A nitriding process method for medium-low carbon steel and nodular cast iron

Abstract

The application discloses a nitriding process method for medium-low carbon steel and nodular cast iron, and relates to the technical field of workpiece surface heat treatment, and comprises the following steps: heating, strong penetration, diffusion and cooling to perform the nitriding of products, accurately controlling the nitrogen potential, controlling the time and the flow of various gases, reducing the whole nitriding process time, improving the nitriding efficiency and the on-time delivery rate of products, and quickly nitriding the medium-low carbon steel and the nodular cast iron, so that the problem of slow nitriding speed in the existing nitriding process for nitriding the medium-low carbon steel and the nodular cast iron is solved, and the medium-low carbon steel and the nodular cast iron can be quickly nitrided.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of workpiece surface heat treatment, in particular to a nitriding process method for medium and low carbon steel and nodular cast iron. BACKGROUND

[0002] Nitriding is a surface nitriding treatment, that is, a chemical heat treatment process for making nitrogen atoms penetrate into the surface layer of a workpiece in a certain medium at a certain temperature. Automobile parts, machine tool parts, large mechanical parts and internal combustion engine parts belong to nitriding products. The existing nitriding treatment process usually adopts one-stage or two-stage nitriding process.

[0003] However, in the process of nitriding treatment by the existing one-stage or two-stage nitriding process, since the carbon steel and nodular cast iron material does not contain Cr, Mo, V and other nitride forming elements, the nitriding speed of such material is slow when gas nitriding.

[0004] Therefore, we need a nitriding process method for medium and low carbon steel and nodular cast iron to solve the problem of slow nitriding speed of the existing nitriding process for medium and low carbon steel and nodular cast iron, so that the medium and low carbon steel and nodular cast iron can be quickly nitrided. SUMMARY

[0005] The purpose of the application is to solve the problem of slow nitriding speed of the existing nitriding process for medium and low carbon steel and nodular cast iron. In order to solve the above problem, the application provides a nitriding process method for medium and low carbon steel and nodular cast iron, which can quickly nitride the medium and low carbon steel and nodular cast iron.

[0006] To achieve the above purpose, the embodiments of the application adopt the following technical solutions:

[0007] The application discloses a nitriding process method for medium-low carbon steel and nodular cast iron, which comprises a temperature rising section, a strong penetration section and a diffusion section.

[0008] In the technical scheme, the four-section nitriding and the control of the nitrogen potential can reduce the whole nitriding process time, improve the nitriding efficiency, and improve the product delivery rate, so that the medium-low carbon steel and the nodular cast iron can be quickly nitrided.

[0009] Further, in the step S100, the temperature rising speed is less than 75 DEG C per hour.

[0010] Further, in the step S200, the ammonia gas is limited in cracking, and the metal surface has low capacity of absorbing nitrogen atoms at low temperature, so the ammonia gas flow is 3-10 m 3 / h.

[0011] Further, in the step S300, the ammonia gas flow is 3-10 m 3 / h.

[0012] Further, in the step S400, the ammonia decomposition rate is 35%-45% to maintain enough nitrogen potential for strong penetration.

[0013] Further, in the step S500, the temperature rising speed is less than 30 DEG C per hour.

[0014] Further, according to the embodiment of the present application, the time for maintaining the temperature in the furnace at 600 DEG C for diffusion in the step S600 is greater than 3 hours.

[0015] Further, according to the embodiment of the present application, the time for maintaining the temperature in the furnace at 600 DEG C for diffusion in the step S600 is greater than 3 hours.

[0016] Further, according to the embodiment of the present application, the time for maintaining the temperature in the furnace at 600 DEG C for diffusion in the step S600 is greater than 3 hours.

[0017] Further, according to the embodiment of the present application, the time for maintaining the temperature in the furnace at 600 DEG C for diffusion in the step S600 is greater than 3 hours. 3 / h.

[0018] Compared with the prior art, the present application can reduce the whole nitriding process time, improve the nitriding efficiency, and improve the product delivery rate in time by the four-stage nitriding treatment and accurate control of the nitrogen potential, and solves the problem of slow nitriding speed of the existing nitriding process for nitriding treatment of medium and low carbon steel and nodular cast iron, and can quickly nitride the medium and low carbon steel and nodular cast iron. BRIEF DESCRIPTION OF DRAWINGS

[0019] The present application will be further described below in combination with the drawings and embodiments.

[0020] Figure 1 is a schematic diagram of a nitriding process method for medium and low carbon steel and nodular cast iron.

[0021] Figure 2 is a flow chart of a nitriding process method for medium and low carbon steel and nodular cast iron.

[0022] Figure 3 is a flow chart of a nitriding process method for medium and low carbon steel and nodular cast iron with added aluminum chloride. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical scheme of the present application clear, complete and the advantages more clear and obvious, the embodiments of the present application will be further described in combination with the drawings. It should be understood that the specific embodiments described herein are part of the embodiments of the present application, not all embodiments, and are only used to explain the embodiments of the present application, and do not limit the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0024] In the description of the present application, it should be noted that the terms "center", "middle", "upper", "lower", "left", "right", "inner", "outer", "top", "bottom", "side", "vertical", "horizontal" and the like indicate the orientation or positional relationship shown in the drawings, which are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "one", "first", "second", "third", "fourth", "fifth", "sixth" are only for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0025] In the description of the present application, it should be noted that unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connection" should be understood broadly, for example, it can be fixed connection, or detachable connection, or integral connection; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through intermediate medium, or it can be the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0026] For the purpose of simplicity and illustration, the principles of the embodiments are mainly described by referring to examples. In the following description, many specific details are presented to provide a thorough understanding of the embodiments. However, it is obvious to those skilled in the art that the embodiments can not be limited to these specific details in practice. In some examples, well-known methods and structures are not described in detail to avoid unnecessarily complicating the embodiments. In addition, all embodiments can be used in combination with each other.

[0027] The present application provides a nitriding process method for medium and low carbon steel and ductile cast iron, comprising:

[0028] The temperature rising section includes:

[0029] S100: hoist the nitriding product into the nitriding furnace for temperature rising, introduce nitrogen as convection medium for protection, the nitrogen flow rate is 3-8m 3 / h, so that the nitriding product in the furnace is uniformly heated, and the heating rate is less than 75℃ per hour.

[0030] Specifically, the product to be nitrided is sent into the nitriding furnace by hoisting, and nitrogen is introduced for protection, the nitrogen flow rate is 3-8m 3 / h, the heating rate is less than 75℃ per hour, which can ensure uniform heating and clean product surface.

[0031] S200: when the temperature in the furnace rises to 450℃, the temperature is kept for more than 2 hours, and ammonia gas is introduced to replace nitrogen gas, so as to provide nitrogen potential for the next stage of the nitrogenization accumulation furnace, the cracking of ammonia gas at low temperature is limited, and the ability of the metal surface to absorb nitrogen atoms at low temperature is very low, the flow rate of ammonia gas is 3-10m 3 / h.

[0032] Specifically, when the temperature in the nitrogenization furnace rises to 450℃, the temperature is kept for more than 2 hours, and the amount of ammonia gas introduced is considered according to the size of the furnace, the flow rate of ammonia gas is 3-10m 3 / h, and considering that the cracking of ammonia gas at low temperature is limited, and the ability of the metal surface to absorb nitrogen atoms at low temperature is very low, a small amount of ammonia gas is used to replace nitrogen gas, and the nitrogen potential for the next stage of the nitrogenization accumulation furnace is provided.

[0033] The strong penetration section includes:

[0034] S300: the temperature in the furnace is raised to 540℃ at a speed less than 45℃ / h, and the flow rate of ammonia gas is maintained, the flow rate of ammonia gas is 3-10m 3 / h.

[0035] Specifically, the temperature in the nitrogenization furnace is raised to 540℃, and the speed of temperature rise is controlled to be less than 45℃ / h, and the stable introduction of ammonia gas is maintained, the flow rate of ammonia gas is 3-10m 3 / h.

[0036] S400: the temperature in the furnace is kept at 540℃ for more than 1 hour, the nitrogen potential in the nitrogenization furnace is controlled by the ammonia decomposition rate or Kn value, the ammonia decomposition rate is 35%-45%, and the nitrogen potential in the furnace is maintained.

[0037] Specifically, the temperature in the nitrogenization furnace is kept at 540℃, and the strong penetration is kept for more than 1 hour, wherein the nitrogen potential in the nitrogenization furnace is controlled by the ammonia decomposition rate or Kn value, the ammonia decomposition rate is controlled to be 35%-45%, and the nitrogen potential in the furnace is maintained.

[0038] Further, the embodiment S400 further includes:

[0039] S410: a small amount of aluminum chloride powder is added from the sampling port of the nitrogenization furnace.

[0040] Specifically, a small amount of aluminum chloride powder is added into the sampling port of the nitriding furnace. At 540°C, a small amount of hydrogen produced by ammonia cracking will react with the aluminum chloride, promoting the enrichment of nitrogen atoms to the metal surface layer. Aluminum atoms will be adsorbed on the surface of the nitriding product and slowly penetrate into the carbon steel. At the same time, it will further form nitrides with the nitrogen atoms cracked by ammonia, forming a high-nitrogen potential layer on the surface of the nitriding product. The high nitrogen potential value in the furnace will cause more activated nitrogen atoms to quickly adsorb to the surface of the nitriding product.

[0041] The diffusion section includes:

[0042] S500: The temperature in the furnace is raised to 600°C to enter the nitriding diffusion stage. The ammonia flow is automatically controlled by the ammonia decomposition rate or Kn value. The temperature rising speed is less than 30°C / h.

[0043] S600: The temperature in the furnace is maintained at 600°C for diffusion. The diffusion time is greater than 3 hours, and the ammonia decomposition rate is maintained at 45%-60% to maintain the nitrogen potential in the furnace.

[0044] Further, the S600 of the embodiment further includes:

[0045] S610: Add aluminum chloride powder and ensure that there is aluminum chloride powder in the furnace to activate and promote penetration.

[0046] Specifically, at a temperature of 600°C, aluminum chloride powder reacts with hydrogen produced by ammonia cracking. Aluminum atoms will be adsorbed on the surface of the nitriding product and slowly penetrate into the carbon steel. At the same time, it will further form nitrides with the nitrogen atoms cracked by ammonia, forming a high-nitrogen potential layer on the surface of the nitriding product. The amount of activated aluminum atoms increases, and the speed of aluminum atoms penetrating the surface of the nitriding product increases. The nitrogen atoms and aluminum atoms on the surface of the metal will quickly diffuse into the metal to form a nitriding layer.

[0047] The cooling section includes:

[0048] S700: Maintain the ammonia decomposition rate at 45%-60% for cooling, and cool to 510°C.

[0049] S800: When the temperature in the furnace is reduced to 510°C, perform heat preservation. The ammonia decomposition rate is maintained at 30%-40% to maintain the nitrogen potential in the furnace. Finally, the surface hardness, wear resistance and corrosion resistance are strengthened. The heat preservation time is greater than 2 hours.

[0050] Specifically, the temperature of the nitriding furnace is maintained at 510°C, the heat preservation time is greater than 2 hours, the ammonia decomposition rate is controlled at 30%-40% to maintain the nitrogen potential in the furnace, and the surface hardness, wear resistance and corrosion resistance of the nitriding layer are further improved. At the same time, it is prepared for product cooling.

[0051] S900: the nitriding of the product is completed, cooling is performed, and the product is discharged from the furnace while being protected by nitrogen gas, the cooling is performed to a temperature of 100°C or lower, and the nitrogen gas flow rate during the protection by nitrogen gas is 3-8m 3 / h.

[0052] The furnace pressure is maintained at 1.5 Kpa during the entire process, and ammonia gas and aluminum chloride react under high temperature conditions:

[0053] 2NH3 = N2 + 3H2 2AlCl3 = 2Al + 3Cl2 H2 + Cl2 = 2HCl

[0054] Active nitrogen atoms and aluminum atoms split from ammonia gas are adsorbed by the surface of the nitriding product, and hydrogen and hydrogen chloride can well activate the metal surface to allow nitrogen and aluminum to better and faster penetrate the metal surface layer.

[0055] Generally, the roughness of the important mating surface of the nitriding product is between Ra0.8-Ra0.4, and the product is cleaned and rinsed before nitriding to make the surface of the nitriding product clean and free of dust and oil, so that nitrogen and aluminum atoms are more easily adsorbed to the metal surface. At a temperature of 500°C or higher, nitrogen atoms and aluminum atoms will penetrate into the metal along the intercrystalline of the nitriding product metal surface.

[0056] As the temperature rises, the kinetic energy of nitrogen atoms and aluminum atoms increases, and the volume of the metal also increases under the condition of heating, and the intercrystalline also increases, so that nitrogen atoms and aluminum atoms are more easily penetrated. Since aluminum is a strong nitride forming element, aluminum atoms penetrate into the intercrystalline of carbon steel and ductile cast iron and form dispersed nitrides with the penetrated nitrogen atoms to strengthen the surface hardness of the nitriding layer. At the same time, due to the penetration of aluminum atoms into the metal surface layer, a large amount of nitrogen atoms are enriched, which accelerates the penetration speed into the metal interior.

[0057] The process uses multi-stage heating and limited heating speed to effectively reduce the deformation of the product.

[0058] Through four-stage nitriding treatment and by adding aluminum chloride, the surface of the product can be activated to accelerate the rapid penetration of nitrogen ions. At the same time, the surface hardness and wear resistance of the nitriding product are improved, and the problem of slow nitriding speed during the existing nitriding process of medium and low carbon steel and ductile cast iron is solved. The medium and low carbon steel and ductile cast iron can be quickly nitrided, and the problem of poor surface hardness and wear resistance of the nitriding product after the rapid nitriding of the medium and low carbon steel and ductile cast iron is solved.

[0059] The technical solutions of the present application will be further described below by listing examples and comparative examples, but the present application is not limited to these examples.

[0060]

Example 1

[0061] The nitriding product is hoisted into the nitriding furnace for heating at a speed of 60°C per hour, nitrogen gas is introduced at a flow rate of 3m 3 / h; the temperature in the furnace is raised to 450°C for heat preservation, ammonia gas is introduced to replace the nitrogen gas at a flow rate of 3m 3 / h; the temperature in the furnace is raised to 540°C at a speed of 30°C / h, and the flow rate of ammonia gas is ensured to be 3m 3 / h; the temperature in the furnace is maintained at 540°C for 2.5 hours of strong penetration, the ammonia decomposition rate is controlled to be 35%, and a small amount of aluminum chloride powder is added from the sampling port of the nitriding furnace; the temperature in the furnace is raised to 600°C for diffusion at a speed of 15°C / h; the temperature in the furnace is maintained at 600°C for diffusion for 4.5 hours, aluminum chloride powder is added, and the ammonia decomposition rate is controlled to be 45%; the ammonia decomposition rate is maintained at 45% for cooling, and the temperature is reduced to 510°C; the temperature in the furnace is maintained at 510°C for heat preservation when the temperature in the furnace is reduced to 510°C, the ammonia decomposition rate is controlled to be 30%, and the heat preservation time is 3.5 hours; the nitriding of the product is completed, and the temperature is reduced to below 100°C, which is protected by nitrogen gas at a flow rate of 3m 3 / h, and the product is discharged.

[0062]

Example 2

[0063] The nitriding product is hoisted into the nitriding furnace for heating at a speed of 65°C per hour, nitrogen gas is introduced at a flow rate of 5m 3 / h; the temperature in the furnace is raised to 450°C for heat preservation, ammonia gas is introduced to replace the nitrogen gas at a flow rate of 6m 3 / h; the temperature in the furnace is raised to 540°C at a speed of 35°C / h, and the flow rate of ammonia gas is ensured to be 6m 3 / h; the temperature in the furnace is maintained at 540°C for 2 hours of strong penetration, the ammonia decomposition rate is controlled to be 40%, and a small amount of aluminum chloride powder is added from the sampling port of the nitriding furnace; the temperature in the furnace is raised to 600°C for diffusion at a speed of 20°C / h; the temperature in the furnace is maintained at 600°C for diffusion for 4 hours, aluminum chloride powder is added, and the ammonia decomposition rate is controlled to be 53%; the ammonia decomposition rate is maintained at 53% for cooling, and the temperature is reduced to 510°C; the temperature in the furnace is maintained at 510°C for heat preservation when the temperature in the furnace is reduced to 510°C, the ammonia decomposition rate is controlled to be 35%, and the heat preservation time is 3 hours; the nitriding of the product is completed, and the temperature is reduced to below 100°C, which is protected by nitrogen gas at a flow rate of 5m 3 / h, and the product is discharged.

[0064]

Example 3

[0065] The nitriding product is hoisted into the nitriding furnace for heating at a speed of 70°C per hour, nitrogen gas is introduced at a flow rate of 8m 3 / h; When the furnace temperature reaches 450℃, it is held for 2.5 hours, and ammonia is introduced to replace the nitrogen gas at a flow rate of 10m³ / h. 3 / h; Raise the furnace temperature to 540℃ at a rate of 40℃ / h, ensuring an ammonia flow rate of 10m³ / h. 3 / h; Maintain the furnace temperature at 540℃ for 1.5 hours of strong percolation, controlling the ammonia decomposition rate at 45%, and add a small amount of aluminum chloride powder into the sampling port of the nitriding furnace; Raise the furnace temperature to 600℃ for diffusion at a rate of 25℃ / h; Maintain the furnace temperature at 600℃ for diffusion for 3.5 hours, add aluminum chloride powder, and control the ammonia decomposition rate at 60%; Maintain the ammonia decomposition rate at 60% and cool down to 510℃; When the furnace temperature drops to 510℃, hold the temperature, controlling the ammonia decomposition rate at 40%, and hold for 2.5 hours; Nitriding of the product is complete, and then cool down to below 100℃, protected by nitrogen gas at a flow rate of 8m³ / h. 3 / h, out of the oven.

[0066] Comparative Example 1

[0067] The existing one-step nitriding process is used.

[0068] Comparative Example 2

[0069] The existing two-stage nitriding process is used.

[0070] Comparative Example 3

[0071] The nitrided product is hoisted into the nitriding furnace for heating at a rate of 80°C per hour, while nitrogen gas is introduced at a flow rate of 8 m³ / h. 3 / h; When the furnace temperature reaches 450℃, it is held for 1.5 hours, and ammonia is introduced to replace the nitrogen gas at a flow rate of 10m³ / h. 3 / h; Raise the furnace temperature to 540℃ at a rate of 50℃ / h, ensuring an ammonia flow rate of 10m³ / h. 3 / h; Maintain the furnace temperature at 540℃ for 0.5 hours of strong percolation, controlling the ammonia decomposition rate at 45%, and add a small amount of aluminum chloride powder into the sampling port of the nitriding furnace; Raise the furnace temperature to 600℃ for diffusion at a rate of 35℃ / h; Maintain the furnace temperature at 600℃ for diffusion for 2.5 hours, add aluminum chloride powder, and control the ammonia decomposition rate at 60%; Maintain the ammonia decomposition rate at 60% and cool down to 510℃; When the furnace temperature drops to 510℃, hold the temperature, controlling the ammonia decomposition rate at 40%, and hold for 1.5 hours; Nitriding of the product is complete, and then cool down to below 100℃, protected by nitrogen gas at a flow rate of 8m³ / h. 3 / h, out of the oven.

[0072] The steps of the above examples 1-3 and comparative examples 1-3 were applied to the same nitriding products to perform nitriding treatment, and the experimental data obtained according to the nitriding time from top to bottom were arranged as shown in Table 1:

[0073] Table 1

[0074]

[0075] As shown in Table 1, after using the nitriding process method of the medium and low carbon steel and the nodular cast iron of the present application, the time for nitriding treatment of the medium and low carbon steel and the nodular cast iron can be significantly reduced, and after rapidly nitriding the medium and low carbon steel and the nodular cast iron, the surface hardness and wear resistance of the nitriding products are better, and the deformation is less.

[0076] Although the above describes the specific embodiments of the present application in order to enable those skilled in the art to understand the present application, the present application is not limited to the scope of the specific embodiments, and for those skilled in the art, all applications created by utilizing the concept of the present application are within the protection scope of the present application as long as various changes are within the spirit and scope of the present application defined and determined by the appended claims.

Claims

1. A process for nitriding medium-low carbon steel and cast iron, characterized in that, Comprising: a temperature rising section, the temperature rising section comprising: S100: hoisting the nitriding product into the nitriding furnace for heating, and introducing nitrogen gas as the convection medium for protection, the nitrogen gas flow being 3-8 m 3 / h to uniformly heat the nitriding product in the furnace; S200: when the temperature in the furnace rises to 450℃, holding for more than 2 hours, and ammonia gas is introduced to replace the nitrogen gas for the next stage of nitrogen potential in the furnace; a strong penetration section, the strong penetration section comprising: S300: the temperature in the furnace is raised to 540℃, the speed of temperature rising is less than 45℃ / h, and the flow of ammonia gas is maintained; S400: the temperature in the furnace is maintained at 540℃ for more than 1 hour for strong penetration, the nitrogen potential in the furnace is controlled by ammonia decomposition rate, the ammonia decomposition rate is 35%-45%, and sufficient nitrogen potential in the furnace is maintained for strong penetration; a diffusion section, the diffusion section comprising: S500: the temperature in the furnace is raised to 600℃ to enter the nitrogen diffusion stage, and the flow of ammonia gas is automatically controlled by ammonia decomposition rate; S600: the temperature in the furnace is maintained at 600℃ for diffusion for more than 3 hours, the ammonia decomposition rate is 45%-60% to maintain the nitrogen potential in the furnace; a temperature decreasing section, the temperature decreasing section comprising: S700: the ammonia decomposition rate is maintained at 45%-60% for temperature decreasing to 510℃; S800: when the temperature in the furnace decreases to 510℃, holding for more than 2 hours, the ammonia decomposition rate is 30%-40% to maintain the nitrogen potential in the furnace, and finally the surface hardness is strengthened, and the wear and corrosion resistance is improved; S900: the nitriding of the product is completed, the temperature is decreased, the furnace is discharged by protection of nitrogen gas; aluminum chloride is added in the strong penetration section and the diffusion section.

2. The process for nitriding medium-low carbon steel and cast iron according to claim 1, characterized in that, The temperature rising speed in the S100 step is less than 75℃ per hour.

3. The process for nitriding medium-low carbon steel and cast iron according to claim 1, characterized in that, The cracking of ammonia gas at low temperature in the S200 step is limited, and the ability of the metal surface to absorb nitrogen atoms at low temperature is very low. The ammonia gas flow rate is 3-10 m 3 / h.

4. The process for nitriding medium-low carbon steel and cast iron according to claim 1, characterized in that, The flow rate of the ammonia gas in the S300 step is 3-10 m 3 / h.

5. The process for nitriding medium-low carbon steel and cast iron according to claim 1, characterized in that, The temperature rising speed in the S500 step is less than 30℃ / h.

6. The process for nitriding medium-low carbon steel and cast iron according to claim 1, characterized in that, The temperature decreasing in the S900 step is decreased to below 100℃.

7. The process for nitriding medium-low carbon steel and cast iron according to claim 1, characterized in that, The nitrogen flow rate for the protection by nitrogen in the S900 step is 3-8 m 3 / h.

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