Ionic nitriding of parts. Ion-plasma nitriding ipa Protective pastes against ion plasma nitriding

With the correct composition and application mode of wear-resistant coatings, the performance of the cutting tool can be significantly improved. However, due to the invariability of the properties of the coating within one layer at the interface with the tool base, the physicomechanical and thermophysical properties (primarily the modulus of elasticity and the coefficient of thermal expansion) change sharply, which leads to the formation of high residual stresses in the coating and a decrease in the strength of its adhesive bond. with the base, which is the most important condition for the successful operation of coated cutting tools.

This, as well as changes in the contact and thermal processes during processing with a coated tool, require the creation of an intermediate transition layer between the tool base and the coating, which increases the resistance of the coated cutting wedge to the applied loads.

The most common method of forming such a layer is ion nitriding. In this case, the nitrided layer formed before the coating application, depending on the specific operating conditions of the tool, must have a certain structure, thickness and microhardness. In practice, HSS tools are usually treated like this.

Figure 4. Schematic diagram of a vacuum-arc installation for combined tool processing, including ion nitriding and coating: 1 - target; 2 - anode; 3 - screen; 4 - vacuum chamber; 5 - neutral atoms; 6 - ions; 7 - electrons; 8 - processed tools

For ionic nitriding and subsequent coating, it is advisable to use an installation based on a vacuum-arc discharge, in which all stages of combined hardening can be realized in one technological cycle without overloading the tools being processed.

The principle of operation of such an installation is as follows (Figure 4).

The target is vaporized by the cathode spots of the vacuum arc and is used as the cathode of the arc discharge. A special screen located between the target and the anode divides the chamber into two zones filled with metal-gas (to the left of the screen) and gas plasma (to the right). This screen is impervious to microdroplets, neutral atoms, and metal ions emitted by cathode spots on the target surface. Only electrons penetrate the screen, ionize the gas supplied to the chamber on the way to the anode and thus form a gas plasma free of metal particles.

Instruments immersed in plasma are heated by electrons when a positive potential is applied to them, and when a negative potential is applied, they are nitrided. At the end of nitriding, the screen is shifted to the side, and after the particles of the metal target begin to enter the surface of the tool, the coating is synthesized.

The deposition of coatings is a very energy-intensive process, accompanied by the effect of a high-energy plasma flow, especially at the moment of ion bombardment. As a result, the characteristics of the layer obtained by ionic nitriding may change significantly.

Therefore, when optimizing the process of combined processing of high-speed tools, it is necessary to take into account the factors not only of the nitriding process, but also of the subsequent process of applying a wear-resistant coating - primarily the application time, on which the coating thickness directly depends. On the one hand, its increase has a beneficial effect on increasing the wear resistance of the tool contact pads, and on the other hand, it leads to a noticeable increase in the number of defects in the coating, a decrease in the adhesion strength of the coating to the tool material, and a decrease in the coating's ability to resist elastoplastic deformations.

The most important conditions for combined processing are the temperature and duration of the nitriding process, the volume fraction of nitrogen in the gas mixture with argon, and the time of the subsequent process of applying a wear-resistant coating. Other factors in this process - nitrogen pressure, reference voltage, arc current at the cathode - mainly affect the characteristics of the coating and should be assigned the same as in the case of deposition of traditional coatings.

Depending on the type of cutting tool and the conditions of its subsequent operation in combined processing, its modes usually vary within the following limits: nitriding temperature 420 ... 510 ° С; atomic fraction of nitrogen N 2 in a gas mixture with argon 10 ... 80%; nitriding time 10 ... 70 min; pressure of the gas mixture ~ 9.75 · 10 -1 Pa; coating time 40 ... 80 min.

The practice of using tools made of high-speed steels after combined hardening in various machining operations shows that the presence of a nitrided layer under the coating, in which there is a brittle nitride zone (? - and? "- phases), significantly limits the effect of the combined treatment.

Such a structure is formed during nitriding in an atmosphere of pure nitrogen using a vacuum arc discharge plasma. The presence of a relatively thick nitride zone (\u003e 0.5 μm) during continuous cutting (turning and drilling) does not provide a significant increase in tool life compared to a tool with a traditional coating, and during interrupted cutting (milling and chiseling) it often leads to chipping of cutting edges already in the first minutes of tool operation.

The introduction of argon into the composition of a nitrogen-containing atmosphere during nitriding preceding the deposition of the coating makes it possible to control the phase composition of the formed layer and, depending on the specific operating conditions of the cutting tool and its service purpose, obtain the required structure.

When operating a high-speed tool with combined processing under interrupted cutting conditions, the optimal structure of the nitrided layer is a viscous solid solution of nitrogen in martensite that is resistant to variable loads, in which the formation of a small amount of dispersed nitrides of alloying components is permissible.

The specified structure can be obtained by nitriding in a medium containing ~ 30% N 2 and 70% Ar.

In the case of tool operation under continuous cutting conditions, the most efficient is the layer consisting of nitrogenous martensite and special nitrides of alloying elements (W, Mo, Cr, V).

In addition, the presence of a very small amount of the β phase is acceptable. This structure increases the resistance of the surface layer of the tool to thermal loads and can be formed by nitriding in an environment containing ~ 60% N 2 and 40% Ar.

The (Ti, Al) N coating applied to the samples nitrided in one-time mixtures containing,%, 60 N 2 + 40 Ar and 30 N 2 + 70 Ar, is characterized by a satisfactory strength of the adhesive bond. The samples did not show any flaking of the coating, nor the obvious cracks that were found in the samples nitrided at 100% N 2.

The creation of a wear-resistant complex on the contact pads of the cutting tool, formed by ion nitriding followed by coating in the vacuum arc discharge plasma, significantly affects the intensity and nature of the tool wear.

Figures 5 and 6 show the experimentally obtained profilograms of tool wear with a coating and with combined machining during longitudinal turning and face milling of structural steel 45. It can be seen that, in comparison with a single-layer coating, nitriding in combination with a coating practically does not change the nature of tool wear, but greatly reduces its intensity.

For the operating conditions under consideration, the low efficiency of the coated tool without nitriding is noted, both in milling and in turning. This is due to the fact that the coating breaks down very quickly and the friction conditions on the flank surface are increasingly approaching those characteristic of an uncoated tool. And this means that the amount of heat released increases, the temperature near the rear surface rises, as a result of which irreversible softening processes begin in the tool material, which lead to catastrophic wear.

Studies of the nature of bluntness of tools with nitriding and coating allow us to conclude that the main contribution to the decrease in the wear rate of high-speed tools is made by the so-called "edge effect", which is as follows.

Already in the first minutes of tool operation, as can be seen from the profilograms of its working surfaces (Figures 5 and 6), the coating collapses to its entire thickness in the areas near the cutting edge. However, further growth of wear centers along the length and depth is restrained by the edges of the contact areas, which retain the wear-resistant combination of the coating and the nitrided layer.

In addition, the surface nitrided layer, which has increased hardness in combination with high heat resistance, is distinguished by a higher resistance to microplastic deformation and helps to slow down softening processes at the rear surface.

Figure 5. Profilograms of worn sections of cutting inserts made of steel R6M5 when turning steel 45: a - R6M5 + (Ti, A1) N; b - P6M5 + nitriding + (Ti, A1) N; processing modes: v \u003d 82 m / min; S \u003d 0.2 mm / rev; / \u003d 1.5 mm (without coolant)

Figure 6. Profilograms of worn sections of cutting inserts made of steel R6M5 during face milling of steel 45: a - R6M5 + (Ti, Al) N; b - P6M5 + nitriding + (Ti, Al) N; processing modes: v \u003d 89 m / min; S \u003d 0.15 mm / tooth; B \u003d 45 mm;

Production experience shows that combined processing, involving preliminary nitriding and subsequent coating, can increase the durability of high-speed tools of the widest range up to 5 and up to 3 times compared to tools without hardening and with traditional coating, respectively.

Figure 7 shows the dependences of the change in wear over time h 3 \u003d f (T) of cutting inserts made of R6M5 steel, which have undergone various types of surface hardening, during turning and face milling of steel 45. It can be seen that the resistance to catastrophic tool wear during turning increases by 2, 6 times, and when milling - 2.9 times compared to a coated tool, but without nitriding.

Figure 7. Dependence of wear on the flank surface of a tool made of steel R6M5 with various options for surface treatment on the cutting time: - * - R6M5 + (Ti, A1) N; - * - Р6М5 + nitriding + (Ti-Al) N; a - turning of steel 45 at v \u003d 82 m / min; S \u003d 0.2 mm / rev; / \u003d 1.5 mm; b - milling steel 45: v \u003d 89 m / min; 5 \u003d 0.15 mm / tooth; B \u003d 45 mm; t \u003d 1.5 mm

Improving the properties of a metal can take place by changing its chemical composition. An example is steel nitriding - a relatively new technology for saturating the surface layer with nitrogen, which began to be applied on an industrial scale about a century ago. The technology under consideration has been proposed to improve some of the qualities of products made from steel. Let's take a closer look at how the steel is saturated with nitrogen.

Purpose of nitriding

Many people compare the process of cementing and nitriding because both are designed to dramatically improve part performance. The nitrogen injection technology has several advantages over carburizing, among which it is noted that there is no need to increase the billet temperature to the values \u200b\u200bat which the atomic lattice is attached. It is also noted that the nitrogen application technology practically does not change the linear dimensions of the workpieces, due to which it can be used after finishing. On many production lines, parts are subjected to nitriding, which have been hardened and grinded, almost ready for release, but some quality needs to be improved.

The purpose of nitriding is associated with a change in the basic performance characteristics during the heating of the part in an environment characterized by a high concentration of ammonia. Due to this effect, the surface layer is saturated with nitrogen, and the part acquires the following performance characteristics:

The wear resistance of the surface is significantly increased due to the increased hardness index.
The value of endurance and resistance to fatigue growth of the metal structure is improved.
In many industries, the use of nitriding is associated with the need to impart anti-corrosion resistance, which is retained in contact with water, steam or air with high humidity.

The above information determines that the results of nitriding are more significant than those of cementation. The advantages and disadvantages of the process largely depend on the technology chosen. In most cases, the transferred performance is retained even when the workpiece is heated to a temperature of 600 degrees Celsius; in the case of cementation, the surface layer loses its hardness and strength after heating to 225 degrees Celsius.

Nitriding process technology

In many ways, the process of nitriding steel surpasses other methods that involve changing the chemical composition of the metal. The nitriding technology of steel parts has the following features:

In most cases, the procedure is carried out at a temperature of about 600 degrees Celsius. The part is placed in a sealed iron muffle furnace, which is placed in the furnace.
When considering nitriding modes, one should take into account the temperature and holding time. For different steels, these indicators will differ significantly. Also, the choice depends on what performance you want to achieve.
Ammonia is fed from a cylinder into the created metal container. The high temperature causes the ammonia to decompose, thereby releasing nitrogen molecules.
Nitrogen molecules penetrate into the metal due to the diffusion process. Due to this, nitrides are actively formed on the surface, which are characterized by increased resistance to mechanical stress.
The procedure for chemical-thermal exposure in this case does not provide for a sharp cooling. Typically, the nitriding furnace is cooled with the ammonia stream and the part so that the surface is not oxidized. Therefore, the technology under consideration is suitable for changing the properties of parts that have already been finished.

The classic process of obtaining the required product with nitriding involves several stages:

Preparatory heat treatment, which consists of quenching and tempering. Due to the rearrangement of the atomic lattice under a given regime, the structure becomes more viscous, and the strength increases. Cooling can take place in water or oil, in a different environment - it all depends on how high-quality the product should be.
Further, mechanical processing is performed to give the desired shape and size.
In some cases, there is a need to protect certain parts of the product. Protection is carried out by applying liquid glass or tin with a layer of about 0.015 mm. As a result, a protective film is formed on the surface.
Steel nitriding is carried out according to one of the most suitable methods.
Work is underway on finishing machining, removing the protective layer.

The resulting layer after nitriding, which is represented by nitride, is from 0.3 to 0.6 mm, due to which there is no need for a hardening procedure. As noted earlier, nitriding is carried out relatively recently, but the process of transforming the surface layer of the metal has already been almost completely studied, which made it possible to significantly increase the efficiency of the technology used.

Metals and alloys subjected to nitriding

There are certain requirements that apply to metals before carrying out this procedure. As a rule, attention is paid to the concentration of carbon. The types of steels suitable for nitriding are very different, the main condition is a carbon fraction of 0.3-0.5%. The best results are obtained when using alloyed alloys, since additional impurities contribute to the formation of additional solid nitrites. An example of the chemical treatment of a metal is the saturation of the surface layer of alloys that contain impurities in the form of aluminum, chromium and others. The alloys under consideration are usually called nitralloys.

Nitrogen is applied using the following steel grades:

If a significant mechanical effect will be exerted on the part during operation, then the brand 38X2MYUA is chosen. It contains aluminum, which causes a decrease in deformation resistance.
Steel 40X and 40XFA are the most widely used in machine tool building.
In the manufacture of shafts, which are often subjected to bending loads, brands 38ХГМ and 30ХЗМ are used.
If during manufacturing it is necessary to obtain high accuracy of linear dimensions, for example, when creating parts of fuel units, then steel grade 30HZMF1 is used. In order to significantly increase the strength of the surface and its hardness, pre-alloying with silicon is carried out.

When choosing the most suitable steel grade, the main thing is to comply with the condition associated with the percentage of carbon, and also take into account the concentration of impurities, which also have a significant effect on the operational properties of the metal.

Main types of nitriding

There are several technologies for nitriding steel. Here's a list as an example:

Ammonia-propane medium. Gas nitriding is very widespread today. In this case, the mixture is represented by a combination of ammonia and propane, which are taken in a ratio of 1 to 1. As practice shows, gas nitriding when using such an environment requires heating to a temperature of 570 degrees Celsius and holding for 3 hours. The resulting nitride layer is characterized by a small thickness, but at the same time the wear resistance and hardness are much higher than when using the classical technology. In this case, nitriding of steel parts makes it possible to increase the hardness of the metal surface up to 600-1100 HV.
Glow discharge is a technique that also involves the use of a nitrogen-containing environment. Its peculiarity lies in the connection of nitrided parts to the cathode; the muffle acts as a positive charge. By connecting the cathode, it is possible to speed up the process several times.
The liquid medium is used a little less often, but it is also characterized by high efficiency. An example is a technology that involves the use of a molten cyanide layer. Heating is carried out to a temperature of 600 degrees, the holding period is from 30 minutes to 3 hours.

In industry, the most widespread is the gaseous environment due to the ability to process a large batch at once.

Catalytic gas nitriding

This type of chemical treatment provides for the creation of a special atmosphere in the oven. Dissociated ammonia is pretreated on a special catalytic element, which significantly increases the amount of ionized radicals. Features of the technology are in the following points:

The preliminary preparation of ammonia makes it possible to increase the proportion of solid solution diffusion, which reduces the proportion of reaction chemical processes during the transition of the active substance from the environment to iron.
Provides for the use of special equipment that provides the most favorable conditions for chemical processing.

This method has been used for several decades, it allows you to change the properties of not only metals, but also titanium alloys. The high costs of installing equipment and preparing the environment determine the applicability of the technology to obtaining critical parts that must have precise dimensions and increased wear resistance.

Properties of nitrided metal surfaces

Quite important is the question of what hardness of the nitrided layer is achieved. When considering hardness, the type of steel being processed is taken into account:

Carbonaceous can have hardness in the range of 200-250HV.
Alloyed alloys after nitriding acquire hardness in the range of 600-800HV.
Nitralloy, which contains aluminum, chromium and other metals, can get hardness up to 1200HV.

Other properties of the steel also change. For example, the corrosion resistance of steel increases, due to which it can be used in an aggressive environment. The process of introducing nitrogen itself does not lead to the appearance of defects, since heating is carried out to a temperature that does not change the atomic lattice.

Nitriding, during which the surface layer of a steel product is saturated with nitrogen, has been used on an industrial scale relatively recently. Such a processing method, proposed for use by academician N.P. Chizhevsky, allows you to improve many characteristics of products made of steel alloys.

The essence of technology

Nitriding of steel, when compared with such a popular method of processing this metal as carburizing, has a number of significant advantages. That is why this technology began to be used as the main method for improving the quality characteristics of steel.

During nitriding, the steel product is not exposed to significant thermal effects, while the hardness of its surface layer increases significantly. It is important that the dimensions of the nitrided parts do not change. This allows this processing method to be applied to steel products that have already been hardened with high tempering and ground to the required geometric parameters. After nitriding, or nitriding as the process is often called, the steel can be polished or other finishing methods immediately.

Steel nitriding means that the metal is heated in an environment characterized by a high ammonia content. As a result of such processing with a surface layer of metal saturated with nitrogen, the following changes occur.

Due to the fact that the hardness of the surface layer of the steel increases, the wear resistance of the part is improved.
The fatigue strength of the product increases.
The surface of the product becomes corrosion resistant. This stability is maintained when steel comes into contact with water, humid air and vapor-air environment.

Carrying out nitriding makes it possible to obtain more stable indicators of steel hardness than when carburizing. Thus, the surface layer of a product that has been nitrided retains its hardness even when heated to a temperature of 550–600 °, while after carburizing, the hardness of the surface layer may begin to decrease even when the product is heated above 225 °. The strength characteristics of the surface layer of steel after nitriding are 1.5–2 times higher than after quenching or carburizing.

How is the nitriding process

Metal parts are placed in a hermetically sealed muffle, which is then installed in a nitriding furnace. In the furnace, the muffle with the part is heated to a temperature that is usually in the range of 500–600 °, and then kept for some time at this temperature regime.

To form the working environment inside the muffle, which is necessary for nitriding, ammonia is fed into it under pressure. When heated, ammonia begins to decompose into constituent elements, this process is described by the following chemical formula:

2NH 3 → 6H + 2N.

Atomic nitrogen released during this reaction begins to diffuse into the metal from which the workpiece is made, which leads to the formation of nitrides on its surface, characterized by high hardness. To fix the result and prevent the surface of the part from oxidizing, the muffle, together with the product and the ammonia that remains in it, is slowly cooled along with the nitriding furnace.

The nitride layer formed on the metal surface during nitriding can have a thickness in the range of 0.3–0.6 mm. This is quite enough to endow the product with the required strength characteristics. Steel processed using this technology does not need to be subjected to any additional processing methods.

The processes occurring in the surface layer of a steel product during nitriding are rather complicated, but have already been well studied by specialists in the metallurgical industry. As a result of such processes, the following phases are formed in the structure of the processed metal:

solid solution Fe 3 N, characterized by nitrogen content in the range of 8-11.2%;
solid solution of Fe 4 N, nitrogen which contains 5.7–6.1%;
nitrogen solution formed in α-iron.

An additional α-phase in the metal structure is formed when the nitriding temperature begins to exceed 591 °. At the moment when the degree of saturation of this phase with nitrogen reaches its maximum, a new phase is formed in the metal structure. Eutectoid decomposition in the metal structure occurs when the degree of its saturation with nitrogen reaches a level of 2.35%.

Valves of high-tech internal combustion engines must undergo a nitriding process

Factors Affecting Nitrogenation

The main factors that affect nitriding are:

the temperature at which such a technological operation is performed;
the pressure of the gas supplied to the muffle;
the duration of the holding of the part in the furnace.

The efficiency of this process is also influenced by the degree of dissociation of ammonia, which, as a rule, is in the range of 15–45%. With an increase in the nitriding temperature, the hardness of the formed layer decreases, but the process of nitrogen diffusion into the metal structure is accelerated. The decrease in the hardness of the surface layer of the metal during nitriding occurs due to the coagulation of the nitrides of the alloying elements included in its composition.

To accelerate the nitriding process and increase its efficiency, a two-stage scheme is used for its implementation. The first stage of nitriding when using this scheme is performed at a temperature not exceeding 525 °. This makes it possible to impart high hardness to the surface layer of the steel product. To perform the second stage of the procedure, the part is heated to a temperature of 600–620 °, while the depth of the nitrided layer reaches the required values, and the process itself is almost doubled. The hardness of the surface layer of a steel product processed by this technology is not lower than the same parameter of products processed by a one-stage technique.

Types of nitrided steels

Nitriding technology can be processed both carbonaceous and those characterized by a carbon content in the range of 0.3–0.5%. The maximum effect when using such a technological operation can be achieved if steels are subjected to it, the chemical composition of which includes alloying elements that form solid and heat-resistant nitrides. These elements include, in particular, molybdenum, aluminum, chromium and other metals with similar characteristics. Steels containing molybdenum are not subject to such a negative phenomenon as temper brittleness, which occurs when the steel product slowly cools. After nitriding, steels of various grades acquire the following hardness:

The alloying elements in the chemical composition of the steel increase the hardness of the nitrided layer, but at the same time reduce its thickness. Such chemical elements as tungsten, molybdenum, chromium and nickel have the most active influence on the thickness of the nitrided layer.

Depending on the scope of application of the product that is subjected to the nitriding procedure, as well as on the conditions of its operation, it is recommended to use certain steel grades for such a technological operation. So, in accordance with the technological problem that needs to be solved, experts advise using products from the following steel grades for nitriding.

38Х2МЮА

This is a steel that, after nitriding, has a high hardness of the outer surface. The aluminum contained in the chemical composition of such steel reduces the deformation resistance of the product, but at the same time contributes to an increase in the hardness and wear resistance of its outer surface. The elimination of aluminum from the chemical composition of steel makes it possible to create products of a more complex configuration from it.

40X, 40XFA

These alloy steels are used for the manufacture of parts used in the field of machine tool construction.

30Х3М, 38ХГМ, 38ХНМЗА, 38ХН3МА

These steels are used for the production of products that are exposed to frequent cyclic bending loads during their operation.

30X3MF1

Products are made from this steel alloy, the accuracy of the geometric parameters of which are subject to high requirements. To impart a higher hardness to parts of this steel (these are mainly parts of fuel equipment), silicon can be added to its chemical composition.

Technological scheme of nitriding

To carry out traditional gas nitriding, innovative plasma nitriding or ion nitriding, the workpiece is subjected to a series of processing steps.

Preparatory heat treatment

Such processing consists in product hardening and its high tempering. Quenching in the framework of this procedure is carried out at a temperature of about 940 °, while the cooling of the workpiece is carried out in oil or water. The subsequent tempering after quenching, passing at a temperature of 600–700 °, makes it possible to endow the processed metal with hardness at which it can be easily cut.

Mechanical restoration

This operation ends with its grinding, which allows bringing the geometric parameters of the part to the required values.

Protection of product areas that do not require nitriding

Such protection is carried out by applying a thin layer (no more than 0.015 mm) of tin or liquid glass. For this, electrolysis technology is used. A film of these materials that forms on the surface of the product does not allow nitrogen to penetrate into its internal structure.

Performing the nitriding itself

The prepared product is processed in a gaseous environment.

Finishing

This stage is necessary in order to bring the geometric and mechanical characteristics of the product to the required values.

The degree of change in the geometric parameters of the part during nitriding, as mentioned above, is very insignificant, and it depends on such factors as the thickness of the surface layer that is saturated with nitrogen; temperature regime of the procedure. The almost complete absence of deformation of the workpiece can be guaranteed by a more advanced technology - ion nitriding. When performing ion-plasma nitriding, steel products are exposed to less thermal stress, due to which their deformation is minimized.

Unlike innovative ion-plasma nitriding, traditional nitriding can be carried out at temperatures up to 700 °. For this, a replaceable muffle or a muffle built into the heating furnace can be used. The use of a replaceable muffle, into which the processed parts are loaded in advance, before installing it in the furnace, can significantly speed up the nitriding process, but is not always an economically viable option (especially in cases where large-sized products are processed).

Types of working environments

Various types of working media can be used to perform nitriding. The most common of these is a gaseous medium consisting of 50% ammonia and 50% propane, or ammonia and endogas, taken in the same proportions. The nitriding process in such an environment is carried out at a temperature of 570 °. In this case, the product is exposed to the gaseous environment for 3 hours. The nitrided layer created by using such a working medium has a small thickness, but high strength and wear resistance.

The method of ion-plasma nitriding, performed in a nitrogen-containing rarefied medium, has recently gained wide acceptance.

Ionic Plasma Nitriding - A Look from the Inside

A distinctive feature of ion-plasma nitriding, which is also called glow discharge processing, is that the workpiece and the muffle are connected to an electric current source, while the product acts as a negatively charged electrode, and the muffle acts as a positively charged one. As a result, an ion flow is formed between the part and the muffle - a kind of plasma, consisting of N 2 or NH 3, due to which both the heating of the treated surface and its saturation with the required amount of nitrogen occur.

In addition to traditional and ion-plasma nitriding, the process of saturating the steel surface with nitrogen can be performed in a liquid medium. As a working medium, which has a heating temperature of about 570 °, in such cases, a melt of cyanide salts is used. The nitriding time, performed in a liquid working medium, can be from 30 to 180 minutes.

Ion-plasma hardening Vacuum ion-plasma methods of hardening the surfaces of parts include the following processes: generation (formation) of a corpuscular flow of matter; its activation, acceleration and focusing; ; condensation and penetration into the surface of parts (substrates). Generation: a corpuscular flow of matter is possible by its evaporation (sublimation) and spraying. Evaporation: the transition of the condensed phase into vapor is carried out as a result of the supply of thermal energy to the evaporated substance. Solids usually melt when heated and then turn into a gaseous state. Some substances pass into a gaseous state bypassing the liquid phase. This process is called sublimation. ...

Using the methods of vacuum ion-plasma technology, it is possible to perform: 1) modification of surface layers: ion-diffusion saturation; (ion nitriding, carburizing, borated, etc.); ionic (plasma) etching (cleaning); ion implantation (implantation); glow discharge annealing; CTO in a non-self-sustaining discharge environment; 2) coating: glow discharge polymerization; ion plating (triode sputtering system, diode sputtering system using hollow cathode discharge); electric arc evaporation; ion-cluster method; cathode sputtering (direct current, high frequency); chemical deposition in a glow discharge plasma.

Advantages of the methods of vacuum ion-plasma hardening high adhesion of the coating to the substrate; uniformity of coating thickness over a large area; varying the composition of the coating in a wide range, within one technological cycle; obtaining high purity of the coating surface; ecological cleanliness of the production cycle.

Ion sputtering Ion sputters are divided into two groups: plasma ionic ones, in which the target is located in a gas-discharge plasma created by means of a glow, arc, and high-frequency discharge. Sputtering occurs as a result of bombardment of the target with ions extracted from the plasma; autonomous sources without focusing and with focusing of ion beams bombarding the target.

Principle spraying system 1 - chamber; 2 - substrate holder; 3 - details (substrates); 4 - target; 5 - cathode; 6 - screen; 7 - working gas supply; 8 - power supply; 9 - pumping out.

Chemical treatment in a glow discharge environment Diffusion installations with a glow discharge are used to carry out the processes of nitriding, carburizing, siliconizing and other types of chemical treatment from the gas phase. The depth of the diffusion layer reaches several millimeters with uniform saturation of the entire surface of the product. The process is carried out at a reduced pressure of 10 -1 - 10 -3 Pa, which ensures the existence of a glow discharge. Advantages of using a glow discharge: high power utilization rate (consumption only for gas ionization and part heating); reducing the duration of the process, due to rapid heating to the saturation temperature; increasing the activity of the gaseous medium and the surface layer; the possibility of obtaining coatings from refractory metals, alloys and chemical compounds. Disadvantages of the process: low pressure in the chamber (10 -1 Pa), low productivity, work in a batch mode, the impossibility of processing long products (for example, pipes), significant energy consumption, high cost of installations.

Ion-diffusion saturation Advantages over the conventional gas nitriding process: reduction of cycle time by 3 -5 times; reduction of deformation of parts by 3 -5 times; the possibility of carrying out controlled nitriding processes to obtain layers with a given composition and structure; the possibility of reducing the temperature of the nitriding process to 350 -400 0 С, which avoids weakening of the materials of the core of the products; reducing the fragility of the layer and increasing its performance; ease of protection of individual areas of parts from nitriding; elimination of the danger of explosion of the furnace; reduction of specific consumption of electrical energy by 1, 5-2 times and working gas by 30 -50 times; improving the working conditions of thermists. Disadvantages: impossibility of accelerating the process by increasing the density of the ion flux, since as a result of overheating of parts, the surface hardness decreases; intensification of the ionic nitriding process; the imposition of a magnetic field in order to increase the current density and reduce the gas pressure; by creating the surface of a part of a given defectiveness (preliminary plastic deformation, heat treatment).

Ionic carburizing unit EVT

Ion cementation Ion cementation creates a high carbon concentration gradient in the boundary layer. The growth rate of the carburized layer of the material is 0.4 ... 0.6 mm / h, which is 3 ... 5 times higher than this indicator for other methods of cementation. The duration of ion cementation to obtain a layer 1 ... 1, 2 mm thick is reduced to 2 ... 3 hours. Due to low consumption of gases, electricity and short processing times, production costs are reduced by 4 ... 5 times. The technological advantages of ionic carburization include high uniformity of carburization, absence of external and internal oxidation, and reduction of warpage of parts. The volume of machining is reduced by 30%, the number of machining steps is reduced by 40%, and the cycle time is reduced by 50%.

Ion-plasma nitriding (IPA) IPA is a type of chemical-thermal treatment of machine parts, tools, stamping and casting equipment, providing diffusion saturation of the surface layer of steel (cast iron) with nitrogen or nitrogen and carbon in nitrogen-hydrogen plasma at a temperature of 450 - 600 ° С , as well as titanium or titanium alloys at a temperature of 800 - 950 ° C in nitrogen plasma. The essence of ion-plasma nitriding is that in a nitrogen-containing gaseous medium discharged to 200-1000 Pa between the cathode, on which the workpieces are located, and the anode, which is played by the walls of the vacuum chamber, an anomalous glow discharge is excited, which forms an active medium (ions, atoms, excited molecules). This ensures the formation of a nitrided layer on the surface of the product, which consists of an outer - nitride zone with a diffusion zone located under it.

Microstructure of the nitrided layer of tool steel 4 X 5 MFS a b Microstructures of steels U 8 (a) and 20 X 13 (b) after ion-plasma nitriding

Installation UA-63 -950/3400 with variable geometry of the working chamber (height 1, 7 or 3, 4 m)

Application of the method of ion-plasma nitriding by this method the following products are processed: nozzles for cars, bearing plates of an automatic drive, dies, punches, dies, molds (Daimler Chrysler); springs for the injection system (Opel); crankshafts (Audi); camshafts (Volkswagen); crankshafts for the compressor (Atlas, USA and Wabco, Germany); gears for BMW (Handl, Germany); bus gears (Voith); strengthening of pressing tools in the production of aluminum products (Nughovens, Scandex, John Davies, etc.). There is a positive experience of industrial use of this method by the CIS countries: Belarus - MZKT, MAZ, Bel. AZ; Russia - Auto. VAZ, Kam. AZ, MMPP Salyut, Ufa Engine-Building Association (UMPO). The following are processed by the IPA method: gears (MZKT); gears and other parts (MAZ); gear wheels of large (over 800 mm) diameter (Bel. AZ); intake and exhaust valves (Auto. VAZ); crankshafts (Kam. AZ).

Metallization of type 1 products is carried out for decorative purposes, to increase the hardness and wear resistance, and to protect against corrosion. Due to the weak adhesion of the coating to the substrate, this type of metallization is impractical to use for parts operating under high loads and temperatures. Metallization technology according to types 1 and 2 a provides for the imposition of a layer of substance on the surface of a product that is cold or heated to relatively low temperatures. These types of metallization include: electrolytic (electroplating); chemical; flame processes for obtaining coatings (spraying); coating by cladding (mechanical-thermal); diffusion, immersion in molten metals. Metallization technology of type 2 b provides for the diffusion saturation of the surface of parts heated to high temperatures with metal elements, as a result of which an alloy is formed in the diffusion zone of the element (Diffusion metallization). In this case, the geometry and dimensions of the part to be metallized practically do not change.

Ion-plasma metallization Ion-plasma metallization has a number of significant advantages over other types of metallization. A high plasma temperature and a neutral environment make it possible to obtain coatings with greater structural homogeneity, lower oxidizability, higher cohesive and adhesive properties, wear resistance, etc. in comparison with these properties of other types of metallization. Using this method of metallization, you can spray various refractory materials: tungsten, molybdenum, titanium, etc., hard alloys, as well as oxides of aluminum, chromium, magnesium, etc. Coating can be done by spraying both wire and powder. Metallization itself consists of three processes: melting of a solid metal of a wire or powder (with ion-plasma metallization), spraying the molten metal and forming a coating. Materials for spraying can be any refractory metals in the form of a wire or powder, but medium-carbon alloyed wires of the type Np-40, Np-ZOKHGSA, Np-ZX 13, etc. can also be used. In the conditions of auto repair enterprises, an alloy of the type can be used as refractory materials. VZK (stellite) or sormite, which has high wear resistance and corrosion resistance.

Ion-plasma nitriding (IPA) is a method of chemical-thermal treatment of steel and cast iron products with great technological capabilities, which makes it possible to obtain diffusion layers of the required composition by using different gaseous media, i.e. the diffusion saturation process is controllable and can be optimized depending on the specific requirements for layer depth and surface hardness. plasma nitriding microhardness alloyed

The temperature range of ionic nitriding is wider than that of gas nitriding and is in the range of 400-600 ° C. Treatment at temperatures below 500 ° C is especially effective in hardening products made of tool alloy steels for cold working, high-speed and maraging steels, because their performance properties are significantly increased while maintaining the hardness of the core at the level of 55-60 HRC.

Parts and tools of almost all industries are subjected to hardening processing by the IPA method (Fig. 1).

Figure: 1.

As a result of IPA, the following characteristics of products can be improved: wear resistance, fatigue endurance, anti-seize properties, heat resistance and corrosion resistance.

In comparison with widely used methods of hardening chemical-thermal treatment of steel parts, such as carburizing, nitrocarburizing, cyanidation and gas nitriding in furnaces, the IPA method has the following main advantages:

· Higher surface hardness of nitrided parts;
· No deformation of parts after processing and high surface finish;
· Increasing the endurance limit and increasing the wear resistance of machined parts;
· Lower processing temperature, due to which no structural transformations occur in steel;
· Possibility of processing blind and through holes;
· Preservation of the hardness of the nitrided layer after heating to 600-650 C;
· The ability to obtain layers of a given composition;
· The ability to process products of unlimited sizes and shapes;
· Lack of environmental pollution;
· Improving the culture of production;
· Reduction of the cost of processing several times.

The advantages of IPA are manifested in a significant reduction in the main production costs.

So, for example, in comparison with gas nitriding in furnaces, IPA provides:

· Reduction of the processing time by 2-5 times, both by reducing the heating and cooling time of the charge, and by reducing the isothermal holding time;
· Reduction of the brittleness of the hardened layer;
· Reduction of the consumption of working gases by 20-100 times;
· Reduction of electricity consumption by 1.5-3 times;
· Exclusion of the depassivation operation;
· Reduction of deformation so as to exclude finishing grinding;
· Simplicity and reliability of screen protection against nitriding of non-strengthened surfaces;
· Improvement of sanitary and hygienic conditions of production;
· Full compliance of technology with all modern environmental protection requirements.

Compared to quenching iPA processing allows:

· Exclude deformation;
· Increase the service life of the nitrided surface by 2-5 times.

The use of IPA instead of carburizing, nitrocarburizing, gas or liquid nitriding, volumetric or HFC quenching saves the main equipment and production space, reduces machine and transport costs, and reduces the consumption of electricity and active gas media.

The principle of operation of the IPA is that in a discharged (p \u003d 200-1000 Pa) nitrogen-containing gas medium between the cathode - parts - and the anode - the walls of the vacuum chamber - an anomalous glow discharge is excited, forming an active medium (ions, atoms, excited molecules), providing the formation of a nitrided layer, consisting of an outer - nitride zone and a diffusion zone located under it.

Technological factors affecting the efficiency of ion nitriding are process temperature, saturation time, pressure, composition and flow rate of the working gas mixture.

Process temperature, the area of \u200b\u200bthe cage involved in heat exchange and the efficiency of heat exchange with the wall (the number of screens) determine the power required to maintain the discharge and ensure the required temperature of the products. The choice of temperature depends on the degree of alloying of the nitrided steel with nitride-forming elements: the higher the degree of alloying, the higher the temperature.

The processing temperature should be at least 10-20 0 С lower than the tempering temperature.

Duration and temperature of the process saturations determine the depth of the layer, the distribution of hardness in depth and the thickness of the nitride zone.

The composition of the saturating medium depends on the degree of alloying of the processed steel and the requirements for the hardness and depth of the nitrided layer.

Process pressure should be such that a tight "fit" of the surface of the products by the discharge and a uniform nitrided layer is obtained. However, it should be borne in mind that the discharge at all stages of the process must be anomalous, i.e., the surface of all parts in the charge must be completely covered with glow, and the discharge current density must be greater than the normal density for a given pressure, taking into account the heating effect gas in the cathode region of the discharge.

With the advent of new generation IPA installations, using compositionally controlled mixtures of hydrogen, nitrogen and argon as a working medium, as well as a pulsating rather than direct current plasma, the manufacturability of the ion nitriding process has significantly increased.

The use of combined heating ("hot" chamber walls) or enhanced thermal protection (triple heat shield), along with the ability to independently regulate the gas composition and pressure in the chamber, make it possible to avoid overheating of the thin cutting edges during the heating of the charge when processing the cutting tool, to precisely regulate the saturation time a , respectively, and the depth of the layer, since heating of products can be performed in a nitrogen-free environment, for example, in a mixture of Ar + H 2.

Effective thermal insulation in the working chamber (triple heat shield) allows the processing of products with low specific energy consumption, thus minimizing temperature differences within the cage during processing. This is evidenced by the distribution of microhardness over the depth of the nitrided layer for the samples located in different places of the cage (Fig. 2).

Figure: 2.

a, b - gear wheel weighing 10.1 kg, 51 pcs., st - 40X, module 4.5, exposure 16 hours, T \u003d 530 0 С;

b, d - gear wheel weighing 45 kg, 11 pcs., st - 38XN3MFA, module 3.25 (outer rim) and 7 mm (inner rim), exposure 16 hours, Т \u003d 555 0 С.

Ionic nitriding is an effective method of hardening processing of parts made of alloyed structural steels: gears, gear rims, gear shafts, shafts, spur, bevel and cylindrical gears, couplings, gear shafts of complex geometric configuration, etc.

Cementation, nitrocarburizing and HFC-hardening justify themselves in the manufacture of heavily loaded parts (gears, axles, shafts, etc.) of low and medium precision that do not require subsequent grinding.

These types of heat treatment are not economically feasible in the manufacture of medium- and low-loaded high-precision parts, because during this treatment, significant warpage is observed and subsequent grinding is required. Accordingly, when grinding, it is necessary to remove a significant thickness of the hardened layer.

IPA can significantly reduce warpage and deformation of parts while maintaining surface roughness in the range of Ra \u003d 0.63 ... 1.2 microns, which allows in the overwhelming majority of cases to use IPA as a finishing treatment.

With regard to machine tool construction, ion nitriding of gear wheels significantly reduces the noise characteristics of machine tools, thereby increasing their competitiveness in the market.

IPA is most effective when processing large-scale similar parts: gears, shafts, axles, toothed shafts, gear shafts, etc. Gears subjected to plasma nitriding have better dimensional stability compared to cemented gears and can be used without additional processing. In this case, the bearing capacity of the lateral surface and the strength of the base of the tooth, achieved by plasma nitriding, correspond to cemented gears (Table 1).

Table 1. Fatigue resistance characteristics of steels depending on the methods of hardening of gear wheels

When strengthening treatment by ion nitriding of parts made of case-hardened, low- and medium-alloyed steels (18KhGT, 20KhNZA, 20KhGNM, 25KhGT, 40Kh, 40KhN, 40KhFA, etc.), it is necessary to improve the forgings at the beginning - volume quenching and tempering to a hardness of 241-285 HB (for some steels - 269-302 HB), then machining and finally ion nitriding. To ensure minimal deformation of the products before nitriding for stress relief, it is recommended to carry out annealing in a protective gas atmosphere, and the annealing temperature should be higher than the nitriding temperature. Annealing should be done before precision machining.

The depth of the nitrided layer formed on these products made of steels 40X, 18XGT, 25XGT, 20X2H4A, etc. is 0.3-0.5 mm at a hardness of 500-800 HV, depending on the steel grade (Fig. 3).

For gears operating under heavier loads, the nitrided layer should be 0.6-0.8 mm with a thin nitride zone or none at all.

Figure: 3.

The optimization of the properties of the hardened layer is determined by the combination of the characteristics of the base material (core hardness) and the parameters of the nitrided layer. The nature of the load determines the depth of the diffusion layer, the type and thickness of the nitride layer:

· Wear - "- or -layer;
· Dynamic load - limited thickness of the nitride layer or no nitride layer at all;
· Corrosion - - layer.

Independent control of the flow rate of each of the components of the gas mixture, pressure in the working chamber and variation of the process temperature allow the formation of layers of different depths and hardness (Fig. 4), thereby ensuring a stable processing quality with a minimum spread of properties from part to part and from charge to charge ( fig. 5).

Figure: 4.

1, 3, 5 - one-step process;
2,4 - two-stage process for the content of N 2 in the working mixture
1,2 - T \u003d 530 0 C, t \u003d 16 hours; 3 - T \u003d 560 0 C, t \u003d 16 hours;
4 - T \u003d 555 0 C, t \u003d 15 hours, 5 - T \u003d 460 0 С, t \u003d 16 hours

Figure: five.

Ionic nitriding is widely known as one of the effective methods of increasing the wear resistance of cutting tools made of high speed steels brands P6M5, P18, P6M5K5, R12F4K5, etc.

Nitriding increases the wear resistance of the tool and its heat resistance. The nitrided surface of the tool, which has a reduced coefficient of friction and improved antifriction properties, provides easier chip evacuation and also prevents chip sticking and formation of wear holes, which makes it possible to increase feed and cutting speed.

The optimal structure of nitrided high-speed steel is high-nitrogen martensite, which does not contain excess nitrides. The specified structure is provided by saturation of the tool surface with nitrogen at a temperature of 480-520 0 С during short-term nitriding (up to 1 hour). In this case, a hardened layer with a depth of 20-40 microns is formed with a surface microhardness of 1000-1200 HV0.5 with a core hardness of 800-900 HV (Fig. 6), and the tool life after ion nitriding increases 2-8 times, depending on its type and type of processed material.

Figure: 6.

The main advantage of ion nitriding of the tool is the possibility of obtaining only a diffusion hardened layer, or a layer with monophase nitride Fe 4 N ("-phase) on the surface, in contrast to the classical gas nitriding in ammonia, where the nitride layer consists of two phases -" +, which is a source of internal stresses at the interface and causes brittleness and flaking of the hardened layer during operation.

Ionic nitriding is also one of the main methods of increasing durability punching tools and casting equipmentfrom steels 5ХНМ, 4Х5МФС, 3Х2В8, 4Х5В2ФС, 4Х4ВМФС, 38Х2МЮА, Х12, Х12М, Х12Ф1.

As a result of ionic nitriding, the following product characteristics can be improved:

· Forging dies for hot stamping and molds for casting metals and alloys - increased wear resistance, reduced metal adhesion.
· Die Casting Molds for Aluminum - The nitrided layer prevents metal from sticking in the liquid jet zone and the mold filling process is less turbulent, which increases the life of the molds, and the casting is of higher quality.

Significantly improves ion nitriding and tool performance for cold (T< 250 0 С) обработки - вытяжка, гибка, штамповка, прессование, резка, чеканка и прошивка.

The main requirements for the high performance of such a tool - high compressive strength, wear resistance and resistance to cold shock loading - are achieved as a result of hardening treatment by ion nitriding.

If high-chromium steel (12% chromium) is used for the tool, then the nitrided layer should only be a diffusion layer, if low-alloy steels, then in addition to the diffusion layer there should be an r-layer - hard and plastic.

A feature of the ion nitriding of high-chromium steels is that by choosing the process temperature, it is possible to maintain the hardness of the core of the product, specified by preliminary heat treatment, within a wide range (Table 2).

To obtain a wear-resistant surface layer while maintaining a viscous die core, it is necessary to first quench with tempering for secondary hardness, dimensional processing, and then ion nitriding.

To eliminate or minimize deformations arising from ion nitriding of a stamping tool, it is recommended to perform annealing in an inert gas atmosphere at a temperature of at least 20 C below the tempering temperature before final machining.

If necessary, apply polishing of nitrided working surfaces.

Table 2. Characteristics of alloy steels after ion-plasma nitriding.

steel grade	Core hardness, HRC	Process temperature	Layer characteristics	Recommended connection layer type
	Depth mm	Pov. tv-st, HV 1	Connecting layer thickness, μm
Hot work steels




Cold work steels

By varying the composition of the saturating medium, the process temperature and its duration, layers of different depths and hardness can be formed (Fig. 7.8).

punch weighing 237 kg
mold weighing 1060 kg.

Figure: 7. Examples of processing die equipment (a, b) and distribution of microhardness over the depth of the nitrided layer (c, d).

Thus, as world experience shows, the use of ion nitriding technology for hardening processing of structural steel products, as well as cutting and stamping tools, this technology is effective and relatively easy to implement, especially with the use of pulsed current plasma.

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