Preparation method of mold surface hardness gradient distribution layer thickness adjustable nitride layer

Abstract

The invention relates to the technical field of nitride layer preparation methods, and particularly discloses a mold surface hardness gradient distribution layer thickness adjustable nitride layer preparation method which comprises the following steps: surface nanocrystallization pretreatment: performing ultrasonic shot peening or surface mechanical grinding treatment on the surface of a mold; a nanocrystalline layer with the depth not smaller than 50 microns and the grain size smaller than 100 nm is formed on the surface of the mold; a nanocrystalline layer is constructed through surface nanocrystallization pretreatment, an efficient diffusion channel is provided for nitrogen atoms, precise regulation and control of multi-field coupling plasma nitriding are matched, excessive generation of a brittle white bright layer can be restrained, the smooth hardness gradient from a base body to a nitriding layer is achieved, and the surface hardness and the core toughness of the mold are both considered.

Description

TECHNICAL FIELD

[0001] The application relates to a nitrogenized layer preparation method, in particular to a nitrogenized layer preparation method with adjustable thickness of a gradient distribution layer of surface hardness of a mold. BACKGROUND

[0002] In the field of mold manufacturing, surface nitriding treatment is a key process for improving the wear resistance and fatigue resistance of the mold. The traditional mold surface nitriding method mainly adopts single plasma nitriding or simple coating technology, which has obvious limitations.

[0003] Firstly, the substrate surface has no effective pretreatment before nitriding, the diffusion channel of nitrogen atoms is insufficient, the thickness of the nitriding layer is uneven, and a brittle white layer is easily formed, which affects the strength and toughness of the mold.

[0004] Secondly, the single film layer has poor adhesion with the substrate, and is easily peeled off under the action of cold and hot cycles, which cannot meet the long-term service requirement.

[0005] In addition, after coating, the surface is prone to defects such as pinholes and microcracks, which reduces the surface finish and corrosion resistance of the mold, and is difficult to meet the strict requirements of the automobile and aerospace fields on high-precision molds.

[0006] Therefore, a nitrogenized layer preparation method capable of realizing gradient performance regulation, improving film-substrate adhesion and optimizing surface quality is needed. SUMMARY

[0007] The application aims to provide a nitrogenized layer preparation method with adjustable thickness of a gradient distribution layer of surface hardness of a mold, so as to solve the problems in the background art.

[0008] To achieve the above-mentioned purpose, the application provides the following technical scheme: a nitrogenized layer preparation method with adjustable thickness of a gradient distribution layer of surface hardness of a mold, the nitrogenized layer preparation method comprising the following steps: Step 1: surface nanocrystallization pretreatment, ultrasonic shot peening or surface mechanical grinding treatment is performed on the mold surface, and a nanocrystalline layer with a depth of not less than 50 microns and a grain size of less than 100 nanometers is formed on the mold surface; The ultrasonic shot peening process parameters are as follows: shot peening pressure 0.4-0.6 MPa, shot diameter 0.1-0.3 mm, strain rate 5x10 3 -1x10 4 s -1 Coverage 100%-150%, treatment time 30-60 minutes; The pretreatment is to clean the mold base, specifically, soaking in 5-8% alkaline degreasing agent at 50-60℃ for 20-30min, then ultrasonic cleaning with anhydrous ethanol for 15min, and finally drying at 80-100℃, wherein the mold base is cold work die steel, hot work die steel or plastic die steel; Step two: multi-field coupled plasma nitriding: the mold after step one pretreatment is placed in a plasma nitriding equipment, vacuumed to ≤5×10 - ³Pa, then argon is introduced and preheated to 400-550℃, then 1:1 volume ratio of N2 and H2 mixed gas is introduced, with total flow rate of 20-30sccm, while a pulse bias of 500-800V and 5-10kHz and a magnetic field of 0.1-0.3T are applied, and the temperature is kept at 400-500℃ for 4-8h; Step three: deposition of multi-layer gradient composite film: the mold after step two nitriding is placed in a multi-arc ion plating equipment, and the following film layers are deposited: (1) chromium primer layer: deposited by pure chromium target, with thickness of 5-8μm; (2) chromium nitride transition layer: deposited by pure chromium target in nitrogen atmosphere, with thickness of 8-12μm; (3) zirconium chromium nitride surface layer: deposited by zirconium chromium alloy target with Zr:Cr atomic ratio of 3:7-5:5 in nitrogen atmosphere, with thickness of 10-15μm; Step four: pulse electron beam surface finishing: the mold surface after step three film deposition is treated by pulse electron beam scanning, with scanning parameters of energy density 10-20J / cm 2 , pulse width 1-5μs, pulse frequency 1-3Hz, and each treatment area is bombarded 2-3 times.

[0009] Preferably, the cold work die steel is selected from Cr12 steel, Cr12MoV steel or DC53 steel; The hot work die steel is selected from H13 steel, 3Cr2W8V steel or 5CrNiMo steel; The plastic die steel is selected from 718H steel, S136 steel or NAK80 steel.

[0010] Preferably, the alkaline degreasing agent is composed of 3-5% NaOH, 2-3% Na2CO3 and the rest water, and the purity of anhydrous ethanol is ≥99.7%.

[0011] Preferably, the surface mechanical grinding parameters are: alumina particles (5-10μm), rotation speed 200-300r / min, time 40-80min and pressure 0.2-0.3MPa.

[0012] Preferably, the projectile of the ultrasonic shot is a high-speed steel projectile, the steel projectile hardness is greater than or equal to 60HRC, and the purity is greater than or equal to 99.5%.

[0013] Preferably, in the multi-field coupled plasma nitriding, the purity of Ar, N2 and H2 is greater than or equal to 99.999%, and the moisture content is less than or equal to 10ppm.

[0014] Preferably, in step three, argon ion sputtering cleaning is performed: vacuum is less than or equal to 3x10 - Pa, Ar flow is 8-12sccm, voltage is 800-1000V, and time is 10-15min.

[0015] Preferably, the purity of the pure chromium target is greater than or equal to 99.95%, and the density is greater than or equal to 95%; the density of the zirconium-chromium alloy target is greater than or equal to 95%, and the oxygen content is less than or equal to 0.05%.

[0016] Preferably, in step three, the deposition temperature is 200-250℃; The target current of the chromium primer layer is 60-80A; The target current of the chromium nitride transition layer is 70-90A, and the N2 flow is 5-8sccm; the target current of the zirconium-chromium nitride surface layer is 90-110A, and the N2 flow is 8-10sccm.

[0017] Preferably, in step four, it is performed under a vacuum of less than or equal to 1x10 -2 Pa, and the distance between the electron beam gun and the surface is 15-20cm.

[0018] Compared with the prior art, the present application has the following beneficial effects: 1. The present application, by surface nanocrystallization pretreatment to construct a nanocrystalline layer, provides an efficient diffusion channel for nitrogen atoms, and cooperates with the precise regulation of multi-field coupled plasma nitriding, can inhibit the excessive generation of brittle white layer, realize the gentle hardness gradient from the matrix to the diffusion layer, and balance the surface hardness and core toughness of the mold.

[0019] 2. The present application adopts a multi-layer gradient composite film layer design, first deposits a chromium primer layer to enhance the bonding with the nitriding matrix, and then gradually transitions through the transition layer and the surface layer to relieve the internal stress caused by the performance difference of the film and the substrate, avoid coating peeling, and meet the stability requirements of long-term service of the mold.

[0020] 3. The present application can eliminate micro defects left by plating through pulse electron beam surface finishing, improve surface density and smoothness, reduce friction loss, and at the same time enhance surface corrosion resistance, which is suitable for application scenarios of high-precision and high-reliability molds. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 The flow chart of the preparation method of the present application. DETAILED DESCRIPTION

[0022] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0023] The materials in the following embodiments can be purchased on the market.

[0024] Embodiment one, a method for preparing a nitrided layer with adjustable thickness of a gradient distribution layer of mold surface hardness, the method for preparing a nitrided layer is as follows: Step one, surface nanocrystallization pretreatment: Cr12MoV cold work die steel is selected as the substrate, which is first placed in a 5% alkaline degreasing agent, the degreasing agent is composed of 3% NaOH, 2% Na2CO3 and the rest deionized water, and is soaked in a 50°C constant temperature water bath for 30 minutes. When operating, the operator needs to wear alkali-resistant gloves and goggles to avoid the solution contacting the skin. After degreasing, the waste water needs to be adjusted to pH 6-9 by adding dilute hydrochloric acid, and then discharged after neutralization treatment; The mold is washed with deionized water for 3 times, and then ultrasonically cleaned with anhydrous ethanol with a purity of 99.7% for 15 minutes. The ultrasonic power is set to 300W, and the water temperature in the cleaning tank is controlled below 25°C to prevent the ethanol from evaporating too quickly. After cleaning, the mold is placed in a 80°C air drying oven for drying for 30 minutes to ensure that there is no residual moisture on the surface; Ultrasonic shot peening is used, and high-speed steel shots with a hardness of 60HRC and a purity of 99.5% are selected. The shot diameter is 0.1mm. Before use, the shots are cleaned with anhydrous ethanol and dried to remove surface oil stains. The shot peening pressure is set to 0.4MPa, the strain rate is 5×10 3 s -1 , the distance between the nozzle and the mold surface is 15cm, the coverage rate is 100%, and the treatment time is 60 minutes; After treatment, the surface grain size is observed by transmission electron microscopy to ensure that it is less than 100nm, and the actual measurement is 80nm. The nanocrystalline layer depth is detected by eddy current thickness gauge to ensure that it is not less than 50μm, and the actual measurement is 50μm.

[0025] Step two, multi-field coupled plasma nitriding: the pretreated mold is placed in a horizontal plasma nitriding furnace cavity, and is fixed by a special tooling fixture made of high-temperature resistant stainless steel (310S) to ensure good contact and uniform heating of the mold and the furnace electrode, and to avoid intermetallic reaction with the mold. After closing the cavity, check the integrity of the furnace door sealing rubber ring (fluorine rubber material), confirm that there is no aging or damage, and then start the two-stage vacuum pump set (mechanical pump + Roots pump). First, start the mechanical pump to vacuum to below 5Pa, then start the Roots pump to vacuum to 5×10 -3Pa, the vacuum pump valve is closed to keep the pressure for 5 min, and the vacuum degree rises by no more than 1*10 -3 Pa, and the sealing is determined to be qualified. The Ar gas supply system is started, Ar gas is introduced into the furnace through a pressure reducing valve (outlet pressure 0.2 MPa) and a mass flow controller (accuracy ±1 sccm), the flow rate is set to 10 sccm, and at the same time, the infrared heating tube in the furnace is started (power 5 kW), and a segmented temperature rising mode is adopted: the temperature rising rate is 3°C / min from room temperature to 200°C, and the temperature rising rate is 5°C / min from 200°C to 400°C, the mold surface temperature is recorded by the thermocouple in the furnace every 50°C to ensure that the deviation from the set temperature is not more than ±3°C. After preheating to 400°C for 30 min to make the mold temperature uniform, the Ar gas mass flow controller is turned off, and when the Ar content detected by the gas analyzer is less than 0.1%, the N2 and H2 gas supply systems are started. Two kinds of gas are introduced into the mixing pipeline through a pressure reducing valve and a mass flow controller, the N2 flow rate is 10 sccm, the H2 flow rate is 10 sccm, the total flow rate is 20 sccm, and the volume ratio is 1:1; before gas mixing, it is treated by a drying and purifying device (molecular sieve + deoxidizer) to ensure that the water content is ≤8 ppm and the oxygen content is ≤5 ppm. The pulse bias power supply is started, the output voltage is set to 500 V, the pulse frequency is 5 kHz, the duty cycle is 50% (pulse on time 100 μs, off time 100 μs), at the same time, the electromagnetic coil outside the furnace is started, an axial magnetic field (consistent with the furnace axis) with a strength of 0.1 T is applied, and the deviation is controlled within ±0.01 T through the magnetic field controller. The temperature is kept for 8 h, and the parameters are checked every 2 h: the temperature in the furnace is 400±5°C, the vacuum degree is 5*10 -3 ±1*10 -3 Pa, N2 flow rate 10±1 sccm, H2 flow rate 10±1 sccm, pulse bias 500±10 V, magnetic field strength 0.1±0.01 T, when the parameters deviate, the control system is adjusted and corrected.

[0026] Step three, deposition of multi-layer gradient composite film: after nitriding, the mold is transferred to a multi-arc ion plating equipment, and argon ion sputtering cleaning is performed first. The vacuum is extracted to 3*10 -3 Pa, 8 sccm of Ar gas with a purity of 99.999% is introduced, a sputtering voltage of 800 V is applied, the cleaning time is 15 min, and the mold self-rotation rate is set to 5 r / min during the cleaning process to ensure uniform cleaning of the surface to remove the trace of oxide film on the nitriding layer surface; After cleaning, the temperature of the deposition cavity of the equipment is raised to 200°C at a rate of 3°C / min. A pure chromium target with a purity of 99.95% and a density of 95% is used, the target current is set to 60 A, and the bias voltage is 100 V. A chromium primer layer is deposited, the thickness is controlled to be 5 μm, the thickness is detected once every 2 μm during deposition, and the accuracy is ensured. After the deposition of the chromium primer layer, the pure chromium target was maintained, 99.999% pure N2 gas was introduced at a flow rate of 5 seem, the target current was adjusted to 70 A, a chromium nitride transition layer was deposited with a thickness of 8 μm, and the atomic ratio of Cr to N in the plasma was monitored by an optical emission spectrometer during the deposition process to ensure the formation of a stable CrN phase; Finally, a zirconium chromium alloy target with an atomic ratio of Zr:Cr of 3:7 (density of 95%, oxygen content of 0.05%) was used, the N2 flow rate was adjusted to 8 seem, and the target current was adjusted to 90 A to deposit a zirconium chromium nitride surface layer with a thickness of 10 μm. After the deposition was completed, the cross-section of the film layer was observed by a metallographic microscope to confirm that there were no delamination and pinhole defects.

[0027] Step four, pulse electron beam surface finishing: the coated mold was placed in a pulse electron beam device, vacuumed to 1 x 10 -2 Pa, the distance between the electron beam gun and the mold surface was adjusted to 15 cm, and the distance was calibrated by a laser range finder. The energy density was set to 10 J / cm 2 , the pulse width was 1 μs, the pulse frequency was 1 Hz, the mold surface was scanned at a scanning rate of 5 mm / s, the adjacent scanning tracks had an overlap rate of 50%, and each region was bombarded twice.

[0028] After finishing, the mold was naturally cooled to room temperature, and the vacuum environment was maintained during the cooling process to avoid surface oxidation. After removal, the surface was wiped with anhydrous ethanol.

[0029] Example two, step one, surface nanocrystallization pretreatment: H13 hot work die steel was used as the substrate, a 6% alkaline degreasing agent by mass fraction was used for soaking at 55°C for 25 min, the degreasing agent was composed of 4% NaOH, 2.5% Na2CO3, and the rest was deionized water. The operation was carried out in a fume hood to avoid the stimulation of NaOH volatile gas to the respiratory tract; Then, 99.8% pure anhydrous ethanol was used for ultrasonic cleaning for 15 min at an ultrasonic frequency of 40 kHz. After cleaning, the surface was dried in a 90°C air drying oven to ensure that the surface was dry and free of residues; Surface mechanical grinding treatment was used, 99.8% pure 5 μm alumina particles were selected, impurities were removed by sieving before use, the grinding speed was set to 200 r / min, the grinding head pressure was 0.2 MPa, the grinding track was a spiral line, and the treatment time was 80 min; After treatment, the grain size was detected by transmission electron microscopy, which was actually measured to be 70 nm, and the nanocrystalline layer depth was detected by an eddy current thickness gauge, which was actually measured to be 55 μm, ensuring that the requirements were met.

[0030] Step 2, Multi-field Coupled Plasma Nitriding: Place the mold into the vertical plasma nitriding furnace and fix it using a suspended fixture. Place an Al2O3 ceramic insulating gasket (2mm thick) between the mold and the fixture to prevent direct current conduction. After closing the furnace, check the sealing surface of the metal spiral wound gasket on the furnace flange. Start the vacuum pump group to evacuate: the mechanical pump evacuates to 10Pa, and the Roots pump evacuates to 4×10 Pa. -3 Pa, maintain pressure for 5 min, vacuum rise ≤ 1 × 10 -3 Pa is considered acceptable.

[0031] Ar gas with a purity of 99.999% was introduced at a flow rate of 10 sccm. An 8kW resistance heating wire was turned on to raise the temperature. The heating rate from room temperature to 200℃ and from 200℃ to 450℃ was 4℃ / min to avoid abnormal microstructural transformation of H13 steel. After heating to 450℃, the temperature was held for 40 minutes. The temperature was monitored by thermocouples at three points (top, middle, and bottom) inside the furnace to ensure that the temperature difference was ≤5℃.

[0032] Turn off the Ar gas and introduce a mixture of N2 and H2 gas at a flow rate of 12.5 sccm and a flow rate of 12.5 sccm for both N2 and H2, for a total flow rate of 25 sccm and a volume ratio of 1:1. The gas is then filtered through a 0.1 μm precision filter to remove impurities, and the moisture content is controlled to be below 9 ppm.

[0033] Turn on the pulse bias power supply, set the voltage to 600V, the frequency to 7kHz, and the duty cycle to 50% (on time 71.4μs, off time 71.4μs). At the same time, turn on the permanent magnet magnetic field system and apply a radial magnetic field with an intensity of 0.2T. The magnetic field uniformity deviation is ≤0.02T.

[0034] The heat treatment lasted for 6 hours. Every 1.5 hours, the purity of N2 and H2 was analyzed by gas chromatography to ensure that there were no impurities in the gas. Every 2 hours, the surface hardness of the mold was checked by a portable hardness tester to ensure that the hardness increased steadily in the early stage of nitriding. If the parameters deviated, the heating power, gas flow rate or bias pressure parameters were adjusted by the PID control system.

[0035] Step 3, Multilayer Gradient Composite Film Deposition: The nitrided mold is transferred to a multi-arc ion plating equipment, where it undergoes argon ion sputtering cleaning and is then evacuated to a vacuum level of 2×10⁻⁶. -3 Pa, 99.999% pure Ar gas is introduced at a flow rate of 10 sccm, 900V sputtering voltage is applied, cleaning time is 12 min, and the mold rotation speed during cleaning is 5 r / min; After cleaning, the temperature of the deposition chamber was raised to 220℃ at a rate of 3℃ / min. A pure chromium target with a purity of 99.95% and a density of 95% was used. The target current was 70A and the bias voltage was 100V. A chromium underlayer was deposited with a thickness of 6μm. The thickness was measured with a film thickness gauge every 2μm of deposition. Next, while maintaining a pure chromium target, N2 gas with a purity of 99.999% was introduced at a flow rate of 6 sccm, and the target current was adjusted to 80 A to deposit a chromium nitride transition layer with a thickness of 10 μm. The ratio of Cr to N atoms was monitored using an optical emission spectrometer. Finally, a zirconium-chromium alloy target with a Zr:Cr atomic ratio of 4:6 (density 95%, oxygen content 0.05%) was used, with an N2 flow rate of 9 sccm and a target current of 100 A to deposit a zirconium-chromium nitride surface layer with a thickness of 12 μm. The quality of the film was observed using a metallographic microscope after deposition.

[0036] Step 4, Pulsed Electron Beam Surface Finishing: Place the mold into the pulsed electron beam equipment and evacuate to 8×10⁻⁶. -3 Adjust the distance between the electron beam gun and the mold surface to 17 cm, and calibrate using a laser rangefinder. Set the energy density to 15 J / cm². 2 The pulse width is 3μs, the pulse frequency is 2Hz, the scanning rate is 5mm / s, the overlap rate of adjacent trajectories is 50%, each region is bombarded twice, and the electron beam gun window is wiped with an alcohol swab before scanning to avoid dust affecting the beam quality.

[0037] After finishing, maintain a vacuum environment and wait for the mold to cool to room temperature. Then, remove it and wipe the surface with anhydrous ethanol.

[0038] Example 3, Step 1, Surface nano-treatment: 718H plastic mold steel was selected and immersed in a 7% (w / w) alkaline degreasing agent at 58°C for 22 minutes. The degreasing agent consisted of 5% NaOH, 3% Na2CO3, and the remainder deionized water. After immersion, it was rinsed three times with deionized water, then ultrasonically cleaned with 99.9% pure anhydrous ethanol for 15 minutes, and subsequently dried in a 95°C forced-air drying oven. Ultrasonic shot peening was employed, using high-speed steel shot with a diameter of 0.2 mm, a hardness of 62 HRC, and a purity of 99.6%. The shot was cleaned and dried with anhydrous ethanol before use. The shot peening pressure was set to 0.5 MPa, and the strain rate to 8 × 10⁻⁶. 3 s -3 The distance between the nozzle and the mold surface is 15cm, the coverage rate is 130%, the processing time is 45min, and the mold is firmly fixed during the shot peening process to avoid vibration and displacement.

[0039] After processing, the grain size was detected by transmission electron microscopy and measured to be 60 nm; the depth of the nanocrystalline layer was detected by eddy current thickness gauge and measured to be 60 μm, which meets the design requirements.

[0040] Step 2, Multi-field Coupled Plasma Nitriding: A horizontal dual-chamber plasma nitriding equipment (pretreatment chamber + nitriding chamber) is used. After the mold undergoes surface cleaning in the pretreatment chamber, it is transferred into the nitriding chamber via a transfer mechanism to avoid secondary contamination. The nitriding chamber is equipped with a water-cooled jacket (water temperature 25±2℃) to prevent the furnace from overheating. The mold is placed on a graphite support platform to ensure uniform heating. The vacuum pump unit (mechanical pump + diffusion pump) was started to evacuate the nitriding chamber, and the basic vacuum level reached 3×10⁻⁶. -3 After holding the pressure for 5 minutes without leakage, introduce Ar gas at a flow rate of 10 sccm and turn on the high-frequency induction heating device (frequency 15 kHz, power 10 kW). The heating rate from room temperature to 300℃ and from 300℃ to 500℃ is 5℃ / min. During the heating process, the surface temperature of the mold is monitored in real time by an infrared thermometer, with a deviation of ≤ ±4℃. After holding at the temperature for 30 minutes, the Ar gas was turned off, and a mixture of N2 and H2 gas was introduced. The N2 flow rate was 14 sccm, the H2 flow rate was 14 sccm, the total flow rate was 28 sccm, and the volume ratio was 1:1. The gas was treated by a low-temperature adsorption dryer, and the moisture content was ≤7ppm. Turn on the pulse bias power supply, set the voltage to 700V, frequency to 8kHz, and duty cycle to 50% (on time 62.5μs, off time 62.5μs), with a bias output ripple coefficient ≤5%; simultaneously turn on the superconducting magnet system, apply a magnetic field of 0.25T, with magnetic field fluctuation ≤0.01T; Heat preservation treatment for 5 hours, record the furnace pressure every hour (3×10). -3 ±1×10 -3 The parameters include Pa, gas flow rate (N2 14±1 sccm, H2 14±1 sccm), and heating power. Gas is sampled every 2 hours using an in-furnace sampling device to detect the purity of N2 and H2 and the content of impurities, ensuring a stable process environment.

[0041] Step 3: Multilayer gradient composite film deposition: After nitriding, the mold is transferred to a multi-arc ion plating equipment for argon ion sputtering cleaning, followed by vacuuming to 1×10⁻⁶. -3 Pa, Ar gas flow rate 11 sccm, sputtering voltage 950V, cleaning time 11 min, mold rotation speed 5 r / min; The deposition chamber temperature was raised to 240℃ at a rate of 3℃ / min. A pure chromium target with a purity of 99.95% and a density of 95% was used. The target current was 75A and the bias voltage was 100V. A 7μm thick chromium underlayer was deposited, and the thickness was checked periodically with a film thickness gauge. Maintain a pure chromium target, introduce N2 gas at a flow rate of 7 sccm, and set a target current of 85 A to deposit an 11 μm thick chromium nitride transition layer. Monitor the Cr to N atomic ratio to ensure the formation of a stable CrN phase. A zirconium-chromium alloy target with a Zr:Cr atomic ratio of 5:5 (density 95%, oxygen content 0.05%) was replaced. The N2 flow rate was 9.5 sccm and the target current was 105 A. A 14 μm thick zirconium-chromium nitride surface layer was deposited. After deposition, the film was inspected with a metallographic microscope and no defects were found.

[0042] Step 4, Pulsed Electron Beam Surface Finishing: The mold is placed in the pulsed electron beam equipment and evacuated to a vacuum level of 5×10⁻⁶. -3Pa, electron beam gun distance from surface 18cm, calibrated with laser rangefinder, energy density set to 18J / cm². 2 The pulse width is 4μs, the pulse frequency is 2Hz, the scan rate is 5mm / s, the overlap rate of adjacent trajectories is 50%, and each region is bombarded 3 times. After finishing, cool to room temperature under vacuum, then remove and wipe the surface with anhydrous ethanol.

[0043] Example 4, Step 1, Surface Nanofiber Pretreatment: Using DC53 cold work die steel as the substrate, the substrate was immersed in an 8% (w / w) alkaline degreasing agent at 60°C for 20 minutes. The degreasing agent consisted of 4.5% NaOH, 2.8% Na2CO3, and the remainder deionized water. After immersion, the substrate was rinsed three times with deionized water, ultrasonically cleaned with anhydrous ethanol (99.7% purity) for 15 minutes, and then dried in a 100°C forced-air drying oven. After drying, the substrate was immediately transferred to the next process to avoid surface oxidation. Surface mechanical grinding is adopted, using 10μm alumina particles with a purity of 99.8%. The particles are sieved before use. The grinding speed is set to 300r / min, the grinding head pressure is 0.3MPa, and the processing time is 40min. Grinding fluid is added periodically during the grinding process to maintain particle dispersion. After processing, the grain size was detected by transmission electron microscopy and measured to be 50 nm; the depth of the nanocrystalline layer was detected by eddy current thickness gauge and measured to be 65 μm, which meets the requirements.

[0044] Step Two: Multi-Field Coupled Plasma Nitriding: The mold is placed in a vacuum plasma nitriding furnace using a tray-type fixture. The fixture surface is covered with an Inconel 600 high-temperature resistant alloy mesh, ensuring tight contact between the mold and the mesh to guarantee uniform current distribution. After closing the furnace door, check the O-ring and pressure ring sealing structure of the furnace body, and start the vacuum pump to evacuate to 2×10⁻⁶. -3 Pa, maintain pressure for 5 min, vacuum rise ≤ 5 × 10 -4 Pa is acceptable; Ar gas flow rate of 10 sccm was introduced, and the electric heating tube with a power of 6 kW was turned on to raise the temperature. The heating rate from room temperature to 300℃ and from 300℃ to 550℃ was 5℃ / min. After raising the temperature to 550℃, the temperature was held for 30 minutes. The temperature of each part of the mold was confirmed to be uniform by using an in-furnace temperature monitoring instrument (accuracy ±1℃). Turn off the Ar gas and introduce a mixture of N2 and H2 gas at a flow rate of 15 sccm for N2 and 15 sccm for H2, with a total flow rate of 30 sccm and a volume ratio of 1:1. The gas is treated by a high-efficiency purification system, with a moisture content of ≤6 ppm and an oxygen content of ≤3 ppm. Turn on the pulse bias power supply, set the voltage to 800V, frequency to 10kHz, and duty cycle to 50% (on time 50μs, off time 50μs), and monitor the bias current through the current transformer; at the same time, turn on the electromagnetic induction coil and apply a magnetic field of 0.3T, and use the Hall sensor to provide real-time feedback on the magnetic field strength, with the deviation controlled within ±0.02T. Heat treatment for 4 hours, with the resistance between the mold and the furnace body checked every hour using an insulation resistance meter to ensure it is ≥10MΩ to prevent leakage. Complete process parameters (temperature 550±5℃, vacuum degree 2×10) are recorded every 2 hours. -3 ±5×10 -4 (Pa, gas flow rate, bias voltage, magnetic field strength), ensure that the parameters are within the set range; Step 3, Multilayer Gradient Composite Film Deposition: After nitriding, the mold is transferred to a multi-arc ion plating equipment, cleaned by argon ion sputtering, and then evacuated to a vacuum of 1×10⁻⁶. -3 Pa, Ar gas flow rate 12 sccm, sputtering voltage 1000V, cleaning time 10 min, mold rotation speed 5 r / min; The deposition chamber temperature was raised to 250℃ at a rate of 3℃ / min. Using a pure chromium target with a purity of 99.95% and a density of 95%, a target current of 80A and a bias voltage of 100V were applied to deposit an 8μm thick chromium underlayer, and the thickness was checked periodically.

[0045] Maintain a pure chromium target, N2 gas flow rate of 8 sccm, target current of 90 A, deposit a 12 μm thick chromium nitride transition layer, and monitor the Cr to N atom ratio; Replace the target with a zirconium-chromium alloy with a Zr:Cr atomic ratio of 3:7 (density 95%, oxygen content 0.05%), N2 flow rate 10 sccm, target current 110 A, deposit a 15 μm thick zirconium-chromium nitride surface layer, and check the film quality with a metallographic microscope after deposition.

[0046] Step 4, Pulsed Electron Beam Surface Finishing: The mold is placed in the pulsed electron beam equipment and evacuated to a vacuum level of 3×10⁻⁶. -3 Pa, electron beam gun distance from surface 20cm, calibrated using laser rangefinder. Energy density set to 20J / cm². 2 The pulse width was 5μs, the pulse frequency was 3Hz, the scanning rate was 5mm / s, the overlap rate of adjacent trajectories was 50%, and each area was bombarded 3 times. After bombardment, the surface was observed with a metallographic microscope to confirm that there were no molten pit defects.

[0047] After finishing, cool to room temperature under vacuum, then remove and wipe the surface with anhydrous ethanol.

[0048] Comparative Example 1: No surface nano-sizing pretreatment, otherwise the same as Example 1.

[0049] Comparative Example 2: Single plasma nitriding, without magnetic field, the rest is the same as Example 2.

[0050] Comparative Example 3: Only a single ZrCrN film layer, without gradient, otherwise the same as Example 3.

[0051] Comparative Example 4: Pulseless electron beam finishing, the rest is the same as Example 4.

[0052] The experimental data for Examples 1-4 and Comparative Examples 1-4 are shown in Table 1. Table 1 – Experimental Data Tables for Examples 1-4 and Comparative Examples 1-4

[0053] Based on the above data, we can conclude that: 1. In Example 1, the thickness of the diffusion layer was increased by 86.7% compared to Comparative Example 1, the surface hardness was increased by 21.4%, and the nitrogen atom diffusion efficiency was significantly improved.

[0054] 2. In Example 2, the thickness of the infiltration layer was increased by 52.4% compared with Comparative Example 2, and the surface hardness was increased by 18.1%, with significant optimization of the uniformity and hardness of the infiltration layer.

[0055] 3. In Example 3, the film-substrate adhesion was increased by 140.9% compared to Comparative Example 3, and the thermal cycling crack rate was reduced to 0, completely solving the coating peeling problem.

[0056] 4. The salt spray corrosion rate of Example 4 was reduced by 93.3% compared with Comparative Example 4, and the surface corrosion resistance and structural stability were significantly enhanced.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a nitrided layer with adjustable thickness and gradient hardness distribution on the surface of a mold, characterized in that: The method for preparing the nitrided layer includes the following steps: Step 1: Surface nanofiber pretreatment, the mold surface is subjected to ultrasonic shot peening or surface mechanical grinding treatment, the mold surface is formed with a nanocrystalline layer with a depth of not less than 50μm and a grain size of <100nm; The ultrasonic shot peening process parameters are: shot peening pressure 0.4-0.6 MPa, shot diameter 0.1-0.3 mm, strain rate 5 × 10⁻⁶. 3 -1×10 4 s -1 Coverage rate 100%–150%, processing time 30–60 minutes; The pretreatment involves cleaning the mold substrate: specifically, soaking it in an alkaline degreasing agent with a mass fraction of 5% to 8% at 50-60°C for 20 to 30 minutes, then ultrasonically cleaning it with anhydrous ethanol for 15 minutes, and finally drying it at 80-100°C. The mold substrate is cold work mold steel, hot work mold steel, or plastic mold steel. Step 2: Multi-field Coupled Plasma Nitriding: Place the mold pretreated in Step 1 into the plasma nitriding equipment and evacuate to a vacuum level of ≤5×10⁻⁶. - The pressure is increased to 3Pa, then argon gas is introduced and preheated to 400-550℃. A 1:1 N2 / H2 mixture is then introduced at a total flow rate of 20-30 sccm, while a pulsed bias voltage of 500-800V and 5-10kHz, along with a magnetic field of 0.1-0.3T, is applied. The mixture is then held at 400-500℃ for 4-8 hours. Step 3: Deposition of multilayer gradient composite film: Place the mold after nitriding in Step 2 into a multi-arc ion plating equipment, and deposit the following films sequentially: (1) Chromium underlayer: pure chromium target deposition with a thickness of 5-8 μm; (2) Chromium nitride transition layer: deposited in a nitrogen atmosphere using a pure chromium target, with a thickness of 8-12 μm; (3) Zirconium chromium nitride surface layer: A zirconium chromium alloy target with a Zr:Cr atomic ratio of 3:7-5:5 is used to deposit the layer in a nitrogen atmosphere, with a thickness of 10-15 μm; Step 4: Pulsed Electron Beam Surface Finishing: The mold surface after film deposition in Step 3 is subjected to pulsed electron beam scanning. The scanning parameters are: energy density 10–20 J / cm². 2 The pulse width is 1-5 μs, the pulse frequency is 1-3 Hz, and each processing area is bombarded 2-3 times.

2. The method for preparing a nitrided layer with adjustable thickness and gradient hardness distribution on the mold surface according to claim 1, characterized in that: The cold work die steel is selected from Cr12 steel, Cr12MoV steel or DC53 steel; The hot work die steel is selected from H13 steel, 3Cr2W8V steel or 5CrNiMo steel. The plastic mold steel is selected from 718H steel, S136 steel or NAK80 steel.

3. The method for preparing a nitrided layer with adjustable thickness and gradient hardness distribution on the mold surface according to claim 1, characterized in that: The alkaline degreasing agent is composed of 3%–5% NaOH, 2%–3% Na2CO3 and the balance water, and the anhydrous ethanol has a purity of ≥99.7%.

4. The method for preparing a nitrided layer with adjustable thickness and gradient hardness distribution on the mold surface according to claim 1, characterized in that: The surface mechanical grinding parameters are: alumina particles (5-10μm), rotation speed 200-300r / min, time 40-80min, and pressure 0.2-0.3MPa.

5. The method for preparing a nitrided layer with adjustable thickness and gradient hardness distribution on the mold surface according to claim 1, characterized in that: The ultrasonic shot peening projectile is a high-speed steel shot with a hardness ≥60HRC and a purity ≥99.5%.

6. The method for preparing a nitrided layer with adjustable thickness and gradient hardness distribution on the mold surface according to claim 1, characterized in that: In the multi-field coupled plasma nitriding process, the purity of Ar, N2, and H2 is ≥99.999%, and the moisture content is ≤10ppm.

7. The method for preparing a nitrided layer with adjustable thickness and gradient hardness distribution on the mold surface according to claim 1, characterized in that: In step three, argon ion sputtering cleaning is performed: vacuum ≤ 3 × 10⁻⁶ - ³Pa, Ar flow rate 8-12sccm, voltage 800-1000V, time 10-15min.

8. The method for preparing a nitrided layer with adjustable thickness and gradient hardness distribution on the mold surface according to claim 1, characterized in that: The pure chromium target has a purity of ≥99.95% and a density of ≥95%; the zirconium-chromium alloy target has a density of ≥95% and an oxygen content of ≤0.05%.

9. The method for preparing a nitrided layer with adjustable thickness and gradient hardness distribution on the mold surface according to claim 1, characterized in that: The deposition temperature in step three is 200-250℃; The current for chromium-based bottom-layer target bonding is 60-80A; The target current for the chromium nitride transition layer is 70–90 A, and the N2 flow rate is 5–8 sccm; the target current for the zirconium nitride surface layer is 90–110 A, and the N2 flow rate is 8–10 sccm.

10. The method for preparing a nitrided layer with adjustable thickness and gradient hardness distribution on the mold surface according to claim 1, characterized in that: In step four, the vacuum level is ≤1×10⁻⁶. -2 The electron beam gun is operated at a distance of 15–20 cm from the surface.

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