A salt bath soft nitriding process for toughening stainless steel parts

Abstract

This invention discloses a salt bath soft nitriding process for toughening stainless steel parts, comprising the following steps: S1, furnace cleaning: after removing grease from the surface of the parts using an alkaline cleaning agent, the parts are placed in a heat treatment furnace; S2, preheating co-diffusion: the temperature of the heat treatment furnace is increased to 300℃-400℃ and preheated for 60-80 minutes, and after drying the moisture, the parts are co-diffused in a salt bath at 550℃-640℃ for 3-10 hours; S3, cooling cleaning: the residual salt on the surface of the parts is removed by rinsing with running water, and then the surface moisture is dried at 100℃-130℃ for 5-8 minutes; S4, diffusion heat treatment: the parts are reloaded into a pre-vacuum nitrogen-protected heat treatment furnace, heated to 605℃-750℃, and maintained for 1-3 hours; S5, cooling treatment: the parts are cooled in the furnace to below 300℃ and then air-cooled to room temperature. A suitable polishing process is then selected according to the shape of the parts. In this invention, post-diffusion heat treatment can effectively reduce surface hardness and improve the toughness of the workpiece.

Description

Technical Field

[0001] This invention relates to the technical field of material heat treatment, and more particularly to a salt bath soft nitriding process for toughening stainless steel parts. Background Technology

[0002] Austenitic stainless steel has a wide range of applications due to its strong corrosion resistance and heat resistance; however, conventional heat treatment cannot strengthen it. The most common method is salt bath nitriding, which gives the surface a high-hardness nitrided layer to improve its wear resistance.

[0003] Nitrocarburizing involves simultaneously introducing nitrogen and carbon elements into the surface of a workpiece, altering the microstructure of the steel material and consequently changing its strength, friction properties, formability, and corrosion resistance under static loads and alternating stresses. After nitrocarburizing, the workpiece acquires a co-diffused microstructure on its surface, offering advantages such as simple equipment, easy operation, rapid infiltration rate, and low production cost.

[0004] The existing salt bath nitriding process for stainless steel parts has the following problems: 1. Conventional nitriding processes usually extend the nitriding time to obtain a deeper nitrided layer, which easily leads to a significant increase in surface nitrogen concentration. This results in significant volume expansion caused by nitrogen atoms, which greatly increases surface compressive stress. The phenomenon is that the nitrided layer is brittle and the sharp edges are prone to point-like chipping. 2. Salt bath soft nitriding, as a nitriding method with a very fast diffusion rate, lacks flexibility in concentration adjustment due to the characteristics of the salt bath itself. Increasing the concentration can be achieved by adding reagents, while decreasing it can only be achieved through consumption and self-decomposition. The lengthy process makes decreasing the concentration impractical for mass production. 3. Conventional salt bath nitriding can generally only obtain an extremely thin (about 0.05 mm) nitrided layer, which is hard and brittle and lacks toughness. Summary of the Invention

[0005] This invention overcomes the shortcomings of the prior art and provides a salt bath soft nitriding process for toughening nitriding of stainless steel parts. It aims to solve the problem of how to improve the nitriding layer formed by the salt bath while reducing the surface nitrogen concentration and improving the toughness of the parts after the salt bath.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a salt bath soft nitriding process for toughening nitriding of stainless steel parts, comprising the following steps:

[0007] S1. Furnace cleaning: After removing the grease from the surface of the parts with an alkaline cleaning agent, place the parts in the heat treatment furnace;

[0008] S2. Preheating co-diffusion: Increase the temperature of the heat treatment furnace to 300℃-400℃ and preheat for 60-80 minutes. After drying the moisture, perform salt bath nitrogen-carbon co-diffusion at 550℃-640℃ for 3-10 hours.

[0009] S3. Cooling and cleaning: Use running water to clean the surface of the parts to remove residual salt, and then dry the surface of the parts at 100℃-130℃ for 5-8 minutes.

[0010] S4. Diffusion heat treatment: Reinstall the parts in a pre-vacuum nitrogen-protected heat treatment furnace, heat to 605℃-750℃, and maintain the treatment time for 1-3 hours.

[0011] S5. Cooling treatment: When the diffusion part is cooled to below 300°C in the furnace, it is taken out of the furnace and air-cooled to room temperature. Then, a suitable polishing process is selected according to the shape of the part.

[0012] In step S3, direct water cooling is used to prevent the formation of oxides on the surface.

[0013] In a preferred embodiment of the present invention, in step S4, the processing temperature during the post-diffusion treatment is 605℃-620℃.

[0014] In a preferred embodiment of the present invention, in step S4, the processing temperature during the post-diffusion treatment is 620℃-650℃.

[0015] In a preferred embodiment of the present invention, in step S4, the processing temperature during the post-diffusion treatment is 650℃-750℃.

[0016] By adjusting the temperature during the diffusion process, the surface nitrogen concentration can be reduced, the brittleness of the nitrided layer can be decreased, and the depth of the nitrided layer can be appropriately increased, thereby reducing the hardness while greatly improving the toughness of the workpiece.

[0017] In a preferred embodiment of the present invention, in step S2, the raw material for the salt bath used for nitrocarburization is a mixed salt, and the components and their mass percentages of the mixed salt are as follows: (NH2)2CO 35%-40%, KOH 35%-45%, Na2CO3 15%-30%; and the melting point of the mixed salt is 447℃-460℃, and the specific gravity in the molten state is 1.70-1.73.

[0018] In a preferred embodiment of the present invention, in step S2, the raw material for the salt bath used in nitrocarbon percolation is a mixed salt, and the mixed salt contains Na + With K + The ratio is 1-2:3-4, the mass fraction of carbon content is 0.75%-0.95%, and the mass fraction of nitrogen content is 0.15%-0.35%.

[0019] In a preferred embodiment of the present invention, in step S2, the salt bath nitriding is divided into three stages, and each stage is heated by 15°C-30°C compared to the previous stage. By gradually increasing the temperature and holding the temperature at each stage, the firmness of the nitrided layer formed on the surface and the depth of nitriding are improved.

[0020] In a preferred embodiment of the present invention, in step S2, starting from the salt bath nitrocarbon co-infiltration, samples are taken from the salt bath furnace every 2-3 hours to determine the cyanide concentration. When the cyanide mass percentage reaches 2wt%, air and sulfur-containing compounds are introduced into the furnace until the cyanide mass percentage is below 2wt%. Through the reaction between sulfides and cyanides, the cyanide content is reduced, preventing harm to the environment and human body.

[0021] In a preferred embodiment of the present invention, in step S3, the workpiece is first cooled in a salt bath at 350-420°C for 5-10 minutes, and then cleaned with flowing water. The method of cleaning the salt residue with flowing water is to first treat it with flowing water at 360-373°C for 2-3 minutes, then reduce the temperature to 260-310°C at a rate of 1-10°C / min, and finally reduce the temperature to room temperature at a rate of 5-15°C / min. High-temperature water and low-temperature water are used in sequence to gradually cool the workpiece and prevent residual stress from being generated, which could lead to breakage and chipping.

[0022] It should be noted that the flowing water at 360-373℃ is water prepared by boiling under a high pressure environment of 21-23MPa.

[0023] In a preferred embodiment of the present invention, the sulfur-containing compound is one or more of sodium sulfide, potassium sulfide, sodium sulfite, or potassium sulfite.

[0024] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0025] (1) The stainless steel surface nitriding process involves placing the stainless steel in a salt bath of a certain concentration and adjusting its co-diffusion temperature to form a nitriding layer on the stainless steel surface. In the existing salt bath process, the concentration of the salt bath can be increased by adding reagents to make its concentration rise rapidly; however, the concentration can only be reduced by consumption and self-decomposition, which is a relatively long process. For mass production, the reduction is not feasible. In this invention, the post-diffusion heat treatment can improve intragranular segregation, eliminate internal stress, and improve the plasticity and toughness of stainless steel workpieces.

[0026] (2) Since nitrogen atoms are interstitial atoms, they are prone to lattice distortion and volume expansion. When nitriding is carried out for a long time, nitrogen atoms on the surface accumulate and the compressive stress increases significantly, making the surface nitrided layer brittle. By increasing the temperature stepwise during co-diffusion and holding it for a period of time, the slow heating can effectively reduce the lattice distortion caused by nitrogen atoms. At the same time, preheating treatment is used before co-diffusion to form an oxide film on the surface of the stainless steel workpiece, which has a certain promoting effect on the nitriding process and can reduce the thermal stress generated during salt bath heating and reduce thermal shock deformation.

[0027] (3) In this invention, after the salt bath nitrogen-carbon co-infiltration is completed, a diffusion heat treatment process is added. The annealing process increases the activation energy of lattice movement, and some defects release excess energy and exist in the form of lower energy. This results in reduced internal stress and improved integrity of the crystal. Optically, this results in improved crystal uniformity. On the other hand, it reduces the lattice distortion caused by nitrogen atoms and the increase in surface brittleness caused by lattice distortion. This solves the problem of sharp edges and corners appearing as point-like chipping caused by increasing the salt bath nitrogen-carbon time in the prior art, as well as other quality problems.

[0028] (4) Traditional salt bath co-infiltration process uses a constant temperature and maintains it for a period of time during co-infiltration, which may result in the nitriding layer formed at a single temperature not being firm enough on the stainless steel surface or floating on the surface. In this invention, a stepped co-infiltration temperature is used, gradually increasing the salt bath temperature and time, which is equivalent to increasing the number of co-infiltrations. As a result, the depth of the infiltrated layer and the surface hardness are significantly increased, and the wear resistance is also significantly improved after the surface wear resistance test.

[0029] (5) By cleaning the residual salt marks on the product surface with flowing water after the salt bath nitrocarburizing, the workpiece is also cooled by water cooling, which can prevent oxides from being generated on the surface of stainless steel workpieces; at the same time, the water temperature is controlled to gradually decrease from a higher temperature to a lower temperature, so that while cooling, the rapid cooling can prevent the workpiece from breaking or cracking and other damage.

[0030] (6) The diffusion heat treatment in this invention can effectively reduce the surface nitrogen concentration and reduce the brittleness of the nitrided layer, while also appropriately increasing the depth of the nitrided layer. When different diffusion temperatures are selected, stainless steel workpieces with different surface hardness can be obtained. The hardness of stainless steel after salt bath soft nitriding is usually 1000-1400HV, which is hard and brittle. However, by adjusting the diffusion temperature to above 750℃ through the process of this invention, a hardness of about 700HV can be obtained, which greatly improves the toughness of the workpiece.

[0031] (7) For general steel, a certain salt bath concentration needs to be maintained in order to obtain single-phase ε. However, when stainless steel is used for long-term nitriding within this concentration range, the concentration is too high. In this invention, diffusion heat treatment is used to perform carbonitriding in a high-concentration salt bath. On the one hand, this can improve the surface structure and prevent problems such as edge chipping and brittleness. On the other hand, the surface hardness and toughness can be adjusted according to the needs without adjusting the concentration of carbonitriding in the salt bath. This makes the salt bath production process smoother, improves work efficiency, and solves the problems of the lack of flexibility in concentration adjustment in the prior art and the lack of operability in mass production. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 This is a flowchart illustrating a preferred embodiment of the present invention;

[0034] Figure 2 This is a curve showing the diffusion temperature versus hardness according to a preferred embodiment of the present invention. Detailed Implementation

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0036] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0037] like Figure 1 The diagram shows a flow chart of a salt bath soft nitriding process for toughening stainless steel parts, which includes the following steps:

[0038] S1. Furnace cleaning: After removing the grease from the surface of the parts with an alkaline cleaning agent, place the parts in the heat treatment furnace;

[0039] S2. Preheating co-infiltration: Increase the temperature of the heat treatment furnace to 300℃-400℃ and preheat for 60-80 minutes. After drying the moisture, perform salt bath nitrocarburizing co-infiltration at 550℃-640℃ for 3-10 hours. The salt bath nitrocarburizing co-infiltration is divided into three stages, with each stage being 15℃-30℃ higher than the previous stage.

[0040] By adopting a stepped co-infiltration temperature, the salt bath temperature and time are gradually increased, which significantly increases the final infiltration layer depth, surface hardness and wear resistance, avoiding the situation that the nitrided layer may be weak or float on the surface that may be caused by constant temperature co-infiltration.

[0041] S3. Cooling and cleaning: First, cool in a salt bath at 350-420℃ for 5-10 minutes, then use running water to clean the surface of the parts to remove residual salt. The method of cleaning the salt with running water is to first treat with running water at 360-373℃ for 2-3 minutes, then reduce to 260-310℃ at a rate of 1-10℃ / min, and finally reduce to room temperature at a rate of 5-15℃ / min. Then, dry the surface moisture of the parts at a temperature of 100℃-130℃ for 5-8 minutes.

[0042] Using water cooling can prevent the formation of an oxide layer. At the same time, gradually reducing the temperature of the flowing water can prevent the workpiece from breaking or cracking due to internal stress caused by rapid cooling.

[0043] S4. Diffusion heat treatment: Reinstall the parts in a pre-vacuum nitrogen-protected heat treatment furnace, heat to 605℃-750℃, and maintain the treatment time for 1-3 hours.

[0044] Diffusion heat treatment can reduce surface hardness and increase toughness on the one hand, and deepen the nitriding layer on the other hand. At the same time, stainless steel with different surface hardness can be obtained at different diffusion temperatures.

[0045] S5. Cooling treatment: When the diffusion part is cooled to below 300°C in the furnace, it is taken out of the furnace and air-cooled to room temperature. Then, a suitable polishing process is selected according to the shape of the part.

[0046] After completing the above steps, depending on the product's color requirements (if black is required), an oxidation process can be added. The resulting stainless steel product is then soaked in rust-preventive oil for preservation.

[0047] Preferably, in step S2, the raw material for the salt bath used for nitrocarburizing is a mixed salt, and the components and their mass percentages of the mixed salt are as follows: (NH2)2CO is 35%-40%, KOH is 35%-45%, and Na2CO3 is 15%-30%; and the melting point of the mixed salt is 447℃-460℃, and the specific gravity in the molten state is 1.70-1.73.

[0048] Preferably, in step S2, the raw material for the salt bath used in nitrocarbon percolation is a mixed salt, and the mixed salt contains Na + With K + The ratio is 1-2:3-4, the mass fraction of carbon content is 0.75%-0.95%, and the mass fraction of nitrogen content is 0.15%-0.35%.

[0049] It should be noted that in step S2, starting from the salt bath nitrocarbon co-percolation, samples are taken from the salt bath furnace every 2-3 hours to determine the cyanide concentration. When the cyanide mass percentage reaches 2wt%, air and sulfur-containing compounds are introduced into the furnace until the cyanide mass percentage is below 2wt%. The sulfur-containing compounds are one or more of sodium sulfide, potassium sulfide, sodium sulfite, or potassium sulfite.

[0050] Example 1

[0051] S1. Furnace cleaning: After removing the grease from the surface of the parts with an alkaline cleaning agent, place the parts in the heat treatment furnace;

[0052] S2. Preheating co-infiltration: Increase the temperature of the heat treatment furnace to 320℃ and preheat for 60 minutes. After drying the moisture, perform salt bath nitrocarburizing co-infiltration at 550℃ for 1 hour. Then, increase the temperature to 570℃ for salt bath nitrocarburizing co-infiltration for 1 hour. Finally, increase the temperature to 600℃ for salt bath nitrocarburizing co-infiltration for 1 hour.

[0053] S3. Cooling and cleaning: First, cool in a salt bath at 350-420℃ for 5-10 minutes, then use running water to clean the surface of the parts to remove the residual salt. The method of cleaning the salt with running water is to first treat with running water at 360℃ for 2 minutes, then reduce to 280℃ at a rate of 5℃ / min, and finally reduce to room temperature at a rate of 10℃ / min. Then, dry the surface of the parts at 120℃ for 7 minutes.

[0054] S4. Diffusion heat treatment: The parts are reinstalled in a pre-vacuum nitrogen-protected heat treatment furnace, heated to 610°C, and maintained for 1 hour.

[0055] S5. Cooling treatment: When the diffusion part is cooled to below 300°C in the furnace, it is taken out of the furnace and air-cooled to room temperature. Then, a suitable polishing process is selected according to the shape of the part.

[0056] Example 2

[0057] S1. Furnace cleaning: After removing the grease from the surface of the parts with an alkaline cleaning agent, place the parts in the heat treatment furnace;

[0058] S2. Preheating co-infiltration: Increase the temperature of the heat treatment furnace to 320℃ and preheat for 60 minutes. After drying the moisture, perform salt bath nitrogen-carbon co-infiltration at 550℃ for 1 hour. Then, increase the temperature to 590℃ for salt bath nitrogen-carbon co-infiltration for 1 hour. Finally, increase the temperature to 630℃ for salt bath nitrogen-carbon co-infiltration for 1 hour.

[0059] S3. Cooling and cleaning: First, cool in a salt bath at 350-420℃ for 5-10 minutes, then use running water to clean the surface of the parts to remove the residual salt. The method of cleaning the salt with running water is to first treat with running water at 360℃ for 2 minutes, then reduce to 280℃ at a rate of 5℃ / min, and finally reduce to room temperature at a rate of 10℃ / min. Then, dry the surface of the parts at 120℃ for 7 minutes.

[0060] S4. Diffusion heat treatment: The parts are reinstalled in a pre-vacuum nitrogen-protected heat treatment furnace, heated to 610°C, and maintained for 1 hour.

[0061] S5. Cooling treatment: When the diffusion part is cooled to below 300°C in the furnace, it is taken out of the furnace and air-cooled to room temperature. Then, a suitable polishing process is selected according to the shape of the part.

[0062] Example 3

[0063] S1. Furnace cleaning: After removing the grease from the surface of the parts with an alkaline cleaning agent, place the parts in the heat treatment furnace;

[0064] S2. Preheating co-infiltration: Increase the temperature of the heat treatment furnace to 320℃ and preheat for 60 minutes. After drying the moisture, perform salt bath nitrogen-carbon co-infiltration at 550℃ for 1 hour. Then, increase the temperature to 590℃ for salt bath nitrogen-carbon co-infiltration for 1 hour. Finally, increase the temperature to 630℃ for salt bath nitrogen-carbon co-infiltration for 1 hour.

[0065] S3. Cooling and cleaning: First, cool in a salt bath at 350-420℃ for 5-10 minutes, then use running water to clean the surface of the parts to remove the residual salt. The method of cleaning the salt with running water is to first treat with running water at 360℃ for 2 minutes, then reduce to 280℃ at a rate of 5℃ / min, and finally reduce to room temperature at a rate of 10℃ / min. Then, dry the surface of the parts at 120℃ for 7 minutes.

[0066] S4. Diffusion heat treatment: The parts are reinstalled in a pre-vacuum nitrogen-protected heat treatment furnace, heated to 700°C, and maintained for 1 hour.

[0067] S5. Cooling treatment: When the diffusion part is cooled to below 300°C in the furnace, it is taken out of the furnace and air-cooled to room temperature. Then, a suitable polishing process is selected according to the shape of the part.

[0068] Measurements of Examples 1, 2, and 3 showed that the surface nitrided layer hardness was 1068 HV, 1105 HV, and 813 HV, respectively, and the fracture toughness was 130 MPa·m, respectively. 1 / 2 128MPa·m 1 / 2 and 142 MPa·m 1 / 2 The nitriding layer depths were 0.102 mm, 0.12 mm, and 0.14 mm, respectively.

[0069] Comparing Example 1 and Example 2, it can be seen that the greater the temperature range of the three stages of co-infiltration during the co-infiltration process, the greater the depth of the resulting nitrided layer.

[0070] Comparing Examples 2 and 3, it can be seen that the higher the temperature of the diffusion heat treatment, the lower the hardness of the resulting stainless steel surface, and relatively, its toughness is also improved.

[0071] Example 4

[0072] S1. Furnace cleaning: After removing the grease from the surface of the parts with an alkaline cleaning agent, place the parts in the heat treatment furnace;

[0073] S2. Preheating co-infiltration: Increase the temperature of the heat treatment furnace to 320℃ and preheat for 60 minutes. After drying the moisture, perform salt bath nitrogen-carbon co-infiltration at 550℃ for 1 hour. Then, increase the temperature to 590℃ for salt bath nitrogen-carbon co-infiltration for 2 hours. Finally, increase the temperature to 630℃ for salt bath nitrogen-carbon co-infiltration for 3 hours.

[0074] S3. Cooling and cleaning: First, cool in a salt bath at 350-420℃ for 5-10 minutes, then use running water to clean the surface of the parts to remove the residual salt. The method of cleaning the salt with running water is to first treat with running water at 360℃ for 2 minutes, then reduce to 280℃ at a rate of 5℃ / min, and finally reduce to room temperature at a rate of 10℃ / min. Then, dry the surface of the parts at 120℃ for 7 minutes.

[0075] S4. Diffusion heat treatment: The parts are reinstalled in a pre-vacuum nitrogen-protected heat treatment furnace, heated to 700°C, and maintained for 1 hour.

[0076] S5. Cooling treatment: When the diffusion part is cooled to below 300°C in the furnace, it is taken out of the furnace and air-cooled to room temperature. Then, a suitable polishing process is selected according to the shape of the part.

[0077] The surface hardness of the stainless steel in Example 4 was measured to be 866 HV, and the fracture toughness was 136 MPa·m. 1 / 2 The nitrided layer depth is 0.18 mm;

[0078] Comparing Examples 3 and 4, it can be seen that as the co-infiltration time in the three stages of the co-infiltration process gradually increases, the surface hardness and nitriding layer depth of the obtained stainless steel both increase.

[0079] Example 5

[0080] S1. Furnace cleaning: After removing the grease from the surface of the parts with an alkaline cleaning agent, place the parts in the heat treatment furnace;

[0081] S2. Preheating co-infiltration: Increase the temperature of the heat treatment furnace to 400℃ and preheat for 80 minutes. After drying the moisture, perform salt bath nitrogen-carbon co-infiltration at 550℃ for 1 hour. Then, increase the temperature to 590℃ for salt bath nitrogen-carbon co-infiltration for 2 hours. Finally, increase the temperature to 640℃ for salt bath nitrogen-carbon co-infiltration for 3 hours.

[0082] S3. Cooling and cleaning: First, cool in a salt bath at 350-420℃ for 5-10 minutes, then use running water to clean the surface of the parts to remove the residual salt. The method of cleaning the salt with running water is to first treat with 360℃ running water for 2 minutes, then reduce to 260℃ at a rate of 1℃ / min, and finally reduce to room temperature at a rate of 5℃ / min. Then, dry the surface of the parts at 130℃ for 8 minutes.

[0083] S4. Diffusion heat treatment: The parts are reinstalled in a pre-vacuum nitrogen-protected heat treatment furnace, heated to 700°C, and maintained for 2 hours.

[0084] S5. Cooling treatment: When the diffusion part is cooled to below 300°C in the furnace, it is taken out of the furnace and air-cooled to room temperature. Then, a suitable polishing process is selected according to the shape of the part.

[0085] Comparative Example 1

[0086] S1. Furnace cleaning: After removing the grease from the surface of the parts with an alkaline cleaning agent, place the parts in the heat treatment furnace;

[0087] S2. Preheating co-infiltration: Increase the temperature of the heat treatment furnace to 400℃ and preheat for 80 minutes. After drying the moisture, perform salt bath nitrogen-carbon co-infiltration at 550℃ for 1 hour. Then, increase the temperature to 590℃ for salt bath nitrogen-carbon co-infiltration for 2 hours. Finally, increase the temperature to 640℃ for salt bath nitrogen-carbon co-infiltration for 3 hours.

[0088] S3. Cooling and cleaning: First, cool in a salt bath at 350-420℃ for 5-10 minutes, then use running water to clean the surface of the parts to remove the residual salt. The method of cleaning the salt with running water is to first treat with 360℃ running water for 2 minutes, then reduce to 260℃ at a rate of 1℃ / min, and finally reduce to room temperature at a rate of 5℃ / min. Then, dry the surface of the parts at 130℃ for 8 minutes.

[0089] S4. Cooling treatment: When the diffusion part is cooled to below 300°C in the furnace, it is taken out of the furnace and air-cooled to room temperature. Then, a suitable polishing process is selected according to the shape of the part.

[0090] Measurements of Example 5 and Comparative Example 1 showed that the surface nitrided layer hardness was 847 HV and 1213 HV, respectively, and the fracture toughness was 129 MPa·m, respectively. 1 / 2 and 92 MPa·m 1 / 2 The nitriding layer depths were 0.151 mm and 0.095 mm, respectively.

[0091] Comparing Example 5 and Comparative Example 1, it can be seen that the surface hardness of the stainless steel workpiece obtained by diffusion heat treatment is significantly lower than that of the stainless steel workpiece obtained by conventional co-diffusion treatment, and the fracture toughness is also significantly improved. At the same time, diffusion heat treatment can also increase the depth of the nitriding layer.

[0092] Comparative Example 2

[0093] S1. Furnace cleaning: After removing the grease from the surface of the parts with an alkaline cleaning agent, place the parts in the heat treatment furnace;

[0094] S2. Preheating co-infiltration: Increase the temperature of the heat treatment furnace to 400℃ and preheat for 80 minutes. After drying the moisture, perform salt bath nitrogen-carbon co-infiltration at 550℃ for 1 hour. Then, increase the temperature to 590℃ for salt bath nitrogen-carbon co-infiltration for 2 hours. Finally, increase the temperature to 640℃ for salt bath nitrogen-carbon co-infiltration for 3 hours.

[0095] S3. Cooling and cleaning: Use running water to clean the surface of the parts to remove residual salt until room temperature, and then dry the surface of the parts at 130°C for 8 minutes.

[0096] S4. Diffusion heat treatment: The parts are reinstalled in a pre-vacuum nitrogen-protected heat treatment furnace, heated to 700°C, and maintained for 2 hours.

[0097] S5. Cooling treatment: When the diffusion part is cooled to below 300°C in the furnace, it is taken out of the furnace and air-cooled to room temperature. Then, a suitable polishing process is selected according to the shape of the part.

[0098] Comparative Example 2 was tested and found to have a surface hardness of 1042 HV and a fracture toughness of 84 MPa·m. 1 / 2 The nitrided layer depth is 0.081 mm;

[0099] Comparing Example 5 and Comparative Example 1, it can be seen that diffusion heat treatment can deepen the nitriding layer to a certain extent, and its surface hardness is also increased. However, before the measurement, slight chipping and cracks were found on the stainless steel surface of Comparative Example 2. It can be seen that the temperature gradient setting can eliminate the residual stress formed on the surface and inside of the stainless steel to the greatest extent, reducing the possibility of cracking, chipping, or pumping of the final product. The difference between the product curvature and the design curvature caused by the residual stress balance is also addressed. Therefore, different holding temperature ranges are set to prevent the temperature difference between the stainless steel surface and the environment due to the different thermal conduction rates, which could lead to surface quality defects and uncontrolled curvature.

[0100] like Figure 2 As shown in the figure, a curve of hardness corresponding to different diffusion temperatures in a salt bath soft nitriding process for toughening stainless steel parts is presented. It can be seen from the figure that the surface hardness after conventional salt bath soft nitriding is usually between 1000-1400 HV. However, by adjusting the temperature during diffusion heat treatment in this invention, the surface hardness of SUS304 stainless steel can be adjusted to about 700 HV at different diffusion heat treatment temperatures.

[0101] The specific correspondences are as follows: When the diffusion temperature range is 605-620℃, the hardness range of the obtained nitrided layer is 1020-1200HV, with a hardness of 1200HV at 605℃, 1105HV at 610℃, and 1020HV at 620℃; When the diffusion temperature range is 620-650℃, the hardness range of the obtained nitrided layer is 916-1020HV, with a hardness of 967HV at 630℃ and 916HV at 650℃; When the diffusion temperature range is 605-620℃, the hardness range of the obtained nitrided layer is 1020-1200HV, with a hardness of 823HV at 700℃ and 753HV at 750℃; When the diffusion temperature is greater than 750℃, the hardness range of the obtained nitrided layer is 700-753HV, and the minimum hardness approaches 700HV.

[0102] Therefore, even after nitriding in a high-concentration salt bath, the hardness and toughness can be adjusted to the required range, eliminating the need to adjust the concentration of the salt bath nitriding, making the salt bath nitriding production process smoother and improving work efficiency.

[0103] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A salt bath soft nitriding process for toughening nitriding of stainless steel parts, characterized in that, Includes the following steps: S1. Furnace cleaning: After removing the grease from the surface of the parts with an alkaline cleaning agent, place the parts in the heat treatment furnace; S2. Preheating co-infiltration: Increase the temperature of the heat treatment furnace to 300℃-400℃ and preheat for 60-80 minutes. After drying the moisture, perform salt bath nitrocarburizing co-infiltration at 550℃-640℃ for 3-10 hours. The salt bath nitrocarburizing co-infiltration is divided into three stages, and each stage is 15℃-30℃ higher than the previous stage. S3. Cooling and cleaning: Cool the parts in a salt bath at 350-420℃ for 5-10 minutes, then use running water to clean the parts to remove residual salt. The method of cleaning the salt with running water is to first treat with running water at 360-373℃ for 2-3 minutes, then reduce the temperature to 260-310℃ at a rate of 1-10℃ / min, and finally reduce the temperature to 150-250℃ at a rate of 5-15℃ / min. Afterwards, dry the surface moisture of the parts at a temperature of 100℃-130℃ for 5-8 minutes. S4. Diffusion heat treatment: Reinstall the parts in a pre-vacuum nitrogen-protected heat treatment furnace, heat to 605℃-750℃, and maintain the treatment time for 1-3 hours. S5. Cooling treatment: When the diffusion part is cooled to below 300°C in the furnace, it is taken out of the furnace and air-cooled to room temperature. Then, a suitable polishing process is selected according to the shape of the part.

2. The salt bath soft nitriding process for toughening stainless steel parts according to claim 1, characterized in that: In step S4, the processing temperature during diffusion heat treatment is 605℃-620℃.

3. The salt bath soft nitriding process for toughening stainless steel parts according to claim 1, characterized in that: In step S4, the processing temperature during diffusion heat treatment is 620℃-650℃.

4. The salt bath soft nitriding process for toughening stainless steel parts according to claim 1, characterized in that: In step S4, the processing temperature during diffusion heat treatment is 650℃-750℃.

5. The salt bath soft nitriding process for toughening stainless steel parts according to claim 1, characterized in that: In step S2, the raw material for the salt bath used for nitrocarburization is a mixed salt, and the components and their mass percentages of the mixed salt are as follows: (NH2)2CO is 35%-40%, KOH is 35%-45%, and Na2CO3 is 15%-30%; and the melting point of the mixed salt is 447℃-460℃, and the specific gravity in the molten state is 1.70-1.

73.

6. The salt bath soft nitriding process for toughening stainless steel parts according to claim 1, characterized in that: In step S2, the raw material for the salt bath used in nitrocarbon percolation is a mixed salt, and the mixed salt contains Na + With K + The ratio is 1-2:3-4, the mass fraction of carbon content is 0.75%-0.95%, and the mass fraction of nitrogen content is 0.15%-0.35%.

7. The salt bath soft nitriding process for toughening stainless steel parts according to claim 1, characterized in that: In step S2, starting from the salt bath nitrocarbon co-diffusion, samples are taken from the salt bath furnace every 2-3 hours to determine the cyanide concentration. When the cyanide mass percentage reaches 2wt%, air and sulfur-containing compounds are introduced into the furnace until the cyanide mass percentage is below 2wt%.

8. The salt bath soft nitriding process for toughening stainless steel parts according to claim 7, characterized in that: The sulfur-containing compound is one or more of sodium sulfide, potassium sulfide, sodium sulfite, or potassium sulfite.

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