An austenitic stainless steel and a hydrogen-assisted ion-carburizing method for improving the uniformity of the carburized layer thereof

Abstract

The application discloses an austenitic stainless steel and a hydrogen-assisted ion carburizing method for improving layer uniformity; first, the austenitic stainless steel workpiece is cleaned and then is placed into an ion carburizing furnace, and the ion carburizing furnace is vacuumized; second, hydrogen is introduced into the ion carburizing furnace, and after high pressure is opened, the passivation film is damaged through physical bombardment and chemical reduction of the hydrogen; then under continuous bombardment of the plasma, the temperature of the austenitic stainless steel workpiece rises, when the temperature rises to a carburizing temperature, acetylene is introduced for carburizing. The application uses hydrogen instead of the traditional argon, effectively inhibits the "edge effect" caused in the carburizing process, and finally obtains a carburized layer with uniform thickness, continuous and compact on the surface of the workpiece.

Description

Technical Field

[0001] This invention relates to the field of chemical heat treatment, and more particularly to an austenitic stainless steel and a hydrogen-assisted ion carburizing method for improving the uniformity of its carburized layer. Background Technology

[0002] Austenitic stainless steel is widely used in food, medical, and chemical industries due to its excellent corrosion resistance. Its superior corrosion resistance stems from alloying elements in the steel, such as Cr, Ni, and Si, which increase the electrode potential of the base metal. Furthermore, Cr forms a dense and stable passivation film (Cr₂O₃) on the stainless steel surface, effectively isolating the base metal from corrosive media. However, austenitic stainless steel has relatively low strength, which cannot meet the requirements of some stress and wear-resistant working conditions. To improve surface wear resistance, surface strengthening treatment is necessary.

[0003] Low-temperature ion carburizing is a chemical heat treatment method for strengthening the surface of austenitic stainless steel. At temperatures below the sensitization temperature (500~550℃), a solid solution-strengthened carburized layer is formed on the surface of austenitic stainless steel through plasma sputtering and deposition, thereby improving the surface hardness and wear resistance. However, due to the presence of a Cr2O3 passivation film on the stainless steel surface, it hinders the penetration of carbon atoms during ion carburizing; therefore, the passivation film needs to be removed before carburizing.

[0004] Currently, there are several methods for removing passivation films:

[0005] (1) Before ion carburizing, the workpiece is pretreated by pickling with hydrochloric acid or hydrogen chloride gas. The process is complicated and improper parameter control can cause over-corrosion of the workpiece and easily corrode the equipment.

[0006] (2) During ion diffusion, argon gas is introduced. The argon gas is ionized into plasma under high voltage and bombarded to remove the passivation film under the acceleration of the electric field, while promoting the diffusion of carbon atoms. This method is also widely used in ion nitriding. However, the electric field of the plasma sheath is distorted at the edges and corners of the workpiece. The electric field strength at the edge of the workpiece is greater than that at the rest, resulting in more ions reaching the edge. This easily leads to the "edge effect", that is, the diffusion layer is thick at the edges and corners of the workpiece, while the diffusion layer in the middle is thin or there is no diffusion layer.

[0007] The method of using argon bombardment to destroy the passivation film of austenitic stainless steel suffers from several drawbacks. Due to the higher electric field intensity at the edges, argon ions preferentially bombard the sample edges, resulting in excessively high temperatures at the workpiece corners. Conversely, the bombardment intensity in the center of the workpiece is low, making it difficult to destroy the passivation film or causing uneven destruction. This leads to a phenomenon where, after ion carburizing, austenitic stainless steel often exhibits a thin or absent carburized layer in the center, while the corners have excessively thick carburized layers. The resulting potential difference due to incomplete carburized layers reduces the workpiece's corrosion resistance, and excessively high corner hardness can cause brittle fracture, severely impacting the workpiece's quality and service life. Summary of the Invention

[0008] The purpose of this invention is to solve the problems of uneven carburization layer and edge effect in austenitic stainless steel after low-temperature ion carburization, and to provide an austenitic stainless steel and a hydrogen-assisted ion carburization method to improve the uniformity of its carburization layer.

[0009] This invention is achieved through the following technical solution:

[0010] A hydrogen-assisted ion carburizing method for improving the uniformity of the carburized layer in austenitic stainless steel includes the following steps:

[0011] S1 The surface of the austenitic stainless steel to be tested is polished and placed on the cathode stage of the glow discharge ion carburizing furnace.

[0012] S2 Start the vacuum pump to draw the gas pressure in the ion carburizing furnace to below 80Pa, start the high-pressure switch, adjust the input voltage and duty cycle, and introduce hydrogen to maintain the furnace pressure at 140~160Pa.

[0013] S3 Observe the arcing of the sample. After there is no obvious arcing, gradually increase the input voltage, duty cycle and hydrogen flow rate. By bombarding and reducing the passivation film with hydrogen, the surface of the workpiece is activated and the workpiece is heated.

[0014] After reaching the carburizing temperature, S4 maintains a continuous flow of hydrogen and increases the flow of acetylene, keeping the furnace pressure at 280~320Pa, and calculates the holding time.

[0015] After the S5 heat treatment is completed, stop the supply of acetylene and hydrogen, turn off the voltage input, extract the remaining gas in the furnace to below 80 Pa, and remove the austenitic stainless steel workpiece from the furnace after it has cooled to below 200°C, thus completing the carburizing of the austenitic stainless steel workpiece.

[0016] The austenitic stainless steel mentioned in step S1 is AISI 304 austenitic stainless steel, and its chemical composition is: C 0.06wt.%; Si 0.41wt.%; Mn 1.20wt.%; P 0.027wt.%; S 0.001wt.%; Cr 18.20wt.%; Ni 8.03wt.%; N 0.04wt.%; balance Fe.

[0017] The ion carburizing furnace described in step S1 uses a pulsed power supply, with the furnace wall as the anode and the cathode platform and workpiece as the cathode.

[0018] The polishing process described in step S1 specifically involves grinding the austenitic stainless steel surface with SiC sandpaper from coarse to fine, then polishing it until there are no obvious scratches on the surface, followed by ultrasonic cleaning and drying.

[0019] The input voltage in step S2 is 580~620V, the duty cycle is 15~25%, and the hydrogen flow rate is 90~110 mL / min.

[0020] The step of gradually increasing the input voltage, duty cycle, and hydrogen flow rate in step S3 is based on step S2: the input voltage is increased by 40-50V each time, the duty cycle is increased by 7-10% each time, and the hydrogen flow rate is increased by 40-50mL each time; this step is repeated until the input voltage, duty cycle, and hydrogen flow rate reach the predetermined values.

[0021] The predetermined values ​​include: an input voltage of 750V, a duty cycle of 41-50%, and a hydrogen flow rate of 230-250mL / min.

[0022] The carburizing temperature in step S4 is 450~500℃, the acetylene flow rate is 45~50mL / min, and the holding time is 6h.

[0023] The ratio of hydrogen flow rate to acetylene flow rate in step S4 is 5:1.

[0024] The thickness of the carburized layer on the austenitic stainless steel workpiece after carburizing in step S4 is 20~55μm.

[0025] Compared with the prior art, the present invention has the following advantages and effects:

[0026] (1) The present invention uses hydrogen to destroy the passivation film, which saves a lot of time and costs compared with acid washing pretreatment, and the treatment method is more environmentally friendly.

[0027] (2) The present invention uses hydrogen as the gas to destroy the passivation film. Compared with argon, it improves the edge effect of the workpiece and austenitic stainless steel can obtain a uniform diffusion layer after ion carburizing.

[0028] (3) In this invention, acetylene is selected as the gas that provides carbon source. Compared with the use of methane, it can provide more carbon atoms and obtain a thicker infiltration layer. Attached Figure Description

[0029] Figure 1 Metallographic images of the carburized layer at the center (left) and corners (right) of the austenitic stainless steel in Example 1.

[0030] Figure 2 Metallographic images of the carburized layer at the center (left) and corner (right) of the austenitic stainless steel in Example 2.

[0031] Figure 3 Metallographic images of the carburized layer at the center (left) and corners (right) of the austenitic stainless steel in Comparative Example 1. Detailed Implementation

[0032] This invention provides a hydrogen-assisted ion carburizing method for improving the uniformity of the carburized layer in austenitic stainless steel. The invention will be further described below with reference to the embodiments, but this is not intended to limit the invention.

[0033] Example 1:

[0034] This example uses a 10×10×5mm austenitic stainless steel block sample with the following chemical composition: C 0.06wt.%; Si 0.41wt.%; Mn 1.20wt.%; P 0.027wt.%; S 0.001wt.%; Cr 18.20wt.%; Ni 8.03wt.%; N 0.04wt.%; balance Fe. The test surface was sanded with SiC sandpaper from coarse to fine, then polished until no obvious scratches remained. After ultrasonic cleaning and drying, the sample was placed on the cathode stage of an ion carburizing furnace.

[0035] Start the vacuum pump to reduce the gas pressure inside the furnace to below 80Pa, turn on the high-pressure switch, adjust the input voltage to 600V and the duty cycle to 20%, and introduce hydrogen gas at a flow rate of 100mL / min to maintain the pressure inside the furnace at 150Pa.

[0036] Observe the arcing behavior of the sample. After no obvious arcing occurs, gradually increase the input voltage by 50V, the duty cycle by 7%, and the hydrogen flow rate by 50mL. Repeat this step until the input voltage is 750V, the duty cycle is 41%, and the hydrogen flow rate reaches 250mL / min. By bombarding and reducing the passivation film with hydrogen, the workpiece surface is activated, causing the workpiece to heat up.

[0037] After reaching the carburizing temperature of 450℃, hydrogen was continuously introduced, and acetylene was introduced at a flow rate of 50 mL / min. The pressure inside the furnace was maintained at 300 Pa, and the holding time was 6 hours.

[0038] After the heat preservation is completed, stop the supply of acetylene and hydrogen, turn off the voltage input, extract the remaining gas in the furnace to below 80 Pa, and remove the workpiece from the furnace after it has cooled to below 200°C.

[0039] The austenitic stainless steel sample was carburized using the method described in Example 1 above, and the results are as follows: Figure 1 As shown, the carburized layer thickness is uniform at the center and corners, with an average thickness of 25.37 μm.

[0040] Example 2:

[0041] This example uses a 10×10×5mm austenitic stainless steel block sample with the following chemical composition: C 0.06wt.%; Si 0.41wt.%; Mn 1.20wt.%; P 0.027wt.%; S 0.001wt.%; Cr 18.20wt.%; Ni 8.03wt.%; N 0.04wt.%; balance Fe. The test surface was sanded with SiC sandpaper from coarse to fine, then polished until no obvious scratches remained. After ultrasonic cleaning and drying, the sample was placed on the cathode stage of an ion carburizing furnace.

[0042] Start the vacuum pump to reduce the gas pressure inside the furnace to below 80Pa, turn on the high-pressure switch, adjust the input voltage to 600V and the duty cycle to 20%, and introduce hydrogen gas at a flow rate of 100mL / min to maintain the pressure inside the furnace at 150Pa.

[0043] Observe the arcing behavior of the sample. After no obvious arcing occurs, gradually increase the input voltage by 50V, the duty cycle by 10%, and the hydrogen flow rate by 50mL. Repeat this step until the input voltage is 750V, the duty cycle is 50%, and the hydrogen flow rate reaches 250mL / min. By bombarding and reducing the passivation film with hydrogen, the workpiece surface is activated, causing the workpiece to heat up.

[0044] After reaching the carburizing temperature of 500℃, hydrogen was continuously introduced, and acetylene was introduced at a flow rate of 50 mL / min. The pressure inside the furnace was maintained at 300 Pa, and the holding time was 6 hours.

[0045] After the heat preservation is completed, stop the supply of acetylene and hydrogen, turn off the voltage input, extract the remaining gas in the furnace to below 80 Pa, and remove the workpiece from the furnace after it has cooled to below 200°C.

[0046] The austenitic stainless steel sample was carburized using the method described in Example 2 above, and the results are as follows: Figure 2 As shown, the carburized layer thickness is uniform at the center and corners, with an average thickness of 55.62 μm.

[0047] Comparative Example 1:

[0048] This example uses a 10×10×5mm austenitic stainless steel block sample with the following chemical composition: C 0.06wt.%; Si 0.41wt.%; Mn 1.20wt.%; P 0.027wt.%; S 0.001wt.%; Cr 18.20wt.%; Ni 8.03wt.%; N 0.04wt.%; balance Fe. The test surface was sanded with SiC sandpaper from coarse to fine, then polished until no obvious scratches remained. After ultrasonic cleaning and drying, the sample was placed on the cathode stage of an ion carburizing furnace.

[0049] Start the vacuum pump to reduce the gas pressure inside the furnace to below 80Pa, turn on the high-pressure switch, adjust the input voltage to 600V and the duty cycle to 20%, and introduce argon gas at a flow rate of 250mL / min to maintain the pressure inside the furnace at 150Pa.

[0050] Observe the arcing behavior of the sample. After no obvious arcing occurs, gradually increase the input voltage by 50V, the duty cycle by 10%, and the argon flow rate by 50mL. Repeat this step until the input voltage is 750V, the duty cycle is 50%, and the argon flow rate reaches 250mL / min. Argon bombardment destroys the passivation film and activates the workpiece surface, causing the workpiece to heat up.

[0051] After reaching the carburizing temperature of 500℃, argon gas was continuously introduced, and the introduction of acetylene and hydrogen gas was increased. The acetylene flow rate was 50 mL / min, the hydrogen flow rate was 250 mL / min, the furnace pressure was maintained at 300 Pa, and the holding time was 6 h.

[0052] After the heat preservation is completed, stop the supply of argon, acetylene and hydrogen, turn off the voltage input, extract the remaining gas in the furnace to below 80 Pa, and remove the workpiece from the furnace after it has cooled to below 200°C.

[0053] The carburized layer of the austenitic stainless steel sample was completed using the method described in Comparative Example 1 above, and the results are as follows: Figure 3 As shown, due to the higher temperature at the edges and corners, the carburized layer is thicker, and the thickness of the carburized layer decreases from the edges and corners of the sample towards the center, with almost no carburized layer in the central area of ​​the sample.

[0054] As described above, the present invention can be implemented well.

[0055] The implementation of the present invention is 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 hydrogen-assisted ion carburizing method for improving the uniformity of the carburized layer in austenitic stainless steel, characterized in that... Includes the following steps: S1 The surface of the austenitic stainless steel to be tested is polished and placed on the cathode stage of the ion carburizing furnace. S2 Start the vacuum pump to pump the gas pressure in the ion carburizing furnace to below 80Pa, adjust the input voltage and duty cycle, and introduce pure hydrogen to maintain the furnace pressure at 140~160Pa. S3 Observe the arcing of the sample. After there is no obvious arcing, gradually increase the input voltage, duty cycle and hydrogen flow rate. By bombarding and reducing the passivation film with hydrogen, the surface of the workpiece is activated and the workpiece is heated. After reaching the carburizing temperature, S4 maintains a continuous flow of pure hydrogen and increases the acetylene flow, keeping the furnace pressure at 280~320Pa, and calculates the holding time. After the S5 heat treatment is completed, stop the supply of acetylene and hydrogen, turn off the voltage input, extract the remaining gas in the furnace to below 80 Pa, and remove the austenitic stainless steel workpiece from the furnace after it has cooled to below 200°C, thus completing the carburizing of the austenitic stainless steel workpiece.

2. The hydrogen-assisted ion carburizing method for improving the uniformity of the carburized layer in austenitic stainless steel according to claim 1, characterized in that: The austenitic stainless steel mentioned in step S1 is AISI 304 austenitic stainless steel, and its chemical composition is: C 0.06wt.%; Si 0.41wt.%; Mn 1.20wt.%; P 0.027wt.%; S 0.001wt.%; Cr 18.20wt.%; Ni 8.03wt.%; N 0.04wt.%; balance Fe.

3. The hydrogen-assisted ion carburizing method for improving the uniformity of the carburized layer in austenitic stainless steel according to claim 1, characterized in that: The ion carburizing furnace described in step S1 uses a pulsed power supply, with the furnace wall as the anode and the cathode platform and workpiece as the cathode.

4. The hydrogen-assisted ion carburizing method for improving the uniformity of the carburized layer in austenitic stainless steel according to claim 1, characterized in that: The polishing process described in step S1 specifically involves grinding the austenitic stainless steel surface with SiC sandpaper from coarse to fine, then polishing until surface scratches appear, followed by ultrasonic cleaning and drying.

5. The hydrogen-assisted ion carburizing method for improving the uniformity of the carburized layer in austenitic stainless steel according to claim 1, characterized in that: The step of gradually increasing the input voltage, duty cycle, and hydrogen flow rate in step S3 is based on step S2: the input voltage is increased by 40-50V each time, the duty cycle is increased by 7-10% each time, and the hydrogen flow rate is increased by 40-50mL each time; this step is repeated until the input voltage, duty cycle, and hydrogen flow rate reach the predetermined values.

6. The hydrogen-assisted ion carburizing method for improving the uniformity of the carburized layer in austenitic stainless steel according to claim 5, characterized in that, The predetermined values ​​include: an input voltage of 750V, a duty cycle of 41-50%, and a hydrogen flow rate of 230-250mL / min.

7. The hydrogen-assisted ion carburizing method for improving the uniformity of the carburized layer in austenitic stainless steel according to claim 1, characterized in that, The carburizing temperature in step S4 is 450~500℃, the acetylene flow rate is 45~50mL / min, and the holding time is 6h.

8. The hydrogen-assisted ion carburizing method for improving the uniformity of the carburized layer in austenitic stainless steel according to claim 1, characterized in that, The input voltage in step S2 is 580~620V, the duty cycle is 15~25%, and the pure hydrogen flow rate is 90~110mL / min; The ratio of hydrogen flow rate to acetylene flow rate in step S4 is 5:

1.

9. The hydrogen-assisted ion carburizing method for improving the uniformity of the carburized layer in austenitic stainless steel according to claim 1, characterized in that, The thickness of the carburized layer on the austenitic stainless steel workpiece after carburizing in step S4 is 20~55μm.

10. An austenitic stainless steel, characterized in that... Obtained by the hydrogen-assisted ion carburizing method according to any one of claims 1-9.

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