Nitrogen expanded austenite layer containing silicon and nitrogen rich nano-phase and its preparation method

Abstract

This invention discloses a nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases and its preparation method; by controlling the Ni / Si content of austenitic stainless steel, at a relatively high nitriding temperature of 450-500℃, γ-rays are deposited on its surface. N Silicon- and nitrogen-rich nanophases with sizes of 3-30 nm precipitate in the nitriding layer, while simultaneously forming spherical Cr-N short-range ordered structures with diameters of 1-6 nm, without causing long-range segregation of Cr elements; the higher nitriding temperature increases the nitrogen diffusion rate, enabling the preparation of thick, high-load-bearing γ-ray crystals in a shorter time. N The nitrided layer exhibits excellent corrosion resistance, remaining bright white after metallographic etching. Its Tafel curve in 3.5 wt.% NaCl solution shows no significant shift compared to the pre-nitriding state. This invention provides a new alloy composition and surface treatment method for improving the nitriding efficiency and surface bearing capacity of corrosion-resistant austenitic stainless steel components.

Description

Technical Field

[0001] This invention relates to a nitrogen-expanded austenitic diffusion layer, particularly to a nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases, and also to a method for preparing the above-mentioned material. Background Technology

[0002] Nitriding was initially applied to the surface hardening of ferritic steels, strengthening the parent phase by forming nitride precipitation at high temperatures of 480-600℃. Since the 1980s, with the widespread use of nitriding in Fe-Cr-Ni austenitic stainless steels, a nitrogen-expanded austenite layer (usually denoted as γ-nitride) has been discovered. N γ (also known as "S phase"). Unlike the nitride precipitation that occurs in ferritic steels after nitriding at 480-600℃, nitrided austenitic stainless steels (SSs) develop a nitrogen-expanded austenitic nitrided layer on the surface after low-temperature nitriding (LTN) without nitride precipitation and with interstitial supersaturation. N The nitrogen concentration inside is hundreds of times the interstitial solubility of nitrogen in stainless steel at thermodynamic equilibrium. This extreme nitrogen supersaturation solid solution increases the surface hardness of austenitic stainless steel from 200 HV to 1200 HV without affecting its corrosion resistance.

[0003] However, on austenitic stainless steel (AISI 304 / 316 SS), a large amount of nitride precipitates in the nitrided layer at nitriding temperatures above 450 °C, resulting in Cr depletion in the parent phase and reducing its corrosion resistance. Therefore, γ N The upper limit of the LTN treatment temperature is limited to 450℃. The formation of an expanded austenite layer of ideal thickness requires a long nitriding treatment time, resulting in low treatment efficiency and a nitrided layer thickness that is often less than 20 μm.

[0004] Increasing the nitriding temperature of austenitic stainless steel can improve nitriding efficiency, increase the thickness of the nitrided layer, and enhance surface bearing capacity. However, it also provides a thermodynamic driving force for the precipitation of CrN, leading to γ-ray dilution. N The long-range segregation of Cr (chromium) reduces corrosion resistance. For the corrosion resistance of nitrogen-expanded austenitic diffusion layers, the core influencing factor is the distribution of Cr, which affects the corrosion resistance of austenitic stainless steel and determines the material's corrosion resistance. Therefore, avoiding γ-ray... N The "long-range segregation" of Cr element in the infiltration layer at high nitriding temperature is the key to improving the thickness and bearing capacity of the infiltration layer. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases, and to provide a method for preparing the above material, which can achieve a greater nitrogen diffusion depth at a higher temperature of 450-500℃ without causing "long-range segregation" of Cr element, thereby improving the thickness and load-bearing capacity of the diffusion layer, shortening the processing time, and improving commercial feasibility.

[0006] Technical solution: The nitrogen-expanded austenite infiltration layer containing silicon-rich and nitrogen-rich nanophases of the present invention is formed by surface nitriding treatment of austenitic stainless steel. The austenitic stainless steel is Fe-xCr-yNi-Si, where 16 < x < 20 wt.%, 16 < y < 20 wt.% or Fe-zCr-wNi-Si, where 16 < z < 20 wt.%, 33 < w < 37 wt.%. Among them, the silicon content is 0.5-2.5 wt.%, and the balance is iron; the γ N In the infiltration layer, silicon-rich and nitrogen-rich nanophases with a size of 3-30 nm are precipitated, and at the same time, a spherical Cr-N short-range ordered structure with a diameter of 1-6 nm is formed.

[0007] Among them, by weight fraction, the austenitic stainless steel is Fe-16.7Cr-17.5Ni-0.5Si, Fe-16.8Cr-17.6Ni-1.1Si, Fe-16.8Cr-18.2Ni-2.2Si, Fe-19.2Cr-34.2Ni-1.2Si or Fe-19.2Cr-34.4Ni-1.9Si, and the balance is iron.

[0008] Among them, the nitriding treatment temperature is 450-500 °C, and the time is 4-20 h. The thickness of the nitrogen-expanded austenite infiltration layer is 10-30 μm, and the surface hardness reaches 975-1244 HV under a higher load of 300 g.

[0009] Among them, the austenitic stainless steel is pretreated, including the following steps: melting silicon powder, nickel powder and austenitic stainless steel, applying magnetic field stirring, turning over and remelting multiple times, high-temperature solid solution annealing, and water quenching to obtain it; preferably, the purity of silicon powder is above 99.9%, the purity of nickel powder is above 99.98%, the austenitic stainless steel includes AISI 304 and RA 330, the high-temperature solid solution annealing temperature is 1020-1060 °C, and the high-temperature solid solution annealing time is 1-2 h; applying a magnetic field to assist stirring when melting the stainless steel to ensure the uniformity of chemical composition. The prepared casting rod is turned over and remelted at least six times to further ensure the uniformity of chemical composition.

[0010] The preparation method of the above nitrogen-expanded austenite infiltration layer containing silicon-rich and nitrogen-rich nanophases includes the following steps:

[0011] (1) Cut out a sample plate with a thickness of 2-3 mm from the pretreated austenitic stainless steel;

[0012] (2) Grind the sample plate in step (1) with sandpaper;

[0013] (3) Perform nitriding treatment at low temperature to obtain a nitrogen-expanded austenite infiltration layer containing silicon-rich and nitrogen-rich nanophases.

[0014] In step (2), one side of the sample plate is gradually polished with P400, P800, and P1200 silicon carbide sandpapers, and the other side is polished with P400 silicon carbide sandpaper.

[0015] Step (3) specifically involves performing a nitriding treatment at 450 - 500 °C for 4 - 20 hours using a Plasma Metal nitriding device with a gas pressure of 0.6 - 1.0 mbar, further preferably 0.7 - 0.9 mbar, a gas mixture of N2:H2 with a volume ratio of 1:1.5 - 2.2, further preferably a volume ratio of 1:2 N2:H2, and a power of 740 - 760 VA.

[0016] The application of the nitrogen-expanded austenite nitrided layer containing silicon-rich and nitrogen-rich nanophases in high-hardness, wear-resistant, and corrosion-resistant devices, for protecting against particle wear and erosion-corrosion on the surface of devices in precision chemical engineering and marine engineering, and for improving the lifespan of the surface of medical devices and implants, etc.

[0017] Principle of the invention: The present invention proposes a nitrogen-expanded austenite nitrided layer containing silicon-rich and nitrogen-rich nanophases. By regulating the Ni / Si content in austenitic stainless steel, silicon-rich and nitrogen-rich phases with a size of 3 - 30 nm precipitate on the surface nitrided layer at a relatively high nitriding temperature of 450 - 500 °C, reducing the nitrogen content in the parent phase, thereby reducing the driving force for the growth of CrN and inhibiting the "long-range segregation" of Cr elements, and only forming a short-range ordered Cr-N structure. Therefore, it can improve the nitrogen diffusion rate, increase the nitrided layer thickness, and at the same time maintain the good corrosion resistance of the material.

[0018] In terms of composition design, the controllable precipitation of silicon-rich and nitrogen-rich nanophases in γ is achieved by regulating the Ni / Si content in austenitic stainless steel. As a strong austenite stabilizing element, Ni inhibits the long-range migration of Cr by reducing the activity of Cr in the parent phase and hindering the substitution diffusion of Cr. Austenitic stainless steels with a medium to high Ni content (18 - 3 N 5 wt.%) can effectively inhibit the premature precipitation of CrN, but a high Ni content (35 wt.% Ni) will reduce the nitrogen diffusion depth and the nitrided layer thickness. Increasing the Si content promotes the precipitation of silicon-rich and nitrogen-rich nanophases, thereby inhibiting the precipitation of CrN, but at the same time it reduces the nitrided layer thickness, and an excessive Si content can still lead to the precipitation of an excessive amount of silicon-rich and nitrogen-rich phases, inducing the formation of Cr-depleted regions and reducing the corrosion resistance of the nitrided layer. By balancing factors such as the nitrided layer thickness, nitrogen supersaturated solid solution, and silicon-rich and nitrogen-rich nanophases, the optimal range of Ni / Si content regulation is selected, Fe-xCr-yNi-Si, 16 < x < 20 wt.%, 16 < y < 20 wt.% or Fe-zCr-wNi-Si, 16 < z < 20 wt.%, 33 < w < 37 wt.%, where the silicon content is 0.5 - 2.5 wt.%, and the balance is iron.

[0019] In the design of nitriding process parameters, by combining the optimal range of Ni / Si content, a higher nitrogen diffusivity is achieved at nitriding temperatures of 450-500°C, improving nitriding efficiency while preventing CrN precipitation and maintaining good corrosion resistance. The synergistic effect of moderately high Ni and Si content inhibits premature CrN precipitation, allowing the designed nitriding temperature to be higher than the surface γ-ray concentration of commonly used austenitic stainless steel. N The 450°C upper temperature limit allows austenitic stainless steel to form a thicker nitrogen-expanded austenitic layer after nitriding at 450-500°C, approximately 10 μm to 30 μm thick. Under a higher load of 300 g, the surface hardness reaches 975-1244 HV without reducing its corrosion resistance. Increasing the nitriding temperature significantly improves nitriding efficiency, shortens nitriding time, and increases γ-ray permeability while maintaining excellent corrosion resistance. N The thickness of the infiltration layer and its surface bearing capacity.

[0020] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0021] (1) The nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases proposed in this invention has high surface hardness. After nitriding at 450-500°C for 4-20 hours, its high surface hardness reaches 1200 HV. 0.1 The above is higher than traditional gamma. N -304 / 316 1030HV 0.1 Its high surface hardness is due to the synergistic effect of nitrogen supersaturation solid solution, silicon-rich and nitrogen-rich nanophase and Cr-N short-range ordered structure (1-6 nm).

[0022] (2) The nitrogen-expanded austenitic nitriding layer containing silicon-rich and nitrogen-rich nanophases proposed in this invention exhibits excellent corrosion resistance. At nitriding temperatures > 450°C, it shows superior corrosion resistance compared to traditional γ-ray distillation. N Unlike the long-range migration of Cr elements caused by the honeycomb-like CrN precipitation in the -304 / 316 infiltration layer, this novel infiltration layer forms a short-range ordered Cr-N structure with a size of 1-6 nm, where only the short-range segregation of Cr elements occurs. Traditional γ-ray distillation... N - At nitriding temperatures above 450°C, the corrosion resistance of 304 / 316 stainless steel deteriorates drastically due to the "long-range migration" of chromium. Compared to the 304 / 316 stainless steel substrate before nitriding, the corrosion current density in the Tafel test using 3.5 wt.% NaCl solution decreases exponentially, and the corrosion voltage is significantly reduced. However, the novel nitrided layer exhibits good overall corrosion resistance, with no significant difference in corrosion voltage / corrosion current density in the Tafel test using 3.5 wt.% NaCl solution compared to the unnitrided layer.

[0023] (3) The nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases proposed in this invention has higher processing efficiency, greater diffusion layer thickness, and surface bearing capacity without reducing corrosion resistance. It can obtain a thick diffusion layer of 18-26 μm in just 4-20 hours at 450-500°C, while traditional γ-ray diffusing technology... N -304 / 316 requires 20-50 hours at 450°C and its thickness is often < 15 μm. Furthermore, this novel γ-ray... N The surface hardness of the infiltrated layer can reach 975-1244 HV under a high load of 300 g. 0.3 It is far higher than that of traditional gamma rays. N -304 / 316 of 500 HV 0.3 . Attached Figure Description

[0024] Figure 1 Metallographic images of Examples 1-5 and Comparative Examples 1-9;

[0025] Figure 2 The thickness of the nitrided layer in the cross section observed under an optical microscope for Examples 1-5 and Comparative Examples 1-9 (2a), and the surface composition-depth distribution of Examples 1-5 and Comparative Examples 1-9 measured by glow discharge spectroscopy (GDOES) (2b-e).

[0026] Figure 3 The XRD patterns of Examples 1-5 and Comparative Examples 1-9 are shown below;

[0027] Figure 4 SAED patterns for Examples 1(b), 4(d), 1(a), 5(c), and 9(e);

[0028] Figure 5 The images are STEM-HAADF images of Example 1 (a1), Example 4 (c1), Comparative Example 5 (b1), and Comparative Example 9 (d1), and STEM-EDS spectra of Fe, Cr, Ni, Si, and N of Example 1 (a2-a6), Example 4 (c2-c6), Comparative Example 5 (b2-b6), and Comparative Example 9 (d2-d6).

[0029] Figure 6 The images show partial STEM-HAADF images of Example 1 (a1-a6), Example 4 (c1-c6), Comparative Example 5 (b1-b6), and Comparative Example 9 (d1-d6) and their corresponding EDS spectra of Fe, Cr, Ni, Si, and N.

[0030] Figure 7 The HRTEM and magnified FFT image (ce) of Example 1, and the HRTEM and magnified FFT image (ab) of Comparative Example 1.

[0031] Figure 8 The HRTEM and magnified FFT image (df) of Example 4, the HRTEM and magnified FFT image (8a-c) of Comparative Example 5, the HRTEM and magnified FFT image (gi) of Comparative Example 9, and the magnified HRTEM image (j) of region B in Comparative Example 9 (g).

[0032] Figure 9 The surface Vickers hardness (a) of Examples 1-5 and Comparative Examples 1-9 before and after nitriding treatment under an indentation load of 0.025 kg; the Vickers hardness (b) of the nitrided 316 SS samples of Examples 1-5 and 450℃ / 10h at indentation loads of 0.1 kg, 0.3 kg, and 1 kg, respectively.

[0033] Figure 10 Tafel curves for 316 SS (a), Example 1 (b), and Example 4 (c) before and after nitriding treatment at 450-500℃ for 4-20 hours. Detailed Implementation

[0034] The technical solution of the present invention will be further described below with reference to the embodiments. The test materials used in the embodiments can all be obtained through conventional means.

[0035] Example 1

[0036] The nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases in this embodiment was obtained by nitriding austenitic stainless steel Fe-17Cr-18Ni-0.5Si at 450-500℃ for 4-20 hours. The sample was named N480-18Ni-0.5Si.

[0037] The preparation and pretreatment of austenitic stainless steel Fe-17Cr-18Ni-0.5Si were as follows: A prototype stainless steel casting rod, approximately 40mm × 20mm × 20mm in size, was prepared by mixing 99.9999% silicon particles and 99.98% nickel particles with commercial austenitic SS grade—AISI 304 and RA330. An 18 wt.% nickel sample was also prepared. Using an Arcast Arc 200 arc melting / casting apparatus, the raw materials were placed in a copper crucible for melting, with magnetic field-assisted stirring to ensure chemical homogeneity. The copper crucible was then tilted, and the molten metal was poured into a rectangular copper mold. The casting rod was turned and remelted at least six times to further ensure chemical homogeneity.

[0038] The method for preparing the nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases in this embodiment is as follows:

[0039] (1) The casting rods are solution annealed in air at 1020-1060℃ for 1-2 h in a box furnace, and then water quenched.

[0040] (2) Cut a sample plate with a thickness of 2-3 mm from the annealing bar, and gradually grind one side of the plate with P400, P800 and P1200 silica sandpaper.

[0041] (3) Nitriding treatment is carried out at 450-500℃ for 4-20 hours using Plasma Metal nitriding equipment, with a gas pressure of 0.6-1.0 mbar, more preferably 0.7-0.9 mbar, and the gas is a mixture of N2:H2 with a volume ratio of 1:1.5-2.2, more preferably 1:2, with a power of 740-760 VA.

[0042] Example 2

[0043] Compared with Example 1, the nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases prepared in this embodiment has a sample composition of Fe-17Cr-18Ni-1Si and is named N480-18Ni-1Si.

[0044] Example 3

[0045] Compared with Example 1, the nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases prepared in this embodiment has a sample composition of Fe-17Cr-18Ni-2Si and is named N480-18Ni-2Si.

[0046] Example 4

[0047] Compared with Example 1, the nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases prepared in this embodiment has a sample composition of Fe-19Cr-35Ni-1Si and is named N480-35Ni-1Si.

[0048] Example 5

[0049] Compared with Example 1, the nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases prepared in this embodiment has a sample composition of Fe-19Cr-35Ni-2Si and is named N480-35Ni-2Si.

[0050] Comparative Example 1

[0051] An austenitic diffusion layer, compared with Example 1, has a temperature change of 430℃ in step (3) of the preparation method, and is named N430-18Ni-0.5Si.

[0052] Comparative Example 2

[0053] An austenitic diffusion layer, compared with Example 2, has a temperature change of 430℃ in step (3) of the preparation method, and is named N430-18Ni-1Si.

[0054] Comparative Example 3

[0055] An austenitic diffusion layer, compared with Example 3, has a temperature change of 430℃ in step (3) of the preparation method, and is named N430-18Ni-2Si.

[0056] Comparative Example 4

[0057] An austenitic diffusion layer, compared with Example 1, the sample composition of the nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophase prepared in this comparative example is Fe-17Cr-18Ni-5Si, and the temperature of step (3) in the preparation method is changed to 430℃, named N430-18Ni-5Si.

[0058] Comparative Example 5

[0059] An austenitic diffusion layer, compared with Example 1, the sample composition of the nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases prepared in this comparative example is Fe-17Cr-18Ni-5Si, named N480-18Ni-5Si.

[0060] Comparative Example 6

[0061] An austenitic diffusion layer, compared with Example 4, has a temperature change of 430℃ in step (3) of the preparation method, and is named N430-35Ni-1Si.

[0062] Comparative Example 7

[0063] An austenitic diffusion layer, compared with Example 5, has a temperature change of 430℃ in step (3) of the preparation method, and is named N430-35Ni-2Si.

[0064] Comparative Example 8

[0065] An austenitic diffusion layer, compared with Comparative Example 4, has a sample composition of Fe-19Cr-35Ni-5Si, named N430-35Ni-5Si.

[0066] Comparative Example 9

[0067] An austenitic diffusion layer, compared with Example 5, has a sample composition of Fe-19Cr-35Ni-5Si, and is named N480-35Ni-5Si.

[0068] Table 1 shows the stainless steel compositions of Examples 1-5 and Comparative Examples 1-9, as confirmed by energy-dispersive X-ray spectroscopy (EDS) in a Jeol 7000 scanning electron microscope (SEM) combined with Oxford Instruments Aztec software.

[0069] Table 1. Chemical composition (wt.%) of nitrogen-expanded austenitic stainless steel with silicon-rich and nitrogen-rich nanophases prepared in Examples 1-5 and Comparative Examples 1-9.

[0070]

[0071] Characterization and performance tests were performed on Examples 1-5 and Comparative Examples 1-9. Characterization tests included cross-sectional metallographic observation under an optical microscope; cutting sample sections from nitrided plates, followed by mounting, grinding, polishing, and etching (50HCl-25HNO3-25H2O) to reveal the cross-sectional morphology; XRD analysis on the sample plate surface using a PROTO AXRD benchtop power diffractometer; compositional depth profile analysis using glow discharge spectroscopy (GDOES); and energy-dispersive X-ray spectroscopy (EDS) using TEM and STEM. Performance tests included surface hardness testing and electrochemical corrosion performance. Results are as follows: Figure 1-9 As shown.

[0072] Figure 1 The images are optical microscope images of the cross-sections of the etched samples in Examples 1-5 and Comparative Examples 1-9, namely Example 1 (a2), Example 2 (b2), Example 3 (c2), Example 4 (e2), Example 5 (f2), Comparative Example 1 (a1), Comparative Example 2 (b1), Comparative Example 3 (c1), Comparative Example 4 (d1), Comparative Example 5 (d2), Comparative Example 6 (e1), Comparative Example 7 (f1), Comparative Example 8 (g1), and Comparative Example 9 (g2).

[0073] Figure 2 The thickness of the nitrided layer on the surface of the nitrided samples in Examples 1-5 and Comparative Examples 1-9, and the surface composition-depth distribution of the nitrided samples, wherein (a) is the thickness of the nitrided layer on the surface of the nitrided sample, and (be) is the depth distribution map of the surface composition of the nitrided sample measured by glow discharge spectroscopy (GDOES) instrument.

[0074] Depend on Figure 1 , Figure 2It is evident that the combination of low silicon (0.5-2 wt.%) and medium-to-high nickel (18-35 wt.%) can form a thick (>25 μm) and corrosion-resistant nitriding layer, breaking through the traditional γ-ray distillation method. N -304 / 316 temperature limit (≤450℃). When the silicon content is ≤2 wt.%, (Examples 1-3, Examples 4-5, Comparative Examples 1-4 and Comparative Examples 6-8), the nitrided layer remains bright and maintains its good corrosion resistance; while the corrosion resistance of the 5 wt.% silicon samples (Comparative Examples 5 and 9) decreases.

[0075] Secondly, the nitriding layer thickness of the 35Ni sample is thinner overall than that of the 18Ni sample (see...). Figure 2 In example a), after nitriding at 480℃ for 10 hours, the nitrided layer thickness of samples containing 0.5-2 wt.% Si (Examples 1-3, Examples 4-5, Comparative Examples 1-4, and Comparative Examples 6-8) significantly increased (18-26 μm), while the thickness of samples containing 5 wt.% Si (Comparative Examples 5 and 9) decreased (see example a). Figure 2 (a) It can also be seen that the depth distribution of nitrogen concentration on the surface exhibits a non-monotonic change with increasing silicon content (see [reference]). Figure 2 When the silicon content increases from 0.5 wt.% to 2 wt.%, the surface nitrogen concentration decreases. When the silicon content further increases to 5 wt.%, the nitrogen concentration rebounds within a depth of 0-5 μm on the surface, which may be related to the evolution of the microstructure.

[0076] Figure 3 X-ray diffraction (XRD) analysis of Examples 1-5 and Comparative Examples 1-9 was performed using a PROTO AXRD benchtop X-ray diffractometer (Cu-Kα radiation, wavelength 0.154 nm) at 2θ angles of 30-60°, step sizes of 0.015°, and dwell times of 1 s. The γ-ray diffraction (XRD) values ​​of 18Ni samples (Examples 1-3 and Comparative Examples 1-5) after nitriding for 10 hours were analyzed. N The peaks are all based on the left shift of the characteristic peak of UNT-18Ni0.5Si. When the Si content is ≤ 2 wt.%, (Examples 1-3 and Comparative Examples 1-3), γ N The peak remained sharp. The high-silicon (5 wt.%) sample (Comparative Example 4) exhibited a sharp γ-ray peak at 430 °C. N (111) Peak broadened significantly (39–44°), possibly related to tissue structure evolution, and further extended to 39.5–46° at 480℃ (Comparative Example 5), connecting γ N (111) and γ N (200) peak. The γ peak of the 35Ni sample (Comparative Examples 6-7) after nitriding at 430℃ for 10 hours. N The peaks are all based on the left shift of the characteristic peak of UNT-35Ni1Si, similar to the 18Ni sample, compared with Comparative Examples 8-9 and Examples 4-5. NPeak intensity decreased. At each processing temperature, Ni and Si content affected the XRD spectra; increasing the Ni level from 18 wt.% to 35 wt.% promoted nitriding formation. Compared to the 18Ni sample (see...),... Figure 3 Compared to sample a), the 35Ni sample (see Figure 3 b) typically exhibits a strong γ(111) XRD peak at 43.4°, γ N The peak is relatively weak. Overall, after nitriding at 430℃, the γ peak is the most prominent. N Primarily, nitriding at 480℃ leads to a more complex nanostructure, resulting in more complex peak shapes. Low Si (0.5–2 wt.%) delays CrN formation, while high Si (5 wt.%) accelerates and reduces γ. N stability.

[0077] Figure 4 To observe Examples 1, 4, Comparative Example 1, Comparative Example 5 and Comparative Example 9 using the FEI Scios 2 HiVac instrument, the transmission electron microscope (TEM) samples were prepared by the FEI Scios 2 HiVac instrument using the focused ion beam (FIB) method (Ga+, 30kV). Figure 4 The symbols (ae) represent the SAED patterns of samples from Comparative Example 1, Example 1, Comparative Example 5, Example 4, and Comparative Example 9, respectively. Figure 4 FCC stripes and tail (along) observed in SAEDs in (a) and (b) <100> The calculated lattice parameters show a high degree of agreement with the FCC-CrN standard values, indicating the presence of CrN, although XRD did not directly detect CrN peaks. In low Ni / Si samples (18Ni0.5Si), Cr-N clusters tend to aggregate along the FCC{100} plane, exhibiting directional stripes (see [reference needed]). Figure 4 In the medium (a, b) and high Ni / Si samples (35Ni5Si), the FCC spots showed significant splitting. Figure 4 (e) reflects the increased difference in lattice parameters between the high-N CrN and low-N parent phases.

[0078] Figure 5 STEM-HAADF images and STEM-EDS spectra of FEI Tecnai F20 (FEG, 200 kV) for Example 1, Comparative Example 5, Example 4, and Comparative Example 9 are shown in Table 2. Figure 5 Size, number density, and spacing of silicon- and nitrogen-rich nanophases were measured using STEM-HAADF images.

[0079] Table 2. Size, number density, and spacing of silicon-rich and nitrogen-rich nanophases in Examples 1, 4, 5, and 9.

[0080]

[0081] Figure 6 Table 3 shows partial STEM-HAADF images of Examples 1 (a1-a6), 4 (c1-c6), Comparative Example 5 (b1-b6), and Comparative Example 9 (d1-d6), and the corresponding EDS spectra of Fe, Cr, Ni, Si, and N. Figure 6 The percentage of chemical composition in the characteristic regions of the STEM-EDS data shown.

[0082] Table 3 shows the percentage of chemical composition in the local image feature regions of STEM-EDS in Examples 1, 4, 5, and 9.

[0083]

[0084] Depend on Figure 5 , Figure 6 It can be seen that two types of nanophases were observed in the nitrided samples: (1) silicon-rich and nitrogen-rich nanophases of 3–30 nm, with a number density of 0.5–2.6 × 10⁻⁶. 3 / μm 2 (See Table 2); (2) Ultrafine Cr-N short-range ordered structures of 1–6 nm (see Table 2); Figure 6 The a1-a6 nanophases exhibit higher density and more uniform distribution (see Table 2). Si / N is enriched in the silicon-rich and nitrogen-rich nanophases (Si: 12–29 at.%, N: 23–52 at.%), while the Fe / Ni / Cr content is lower (see Table 3). Cr / N is significantly enriched in the Cr-N short-range ordered structure (see Table 3). Figure 6 In particular, N480-18Ni0.5Si exhibits a Cr-N structure oriented along the FCC{100} plane (see...). Figure 6 (a1-a6). Neither of the two nanostructures induced significant long-range Cr segregation.

[0085] Figure 7 The image shows the HRTEM and magnified FFT image (ce) of Example 1, and the HRTEM and magnified FFT image (ab) of Comparative Example 1.

[0086] Figure 8 The HRTEM and magnified FFT images of Example 4 ( Figure 8 (middle df), HRTEM and magnified FFT images of scale 5 ( Figure 8 (middle ac), HRTEM and magnified FFT images of scale 9 ( Figure 8 (Gi), compared to ratio 9 ( Figure 8 HRTEM magnification of region B in (g) Figure 8 (j).

[0087] Depend on Figure 7, Figure 8 As can be seen, in the high Ni / Si samples (Example 4 and Comparative Example 9), the silicon-rich and nitrogen-rich nanophases are more pronounced, and CrN is transformed into discrete spherical particles (see...). Figure 8 (d, g). High Si promotes the precipitation of silicon-rich and nitrogen-rich nanophases, thus reducing the N concentration in the parent phase and delaying the formation of CrN (see d, g). Figure 8 (j).

[0088] Figure 9 The Vickers hardness of the nitrided samples in Examples 1-5 and Comparative Examples 1-9 was obtained by testing with a Future-Tech Corp FM-700 hardness tester. Figure 9 In Figure a, the surface Vickers hardness of Examples 1-5 and Comparative Examples 1-9 before and after nitriding treatment under an indentation load of 0.025 kg is represented by a. Figure b shows the Vickers hardness of the 316 SS samples from Examples 1-5 and the samples nitrided at 450℃ / 10 hours under indentation loads of 0.1 kg, 0.3 kg, and 1 kg, respectively. This is in contrast to low-N austenitic SSs (which typically have a hardness between 160 and 230 HV). 0.025 Compared to the previous treatment, the average microhardness after nitriding reached 1401-1765 HV. 0.025 (See Figure 9 (a) The nitrided surface maintains high Vickers hardness values ​​under indentation loads of 0.1 kg, 0.3 kg, and 1 kg (see [reference]). Figure 9 (b) A 10 μm thick γ-ray film obtained by nitrogen co-diffusion with 316SS at 450℃ / 10h. N Compared to the -316 nitrided layer, the thick γ-ray layer obtained by the present invention after nitrogen co-diffusion at 480℃ / 10h N The nitrided layer exhibits significantly higher hardness under high loads (especially HV). 0.3 ), and 1030 HV of 316SS nitrided at 450℃ / 10h. 0.1 505 HV 0.3 Compared to 253 HV1, N480-18Ni0.5Si has a significantly higher surface hardness of 1331 HV. 0.1 1157 HV 0.3 and 425 HV1 (see Figure 9 (b) Under a 0.3 kg load, most bright γ N The average surface hardness of (Examples 1-5) is more than twice that of nitrided 316 stainless steel at 450°C / 10 hours (see...). Figure 9 (b) Under a relatively high indentation load of 1 kg, the Vickers hardness values ​​of N480-18Ni0.5Si, N480-18Ni1Si, and N480-35Ni1Si exceed 400 HV—approximately twice that of the untreated SS substrate.

[0089] Figure 10 The corrosion resistance of 316 SS, Example 1, and Example 4 was measured using an electrochemical workstation (CHI660E, Shanghai Chenhua) in a 3.5 wt.% NaCl solution at room temperature and a three-electrode system. Tafel curves were obtained before and after nitriding at 480℃ for 10 hours for three different samples. The corrosion resistance of 316 SS decreased after prolonged nitriding at 480℃. Figure 10 Unlike c), the nitrogen-expanded austenitic diffusion layers of silicon-rich and nitrogen-rich nanophases formed in Examples 1 and 4 exhibit good corrosion resistance. After nitriding at 480℃ for 10 hours, the relative positions of the polarization curves of 18Ni0.5Si and 35Ni1Si did not change significantly. Furthermore, in the cathode region, with γ... N The corrosion current density of the predominantly treated N480-18Ni0.5Si was lower than that of the untreated stainless steel, while N480-35Ni1Si exhibited a corrosion current density similar to that of untreated stainless steel.

[0090] This invention achieves γ-ray formation on the surface of austenitic stainless steel by controlling the Ni / Si content at a relatively high nitriding temperature of 450-500℃. N Silica- and nitrogen-rich nanophases with sizes of 3-30 nm precipitate in the nitriding layer, while simultaneously forming spherical Cr-N short-range ordered structures with diameters of 1-6 nm, without inducing long-range segregation of Cr. The higher nitriding temperature increases the nitrogen diffusion rate, thus enabling the preparation of thick (10-30 μm) and high-load-bearing γ-ray crystals. N Permeation layer (> 950 HV under 300 g high load) 0.3 The process is efficient and requires a short time (< 20 h). Furthermore, the nitrided layer exhibits excellent corrosion resistance, displaying a bright white color after metallographic etching. The electrochemical Tafel curve in 3.5 wt.% NaCl solution shows no significant shift compared to the pre-nitriding state. This invention provides a new alloy composition and surface treatment method for improving the nitriding efficiency and surface load-bearing capacity of corrosion-resistant austenitic stainless steel components.

Claims

1. A nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases, characterized in that, The nitrogen-expanded austenite infiltration layer is formed by nitriding treatment on the surface of austenitic stainless steel, and the austenitic stainless steel is Fe-xCr-yNi-Si, where 16 < x < 20 wt.%, 16 < y < 20 wt.% or Fe-zCr-wNi-Si, where 16 < z < 20 wt.%, 33 < w < 37 wt.%. Among them, the silicon content is 0.5 - 2.5 wt.%, and the balance is iron.

2. The nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases according to claim 1, characterized in that, Surface γ N Silicon- and nitrogen-rich nanophases with a size of 3-30 nm are precipitated in the infiltration layer, while spherical Cr-N short-range ordered structures with a diameter of 1-6 nm are formed.

3. The nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases according to claim 1, characterized in that, By weight fraction, the austenitic stainless steel is Fe-16.7Cr-17.5Ni-0.5Si, Fe-16.8Cr-17.6Ni-1.1Si, Fe-16.8Cr-18.2Ni-2.2Si, Fe-19.2Cr-34.2Ni-1.2Si or Fe-19.2Cr-34.4Ni-1.9Si, and the balance is iron.

4. The nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases according to claim 1, characterized in that, The nitriding treatment temperature is 450 - 500 °C, and the time is 4 - 20 h.

5. The nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases according to claim 1, characterized in that, The thickness of the nitrogen-expanded austenite infiltration layer is 10 - 30 μm.

6. The nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases according to claim 1, characterized in that, The austenitic stainless steel undergoes pretreatment, including the following steps: melting silicon powder, nickel powder and austenitic stainless steel, applying magnetic field stirring, turning over and remelting multiple times, performing high-temperature solid solution annealing, and then water quenching to obtain it.

7. The nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases according to claim 5, characterized in that, The purity of silicon powder is above 99.9%, the purity of nickel powder is above 99.98%, the austenitic stainless steel includes AISI 304 and RA 330, the high-temperature solid solution annealing temperature is 1020 - 1060 °C, and the high-temperature solid solution annealing time is 1 - 2 h.

8. A method for preparing a nitrogen-expanded austenitic diffusion layer containing silicon-rich and nitrogen-rich nanophases as described in claim 1, characterized in that, It includes the following steps: (1) Cutting out a sample plate with a thickness of 2 - 3 mm from the pretreated austenitic stainless steel; (2) Grinding the sample plate in step (1) with sandpaper; (3) Performing nitriding treatment at low temperature to obtain a nitrogen-expanded austenite infiltration layer containing silicon-rich and nitrogen-rich nanophases.

9. The preparation method according to claim 7, characterized in that, Step (3) is specifically to perform nitriding treatment at 450 - 500 °C for 4 - 20 hours, using a Plasma Metal nitriding device, with a pressure of 0.6 - 1.0 mbar, a gas of N2:H2 mixed at a volume ratio of 1:1.5 - 2.2, and a power of 740 - 760 VA.

10. An austenitic stainless steel component containing the nitrogen-expanded austenite infiltration layer containing silicon-rich and nitrogen-rich nanophases as described in claim 1.

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