A method for controlling the thickness of nitrogen-supersaturated austenite phase layer during plasma nitrocarburizing process

Abstract

This invention discloses a method for controlling the thickness of the nitrogen-supersaturated austenite phase layer during plasma nitriding, comprising the following steps: placing a 316L stainless steel workpiece in a nitriding furnace and performing plasma nitriding for 6 hours under a furnace pressure of 130 Pa; and adjusting the γ-ray distillation temperature, nitrogen volume ratio, and workpiece cathode voltage to control the thickness of the nitrogen-supersaturated austenite phase layer during plasma nitriding. N The phase layer thickness δ is controlled; the calculation formula for δ is as follows: where T is the diffusion thermodynamic temperature, c is the nitrogen volume ratio, U is the workpiece cathode voltage, and ±Δσ is the error term. This invention controls the γ phase thickness δ during the plasma nitrocarburizing process by adjusting the diffusion thermodynamic temperature, nitrogen volume ratio, and workpiece cathode voltage. N Phase layer thickness and γ N The ratio of the phase layer thickness to the thickness of the nitrocarbon co-diffusion modified layer can be controlled to improve the surface hardness and pitting resistance of the workpiece.

Description

Technical Field

[0001] This invention relates to the field of ion plating technology, and specifically to a method for controlling the thickness of the nitrogen-supersaturated austenite phase layer during the plasma nitrocarburizing process. Background Technology

[0002] When ion plating is performed at low temperatures, the resulting coating is primarily composed of metastable supersaturated phases. This coating can improve the hardness of the substrate without affecting its corrosion resistance. Metastable supersaturated phases are commonly referred to as "expanded austenite" or "S" phases, such as nitrogen-supersaturated austenite phase γ. N γ-carbon supersaturated austenite phase C In the austenitic FCC (face-centered cubic) lattice, interstitial nitrogen and carbon atoms are dissolved in the octahedral interstitial positions at the center of the unit cell. N Phase layer and γ C Although the phase layers have similar structures, the lattice expansion associated with N is greater than that caused by C, γ N Phase layer compared to γ C The phase layer will produce greater expansion and higher density of dislocations on the crystal plane, thus γ N Phase layer and γ C The phase layer has higher hardness compared to the previous one. Furthermore, when pitting corrosion occurs on the passivation film of stainless steel, γ... N Nitrogen atoms dissolved on the phase surface are released during corrosion, which can consume the H atoms in the pitting pits. + Formation of NH 4+ This inhibits the growth of pitting corrosion, thus γ N Phase layer and γ C Compared to other phase layers, it also has better resistance to pitting corrosion.

[0003] Therefore, it is necessary to develop a method that can regulate γ during the plasma nitrocarbon co-infiltration (PNC) process. N The method of phase layer thickness is of great significance. Summary of the Invention

[0004] The purpose of this invention is to provide a method for controlling the thickness of the nitrogen-supersaturated austenite phase layer during the plasma nitrocarburizing process.

[0005] The technical solution adopted in this invention is:

[0006] A method for controlling the thickness of the nitrogen-supersaturated austenite phase layer during plasma nitrocarburizing process includes the following steps:

[0007] A 316L stainless steel workpiece was placed in a nitriding furnace and subjected to plasma nitrocarburizing at a furnace pressure of 130 Pa for 6 hours. The γ-ray diffusing temperature, nitrogen volume ratio, and cathode voltage of the workpiece were adjusted to control the γ-ray diffusing temperature. NThe phase layer thickness δ is controlled; the formula for calculating δ is as follows: In the formula, T is the thermodynamic temperature of diffusion coating (in K), c is the volume percentage of nitrogen (expressed as a decimal, for example, 63% is expressed as 0.63), U is the cathode voltage of the workpiece (in V), and ±Δσ is the error term.

[0008] Preferably, the specific operation of the plasma nitrocarbon co-diffusion is as follows:

[0009] 1) Hydrogen pretreatment: Place the 316L stainless steel workpiece in a nitriding furnace, then evacuate and introduce hydrogen gas, and then perform hydrogen plasma sputtering on the surface of the workpiece.

[0010] 2) First stage low-temperature plasma nitriding: Methane, nitrogen, hydrogen and argon are introduced into the nitriding furnace, and then plasma nitriding is performed on the workpiece treated in step 1).

[0011] 3) Second stage low-temperature plasma nitrocarburizing: Methane, nitrogen and hydrogen are introduced into the nitriding furnace, and the workpiece treated in step 2) is then subjected to plasma nitrocarburizing, and then cooled to room temperature with the furnace.

[0012] Preferably, the workpiece described in step 1) has been polished and cleaned before hydrogen plasma sputtering.

[0013] Preferably, the specific operation of vacuuming in step 1) is to vacuum until the pressure inside the furnace is 7Pa to 8Pa.

[0014] Preferably, the operating parameters for hydrogen plasma sputtering in step 1) are: hydrogen flow rate of 180 mL / min to 220 mL / min, furnace pressure maintained at 60 Pa to 80 Pa, workpiece cathode voltage of 580 V to 600 V, furnace temperature of 280 °C to 320 °C, and sputtering time of 50 min to 70 min. Using hydrogen plasma to bombard the surface of 316L stainless steel workpieces effectively removes the oxide film, reducing the obstruction of active atoms to the substrate during nitrocarburizing. Furthermore, hydrogen molecules have a smaller diameter than argon molecules, resulting in lower energy gain during the diffusion process, leading to less sputtering loss to the substrate and reducing the impact on the workpiece's corrosion resistance. The sputtering time is controlled at 50 min to 70 min to effectively remove the passivation film from the workpiece surface and maintain its active state; however, excessively long sputtering times increase costs.

[0015] Preferably, the operating parameters for plasma nitrocarburizing in step 2) are: methane flow rate of 100 mL / min to 200 mL / min, nitrogen flow rate of 400 mL / min to 600 mL / min, hydrogen flow rate of 200 mL / min to 250 mL / min, argon flow rate of 40 mL / min to 60 mL / min, furnace pressure maintained at 130 Pa, workpiece cathode voltage of 630 V to 880 V, furnace temperature of 380 °C to 450 °C, and plasma nitrocarburizing time of 25 min to 35 min. Simultaneous introduction of hydrogen and argon as infiltration aids allows nitrogen on the substrate surface to be affected by the stress induced by hydrogen, increasing the driving force for nitrogen diffusion and thus enabling deeper nitrogen penetration. Furthermore, the higher nitrogen concentration produced by the presence of hydrogen further allows carbon to diffuse deeper into the substrate, resulting in a thicker nitrocarburized modified layer on the 316L stainless steel workpiece. The presence of argon can increase the concentration of high-energy particles in the plasma for a short period of time, but this should be controlled within 25 to 35 minutes to prevent the strong sputtering effect of argon from increasing defects on the workpiece surface and thus affecting the workpiece performance.

[0016] Preferably, the operating parameters for the plasma nitrocarburizing in step 3) are: methane flow rate of 100 mL / min to 200 mL / min, nitrogen flow rate of 500 mL / min to 600 mL / min, furnace pressure maintained at 130 Pa, workpiece cathode voltage of 630 V to 880 V, furnace temperature of 380 °C to 450 °C, and plasma nitrocarburizing time of 265 min to 275 min. Stopping the argon gas flow and increasing the hydrogen gas flow rate allows the hydrogen-containing gas mixture to react with the carbon on the surface of the 316L stainless steel workpiece to form CH3 free radicals, thereby achieving decarburization and ensuring that nitrogen atoms can continue to penetrate into the matrix, increasing the nitrogen-rich γ layer within the modified layer. N The thickness of the phase layer.

[0017] Preferably, the methane flow rates in steps 2) and 3) are the same.

[0018] Preferably, the nitrogen flow rates in steps 2) and 3) are the same.

[0019] Preferably, the thickness of the nitrocarbon co-diffusion modified layer formed by the first stage of low-temperature plasma nitrocarbon co-diffusion in step 2) and the second stage of low-temperature plasma nitrocarbon co-diffusion in step 3) is greater than 20 μm. Controlling the total plasma nitrocarbon co-diffusion time to 6 hours can yield a nitrocarbon co-diffusion modified layer with a thickness greater than 20 μm. As the thickness of the nitrocarbon co-diffusion modified layer increases, the path of active nitrogen atoms penetrating into the substrate becomes longer, and the diffusion rate slows down with prolonged plasma nitrocarbon co-diffusion time. Excessive plasma nitrocarbon co-diffusion time can cause phase transitions and the precipitation of compounds, resulting in higher costs and poor diffusion effects.

[0020] The beneficial effects of this invention are: This invention adjusts the thermodynamic temperature of the diffusion plating, the nitrogen volume ratio, and the cathode voltage of the workpiece to control the γ-ray dispersion during the plasma nitrocarburizing process. N Phase layer thickness and γ N The ratio of the phase layer thickness to the thickness of the nitrocarbon co-diffusion modified layer can be controlled to improve the surface hardness and pitting resistance of the workpiece.

[0021] Specifically:

[0022] 1) This invention yields the thermodynamic temperature of diffusion plating, the nitrogen volume ratio, the workpiece cathode voltage, and the γ during the plasma nitrocarburizing process. N The nonlinear functional relationship between phase layer thicknesses, δ=f(T,c,U), allows γ to be adjusted by modifying the process parameters in the function. N The phase layer thickness is good, the process stability is good, it is easy to control, and the preparation cost is low;

[0023] 2) This invention utilizes the discharge states of hydrogen and argon during plasma nitrocarburizing by adjusting the timing of their addition, the gas combination, and their ratio. This facilitates the infiltration of nitrogen atoms by the discharge states of hydrogen and argon during the nitrocarburizing process. Furthermore, it utilizes the nitrogen atoms in the plasma during the nitrocarburizing process... 2+ The difference in the degree of dissociation between nitrogen ions and carbon-based molecules allows for the selection of appropriate hydrogen and argon flow rates and holding times to enhance γ-ray diffraction. N Phase layer thickness and γ N The proportion of the phase layer thickness to the thickness of the nitrocarbon co-diffusion modified layer improves the surface hardness and pitting resistance of the workpiece.

[0024] 3) This invention utilizes hydrogen as the primary nitriding aid gas. By adjusting the hydrogen flow rate to maintain a constant total gas pressure within the nitriding furnace, and by changing the flow rates of methane and nitrogen, the gas volume ratio is adjusted, thereby adjusting γ. N The thickness of the phase layer (nitrogen-rich layer) is adjusted, and the proportion of methane gas is adjusted to avoid excessive carbon potential causing the precipitation of carbon compounds (Fe3C) in the nitrogen-carbon co-diffusion modified layer, which would affect the surface properties of the workpiece. Attached Figure Description

[0025] Figure 1 Metallographic cross-sectional views of 316L stainless steel workpieces in groups A, B, C, D, E and F of Example 1.

[0026] Figure 2 The images show the XRD patterns of 316L stainless steel workpieces from groups A, B, C, D, E, and F in Example 1.

[0027] Figure 3 The graph shows the fitting of the δ=f(T,c,U) function in Example 1.

[0028] Figure 4Metallographic cross-sectional views of the 316L stainless steel workpieces in groups G, H, I and J in Example 2.

[0029] Figure 5 The graph shows the fitting of the δ=f(T,c,U) function in Example 2.

[0030] Figure 6 Metallographic cross-sectional views of the 316L stainless steel workpieces in groups K, L, M and N of Example 3.

[0031] Figure 7 The graph shows the fitting of the δ=f(T,c,U) function in Example 3. Detailed Implementation

[0032] The present invention will be further explained and described below with reference to specific embodiments.

[0033] Example 1:

[0034] A method for controlling the thickness of the nitrogen-supersaturated austenite phase layer during plasma nitrocarburizing is proposed. This involves grouping experiments under different experimental conditions, varying the volume ratios of methane and nitrogen in the diffusion gas, and adjusting the hydrogen flow rate to maintain a constant total furnace pressure. The method verifies the effect of γ-ray dilution. N The phase layer thickness δ has a nonlinear functional relationship with the diffusion thermodynamic temperature, nitrogen volume ratio, and workpiece cathode voltage, specifically including the following steps:

[0035] 1) Hydrogen pretreatment: The 316L stainless steel workpiece (the chemical composition of 316L stainless steel is shown in Table 1) is polished and cleaned, then placed on the cathode plate of the PN-IV type multi-functional glow discharge ion nitriding furnace. After the furnace pressure is evacuated to 7.6Pa, hydrogen gas is introduced, and then hydrogen plasma sputtering is performed on the surface of the workpiece. The operating parameters of hydrogen plasma sputtering are: hydrogen flow rate of 200mL / min, furnace pressure maintained at 70Pa, workpiece cathode voltage of 600V, furnace temperature of 300℃, and sputtering time of 60min.

[0036] Table 1 Chemical composition of 316L stainless steel

[0037]

[0038] 2) First stage low-temperature plasma nitrocarburization (flow rate and volume ratio of methane and nitrogen are shown in Table 2): The N2+CH4+H2+Ar ion nitrocarburization process is adopted, with N2 as the nitrogen source and CH4 as the carbon source. A mixed gas composed of H2 and Ar is selected as the infiltration aid gas to perform plasma nitrocarburization on the workpiece after step 1). The operating parameters of plasma nitrocarburization are: methane flow rate of 100mL / min~200mL / min, nitrogen flow rate of 400mL / min~600mL / min, hydrogen flow rate of 200mL / min~250mL / min, argon flow rate of 50mL / min, furnace pressure maintained at 130Pa, workpiece cathode voltage of 730V, furnace temperature of 450℃, and plasma nitrocarburization time of 30min.

[0039] 3) Second stage low-temperature plasma nitrocarburization (the flow rate and volume ratio of nitrogen and methane are shown in Table 2): The N2+CH4+H2 ion nitrocarburization process is adopted, with N2 as the nitrogen source, CH4 as the carbon source, and H2 as the infiltration aid gas to perform plasma nitrocarburization on the workpiece after step 2). The operating parameters of plasma nitrocarburization are: methane flow rate of 100mL / min~200mL / min, nitrogen flow rate of 400mL / min~600mL / min, hydrogen flow rate adjusted to maintain the furnace pressure at 130Pa, workpiece cathode voltage of 730V, furnace temperature of 450℃, and plasma nitrocarburization time of 330min, followed by furnace cooling to room temperature.

[0040] Table 2. Flow rate and volume percentage (c%) of methane and nitrogen.

[0041]

[0042] Experimental comparative analysis:

[0043] 1) Metallographic cross-sectional views of the 316L stainless steel workpieces in groups A, B, C, D, E, and F in this embodiment are as follows: Figure 1 As shown.

[0044] Depend on Figure 1 It can be seen that the γ of the 316L stainless steel workpieces in groups A, B, C, D, E, and F is... N The phase layer thicknesses were 10.0 μm, 8.4 μm, 6.3 μm, 5.8 μm, 4.2 μm and 3.7 μm, respectively.

[0045] 2) The X-ray diffraction (XRD) patterns of the 316L stainless steel workpieces in groups A, B, C, D, E, and F of this embodiment are shown below. Figure 2 As shown.

[0046] Depend on Figure 2It can be seen that the γ of the 316L stainless steel workpieces in groups A, B, C, D, E, and F is... NC (111) The phase peak positions are at 41.187°, 41.314°, 41.627°, 41.702°, 41.732° and 42.618° respectively. The greater the interstitial atom content in the expanded austenite, the greater the lattice distortion and the greater the peak shift. The interstitial atom content in the A group of the diffusion layer is the largest, that is, the γ-phase peak in the nitrogen-carbon co-diffusion modified layer is the largest. N The proportion of phase layers is higher, which is consistent with the metallographic cross-section diagram.

[0047] It can be seen that γ N The phase layer thickness δ exhibits a nonlinear functional relationship with the diffusion thermodynamic temperature, nitrogen volume ratio, and workpiece cathode voltage, i.e. In this embodiment, T = 723.15 K, U = 730 V, time is 6 hours, and air pressure is 130 Pa. Therefore, the functional relationship δ = f(T, c, U) can be simplified to δ = f(c) = 56.417c. 3.922 Substituting ±Δσ into the c values ​​corresponding to experimental groups A, B, C, D, E, and F in this embodiment (c = 0.63, 0.61, 0.60, 0.57, 0.52, and 0.47 respectively), the calculated δ values ​​are 9.19 μm, 8.11 μm, 7.60 μm, 6.22 μm, 4.34 μm, and 2.92 μm (the fitting graph of the function δ = f(T, c, U) is shown in the figure). Figure 3 As shown in the figure, the average error term is approximately 0.52 μm compared with the metallographic test results.

[0048] In summary, γ N The semi-empirical formula δ, which relates phase layer thickness δ to nitrogen content process parameters δ=f(T,c,U), is consistent with the experimental results.

[0049] Example 2:

[0050] A method for controlling the thickness of the nitrogen-supersaturated austenite phase layer during the plasma nitrocarburizing process, except for temperature, is the same as that in Example 1 (the ion nitrocarburizing process parameters are shown in Table 3).

[0051] Table 3 Ion Nitrogen-Carbon Co-diffusion Process Parameters

[0052]

[0053] Experimental comparative analysis:

[0054] Metallographic cross-sectional views of the 316L stainless steel workpieces in groups G, H, I, and J in this embodiment are shown below. Figure 4 As shown.

[0055] Depend on Figure 4 It can be seen that the γ of 316L stainless steel workpieces in groups G, H, I and J is...N The phase layer thicknesses were 10.0 μm, 7.9 μm, 5.8 μm and 3.2 μm, respectively.

[0056] It can be seen that γ N The phase layer thickness δ exhibits a nonlinear functional relationship with the diffusion thermodynamic temperature, nitrogen volume ratio, and workpiece cathode voltage, i.e. In this embodiment, c = 0.63, U = 730V, time is 6 hours, and air pressure is 130Pa. Therefore, the functional relationship δ = f(T, c, U) can be simplified to... Substituting the values ​​of T (723.15K, 693.15K, 673.15K, and 653.15K respectively) for the experimental groups G, H, I, and J in this embodiment, the calculated values ​​of δ are 9.55μm, 7.15μm, 5.85μm, and 4.73μm (the fitting graph of the function δ=f(T,c,U) is shown in the figure). Figure 5 As shown in the figure, the average error term is approximately 0.61 μm compared with the metallographic test results.

[0057] In summary, γ N The semi-empirical formula δ, which relates phase layer thickness δ to temperature process parameters δ=f(T,c,U), is consistent with the experimental results.

[0058] Example 3:

[0059] A method for controlling the thickness of nitrogen-supersaturated austenite phase layer in the plasma nitrocarburizing process, except for voltage, is the same as that in Example 1 (the ion nitrocarburizing process parameters are shown in Table 4).

[0060] Table 4. Ion Nitrogen-Carbon Co-diffusion Process Parameters

[0061]

[0062] Experimental comparative analysis:

[0063] Metallographic cross-sectional views of the 316L stainless steel workpieces in groups K, L, M, and N in this embodiment are shown below. Figure 6 As shown.

[0064] Depend on Figure 6 It can be seen that the γ of 316L stainless steel workpieces in groups G, H, I and J is... N The phase layer thicknesses were 11.0 μm, 10.0 μm, 7.9 μm and 3.7 μm, respectively.

[0065] It can be seen that γ N The phase layer thickness δ exhibits a nonlinear functional relationship with the diffusion thermodynamic temperature, nitrogen volume ratio, and workpiece cathode voltage, i.e. In this embodiment, c = 0.63, T = 693.15 K, time is 6 h, and air pressure is 130 Pa. That is, the functional relationship δ = f(T, c, U) can be simplified to δ = f(c) = 5.077 × (0.00158 U). 2.4 Substituting ±Δσ into the U values ​​corresponding to experimental groups G, H, I, and J in this embodiment (U = 880V, 830V, 730V, and 630V respectively), the calculated δ values ​​are 11.19μm, 9.7μm, 7.1μm, and 4.92μm (δ=f(T,c,U) function fitting graph is shown in the figure). Figure 7 As shown in the figure, the average error term is approximately 0.71 μm compared with the metallographic test results.

[0066] In summary, γ N The semi-empirical formula for phase layer thickness δ and voltage process parameters δ=f(T,c,U) is consistent with the experimental results.

[0067] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for controlling the thickness of the nitrogen-supersaturated austenite phase layer during plasma nitrocarburizing process, characterized in that, Includes the following steps: A 316L stainless steel workpiece was placed in a nitriding furnace and subjected to plasma nitrocarburizing at a furnace pressure of 130 Pa for 6 hours. The γ-ray diffusing temperature, nitrogen volume ratio, and cathode voltage of the workpiece were adjusted to control the γ-ray diffusing temperature. N The phase layer thickness δ is controlled; the formula for calculating δ is as follows: In the formula, T is the thermodynamic temperature of diffusion coating, c is the volume percentage of nitrogen, U is the cathode voltage of the workpiece, and ±Δσ is the error term. The specific operation of the plasma nitrocarbon co-diffusion is as follows: 1) Hydrogen pretreatment: Place the 316L stainless steel workpiece in a nitriding furnace, then evacuate and introduce hydrogen gas, and then perform hydrogen plasma sputtering on the surface of the workpiece. 2) First stage low-temperature plasma nitriding: Methane, nitrogen, hydrogen and argon are introduced into the nitriding furnace, and then plasma nitriding is performed on the workpiece treated in step 1). 3) Second stage low temperature plasma nitrocarburizing: Methane, nitrogen and hydrogen are introduced into the nitriding furnace, and then the workpiece treated in step 2) is subjected to plasma nitrocarburizing, and then cooled to room temperature with the furnace. The operating parameters for the plasma nitrocarburizing process in step 2) are as follows: methane flow rate is 100 mL / min to 200 mL / min, nitrogen flow rate is 400 mL / min to 600 mL / min, hydrogen flow rate is 200 mL / min to 250 mL / min, argon flow rate is 40 mL / min to 60 mL / min, furnace pressure is maintained at 130 Pa, workpiece cathode voltage is 630 V to 880 V, furnace temperature is 380 °C to 450 °C, and plasma nitrocarburizing time is 25 min to 35 min. Step 3) The operating parameters for plasma nitrocarburization are as follows: methane flow rate is 100 mL / min to 200 mL / min, nitrogen flow rate is 500 mL / min to 600 mL / min, furnace pressure is maintained at 130 Pa, workpiece cathode voltage is 630 V to 880 V, furnace temperature is 380 °C to 450 °C, and plasma nitrocarburization time is 265 min to 275 min.

2. The method according to claim 1, characterized in that: The operating parameters for hydrogen plasma sputtering in step 1) are as follows: hydrogen flow rate is 180 mL / min to 220 mL / min, furnace pressure is maintained at 60 Pa to 80 Pa, workpiece cathode voltage is 580 V to 600 V, furnace temperature is 280 °C to 320 °C, and sputtering time is 50 min to 70 min.

3. The method according to claim 1 or 2, characterized in that: Step 1) The workpiece was polished and cleaned before hydrogen plasma sputtering.

4. The method according to claim 1 or 2, characterized in that: The specific operation of vacuuming in step 1) is as follows: vacuuming until the pressure inside the furnace is 7Pa to 8Pa.

5. The method according to claim 1 or 2, characterized in that: The methane flow rates in steps 2) and 3) are the same.

6. The method according to claim 1 or 2, characterized in that: The nitrogen flow rate is the same in steps 2) and 3).

7. The method according to claim 1 or 2, characterized in that: Step 2) The thickness of the nitrocarbon co-infiltration modified layer formed by the first low-temperature plasma nitrocarbon co-infiltration and Step 3) the second low-temperature plasma nitrocarbon co-infiltration is greater than 20 μm.

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