A steel surface treatment process of surface nanocrystallization coupled with vacuum carburizing

Abstract

This invention discloses a surface treatment process for steel using surface nano-sizing coupled with vacuum carburizing, comprising the following steps: taking a steel part, performing pre-oxidation and carburizing treatments sequentially to form a hardened layer on the surface; directly heating the carburized steel part from the carburizing temperature to the austenitizing temperature, holding it at that temperature, and then rapidly cooling and quenching it; subjecting the quenched steel part to deep cryogenic treatment and medium-temperature tempering sequentially to obtain a primary part; performing surface nano-sizing treatment on the primary part to form a residual compressive stress layer with a thickness of millimeters to obtain an intermediate part; and performing a second cyclic cold tempering treatment on the intermediate part to obtain a finished part. This invention achieves optimized control of the surface hardness distribution of experimental steel, as well as further improvement of the outermost layer hardness. Simultaneously, through surface nano-sizing, the hardness gradient of the carburized steel transitions uniformly from the surface to the core, eliminating hardness pits in the steel.

Description

Technical Field

[0001] This invention relates to the field of thermochemical treatment, and more specifically to a surface treatment process for steel that involves surface nano-sizing coupled with vacuum carburizing. Background Technology

[0002] Surface-modified steel refers to steel that has undergone surface processing or heat treatment. Its characteristic is that the surface layer from the outer surface to a certain depth has properties different from the substrate. The aspects and degree of influence of these properties vary depending on the surface processing or heat treatment process. The bending fatigue performance, contact fatigue performance, and adhesion properties of surface-modified steel are closely related to the load-bearing capacity, service life, and reliability of the formed workpieces.

[0003] In existing technologies, this type of steel undergoes strong diffusion and infiltration heat treatment. However, under pulsed current, the carbon enriched on the surface tends to diffuse towards the core for a prolonged period, leading to a decrease in carbon content in the outermost 5-10 μm layer. After quenching, martensite is not formed, and retained austenite results in a softened layer (hardness pit) within the 5-10 μm surface region, affecting the surface hardness of the steel. Simultaneously, due to tensile stress caused by contact loads, cracks initiate in the hardened layer on the workpiece surface during service, resulting in a rapid crack initiation rate. This severely impacts fatigue performance and tribological properties, shortening the workpiece's service life.

[0004] To further eliminate the softened layer on the steel surface, existing technologies can employ methods such as direct heating to the austenitizing temperature, multiple quenching, and tempering to improve the isotropic nature of the in-plane microstructure and enhance batch stability. However, with the increase in processing methods, the time and energy costs also increase accordingly. The surface material remains susceptible to stress and cracking, making it difficult to further improve fatigue resistance. Summary of the Invention

[0005] The purpose of this invention is to provide a surface treatment process for steel by surface nano-coupling coupled with vacuum carburizing, so as to solve the technical problem of hardness pits in steel parts in the prior art.

[0006] To solve the above-mentioned technical problems, the present invention specifically provides the following technical solution:

[0007] This invention provides a surface treatment process for steel involving surface nano-sizing coupled with vacuum carburizing, comprising the following steps:

[0008] S100. Take a steel part and perform pre-oxidation and carburizing treatments on the steel part in sequence to form a hardened layer on the surface of the steel part.

[0009] S200: The carburized steel parts are directly heated from the carburizing temperature to the austenitizing temperature, held at the temperature, and then rapidly cooled and quenched. The quenched steel parts are then subjected to deep cryogenic treatment and medium-temperature tempering in sequence, and this process is repeated multiple times to obtain the primary part.

[0010] S300. The primary part is subjected to surface nano-processing, so that a residual compressive stress surface layer with a thickness of millimeters is attached to the primary part to obtain an intermediate part. The hardness gradient of the intermediate part is uniformly transitioned from the surface layer to the core, eliminating hardness pits.

[0011] S400. Perform a second cycle of cold return treatment on the intermediate component to obtain the finished product.

[0012] As a preferred embodiment of the present invention, in S300, the surface nano-processing conditions satisfy:

[0013] The two sides of the intermediate part are subjected to surface nano-sizing treatment with a total single-side indentation depth of 120 μm, a single-pass indentation depth of 10-15 mm, a rotation speed of 1000 r / min, and a scanning processing rate of 4 mm / min.

[0014] In a preferred embodiment of the present invention, in S100, the temperature range of the pre-oxidation is 800°C to 950°C.

[0015] As a preferred embodiment of the present invention, in S100, the carburizing process is pulsed vacuum carburizing;

[0016] The specific conditions for the carburizing treatment are: a temperature range of 930℃ to 980℃, a holding time of 10h to 40h, and a hardened layer with a depth range of 1.4mm to 2.5mm formed on the surface of the steel part.

[0017] In a preferred embodiment of the present invention, during the carburizing process, the strong carburizing stage and the diffusion stage are repeated sequentially, and the time ratio of the stage to the diffusion stage is 1:2 to 1:3.

[0018] As a preferred embodiment of the present invention, in S200, the carburized steel part is directly heated from the carburizing temperature to the austenitizing temperature in a vacuum environment.

[0019] The austenitizing temperature is 1000℃~1150℃;

[0020] The required heat preservation time is as follows: until the temperature inside and outside of the steel part is uniform, so that the steel part is completely austenitized, and until the austenite grains no longer grow.

[0021] In a preferred embodiment of the present invention, in S200, the cryogenic treatment includes the following steps:

[0022] The temperature of the steel part is lowered to -120℃ to -190℃ at a cooling rate of 2.5 to 6.0℃ / min, and then held at that temperature for 2 to 4 hours. After that, the steel part is warmed back to room temperature at a heating rate of 2.0 to 10.0℃ / min.

[0023] In a preferred embodiment of the present invention, in S200, the temperature of the medium-temperature tempering is 460℃-540℃, and the holding time is 1h-4h.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] This invention utilizes a self-developed novel surface nano-sizing process to treat the surface of steel parts, making the material surface nano-sized. The resulting compressive stress causes the residual austenite to transform into martensite, thereby increasing the hardness of the subsurface layer and making the hardness gradient after carburizing transition uniformly from the surface to the core, thus eliminating the hardness pits of the material and obtaining a modified layer structure with better surface fatigue resistance.

[0026] This invention combines vacuum carburizing with surface nano-sizing to achieve a residual compressive stress surface layer with a thickness of millimeters. The residual compressive stress can offset some of the tensile stress caused by contact loads, thereby reducing the possibility of crack initiation. Under compressive stress, the stress concentration at the crack tip is reduced, slowing down the rate of crack propagation along the material. Under contact loads, the residual compressive stress helps to distribute contact stress more evenly, improving the load-bearing capacity of the material in the contact area, reducing fatigue damage, and extending the fatigue life of the material.

[0027] This invention combines vacuum carburizing with surface nano-sizing and secondary cyclic cooling treatment, which makes it easy to obtain a gradient structure layer and a larger strain / hardness gradient, and achieve a residual compressive stress surface layer with a thickness of millimeters, thereby further improving the surface hardness of the material. Attached Figure Description

[0028] To more clearly illustrate the embodiments of the present invention or the technical solutions in 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 merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0029] Figure 1 A schematic flowchart of the steel surface treatment process of surface nano-coupling coupled vacuum carburizing is provided for the present invention.

[0030] Figure 2 The present invention provides a cross-sectional microhardness distribution diagram of the surface-modified structure test steel shown in Example 1. Detailed Implementation

[0031] 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.

[0032] like Figure 1 As shown, this invention provides a surface treatment process for steel using surface nano-sizing coupled with vacuum carburizing, comprising the following steps:

[0033] S100. Take a steel part and perform pre-oxidation and carburizing treatments on the steel part in sequence to form a hardened layer on the surface of the steel part.

[0034] S200, Heating, Quenching and Tempering: The carburized steel parts are directly heated from the carburizing temperature to the austenitizing temperature, held at that temperature and then rapidly cooled and quenched. The quenched steel parts are then subjected to deep cryogenic treatment and medium-temperature tempering in sequence, and this process is repeated multiple times to obtain the primary part.

[0035] S300. Perform surface nano-processing on the primary component to attach a residual compressive stress surface layer with a thickness of millimeters to obtain the intermediate component.

[0036] S400. Perform a second cycle of cold return treatment on the intermediate component to obtain the finished product.

[0037] This invention performs carburizing treatment and subsequent deep quenching and tempering treatment on modified layer steel parts, which can synthesize a hardened layer on the surface of the steel parts, so that the surface hardness of the primary part is greater than the hardness of the steel parts.

[0038] However, the problem is that the tensile stress caused by the contact load on the primary component can increase the possibility of crack initiation. The stress concentration at the crack tip can spread and cause the crack on the workpiece surface to propagate along the material, resulting in a reduction in the fatigue life of the material.

[0039] To improve the fatigue life of materials and slow down crack initiation and propagation rates, this invention utilizes a self-developed novel surface nano-sizing process. This process nano-sizing of the material surface generates compressive stress that eliminates residual austenite, transforming it into martensite to increase hardness. This ensures a uniform hardness gradient from the surface to the core after carburizing, eliminating hardness pits in the material. Combined with surface vacuum carburizing, this aims to obtain a modified layer structure with superior surface fatigue resistance. After surface nano-sizing of the carburized and tempered primary part, the surface strain rate is increased, the isotropic in-plane microstructure is improved, batch stability is enhanced, and the thickness of the nano-surface layer can be controlled.

[0040] Specifically, after surface nano-sizing treatment, a residual compressive stress layer with a thickness of millimeters is attached to the primary component. Firstly, the presence of residual compressive stress improves the load-bearing capacity of the material in the contact area. When an external load is applied to the material surface, the residual compressive stress can work together with the external load to improve the overall load-bearing capacity of the material.

[0041] It is evident that residual compressive stress on the surface is a highly favorable condition for improving contact fatigue performance. The reasons include, but are not limited to, the residual compressive stress on the surface can offset some of the tensile stress caused by contact load, thereby reducing the possibility of crack initiation; under compressive stress, the stress concentration at the crack tip is reduced, slowing down the rate of crack propagation along the material; under contact load, residual compressive stress helps to distribute contact stress more evenly, reducing local stress concentration, thereby reducing fatigue damage.

[0042] When an external load is applied to the surface of a material, the residual compressive stress can work together with the external load to improve the overall load-bearing capacity of the material. The surface residual compressive stress can significantly extend the fatigue life of the material.

[0043] After surface nano-sizing, the workpiece undergoes a second cycle of cold recovery treatment, which makes it easier to obtain a residual compressive stress surface layer with a larger strain / hardness gradient and a thickness of millimeters, thereby further improving the surface hardness.

[0044] In a preferred embodiment of the present invention, the surface nano-sizing treatment satisfies the following conditions:

[0045] The two sides of the intermediate part are subjected to surface nano-sizing treatment with a total single-side indentation depth of 120 μm, a single-pass indentation depth of 10-15 mm, a rotation speed of 1000 r / min, and a scanning processing rate of 4 mm / min.

[0046] This invention uses carburizing to form a thick hardened layer on the surface of the workpiece. After carburizing, surface nano-sizing is used to optimize the workpiece, generating a residual compressive stress surface layer with controllable nano-layer thickness on the steel surface. On the one hand, this can offset some of the tensile stress caused by contact load, thereby reducing the possibility of crack initiation. On the other hand, under compressive stress, the stress concentration at the crack tip is reduced, slowing down the rate of crack propagation along the material. At the same time, under contact load, the residual compressive stress helps to distribute contact stress more evenly, reducing local stress concentration and thus reducing fatigue damage.

[0047] In addition, the present invention employs the following two methods to avoid cracking caused by high internal stress due to severe lattice distortion and a large amount of carbide precipitation after carburizing: The first method is to eliminate stress through heat treatment. After carburizing, the steel is directly heated to the austenitizing temperature. The holding time is required until the internal and external temperatures of the steel part are uniform, so that the steel part is completely austenitized and the austenite grains stop growing. After the holding time is completed, rapid cooling and quenching are performed directly. At this time, the carbides caused by carburizing and the carbon atoms in the interstitial lattice will all dissolve into the austenite lattice and rearrange the internal grains, thereby eliminating internal stress more thoroughly.

[0048] The existing technology avoids workpiece cracking by slow cooling in the furnace. During the slow cooling process, network carbides are easily precipitated along the grain boundaries, which is an important origin of cracks. The second method to eliminate internal stress is the cooling method. After the workpiece is heated and quenched, it needs to be subjected to deep cryogenic treatment immediately. By rapidly cooling the workpiece, the internal grains can be rearranged to eliminate the internal stress caused by lattice distortion.

[0049] In cryogenic treatment, cryogenically cooled parts are generally brought to room temperature before being removed from the cryogenic treatment equipment. However, depending on the workpiece requirements, they can also be removed directly from the cryogenic equipment and allowed to warm naturally in the air. Post-treatment of cryogenically cooled parts should remove condensate from the surface of the workpiece. It is also worth noting that cryogenically cooled parts should be tempered promptly. The tempering process and parameters are determined according to the performance requirements of the workpiece.

[0050] Preferably, cryogenic treatment involves lowering the workpiece temperature to below -100°C, holding it at that temperature, and then restoring it to room temperature. Different cooling and heating rates affect the elimination of internal stress in the workpiece. Too rapid a temperature change can easily lead to unevenness within the material, causing cracking; too slow a change makes it difficult to remove internal stress. Therefore, the cooling rate of cryogenic treatment is limited to between 2.5 and 6.0°C / min. Cooling is stopped at -120°C to -190°C, and the workpiece is held at that temperature for 2 to 4 hours before being heated. The heating rate is limited to between 2.0 and 10.0°C / min to allow the workpiece to return to room temperature.

[0051] The specific cooling temperature and holding time are related to the material and size of the workpiece and can be adjusted according to actual production targets.

[0052] Furthermore, pre-oxidation treatment is a necessary condition for successful carburizing. It primarily addresses the issue of dense Cr-containing oxide films affecting the carburizing effect. The pre-oxidation treatment temperature range is 800℃~950℃, during which Fe oxides continue to form on the surface of the workpiece with a dense oxide film, loosening the film and facilitating the penetration of carbon elements during the carburizing process.

[0053] To improve carburizing efficiency, shorten carburizing time, and meet the carburizing requirements of workpieces with different carburized layers, the effective carburized layer thickness is 1.4mm to 2.5mm. The specific conditions for carburizing are: a temperature range of 930℃ to 980℃ and a holding time of 10h to 40h. During the carburizing process, the strong carburizing stage and the diffusion stage are repeated sequentially, with precise control over the time of the strong carburizing stage and the diffusion stage. The time ratio of the strong carburizing stage to the diffusion stage is 1:2 to 1:3. Under pulsed current, the carbon enriched on the surface diffuses towards the core for a relatively long time. The main purpose is to increase the depth of the hardened layer on the steel surface, increase the surface hardness of the workpiece to be further formed, and thus further improve the friction and wear resistance and hot hardness of the surface modified layer steel.

[0054] Because this material contains a very high concentration of carbon, the large amount of carbon infiltrated into the surface will cause severe lattice distortion, resulting in significant internal stress and making it prone to cracking. If the tempering temperature is too low, the energy will be insufficient to reduce internal stress and stabilize the microstructure. If the tempering temperature is too high and the tempering time is too long, more carbon in the matrix will precipitate from the grain boundaries, causing carbide growth and forming a network of carbides at the grain boundaries, leading to intergranular cracking.

[0055] Therefore, in this invention, the temperature of the medium-temperature tempering is strictly controlled between 460℃ and 540℃, and the time is 1h to 4h, which improves the overall hardness (from surface to core) and prevents the material from cracking. At the same time, tempering after quenching further improves the overall hardness (from surface to core), increases toughness, and enhances comprehensive mechanical properties.

[0056] Repeated deep cryogenic treatment and medium-temperature tempering processes on quenched workpieces also play a very important role in solving the problems of softened layer and pits.

[0057] The present invention also generally includes other steps:

[0058] 1) Before carburizing, perform pre-heat treatment. For gears with high requirements for distortion control, quenching and tempering is preferred. After quenching and tempering, the microstructure is tempered sorbite, and the cementite is spheroidal with a uniform and fine microstructure.

[0059] 2) After the pre-heat treatment, prepare the workpiece. Before carburizing, ensure the workpiece surface is clean, remove oxide scale, rust, oil and dirt, and perform non-destructive testing to ensure there are no cracks before carburizing. Take necessary anti-seepage protection measures for the anti-seepage parts.

[0060] 3) Repeat cryogenic treatment and medium-temperature tempering two or three times. The number of cryogenic treatments and temperings can be determined according to the shape and size of the workpiece, generally 2 to 4 times. Tempering can stabilize the structure, eliminate internal stress, and at the same time precipitate carbides to play a strengthening role.

[0061] The above method will be further illustrated by the following examples:

[0062] Example 1:

[0063] I. Material Selection:

[0064] The selected chemical composition of the Cr-containing stainless steel workpiece is C, Cr, Mo, V, Ni, Co and Nb elements, as well as incidental impurity elements. In this example, the C element content is 0.16%, the Cr element content is 13.5%, the Mo element content is 4.5%, the Co element content is 13.0%, the Ni element content is 2.2%, the V element content is 0.58%, and the Nb element content is 0.03%.

[0065] II. Heat Treatment Process

[0066] 1. Preparation

[0067] For gear workpieces with high requirements for distortion control, quenching and tempering is the first choice. After quenching and tempering, the microstructure is tempered sorbite, which is prepared as cementite. The cementite is spheroidal and the microstructure is uniform and fine.

[0068] Before carburizing, ensure the workpiece surface is clean, removing oxide scale, rust, oil, and stains;

[0069] Perform non-destructive testing to ensure there are no cracks before carburizing;

[0070] Seepage prevention and protection measures should be taken for seepage prevention areas.

[0071] 2. Pre-oxidation treatment

[0072] The workpiece is pre-oxidized at a temperature of 850℃. Under high temperature conditions, Fe oxides continue to form on the surface of the workpiece with a dense oxide film, so that the dense oxide film is loose and porous. The dense oxide film is mainly a Cr-containing dense oxide film.

[0073] 3. Carburizing

[0074] The workpiece was subjected to pulsed vacuum carburizing. During the carburizing process, the carburizing temperature was 960℃ and the carburizing time was 28h.

[0075] The strong carburizing stage and the diffusion stage are repeated in sequence, with a time ratio of 1:2 between the strong carburizing stage and the diffusion stage. The workpiece surface gradually hardens, and the effective carburized layer thickness obtained is 1.5 mm.

[0076] 4. Heating and quenching

[0077] In a vacuum furnace, the carburized workpiece is heated directly from the carburizing temperature to the austenitizing temperature, i.e., 1080℃. After holding at this temperature until the internal and external temperatures of the workpiece are uniform, it is then rapidly cooled and quenched using quenching oil.

[0078] 5. Cryogenic treatment

[0079] After quenching, the workpiece is cooled to -120℃ at a rate of 4℃ / min and held at that temperature for 3 hours.

[0080] The workpiece is cooled to room temperature (25℃) at a heating rate of 6℃ / min.

[0081] 6. Medium-temperature tempering

[0082] The cryogenically treated workpiece is reheated to 480℃ and held for 2.5 hours. Steps 5 and 6 are repeated three times to obtain a primary part, which is then used for the next step.

[0083] 8. Cleaning and testing

[0084] The surface oil stains of the primary part were cleaned in time, and its hardness distribution from the surface to the core and the surface carbon content were tested. The mass fraction of surface carbon of the primary part was 1.2%, which met the requirements.

[0085] 9. Surface nano-treatment

[0086] The two sides of the primary part are subjected to surface nano-sizing treatment with a total single-sided indentation depth of 120μm, a single-pass indentation depth of 10-15mm, a rotation speed of 1000r / min, and a scanning processing rate of 4mm / min to obtain the intermediate part.

[0087] 10. Cooling

[0088] The intermediate part is subjected to a second cycle of cold recovery treatment to obtain the finished part, namely the surface modified structural test steel of the present invention.

[0089] The surface hardness of the primary component, intermediate component, and finished product was tested, and the results are shown in [the table below]. Figure 2 As shown.

[0090] exist Figure 2 In the diagram, red represents the hardness after vacuum carburizing and tempering (primary part), green represents the hardness after surface nano-sizing (intermediate part), and blue represents the hardness after surface nano-sizing and tempering (finished part).

[0091] It is evident that vacuum carburizing combined with large surface deformation treatment enabled the optimization and control of surface hardness distribution in the experimental steel, as well as a further improvement in the hardness of the outermost layer.

[0092] This embodiment utilizes a self-developed novel surface nano-sizing process to optimize the surface hardness gradient distribution, explore the possibility of further improving surface hardness, and obtain a modified layer structure with superior surface fatigue resistance. By utilizing surface nano-sizing, the compressive stress generated eliminates residual austenite and transforms it into martensite to increase hardness, making the hardness gradient after carburizing transition uniformly from the surface to the core, thereby eliminating hardness pits.

[0093] The above embodiments are merely exemplary embodiments of this application and are not intended to limit this application. The scope of protection of this application is defined by the claims. Those skilled in the art can make various modifications or equivalent substitutions to this application within its substance and scope of protection, and such modifications or equivalent substitutions should also be considered to fall within the scope of protection of this application.

Claims

1. A surface treatment process for steel using nano-coupling coupled with vacuum carburizing, characterized in that, Includes the following steps: S100. Take a steel part and perform pre-oxidation and carburizing treatments on the steel part in sequence to form a hardened layer on the surface of the steel part. S200: The carburized steel parts are directly heated from the carburizing temperature to the austenitizing temperature, held at the temperature and then rapidly cooled and quenched. The quenched steel parts are then subjected to deep cryogenic treatment and medium-temperature tempering in sequence to obtain the primary parts. S300. The primary part is subjected to surface nano-processing, so that a residual compressive stress surface layer with a thickness of millimeters is attached to the primary part to obtain an intermediate part. The hardness gradient of the intermediate part is uniformly transitioned from the surface layer to the core, eliminating hardness pits. The surface nano-processing conditions must meet the following requirements: The two sides of the intermediate part are subjected to surface nano-sizing treatment with a total single-side indentation depth of 120 μm, a single-pass indentation depth of 10-15 mm, a rotation speed of 1000 r / min, and a scanning processing rate of 4 mm / min. S400. Perform a second cycle of cold recovery on the intermediate component to obtain the finished product; The cryogenic treatment includes the following steps: The temperature of the steel part is lowered to -120℃ to -190℃ at a cooling rate of 2.5~6.0℃ / min, and held at that temperature for 2h~4h. Then, the steel part is warmed back to room temperature at a heating rate of 2.0~10.0℃ / min. The temperature for the intermediate-temperature tempering is 460℃-540℃, and the holding time is 1h~4h.

2. The steel surface treatment process of surface nano-coupling coupled vacuum carburizing according to claim 1, characterized in that, In S100, the pre-oxidation temperature range is 800℃~950℃.

3. The steel surface treatment process of surface nano-coupling coupled vacuum carburizing according to claim 1, characterized in that, In S100, the carburizing process is pulsed vacuum carburizing; The specific conditions for the carburizing treatment are: a temperature range of 930℃ to 980℃, a holding time of 10h to 40h, and a hardened layer with a depth range of 1.4mm to 2.5mm formed on the surface of the steel part.

4. The steel surface treatment process of surface nano-coupling coupled vacuum carburizing according to claim 1, characterized in that, During the carburizing process, the strong carburizing stage and the diffusion stage are repeated sequentially, and the time ratio of the two stages is 1:2 to 1:

3.

5. The steel surface treatment process of surface nano-coupling coupled vacuum carburizing according to claim 1, characterized in that, In S200, the carburized steel part is directly heated from the carburizing temperature to the austenitizing temperature in a vacuum environment. The austenitizing temperature is 1000℃~1150℃; The required heat preservation time is as follows: until the temperature inside and outside of the steel part is uniform, so that the steel part is completely austenitized, and until the austenite grains no longer grow.

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