A nitriding agent for surface nitriding of steel and a method for surface nitriding of steel

Abstract

The application provides a kind of steel surface nitriding agent for steel surface nitriding treatment and a steel surface nitriding method, and belongs to the technical field of steel surface treatment.The steel surface nitriding agent comprises a base nitriding agent and an accelerator.The base nitriding agent comprises the following components by weight: 55-65 parts of sodium cyanate, 14-16 parts of potassium cyanate, 14-16 parts of potassium chloride, 7-9 parts of lithium carbonate and 1.8-2.2 parts of yttrium chloride.The accelerator comprises the following components by weight: 55-65 parts of sodium nitrate, 33-37 parts of sodium bicarbonate, 2.8-3.2 parts of lithium chloride and 1.8-2.2 parts of sodium peroxide.The steel surface nitriding agent provided by the application can improve the wear resistance and corrosion resistance of steel after steel surface nitriding treatment.

Description

Technical Field

[0001] This invention belongs to the field of steel surface treatment technology, and particularly relates to a nitriding agent for nitriding steel surface and a method for nitriding steel surface. Background Technology

[0002] In the oil extraction process, oil pumps are used to extract crude oil. High-density filter pipes are used to filter out hard particulate impurities such as sand. To improve the wear resistance and corrosion resistance of the pump barrel, thereby extending its service life, the inner wall of the pump barrel is usually hard chrome plated, and the plunger is treated with spray welding (hard alloy powder). However, the heavy metal chromium produced during the chrome plating process poses a risk of cancer and gene mutations when absorbed by the human body. In September 2017, the European Union implemented the RoHS regulation, a mandatory standard to eliminate heavy metal chromium from electrical and electronic products, aiming to prohibit the use of hazardous substances including chromium.

[0003] The filter sand pipe is welded from composite materials, with the frame made of materials commonly used in oil casing. The filter screen is made of 304 / 316L austenitic stainless steel. The main failure mode during use is the accelerated impact of hard particles such as sand on the filter screen surface after oil and gas wells flow naturally or are pressurized by the pump. This causes wear and erosion, ultimately leading to filter screen breakage and loss of filtration function. While 304 / 316L austenitic stainless steel has excellent corrosion resistance, ensuring the filter screen surface remains uncorroded in the downhole environment, its relatively low strength and surface hardness make it prone to rapid deformation, wear, and breakage under pressure from hard objects or abrasive impacts from particles. Furthermore, the thin-walled nature of the filter sand pipe further exacerbates the failure process. Summary of the Invention

[0004] This invention provides a nitriding agent for nitriding steel surfaces and a method for nitriding steel surfaces. After nitriding steel surfaces with the nitriding agent provided by this invention, the steel can have good wear resistance and corrosion resistance.

[0005] To achieve the above objectives, the present invention provides a nitriding agent for nitriding treatment of steel surfaces, comprising a base nitriding agent and an accelerator; the base nitriding agent comprises the following components in parts by weight: 55-65 parts sodium cyanate, 14-16 parts potassium cyanate, 14-16 parts potassium chloride, 7-9 parts lithium carbonate, and 1.8-2.2 parts yttrium chloride; the accelerator comprises the following components in parts by weight: 55-65 parts sodium nitrate, 33-37 parts sodium bicarbonate, 2.8-3.2 parts lithium chloride, and 1.8-2.2 parts sodium peroxide.

[0006] This invention provides a method for nitriding a steel surface, comprising the following steps:

[0007] 1) Add the basic nitriding agent and rare earth nitriding agent described in the above scheme to the nitriding furnace, and transfer the pretreated steel into the nitriding furnace for basic nitriding;

[0008] 2) After the basic nitriding is completed, the nitriding agent and nitrogen gas described in the above scheme are added to the nitriding furnace to enhance nitriding;

[0009] 3) After the nitriding process is completed, the steel is transferred to an oxidation furnace for oxidation treatment.

[0010] 4) After oxygen infiltration is completed, the steel is cooled, cleaned and sealed in sequence.

[0011] Preferably, when the steel is 42CrMo steel, the temperature for basic nitriding in step 1) is 510-530°C and the time is 170-190 min;

[0012] When the steel is austenitic stainless steel, the temperature for basic nitriding in step 1) is 410-430℃ and the time is 90-110min.

[0013] Preferably, when the steel is 42CrMo steel, the temperature for strengthening nitriding in step 2) is 510-530°C and the time is 15-25 min;

[0014] Preferably, when the steel is austenitic stainless steel, the temperature for strengthening nitriding in step 2) is 410-430°C and the time is 15-25 min.

[0015] Preferably, the rare earth permeation catalyst in step 1) comprises the following components in parts by weight: 55-65 parts of yttrium chloride and 35-45 parts of lanthanum carbonate.

[0016] Preferably, the oxygen permeating agent used in step 3) comprises the following components in parts by weight: 33-37 parts sodium nitrate, 28-32 parts sodium nitrite, 14-16 parts sodium hydroxide, 14-16 parts sodium carbonate, and 4.5-5.5 parts potassium nitrate.

[0017] Preferably, the temperature during the oxygen permeation treatment in step 3) is 390–410°C and the time is 25–35 min.

[0018] Preferably, the sealing agent used in step 4) is a mixture of straight-chain alkanes and monocyclic cycloalkanes; the sealing temperature is 170–190°C and the time is 50–70 min.

[0019] Preferably, the pretreatment method in step 1) is as follows: the steel is cleaned and preheated to obtain pretreated steel; the temperature during preheating is 390-410℃ and the time is 25-35min.

[0020] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0021] The nitriding agent for steel surface nitriding provided by the present invention includes a basic nitriding agent and an accelerator. The addition of the accelerator can instantly increase the nitrogen potential at the interface between the molten salt and the workpiece surface during nitriding, promote the formation of a high nitrogen content solid solution on the workpiece surface, obtain iron-chromium nitrides on the surface of austenitic steel, and form Fe2N on the surface of 42CrMo steel, thereby providing wear resistance and / or corrosion resistance of the steel.

[0022] This invention further provides a method for nitriding steel surfaces, employing a two-stage approach of basic nitriding and enhanced nitriding. For austenitic steels, it comprehensively treats the properties of stainless steel and the combining characteristics of iron, chromium, and nitrogen, ultimately achieving a significant improvement in surface wear resistance while maintaining the corrosion resistance of the stainless steel filter pipe. It also inhibits Cr precipitation and significantly reduces CrN and Cr... 23 The formation of C3 avoids the formation of chromium-depleted zones, allowing the workpiece to form a nitride-free diffusion layer during the infiltration process. For 42CrMo steel, a comprehensive treatment is performed using process material properties and the iron-nitrogen phase diagram, ultimately achieving the goal of significantly improving surface wear resistance while ensuring the corrosion resistance of the inner wall of the ion pump cylinder. Attached Figure Description

[0023] Figure 1 Iron-nitrogen phase diagram;

[0024] Figure 2 This is a diagram of the diffusion layer structure of the workpiece after treatment in Example 1;

[0025] Figure 3 The XRD pattern of the workpiece after processing in Example 1;

[0026] Figure 4 This is a diagram showing the state of the workpiece before the salt spray test.

[0027] Figure 5 This is a diagram showing the state of the workpiece after the salt spray test. Detailed Implementation

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] This invention provides a nitriding agent for nitriding treatment of steel surfaces, comprising a base nitriding agent and an accelerator; the base nitriding agent comprises the following components in parts by weight: 55-65 parts sodium cyanate, 14-16 parts potassium cyanate, 14-16 parts potassium chloride, 7-9 parts lithium carbonate, and 1.8-2.2 parts yttrium chloride; the accelerator comprises the following components in parts by weight: 55-65 parts sodium nitrate, 33-37 parts sodium bicarbonate, 2.8-3.2 parts lithium chloride, and 1.8-2.2 parts sodium peroxide.

[0030] The nitriding agent for steel surface nitriding provided by this invention is a molten salt nitriding agent. This nitriding agent has excellent fluidity at low temperatures. The purpose of using the molten salt nitriding agent for nitriding treatment is to allow the effective ions in the molten salt nitriding agent to diffuse into the inner surface of the steel, thereby forming a diffusion layer of a certain thickness. When the steel is 42CrMo steel, this diffusion layer consists of dense iron-chromium oxide, a compound layer with gradient hardness and phase change, a solid solution of nitrogen and carbon in α-iron, and a trace amount of rare earth lanthanum-yttrium forming an interstitial solid solution, exhibiting high wear resistance, corrosion resistance, and impact toughness. When the steel is austenitic stainless steel, the diffusion layer consists of dense iron-chromium oxide, a solid solution of nitrogen and carbon in austenite, and a trace amount of rare earth lanthanum-yttrium forming an interstitial solid solution, exhibiting high wear resistance, corrosion resistance, and impact toughness.

[0031] This invention provides a method for nitriding a steel surface, comprising the following steps:

[0032] 1) Add the basic nitriding agent and rare earth nitriding agent described in the above scheme to the nitriding furnace, and transfer the pretreated steel into the nitriding furnace for basic nitriding;

[0033] 2) After the basic nitriding is completed, the nitriding agent and nitrogen gas described in the above scheme are added to the nitriding furnace to enhance nitriding;

[0034] 3) After the nitriding process is completed, the steel is transferred to an oxidation furnace for oxidation treatment.

[0035] 4) After oxygen infiltration is completed, the steel is cooled, cleaned and sealed in sequence.

[0036] This invention adds the basic nitriding agent and rare earth nitriding catalyst described above to a nitriding furnace, and then transfers the pretreated steel into the nitriding furnace for basic nitriding. In this invention, the pretreatment method involves cleaning and preheating the steel to obtain pretreated steel. Preferably, the cleaning method uses ultrasonic cleaning combined with a water-based cleaning agent to remove oil from the steel surface, followed by rinsing with clean water and hot water. The preheating temperature is preferably 390–410°C, and the preheating time is preferably 25–35 minutes. In this invention, preheating not only removes moisture from the workpiece surface but also increases the workpiece temperature, preventing the formation of molten salt crystals in the nitriding furnace and forming a small oxide layer on the workpiece surface, thus promoting the next nitriding step.

[0037] In this invention, the rare earth nitriding catalyst preferably comprises 55-65 parts by weight of yttrium chloride and 35-45 parts by weight of lanthanum carbonate. In this invention, during basic nitriding, the nitrogen potential content at the surface of the steel and the nitriding agent is controlled to be below 25% (characterized by cyanate content).

[0038] In this invention, when the steel is 42CrMo steel, the preferred temperature for basic nitriding is 510–530°C, and the preferred time is 170–190 min. When the steel is austenitic stainless steel, the preferred temperature for basic nitriding is 410–430°C, and the preferred time is 90–110 min. This invention does not specifically limit the source of the 42CrMo steel and austenitic stainless steel; commercially available products conventional in the art can be used.

[0039] The sensitization temperature range of austenitic stainless steel is 450℃ to 850℃, with 550℃ being the nose temperature, meaning that at this temperature, Cr... 23 C3 formation is most intense. The basic nitriding process in this invention is performed at 410–430℃ to avoid the sensitization curve and prevent Cr formation. 23 The formation of C3 and the incorporation of sufficient nitrogen atoms into the workpiece are key challenges at this stage. One difficulty lies in the reduced fluidity of the molten salt penetrant at excessively low temperatures, resulting in numerous pores in the filter sand tube. The density of the molten penetrant in its molten state is approximately 1.80 g / cm³. 3 The molten salt displacer replacement rate on the inner wall of the pores is low, resulting in poor penetration. The inner wall is the main wear surface during the sand filtration process; furthermore, low temperature reduces the diffusion coefficient of nitrogen atoms, thus affecting the penetration depth. In this invention, firstly, the precipitation of Cr in the workpiece is suppressed by low temperature; secondly, CrN and Fe in the workpiece are suppressed by low temperature and low concentration. X The formation of nitrogen (N). Introducing rare earth elements increases the diffusion depth and improves the surface toughness of the workpiece after nitrogen atoms are incorporated during the basic nitriding stage. The core of this stage is to allow nitrogen from the workpiece surface to appear in the workpiece lattice in a solid solution form.

[0040] After basic nitriding is completed, this invention adds a nitriding agent and nitrogen gas to the nitriding furnace for enhanced nitriding. In this invention, the nitrogen potential content in the nitriding furnace is preferably increased to the theoretical limit of 43% before enhanced nitriding. The principle for adding the nitriding agent is to maintain the liquid surface in a foamy state while simultaneously introducing nitrogen gas into the crucible and at the liquid surface. Once the steel is removed, the addition of the nitriding agent and the introduction of nitrogen gas are stopped. In this invention, adding an accelerator during enhanced nitriding can instantly increase the nitrogen potential at the interface between the molten salt and the workpiece surface. In this invention, when the steel is 42CrMo steel, the preferred temperature for enhanced nitriding is 510–530°C, and the preferred time is 15–25 minutes. Controlling the temperature within this range when the steel is 42CrMo steel not only facilitates good fluidity of the molten salt nitriding agent but also minimizes the risk of cyanate decomposition. This invention is suitable for mass industrial production. When the steel is austenitic stainless steel, the preferred temperature for strengthening nitriding is 410–430°C, and the preferred time is 15–25 minutes. In this invention, the temperature is maintained at 410–430°C during the strengthening nitriding stage of the austenitic stainless steel to further suppress Cr precipitation.

[0041] In this invention, the accelerator and nitrogen flow are used to instantly increase the nitrogen potential, and their mechanism of action is as follows:

[0042] Normal cyanate decomposition provides a nitrogen source:

[0043] 4CNO – →CO3 2- +2CN – +CO+2[N] (1)

[0044] Fe + [N] → Fe X N (2)

[0045] Existing molten salt infiltration agents are eutectic melts of various raw materials. As can be seen from the chemical formula (1), the cyanate content is the only means to characterize the nitrogen potential of the molten salt system. However, the molten salt system is a liquid phase, and the cyanate content cannot be increased indefinitely. The cyanate content limit of pure NaCNO is 41.6%. Considering the mutual solubility in the molten state, the cyanate content test results in actual production have never exceeded 45%.

[0046] During normal production, the two reactions occur simultaneously. The [N] on the right side of equation (1) begins to be consumed, which accelerates the consumption of cyanate on the left side of equation (1).

[0047] The consumption of cyanate ions is not only used to provide nitrogen potential, but also occurs through natural decomposition, as shown in reactions (3) and (4).

[0048] 4CNO – +O2→CO3 2- +CO+N2↑ (3)

[0049] 4CNO – →CO3 2- +2CN – +CO+N2↑ (4)

[0050] In summary, as the sole source of nitrogen potential for molten salt permeation agents, the content of cyanate ions determines the level of nitrogen potential in molten salts. However, as can be seen from chemical formulas (1-4), the higher the concentration of cyanate ions, the higher the chemical potential, the more vigorous the reaction, and the faster the decomposition rate. Therefore, the existing molten salt system has an upper limit on the amount of nitrogen it can provide.

[0051] The main determinants of nitrogen diffusion are temperature and the interface nitrogen potential. Higher temperatures result in a higher diffusion coefficient and easier diffusion. For austenitic stainless steel, this invention uses an ultra-low temperature of 410–430°C to suppress Cr precipitation. This significantly reduces the nitrogen diffusion coefficient and decreases the fluidity of the diffusion agent, necessitating an increase in the nitrogen potential to enhance the diffusion driving force. Simply increasing the static cyanate content of the molten salt has an upper limit; under this limit, a single diffusion layer, i.e., a nitrogen-containing solid solution, will form on the workpiece surface. The higher the nitrogen content in the nitrogen-containing solid solution, the stronger the pinning effect of nitrogen atoms, and the higher the hardness.

[0052] For 42CrMo steel, from Figure 1 According to the iron-nitrogen phase diagram, to form high-hardness Fe₂N, the mass fraction of nitrogen must exceed 11.1%. Therefore, in existing molten salt nitriding (or liquid nitriding) processes, it is generally believed that high-nitrogen iron-nitrogen compounds, i.e., Fe₂N, will not form; only Fe₂N can be formed. 2-3 N or Fe4N.

[0053] This invention utilizes an accelerator and nitrogen gas for enhanced nitriding. Its principle is to use the accelerator to remove CN from the molten salt. – Rapid oxidation to CNO in a short period of time – Using nitrogen to suppress CNO –Natural decomposition consumes cyanate, pushing the cyanate content above 55%, ultimately creating an ultra-high nitrogen potential at the interface between the molten salt and the workpiece. This promotes the formation of a high-nitrogen solid solution on the workpiece surface. Simultaneously, process time control prevents excessive nitrogen potential, which could lead to the formation of iron-chromium nitrides on the surface of austenitic stainless steel. When the workpiece is 42CrMo steel, Fe2N forms on the surface, overcoming general industry limitations. Fe2N is a brittle phase; in conventional long-term gas nitriding, a Fe2N-dominated nitriding layer forms. This layer is typically ground off before use to prevent excessive brittleness and subsequent cracking and other failures during use. In this invention, by controlling the strengthening nitriding stage time to 15-25 minutes, the short nitriding time prevents the formation of a continuous, dense, brittle Fe2N layer on the surface. Instead, it allows Fe2N to be uniformly and diffusely distributed within the Fe2N-dominated solid solution. 2-3 The outer side of the compound layer with N as the main phase plays a role in increasing the hardness of the outer side of the compound. This method is very suitable for thin-walled parts such as ion pump barrels, which can improve wear resistance by increasing the hardness of the outermost surface at a low distance, without causing excessive hardness and reducing the overall toughness of the workpiece.

[0054] The main component of the accelerator is sodium nitrate, which is effective against CN. - The redox reaction is shown in equation (5):

[0055] CN – +NaNO3→CNO – +Na + +CO+NO2 + (5).

[0056] In existing nitriding processes, mixed oxygen or compressed air is generally used. The purposes are twofold: first, to agitate the molten nitriding agent using air; and second, to utilize the oxidizing properties of oxygen to react with CN. - Redox reactions are carried out. This method generally has limited reducing power. According to formula (3), oxygen itself can decompose cyanate ions, but cannot produce active [N] atoms. This is also why the industry does not use pure oxygen for gaseous redox reactions. - The consumption rate is greater than that of CNO. - The formation rate is relatively low, and final ventilation is usually only a supplementary means. In this invention, the nitrogen gas is introduced by using pure nitrogen + air, wherein the flow rate ratio of pure nitrogen to air is preferably 3 to 4:1. In this invention, the main purpose of using this method for nitrogen introduction is to act as a protective gas and to agitate the air, thereby reducing the decomposition of cyanate ions by O2 in the air.

[0057] After nitriding is completed, the steel is transferred to an oxidation furnace for oxidation treatment. In this invention, the oxidation agent preferably comprises the following components by weight: 33-37 parts sodium nitrate, 28-32 parts sodium nitrite, 14-16 parts sodium hydroxide, 14-16 parts sodium carbonate, and 4.5-5.5 parts potassium nitrate. In this invention, the oxidation treatment temperature is preferably 390-410°C, and the treatment time is preferably 25-35 minutes. In this invention, the oxidation treatment oxidizes the surface layer, forming a dense Fe3O4 layer.

[0058] After oxygen infiltration, the steel is sequentially subjected to cooling cleaning and sealing treatments. In this invention, the cooling method is preferably air cooling; the cleaning method is preferably ultrasonic cleaning. In this invention, the sealing agent used for the sealing treatment is preferably a mixture of straight-chain alkanes and monocyclic cycloalkanes; the sealing treatment temperature is preferably 170–190°C, and the time is preferably 50–70 minutes. In this invention, the sealing agent selected for the sealing treatment is an ion stabilizer, which can seal micro-defects formed in the preceding processes, thereby reducing the surface friction coefficient of the workpiece.

[0059] When the steel is austenitic stainless steel, after all processes are completed, the final diffusion layer structure from the surface to the core is as follows:

[0060] 1. Oxide layer, composition: Fe3O4

[0061] 2. Diffusion layer, composition: nitrogen-containing solid solution

[0062] 3. Matrix

[0063] When the steel is 42CrMo steel, after all processes are completed, the final infiltration layer structure from the surface to the core is as follows:

[0064] 1. Oxide layer, composition: Fe3O4

[0065] 2. Compound layer, composition: main component Fe 2-3 N and Fe2N are diffusely distributed on the outer side of the compound layer, while Fe4N is uniformly distributed on the inner side of the compound layer, with the content decreasing gradually with increasing depth.

[0066] 3. Diffusion layer, composition: mainly nitrogen-containing solid solution, with a small amount of Fe4N on the outer side.

[0067] 4. Matrix.

[0068] To further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0069] In each embodiment, the workpiece to be processed is an oil pump barrel or a filter tube. The oil pump barrel is made of 42CrMo material, and the filter tube is made of 304 austenitic stainless steel.

[0070] The water-based cleaning agent is Aisen ES503, whose main components are anionic surfactant (CAS#68439-50-9), sodium bicarbonate (CAS#144-55-8), and additives.

[0071] Example 1

[0072] The surface of the oil pump barrel was cleaned by ultrasonic cleaning (50kW power, 60min) with water-based cleaning agent. After rinsing with clean water and hot water, it was placed in a preheating furnace for preheating treatment at 400℃ for 30min to obtain the pretreated workpiece.

[0073] Place the crucible containing the basic nitriding agent (composed of the following components by mass concentration: 60% sodium cyanate, 15% potassium cyanate, 15% potassium chloride, 8% lithium carbonate, and 2% yttrium chloride) and the rare earth nitriding catalyst (composed of the following components by mass concentration: 60% yttrium chloride and 40% lanthanum carbonate, with the rare earth nitriding catalyst added at 3.5% of the basic nitriding agent) into the No. 1 nitriding furnace. Place the pretreated workpiece into the crucible and maintain the temperature at 520°C for 180 minutes to perform the basic nitriding process.

[0074] The nitrogen potential content in furnace No. 2 was increased to the theoretical limit of 43%. The workpiece was then transferred from furnace No. 1 into furnace No. 2 into a crucible containing a base nitriding agent (composed of the following components by mass concentration: sodium cyanate 60%, potassium cyanate 15%, potassium chloride 15%, lithium carbonate 8%, yttrium chloride 2%) for enhanced nitriding. The specific operation was as follows: An accelerator (composed of the following components by mass concentration: sodium nitrate 60%, sodium bicarbonate 35%, lithium chloride 3%, sodium peroxide 2%) was immediately and continuously added to maintain a foamy state on the liquid surface. Simultaneously, nitrogen gas (pure nitrogen + air, preferably a nitrogen-to-air flow ratio of 3:1, nitrogen flow rate 25 L / min) was introduced into the crucible and onto the liquid surface. The holding temperature was 520℃, and the enhancement time was 20 min. After the workpiece was removed, the addition of the accelerator and the nitrogen flow were stopped.

[0075] The workpiece was placed from nitriding furnace No. 2 into oxygen-diffusion furnace for oxygen-diffusion treatment (oxygenating agent: 35% sodium nitrate, 30% sodium nitrite, 15% sodium hydroxide, 15% sodium carbonate, and 5% potassium nitrate), held at 400℃ for 30 minutes. The surface was oxidized, forming a dense Fe3O4 layer.

[0076] After oxygen permeation treatment, the workpiece is air-cooled for 20 minutes, followed by cooling with deionized water, post-cleaning, and ultrasonic cleaning. After cleaning, the workpiece is placed in an ion stabilization furnace (the sealing agent consists of straight-chain alkanes and monocyclic cycloalkanes in a 3:2 mass ratio) for sealing treatment. The holding temperature is 180℃, and the holding time is 60 minutes.

[0077] The structure of the nitrided layer on the workpiece after nitriding treatment was measured, specifically as follows: Figures 2-3 As shown, from the surface to the core, the layers are as follows: oxide layer (composition: Fe3O4); compound layer (composition: mainly Fe). 2-3 N and Fe2N are diffusely distributed on the outer side of the compound layer, while Fe4N is uniformly distributed on the inner side of the compound layer, with the content decreasing gradually with increasing depth; diffusion layer (composition: mainly nitrogen-containing solid solution, with a small amount of Fe4N on the outer side); matrix.

[0078] Example 2

[0079] The surface of the oil pump barrel was cleaned by ultrasonic cleaning (50kW power, 60min) with water-based cleaning agent. After rinsing with clean water and hot water, it was placed in a preheating furnace for preheating treatment at 400℃ for 30min to obtain the pretreated workpiece.

[0080] Place the crucible containing the base penetrant (composed of the following components by mass concentration: sodium cyanate 58.8%, potassium cyanate 16%, potassium chloride 16%, lithium carbonate 7%, yttrium chloride 2.2%) and the rare earth penetrant (composed of the following components by mass concentration: yttrium chloride 55% and lanthanum carbonate 45%, with the rare earth penetrant added at 3% of the base penetrant) into nitriding furnace No. 1. Place the pretreated workpiece into the crucible and perform basic nitriding treatment at a temperature of 530°C for 170 minutes.

[0081] The nitrogen potential content in furnace No. 2 was increased to the theoretical limit of 43%. The workpiece was then transferred from furnace No. 1 into furnace No. 2 into a crucible containing a base nitriding agent (composed of the following components by mass concentration: sodium cyanate 58.8%, potassium cyanate 16%, potassium chloride 16%, lithium carbonate 7%, and yttrium chloride 2.2%) for enhanced nitriding. The specific operation was as follows: An accelerator (composed of the following components by mass concentration: sodium nitrate 58%, sodium bicarbonate 37%, lithium chloride 2.8%, and sodium peroxide 2.2%) was immediately and continuously added to maintain a foamy state on the liquid surface. Simultaneously, nitrogen gas (pure nitrogen + air, with a nitrogen to air flow rate ratio of 3:1 and a nitrogen flow rate of 25 L / min) was introduced into the crucible and onto the liquid surface. The holding temperature was 530℃, and the enhancement time was 20 minutes. After the workpiece was removed, the addition of the accelerator and the nitrogen flow were stopped.

[0082] The workpiece was placed from nitriding furnace No. 2 into oxygenation furnace for oxygenation treatment (the oxygenating agent is 35% sodium nitrate, 30% sodium nitrite, 15% sodium hydroxide, 15% sodium carbonate and 5% potassium nitrate). The holding temperature was 410℃ and the holding time was 25min. The surface was oxidized to form dense Fe3O4.

[0083] After oxygen permeation treatment, the workpiece is air-cooled for 20 minutes, followed by cooling with deionized water, post-cleaning, and ultrasonic cleaning. After cleaning, the workpiece is placed in an ion stabilization furnace (the sealing agent consists of straight-chain alkanes and monocyclic cycloalkanes in a 3:2 mass ratio) for sealing treatment. The holding temperature is 170℃, and the holding time is 70 minutes.

[0084] The structure of the nitrided layer on the workpiece was measured, and from the surface to the core, it consisted of: an oxide layer (composition: Fe3O4); a compound layer (composition: mainly Fe). 2-3 N and Fe2N are diffusely distributed on the outer side of the compound layer, while Fe4N is uniformly distributed on the inner side of the compound layer, with the content decreasing gradually with increasing depth; diffusion layer (composition: mainly nitrogen-containing solid solution, with a small amount of Fe4N on the outer side); matrix.

[0085] Example 3

[0086] The filter sand tube was cleaned by ultrasonic cleaning (50kW power, 80min) with water-based cleaning agent to remove oil from the surface. After rinsing with clean water and hot water, it was placed in a preheating furnace for preheating treatment at 400℃ for 30min to obtain the pretreated workpiece.

[0087] Place the crucible containing the base penetrant (composed of the following components by mass concentration: 60% sodium cyanate, 15% potassium cyanate, 15% potassium chloride, 8% lithium carbonate, and 2% yttrium chloride) and the rare earth catalyst (composed of the following components by mass concentration: 65% yttrium chloride and 35% lanthanum carbonate, with the rare earth catalyst added at 5% of the base penetrant) into nitriding furnace No. 1. Place the pretreated workpiece into the crucible and transfer it from the preheating furnace into nitriding furnace No. 1. Maintain the temperature at 420°C for 120 minutes for the base nitriding treatment.

[0088] The nitrogen potential content in furnace No. 2 was increased to the theoretical limit of 43%. The workpiece was then transferred from furnace No. 1 into furnace No. 2 into a crucible containing a basic nitriding agent (composed of the following components by mass concentration: sodium cyanate 60%, potassium cyanate 15%, potassium chloride 15%, lithium carbonate 8%, yttrium chloride 2%) for enhanced nitriding. The specific operation was as follows: An accelerator (composed of the following components by mass concentration: sodium nitrate 60%, sodium bicarbonate 35%, lithium chloride 3%, sodium peroxide 2%) was immediately and continuously added to maintain a foamy state on the liquid surface. Simultaneously, nitrogen gas (pure nitrogen + air, preferably with a nitrogen-to-air flow rate ratio of 4:1 and a nitrogen flow rate of 30 L / min) was introduced into the crucible and onto the liquid surface. The holding temperature was 420℃, and the enhancement time was 25 min. After the workpiece was removed, the addition of the accelerator and the nitrogen flow were stopped.

[0089] The workpiece was placed from the No. 2 nitriding furnace into the oxygen diffusion furnace for oxygen diffusion treatment (the oxygen diffusion agent is 35% sodium nitrate, 30% sodium nitrite, 15% sodium hydroxide, 15% sodium carbonate and 5% potassium nitrate). The holding temperature was 400℃ and the holding time was 30min. The surface was oxidized to form dense Fe3O4.

[0090] After oxygen permeation treatment, the workpiece is air-cooled for 20 minutes, followed by cooling with deionized water, post-cleaning, and ultrasonic cleaning. After cleaning, the workpiece is placed in an ion stabilization furnace (the stabilizer is a sealing agent composed of straight-chain alkanes and monocyclic cycloalkanes in a 3:2 mass ratio) for a sealing process. The holding temperature is 180℃, and the holding time is 60 minutes.

[0091] The structure of the nitrided layer of the workpiece was measured. From the surface to the core, it consists of: oxide layer (composition: Fe3O4); diffusion layer (composition: nitrogen-containing solid solution); and matrix.

[0092] Comparative Example 1

[0093] The surface of the ion pump barrel is degreased and cleaned by first acid washing in an acid-containing cleaning tank, followed by rinsing with clean water and then rinsing with hot water.

[0094] After rinsing, place the pump cylinder into the electroplating tank frame, insert the electroplating electrode into the pump cylinder, and pass the flowing electroplating solution into the pump cylinder to perform electroplating (current density ≥ 50A / dm³). 2 The plating bath temperature is 58–60℃, the electroplating time is 12 hours, and the thickness is controlled to be ≥30μm.

[0095] Performance testing

[0096] 1. Wear resistance test

[0097] The workpieces treated in Examples 1-3 and Comparative Example 1 were tested. Three workpieces were tested in each group, and the average value of the measurement data was taken. The specific test results are shown in Table 1.

[0098] Test Items: *Comparative test of friction and wear. Test Standards: GB / T 12444-2006, GB / T6062-2009, GB / T 11378-2005

[0099] Testing equipment: High-temperature friction and wear testing machine; Model: HT-600

[0100] Test conditions: The friction pair is a 6mm SiC ball, the applied force W = 1025g, the rotation diameter = 5x10-3m, the rotation speed 120rpm, and the number of rotations = 7200 times.

[0101] Table 1

[0102]

[0103] As can be seen from Table 1, the pump barrels prepared using Examples 1 or 2 have a wear volume and wear rate that are an order of magnitude higher than those prepared using the electroplating chromium process, significantly improving the wear resistance of the product.

[0104] 2. Neutral Salt Spray Test (NSS): Standard ASTM B117-2011

[0105] Neutral salt spray tests were performed on the workpieces treated in Examples 1-3 and Comparative Example 1, respectively. The workpiece state diagrams before and after the test are shown below. Figures 4-5 As shown (where Figure 4 and 5 (1-1, 2-1, and 3-1 are workpieces from Examples 1 to 3, respectively), and the test results are shown in Table 2.

[0106] Table 2

[0107] Example 1 450 hours Level 10 Example 2 480 hours Level 10 Example 3 500 hours Level 10 Comparative Example 1 72 hours Level 9

[0108] As can be seen from Table 2, the method provided by this invention has better corrosion resistance.

[0109] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for nitriding steel surfaces, characterized in that, Includes the following steps: 1) Add basic nitriding agent and rare earth nitriding agent to the nitriding furnace, and transfer the pretreated steel into the nitriding furnace for basic nitriding; 2) After the basic nitriding is completed, a nitriding agent is added to the nitriding furnace to keep the liquid surface in a foamy state, and nitrogen gas is introduced to enhance nitriding; the nitrogen gas flow rate is 25~30L / min; 3) After the nitriding process is completed, the steel is transferred to an oxidation furnace for oxidation treatment. 4) After oxygen infiltration is completed, the steel will be cooled, cleaned, and sealed in sequence; The penetrant comprises a base penetrant and an accelerator; the base penetrant comprises the following components in parts by weight: 55-65 parts sodium cyanate, 14-16 parts potassium cyanate, 14-16 parts potassium chloride, 7-9 parts lithium carbonate, and 1.8-2.2 parts yttrium chloride; the accelerator comprises the following components in parts by weight: 55-65 parts sodium nitrate, 33-37 parts sodium bicarbonate, 2.8-3.2 parts lithium chloride, and 1.8-2.2 parts sodium peroxide; The amount of rare earth penetration enhancer added in step 1) is 3% to 5% of the mass of the basic penetration enhancer; When the steel is 42CrMo steel, the temperature for basic nitriding in step 1) is 510~530℃ and the time is 170~190min; When the steel is austenitic stainless steel, the temperature for basic nitriding in step 1) is 410~430℃ and the time is 90~110min; When the steel is 42CrMo steel, the temperature for strengthening nitriding in step 2) is 510~530℃ and the time is 15~25min; When the steel is austenitic stainless steel, the temperature for strengthening nitriding in step 2) is 410~430℃ and the time is 15~25min.

2. The method according to claim 1, characterized in that, The rare earth penetration catalyst mentioned in step 1) comprises the following components in parts by weight: 55-65 parts of yttrium chloride and 35-45 parts of lanthanum carbonate.

3. The method according to claim 1, characterized in that, The oxygen permeating agent used in step 3) comprises the following components in parts by weight: 33-37 parts sodium nitrate, 28-32 parts sodium nitrite, 14-16 parts sodium hydroxide, 14-16 parts sodium carbonate, and 4.5-5.5 parts potassium nitrate.

4. The method according to claim 1, characterized in that, In step 3), the oxygen permeation treatment is performed at a temperature of 390~410℃ for 25~35 minutes.

5. The method according to claim 1, characterized in that, The sealing agent used in step 4) is a mixture of straight-chain alkanes and monocyclic cycloalkanes; the sealing temperature is 170~190℃ and the time is 50~70min.

6. The method according to claim 1, characterized in that, The pretreatment method described in step 1) is as follows: the steel is cleaned and preheated to obtain pretreated steel; the temperature during preheating is 390~410℃ and the time is 25~35min.

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