Controllable gradient and non-gradient nitriding layer, and preparation method and application thereof

Abstract

The application discloses a controllable gradient and non-gradient nitriding layer, a preparation method and application thereof, and belongs to the technical field of metal material surface strengthening. The application realizes controllable transformation of the structure type of the nitriding layer by adjusting and controlling the component characteristics of a base material and ion nitriding process parameters. The application realizes designable adjustment and control of the structure of the nitriding layer by establishing the corresponding relationship between the material component and the nitriding behavior, so that the target structure type can be selected according to different service requirements, and the application has wide application prospects in the field of metal material surface strengthening.

Description

Technical Field

[0001] This invention relates to the field of surface strengthening technology for metallic materials, and in particular to a controllable gradient and non-gradient nitriding layer, its preparation method, and its application. Background Technology

[0002] Ion nitriding is a chemical heat treatment process that strengthens and modifies the surface of stainless steel using glow discharge plasma technology. Ion nitriding is carried out in a vacuum chamber. A DC voltage is applied between the workpiece (cathode) and the chamber wall (anode), ionizing the rarefied nitrogen-containing gas to generate nitrogen-hydrogen plasma. Active nitrogen ions bombard and heat the workpiece surface under the influence of the electric field, penetrating the matrix through adsorption, dissolution, and diffusion processes, thus forming a surface-modified layer with excellent properties. Before ion nitriding, the stainless steel surface usually requires activation treatment, such as sandblasting, pickling, or pre-plating with titanium, to remove the inherent Cr passivation film that hinders nitrogen penetration. By controlling process parameters, such as operating within a lower temperature range, a diffusion layer dominated by nitrogen supersaturated solid solution, i.e., expanded austenite or S phase, can be formed on the surface of austenitic stainless steel. This diffusion layer can significantly increase the surface hardness to above HV 1000, greatly improving wear resistance and fatigue strength while maintaining or even enhancing the material's original corrosion resistance. Ion nitriding technology features low processing temperature, minimal workpiece deformation, uniform and dense nitrided layer, and is clean and environmentally friendly. It is widely used in the manufacture of high-performance stainless steel parts in fields such as medical devices, precision molds, chemical pumps and valves, and aerospace.

[0003] Ion nitriding can be used to directionally construct a nitrided layer with a specific hardness distribution on the material surface by adjusting process parameters, including gradient hardness or uniform hardness.

[0004] To achieve a gradient in the hardness of the nitrided layer, nitriding is typically performed at relatively high temperatures (e.g., 500-580°C) for extended periods. This promotes the deep diffusion of nitrogen atoms into the substrate, creating a significant nitrogen concentration gradient. Correspondingly, the strengthening effect decreases with increasing depth, resulting in a smooth hardness gradient between the high-hardness surface region and the core substrate. This type of nitrided layer exhibits higher bonding strength with the substrate and superior fatigue resistance and load-bearing capacity. For example, Chinese invention patent CN121472769A discloses a method for preparing a nitrided layer with adjustable thickness and a gradient hardness distribution on the mold surface. The preparation method of this invention includes the following steps: surface nano-sizing pretreatment, ultrasonic shot peening or surface mechanical grinding treatment of the mold surface, forming a nanocrystalline layer with a depth of not less than 50 μm and a grain size of <100 nm on the mold surface; the nanocrystalline layer is constructed by surface nano-sizing pretreatment, providing an efficient diffusion channel for nitrogen atoms. With the precise control of multi-field coupled plasma nitriding, the excessive formation of brittle white bright layer can be suppressed, and a gentle hardness gradient from the matrix to the nitrided layer can be achieved, taking into account both the surface hardness and core toughness of the mold.

[0005] To obtain a uniform, high-hardness nitrided layer, it is usually necessary to form a specific phase on the surface. For austenitic stainless steel, a low-temperature treatment below 450°C and a short treatment time are typically used to suppress the precipitation of Cr nitrides, promoting nitrogen supersaturation and solidification in the austenite lattice to form a uniform, high-hardness, and highly corrosion-resistant expanded austenite (S phase) layer. For tool steels, a continuous, dense, and extremely hard compound layer can be formed on the surface by controlling the nitrogen potential of the atmosphere. For example, Chinese invention patent CN121344522A provides a method for controlling the nitriding penetration layer of a die-casting mold. This solution combines dynamically adjusted nitrogen-hydrogen gas ratios with a zoned heating program based on the mold structure, and precisely controls the process transition point based on real-time monitoring of the nitriding reaction rate. This effectively solves the problems of uneven nitriding layer depth and unsatisfactory hardness caused by differences in heat capacity and heat dissipation conditions in different parts of irregular die-casting molds during traditional nitriding. As a result, a high-quality nitriding layer with uniform thickness and dense structure can be obtained simultaneously on the entire mold surface, which is beneficial to improving the uniformity of hardness distribution, thermal fatigue resistance, wear resistance and overall service life of the die-casting mold.

[0006] Because nitrogen diffusion and nitrogen solid solution behaviors differ during the formation of gradient and uniform layers, involving numerous parameters, existing processes typically design corresponding processing flows based on specific requirements, limiting process flexibility. This paper proposes a method for preparing controllable gradient and non-gradient nitriding layers. Through simple raw material and parameter control, this method enables the preparation of nitriding layers with two different hardness characteristics within a single process system, adapting to diverse application needs. This is of significant importance in the field of surface strengthening of metallic materials. Summary of the Invention

[0007] In view of the above-mentioned deficiencies of the prior art, the purpose of the present invention is to achieve a controllable transformation of the nitriding layer structure type by adjusting the composition characteristics of the substrate and the parameters of the ion nitriding process, design and construct the correspondence between the material composition and the nitriding behavior, realize the designable control of the nitriding layer structure, and thus select the target structure type according to different service requirements.

[0008] To achieve the above objectives, the technical solution provided by the present invention is as follows: In a first aspect of the present invention, a method for preparing controllable gradient and non-gradient nitriding layers is provided, comprising the following steps: After surface pretreatment of the steel substrate, ion nitriding is carried out in a nitrogen-containing atmosphere. By synergistically controlling the alloy composition and process parameters of the steel substrate, nitriding layers of different structural types are formed on the material surface. When the nitriding layer is a non-gradient nitriding layer, the mass fraction of Cr element in the steel substrate is controlled to be 18 wt.%~25 wt.%, and ion nitriding treatment is carried out at 480~540℃. When the nitriding layer is a gradient nitriding layer, the mass fraction of Cr in the steel substrate is controlled to be 1 wt.%~5 wt.%, and ion nitriding treatment is carried out at 480~540℃.

[0009] Preferably, the surface pretreatment includes surface polishing and / or ultrasonic cleaning.

[0010] Preferably, when the target nitriding layer is a non-gradient nitriding layer, the ion nitriding treatment time is 6-12 h and the reaction gas pressure is 300-600 Pa.

[0011] Preferably, when the target nitriding layer is a gradient nitriding layer, the ion nitriding treatment time is 6-12 h and the reaction gas pressure is 300-600 Pa.

[0012] Preferably, in the non-gradient nitriding layer, the Vickers hardness remains relatively stable within the nitriding layer region, the fluctuation range of the Vickers hardness (HV0.1) does not exceed ±10% of its average value, and the decrease range at the interface between the nitriding layer and the steel substrate is ≥30%.

[0013] Preferably, the non-gradient nitriding layer comprises a CrN phase and a γ′-Fe4N phase, wherein the volume fraction of the CrN phase is ≥60%; the phase structure of the gradient nitriding layer comprises a γ′-Fe4N phase and an ε-Fe3N phase.

[0014] Preferably, the nitrogen concentration in the gradient nitriding layer decreases along the depth direction, forming a diffusion transition zone.

[0015] In this solution, there are various methods for controlling the Cr element in the steel substrate. An active adjustment method can be used, introducing Cr into the substrate; or a passive method can be used, selecting commercially available steel that meets the Cr content range. By employing either of these methods, the Cr content can be controlled within the relevant range, thus achieving the objective of this invention.

[0016] In a second aspect of the present invention, a controllable gradient and non-gradient nitriding layer is provided, which is prepared using the preparation method of the first aspect of the present invention.

[0017] In a third aspect of the invention, the application of the controllable gradient and non-gradient nitriding layers of the second aspect of the invention is provided, including: as a protective layer for surface strengthening of metallic materials.

[0018] Preferably, the non-gradient nitriding layer is used to improve the surface hardness and / or wear resistance of the substrate, and the gradient nitriding layer is used to improve the fatigue resistance and / or impact resistance of the substrate.

[0019] Based on the above technical solutions, the design concept and principle of this invention are as follows: This invention achieves controllable transformation of the nitriding layer structure type by adjusting the Cr composition characteristics of the substrate and the ion nitriding process parameters. When the Cr content in the substrate is high, a dense compound layer preferentially forms within a specific designed temperature range, exhibiting a sudden change in hardness distribution. When the substrate is a medium- or low-Cr alloy steel, nitrogen continuously diffuses along the depth direction, forming a gradient-distributed diffusion layer structure. This invention establishes a correspondence between material composition and nitriding behavior, enabling the designable control of the nitriding layer structure, thereby allowing the selection of the target structure type according to different service requirements.

[0020] This invention discovers that although both Cr content and nitriding temperature affect the formation behavior of nitride phases, a controllable transformation of the nitrided layer structure type can only be achieved under the synergistic effect of a specific composition range and temperature window designed in this invention. Therefore, this invention utilizes the decisive influence of a specific Cr content on nitrogen atom diffusion behavior within a designed temperature range to control the nitrided layer structure type. Compared to traditional ion nitriding processes that require trying various process parameters such as temperature, time, and atmosphere to optimize results for a given material, this significantly reduces process complexity.

[0021] Specifically, when the Cr mass fraction is between 18 wt.% and 25 wt.% and the nitriding temperature is controlled within the range of 480 to 540°C, Cr preferentially reacts with nitrogen to form the CrN phase, creating a dense compound layer on the material surface. This compound layer significantly hinders further diffusion of nitrogen atoms, thus inhibiting the development of the diffusion layer. This results in abrupt hardness distribution at the interface, making it suitable for applications requiring extremely high surface hardness and wear resistance. When the Cr mass fraction is within the design range of 2 wt.% to 5 wt.%, the Cr content is relatively low, and its binding effect on nitrogen is significantly weakened. Nitrogen atoms can continuously diffuse into the matrix, forming a continuous diffusion layer, exhibiting a gradient distribution of hardness along the depth direction. When the above composition and temperature combination conditions are deviated from, the nitriding layer structure will change significantly. In this case, an excessively thick and brittle compound layer may form, or the strengthening effect may be insufficient due to poor hardenability. It is difficult to obtain a stable gradient or non-gradient structure simultaneously, thus failing to achieve the structural control effect of this invention.

[0022] Based on the above design, by simply determining the Cr content of the workpiece material and controlling the nitriding temperature within this critical window, the target nitriding layer structure can be predicted and obtained, significantly improving the flexibility and specificity of the process.

[0023] Compared with the prior art, the present invention has the following advantages and beneficial effects: This invention provides controllable gradient and non-gradient nitriding layers, their preparation methods, and applications. By adjusting the compositional characteristics of the substrate and the parameters of the ion nitriding process, the structural type of the nitriding layer can be controllably transformed. By establishing a correspondence between material composition and nitriding behavior, this invention enables the design and controllability of the nitriding layer structure, allowing for the selection of target structural types according to different service requirements. This invention has broad application prospects in the field of surface strengthening of metallic materials. Attached Figure Description

[0024] Figure 1 A comparison diagram of the cross-sectional structures of gradient nitriding layer and non-gradient nitriding layer; Figure 2 The hardness distribution curves of gradient nitrided layers and non-gradient nitrided layers as a function of depth are shown. Figure 3 Schematic diagram of the structural types of gradient nitriding layer and non-gradient nitriding layer; Figure 4 The X-ray diffraction patterns are shown for the gradient nitrided layer and the non-gradient nitrided layer. Detailed Implementation

[0025] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0026] Example 1 This embodiment provides a method for preparing controllable gradient and non-gradient nitriding layers, specifically a non-gradient nitriding layer, the steps of which are as follows: F53 duplex stainless steel with a Cr content of approximately 22 wt.% was selected as the substrate. After surface pretreatment, the substrate was placed in an ion nitriding apparatus. Ion nitriding was performed under a nitrogen atmosphere at 500℃, 400 Pa, and for 9 h to obtain a non-gradient nitrided layer with a thickness of approximately 80 μm.

[0027] Example 2 This embodiment provides a method for preparing controllable gradient and non-gradient nitriding layers, specifically a gradient nitriding layer, with the following steps: 35CrMnSiA steel with a Cr content of approximately 1 wt.% was selected as the substrate. After surface pretreatment, the substrate was placed in an ion nitriding apparatus. Ion nitriding was performed under a nitrogen atmosphere at 500℃, 400 Pa, and for 9 h to obtain a gradient nitrided layer.

[0028] Example 3 This embodiment provides a method for preparing a non-gradient nitriding layer.

[0029] A duplex stainless steel with a Cr content of approximately 18 wt.% was selected as the substrate. After surface pretreatment, the substrate was placed in an ion nitriding apparatus. Ion nitriding was performed under a nitrogen-containing atmosphere at 480°C, 300 Pa, and for 6 hours. This example illustrates that a non-gradient nitrided layer can still be formed under the lower limit of the parameter range of this invention.

[0030] Example 4 This embodiment provides a method for preparing a non-gradient nitriding layer.

[0031] A duplex stainless steel with a Cr content of approximately 25 wt.% was selected as the substrate. After surface pretreatment, the substrate was placed in an ion nitriding apparatus. Ion nitriding was performed under a nitrogen-containing atmosphere at 540°C, 600 Pa, and for 12 h. This example illustrates that a non-gradient nitrided layer dominated by a compound layer can still be formed even under the upper limit of the parameter range of this invention.

[0032] Example 5 This embodiment provides a method for preparing a non-gradient nitriding layer.

[0033] A duplex stainless steel with a Cr content of approximately 20 wt.% was selected as the substrate. After surface pretreatment, the substrate was placed in an ion nitriding apparatus. Ion nitriding was performed under a nitrogen atmosphere at 525°C, 300 Pa, and for 12 h. This example illustrates that the present invention can still achieve the preparation of a non-gradient nitrided layer under intermediate Cr contents and different combinations of pressure and time.

[0034] Example 6 This embodiment provides a method for preparing a gradient nitriding layer.

[0035] Alloy steel with a Cr content of approximately 2 wt.% was selected as the substrate. After surface pretreatment, the substrate was placed in an ion nitriding apparatus. Ion nitriding was performed under a nitrogen-containing atmosphere at 480°C, 300 Pa, and for 6 hours. This example illustrates that a gradient nitrided layer can still be formed under the lower limit of the parameters of this invention.

[0036] Example 7 This embodiment provides a method for preparing a gradient nitriding layer.

[0037] Alloy steel with a Cr content of approximately 5 wt.% was selected as the substrate. After surface pretreatment, the substrate was placed in an ion nitriding apparatus. Ion nitriding was performed under a nitrogen-containing atmosphere at 540 °C, 600 Pa, and for 12 h. This example illustrates that a gradient nitrided layer can still be formed under the upper limit of the parameter range of the present invention.

[0038] Example 8 This embodiment provides a method for preparing a gradient nitriding layer.

[0039] Alloy steel with a Cr content of approximately 3 wt.% was selected as the substrate. After surface pretreatment, the substrate was placed in an ion nitriding apparatus. Ion nitriding was performed under a nitrogen-containing atmosphere at 500°C, 300 Pa, and for 12 h. This example illustrates that the present invention can still achieve the preparation of a gradient nitrided layer under intermediate Cr contents and different combinations of pressure and time.

[0040] Example 9 This embodiment provides a non-gradient nitriding layer and a gradient nitriding layer, prepared using the methods described in Examples 1 and 2, respectively. This embodiment characterizes the cross-sectional structures of the two nitriding layers, as follows: Figure 1 As shown, scanning electron microscopy was used for observation. (a) and (b) illustrate the structures of the gradient nitriding layer and the non-gradient nitriding layer obtained when the substrates are 35CrMnSiA steel and F53 duplex stainless steel, respectively. The low-Cr content material forms a distinct diffusion layer structure, while the high-Cr content material forms a dense and continuous compound layer structure, indicating a significant difference in the nitriding layer structure under different Cr contents. This result confirms the technical solution of this invention, which achieves controllable transformation of the nitriding layer structure type by adjusting the Cr content.

[0041] Example 10 This embodiment tests the hardness distribution of the non-gradient nitriding layer and the gradient nitriding layer of Example 9 to study the effect of the two nitriding layers in practical applications. This embodiment can also be regarded as an application embodiment of controllable gradient and non-gradient nitriding layers.

[0042] The test was conducted using a Vickers microhardness tester, such as... Figure 2 As shown, (a) and (b) present the hardness distribution curves of the gradient nitriding layer and the non-gradient nitriding layer, respectively. Figure 2 It is evident that the hardness in the gradient nitriding layer decreases continuously along the depth direction, while the non-gradient nitriding layer exhibits a significant abrupt decrease at the interface. This result demonstrates that different nitriding layer structures exhibit significant differences in mechanical properties, further validating the structural control effect of this invention.

[0043] Figure 3 This is a schematic diagram of the nitriding layer structure type of the present invention. This diagram is combined with... Figure 1 Cross-sectional structural features and Figure 2The hardness distribution results are used to summarize the different nitriding layer structures. The non-gradient nitriding layer exhibits an abrupt transition structure between the dense surface compound layer and the matrix, corresponding to an abrupt hardness distribution; the gradient nitriding layer exhibits a gradual transition from the diffusion layer to the matrix structure, corresponding to a continuous hardness gradient distribution. This schematic diagram summarizes the technical principle of the invention from both structural and performance perspectives, namely, achieving designable and controllable nitriding layer structure types through the synergistic control of material composition and process parameters.

[0044] In addition, X-ray diffraction was used to characterize the nitrided layers of the high-Cr content material (F53 duplex stainless steel) and the low-Cr content material (35CrMnSiA) in Examples 1 and 2, and the results are as follows: Figure 4 As shown, in the low-Cr content material (35CrMnSiA), γ′-Fe4N and ε-Fe3N phases were mainly detected, while no obvious CrN phase was observed. This indicates that nitrogen mainly exists in the form of iron-based nitrides and is distributed by diffusion, thus forming a gradient nitriding layer structure. In the high-Cr content material (F53), in addition to the γ′-Fe4N phase, the characteristic peaks of the CrN phase were also clearly detected, indicating that Cr preferentially reacts with nitrogen to form stable nitrides and forms a dense compound layer on the surface. This compound layer hinders the diffusion of nitrogen into the matrix, resulting in a sudden change in hardness distribution in the thickness direction of the nitrided layer.

[0045] The above results indicate that the Cr element content has a significant impact on the formation of the nitride phase and further determines the structural type of the nitrided layer. Combined with... Figure 1 Cross-sectional structure and Figure 2 The hardness distribution results further confirm that under high Cr content conditions, the formation of the CrN phase is the main reason for the formation of the non-gradient nitriding layer structure; while under low Cr content conditions, due to the lack of the CrN phase, nitrogen exists in a diffusion manner, thus forming a gradient nitriding layer structure.

[0046] Comparative Example 1 This comparative example provides a method for preparing a nitrided layer, serving as a control group for Example 1. The steps are as follows: F53 duplex stainless steel with a Cr content of approximately 22 wt.% was selected as the substrate. After surface pretreatment, the substrate was placed in an ion nitriding apparatus. Ion nitriding was performed under a nitrogen atmosphere at 450℃ and 400 Pa for 9 h to obtain a nitrided layer.

[0047] In this comparative example, under relatively low temperature (450℃) conditions, the diffusion ability and reactivity of nitrogen atoms are relatively low. Simultaneously, the thermodynamic and kinetic conditions required for Cr atoms to form a stable CrN phase are insufficient, making it difficult for the CrN phase to form in large quantities. Therefore, it is difficult to form a dense compound layer dominated by CrN on the material surface.

[0048] Under these conditions, nitrogen tends to exist in austenite in solid solution form, forming an extended austenite (S phase) structure, rather than forming a stable nitride phase. Due to the lack of a dense compound layer to hinder nitrogen diffusion, the nitrided layer structure is difficult to form obvious interface separation, thus making it difficult to exhibit the abrupt hardness distribution characteristics described in Example 1.

[0049] Therefore, the non-gradient nitriding layer structure described in this invention cannot be achieved under these temperature conditions.

[0050] Comparative Example 2 This comparative example provides a method for preparing a nitrided layer, serving as a control group for Example 2. The steps are as follows: 35CrMnSiA steel with a Cr content of approximately 1 wt.% was selected as the substrate. After surface pretreatment, the substrate was placed in an ion nitriding apparatus. Ion nitriding was performed under a nitrogen atmosphere at 600 °C, 400 Pa, and 9 h to obtain a nitrided layer.

[0051] In this comparative example, under higher temperatures (600 °C), the diffusion capacity of nitrogen atoms is significantly enhanced. Although a diffusion-dominated nitriding layer structure can still be formed, the excessively rapid diffusion rate easily leads to problems such as excessively deep nitriding layers, coarsening of the microstructure, and uneven nitrogen distribution. Simultaneously, high temperatures may promote matrix microstructure transformation and local stress accumulation, thereby affecting the microstructure stability and mechanical properties of the nitrided layer. Therefore, although a gradient distribution characteristic may be formed under these conditions, the resulting nitrided layer structure is difficult to maintain stability and uniformity, and the gradient distribution is uncontrollable, failing to achieve the controllable gradient nitriding layer preparation effect described in this invention. Thus, it is evident that when the nitriding temperature exceeds the range defined in this invention, although a gradient distribution trend may appear, its microstructure and properties are difficult to control stably, indicating that the temperature window defined in this invention plays a crucial role in obtaining a stable and controllable gradient nitrided layer.

[0052] Comparative Example 3 This comparative example provides a method for preparing a nitrided layer, serving as a comparison group for Examples 1 and 2. The steps are as follows: Alloy steel with a Cr content of approximately 10 wt.% was selected as the substrate. After surface pretreatment, the substrate was placed in an ion nitriding apparatus. Ion nitriding was performed under a nitrogen atmosphere at 500℃, 400 Pa, and for 9 h to obtain a nitrided layer.

[0053] When the mass fraction of Cr is in the middle range (approximately 5 wt.% to 18 wt.%), the material possesses both a certain ability to form nitrides and a strong nitrogen diffusion capacity. During nitriding, some Cr reacts with nitrogen to form nitride phases such as CrN. However, due to insufficient Cr content to form a continuous and dense compound layer, these nitrides are usually dispersed or locally precipitated. Simultaneously, nitrogen atoms can still diffuse into the matrix, forming a diffusion layer structure. Therefore, within this composition range, the nitrided layer typically exhibits a composite structure with localized compound layers and diffusion layers, with indistinct interface transitions and poor microstructural stability. Consequently, the nitrided layer does not exhibit a significant abrupt change in hardness along its thickness direction, nor does it show a continuous and stable gradient distribution. Therefore, under these compositional conditions, it is difficult to achieve the controllable transformation of the gradient or non-gradient nitrided layer described in this invention.

[0054] Based on the above test results, if the composition and temperature combination conditions of this invention are deviated from, the nitriding layer structure will change significantly. This results in an excessively thick and brittle compound layer, or insufficient strengthening effect due to poor hardenability, making it difficult to simultaneously obtain stable gradient or non-gradient structures, thus failing to achieve the structural control effect of this invention. This invention achieves controllable transformation of the nitriding layer structure type by regulating the substrate composition characteristics and ion nitriding process parameters. It designs and constructs the correspondence between material composition and nitriding behavior, enabling the designable control of the nitriding layer structure. This allows for the selection of the target structure type according to different service requirements, and has broad application prospects in the field of surface strengthening of metallic materials.

[0055] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for preparing controllable gradient and non-gradient nitriding layers, characterized in that, Includes the following steps: After surface pretreatment of the steel substrate, ion nitriding is carried out in a nitrogen-containing atmosphere. By synergistically controlling the alloy composition and process parameters of the steel substrate, nitriding layers of different structural types are formed on the material surface. When the nitriding layer is a non-gradient nitriding layer, the mass fraction of Cr element in the steel substrate is controlled to be 18 wt.%~25 wt.%, and ion nitriding treatment is carried out at 480~540℃. When the nitriding layer is a gradient nitriding layer, the mass fraction of Cr in the steel substrate is controlled to be 1 wt.%~5 wt.%, and ion nitriding treatment is carried out at 480~540℃.

2. The method for preparing controllable gradient and non-gradient nitriding layers according to claim 1, characterized in that: The surface pretreatment includes surface polishing and / or ultrasonic cleaning.

3. The method for preparing controllable gradient and non-gradient nitriding layers according to claim 1, characterized in that: When the target nitriding layer is a non-gradient nitriding layer, the ion nitriding treatment time is 6~12 h and the reaction gas pressure is 300~600 Pa.

4. The method for preparing controllable gradient and non-gradient nitriding layers according to claim 1, characterized in that: When the target nitriding layer is a gradient nitriding layer, the ion nitriding treatment time is 6~12 h and the reaction gas pressure is 300~600 Pa.

5. The method for preparing controllable gradient and non-gradient nitriding layers according to claim 1, characterized in that: In the non-gradient nitriding layer, the Vickers hardness remains relatively stable within the nitriding layer region, with fluctuations not exceeding ±10% of its average value, and a decrease of ≥30% at the interface between the nitriding layer and the steel substrate.

6. The method for preparing controllable gradient and non-gradient nitriding layers according to claim 1, characterized in that: The non-gradient nitriding layer includes a CrN phase and a γ′-Fe4N phase, wherein the volume fraction of the CrN phase is ≥60%; the phase structure of the gradient nitriding layer includes a γ′-Fe4N phase and an ε-Fe3N phase.

7. The method for preparing controllable gradient and non-gradient nitriding layers according to claim 1, characterized in that: In the gradient nitriding layer, the nitrogen concentration decreases along the depth direction, forming a diffusion transition zone.

8. A controllable gradient and non-gradient nitriding layer, characterized in that: It is prepared by any one of the preparation methods described in claims 1 to 7.

9. An application of the controllable gradient and non-gradient nitriding layers as described in claim 8, characterized in that, include: It is used as a protective layer for surface strengthening of metallic materials.

10. The application of the controllable gradient and non-gradient nitriding layers according to claim 9, characterized in that: The non-gradient nitriding layer is used to improve the surface hardness and / or wear resistance of the substrate, while the gradient nitriding layer is used to improve the fatigue resistance and / or impact resistance of the substrate.

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