A salt bath nitriding process using different stirring gases to control the composition of the salt bath

Abstract

The present invention discloses a salt bath nitriding process using different stirring gases to control the composition of the salt bath, comprising the following steps: preparing a salt bath solution, immersing the stainless steel parts after surface cleaning in the salt bath solution for salt bath nitriding, and monitoring the concentration of the salt bath byproduct CN ions in real time during the salt bath nitriding process; 0.5h after the start of the salt bath nitriding process, different types of stirring gases are selected and stirred based on the concentration of CN ions in the salt bath solution, so that the concentration of the salt bath byproduct CN ions is maintained between 1.5% and 3.0%; wherein the stirring gas includes: pure nitrogen, compressed air and pure oxygen; and the CN ion concentration is observed every 0.5h. The present invention can effectively control the CN ion concentration of the salt bath solution in the salt bath nitriding process by introducing harmless gas to ensure the effect of part nitriding.

Description

Technical Field

[0001] The invention relates to the technical field of parts processing, and in particular to a salt bath nitriding process for controlling salt bath components by using different stirring gases. Background Art

[0002] Salt bath nitriding, also known as liquid salt bath nitriding or QPQ (Quench-Polish-Quench) process, is a thermochemical surface treatment method that immerses the workpiece in a molten salt bath containing nitriding salts, diffuses nitrogen atoms to the metal surface at high temperature, and forms a nitride layer to improve the hardness, wear resistance, corrosion resistance of the workpiece, and reduce the friction coefficient of the workpiece, thereby improving the overall performance of the workpiece. Salt bath nitriding is widely used in various industrial fields, including automotive parts, hydraulic machinery, gears, tool and mold manufacturing, etc.

[0003] In the process of salt bath nitriding, nitriding salt is generally used as raw material, and CN- ions are decomposed at high temperature. The ions have strong reducing properties and can ensure the brightness of the surface of the nitrided parts. If the concentration of the by-product in the salt bath is very low or almost non-existent, it will not have much effect on the nitriding of conventional steel parts. However, it will have a significant effect on the nitriding of stainless steel parts, causing the nitrided surface to be oxidized, roughened, or even peeled. It is necessary to maintain a certain concentration of CN- ions in the salt bath, but the raw materials of this ion (NaCN, KCN, CH3CN, etc.) are all highly toxic and difficult to supplement through raw materials. Therefore, it is necessary to propose a new method to control the concentration of CN- ions in the salt bath nitriding process under the premise of meeting production safety standards, so as to achieve the best steel nitriding effect. Summary of the invention

[0004] The present invention overcomes the shortcomings of the prior art and provides a salt bath nitriding process using different stirring gases to control the composition of the salt bath and a ventilation system for the stirring gas in the salt bath nitriding process.

[0005] To achieve the above object, the technical solution adopted by the present invention is: a salt bath nitriding process using different stirring gases to control the composition of the salt bath, characterized in that it includes the following steps:

[0006] Prepare a salt bath solution, immerse the cleaned stainless steel parts in the salt bath solution for salt bath nitriding, and monitor the concentration of the salt bath byproduct CN- ions in real time during the salt bath nitriding process;

[0007] 0.5 h after the start of the salt bath nitriding process, different types of stirring gases are selected based on the concentration of CN- ions in the salt bath solution and stirred to keep the concentration of CN- ions as a byproduct of the salt bath between 1.5% and 3.0%; wherein the stirring gases include: pure nitrogen, compressed air and pure oxygen; the CN- ion concentration is observed every 0.5 h.

[0008] In a preferred embodiment of the present invention, when the concentration of CN- ions in the salt bath solution is lower than 1.5%, pure nitrogen is used as a stirring gas to increase the concentration of CN- ions in the salt bath solution, and when the concentration of CN- ions in the salt bath solution reaches between 2.0% and 2.5%, the introduction of pure nitrogen is stopped.

[0009] In a preferred embodiment of the present invention, when the concentration of CN- ions in the salt bath solution is between 3% and 4%, compressed air is used as a stirring gas to reduce the concentration of CN- ions in the salt bath solution, and the introduction of compressed air is stopped when the concentration of CN- ions in the salt bath solution is reduced to between 2.0% and 2.5%.

[0010] In a preferred embodiment of the present invention, when the concentration of CN- ions in the salt bath solution is higher than 4%, pure oxygen is used as a stirring gas to reduce the concentration of CN- ions in the salt bath solution, and the introduction of compressed air is stopped when the concentration of CN- ions in the salt bath solution is reduced to between 2.0% and 2.5%.

[0011] In a preferred embodiment of the present invention, the components in the salt bath solution include: 5-15% urea, 40-60% potassium nitrate, 20-40% potassium carbonate, and 5-15% lithium chloride.

[0012] In a preferred embodiment of the present invention, the pressure of the stirring gas introduced into the salt bath solution is set between 0.0180-0.022 MPa.

[0013] In a preferred embodiment of the present invention, the flow rate of the stirring gas introduced into the salt bath solution is set at 400-450 L / h.

[0014] In a preferred embodiment of the present invention, the specific method for real-time monitoring of CN- ion concentration includes: inserting the sampling pipeline of the online cyanide analyzer into the salt bath solution, and measuring and calculating the CN- ion concentration in real time through the sensor or chemical reaction module inside the online cyanide analyzer.

[0015] A ventilation system for stirring gas in a salt bath nitriding process, comprising: a salt bath circulation pipeline connected to the bottom of a salt bath solution, and a nitrogen delivery pipeline, an air delivery pipeline and an oxygen delivery pipeline connected to the salt bath circulation pipeline;

[0016] In the direction of the gas inlet ends of the nitrogen delivery pipeline, the air delivery pipeline and the oxygen delivery pipeline toward the salt bath circulation pipeline, a ventilation control switch, a pressure regulating device, a pressure monitoring device and a gas flow control device are sequentially arranged;

[0017] A nitrogen emission control device is also provided between the pressure monitoring device and the gas flow control device in the nitrogen delivery pipeline, a nitrogen emission pipeline connected to the nitrogen delivery pipeline is also provided in the nitrogen delivery pipeline between the nitrogen emission control device and the gas flow control device, and a gas emission control valve is provided in the nitrogen emission pipeline;

[0018] In the direction from the gas inlet end to the gas outlet end of the salt bath circulation pipeline, there are arranged in sequence: a gas flow total regulating device, a total control switch, a gas flow total monitoring device and a pressure total monitoring device.

[0019] In a preferred embodiment of the present invention, the pressure regulating device is a pressure reducing valve, which is used to control the pressure of the incoming gas between 0.0180-0.022MPa; the pressure monitoring device and the total pressure monitoring device are both set as pressure gauges with a monitoring range between 0-0.1MPa; the total gas flow monitoring device is set as a gas flow meter with a monitoring range between 0-30NL / min.

[0020] The present invention solves the defects existing in the background technology and has the following beneficial effects:

[0021] (1) The present invention changes the concentration of CN- ions during the salt bath nitriding process to keep it within a specified appropriate range, thereby ensuring the effect of part nitriding and avoiding problems such as rough oxidation and even peeling of the surface of stainless steel parts caused by low CN- ion concentration during the nitriding process. It also avoids the problem of uneven formation of the nitrided layer on the surface of the parts and even the formation of a loose layer due to excessive CN- ion concentration, which affects the performance of the parts.

[0022] (2) The present invention introduces harmless pure nitrogen, compressed air or pure oxygen into the salt bath solution at different CN- ion concentrations, which can significantly and effectively change the concentration of CN- ions to ensure the nitriding effect of parts. These gases are easy to obtain and the operation is safe. There is no operation involving hazardous chemicals, no wastewater, waste gas and solid waste generated, and it is completely green and environmentally friendly. At the same time, these gases are easy to obtain and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative work.

[0024] Figure 1 It is a structural schematic diagram of the ventilation system in the present invention. DETAILED DESCRIPTION

[0025] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited to the specific embodiments disclosed below.

[0027] Application Overview

[0028] It has been found that in the salt bath nitriding process, salt bath nitrocarburizing has the advantages of high efficiency and low cost. The salt bath nitriding process uses mixed salts containing cyanate ions as the raw materials for the salt bath solution. Therefore, in the salt bath process, an intermediate product of CN- ions will be produced under high temperature reaction. After production, this ion will be oxidized and consumed. The reaction formula is as follows:

[0029] △

[0030] 2CN-+O2→2CNO-

[0031] When the salt bath is stirred with compressed air, the above reaction process will inevitably occur. If the salt bath is not stirred, the above reaction will also occur, but the speed will be slower.

[0032] The main component of conventional liquid nitrocarburizing salt bath is CNO- ions, which usually exhibit two characteristics:

[0033] Reducing property: Due to the presence of "CN" group in this component, it has certain reducing property, which can reduce the oxides on the surface of parts and make the surface of parts shiny;

[0034] Oxidation: Due to the presence of "O" atoms, the component will sometimes show oxidation on the surface, which will cause the surface of the part to coarsen;

[0035] At the same time, CN- ions play an important role in the quality of nitrided parts. The reason is that CN- ions have strong reducing properties and can ensure the brightness of the surface of nitrided parts. Studies have shown that if the concentration of the by-product in the salt bath is very low or almost non-existent, it has little effect on the nitriding of conventional steel parts. However, it has a significant impact on the nitriding of stainless steel parts, causing the nitrided surface to be oxidized and rough, or even peeling.

[0036] Therefore, it is necessary to maintain a certain concentration of CN- ions in the salt bath, but the raw materials of this ion are highly toxic and difficult to replenish through raw materials. The research of this patented process is to maintain the ion concentration within a certain range without using toxic raw materials, so that the salt bath is suitable for salt bath soft nitriding treatment of any steel, and will not cause adverse phenomena such as rough peeling of the surface of parts.

[0037] Exemplary Process

[0038] like Figure 1 As shown, a salt bath nitriding process using different stirring gases to control the composition of the salt bath includes the following steps:

[0039] Prepare a salt bath solution, immerse the stainless steel parts after surface cleaning in the salt bath solution for salt bath nitriding, and monitor the concentration of the salt bath byproduct CN- ions in real time during the salt bath nitriding process;

[0040] 0.5 hours after the start of the salt bath nitriding process, different types of stirring gases are selected based on the concentration of CN- ions in the salt bath solution and stirred to keep the concentration of CN- ions as a byproduct of the salt bath between 1.5-3.0%; wherein the stirring gases include: pure nitrogen, compressed air and pure oxygen.

[0041] Specifically, the components of the salt bath solution include: 5-15% urea, 40-60% potassium nitrate, 20-40% potassium carbonate, 5-15% lithium chloride, and the temperature is maintained at 570-600°C during the salt bath nitriding process; the specific method for cleaning stainless steel parts is: use an alkaline detergent, including but not limited to NaOH solution, Na2CO3 solution, etc., and use a cleaning brush to flush the surface of the stainless steel parts until there is no greasy feeling when touched by hand to complete the cleaning of the surface of the stainless steel parts.

[0042] Urea (CO(NH2)2) serves as a nitrogen source and carbon source in the salt bath process. It decomposes at high temperature to produce ammonia (NH3), carbon dioxide (CO2) and isocyanic acid (HNCO). The latter can further decompose or react with metal ions to form cyanate ions (OCN-) and briefly generate cyanide ions (CN-) under reducing conditions; potassium nitrate (KNO3) serves as an oxidant to ensure that the final product is cyanate ions (CNO-) rather than cyanide. The oxygen provided by nitrate helps to oxidize any generated cyanide ions to cyanate ions; potassium carbonate (K2CO3) provides an alkaline environment, helps stabilize cyanate ions, and adjusts the pH value; lithium chloride (LiCl) serves as a diluent and co-solvent to help maintain the fluidity of the salt bath and improve thermal conductivity.

[0043] Under the high-temperature reaction of urea, potassium nitrate, potassium carbonate and lithium chloride, CN- ions and active nitrogen atoms are generated simultaneously. Eventually, the active nitrogen atoms combine with the metal in the stainless steel to form a dense nitride layer on the surface of the stainless steel. This nitride layer can significantly improve the surface hardness and wear resistance of the stainless steel and improve the overall performance of the stainless steel.

[0044] Specifically, pure nitrogen can be separated and extracted from the air by a nitrogen generator in the prior art, compressed air can be obtained by compressing an air compressor and drying it through a drying device, and pure oxygen can be bottled industrial pure oxygen that can be purchased on the market. These gases are easy to obtain and safe to operate, and the production cost is low.

[0045] When the concentration of CN- ions produced by high-temperature decomposition in the salt bath is lower than 1%, the salt bath will show obvious oxidizing properties, which is very unfavorable for the nitriding of stainless steel parts, and the surface is prone to rough peeling; when the concentration of CN- ions in the salt bath is higher than 4%, the nitriding reaction of CNO- ions begins to slow down, and the nitriding speed slows down; therefore, it is necessary to maintain the concentration of CN- ions between 1.5% and 3% for nitriding reaction, which can ensure the salt bath nitriding effect and efficiency of stainless steel parts, and can also reserve sufficient reaction time to make process adjustments to control the concentration of CN- ions in the salt bath solution to ensure the quality of salt bath nitriding of stainless steel parts.

[0046] Specifically, when the concentration of CN- ions in the salt bath solution is lower than 1.5%, pure nitrogen is used as a stirring gas to increase the concentration of CN- ions in the salt bath solution, and when the concentration of CN- ions in the salt bath solution reaches between 2.0% and 2.5%, the introduction of pure nitrogen is stopped;

[0047] Nitrogen is an inert gas with inactive chemical properties. It is difficult to react chemically with other substances. Continuously introducing a large amount of nitrogen into the salt bath solution can effectively dilute the concentration of oxygen in the salt bath solution, thereby reducing the contact opportunity between cyanide ions and oxygen, reducing the probability and rate of oxidation reaction of CN- ions, and reducing the consumption of CN- ions and the generation of CNO- ions. At the same time, with the continuous high-temperature salt bath, CN- ions are continuously decomposed in the salt bath pool, and the concentration of CN- ions in the salt bath solution will gradually increase, so as to control the concentration of cyanide ions in the salt bath solution within the specified range to ensure the quality of the salt bath nitriding process.

[0048] Specifically, when the concentration of CN- ions in the salt bath solution is between 3% and 4%, compressed air is used as a stirring gas to reduce the concentration of CN- ions in the salt bath solution, and when the concentration of CN- ions in the salt bath solution is reduced to between 2.0% and 2.5%, the introduction of compressed air is stopped;

[0049] When compressed air is introduced into the salt bath solution, the oxygen content in the solution will increase, thereby increasing the contact opportunity between CN- ions and oxygen, accelerating the oxidation of CN- ions, accelerating the consumption of CN- ions, and gradually reducing the concentration of CN- ions in the salt bath solution to keep it within the specified safety range to ensure the effect of salt bath nitriding of stainless steel parts.

[0050] Specifically, when the concentration of CN- ions in the salt bath solution is higher than 4%, pure oxygen gas is used as a stirring gas to reduce the concentration of CN- ions in the salt bath solution, and when the concentration of CN- ions in the salt bath solution is reduced to between 2.0% and 2.5%, the introduction of compressed air is stopped;

[0051] Compared with the introduction of compressed air, pure oxygen is introduced into the salt bath solution as the stirring gas. The oxygen content in the salt bath solution increases faster and more. Compared with the introduction of compressed air, CN- ions have more opportunities to contact oxygen, and the oxidation reaction rate of CN- ions is faster, that is, the concentration of CN- ions decreases faster, which can make the concentration of CN- ions return to the specified safety range faster, thereby improving the quality of the nitrided layer on the surface of stainless steel parts and improving the effect of salt bath nitriding.

[0052] Specifically, the pressure of the stirring gas introduced into the salt bath solution is set between 0.0180-0.022MPa; when the gas with too high pressure is introduced into the salt bath, the salt bath solution will roll violently, and even splash out of the salt bath tank, causing equipment damage and burns to personnel. Too low pressure will cause the gas to be confined to the local range of the salt bath solution, resulting in insufficient contact and reaction with the stainless steel parts, and the purpose of controlling the CN- ion concentration cannot be achieved, affecting the quality and rate of nitride formation on the surface of the stainless steel parts. The introduction of stirring gas with a pressure between 0.0180-0.022MPa can produce a significant effect of controlling the CN- ion concentration and maintain the stability of the salt bath process.

[0053] Specifically, the flow rate of the stirring gas introduced into the salt bath solution is set at 400-450L / h; the salt bath solution will also form nitride salts at high temperatures. If the gas with too high a flow rate enters the salt bath, it will accelerate the decomposition reaction of the nitride salts, resulting in unstable quality of the nitride layer produced on the surface of the stainless steel, which may produce a loose layer and affect the nitriding effect; if the gas flow rate is too low, the thickness and hardness of the nitride layer will not meet the standards, and the nitride layer will be uneven, and even the problem of partial non-nitridation on the surface of the stainless steel will occur.

[0054] Specifically, the specific method for real-time monitoring of CN- ion concentration includes: inserting the sampling pipeline of the online cyanide analyzer into the salt bath solution, and measuring and calculating the CN- ion concentration in real time through the sensor or chemical reaction module inside the online cyanide analyzer; referring to the prior art, the monitoring principle of the online cyanide analyzer is based on the electrochemical method, utilizing the electrochemical reaction of cyanide ions on the electrode, and calculating and determining its concentration by measuring the changes in electrode position or current.

[0055] Exemplary Systems

[0056] A ventilation system for stirring gas in a salt bath nitriding process, comprising: a salt bath circulation pipeline connected to the bottom of a salt bath solution, and a nitrogen delivery pipeline, an air delivery pipeline and an oxygen delivery pipeline connected to the salt bath circulation pipeline;

[0057] In the direction of the gas inlet ends of the nitrogen delivery pipeline, the air delivery pipeline and the oxygen delivery pipeline toward the salt bath circulation pipeline, there are arranged in sequence: a ventilation control switch, a pressure regulating device, a pressure monitoring device and a gas flow control device, which are respectively used for pipeline opening and closing control, air pressure regulation, air pressure measurement and limiting the gas flow rate of the gas introduced to ensure the safety and stability of the salt bath process. This structural setting facilitates independent control and adjustment of the output of different gases.

[0058] A nitrogen emission control device is also provided between the pressure monitoring device and the gas flow control device in the nitrogen delivery pipeline. A nitrogen emission pipeline connected to the nitrogen delivery pipeline is also provided in the nitrogen delivery pipeline between the nitrogen emission control device and the gas flow control device. A gas emission control valve is provided in the nitrogen emission pipeline. The purpose of this design is to facilitate the connection of a gas storage tank at the gas outlet of the nitrogen emission pipeline to achieve the purpose of recovering nitrogen. Nitrogen has a wide range of industrial uses, and at the same time, the preparation cost is relatively high. The recovery of residual nitrogen in the pipeline can effectively save cost expenses.

[0059] In the direction from the gas inlet end to the gas outlet end of the salt bath circulation pipeline, there are arranged in sequence: a gas flow total regulating device, a total control switch, a gas flow total monitoring device and a pressure total monitoring device; this facilitates the control of the gas flow of the entire system during the salt bath process to ensure the safety of the process.

[0060] Specifically, the pressure regulating device is a pressure reducing valve, which is used to control the pressure of the incoming gas between 0.0180-0.022MPa; the pressure monitoring device and the total pressure monitoring device are both set as pressure gauges with a monitoring range between 0-0.1MPa; the total gas flow monitoring device is set as a gas flow meter with a monitoring range between 0-30NL / min.

[0061] Embodiment 1

[0062] Using sodium hydroxide solution as a detergent, clean the surface of the stainless steel parts until the surface is not greasy to the touch, then prepare a salt bath solution in a salt bath using 10% urea (CO(NH2)2), 50% potassium nitrate (KNO3), 30% potassium carbonate (K2CO3), and 10% lithium chloride (LiCl) as raw materials, and extend the sampling tube of the online cyanide analyzer into the bottom of the salt bath, then immerse the stainless steel parts in the salt bath solution, and heat the salt bath solution to 580°C for nitriding;

[0063] 0.5 hours after the start of the salt bath nitriding process, observe the CN- ion concentration value obtained by the online cyanide analyzer. If the concentration value fluctuates between 1-3%, keep the main control switch in the ventilation system closed, without introducing stirring gas, and observe the CN- ion concentration every 0.5 hours.

[0064] Embodiment 2

[0065] Using sodium hydroxide solution as a detergent, clean the surface of the stainless steel parts until the surface is not greasy to the touch, then prepare a salt bath solution in a salt bath using 10% urea (CO(NH2)2), 50% potassium nitrate (KNO3), 30% potassium carbonate (K2CO3), and 10% lithium chloride (LiCl) as raw materials, and extend the sampling tube of the online cyanide analyzer into the bottom of the salt bath, then immerse the stainless steel parts in the salt bath solution, and heat the salt bath solution to 580°C for nitriding;

[0066] 0.5 h after the start of the salt bath nitriding process, observe the CN- ion concentration value obtained by the online cyanide analyzer. If the concentration value is lower than 1.5%, open the ventilation system, and introduce pure nitrogen with a gas flow rate of 450 L / h and a gas pressure of 0.02 MPa while continuously stirring until the CN- ion concentration reaches 2.5%. Stop the gas introduction and observe the CN- ion concentration every 0.5 h.

[0067] Embodiment 3

[0068] Using sodium hydroxide solution as a detergent, clean the surface of the stainless steel parts until the surface is not greasy to the touch, then prepare a salt bath solution in a salt bath using 10% urea (CO(NH2)2), 50% potassium nitrate (KNO3), 30% potassium carbonate (K2CO3), and 10% lithium chloride (LiCl) as raw materials, and extend the sampling tube of the online cyanide analyzer into the bottom of the salt bath, then immerse the stainless steel parts in the salt bath solution, and heat the salt bath solution to 580°C for nitriding;

[0069] 0.5 h after the start of the salt bath nitriding process, observe the CN- ion concentration value obtained by the online cyanide analyzer. If the concentration value is between 3-4%, open the ventilation system, introduce compressed air with a gas flow rate of 450L / h and a pressure of 0.02MPa, and continue stirring until the CN- ion concentration reaches 2.5%. Stop the gas introduction and observe the CN- ion concentration every 0.5 h.

[0070] Embodiment 4

[0071] Using sodium hydroxide solution as a detergent, clean the surface of the stainless steel parts until the surface is not greasy to the touch, then prepare a salt bath solution in a salt bath using 10% urea (CO(NH2)2), 50% potassium nitrate (KNO3), 30% potassium carbonate (K2CO3), and 10% lithium chloride (LiCl) as raw materials, and extend the sampling tube of the online cyanide analyzer into the bottom of the salt bath, then immerse the stainless steel parts in the salt bath solution, and heat the salt bath solution to 580°C for nitriding;

[0072] 0.5 h after the start of the salt bath nitriding process, observe the CN- ion concentration value obtained by the online cyanide analyzer. If the concentration value is higher than 4%, open the ventilation system, and introduce pure oxygen with a gas flow rate of 450 L / h and a gas pressure of 0.02 MPa, and continue stirring until the CN- ion concentration reaches 2.5%. Stop the gas introduction and observe the CN- ion concentration every 0.5 h.

[0073] According to the above-mentioned embodiments 2 to 4, the relationship between the concentration change of CN- ions and time from the beginning of the introduction of gas to the end of the introduction is shown in the following figure:

[0074] Table 1 Data on the change of CN- ion concentration over time after pure nitrogen was introduced in Example 2

[0075] 0.5h 1.0h 1.5h 2.0h 2.5h 3.0h CN-Concentration 1.506％ 1.514％ 1.522％ 1.529％ 1.539％ 1.547％

[0076] Table 2 Data on the change of CN- ion concentration over time after compressed air was introduced in Example 3

[0077] 0.5h 1.0h 1.5h 2.0h 2.5h 3.0h CN-Concentration 3.012％ 2.985％ 2.951％ 2.920％ 2.887％ 2.856％

[0078] Table 3 Data on the change of CN- ion concentration over time after pure oxygen was introduced in Example 4

[0079] 0.5h 1.0h 1.5h 2.0h 2.5h 3.0h CN-Concentration 4.010％ 3.947％ 3.876％ 3.811％ 3.761％ 3.702％

[0080] It can be seen from the above table that when pure nitrogen (N2) is used for stirring, the concentration of CN- ions in the salt bath can increase by 0.3-0.5% every 24 hours according to test tracking. When industrial pure oxygen (O2) is used for stirring, the CN- ion concentration can be reduced by about 0.12% per hour. When compressed air is used for stirring, the CN- ion concentration can be reduced by about 0.06% per hour. After the above gas is introduced, the surface of the stainless steel parts that are nitrided has no obvious oxidation, and the nitrided layer is not loose and evenly covers the surface of the parts. The present application can effectively control the concentration of CN- ions and is suitable for the salt bath nitriding process of any steel.

[0081] The above is based on the ideal embodiment of the present invention. Through the above description, relevant personnel can make various changes and modifications without departing from the technical concept of the present invention. The technical scope of the present invention is not limited to the content in the specification, and the technical scope must be determined according to the scope of the claims.

Claims

1. A salt bath nitriding process using different stirring gases to control the composition of the salt bath, characterized in that: The steps include: Prepare a salt bath solution, immerse the cleaned stainless steel parts in the salt bath solution for salt bath nitriding, and monitor the concentration of the salt bath byproduct CN- ions in real time during the salt bath nitriding process; 0.5 h after the start of the salt bath nitriding process, different types of stirring gases are selected based on the concentration of CN- ions in the salt bath solution and stirred to keep the concentration of CN- ions as a byproduct of the salt bath between 1.5% and 3.0%; wherein the stirring gases include: pure nitrogen, compressed air and pure oxygen; the CN- ion concentration is observed every 0.5 h.

2. A salt bath nitriding process using different stirring gases to control the composition of the salt bath according to claim 1, characterized in that: When the concentration of CN- ions in the salt bath solution is lower than 1.5%, pure nitrogen is used as stirring gas to increase the concentration of CN- ions in the salt bath solution. When the concentration of CN- ions in the salt bath solution reaches between 2.0% and 2.5%, the introduction of pure nitrogen is stopped.

3. A salt bath nitriding process using different stirring gases to control salt bath composition according to claim 2, characterized in that: When the concentration of CN- ions in the salt bath solution is between 3% and 4%, compressed air is used as stirring gas to reduce the concentration of CN- ions in the salt bath solution. When the concentration of CN- ions in the salt bath solution is reduced to between 2.0% and 2.5%, the introduction of compressed air is stopped.

4. A salt bath nitriding process using different stirring gases to control the composition of the salt bath according to claim 3, characterized in that: When the concentration of CN- ions in the salt bath solution is higher than 4%, pure oxygen is used as a stirring gas to reduce the concentration of CN- ions in the salt bath solution. When the concentration of CN- ions in the salt bath solution is reduced to between 2.0% and 2.5%, the introduction of compressed air is stopped.

5. The salt bath nitriding process using different stirring gases to control the composition of the salt bath according to claim 1, characterized in that: The components of the salt bath solution include: 5-15% urea, 40-60% potassium nitrate, 20-40% potassium carbonate and 5-15% lithium chloride.

6. A salt bath nitriding process using different stirring gases to control salt bath composition according to claim 1, characterized in that: The pressure of the stirring gas introduced into the salt bath solution is set between 0.0180-0.022 MPa.

7. The salt bath nitriding process using different stirring gases to control the composition of the salt bath according to claim 1, characterized in that: The flow rate of the stirring gas introduced into the salt bath solution was set at 400-450 L / h.

8. The salt bath nitriding process using different stirring gases to control the composition of the salt bath according to claim 1, characterized in that: The specific method for real-time monitoring of CN- ion concentration includes: inserting the sampling pipeline of the online cyanide analyzer into the salt bath solution, and measuring and calculating the CN- ion concentration in real time through the sensor or chemical reaction module inside the online cyanide analyzer.

9. A ventilation system for stirring gas in a salt bath nitriding process, suitable for a salt bath nitriding process using different stirring gases to control salt bath composition as claimed in any one of claims 1 to 8, characterized in that: include: a salt bath circulation pipeline connected to the bottom of the salt bath solution, and a nitrogen delivery pipeline, an air delivery pipeline and an oxygen delivery pipeline connected to the salt bath circulation pipeline; In the direction of the gas inlet ends of the nitrogen delivery pipeline, the air delivery pipeline and the oxygen delivery pipeline toward the salt bath circulation pipeline, a ventilation control switch, a pressure regulating device, a pressure monitoring device and a gas flow control device are sequentially arranged; A nitrogen emission control device is also provided between the pressure monitoring device and the gas flow control device in the nitrogen delivery pipeline, a nitrogen emission pipeline connected to the nitrogen delivery pipeline is also provided in the nitrogen delivery pipeline between the nitrogen emission control device and the gas flow control device, and a gas emission control valve is provided in the nitrogen emission pipeline; In the direction from the gas inlet end to the gas outlet end of the salt bath circulation pipeline, there are arranged in sequence: a gas flow total regulating device, a total control switch, a gas flow total monitoring device and a pressure total monitoring device.

10. The salt bath nitriding process using different stirring gases to control the composition of the salt bath according to claim 1, characterized in that: The pressure regulating device is a pressure reducing valve, which is used to control the pressure of the incoming gas between 0.0180-0.022MPa; the pressure monitoring device and the total pressure monitoring device are both set as pressure gauges with a monitoring range between 0-0.1MPa; the total gas flow monitoring device is set as a gas flow meter with a monitoring range between 0-30NL / min.

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