A method for preparing a sodium cold corrosion resistant coating on the surface of nuclear stainless steel in situ

Abstract

The present invention discloses a method for preparing a sodium cold corrosion resistant coating on the surface of nuclear stainless steel in situ, comprising the following steps: 1. degreasing, derusting and cleaning the surface of the stainless steel to obtain a clean stainless steel substrate; 2. spraying chromium powder on the surface of the stainless steel and using laser alloying to prepare a chromium-rich coating; 3. spraying nano-CrN powder on the surface of the chromium-rich coating and using laser alloying to form a Cr-CrN enriched coating; 4. using laser heat treatment to obtain a Cr-Cr2N sodium cold corrosion resistant coating. The present invention first uses laser alloying to prepare a chromium-rich layer on the surface of austenitic stainless steel, then uses laser alloying to introduce CrN into the chromium-rich layer to form a Cr-CrN enriched composite coating, and uses laser heat treatment to induce the formation of a Cr2N enhanced coating. Without affecting the grain growth of the austenitic stainless steel, the stainless steel surface is quickly coated with a sodium cold corrosion resistant coating, thereby improving the sodium cold service resistance of the stainless steel.

Description

Technical Field

[0001] The invention belongs to the technical field of surface modification of key metal materials in the nuclear industry, and particularly relates to a method for composite in-situ preparation of a sodium cold corrosion resistant coating on the surface of nuclear stainless steel. Background Art

[0002] Sodium-cooled fast neutron reactors (SFRs), which use liquid sodium as a coolant, are the preferred reactor type for fourth-generation advanced nuclear energy systems. Small SFRs offer advantages such as high power density, inherent safety, miniaturization, and the ability to operate without refueling throughout their lifecycle. They are suitable for use in remote areas such as offshore islands and reefs. They can also be coupled with renewable energy sources such as wind and photovoltaics to enhance microgrid stability, making them a preferred technology for small advanced reactors. The development of SFRs is crucial for establishing a closed nuclear fuel cycle, fully utilizing uranium resources, and minimizing nuclear waste.

[0003] The technological maturity of sodium-cooled fast reactors (SFRs) has been largely validated by engineering projects. In recent years, my country has been at the forefront of global research and development in SFRs. Construction began on the 600,000-kilowatt (MW) sodium-cooled fast reactor demonstration project in Xiapu, Fujian Province, in 2017, with completion and commissioning scheduled for 2023. SFRs utilize a three-circuit sodium-sodium-water / steam heat transfer system. The heat exchanger in the reactor vessel is a key component of the fast reactor, consisting primarily of an intermediate heat exchanger and an independent heat exchanger. Under actual operating conditions, the heat exchanger's maximum service temperature is approximately 600°C. Such harsh operating conditions can lead to several failure modes in forgings, including creep-fatigue damage, persistent fracture, corrosion, and radiation embrittlement. This requires that the tubesheet forgings possess both excellent high-temperature strength and resistance to sodium cold corrosion. 316 austenitic stainless steel has advantages such as excellent corrosion resistance, good structural stability, high operating temperature (up to 700°C), good weldability, and excellent processing properties. It can serve in harsh environments such as high temperature, high pressure, corrosive atmosphere, and physical radiation radiation. Therefore, it is selected as the material for upper and lower tube sheets and tubes of heat exchangers both domestically and internationally. However, 316 austenitic stainless steel cannot resist liquid sodium corrosion.

[0004] Currently, the common technique for preparing sodium cold corrosion-resistant coatings on 316 stainless steel components in sodium-cooled fast reactors is chromizing and nitriding. This involves applying a composite process of solid embedding and gas nitriding to create a Cr2N-reinforced wear-resistant and corrosion-resistant coating on the surface of austenitic stainless steel. Operating experimental fast reactors (EFBRs) and demonstration fast reactors (DFBRs) under construction in China require a large number of stainless steel core components requiring chromizing and nitriding. This process is crucial for preparing wear-resistant and corrosion-resistant coatings on components in these reactors. Patents related to the preparation of corrosion-resistant coatings on austenitic stainless steel materials mainly include: Chromizing and Nitriding Process for Austenitic Steel Parts (CN 101333639A), an invention patent applied for by the China Institute of Atomic Energy in 2008. The process includes pretreatment of austenitic stainless steel parts, chromizing, cleaning, and nitriding. The most important of these processes are chromizing and nitriding. The chromizing process adopts a solid embedding chromizing method. The solid chromizing agent used is an iron-chromium alloy powder with a chromium mass fraction of 58-70%. The chromizing temperature is 1100±10℃ and the heat is maintained for 17-24 hours. The nitriding process requires a nitriding furnace with a temperature of 1100±20℃ for 4-5 hours.

[0005] The characteristics of the solid diffusion process require that the parts be embedded in a box with solid diffusion powder. The box containing the parts needs to be placed in a furnace and heated to 1100°C, undergoing a process of heating-holding-cooling, and the high-temperature holding time is 17-24 hours. Although the chromium nitriding layer can be successfully obtained, it will inevitably cause the grain size of the austenitic stainless steel parts substrate to be coarse, affecting the mechanical properties of the substrate. This is also the bottleneck of the solid embedding technology. Since the diffusion temperature of the solid embedding diffusion technology exceeds 1000°C, and the nitriding temperature also exceeds 1000°C, the holding time is long, and the key large-size thin plate parts of the sodium-cooled fast reactor are difficult to be welded. The parts need to go through two heating-insulation-cooling processes, and deformation problems are inevitable after diffusion. The traditional solid embedding infiltration method requires the parts to be embedded in a solid chromizing agent. After high-temperature sintering, the infiltrant is easy to agglomerate, and the infiltrant needs to be broken with tools before the workpiece is removed, which is easy to damage the workpiece. In addition, the workload is large, the cycle is long, the dust pollution is serious, and the process is environmentally friendly. Since the infiltration process requires the parts to be embedded in a box with a solid infiltrant, and then the box is placed in a furnace for heating and insulation, as some nuclear reactor parts are too large, strict requirements are placed on the furnace cavity size, and the amount of solid infiltrant used is large, resulting in a large amount of powder waste.

[0006] The combined process of solid embedding chromizing and gas nitriding is the mainstream technology for preparing sodium corrosion-resistant cold corrosion coatings on austenitic stainless steel. Solid embedding chromizing is a chemical heat treatment chromizing method. This method embeds the workpiece with a solid powder nitriding agent and places it in a sealed container. The sealed container is then heated, kept warm, and diffusion annealed to prepare a chromized layer on the surface of the stainless steel. Therefore, this method has the advantages of simple operation, low requirements for production equipment, uniform nitriding layer thickness, and low production cost. However, due to the diffusion temperature exceeding 1000°C, this process has the following inherent disadvantages:

[0007] 1) Since the diffusion process requires the entire part to be placed in a furnace and heated to 1100°C, it is easy to cause the grain size of the austenitic stainless steel part substrate to coarsen, affecting the mechanical properties of the substrate;

[0008] 2) Since the diffusion process requires the parts to be embedded in a box with a solid diffusion agent, and then the box is placed in a furnace for heating and insulation, as the process size increases, stringent requirements are placed on the furnace cavity size;

[0009] 3) Due to the high diffusion temperature and long high temperature time, large-sized thin plate parts are prone to deformation after diffusion;

[0010] 4) The traditional embedding infiltration method has a complex preparation process, a long cycle, serious dust pollution, and poor environmental protection.

[0011] At present, in response to the above-mentioned problems existing in the solid embedding infiltration technology in the process of metal material diffusion, although some researchers have explored low-temperature solid embedding infiltration processes from the aspects of catalysts, infiltrants, and metal substrate surface activation, due to the inherent process characteristics of solid embedding infiltration technology that relies on high temperature to drive diffusion to form a diffusion layer, there is no effective method to significantly reduce the diffusion temperature, and it is impossible to solve the problems of coarse metal substrate grains, workpiece deformation, and low processing efficiency.

[0012] In view of the problems existing in the preparation of Cr2N reinforced wear-resistant and corrosion-resistant coating on the surface of austenitic stainless steel using traditional solid embedding method and nitriding composite process, such as long chromizing and nitriding time and high diffusion temperature, which result in long coating production cycle, grain growth of stainless steel substrate, and deformation of workpiece, it is necessary to propose a method for composite in-situ preparation of sodium cold corrosion-resistant coating on the surface of nuclear stainless steel. Summary of the Invention

[0013] The technical problem to be solved by the present invention is to address the shortcomings of the above-mentioned existing technologies and provide a method for preparing a sodium cold corrosion-resistant coating on the surface of nuclear stainless steel in situ. This method first uses laser alloying to prepare a chromium-rich layer on the surface of austenitic stainless steel. Then, CrN is introduced into the chromium-rich layer using laser alloying to form a Cr-CrN-enriched composite coating. Laser heat treatment technology is then used in a multi-cycle cyclic manner to induce the formation of a Cr2N-enhanced coating. Without affecting the grain growth of the austenitic stainless steel substrate, the Cr2N-enhanced wear-resistant and corrosion-resistant coating is quickly obtained on the stainless steel surface, thereby improving the sodium cold service performance of the austenitic stainless steel.

[0014] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for preparing a sodium cold corrosion resistant coating on the surface of nuclear stainless steel in situ, characterized in that the method comprises the following steps:

[0015] Step 1: Degreasing, derusting and cleaning the surface of the nuclear stainless steel in sequence to obtain a clean stainless steel substrate;

[0016] Step 2: spraying chromium powder onto the surface of the clean stainless steel substrate obtained in step 1, and then treating it with laser alloying technology to obtain a chromium-rich coating on the surface of the clean stainless steel substrate;

[0017] Step 3: spraying nano-CrN powder onto the surface of the chromium-rich coating obtained in step 2, and then treating it with laser alloying technology to obtain a Cr-CrN enriched coating on the surface of the clean stainless steel substrate;

[0018] Step 4: Scan the Cr-CrN enriched coating formed in step 3 using laser heat treatment technology to induce the formation of Cr2N phase in the coating, thereby obtaining a Cr-Cr2N sodium cold corrosion resistant coating on the surface of the clean stainless steel substrate.

[0019] The present invention firstly uses a laser alloying method to prepare a metastable supersaturated chromium-rich layer on the surface of austenitic stainless steel, wherein the surface has no crack defects and the chromium-rich layer is metallurgically bonded to the austenitic stainless steel substrate. Then, CrN is introduced into the chromium-rich layer by the laser alloying method, and nano-CrN powder is solid-dissolved in the chromium-rich coating to form a Cr-CrN enriched composite coating. Laser heat treatment technology is used in a multi-cycle cycle mode to in-situ form a Cr2N reinforcement phase in the induced Cr-CrN enriched coating. Without affecting the grain growth of the austenitic stainless steel substrate, the Cr-Cr2N sodium cold corrosion resistant coating is quickly obtained on the stainless steel surface, thereby improving the sodium cold service resistance of the austenitic stainless steel.

[0020] The above-mentioned method for preparing a sodium cold corrosion resistant coating on the surface of nuclear stainless steel by composite in-situ is characterized in that the stainless steel in step 1 is 316L austenitic stainless steel or 316H austenitic stainless steel. The present invention is applicable to various austenitic stainless steels.

[0021] The aforementioned method for preparing a sodium cold corrosion-resistant coating on a nuclear stainless steel surface in situ composite is characterized in that the chromium powder in step 2 has a purity greater than 99.9% and is spherical with a diameter of 5 to 10 μm. The chromium powder is sprayed by mixing the chromium powder, ethyl acetate, and varnish. By controlling the parameters of the chromium powder, the present invention achieves a uniformly dissolved iron-chromium austenite phase after laser alloying. By spraying the mixture of chromium powder, ethyl acetate, and varnish, the chromium powder adheres to the clean stainless steel substrate surface, facilitating laser alloying.

[0022] The above-mentioned method for preparing a sodium cold corrosion-resistant coating on the surface of nuclear stainless steel in situ composite is characterized in that the laser alloying technology treatment in step 2 refers to spraying chromium powder and laser alloying in a cycle of 4 to 8 times under the protection of argon gas with a mass purity greater than 99.999%, the thickness of the chromium-rich coating is greater than 100 μm, and the mass content of Cr in the chromium-rich coating is greater than 60%. The present invention cycles through 4 to 8 times of spraying chromium powder and laser alloying to increase the thickness of the chromium-rich coating to greater than 100 μm, meeting the thickness index, and controlling the mass content of Cr in the chromium-rich coating to greater than 60%, forming a non-equilibrium supersaturated iron-chromium solid solution, which is a necessary condition for preparing the Cr2N phase. The laser alloying treatment technology can not only obtain a chromium-rich coating with a thickness greater than 100 μm in a short time, but also prevent the growth of substrate grains, solving the shortcomings of the traditional solid embedding infiltration process, which has a long infiltration process time and coarse substrate grains.

[0023] The aforementioned method for preparing a sodium cold corrosion-resistant coating on a nuclear stainless steel surface by composite in-situ coating is characterized in that the nano-CrN powder described in step 3 has a mass purity greater than 99.9%, a polygonal morphology, and an average particle size of 20 nm; the nano-CrN powder is sprayed by mixing the nano-CrN powder, deionized water, and a dispersant. The present invention controls the parameters of the nano-CrN powder to facilitate its dispersion within the iron-chromium austenite structure during laser alloying and to facilitate reaction with high concentrations of chromium to form Cr2N. By spraying the nano-CrN powder, deionized water, and dispersant, the nano-CrN powder adheres to the surface of the chromium-rich coating, facilitating laser alloying. Sodium lauryl sulfate is used as the dispersant.

[0024] The above-mentioned method for preparing a sodium cold corrosion-resistant coating on the surface of nuclear stainless steel in situ composite is characterized in that the laser alloying technology treatment in step 3 refers to spraying nano-CrN powder and laser alloying in a cycle of 4 to 8 times under the protection of argon gas with a mass purity greater than 99.999%, and the CrN mass content in the Cr-CrN enriched coating is greater than 60%. The present invention controls the cycle of spraying nano-CrN powder and laser alloying 4 to 8 times to dissolve the nano-CrN particles in the supersaturated Cr-rich layer, forming a Cr-CrN composite alloyed layer with a CrN content greater than 60wt%. This increases the probability of solid solution diffusion reaction between CrN and Cr element, and the supersaturated solid solution CrN-Cr phase with non-solidification equilibrium characteristics under laser induction is more likely to produce a Cr2N phase.

[0025] The aforementioned method for in-situ composite preparation of a sodium cold corrosion-resistant coating on the surface of nuclear stainless steel is characterized in that the laser used in the laser alloying technology in steps 2 and 3 is a semiconductor continuous output high-power laser with a laser wavelength of 850nm to 900nm, and a rectangular laser spot with a length × width of 6mm × 8mm. The semiconductor rectangular spot in the present invention is the optimal choice for alloying CrN with Cr. The semiconductor laser energy density is uniformly distributed, and the semiconductor laser wavelength has a high photoelectric conversion efficiency, which is beneficial to the absorption of laser energy by austenitic stainless steel and increases the efficiency of CrN and Cr alloying.

[0026] The aforementioned method for preparing a composite in-situ sodium cold corrosion-resistant coating on a nuclear stainless steel surface is characterized in that the laser heat treatment technique in step 4 involves reciprocating irradiation scanning of the Cr-CrN enriched coating 50 or more times. The reciprocating scanning not only controls the temperature rise of the austenitic stainless steel substrate but also provides continuous reaction energy for inducing the Cr-CrN reaction to form Cr2N. The rapid laser heating effect is utilized to induce the in-situ formation of the Cr2N phase in the Cr-CrN enriched coating.

[0027] The above-mentioned method for preparing a sodium cold corrosion-resistant coating on the surface of nuclear stainless steel in situ is characterized in that the laser heat treatment technology used in step 4 is a fiber continuous output high-power laser with a laser wavelength of 900nm to 1064nm, the laser spot is rectangular, and the length × width of the spot is 2mm × 15mm. The laser scanning speed is 50mm / s to 100mm / s, and the temperature of the laser heat treatment layer is 1040℃ to 1060℃. The high energy density of the fiber laser photons in the present invention can provide high-density energy for rapid cleaning and can also radiate the metal material, stimulating the electronic transition of Cr and CrN and increasing the reaction probability. The laser heat treatment technology is a selective surface heat treatment technology. The laser selected for heat treatment is a fiber continuous output high-power laser, which scans back and forth. The temperature of the laser heat treatment layer is controlled at 1040℃ to 1060℃. The Cr-CrN composite alloy layer containing supersaturated Cr elements is scanned back and forth, and the laser rapid heating effect is used to induce the in-situ formation of a solid solution Cr2N phase in the alloy layer.

[0028] The aforementioned method for preparing a composite in-situ sodium cold corrosion-resistant coating on a nuclear stainless steel surface is characterized in that, prior to the laser heat treatment in step 4, a layer of graphite coating composed of -500 mesh graphite particles and ethanol is sprayed on the surface of the Cr-CrN enriched coating. The laser heat treatment in step 4 is performed under nitrogen protection with a purity greater than 99.99%. To improve the absorption rate of laser energy, a light-absorbing layer, namely, graphite coating, is sprayed on the surface of the Cr-CrN composite alloy layer before the laser heat treatment.

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] 1. The present invention first uses a laser alloying method to prepare a chromium-rich layer on the surface of austenitic stainless steel, then uses a laser alloying method to introduce CrN into the chromium-rich layer to form a Cr-CrN enriched composite coating, and uses a multi-cycle laser heat treatment technology to induce the formation of a Cr2N enhanced coating. Without affecting the grain growth of the austenitic stainless steel substrate, the stainless steel surface is quickly coated with a Cr2N enhanced wear-resistant and corrosion-resistant coating, thereby improving the sodium cold service resistance of the austenitic stainless steel.

[0031] 2. The present invention uses laser alloying, that is, the thermal effect of the interaction between laser and metal substrate, to rapidly melt, solidify and form new alloy substances on the surface of austenitic stainless steel to change its physical and chemical properties, thereby realizing the preparation of Cr-Cr2N sodium corrosion-resistant cold corrosion coating. Its outstanding advantages are transient process and regional scannable selectivity, and the ability to obtain a diffusion layer while ensuring that the performance of the substrate remains unchanged.

[0032] 3. The laser alloying and laser heat treatment technologies in the present invention both utilize the rapid heating characteristics of the high-energy laser beam to selectively treat the surface of austenitic stainless steel, avoiding overall heating of the parts and preventing deformation. In addition, laser is a high-energy beam clean energy with high processing freedom and high efficiency in preparing the chromized layer. It can avoid the large-scale use of chromizing agents and avoid dust pollution, making it a more environmentally friendly and efficient new technology.

[0033] 4. The present invention utilizes laser technology to solve the problem of using a non-standard large diffusion furnace required for large-sized workpieces, making it easier to quickly prepare the diffusion layer and reduce costs.

[0034] 5. The present invention aims at large-sized and easily deformed thin-plate workpieces of austenitic stainless steel, and proposes a method for in-situ composite preparation of sodium cold corrosion-resistant coating on the surface of nuclear stainless steel, which solves the problems of coarsening of part substrate structure, deformation of large-sized workpieces, and complex preparation process of Cr2N-reinforced coating in traditional solid embedding infiltration technology.

[0035] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a schematic diagram of the laser alloying process in step 2 of the present invention.

[0037] Figure 2 This is a schematic diagram of the laser alloying process in step three of the present invention.

[0038] Figure 3 This is a schematic diagram of scanning using laser heat treatment technology in step four of the present invention.

[0039] Figure 4 This is a microstructure diagram of the Cr-Cr2N sodium cold corrosion resistant coating prepared in Example 1 of the present invention.

[0040] Figure 5 This is the EDS scanning spectrum of the Cr-Cr2N sodium cold corrosion resistant coating prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0041] Figure 1 This is a schematic diagram of the laser alloying process in step 2 of the present invention. Figure 1 It can be seen that a chromium-rich coating is obtained on the surface of nuclear stainless steel by laser alloying.

[0042] Figure 2 This is a schematic diagram of the laser alloying process in step 3 of the present invention. Figure 2 It can be seen that a Cr-CrN enriched coating is obtained on the surface of nuclear stainless steel by laser alloying.

[0043] Figure 3 This is a schematic diagram of the laser heat treatment technology used in step 4 of the present invention. Figure 3 It can be seen that the Cr-Cr2N sodium cold corrosion resistant coating is obtained on the surface of nuclear stainless steel by laser heat treatment.

[0044] Example 1

[0045] This embodiment includes the following steps:

[0046] Step 1: Degreasing, derusting and cleaning the surface of the core 316L austenitic stainless steel in sequence to obtain a clean stainless steel substrate;

[0047] Step 2: Under the protection of argon with a mass purity greater than 99.999%, chromium powder, ethyl acetate and varnish are mixed in a ratio of 100g:500mL:1g and sprayed on the surface of the clean stainless steel substrate obtained in step 1, and then treated by laser alloying technology. The above process is repeated 4 times to obtain a chromium-rich coating on the surface of the clean stainless steel substrate; the mass purity of the chromium powder is greater than 99.9%, the morphology is spherical, and the diameter is 5μm to 10μm; the laser used in the laser alloying technology is a semiconductor continuous output high-power laser with a laser wavelength of 850nm to 900nm, a rectangular laser spot with a length × width of 6mm × 8mm, a laser power of 3000W, and a scanning speed of 20mm / s;

[0048] Step 3. Under the protection of argon with a mass purity greater than 99.999%, nano-CrN powder, deionized water and sodium lauryl sulfate are ultrasonically mixed in a ratio of 100g:500mL:50g and then sprayed on the surface of the chromium-rich coating obtained in step 2, and then treated by laser alloying technology. The above process is repeated 4 times to obtain a Cr-CrN enriched coating on the surface of the clean stainless steel substrate; the mass purity of the nano-CrN powder is greater than 99.9%, the morphology is polygonal, and the average particle size is 20nm; the laser used in the laser alloying technology is a semiconductor continuous output high-power laser with a laser wavelength of 850nm to 900nm, a rectangular laser spot with a length × width of 6mm × 8mm, a laser power of 5000W, and a scanning speed of 40mm / s;

[0049] Step 4. Under the protection of nitrogen with a mass purity greater than 99.99%, a layer of graphite coating prepared by graphite particles with a particle size of -500 mesh and ethanol in a ratio of 100g:500mL is sprayed on the surface of the Cr-CrN enriched coating obtained in step 3, and then a laser heat treatment technology is used to scan back and forth 50 times to induce the formation of a Cr2N phase in the coating, and a Cr-Cr2N sodium corrosion-resistant cold coating is obtained on the surface of a clean stainless steel substrate; the laser used in the laser heat treatment technology is a fiber continuous output high-power laser with a laser wavelength of 900nm to 1064nm, a rectangular laser spot with a length × width of 2mm × 15mm, a laser scanning speed of 50mm / s, and a temperature of the laser heat treatment layer of 1040°C to 1060°C.

[0050] Figure 4 The microstructure of the Cr-Cr2N sodium cold corrosion resistant coating prepared in this embodiment is shown in FIG. Figure 4 It can be seen that the 1 marked in the figure represents the Cr2N formed by the reaction, and the 2 marked represents the unconverted CrN, and a Cr-Cr2N sodium cold corrosion resistant coating is obtained on the surface of the clean stainless steel substrate.

[0051] Figure 5 The EDS scanning spectrum of the Cr-Cr2N sodium cold corrosion resistant coating prepared in this embodiment is shown in FIG. Figure 5 As can be seen from the figure, in this embodiment, a 100 μm thick Cr—Cr2N chromized layer is in situ obtained on the surface of the 316L stainless steel substrate through a combined process of laser alloying and laser heat treatment.

[0052] After testing, the thickness of the metastable supersaturated chromium-rich coating prepared in step 2 of this embodiment is greater than 100 μm, the mass content of Cr is greater than 60%, and there are no crack defects on the surface. The chromium-rich coating is metallurgically bonded to the austenitic stainless steel substrate; in the Cr-CrN enriched coating prepared in step 3 of this embodiment, CrN particles are solid dissolved in the supersaturated chromium-rich coating, and the CrN content is greater than 60 wt%. A solid solution Cr2N phase is formed in situ in the Cr-Cr2N sodium cold corrosion resistant coating prepared in step 4 of this embodiment.

[0053] Example 2

[0054] This embodiment includes the following steps:

[0055] Step 1: Degreasing, derusting and cleaning the surface of the nuclear 316H austenitic stainless steel in sequence to obtain a clean stainless steel substrate;

[0056] Step 2: Under the protection of argon with a mass purity greater than 99.999%, chromium powder, ethyl acetate and varnish are mixed in a ratio of 100g:500mL:1g and sprayed on the surface of the clean stainless steel substrate obtained in step 1, and then treated by laser alloying technology. The above process is repeated 6 times to obtain a chromium-rich coating on the surface of the clean stainless steel substrate; the mass purity of the chromium powder is greater than 99.9%, the morphology is spherical, and the diameter is 5μm to 10μm; the laser used in the laser alloying technology is a semiconductor continuous output high-power laser with a laser wavelength of 850nm to 900nm, a rectangular laser spot with a length × width of 6mm × 8mm, a laser power of 3000W, and a scanning speed of 20mm / s;

[0057] Step 3. Under the protection of argon with a mass purity greater than 99.999%, nano-CrN powder, deionized water and sodium lauryl sulfate are ultrasonically mixed in a ratio of 100g:500mL:50g and then sprayed on the surface of the chromium-rich coating obtained in step 2, and then treated by laser alloying technology. The above process is repeated 6 times to obtain a Cr-CrN enriched coating on the surface of the clean stainless steel substrate; the mass purity of the nano-CrN powder is greater than 99.9%, the morphology is polygonal, and the average particle size is 20nm; the laser used in the laser alloying technology is a semiconductor continuous output high-power laser with a laser wavelength of 850nm to 900nm, a rectangular laser spot with a length × width of 6mm × 8mm, a laser power of 5000W, and a scanning speed of 40mm / s;

[0058] Step 4. Under the protection of nitrogen with a mass purity greater than 99.99%, a layer of graphite coating prepared by graphite particles with a particle size of -500 mesh and ethanol in a ratio of 100g:500mL is sprayed on the surface of the Cr-CrN enriched coating obtained in step 3, and then a laser heat treatment technology is used to scan back and forth 80 times to induce the formation of a Cr2N phase in the coating, and a Cr-Cr2N sodium corrosion-resistant cold coating is obtained on the surface of a clean stainless steel substrate; the laser used in the laser heat treatment technology is a fiber continuous output high-power laser with a laser wavelength of 900nm to 1064nm, a rectangular laser spot with a length × width of 2mm × 15mm, a laser scanning speed of 80mm / s, and a temperature of the laser heat treatment layer of 1040°C to 1060°C.

[0059] After testing, the thickness of the metastable supersaturated chromium-rich coating prepared in step 2 of this embodiment is greater than 100 μm, the mass content of Cr is greater than 60%, and there are no crack defects on the surface. The chromium-rich coating is metallurgically bonded to the austenitic stainless steel substrate; in the Cr-CrN enriched coating prepared in step 3 of this embodiment, CrN particles are solid dissolved in the supersaturated chromium-rich coating, and the CrN content is greater than 60 wt%. A solid solution Cr2N phase is formed in situ in the Cr-Cr2N sodium cold corrosion resistant coating prepared in step 4 of this embodiment.

[0060] Example 3

[0061] This embodiment includes the following steps:

[0062] Step 1: Degreasing, derusting and cleaning the surface of the core 316L austenitic stainless steel in sequence to obtain a clean austenitic stainless steel substrate;

[0063] Step 2: Under the protection of argon with a mass purity greater than 99.999%, chromium powder, ethyl acetate and varnish are mixed in a ratio of 100g:500mL:1g and sprayed on the surface of the clean stainless steel substrate obtained in step 1, and then treated by laser alloying technology. The above process is repeated 8 times to obtain a chromium-rich coating on the surface of the clean stainless steel substrate; the mass purity of the chromium powder is greater than 99.9%, the morphology is spherical, and the diameter is 5μm to 10μm; the laser used in the laser alloying technology is a semiconductor continuous output high-power laser with a laser wavelength of 850nm to 900nm, a rectangular laser spot with a length × width of 6mm × 8mm, a laser power of 3000W, and a scanning speed of 20mm / s;

[0064] Step 3. Under the protection of argon with a mass purity greater than 99.999%, nano-CrN powder, deionized water and sodium lauryl sulfate are ultrasonically mixed in a ratio of 100g:500mL:50g and then sprayed on the surface of the chromium-rich coating obtained in step 2, and then treated by laser alloying technology. The above process is repeated 8 times to obtain a Cr-CrN enriched coating on the surface of the clean stainless steel substrate; the mass purity of the nano-CrN powder is greater than 99.9%, the morphology is polygonal, and the average particle size is 20nm; the laser used in the laser alloying technology is a semiconductor continuous output high-power laser with a laser wavelength of 850nm to 900nm, a rectangular laser spot with a length × width of 6mm × 8mm, a laser power of 5000W, and a scanning speed of 40mm / s;

[0065] Step 4. Under the protection of nitrogen with a mass purity greater than 99.99%, a layer of graphite coating prepared by graphite particles with a particle size of -500 mesh and ethanol in a ratio of 100g:500mL is sprayed on the surface of the Cr-CrN enriched coating obtained in step 3, and then a laser heat treatment technology is used to scan back and forth 120 times to induce the formation of a Cr2N phase in the coating, and a Cr-Cr2N sodium corrosion-resistant cold coating is obtained on the surface of a clean stainless steel substrate; the laser used in the laser heat treatment technology is a fiber continuous output high-power laser with a laser wavelength of 900nm to 1064nm, a rectangular laser spot with a length × width of 2mm × 15mm, a laser scanning speed of 100mm / s, and a temperature of the laser heat treatment layer of 1040°C to 1060°C.

[0066] After testing, the thickness of the metastable supersaturated chromium-rich coating prepared in step 2 of this embodiment is greater than 100 μm, the mass content of Cr is greater than 60%, and there are no crack defects on the surface. The chromium-rich coating is metallurgically bonded to the austenitic stainless steel substrate; in the Cr-CrN enriched coating prepared in step 3 of this embodiment, CrN particles are solid dissolved in the supersaturated chromium-rich coating, and the CrN content is greater than 60 wt%. A solid solution Cr2N phase is formed in situ in the Cr-Cr2N sodium cold corrosion resistant coating prepared in step 4 of this embodiment.

[0067] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent variation made to the above embodiment based on the technical essence of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preparing an in-situ sodium cold corrosion resistant coating on the surface of nuclear stainless steel, characterized in that: The method comprises the following steps: Step 1: Degreasing, derusting and cleaning the surface of the nuclear stainless steel in sequence to obtain a clean stainless steel substrate; Step 2: spraying chromium powder onto the surface of the clean stainless steel substrate obtained in step 1, and then treating it with laser alloying technology to obtain a chromium-rich coating on the surface of the clean stainless steel substrate; Step 3: spraying nano-CrN powder onto the surface of the chromium-rich coating obtained in step 2, and then treating it with laser alloying technology to obtain a Cr-CrN enriched coating on the surface of the clean stainless steel substrate; Step 4: Scan the Cr-CrN enriched coating formed in step 3 using laser heat treatment technology to induce the formation of a Cr2N phase in the coating, and obtain a Cr-Cr2N sodium cold corrosion resistant coating on the surface of the clean stainless steel substrate; the laser heat treatment technology is to reciprocately irradiate and scan the Cr-CrN enriched coating more than 50 times; the laser used in the laser heat treatment technology is a fiber continuous output high-power laser with a laser wavelength of 900nm~1064nm, a rectangular laser spot with a length × width of 2mm×15mm, a laser scanning speed of 50mm / s~100mm / s, and a temperature of the laser heat treatment layer of 1040℃~1060℃.

2. The method for preparing a composite in-situ sodium cold corrosion resistant coating on the surface of nuclear stainless steel according to claim 1, characterized in that: The stainless steel in step 1 is 316L austenitic stainless steel or 316H austenitic stainless steel.

3. The method for preparing a composite in-situ sodium cold corrosion resistant coating on the surface of nuclear stainless steel according to claim 1, characterized in that: The chromium powder in step 2 has a mass purity greater than 99.9%, a spherical shape, and a diameter of 5µm to 10µm; and the chromium powder spraying is performed by mixing the chromium powder, ethyl acetate, and varnish and then spraying.

4. The method for preparing a composite in-situ sodium cold corrosion resistant coating on the surface of nuclear stainless steel according to claim 1, characterized in that: The laser alloying technology treatment in step 2 refers to 4 to 8 cycles of spraying chromium powder and laser alloying, the thickness of the chromium-rich coating is greater than 100 μm, and the mass content of Cr in the chromium-rich coating is greater than 60%.

5. The method for preparing a sodium cold corrosion resistant coating on the surface of nuclear stainless steel in situ according to claim 1, characterized in that: The nano-CrN powder in step 3 has a mass purity greater than 99.9%, a polygonal morphology, and an average particle size of 20 nm; the nano-CrN powder spraying is performed by mixing the nano-CrN powder, deionized water, and a dispersant and then spraying.

6. The method for preparing a sodium cold corrosion resistant coating on the surface of nuclear stainless steel in situ according to claim 1, characterized in that: The laser alloying technology treatment in step 3 refers to spraying nano-CrN powder and laser alloying for 4 to 8 cycles under the protection of argon gas with a mass purity greater than 99.999%, and the mass content of CrN in the Cr-CrN enriched coating is greater than 60%.

7. The method for preparing a sodium cold corrosion resistant coating on the surface of nuclear stainless steel in situ according to claim 1, characterized in that: The laser used in the laser alloying technology in step 2 and step 3 is a semiconductor continuous output high-power laser with a laser wavelength of 850nm~900nm. The laser spot is rectangular with a length×width of 6mm×8mm.

8. The method for preparing a composite in-situ sodium cold corrosion resistant coating on the surface of nuclear stainless steel according to claim 1, characterized in that: Before the laser heat treatment in step 4, a layer of graphite coating prepared by graphite particles with a particle size of -500 mesh and ethanol is sprayed on the surface of the Cr-CrN enriched coating. The laser heat treatment in step 4 is carried out under the protection of nitrogen with a mass purity greater than 99.99%.

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