SU1095673A1 - Method of nitriding metal articles - Google Patents

Abstract

Ь METHOD OF NITROGENING METAL PRODUCTS by using a volumetric electric discharge in gas between the surface of the product and the electrode connected with it, including filling the space between the product and the electrode with nitrogen or ammonia and the nitric mixture and supplying the difference of electric potentials to the electrode and product In order to increase the rate of metal saturation with nitrogen, a beam of ionizing radiation is introduced into the space between the surface of the products and the electrode, which initiates ignition between surface of the article and the electrode non-self Tel'nykh volumetric discharge in the gas. 2. The method according to claim 1, in connection with the fact that the magnitude of the potential difference applied to the electrode and the product takes a lower voltage of self-sustained discharge in the gas. 3. The method according to claim 1, which is also distinguished by the fact that pulses of ionizing radiation, which repeat in time, are used to initiate the discharge. (L 4. Method according to claim 1, which is distinguished by the use of a beam of reactive electrons as ionizing radiation. 5. The method according to claim 1, which is also distinguished by the fact that saturation of the surface of the product with nitrogen is increased gas pressure. 6. The method according to claim 5, characterized by the fact that saturation is carried out at a gas pressure of 1 to 3 at. 00 00

Description

This invention relates to chemical heat treatment, in particular to the nitriding of metal products. Known methods for nitriding metal surfaces in a glow discharge are known. The essence of these methods, for example, Cl3j is that in a glowing discharge of gas ions, the cathode surface of the glowing discharge and the cathode surface in the electric field cause the cathode sputtering of the metal (i.e. the evaporation of individual metal atoms from the solid surface cathode). Free iron atoms pulled out of a steel cathode interact with active (excited or ionized) ammonia or nitrogen molecules located at the surface to form iron nitrides. A portion of the nitrides condenses on the surface of the cathode, forming a surface nitride layer with a thickness of up to 8 microns. Then, under the action of gas ion bombardment, part of the iron nitrides in the nitride layer decomposes with the release of nitrogen atoms, which diffuse into the metal. Thus, when nitriding in glowing discharge, a stream of nitrogen atoms from nitride is added to the nitrogen atom flux caused by thermal chemisorption. lay The intensity of the electric field in the cathode layer of the discharge is / VIO - 10 V / cm. This electric field facilitates the transition of electrons from a metal to gas molecules at the surface, i.e. increases the rate of chemisorption. The closest to the invention according to the technical solution (prototype) is the method of nitriding steel in glow discharge 2. According to this method, nitriding is carried out as follows. The product to be nitrided is placed in a hermetic container and connected to the negative terminal of a constant voltage source. Over the surface of the product set isolated from the anode. Air is pumped out of the container to a vacuum of 0.01 Torr and at the same time the container is heated to 250 ° C. Then the container is filled with nitrogen or ammonia and a mixture of nitrogen and hydrogen to a pressure of 0.1-0.2 torr and a voltage of 1100-1400 V is applied to the electrodes. In this case, an anomalous glow discharge occurs in the gas between the cathode (product) and the anode. In an anomalous glow discharge with such a gas pressure, the discharge current density can reach 0.1 A / cm, and the energy of the ions bombarding the cathode can be up to 1 keV. Under the action of intense bombardment of the cathode surface by such high-energy ions, sputtering of oxide films and soils on the cathode surface occurs. This is a quick (560 min) cleaning (activation) of the product surface. After activation of the surface proceed to the actual nitriding. For this, the voltage on the electrodes is reduced to 600-900 V, the gas pressure in the container is increased to 6-8 Torr, and the temperature is increased to 470-580s. At the same time, the charge current of the discharge decreases to 0.5–20 mA / cm and the energy of the ions bombarding the cathode decreases. But as the pressure of the gas increases, the speed of the gas being assessed with nitrogen is increased. In this mode, nitriding is carried out for several hours. At the same time, the rate of nitriding, for example, of the ZOCHMUA steel, reaches 500 microns in 14 hours at a specific discharge power of 10 W / cm and specific power consumption of discharge for azoTation of a layer 500 microns thick to -5-105 J / cm2 / -0.14 kW- h / cm After the surface is treated with nitrogen, the voltage is disconnected, the container is removed from the furnace, it is cooled and the pressure in it is increased to atmospheric. Then remove the product from the container. Nitriding in the glow discharge, in addition to speeding up the process, allows the process to be conducted not only in ammonia, but also in pure nitrogen without hydrogen. This makes it possible to prevent hydrogen embrittlement of the metal. The glowing discharge nitride is low in the rate of saturation of the surface layers of the metal with nitrogen; it is only 500 µm in 14 hours. The low rate of nitriding in glow discharge is due to the low operating pressure of the gas. Indeed, both the solubility of gases in a metal, and the rate of diffusion of gas in a metal, and the rate of gas chemisorption increase with increasing gas pressure. Therefore, the application in the known method of low pressure (6-8 torr) extremely negatively affects the rate of nitriding. But in a known method, the gas pressure cannot be raised higher, because the glow discharge can exist only at low gas pressures. The aim of the invention is to increase the rate of saturation of the metal with nitrogen. This goal is achieved by the fact that in the method of nitriding metal products using a volume electric discharge in a gas, where the space between the surface of the product and the associated electrode is filled with nitrogen or ammonia and a nitrogen-hydrogen mixture, the electric difference is applied to the electrode and the product according to the invention, a beam of ionizing radiation is introduced into the space between the surface of the product and the electrode, which initiates the ignition between the surface of the product and the Heca electrode Low in gas. In this case, the magnitude of the potential difference applied to the electrode and the product takes a lower voltage to burn the self-sustained discharge in the gas. In addition, pulsed ionizing radiation, repeated in time, is used to initiate the discharge. A beam of relativistic electrons is used as ionizing radiation. The product surface is filled with nitrogen at an elevated gas pressure, which varies from 1 to 3 atmospheres. The non-self-sustaining high-current discharge, called in some works by the electron-ionization discharge CsJ, is a type of volumetric discharge in gas and, in some respects, is similar to tleyush 1st discharge. In the electron-discharge discharge, the current between the electrodes arises as a result of the ionization of the gas by an external source of ionizing radiation (a beam of reactivistic electrons, ultraviolet radiation, etc.). At the same time, the current density of the discharge Er can be extremely high and is limited only by the power source from. In real installations, which are widely used to excite gas from ionization lasers, 3 reaches A / cm. The electro-ionization734 discharge can burn at virtually any gas pressure (from fractions of a torus to ten atmospheres). The cathode region in an electron-ionization discharge is generated in a thin (10 cm) cathode layer with a high electric field strength (up to 10 V / cm), which increases with increasing Lp. Positive ions, as in a glow discharge, are accelerated in the electric field of the cathode layer and bombard the surface of the cathode ;. At the same time, the ion flux density 3j is determined by the discharge current density (3-Jp / O and achieves what is more than the maximum possible ion flux density in a glow discharge. The electro-ionization discharge, in comparison with the glow, allows to achieve more than 1 ° pressure gas P (up to 10 torr), greater current density of discharge Zp (up to 10 A / cm), greater intensity of the electric field at the cathode surface E (up to l / S V / cm). When electrization nitriding in the discharge at such large E increases dramatically compared to nitriding in glow discharges, The number of electrons that overcome the surface potential barrier and transfer from the metal to the gas molecules located at the surface. At high E, and at high gas pressures P, the rate of production of negative ammonia ions (Schr at the surface of the metal in the ionization gap can be much higher than in the glow discharge. As a result, the rate of ammonia chemisorption increases, the first stage of which is the production of negative ions at the metal surface. An increase in the rate of chemisorption leads to an increase in the rate of nitriding. When nitriding in an electron-ionized discharge initiated by a beam of reactive electrons, the electrons of the beam, passing through the gas of the discharge gap, fall on the cathode surface. In this case, electrons with energy / 1 MeV penetrate into the steel to a depth of 700 μm. At the same time, electrons of such energy are capable of generating dislocations in the crystal lattice of a metal. With an increase in the density of dislocations to a depth of 700 microns, the rate of diffusion of nitrogen atoms in the metal to this depth also increases. This also leads to an increase in the rate of nitriding. S10 With the purpose of providing large Ir and E, with a small average time of discharge power, it is recommended in this invention to use pulsed electron-ionization discharges with a high pulse repetition rate for nitriding. This gives a number of benefits. First, at a small average power (the power of the installation consumed from the power supply network is achieved, repetitive discharge pulses with a high frequency with extremely large Jp and Ej are reached. And since the electron flux density from the cathode grows with increasing Е | faster than the exponential law then the rate of chemisorption increases many times during such a pulse. As a result, the time-averaged rate of chemisorption is also large. Secondly, for large Dp in a pulse, almost all gas molecules are activated in the cathode layer, i.e. and all molecules become capable of entering into chemical bonds with iron. This ensures the maximum rate of chemisorption of gas molecules. Thirdly, in the intervals between pulses, when the rate of chemisorption decreases, nitrogen is resorbed from the surface layer into the metal, and the next pulse The density of nitrogen in the surface layer is less than the density of saturation, which makes it possible for the next portion of nitrogen to chemisorb 1 to the free space. Samples are taken from steel 38Kh1TSOA (GOST 4543-48), Armko iron, HF 45-5 cast iron (GOST 7293-54) and VTZ-1 titanium alloy (OST 1900 13-71. Samples are taken in the form of ground plates 50x50x5 mm. The plates are degreased with gasoline and acetone. Then place the plate in a 5 l container on a flat copper plate stand on a ceramic insulator. Honey on the plate is attached to the negative terminal of a high voltage source. A flat anode in the form of aluminum foil 50 µm thick is installed over the surface of the plate to be nitrided. pulled on a frame with The distance from the surface of the plate to the foil is 3 mm. Opposite the anode at a distance of 20 mm from it in the upper edge of the container there is a window 0 60 mm thicked with titanium foil 50 µm thick and sealed on the vacuum. The metal case of the container is connected to the positive terminal of the high voltage source and grounded. The container is evacuated from the container and simultaneously heated to 500 + 50 ° C. (The container body is heated by an electric heater). Then the container is filled with nitrogen or partially dissociated ammonia (the degree of dissociation is 30%) to the pressure P indicated in the table. A voltage is applied to the electrodes of the container and indicated in the table. A beam of relativistic electrons from the ELITE-1.5 accelerator is directed into it through the upper window of the container. The electron energy of the beam is 1 MeV, the pulse duration of the beam is 3 μs, the pulse repetition frequency is 100 Hz, the current density of the beam at the entrance to the container is 0.1 A / cm per pulse, the average time beam power is 1 kW. The beam electrons pass through the titanium foil of the upper window of the container, then penetrate the aluminum foil of the anode and ionize the gas between the anode and the surface of the processed plate, which acts as a cathode. Then the beam electrons bombard the surface of this plate. The ionizing capacity of an electron beam in a gas (gas ionization rate) V depends on the gas pressure P and is given in the table. When a gas is ionized between the surface of the plate and the anode, a pulsed electroionization discharge arises with a pulse duration equal to the duration of the beam pulse. The amplitude of the discharge current Er and the discharge current density (pulsed 3, time average apt) are given in the table, which also shows the time average power of the discharge W per second 1 cm cathode. Under the action of joint bombardment of the plate surface with relativistic electrons of the beam and plasma ions, the discharge of the plate surface is degassed and activated. The activation continues for 2-3 minutes while purging the gas through the container at a flow rate of 5 l / min. at the same time, nitriding of the surface of the plate begins. The rate of saturation of the metal with nitrogen increases as the surface is activated. Nitriding in pulsed electron-discharge discharges with a frequency of 100 Hz is continued for the time indicated in the table when gas is blown through the container. Then, the accelerator and the high voltage source are turned off, the container is cooled and the nitrated plate is removed from it. The results are tabulated.

The invention has the following advantages over known methods:

- the working pressure of the gas increases to normal (atmospheric) and high pressure pressures, which creates ease of operation of the installations;

- the rate of nitriding increases;

- the rate of saturation of the metal with nitrogen remains high even with large thicknesses of the metal layer saturated with nitrogen;

- The energy consumption for nitriding is reduced.

Evaluation of the economic effect of the use of the invention is carried out as follows. When nitriding in electroionization discharge de co

With an average discharge power of 19 W / cm, a layer with a thickness of 500 µm is filled with nitrogen in 20 minutes (see table). Consequently, the specific energy consumption of the discharge for nitriding of a layer of steel of such thickness is 2.3-10 J / cm. In addition, the electron beam energy from the accelerator S is consumed in an amount of W, -t it / S 0.28-10. Tzh / cm, where VI is 1 kW of accelerator power, 5 25 cm surface area of the workpiece, t 20 min is the nitriding time, .4 0.6 - Accelerator efficiency. So, the total specific energy consumption for creating a discharge when nitriding a layer of steel with a thickness of 500 µm is 2.58-10 J / cm 0.072 kWh / s. At the same time, during nitriding in a glow discharge, the energy consumption of a discharge for nitriding a steel layer 500 microns thick make up. 5-10 J / cm 0.14 kWh / cm. This creates an energy saving of 680, which means that when the cost of electricity is 2 kopeks per 1 kWh, it saves 13.6 rubles / m. In addition, as a result of shortening the nitriding time

14 hours to 20 minutes reduces energy consumption of furnaces by 40 times.

: heated containers.

Gas pressure P, ata - 13 Voltage and, kV4.5 13.5 1.5 4, Ionization rate V 10 ion pairs / cm, s 39 Amplitude of discharge current 1, kA 0.35 0.6 0.3 0 , Impulse current density of discharge 1p, A / cm 14 24 1 11 11 1 4.5 1.5. 4.5 1.5 4.5 1.5 0.35 0.3 0.35 0.3 0.35 0.3 14 12 14 12 14 12

Sample material

Parameter Gas type Steel 38XMICA NHj NHj N, JN, Time average discharge current density; ) R.Sr, mA / cm2 4.2 7.2 3.6 6 4.2 Average time specific power of discharge, W.BT/CM 19 97 5.4 27 19 Cast iron HF45-5 Alloy vtz-1 Iron Armco N2 W N NHj. 3,6 4,2 3, b 4,2 3,6 5,4 19 5,4 19 5,4

Claims (6)

1. METHOD OF NITROGENING METAL PRODUCTS using a volume electric discharge in gas between the surface of the product and the electrode mating with it, including filling the space between the product and the electrode with nitrogen or ammonia and a hydrogen-hydrogen mixture and supplying to the electrode and the product a difference in electric potentials, characterized in that, in order to increase the rate of metal saturation with nitrogen, a beam of ionizing radiation is introduced into the space between the surface of the product and the electrode, initiating ignition between the surface Tew products and electrode non-self-contained volume discharge in the gas. 2. The method according to π. 1, with the fact that the magnitude of the difference in potentials supplied to the electrode and the product is taken to be lower than the voltage of the self-discharge in the gas. 3. The method of pop. 1, characterized in that in order to initiate a discharge, pulses of ionizing radiation are used that are repeated in time. 4. The way to pop. 1, about l and ch. I TeMj that a beam of relativistic electrons is used as ionizing radiation. 5. The method of pop. 1, characterized in that the saturation of the surface of the product with nitrogen is carried out at elevated gas pressure. 6. The method according to claim 5, characterized in that the saturation is carried out at a gas pressure of from 1 to 3 atmospheres. SO SP Od m 00 1095673. 2