CN107620031B - Nitriding system and method for austenitic stainless steel based on hollow cathode ion source - Google Patents

Abstract

The invention relates to a nitriding treatment system and method for austenitic stainless steel based on a hollow cathode ion source, and belongs to the technical field of nitriding treatment of austenitic stainless steel. The system includes a power supply system, a vacuum nitriding furnace, a hollow cathode device placed in the furnace, an air extraction system, a gas supply system, a measurement and control system, as well as connecting pipes and valves; the hollow cathode device in the vacuum nitriding furnace has multiple structures. The metal plates are arranged in parallel in the furnace, each metal plate is provided with a plurality of long grooves arranged at intervals, and each groove is provided with through holes arranged at intervals. In this method, multiple parts are cleaned and placed between two metal plates installed with a hollow cathode device in a vacuum nitriding furnace, after nitriding treatment, the temperature is raised to a temperature of 500°C-550°C, and the holding time is 0.5h-1.5h , and then cool down and take out the parts. The invention improves the gas ionization rate, increases the plasma concentration, can improve the surface hardness and wear resistance of austenitic stainless steel, does not affect its corrosion resistance, and improves the nitriding efficiency.

Description

Nitriding system and method for austenitic stainless steel based on hollow cathode ion source

technical field

The invention belongs to the technical field of nitriding treatment of austenitic stainless steel, and particularly relates to a nitriding treatment system and method for austenitic stainless steel based on a hollow cathode ion source.

Background technique

Due to the presence of a passivation film (such as Cr ₂ O ₃ ) on the surface of austenitic stainless steel, it is difficult for nitrogen to penetrate in conventional gas nitriding at relatively low temperatures. During gas nitriding, the depassivation process of the workpiece surface becomes an essential step. For example, after pre-heat treatment of pickling and shot blasting, the passive film on the surface is removed, and its regeneration should be inhibited. In the case of DC ion nitriding, the oxide layer can be effectively removed from the stainless steel surface due to the strong cathode sputtering effect. Therefore, nitrogen can also be effectively transported at temperatures below 400 °C.

A large number of experimental studies and theoretical analysis show that the key to the ion nitriding treatment technology of austenitic stainless steel is to control a sufficiently low nitriding temperature or a short nitriding time to suppress the formation of CrN as much as possible, thereby producing a single S-phase hardening. Floor. A single S-phase layer improves hardness and wear resistance while ensuring the original corrosion resistance of austenitic stainless steel. The S phase is an intermediate phase in a metastable state. As the processing temperature increases, the structure and properties of the nitrided layer of austenitic stainless steel change accordingly. The S phase is decomposed, the lattice lattice undergoes incoherent transformation, and it is transformed into a stable chromium nitride equilibrium phase to precipitate.

Plasma nitriding technology is widely used in the field of surface strengthening treatment of austenitic stainless steel due to its excellent performance in the treatment of austenitic stainless steel materials, and hollow cathode plasma nitriding is a further improvement of traditional plasma nitriding. At present, some ion nitriding based on hollow cathode structure improves the traditional ion nitriding but also produces certain limitations. For example, Ke Yueqian et al. studied the ion nitriding of two types of parts of austenitic stainless steel mold ejector rod and exhaust valve. Both types of parts were quenched and tempered, and a certain specification was set at a suitable distance around the parts. The wire mesh is used as the auxiliary cathode, so that the discharge between the two types of parts samples and the auxiliary cathode forms a hollow cathode structure. The nitriding rate was accelerated and a uniform hardened layer was obtained with cathode assistance; Zhao Yanhui et al. also used a hollow cathode structure when nitriding 316 austenitic stainless steel, that is, discharge at the air inlet above the vacuum furnace cavity Two flat plates with a small area are set at both ends of the device to form a hollow cathode, and a heating plate is set on the wall of the vacuum furnace. The austenitic stainless steel sample is placed on the sample plate in the center of the vacuum furnace. After arc plasma nitriding for 1h, the surface hardness of the sample was increased by more than four times, the wear rate was reduced by two orders of magnitude, and the corrosion resistance was also greatly improved; K.Nikolov et al. studied the nitrogen of 304 austenitic stainless steel plate using a hollow cathode structure Chemical treatment, the hollow cathode structure is two substrates (350mm×150mm, thickness 0.1-0.2mm) are installed face to face in parallel with a distance of 30mm, one side between the two plates is an anode block and the rest is an exhaust block, (the hollow cathode structure The configuration length is 140mm, the width is 30mm, and the height is 150mm) and the longest nitridation is 2h at 400-510℃, and finally a thicker nitrided layer is obtained, which improves the corrosion resistance of the plate sample.

The above-mentioned various hollow cathode plasma nitriding methods have shortcomings:

The research of Ke Yueqian et al. is to set up a hollow cathode structure for each different austenitic stainless steel parts, and the parts are also part of the hollow cathode structure. The surface of the sample will be over-burned or small holes will be formed, which is not conducive to the nitrogen of a large number of workpieces. In addition, the preparation work before nitriding is cumbersome, the utilization rate of hollow cathode equipment is low, and it is easy to cause waste; the hollow cathode used by Zhao Yanhui and others has a small coverage area, which will affect the gas ionization rate and reduce the nitriding efficiency. At the same time, the sample cannot be heated, which requires the addition of a heating plate, which not only requires a plate but also increases energy consumption; the hollow cathode structure of K.Nikolov et al. The size of the workpiece or sample is strictly required, and it can only handle plate or thin samples. The installation of exhaust block will also affect the gas diffusion and reduce the efficiency.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the deficiencies in the prior art that the low-temperature nitriding of austenitic stainless steel takes too long and the nitriding rate is low, and proposes a nitriding treatment system and method for austenitic stainless steel based on a hollow cathode ion source. . The invention improves the hollow cathode discharge structure in the hollow cathode ion source nitriding system. On this basis, the high-temperature rapid nitriding treatment technology is performed on the austenitic stainless steel workpiece. The method improves the corrosion resistance without affecting the corrosion resistance. Austenitic stainless steel surface hardness and wear resistance, while improving nitriding efficiency. The present invention proposes an austenitic stainless steel nitriding treatment system based on a hollow cathode ion source. The system includes a power supply system, a vacuum nitriding furnace, a hollow cathode device placed in the furnace, an air extraction system, an air supply system and a measurement and control system. system, as well as connecting pipes and valves between various parts; among them, the power supply system is connected to the vacuum nitriding furnace, the gas supply system is connected to the measurement and control system through the valve, and then connected to the vacuum nitriding furnace and hollow cathode device; The bottom of the nitrogen furnace body is connected with the gas extraction system through a valve; it is characterized in that, the hollow cathode device structure in the vacuum nitriding furnace is a plurality of metal plates arranged in parallel in the furnace, and each metal plate has a plurality of spaced arrangements There are long grooves and through holes arranged at intervals are opened in each groove.

A method for nitriding treatment based on the above-mentioned hollow cathode device, characterized in that it comprises the following steps:

1) Smooth and polish the surfaces of multiple parts to be treated; clean the surfaces of the parts that have been polished with alcohol to wash off the oil stains on the surfaces of the parts;

2) Put multiple parts between the two metal plates where the hollow cathode device is installed in the vacuum nitriding furnace, connect with the cathode of the power supply, cover the furnace cover, turn on the air pump to vacuumize the nitriding furnace, and pass the cooling water; Vacuum to 5-15Pa, adjust the working voltage and duty ratio, then open the ammonia bottle, and adjust the flowmeter to maintain the gas pressure at 400-450Pa;

3) Heat up the parts in the vacuum nitriding furnace, adjust the current and pressure to ensure the stable discharge of the hollow cathode, until the temperature rises to 500℃-550℃;

4) After heating up to the required temperature, keep the parts warm and start timing. The holding time is 0.5h-1.5h to ensure that the fluctuation range is controlled at 1-2°C;

5) When the set thermal insulation value is reached and the thermal insulation is over, the parts are cooled with the furnace;

6) When the parts are cooled to below 300°C, after the cooling stage is over, open the furnace and take out the parts, and the nitriding treatment is over.

Features and beneficial effects of the present invention:

The invention realizes the hollow cathode discharge by means of the improved hollow cathode electrode, and the austenitic stainless steel parts can be rapidly heated under the nitriding medium atmosphere with high activity and high concentration. It is expected that rapid nitriding can be performed at high temperature, and CrN cannot be aggregated and precipitated in a short period of time. Therefore, high-temperature rapid nitriding treatment technology can improve the surface hardness and wear resistance of austenitic stainless steel without affecting its corrosion resistance, while improving nitriding. efficiency.

The plate-shaped hollow cathode in the system of the invention has grooves and holes, and the grooves increase the area of the cathode plate, so that the discharge area increases under the same parameters, the temperature of the parts and the heating rate are both larger, and the diffusion of nitrogen atoms can be accelerated. The increase of the speed and area can increase the ionization rate of gas ionization and improve the nitriding efficiency, and the hole structure is conducive to gas diffusion to accelerate gas ionization, so that the metal plate can be hit by a large number of ions, and the nitriding quality is improved; the plate of the present invention The hollow cathode structure can be flexibly placed in the nitriding furnace, and the requirements for the external dimensions of the parts are small, which improves the utilization rate of the space in the nitriding furnace. These features make the nitridation efficiency of the plate-shaped hollow cathode structure with grooves and holes more than 1.5 times higher than that of the existing hollow cathode structure.

Description of drawings

FIG. 1 is a schematic diagram of the composition and structure of the austenitic stainless steel nitriding treatment system based on the hollow cathode ion source of the present invention.

Among them: 1.1 is the power supply system, 1.2 is the vacuum nitriding furnace, 1.3 is the hollow cathode equipment placed in the furnace, 1.4 is the measurement and control system, 1.41 is the display system for measuring temperature and pressure in the measurement and control system, and 1.5 is the ammonia cylinder ( Air supply system), 1.6 is the vacuum pump (extraction system).

FIG. 2 is a schematic structural diagram of a hollow cathode device of the system of the present invention.

Among them: 2.1 is the groove on the hollow cathode plate, and 2.2 is the through hole in the groove on the plate.

FIG. 3 is a SEM image of the nitrided layer of the part after the nitridation treatment according to the embodiment of the present invention.

FIG. 4 is an electrochemical corrosion polarization curve diagram after nitriding treatment according to an embodiment of the present invention.

Detailed ways

A system and method for nitriding austenitic stainless steel based on a hollow cathode ion source proposed by the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

An austenitic stainless steel nitriding treatment system based on a hollow cathode ion source proposed by the present invention, the structure is shown in Figure 1, including a power supply system 1.1, a vacuum nitriding furnace 1.2, a hollow cathode device 1.3 placed in the furnace, and a vacuum pump (extraction system) 1.6, ammonia cylinder (gas supply system) 1.5 and measurement and control system 1.4, as well as the connecting pipes and valves between the various parts; wherein, the power supply system 1.1 is connected with the vacuum nitriding furnace 1.3, and the measurement and control system 1.4 is It is composed of the pressure, temperature, voltage, current gas flow display 1.14 and the computer for detection and control. The ammonia cylinder (gas supply system) 1.5 is connected to the measurement and control system through a valve, and then connected to the vacuum nitriding furnace and the hollow cathode device; the bottom of the vacuum nitriding furnace body is connected to the vacuum pump (extraction system) 1.6 through a valve.

A kind of austenitic stainless steel nitriding treatment system based on the hollow cathode ion source of the present embodiment is different from the prior art except that the structure of the hollow cathode device is different from the prior art, and other components can all be realized by conventional devices, and the description is as follows:

The power supply system of the present invention adopts a conventional intermediate frequency pulse power supply to ensure that the hollow cathode discharge is generated under the conditions of different vacuum degrees. The structure of the hollow cathode device in the vacuum nitriding furnace used is a plurality of metal plates arranged in parallel in the furnace, each metal plate has a plurality of long grooves 2.1 arranged at intervals, and each groove is provided with a space The arranged through holes 2.2 are shown in Figure 2. The length and width of the metal plate can be determined according to the nitriding furnace and the parts, and the spacing between the metal plates is adjusted according to the placement position and quantity of each part. The thickness is 15-20mm, the groove width is 7-10mm, the depth is 5-7mm, the groove interval can be consistent with the width, the through hole is opened at the bottom of the groove, the diameter is 1mm smaller than the groove width or the same, the through hole spacing can be Consistent with the diameter, the size and spacing of the metal plates can be determined according to the number of nitriding parts and the space in the nitriding furnace. The size of the metal plate used in this example is 500mm×500mm×15mm, the groove width is 10mm, the depth is 5mm, and the groove interval is 10mm; the diameter of the through hole is 9mm, and the interval is 9mm; Placement and quantity adjustment. Multiple workpieces to be nitrided are placed between two adjacent metal plates, because the metal plates have round holes, which can ensure that the metal plates can be hit by a large number of ions, improve the quality of nitriding, and the hollow cathode of the metal plates The structure can be radiantly heated.

A method for nitriding treatment based on the above-mentioned hollow cathode device proposed by the present invention, the embodiment is carried out in a hollow cathode ion source nitriding furnace equipment, including the following:

1) The object to be processed in this embodiment is a plurality of parts made of austenitic stainless steel (the method of the present invention can be applied to austenitic stainless steel of any composition and shape, and this embodiment uses a round cake-shaped austenite with a diameter of 20 mm and a thickness of 6 mm, The two metal plates are 50mm away from the part), and the surface of the part is smoothed and polished with SiC sandpaper with labels of 240#, 400#, 800#, 1000#, 1500# and 2000# in turn; Cleaning, washing off the oil stains on the surface of the parts, etc.;

2) Put multiple parts between the two metal plates where the hollow cathode device is installed in the vacuum nitriding furnace, connect with the cathode of the power supply, cover the furnace cover, turn on the air pump to vacuumize the nitriding furnace, and pass the cooling water; Vacuum to 5-15Pa (10Pa in this example, the value can be seen from the pressure indicator, pressure, temperature, voltage, current gas flow, etc. can be displayed on the computer of the control system), adjust the working voltage (700-900V) And the duty ratio (70%-80%), then open the ammonia bottle, adjust the flowmeter to maintain the gas pressure at 400-450Pa (450Pa in this embodiment);

3) Heat up the parts in the vacuum nitriding furnace, and adjust the current and pressure (by adjusting the flowmeter on the ammonia cylinder to control the pressure to ensure the stable discharge of the hollow cathode, the heating rate is 15-25 degrees/min, and the heating rate is When it is slow, the current and pressure can be increased, and when the discharge is unstable, the current and pressure can be reduced), until the temperature rises to 500°C-550°C (the parameter in this embodiment is 530°C);

4) When the temperature is raised to the required temperature (530°C), the parts are kept warm and the timing is started. During the heat preservation process, adjust the current, voltage or air pressure (when the temperature continues to rise in the heat preservation stage, it means that the current density is too large and needs to be reduced; when the temperature drops, it means that the current density is too small and needs to be increased, then the voltage can be increased or air pressure) to ensure that its fluctuation range (the temperature fluctuation range is controlled at 1-2°C) is not too large;

5) When the set thermal insulation value is reached and the thermal insulation is over, the parts are cooled with the furnace;

6) When the parts are cooled to below 300°C, after the cooling stage is over, open the furnace and take out the parts, and the nitriding treatment is over. When shutting down the device, turn off the gas first, then the flowmeter, and finally the power.

The working principle of the present invention: It can be known from the mechanism of the hollow cathode discharge that when the hollow cathode discharge is generated, the electrons oscillate at the same amplitude of d/2 between the two polar plates, and at this time, it is just larger than but close to the electrons in the The mean free path of the gas is

d=2ke

Among them, d is the distance between the two cathode plates; e is the mean free path of electrons; k is the intensity coefficient.

The electron mean free path is:

In the formula, K is the Boltzmann constant, T is the working temperature, r is the molecular radius of the working gas, and P is the working pressure.

When the value of k is greater than and close to 1/2, that is, d is greater than and close to λe, the electrons obtain the maximum kinetic energy and potential energy conversion. The probability of ionization is highest if it collides with gas atoms in the negative glow region. Although the collision occurs because the cathode drop regions of the two poles are relatively close, many high-energy electrons can drop from one cathode to the other cathode drop region. the strongest distance. When the K value increases, it means that the distance increases. At this time, if the electrons accelerated from one cathode potential drop collide or excite in the negative glow region, and their energy is consumed, they enter the cathode potential drop of the other plate. The number of high-energy particles in the hollow cathode decreases due to the increase of the spacing, and even if the proportion of electrons that can generate "oscillation" decreases, the discharge intensity of the hollow cathode decreases.

The key to hollow cathode ignition is to establish a stable plasma sheath (cathode drop zone) on the cathode surface, otherwise, self-sustained discharge cannot be formed. The formula for the cathode drop zone is

d _c =[(K _i V _c ² )(1+γ)/(nj ₀ )] ^1/3

d _c is the width of the cathode drop zone; Ki is the electron drift rate; V _c is the voltage of the cathode drop zone; γ is the second ionization coefficient of Townsend; _{j 0} is the initial current density. It can be seen that the width of the cathode drop region is a monotonically increasing trend with the increase of the voltage V. Increasing the voltage (the operating voltage of the device is 700-900V) speeds up the movement of electrons in the cathode space, which is conducive to the generation of the hollow cathode effect. Under this voltage, the hollow cathode structure can be compared with the traditional ion nitridation ionization rate (10%-20% ) 3-6 times higher, increasing the plasma density.

The results obtained by the nitriding treatment of the present embodiment are analyzed and compared as follows:

Figure 3 shows the SEM image of the nitriding layer, in which: (a) the part is kept at 530°C when the heating is stopped for 0h, (b) the part is kept at 530°C for 1h, (c) the part is kept at 530°C for 5h , (d) is the traditional ion nitriding heat preservation of the parts at 450 ℃ for 10h. When the temperature just reaches the temperature (ie, 0h), a thin nitride layer of 2.6 μm is formed on the surface. At 530℃ for 1h, the nitrided layer is about 8μm, which has good corrosion resistance. The thickness of the nitrided layer of the traditional ion nitriding for 10h is also about 8 μm, indicating that the thickness of the nitrided layer produced by the hollow cathode-based ion nitridation for 1h is the same as that of the traditional ion nitridation for 10h. After holding for 5h, the thickness of the nitrided layer reached 18μm, but accompanied by the precipitation of black phase.

New and wider diffraction peaks S(111) and S(200) appeared in the samples subjected to the nitriding treatment of the method of the present invention. When the holding time is extended to 5h, the XRD pattern is mainly composed of CrN and martensite α phase. With the prolongation of nitriding treatment time, the metastable S phase changes at high temperature: S→CrN+α, and nitride precipitates and defects appear.

With the prolongation of treatment time, the hardness increased continuously. The surface hardness value of the nitriding 1h specimen increased to 800-900HV _0.1 . When the treatment time reaches 5h, the hardness value reaches about 1100HV _0.1 . When nitriding at 450°C for 10h, the hardness value is 800-900HV _0.1 , and there is no difference between the hardness at 530°C for 1h.

As shown in Figure 4, the untreated and nitrided parts were subjected to electrochemical corrosion test, in which: 4.1 is the polarization curve of the part without treatment, 4.2 is the polarization curve of the part treated at 530 °C for 0 h, and 4.3 is the part held at 530 °C. The polarization curve of 1h treatment, 4.4 is the polarization curve of the part treated at 530℃ for 5h, and 4.5 is the polarization curve of the part treated by traditional ion nitriding at 450℃ for 10h. In the corrosive medium containing chloride ions, the pitting potential of the parts treated for 0h and 1h for a short time increased by 0.2V, and the corrosion current was lower than that of the untreated parts even at a voltage of 0.8V. This is the rapid nitriding to prepare a thick S-phase layer on the surface, which has better corrosion resistance and can prevent the intrusion of corrosive liquids, especially chloride ions. The traditional ion nitriding at 450°C for 10h, the self-corrosion potential of the parts is lower than the parts treated by 530°C hollow cathode nitriding for 0h and 1h, indicating that the corrosion resistance is worse than that of 530°C hollow cathode nitriding for 0h and 1h, and the voltage is above 0.5V When the corrosion current is lower than the corrosion current of nitriding parts at 530℃ for 1h. The corrosion resistance of the nitriding 5h parts is reduced because of the formation of various phases such as CrN and α in the surface layer, the formation of galvanic corrosion between the different phases, and the deterioration of the corrosion resistance.

No pitting corrosion phenomenon is found on the surface of the nitriding 0.5-1.5h sample in the present invention, indicating that the high-temperature rapid nitriding process improves the pitting corrosion resistance of the austenitic stainless steel. High-temperature rapid nitriding can effectively treat AISI304 austenitic stainless steel, and nitride layers with different thicknesses and structures are prepared on the surface, which significantly improves the hardness of the matrix. At a high temperature of 530°C and a short processing time (0.5-1.5h), a single S-phase nitride layer without CrN precipitation can be prepared, and the thickness of the nitride layer is not less than that of traditional ion nitridation at 450°C for 10h; electrochemical Corrosion experiments show that compared with the traditional ion nitridation at 450℃ for 10h, the nitrided parts at high temperature and for a short time (0.5-1.5h) improve the pitting corrosion resistance in NaCl solution.

Claims (2)

1. An austenitic stainless steel nitriding treatment system based on a hollow cathode ion source, the system includes a power supply system, a hollow cathode device placed in the furnace, an air extraction system, an air supply system and a measurement and control system, as well as a connection with each part. The connecting pipes and valves between the two parts; among them, the power supply system is connected with the vacuum nitriding furnace, the gas supply system is connected with the measurement and control system through the valve, and then connected to the vacuum nitriding furnace and the hollow cathode device; the bottom of the vacuum nitriding furnace is connected with the valve through the valve. The air extraction system is connected; it is characterized in that, the hollow cathode device structure in the vacuum nitriding furnace is a plurality of metal plates arranged in parallel in the furnace, and each metal plate has a plurality of long grooves arranged at intervals and each There are through holes arranged at intervals in each of the grooves, and a plurality of workpieces to be nitrided are respectively placed between two adjacent metal plates; The thickness of the metal plate is 15-20mm, the groove width of the metal plate is 7-10mm, the depth is 5-7mm, the groove interval is consistent with the width, the through hole is opened at the bottom of the groove, and the diameter is 1mm smaller than the groove width Or the same, the via spacing is the same as the diameter. 2. A method for nitriding treatment based on the hollow cathode device as claimed in claim 1, characterized in that, comprising the following steps: 1) Smooth and polish the surfaces of multiple parts to be treated; clean the surfaces of the parts that have been polished with alcohol to wash off the oil stains on the surfaces of the parts; 2) Put multiple parts between the two metal plates where the hollow cathode device is installed in the vacuum nitriding furnace, connect with the cathode of the power supply, cover the furnace cover, turn on the air pump to vacuumize the nitriding furnace, and pass the cooling water; Vacuum to 5-15Pa, adjust the working voltage and duty ratio, then open the ammonia bottle, and adjust the flowmeter to maintain the gas pressure at 400-450Pa; 3) Heat up the parts in the vacuum nitriding furnace, adjust the current and pressure to ensure the stable discharge of the hollow cathode, until the temperature rises to 500℃-550℃; 4) After heating up to the required temperature, keep the parts warm and start timing. The holding time is 0.5h-1.5h to ensure that the fluctuation range is controlled at 1-2°C; 5) When the set thermal insulation value is reached and the thermal insulation is over, the parts are cooled with the furnace; 6) When the parts are cooled to below 300°C, after the cooling stage is over, open the furnace and take out the parts, and the nitriding treatment is over.

Images