A surface strengthening treatment method for austenitic stainless steel

Abstract

The present invention discloses a surface strengthening treatment method for austenitic stainless steel, which belongs to the technical field of surface modification of metal materials. The method comprises: first performing laser shock treatment on austenitic stainless steel, and then performing ion nitrocarburizing treatment on the austenitic stainless steel after laser shock treatment; the impact energy of the laser shock treatment is 0-30J and is not 0. The present invention first performs laser shock treatment on austenitic stainless steel, and then performs ion nitrocarburizing treatment on the austenitic stainless steel after laser shock treatment. The laser shock treatment and the ion nitrocarburizing treatment work together to effectively improve the hardness and friction and wear performance while taking into account the corrosion resistance of stainless steel, thereby obtaining austenitic stainless steel having high hardness, good friction and wear performance, and good corrosion resistance.

Description

Technical Field

[0001] The invention relates to the technical field of metal material surface modification, in particular to a surface strengthening treatment method for austenitic stainless steel. Background Art

[0002] Austenitic stainless steel boasts excellent corrosion resistance, good ductility, and toughness, making it widely used in industries such as marine, nuclear energy, chemical, medical, and food. However, due to its low hardness and poor wear resistance, austenitic stainless steel cannot be used in applications requiring both good wear resistance and excellent corrosion resistance. This significantly limits its application range and forces manufacturers to seek expensive alternative materials and production technologies, significantly increasing costs. Therefore, it is of great significance to study how to use surface modification technology to enable stainless steel to meet the industrial requirements of both corrosion resistance and wear resistance.

[0003] Nitrocarburizing of stainless steel is a process of improving the hardness, wear resistance and corrosion resistance of stainless steel by co-diffusion of nitride and carbon elements on the surface of stainless steel. This treatment method can form a layer of a mixture of nitrides and carbides on the surface of stainless steel, thereby improving the wear resistance of the material. However, simply performing ion nitrocarburizing on the surface of stainless steel has limited improvement on the hardness and friction and wear properties of stainless steel, and also affects the corrosion resistance of stainless steel. How to reduce the adverse effects of ion nitrocarburizing and obtain stainless steel with good corrosion resistance, good wear resistance and higher hardness is a difficult problem that technicians in this field need to solve at this stage. Summary of the Invention

[0004] The purpose of the present invention is to provide a surface strengthening treatment method for austenitic stainless steel to solve the above problems existing in the prior art.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] One of the technical solutions of the present invention is a method for surface strengthening treatment of austenitic stainless steel, which includes first performing laser shock treatment on the austenitic stainless steel and then performing ion nitrogen and carbon co-diffusion treatment on the austenitic stainless steel after laser shock treatment; the impact energy of the laser shock treatment is 0-30J and is not 0.

[0007] Further preferably, the impact energy of the laser shock treatment is 0-25J or 25-30J, and is not 0 or 25J.

[0008] Further preferably, the impact energy of the laser shock treatment is 8-22J or 27-30J.

[0009] Further preferably, the impact energy of the laser shock treatment is 20J or 30J.

[0010] More preferably, the impact energy of the laser shock treatment is 20J.

[0011] Furthermore, the number of shock layers in the laser shock treatment is 1 or 2, and the overlap rate is 40-60%.

[0012] Furthermore, the laser spot diameter of the laser shock treatment is 3-8 mm, the multiple shock frequency is 5-10 Hz, the pulse width is 16-25 ns, and the laser wavelength is 1064 nm.

[0013] Furthermore, before laser shock treatment, a 0.1 mm thick black tape was pasted on the surface of the austenitic stainless steel to be impacted as an absorption protection layer, and then a 1 mm thick running water was used as a constraint layer.

[0014] Furthermore, the holding temperature of the ion nitrocarburizing treatment is 450-520° C., and the holding time is 4-8 hours.

[0015] Furthermore, during the heat preservation process of the ion nitrocarburizing treatment, a mixed gas of ammonia, argon and methane was introduced at a pressure of 300 Pa. The flow ratio of the ammonia, argon and methane was 20:9:1.

[0016] Furthermore, the heating process of the ion nitrocarburizing treatment includes two stages: room temperature to 300°C, and 300°C to the holding temperature. In the stage from room temperature to 300°C, a mixed gas of argon and methane is introduced, with the flow ratio of argon to methane being 9:1 and the gas pressure being 200Pa; when the temperature is raised to 300°C, ammonia is introduced, and the flow rate of ammonia gradually increases. That is, in the stage from 300°C to the holding temperature, a mixed gas of ammonia, argon and methane is introduced, so that the flow ratio of ammonia, argon and methane and the gas pressure in the furnace gradually increase with the increase in temperature. When the temperature reaches the holding temperature, the gas pressure in the furnace reaches 300Pa, and the flow ratio of ammonia, argon and methane reaches 20:9:1.

[0017] Furthermore, the austenitic stainless steel is pretreated before the laser shock treatment, including: polishing the austenitic stainless steel, and then ultrasonically cleaning it in anhydrous ethanol.

[0018] Furthermore, the polishing is specifically polishing until the surface roughness of the austenitic stainless steel is ≤Ra3.2.

[0019] Furthermore, the polishing is performed sequentially using SiC sandpaper with mesh sizes of 180 to 2000.

[0020] Furthermore, the ultrasonic cleaning time is not less than 15 minutes.

[0021] Furthermore, before the ion nitrocarburizing treatment is performed on the austenitic stainless steel, the austenitic stainless steel after the laser shock treatment is polished again until the surface roughness of the austenitic stainless steel is ≤Ra1.6.

[0022] The second technical solution of the present invention: an austenitic stainless steel obtained by treating it according to the above-mentioned austenitic stainless steel surface strengthening treatment method.

[0023] The present invention first performs laser shock treatment on austenitic stainless steel, and then performs ion nitrocarburizing on the treated austenitic stainless steel. Under the action of the laser shock, the austenitic stainless steel undergoes a high strain rate dynamic response, resulting in an increase in surface dislocations, grain refinement, and even the formation of nanocrystals. The increased dislocation density and grain boundaries provide more channels for the diffusion of nitrogen and carbon atoms during the subsequent ion nitrocarburizing process, reducing the atomic diffusion activation energy, facilitating the diffusion and reaction of elements, improving diffusion efficiency, forming a stable infiltration layer, and promoting the formation of physical phases.

[0024] The present invention discovered that the laser shock energy determines the degree of grain nanocrystalization, dislocation density, and defect level in the stainless steel surface layer. These, in turn, influence the formation of the S phase. While the nanocrystalization or dislocation density within a certain range of stainless steel grains promotes S-phase formation, it inhibits S-phase formation within another range of grain nanocrystalization or dislocation density. Consequently, the overall performance of the stainless steel nitrocarburized layer subjected to different laser shock energies exhibits a quasi-sinusoidal variation (increases first, then decreases, and then increases again).

[0025] The present invention discloses the following technical effects:

[0026] The present invention first performs laser shock treatment on austenitic stainless steel, and then performs ion nitrocarburizing on the treated austenitic stainless steel. The synergistic effect of laser shock treatment and ion nitrocarburizing can effectively improve the hardness and friction and wear properties while also taking into account the corrosion resistance of the stainless steel. The result is austenitic stainless steel with high hardness, good friction and wear properties, and good corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1The cross-sectional SEM images of the stainless steel samples of Examples 1-3 and Comparative Example 1 after grinding, polishing, and corrosion by corrosive liquid are shown, wherein (a) is Comparative Example 1, (b) is Example 1, (c) is Example 2, and (d) is Example 3;

[0029] Figure 2 The cross-sectional EDS images of the stainless steel samples of Examples 1-3 and Comparative Example 1 after grinding, polishing, and corrosion by corrosive liquid are shown, wherein (a) is Comparative Example 1, (b) is Example 1, (c) is Example 2, and (d) is Example 3;

[0030] Figure 3 The cross-sectional and surface microhardness change curves of stainless steel samples after ion nitrocarburizing with different laser shock energies in Examples 1-3 and Comparative Example 1 are shown;

[0031] Figure 4 XRD patterns of stainless steel samples after ion nitrocarburizing with different laser shock energies in Examples 1-3 and Comparative Example 1;

[0032] Figure 5 The friction coefficient change curves of stainless steel samples impacted with different laser shock energies in Examples 1-3 and Comparative Example 1 after ion nitrocarburizing;

[0033] Figure 6 Electrochemical impedance spectroscopy (EIS) graphs of stainless steel samples after ion nitrocarburizing with different laser shock energies in Examples 1-3 and Comparative Example 1;

[0034] Figure 7 is the equivalent circuit diagram of the impedance specimen;

[0035] Figure 8 Polarization curves of stainless steel samples after ion nitrocarburizing with different laser shock energies in Examples 1-3 and Comparative Example 1. DETAILED DESCRIPTION

[0036] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0037] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0038] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0039] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the present invention. The present description and examples are intended to be illustrative only.

[0040] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0041] The austenitic stainless steel used in the following examples is 316 austenitic stainless steel that has not been solution treated, and its chemical composition is shown in Table 1:

[0042] Table 1 Chemical composition of 316 austenitic stainless steel (mass fraction, %)

[0043]

[0044] In the following examples, a LAMBER-H40 laser system was used to impact harden austenitic stainless steel. The system consists of an Nd:YAG pulse laser external optical path, an external control program, a multi-axis robotic arm, a water confinement layer device, and a feedback controller.

[0045] In the following examples, an LDMC-100AZ fully automatic ion nitriding furnace was used to perform ion nitrocarburizing treatment on austenitic stainless steel.

[0046] Example 1

[0047] Stainless steel pretreatment: 316 austenitic stainless steel was processed into specimens with a size of 50 mm × 50 mm × 5 mm, and polished in sequence using 180-2000 mesh SiC sandpaper to a surface finish of Ra3.2 (to ensure uniformity of laser impact energy absorption). The specimens were then placed in anhydrous ethanol and ultrasonically cleaned for 15 minutes. After being taken out and blown dry, they were placed in a vacuum specimen bag and evacuated for later use.

[0048] Laser shock treatment: Pretreated 316 stainless steel specimens were laser-shock treated using 20J of shock energy (laser single pulse energy). The number of shock layers was one, with a 50% overlap ratio. The laser spot diameter was 4mm, the frequency of multiple shocks was 6Hz, the pulse width was 16ns, and the laser wavelength was 1064nm. Before the test, a 0.1mm-thick layer of black tape was applied to the surface of the specimen to be impacted as an absorbent protective layer, and a 1mm-thick layer of running water was used as a restraining layer.

[0049] After the laser shock treatment, the 316 austenitic stainless steel sample was cut into small samples of 20 mm × 20 mm × 5 mm and polished to a finish of Ra1.6 (to ensure the uniformity of the structure of the later ion nitrocarburized layer).

[0050] Ion Nitrocarburizing: Small samples of laser-shocked and cut 316 austenitic stainless steel were placed in an ion nitriding furnace for ion nitrocarburizing. The ion nitrocarburizing process was conducted at a holding temperature of 480°C for 5 hours. A mixture of ammonia, argon, and methane (with a flow ratio of 20:9:1) was introduced during the process, and the furnace pressure was maintained at 300 Pa. The heating process consisted of two stages: from room temperature to 300°C, and then from 300°C to the holding temperature. A mixed gas of argon and methane (the flow ratio of argon and methane is 9:1) is introduced from room temperature to 300°C, and the pressure in the furnace is 200Pa; when the temperature is raised to 300°C, ammonia is introduced, and the flow rate of ammonia gradually increases, that is, a mixed gas of ammonia, argon and methane is introduced from 300°C to the insulation temperature, so that the flow ratio of ammonia, argon and methane and the pressure in the furnace gradually increase with the increase of temperature. When the temperature reaches 480°C, the pressure in the furnace reaches 300Pa, and the flow ratio of ammonia, argon and methane reaches 20:9:1.

[0051] Example 2

[0052] The same as Example 1, except that the impact energy of the laser shock treatment is 25J.

[0053] Example 3

[0054] The same as Example 1, except that the impact energy of the laser shock treatment is 30J.

[0055] Comparative Example 1

[0056] Stainless steel pretreatment: 316 austenitic stainless steel was processed into specimens with a size of 50 mm × 50 mm × 5 mm, and polished in sequence with 180-2000 mesh SiC sandpaper to a surface finish of Ra3.2. The specimens were then placed in anhydrous ethanol and ultrasonically cleaned for 15 min. After being taken out and blown dry, they were placed in a vacuum specimen bag and evacuated for later use.

[0057] The pretreated 316 austenitic stainless steel specimens were cut into small samples of 20 mm × 20 mm × 5 mm and polished to a surface finish of Ra1.6.

[0058] Ion Nitrocarburizing: After pretreatment and re-cutting and polishing, small samples of 316 austenitic stainless steel were placed in an ion nitriding furnace for ion nitrocarburizing. The ion nitrocarburizing treatment was performed at a holding temperature of 480°C for 5 hours. During this process, a mixture of ammonia, argon, and methane (with a flow ratio of 20:9:1) was introduced into the furnace. The pressure in the furnace was maintained at 300 Pa. The heating process consisted of two stages: from room temperature to 300°C, and then from 300°C to the holding temperature. A mixed gas of argon and methane (the flow ratio of argon and methane is 9:1) is introduced from room temperature to 300°C, and the pressure in the furnace is 200Pa; when the temperature is raised to 300°C, ammonia is introduced, and the flow rate of ammonia gradually increases, that is, a mixed gas of ammonia, argon and methane is introduced from 300°C to the insulation temperature, so that the flow ratio of ammonia, argon and methane and the pressure in the furnace gradually increase with the increase of temperature. When the temperature reaches 480°C, the pressure in the furnace reaches 300Pa, and the flow ratio of ammonia, argon and methane reaches 20:9:1.

[0059] Compared with Example 1, in this comparative example, the laser shock treatment is omitted (or the shock energy of the laser shock treatment is 0 J), and the 316 austenitic stainless steel is directly subjected to ion nitrocarburizing treatment.

[0060] Effect verification

[0061] 1. Morphology, structure and element distribution

[0062] The cross sections of the samples after ion nitrocarburizing in Examples 1-3 and Comparative Example 1 were ground and polished, and the cross sections were wiped and etched with a marble solution (CuSO4 4 g + HCl 20 mL + H2O 20 mL) as an etching solution to prepare metallographic samples.

[0063] The JEOL JSM-7600F field emission scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were used to observe the microstructure and analyze the composition of the surface layer of the samples of each embodiment and comparative example after being corroded by the corrosive solution.

[0064] Figure 1 These are cross-sectional SEM images of the stainless steel samples of Examples 1-3 and Comparative Example 1 after grinding, polishing, and corrosion with corrosive liquid, wherein (a) is Comparative Example 1, (b) is Example 1, (c) is Example 2, and (d) is Example 3. Figure 2The cross-sectional EDS images of the stainless steel samples of Examples 1-3 and Comparative Example 1 after grinding, polishing and corrosion by the corrosive solution are shown in Figure 1, where (a) is Comparative Example 1, (b) is Example 1, (c) is Example 2, and (d) is Example 3. Figure 1 It can be seen from the SEM images in that the stainless steel sample without laser shock (Comparative Example 1) and the stainless steel sample subjected to laser shock with shock energies of 20J (Example 1), 25J (Example 2) and 30J (Example 3), after ion nitrogen and carbon diffusion at 480°C and heat preservation for 5 hours, the thickness of the nitrogen and carbon diffusion layer is 27μm, 37μm, 32μm and 40μm respectively. That is to say, with the increase of laser shock energy, the thickness of the diffusion layer on the surface of the stainless steel sample first increases and then decreases slightly and then increases again. From the morphology point of view, the nitrogen and carbon diffusion layers of (a) and (b) are relatively uniform, without obvious stratification and cracks, while (c) and (d) have obvious stratification and interfaces, but the diffusion layer thickness of (d) is thicker and more uniform. In addition, from Figure 2 The EDS line scans clearly show that, compared to the stainless steel substrate, the Fe content in the nitrocarburized layer of stainless steel samples impacted at different laser shock energies significantly decreases, while the Cr content increases. With increasing laser shock energy, the N content in the nitrocarburized layer on the stainless steel surface first increases, then slightly decreases, and then increases again. The C content in (a) is very low and almost undetectable (because without laser shock, the surface dislocations are minimal, resulting in minimal C diffusion). In contrast, the C content in the nitrocarburized layer in (b), (c), and (d) increases significantly, indicating that the C content in the nitrocarburized layer gradually increases with increasing laser shock energy. The N, C (except for (a)), Fe, and Cr content in the nitrocarburized layers of (a), (b), (c), and (d) exhibit a jagged pattern. Areas where the N content decreases also experience a decrease in C (except for (a)), while the Fe and Cr contents increase, and vice versa.

[0065] 2. Microhardness analysis

[0066] The cross section and surface of the sample after ion nitrocarburizing were polished to a mirror surface, and the microhardness of the cross section and surface of the nitrocarburized layer was tested using a HXD-1000 micro Vickers hardness tester with a test pressure of 50 g and a holding time of 15 s.

[0067] Figure 3 The cross-section and surface microhardness change curves of stainless steel samples after ion nitrocarburizing with different laser shock energies in Examples 1-3 and Comparative Example 1 are shown in FIG. Figure 3It can be seen that the highest hardness of the stainless steel samples with impact energies of 0J, 20J, 25J, and 30J is on the outer surface of the sample, reaching 1100, 1207, 981, and 1250.5 HV0.05, respectively, while the substrate hardness is around 190 HV0.05. In other words, the maximum hardness of the nitrocarburized layer is 5.5 to 6.5 times that of the substrate. Aside from the surface (where the surface hardness is highest at 30J), at the same distance from the sample surface, the stainless steel sample with a laser impact energy of 20J has the highest hardness (i.e., the hardness curve is on the far right). The hardness gradient of the nitrocarburized layer is also relatively small, and the hardness gradient is almost zero at 25-35μm, which greatly reduces the risk of nitrocarburized layer shedding. The stainless steel sample with an impact energy of 30J also has a higher hardness, second only to the sample with a laser impact energy of 20J, but its hardness gradient is large, making it prone to shedding. The samples with the lowest hardness are those with impact energies of 0J and 25J, and their hardness curves are almost the same.

[0068] 3. Phase analysis of the seepage layer

[0069] Before phase analysis, the surface of the ion nitrocarburized sample was polished to a mirror finish. Phase analysis of the carburized layer of the stainless steel sample was performed using a Rigaku Ultima IV Cu Ka target X-ray diffractometer. Based on the relevant tissue PDF card, the angle was selected between 30° and 100°. The speed was selected at 5° / min, the high voltage was selected at 40kV, and the test current was selected at 40mA.

[0070] Figure 4 The XRD patterns of stainless steel samples after ion nitrocarburizing with different laser shock energies in Examples 1-3 and Comparative Example 1 are shown. Figure 4 It can be seen that the diffusion layers of stainless steel samples that have been laser impacted with impact energies of 0J, 20J, 25J and 30J are composed of three phases: γ phase, S phase and Cr2N phase. Among them, the phase composition of the diffusion layers of stainless steel samples with impact energies of 20J and 30J is not much different. They are mainly composed of S phase, with a small amount of γ phase and Cr2N phase. The phase composition of the diffusion layers of stainless steel samples with impact energies of 0J and 25J is not much different. The amount of γ phase increases significantly, the amount of S phase decreases significantly, and there is also a small amount of Cr2N phase. The γ phase is the austenite phase, which has low hardness and poor wear resistance, but good corrosion resistance. The S phase is a saturated γ phase. N Phase, has good corrosion resistance and wear resistance, while Cr2N phase has high hardness but poor corrosion resistance. A comparative analysis of the content of each phase in the carburized layer of stainless steel samples with impact energy of 20J and 30J and stainless steel samples with impact energy of 0J and 25J found that the absolute content of S phase in the former is much higher than that in the latter, the content of Cr2N phase is basically the same, and the absolute content of γ phase in the latter is higher than that in the former.

[0071] S phase is divided into SN phase (formed by diffusion of N element into γ phase), S C (formed by the diffusion of C elements into the γ phase) and S NC Phase ((N and C elements diffuse into the γ phase at the same time), the S phase formed in the austenitic stainless steel after laser shock treatment + ion nitrocarburizing treatment in the present invention is S NC Phase. S N High phase hardness, poor toughness, easy to fall off; S C Good toughness, low hardness, slightly poor wear resistance. NC The phase combines the advantages of the two very well, so that the diffusion layer has both high hardness and good toughness, as well as good corrosion resistance.

[0072] 4. Friction and wear performance analysis

[0073] Before the friction and wear tests, the surfaces of the samples after ion nitrocarburizing were first polished to a mirror finish. The experimental equipment used was an HT-2005 testing machine with a load of 10N, a wear time of 1800s, a WC ball with a diameter of 6mm, and a rotation speed of 318rpm.

[0074] Figure 5 The friction coefficient change curve of stainless steel samples after ion nitrocarburizing with different laser shock energies in Examples 1-3 and Comparative Example 1 is shown. There are many factors that affect the change of friction coefficient, mainly surface hardness, phase structure and its ratio, roughness, etc. Figure 5 It can be seen that the friction coefficient of the stainless steel sample with a laser impact energy of 30J is the lowest, about 0.53 to 0.65, which is related to its higher hardness and thickest carburized layer. The second is the sample with a laser impact energy of 20J, with a friction coefficient of about 0.52 to 0.68. Figure 5 It can be seen that the friction coefficients of all samples increase with increasing friction time. The stainless steel sample with an impact energy of 25J has the highest friction coefficient, approximately 0.73 at steady state. This is due to its lower hardness and roughness (polished before the experiment). The friction coefficient of the sample without laser shock increases first and then decreases. The large fluctuations in the friction coefficients of the stainless steel samples are due to the high surface roughness caused by laser shock.

[0075] 5. Electrochemical analysis

[0076] Before the electrochemical test, the surface of the sample after ion nitrocarburizing was first polished to a mirror finish. Tafel polarization curves and Nyquist plots were performed on the stainless steel samples of Examples 1-3 and Comparative Example 1 using a Shanghai Chenhua CHI660E electrochemical workstation. The stainless steel sample served as the working electrode, the auxiliary electrode was a platinum electrode, the reference electrode was a saturated calomel electrode (SCE), and the solution was a 3.5% NaCl solution.

[0077] Figure 6 The electrochemical impedance spectroscopy (EIS) graphs of the stainless steel samples after ion nitrocarburizing with different laser shock energies in Examples 1-3 and Comparative Example 1 are shown. Figure 6 It can be seen that in 3.5% NaCl solution, the Nyquist plots of the surface passivation film of the stainless steel samples after nitrocarburizing with different laser shock energies of 0J, 20J, 25J and 30J all show a single capacitive arc. The largest capacitive arc is the sample with a laser shock energy of 20J, followed by the sample with an impact energy of 30J. The capacitive arc is the smallest for the samples with impact energies of 0J and 25J. The larger the curvature radius of the capacitive arc, the greater the capacitance of the passivation film, the greater the density, and the better the corrosion resistance. Figure 7 The equivalent circuit diagram of the impedance sample shown in the figure is used to fit the EIS data using ZsimpWin software, where Rs is the solution resistance, Rt is the charge transfer resistance, CPE represents the constant phase angle element of the double layer capacitance characteristic, and its impedance (Z CPE )for:

[0078] Z CPE =(j ω -n ) / Y0 (1)

[0079] Where: ω is the angular frequency, rad / s; Y0 is the CPE parameter, F / (sn -1 cm 2 ); n is a dimensionless index that indicates the degree to which CPE deviates from pure capacitance. The closer its value is to 1, the closer its passivation film is to the ideal capacitance. Figure 7 The fitting curve fits the measured curve very well, and the fitting error of each component is within 5%, indicating that the equivalent circuit basically reflects the actual corrosion behavior.

[0080] Table 2

[0081]

[0082] The fitting parameters of the equivalent circuit are shown in Table 2. As can be seen from Table 2, when the laser shock energy is 20J, the stainless steel nitrocarburized layer passivation film has the highest charge transfer resistance and the lowest double-layer capacitance, followed by the stainless steel sample with a laser shock energy of 30J. The stainless steel samples with laser shock energies of 0J and 25J have the lowest charge transfer resistance and the highest double-layer capacitance. This indicates that under these nitrocarburizing conditions, the passivation film formed on the surface of the stainless steel sample with a laser shock energy of 20J is thicker, has the best density and corrosion resistance, and can effectively hinder the penetration and migration of reactive ions. The stainless steel sample with a laser shock energy of 30J is the second best, while the stainless steel samples with laser shock energies of 0J and 25J have the worst corrosion resistance, which is consistent with the results of the polarization curve test.

[0083] Figure 8 Table 3 shows the polarization curves of stainless steel samples after ion nitrocarburizing with different laser shock energies in Examples 1-3 and Comparative Example 1. Figure 8 The electrochemical parameters obtained by fitting the polarization curve are Figure 8 The polarization curves of the stainless steel samples with different laser shock energies and the electrochemical parameters fitted by the polarization curves in Table 3 clearly show that there is no obvious passivation zone in the polarization curves of the nitrocarburized layer. The self-corrosion current (I P ) (0.948μA and 0.852μA, respectively) and self-corrosion potential (E corr The lowest values ​​were for the stainless steel samples with laser shock energies of 0 J and 25 J (-0.324 V and -0.335 V, respectively), followed by the stainless steel samples with laser shock energies of 0 J and 25 J. This means that the stainless steel samples with laser shock energies of 20 J and 30 J have the best corrosion resistance, which confirms the results of electrochemical impedance spectroscopy.

[0084] Table 3

[0085]

[0086] The combined treatment process of laser shock and ion nitrocarburizing has a significant effect on the corrosion resistance, friction and wear properties, and microhardness of stainless steel. Based on the above results, the following analysis is made: laser shock causes dislocations and other defects to entangle or aggregate at specific locations, thereby affecting the diffusion of N and C elements and the formation of nitrogen carbides, causing nitrogen and carbon atoms to aggregate along specific directions or interfaces, forming Figure 2The jagged distribution of N and C elements in the nitrocarburized layer is shown in Figure 2. The reason why the changes in N and C elements are opposite to those in Cr and Fe elements may be that during nitrocarburizing, N and C atoms enter the metal lattice and form a substantial solid solution compound with the metal atoms. In this compound, N and C atoms replace some of the Cr and Fe atoms, resulting in a relative decrease in the Cr and Fe elements in the nitrocarburized layer. In terms of phase composition, a comparative analysis of the phase content of the nitrocarburized layer of stainless steel samples with impact energies of 20J and 30J and those with impact energies of 0J and 25J revealed that the S phase content in the former was much higher than that in the latter, the Cr2N content was basically the same, and the absolute content of the γ phase in the latter was higher than that in the former. That is to say, in the composite strengthening of 316 stainless steel by laser shock plus ion nitrocarburizing (keeping at 480℃ for 5h), the generation or decomposition of S phase may be related to the magnitude of the impact energy. Using an impact energy value within a certain range (such as 20J) for laser shock is conducive to the generation of S phase or inhibits its decomposition, while using an impact energy value within another range (such as 25J) for laser shock may slow down the generation of S phase or promote its decomposition process.

[0087] Through the performance data analysis of the cross-section microhardness, friction and wear test and electrochemical experiment of the nitrocarburized layer, it was found that the stainless steel sample with an impact energy of 20J not only had the highest hardness and the lowest friction coefficient, but also had the best corrosion resistance, followed by the sample with a laser impact energy of 30J. The sample with the smallest hardness, the lowest friction coefficient and the worst corrosion resistance was the sample with an impact energy of 25J. From the phase analysis, the present invention found that this may be related to the content of S phase. The content of S phase in the stainless steel samples with impact energies of 20J and 30J was significantly higher than that in the stainless steel samples with impact energies of 0J and 25J. This may be the reason why it has both high hardness and wear resistance and good corrosion resistance. Under the process conditions of the present invention, within the range of 0-30J laser impact energy, as the laser impact energy increases, the cross-section hardness, friction coefficient and corrosion resistance of the stainless steel nitrocarburized layer all show a sine curve change (first increase, then decrease and then increase), with obvious peaks and troughs. Compared with the previous result analysis, the change in the performance of the stainless steel nitrocarburized layer may be mainly related to the content and distribution of S phase.

[0088] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for surface strengthening of austenitic stainless steel, characterized in that: First, the austenitic stainless steel is subjected to laser shock treatment, and then the austenitic stainless steel after the laser shock treatment is subjected to ion nitrocarburizing treatment; the impact energy of the laser shock treatment is 20J; The holding temperature of the ion nitrocarburizing treatment is 450-520°C and the holding time is 4-8h; During the heat preservation process of the ion nitrocarburizing treatment, a mixed gas of ammonia, argon and methane is introduced at a pressure of 300 Pa; The flow ratio of ammonia, argon and methane is 20:9:

1.

2. The surface strengthening treatment method for austenitic stainless steel according to claim 1, characterized in that: The number of impact layers in the laser impact treatment is 1 or 2, and the overlap rate is 40-60%.

3. The surface strengthening treatment method for austenitic stainless steel according to claim 1, characterized in that: Before laser shock treatment, the austenitic stainless steel is pretreated, including grinding the austenitic stainless steel and then ultrasonically cleaning it in anhydrous ethanol.

4. The surface strengthening treatment method for austenitic stainless steel according to claim 3, characterized in that: The grinding is specifically grinding until the surface roughness of the austenitic stainless steel is ≤ Ra3.

2.

5. The surface strengthening treatment method for austenitic stainless steel according to claim 3, characterized in that: The ultrasonic cleaning time is not less than 15 minutes.

6. Austenitic stainless steel obtained by treatment according to the surface strengthening treatment method of austenitic stainless steel according to any one of claims 1 to 5.

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