing process used in this invention utilizes a highly active catalyst, AlCl3, in the aluminizing agent, allowing the aluminizing reaction to proceed at a lower temperature. This effectively protects the substrate properties and is convenient to operate with simple equipment requirements. This invention combines electroplating nickel with solid powder aluminizing, a highly effective method for obtaining a Ni-Al coating on the surface of ordinary low-carbon steel. In the subsequent solid powder aluminizing, the aluminizing agent first undergoes a chemical reaction to generate aluminum atoms, which then deposit on the nickel-plated low-carbon steel surface and diffuse inwards into the nickel layer. Through heat preservation treatment, a Ni-Al intermetallic compound coating of a certain thickness is finally formed, with an interdiffusion layer between the coating and the substrate. This gradient structure is beneficial for improving the hardness of the low-carbon steel substrate. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the apparatus used for nickel electroplating in this invention;

[0020] Figure 2 This is a schematic diagram of the structure during solid powder embedding and aluminizing in this invention;

[0021] Figure 3 The image shows the XRD pattern of the Ni-Al gradient coating prepared in this invention.

[0022] Figure 4 This is a cross-sectional morphology diagram of the Ni-Al gradient coating prepared in this invention;

[0023] Figure 5 The cross-sectional composition variation curve of the Ni-Al gradient coating prepared in this invention;

[0024] Figure reference numerals: 1. DC regulated power supply, 2. Butterfly clip, 3. Thermometer, 4. Fixing rod, 5. Base, 6. Cathode sample, 7. Magnetic ball, 8. Magnetic heating stirrer, 9. Electroplating solution, 10. Anode nickel plate, 11. Alumina crucible, 12. Crucible lid, 13. Aluminizing agent powder, 14. Sample. Detailed Implementation

[0025] To enable those skilled in the art to better understand the technical solutions provided by the present invention, further detailed descriptions are provided below in conjunction with the accompanying drawings and specific embodiments.

[0026] A method for preparing a Ni-Al gradient coating that can improve the hardness of low-carbon steel includes the following steps:

[0027] Step 1: Perform simple surface degreasing and rust removal pretreatment on the low-carbon steel sample;

[0028] Step 2: Place the low-carbon steel sample into the electroplating solution for nickel plating. The electroplating solution consists of nickel sulfate, nickel chloride, boric acid and water. The electroplating current is 0.02~0.03A, the electroplating time is 2~3 hours, and the current used is direct current. The sample is placed upright in the solution during nickel plating.

[0029] Step 3: Solid powder embedding and aluminizing of the nickel-plated sample: Clean and dry the nickel-plated sample for later use. The aluminizing agent consists of 5wt%~10wt% aluminum powder, 3wt%~8wt% AlCl3 catalyst, and the remaining proportion of Al2O3 powder filler. Place the nickel-plated sample and the aluminizing agent into an alumina crucible and heat it in a vacuum tube furnace to 550~700°C. o C, hold at this temperature for 15-25 hours, then remove and clean after cooling to room temperature in the furnace. The Al2O3 powder filler needs to be heated to 1050°C before use. oC is calcined at high temperature for 2 hours. The AlCl3 catalyst does not need to be pretreated and can be taken directly from the reagent bottle to avoid volatilization. The nickel plating sample should be placed in the middle of the aluminizing agent powder in the crucible. The alumina crucible should also be sealed with an adhesive and dried in a drying oven for 2 hours. Then it is placed in a vacuum tube furnace and purged with argon three times to clean the furnace. Then the heating program is started.

[0030] The specific operating method is as follows:

[0031] I. Sample Pretreatment

[0032] All samples must undergo the following two pretreatment steps before electroplating:

[0033] Step 1: The samples were polished with 120, 240, 320, 400, 600 and 800 grit metallographic sandpaper respectively, and then polished on a polishing machine;

[0034] Step 2: After grinding and polishing, the sample is ultrasonically cleaned with distilled water, alcohol and acetone in sequence to remove oil or impurities, and then dried for later use.

[0035] II. Nickel plating

[0036] 2.1 Preparation of Electroplating Solution

[0037] The basic components of the electroplating solution are: nickel sulfate (NiSO4•6H2O), nickel chloride (NiCl2•6H2O) and boric acid (H3BO3), and the specific contents of each solute are shown in Table 1.

[0038] Weigh each component according to Table 1 using an electronic balance and dissolve it in deionized water. To ensure complete dissolution, place the solution in a water bath at 70°C. o The solution is kept at a constant temperature for 1 hour, then cooled, filtered, and transferred to a container. To ensure the uniformity of the electroplating solution during use, the solution needs to be continuously stirred in the container using a magnetic stirrer and magnetic heating throughout the entire electroplating process.

[0039] Table 1. Composition and content of electroplating solution

[0040] Element Content (g / L) <![CDATA[NiSO4•6H2O]]> 240 <![CDATA[NiCl2•6H2O]]> 20 <![CDATA[H3BO3]]> 30

[0041] 2.2 Preparation of the nickel electroplating equipment

[0042] As attached Figure 1 As shown, the electroplating apparatus of the present invention is as follows: Figure 1As shown, it consists of components such as butterfly clip 2, DC regulated power supply 1, and magnetic heating stirrer 8. Fixing rod 4 is fixed on base 5 and connected to thermometer 3 to ensure that the temperature of electroplating solution 9 is stable within a certain range. Cathode sample 6 and anode nickel plate 10 are located on both sides of the electroplating solution in the container, and the electroplating solution 9 is kept in a uniform state by magnetic stirrer 8 and magnetic stirrer 7.

[0043] 2.3 Electroplating process parameters

[0044] The first step in preparing the Ni-Al gradient coating, the electroplating of the nickel layer, is completed using a traditional DC electroplating process. The electroplating apparatus described above is used, and the specific process parameters are set as shown in Table 2.

[0045] Table 2 Electroplating Nickel Process Parameters

[0046] name Parameter value <![CDATA[Current density (A / dm 2 ).]]> 2-3 pH value 4.0 <![CDATA[Bath temperature ( o °C)]]> 45 Stirring speed (r / min) 400 Electroplating time (h) 2-3

[0047] III. Solid Powder Embedding and Aluminizing

[0048] This invention employs a low-temperature aluminizing process, specifically at 550-700°C. o Aluminizing is performed within the range of C, and the Ni-Al coating prepared by this method is an aluminum-rich phase Ni2Al3. The aluminizing agent consists of aluminum powder, alumina powder (filler), and AlCl3 (catalyst). The aluminum powder has a particle size of 200 mesh and a purity of 99%, the alumina powder has a particle size of 300 mesh and an analytical grade purity, and the AlCl3 is also analytical grade. The aluminum powder content is selected from 5wt% to 10wt%, the catalyst AlCl3 is 3wt% to 8wt%, and the remaining proportion is alumina powder as filler, as detailed in Examples 1 and 2.

[0049] Before use, alumina powder needs to be heated to a high temperature of 1050°C. o The sample was calcined at C for 2 hours to remove low-melting-point impurities. After cooling to room temperature, it was removed and mixed with aluminum powder in a glass mortar. It was then placed in a drying oven for 2 hours to dry. After the temperature decreased, it was removed, and pre-weighed and ground AlCl3 was added. All powders were quickly ground and mixed until homogeneous using a mortar and pestle. The sample was placed as follows... Figure 2As shown, first, half of the aluminizing agent powder 13 is placed into the alumina crucible 11, then the sample 14 is placed on the aluminizing agent powder 13, and then covered with an equal amount of aluminizing agent powder 13. During the process of adding the aluminizing agent powder 13, the alumina crucible 11 should be continuously shaken up and down until the total amount of aluminizing agent powder 13 is flush with the upper edge of the alumina crucible 11, in order to expel as much air as possible from the inside of the alumina crucible and reduce oxidation. Then, aluminum dihydrogen phosphate is used as a binder to fill the gap between the crucible lid 12 and the alumina crucible 11 body. Subsequently, the crucible is placed in a drying oven and kept at a certain temperature for a certain period of time to further remove moisture. After cooling to room temperature, it is taken out and placed in a vacuum tube furnace. Argon gas is first charged and released into the tube furnace three times to clean the furnace (to facilitate the removal of oxygen from the furnace), and then heating treatment is performed. The aluminizing experiment of this invention adopts a segmented heating program, that is, first heating to 150°C. o Temperature C, keep warm for two hours, then raise the temperature to 550~700°C. o The sample is heated to C for 15-25 hours to accelerate the aluminizing reaction. Afterward, the sample is cooled to room temperature in the furnace, then removed and cleaned to obtain a low-carbon steel product with a Ni-Al gradient coating, consisting of an outer Ni2Al3 intermetallic compound coating, a middle interdiffusion transition layer, and an inner low-carbon steel substrate.

[0050] Example 1

[0051] The Q235 low-carbon steel samples were pretreated and then electroplated with nickel. The electroplating solution preparation is shown in Table 1, and the electroplating process parameters are shown in Table 2. Among them, the current density was selected as 2 A / dm³. 2 The electroplating time was selected as 2 hours. After the nickel electroplating was completed, the sample was simply cleaned, and then solid powder embedding aluminizing was performed. The aluminizing agent composition was selected as follows: 8 wt% aluminum powder, 3 wt% AlCl3 and the remaining proportion of alumina powder. Other test operation procedures were as described above for solid powder embedding aluminizing.

[0052] After the experiment, the samples were ultrasonically cleaned to remove the adhering exudate, followed by XRD phase analysis, SEM morphology analysis, and EDS composition analysis. Typical Ni-Al coating phases and morphologies are shown in the attached figure. Figure 3 and 4 As shown, analysis revealed that all prepared Ni-Al coatings were aluminum-rich Ni2Al3 phases. The interface morphology showed that the sample surface was the prepared Ni2Al3 coating, and an interdiffusion region existed between the surface layer and the internal low-carbon steel substrate. Figure 5 The curve showing the change in cross-sectional composition is shown.

[0053] In the experiments of this embodiment, different aluminizing process parameters, such as aluminizing time and aluminum powder content in the aluminizing agent, all affect the final coating structure: with the same aluminizing agent composition, different aluminizing times only affect the coating thickness and have no effect on the coating phase type; with the same aluminizing time, different aluminum powder contents in the aluminizing agent only affect the coating thickness and have no effect on the coating phase type; the longer the aluminizing time and the higher the aluminizing temperature, the thicker the surface Ni2Al3 coating and the thinner the intermediate diffusion layer. Nanoindentation hardness testing was performed on the prepared Ni2Al3 coating + interdiffusion layer Ni-Al gradient coating. The results showed that the average nanoindentation hardness H of the Ni2Al3 coating was 15.05 GPa, the average hardness H of the interdiffusion layer was 6.61 GPa, and the average hardness H of the low-carbon steel substrate was 0.25 GPa, with the coating hardness reaching 60 times that of the substrate. Therefore, the use of Ni-Al gradient coating can significantly improve the hardness of low-carbon steel substrate, making it more suitable for applications requiring high wear resistance.

[0054] Example 2

[0055] The Q235 low-carbon steel samples were pretreated and then electroplated with nickel. The electroplating solution preparation is shown in Table 1, and the electroplating process parameters are shown in Table 2. Among them, the current density was selected as 3A / dm³. 2 The electroplating time was selected as 3 hours. After the nickel electroplating was completed, the sample was simply cleaned, and then solid powder embedding aluminizing was performed. The aluminizing agent composition was selected as follows: 10 wt% aluminum powder, 8 wt% AlCl3 and the remaining proportion of alumina powder. Other test operation procedures were the same as those described above for solid powder embedding aluminizing.

[0056] After the experiment, the samples were ultrasonically cleaned to remove the adhering exudate, followed by XRD phase analysis and SEM morphology examination. The typical Ni-Al coating phase and morphology are shown in the attached figure. Figure 3 and 4As shown, the prepared Ni-Al coating surface is still composed of the aluminum-rich Ni2Al3 phase. The interface morphology shows that the sample surface is a Ni2Al3 coating, and there is still an interdiffusion region between the surface layer and the internal low-carbon steel substrate. The difference is that the thickness of the Ni2Al3 coating and the interdiffusion layer are different under different parameter conditions. The results of this example still lead to the conclusion that different aluminizing process parameters, such as aluminizing time and aluminum powder content in the aluminizing agent, only affect the coating thickness and have no effect on the coating phase type; the longer the aluminizing time and the higher the aluminizing temperature, the thicker the surface Ni2Al3 coating and the thinner the intermediate diffusion layer. Nanoindentation hardness tests were performed on the prepared Ni-Al gradient coating composed of Ni2Al3 coating and interdiffusion layer. The results show that the average nanoindentation hardness H of the Ni2Al3 coating is 15.25 GPa, the average hardness H of the interdiffusion layer is 6.7 GPa, and the average hardness H of the low-carbon steel substrate is 0.25 GPa. The coating hardness is still much higher than the substrate hardness. Therefore, it can be seen that Ni-Al gradient coating can significantly improve the hardness value of low carbon steel substrate.

[0057] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the entire scope of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still make formal modifications to the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. All formal modifications and equivalent substitutions made within the scope of the concept and teaching of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a Ni-Al gradient coating that can improve the hardness of low-carbon steel, comprising the steps of pre-treating the surface of a low-carbon steel sample by degreasing and derusting; electroplating nickel on the surface of a low-carbon steel substrate; and embedding aluminizing with solid powder on the surface of the nickel-plated sample, characterized in that: In the nickel electroplating step, the electroplating solution used consists of nickel sulfate, nickel chloride, boric acid, and deionized water, with a pH of 4.0 and a current density of 2~3 A / dm³. 2 The electroplating time is 2-3 hours. After nickel plating, the sample needs to be cleaned and dried. In the solid powder embedding aluminizing step, the aluminizing agent used consists of 5-10 wt% aluminum powder, 3-8 wt% catalyst AlCl3, and the remaining filler Al2O3 powder. The filler Al2O3 powder is placed in a 1050°C solution before use. o High-temperature calcination was carried out at temperature C for 2 hours. When embedding aluminizing, the nickel-plated sample and the aluminizing agent should be placed together in an alumina crucible, and the alumina crucible should be heated in a vacuum tube furnace. The alumina crucible should first be heated to 150°C. o Hold at temperature C for 2 hours, then raise the temperature to 550~700°C. o C, hold at the temperature for 15~25 h, cool to room temperature with the furnace, then remove and clean to obtain a low-carbon steel product with a Ni-Al gradient coating, consisting of an outer Ni2Al3 intermetallic compound coating, a middle interdiffusion transition layer of Al, Ni and Fe elements, and an inner low-carbon steel substrate.

2. The method for preparing a Ni-Al gradient coating that can improve the hardness of low-carbon steel according to claim 1, characterized in that: The specific operation method of the degreasing and rust removal pretreatment step is as follows: first, use metallographic sandpaper to grind the sample surface, then polish it on a polishing machine, then use distilled water, alcohol and acetone in sequence to ultrasonically clean the sample surface, and finally dry the sample.

3. The method for preparing a Ni-Al gradient coating that can improve the hardness of low-carbon steel according to claim 1, characterized in that: In the nickel plating process, the sample must be placed upright in the plating solution.

4. The method for preparing a Ni-Al gradient coating that can improve the hardness of low-carbon steel according to claim 1, characterized in that: In the nickel electroplating step, the concentration of NiSO4•6H2O in the electroplating solution is 240 g / L, the concentration of NiCl2•6H2O is 20 g / L, and the concentration of H3BO3 is 30 g / L. Furthermore, the electroplating solution needs to be continuously stirred during nickel electroplating.

5. The method for preparing a Ni-Al gradient coating that can improve the hardness of low-carbon steel according to claim 1, characterized in that: In the nickel electroplating step, the temperature of the electroplating solution is 40-50°C.

6. The method for preparing a Ni-Al gradient coating that can improve the hardness of low-carbon steel according to claim 1, characterized in that: After the Al2O3 filler powder is calcined at high temperature, it is cooled to room temperature and then mixed with aluminum powder. The mixed powder is then placed in a drying oven for 2 hours. After cooling to room temperature again, it is ground and mixed with the pre-weighed and ground AlCl3 filler in a glass mortar within 1-5 minutes until uniform.

7. The method for preparing a Ni-Al gradient coating that can improve the hardness of low-carbon steel according to claim 1, characterized in that: In the solid powder embedding aluminizing step, the nickel-plated sample should be placed in the middle of the aluminizing agent powder in the alumina crucible. After the nickel-plated sample and the aluminizing agent are placed in the alumina crucible, the alumina crucible needs to be sealed with an adhesive and placed in a drying oven for 2 hours. After that, it is placed in a vacuum tube furnace and the vacuum tube furnace is continuously purged with argon three times before the heating program is started.

8. The method for preparing a Ni-Al gradient coating that can improve the hardness of low-carbon steel according to claim 7, characterized in that: The adhesive is aluminum dihydrogen phosphate.

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