Chemical-thermal treatment (XTO) is called the process of change chemical composition, microstructure and properties of surface layers of steel parts. The change in the chemical composition of the surface layers is achieved as a result of their interaction with the environment, solid, liquid or gaseous, in which heating is carried out. As a result of a change in the chemical composition of the surface layer, its phase composition and microstructure also change.
The main parameters of the CTO are the heating temperature and the holding time. The main processes of any type of CTO are: dissociation-absorption-diffusion.
Dissociation- obtaining a saturating element in a more active, atomic state: 2NH 3 \u003d 2N + 3H 2; CH 4 \u003d C + 2H 2, etc.
Absorption- capture by the surface of the part of the atoms of the saturating element.
Diffusion- movement of an atom captured by the surface into the depth of the product.
Thermodynamics and kinetics of CTO. Theoretical and experimental data obtained using precision methods for studying the phase and chemical composition of diffusion layers indicate that in many cases the formation of a diffusion layer proceeds under conditions that differ significantly from equilibrium (at high supersaturations). In this regard, it is possible to single out a set of physicochemical and kinetic factors that determine the mechanism for the formation of a diffusion layer with a nonequilibrium composition and structure.
Physico-chemical factors: thermodynamic functions of phases in a system of interacting elements; equilibrium composition of the saturating medium; the degree of non-equilibrium state of the environment; adsorption characteristics of elements and compounds; structural and energy conditions for the formation of a phase on the surface of the alloy; the degree of plastic deformation in the diffusion zone.
Kinetic factors: the ratio between the rate of entry of an element to the surface and the rate of its diffusion in a solid material; the reaction rate of the interaction at the interface between the alloy and the saturating medium; heating rate to the process isotherm and cooling rate after saturation; the duration of the diffusion process on the isotherm; the ratio between the rate of diffusion of a foreign element in the surface layer and the rate of self-diffusion of the component of the saturable alloy.
It is not possible to take into account the influence of all these factors on the mechanisms of formation of diffusion layers using theoretical research methods, therefore, in the analysis of saturation processes, methods of experiment planning are widely used. For example, they were used to study the influence of the theoretical parameters of the saturation process on the rate of formation of a diffusion layer, its phase and chemical composition, to optimize many methods of applying diffusion coatings in terms of properties, to study the correlation between the structural characteristics of the modified layer and its properties, etc.
When analyzing the kinetic regularities of ChTO processes, the empirical dependence of the depth of the diffusion layer on the duration of the process is usually used: (9.1)
where k and n are constants determined experimentally. 206-208 p..
Cementation- This is a chemical-thermal treatment, in which the surface of steel parts is saturated with carbon. Products are heated in an environment that easily releases carbon. As a rule, steels with a low carbon content (0.1-0.2% C) are subjected to carburizing. Having chosen the mode, the layer is saturated with carbon to the desired depth.
Depth of grouting conditionally consider the distance from the surface of the part to half of the zone, where in the structure, along with pearlite, there is approximately the same amount of ferrite. The depth of the cemented layer is usually 1-2 mm, but can be more if desired.
Degree of cementation is the average carbon content in the surface layer (usually not more than 1.2% C).
After carburizing, the products are hardened with low tempering. This ensures that high hardness products are obtained in the surface layer while maintaining a soft and viscous core. On the surface after cementation, compressive stresses arise, which increase the endurance limit and durability of the parts. Carburizing is carried out in solid, liquid and gaseous carburizers.
The most common is gas carburizing, which has a number of advantages over other methods.
At gas carburizing parts are heated in hermetic furnaces and atmosphere of carbonaceous gases. For gas carburizing, natural gas is used (contains up to 92-96% methane) or artificial gases obtained by pyrolysis of liquid hydrocarbons - kerosene, benzene: CH 4 \u003d C + 2H 2; 2CO \u003d C + CO 2. Compared to carbon monoxide, methane is a more active carburetor. Fe almost does not dissolve carbon, therefore, during carburizing, the products are heated to temperatures above A s (930-950 °C). At such temperatures, the steel acquires the structure of austenite, which dissolves up to 2% C. The depth of the carburized layer depends not only on the temperature at which the process was carried out, but also on the holding time at this temperature.
Typically, the carburization rate is about 0.1 mm per hour of exposure. Since the depth of the carburized layer is rarely required to be more than 1.0-1.5 mm, the process is carried out in 8-12 hours. With long exposures or a significant increase in the carburizing temperature, austenite grains can grow strongly, which significantly worsens the properties of the carburized layer and will require additional hardening to correct .
Gas carburizing of low-carbon steels (containing less than 0.2% C) is carried out at temperatures of 920 - 950 °C, while the optimal concentration of carbon in the modified
layer thickness (0.5 - 2 mm) is 0.8 - 0.9% (but not more than 1.2%). Since by means of diffusion saturation it is only possible to fix the required concentration profile, then in order to obtain high hardness and wear resistance of the surface layer with a relatively viscous core, the carburization is subjected to hardening (850 - 900 ° C) and such tempering (180 - 200 ° C). As a result of such heat treatment, the cemented layer acquires the structure of high-carbon martensite or martensite with carbide inclusions and a small amount of residual austenite. The surface layer after the three-stage XTO has a hardness of HRC 58 - 62, and the matrix - HRC 25 - 35.
Cementation is applied to the contact working surfaces of gears, shafts, piston pins, valves, cam washers and other parts. The degree of hardening depends on the steel grade, the carbon content in the carburized layer, the depth of the carburized layer, strength, toughness and hardness of the matrix.
Liquid grouting is a process of diffusion saturation of the surface layer of a material with carbon from a liquid medium and is carried out in a molten salt c. the addition of silicon carbide (for example, 75 - 85% Na 2 CO 3, 10 - 15% NaCl, 5 - 10% SiC). Silicon carbide reacts with soda to form free carbon, which diffuses into the material. The saturation process is carried out at 815 - 850 ° C (depending on the composition of the steel) and is mainly used for hardening steels to shallow depths (up to 200 microns).
Carburizing with a solid carburetor . With this method of cementation, the products are placed in metal boxes, pouring them with a solid carburetor - a mixture of charcoal (75-80% by volume) with activators, which are BaCO 3 and Na 2 CO 3. The boxes are closed with lids, which are coated with refractory clay for greater tightness. Then they are placed in an oven, where they are heated to the desired temperature (900-950 ° C). After the end of the process, the boxes are removed from the furnace, cooled and the parts are removed from them.
hard carburizing process Compared to gas, it has a number of disadvantages: its implementation takes more time (many auxiliary operations); difficult to automate and control; more service personnel are required; the equipment is bulky, etc. All this makes gas carburizing a cheaper and more modern process and reduces the use of hard carburizing.
After cementation, a layer of hypereutectoid steel is formed on the surface of the product, consisting of perlite and secondary cementite. Gradually, as one moves away from the surface, the carbon content decreases and the next zone consists only of perlite. Then ferrite grains appear, their number increases with distance from the surface, and, finally, the structure becomes corresponding to the composition of the steel. Immediately after cementation, the product does not acquire the required properties. This is achieved by heat treatment. All parts, regardless of the method of carburizing, must be subjected to hardened with low tempering.
If the steel is hereditarily fine-grained or non-critical products, then hardening is carried out once from 820-850 ° C. This ensures the production of martensite in the cemented layer and partial recrystallization and grinding of the grain of the core. During gas carburizing, the products are cooled to these temperatures at the end of the process and then quenched.
For more critical products, a different heat treatment mode is used:
1) hardening (or normalization) from 880-900 °C to correct the structure of the core;
2) the second hardening from 760-780 °C to obtain finely acicular martensite in the surface layer.
Vacation low is always carried out at 160-180 o C, as a result, a tempered martensite structure is obtained in the surface layer, internal stresses are partially removed.
In the surface layer tempered martensite gradually passes to troostite, sorbite, and ferrite with a small amount of pearlite is preserved in the core of the product, as before carburizing.
After cementation and heat treatment, the hardness of the surface layers is 60-63 HRC.
A variety of parts are carburized: gears, piston pins, worms, axles and other parts, sometimes of considerable size (for example, large rings and ball bearing rollers).
Nitriding. Nitriding is called XTO, in which the surface layer of the part is saturated with nitrogen. This not only increases hardness and wear resistance, but also increases corrosion resistance.
For the first time, nitriding was carried out by Chizhevsky N.P. in 1913
According to the Fe-N state diagram, nitrogen forms several phases with iron:
1) -phase - nitrogenous ferrite containing at 20 ° C about 0.015% N 2, at 591 ° C - 0.42% N 2;
2) ’-phase - a solid solution based on iron nitride Fe 4 N (5.6-5.95% "N a);
3) -phase - a solid solution based on iron nitride;
4) -phase - a solid solution of nitrogen in -iron. There is a temperature above the eutectoid transformation (591 °C).
When nitriding, the product is loaded into hermetic furnaces, where ammonia NH 3 enters at a certain rate, which, when heated, dissociates according to the reaction 2NH 3 - 2N + 6H. Atomic nitrogen, which has a high activity, is absorbed by the surface and diffuses into the depth of the part. The structure of the nitrided layer (from the surface to the depth of the product) consists of phases: + ’ - ’ - + ’ - + ’ ex. The phases obtained in the nitrided layer of carbon steel do not provide a sufficiently high hardness and the resulting layer is brittle. Therefore, alloyed steels containing aluminum, molybdenum, chromium, titanium and other elements are used for nitriding. The nitrides of these elements are very dispersed and have high hardness and thermal stability. Typical nitrided steels are 38XM10A and 35XM10A.
Depending on the operating conditions of the parts, two types of nitriding are distinguished: to increase surface hardness and wear resistance (“hard” nitriding) and to improve corrosion resistance (anti-corrosion nitriding).
At " hardness" nitriding parts are nitrided at 500-520 ° C, the process continues from 21 to 90 hours (nitriding rate is about 0.01 mm per hour). The nitrogen content in the surface layer reaches 10-12%, the layer thickness is about 0.3-0.6 mm, the hardness reaches 1000-1200 HV. Nitriding is applied to motor and pump cylinders, gears, injection molds, dies, punches, etc.
At anticorrosivenitriding nitriding is subjected to products made of both alloyed and carbon steels. In this case, nitriding is carried out at 650-700 °C. The diffusion rate increases, the duration of the process is reduced to several hours. A layer of -phase (0.01-0.03 mm) is formed on the surface of the products, which has a high resistance to corrosion.
Nitriding is the final, final operation in the manufacture of a part. Details are subjected to nitriding after final mechanical and heat treatment - hardening with high tempering. After such heat treatment, a sorbitol structure is obtained in the details, which will remain in the core of the product after nitriding and provide increased strength and toughness.
Comparing carburizing and nitriding, the following can be noted:
1) the duration of cementation is less than the duration of nitriding;
2) the hardened layers are deeper and allow higher specific pressures during operation;
3) the hardness of the cemented layer is 1.5-2 times less than the nitrided one and is retained when heated only up to 180 - 250 ° C, while the nitrided layer retains its hardness up to 600 - 650 ° C
Cyaniding and nitrocarburizing.cyanidation is called CTO, in which the surface is saturated with both carbon and nitrogen. The cyanated layer has high hardness, wear resistance. Fatigue strength and corrosion resistance are also improved. The joint diffusion of carbon and nitrogen occurs faster than each of these elements separately, so the duration of cyanidation is usually 0.5-2 hours. Cyanidation is high-temperature at 800-950 ° C and low-temperature at 540-560 ° C. During high-temperature cyanidation, the surface is saturated with more carbon than nitrogen, i.e. this process approaches cementation. After such cyanidation, the products are subjected to hardening with low tempering. The surface layer after deep cyanidation contains 0.8 - 1.2% C and 0.2-0.3% N. Low-temperature cyanidation is applied to parts that have already undergone heat treatment, as in nitriding. With such cyanidation, the surface is saturated mainly with nitrogen, the layer depth is 0.015-0.03 mm.
By analogy with carburizing, cyanidation is divided into liquid and gas, gas cyanidation is called nitrocarburizing.
Liquid cyanidation, which provides high productivity, is most often used for processing steels. It is carried out in molten cyanide salts, which are suppliers of active carbon and nitrogen atoms, such as Na(CN) or Ca(CN) 2 .
The main disadvantage of liquid cyanidation is the toxicity of cyanide salts. This drawback is not present in gas cyanidation.
Nitrocarburizing - gas cyanidation is carried out in gas mixtures containing 70-80% cementing gas and 20-30% ammonia. The composition of the gas and the temperature determine the ratio of carbon and nitrogen in the cyanidated layer. The layer depth depends on the process temperature and the holding time.
Compared to gas carburizing, nitrocarburizing has a number of advantages: less deformation and warping of products, more wear resistance and corrosion resistance.
Nitrocarburizing is carried out as follows: a carburetor is loaded into the container, which includes cyanide and carbonic salts (for example, 30-40% K 4 Fe (CN) 6, 10% Na 2 CO 3 and charcoal), which, when heated, decompose with the release CO 2 and nitrogen. The saturation process can be limited by the kinetics of chemical reactions, delivery of CO 2 and nitrogen to the surface of the part or by diffusion of C and N into the matrix, so the productivity of this method is low.
Processing of parts is carried out in an environment of carburizing and nitriding gases (for example, ammonia 2 - 6% with propane or lighting gas).
The main characteristics of the physicochemical state of the surface layer hardened during cyanidation are hardness, thickness, and also the fixed values of carbon and nitrogen concentrations. These characteristics are especially affected by the process temperature (as the temperature rises, the carbon content in the surface layer increases, and as the temperature decreases, the nitrogen concentration increases). Since cyanidation is, in fact, a superposition of carburizing and nitriding, it can be said that at high temperature the process is closer to carburizing, and at low - to nitriding, so cyanidation is divided into high-temperature (800 - 950 ° C) and low-temperature (500 - 600 °C).
Low temperaturecyanidation subjected to cutting tools made of high-speed steel (milling cutters, taps, drills, countersinks), as well as carbon steels. The essence of the process of cyanidation of carbon steels is the saturation of steels with nitrogen and carbon, which is carried out in cyanide salts (40% K, CN + 60% NaCN) by passing dry air. As a result of such treatment, which is carried out at 570 ° C for 0.5 - 3 hours, a thin (10 - 15 microns) Fe 3 (CN) carbonitride layer is formed on the surface of the part, which is less brittle than pure carbides and nitrides (Fe 3 C and Fe 3 N) and at the same time has good wear resistance. Between this layer and the matrix, a sublayer of nitrogenous hard ferrite is formed (on alloyed steels, the hardness reaches 600–1000 HV) with a thickness of 200–500 μm.
High temperature cyanidation used for processing simple and alloyed medium and low carbon steels. Saturation is usually carried out in molten salts of the following compositions: 40% NaCN, 40% NaCl, 20% Na 2 CO 3 (melt temperature 820 - 850 ° C) or 6% NaCN, 80% BaCl 2, 14% NaCl (900 - 950 ° C). An increase in the content of cyanide salts contributes to an increase in the concentration of C and N in the surface layer.
The thickness of the modified zone depends on the melt composition, temperature and duration of the process. The average rate of high-temperature nitrocarburizing is 80 - 100 µm/h. For structural steels = 15 - 500 microns, and the hardness exceeds HRS e 58 (for high-speed steels - 10 - 60 microns and HRS e 60 - 72, respectively). The cyanated layer, compared to the cemented layer, has greater hardness and higher resistance to wear and corrosion. 208-214 c..
Removal of oxygen
Oxygen remaining after deaeration can be completely removed by adding a chemical oxygen scavenger such as hydrazine, diethylhydroxylamine or sodium sulfite to the feed water.
As a result of the reaction of treatment with hydrazine, water and nitrogen are obtained, which is a neutral gas and does not interact with the metal of the system. These reaction products do not increase the solids content of the boiler water as is the case with other oxygen scavengers such as sodium sulfite.
N 2 H 4 + O 2 → 2H 2 O + N 2
Entering hydrazine has advantages. With proper water treatment and the required concentration of hydrazine, after the operation of the boiler plant, a protective film of black iron oxide - magnetite Fe 3 O 4 is formed within a short time. At the same time, red iron oxide Fe 2 O 3 - hematite, which does not protect the metal surface, slowly turns into magnetite. This magnetite film passivates the metal surface.
Assuming there is no excess hydrazine, oxygen will not be removed from the system. In this case, the magnetite film will turn into hematite and the corrosion of the metal will begin again.
Due to the fact that hydrazine is volatile, some of it is carried away with the steam. Therefore, the metal of the condensate system can also be protected. As a result of a series of reactions similar to the one above for metals containing iron, non-ferrous metals are also less susceptible to corrosion. For example, copper oxide CuO is converted into protective oxide Cu 2 O.
4CuO + N 2 H 4 → 2Cu 2 O + 2H 2 O + N 2
The last of the oxygen scavengers that have appeared in marine practice is diethylhydroxylamine, also known as DEHA. In addition to absorbing oxygen, DEHA forms a passivating magnetite film that prevents corrosion.
As a result of the oxygen scavenging reaction using DEHA, acetic acid, nitrogen and water are formed. In boiler water, the basic alkalinity is neutralized with acetic acid and removed by blowing in the form of sodium acetate.
4(C 2 H 5) 2NOH + 9O 2 → 8CH 3 COOH + 2N 2 + 6H 2 O
Another feature of DEHA is its volatility, similar to that of morpholine. It takes place in the feed water, boiler and condensate system where oxygen is taken up,
passivation of the metal surface and neutralization of the condensate by the residual content of DEHA.
An alternative oxygen scavenger is sodium sulfite (Na 2 SO 3). This compound reacts with oxygen dissolved in water to form a more stable compound - sodium sulfate (Na 2 SO 4). This process effectively removes dissolved oxygen while adding soluble solids to the water. Therefore, sodium sulfite is generally not recommended for high pressure boilers where the requirement is to minimize the content of soluble solids.
Sodium sulfite is non-volatile and is not a metal passivator. It remains in the boiler water and does not contribute to the protection of the condensate system.
Na 2 SO 3 + 1/2O 2 = Na 2 SO 4
(Sodium Sulfite) + (Oxygen) = (Sodium Sulfate)
Condensate pH control
As explained earlier, CO 2 in gaseous form reacts with the condensate to form carbonic acid. In the absence of chemical water treatment, this carbonic acid reduces the pH level of the condensate. The pH level can be maintained within predetermined limits, safe under corrosion conditions, by continuous dosed input of a neutralizing amine, such as morpholine or cyclohexamine.
ACID CORROSION
Acid corrosion of tubes and drums of boilers usually manifests itself in the form of a general thinning of the entire metal surface.
Acid corrosion, except when caused by the presence of CO 2 , occurs when water containing salts entrained in the evaporator enters the feed water, or sea water leaks in the condenser. When magnesium chloride (MgCl 2) contained in sea water enters the boiler water system, it dissociates with the formation of magnesium ions (Mg + 2) and chlorine (Cl -). Chlorine ions (Cl -) interact with hydrogen ions, which leads to a decrease in the pH level of boiler water and acid corrosion of the metal surface.
Magnesium ions (Mg +2) interact with phosphates (PO 4 -3) and hydroxyl ions (OH -), if present in water, to form sludge. Magnesium ions only react with phosphate ions to form magnesium phosphate, a soft, sticky residue that tends to hold all other deposits on the pipe surface.
All deposits on the surface of pipes impair heat transfer and contribute to the formation of conditions for their destruction. Water held on a stressed heat transfer surface beneath these deposits will increase in acid or alkali concentration. In this case, corrosion rates become very high and severe local damage occurs within a very short time.
HYDROGEN embrittlement
This type of corrosion consists in the embrittlement or cracking of the pipe metal, violation of the metal structure.
Hydrogen ions are formed as a result of an increase in the concentration of acids under a layer of solid sediment. Hydrogen ions (H +) are the smallest of all elements and can penetrate through the grain boundaries of pipe metal. They react with the carbon atoms present in the steel to form methane.
Methane (CH 4) has large gas molecules that cause pressure to build up inside the metal. The high pressure, along with bond loosening caused by washing out of the graphite, encourages the steel grains to separate. Thus, cracks form in the metal.
Hydrogen embrittlement develops very rapidly. Pipe metal collapses when the damaged part can no longer withstand internal pressure.
ALKALINE CORROSION
Alkaline corrosion is characterized by a non-uniform nature of the destruction of the metal. It is often referred to as "alkaline cracking".
Alkaline corrosion is the result of an excess of free hydroxyl (OH) ions in the boiler water, as indicated by a very high pH value.
Like acid corrosion, alkaline corrosion can occur under a layer of deposits that form on heat transfer surfaces, contributing to an increase in the concentration of hydroxyl ions and causing the development of localized corrosion.
Alkali corrosion occurs in horizontal or inclined pipes when, due to strong boiling or separation of steam and water, their inner surface is covered with steam. Boiler water containing hydroxyl ions can splash onto steam-covered heating surfaces, on which, after the water has evaporated, the concentration of hydroxyl ions increases.
CAUSTIC CRACKING
This form of corrosion is a type of intergranular cracking. When water containing a high concentration of caustic alkali comes into contact with steel under mechanical stress, intergranular corrosion occurs. (Metals can be subjected to residual stress relaxation). Corrosion of this type takes place at the boundaries of the crystals of a metal or alloy.
CORROSION CRACKING
Stress corrosion cracking appears as a series of thin cracks in the pipe walls. The presence of these cracks is exacerbated by conditions that promote the development of corrosion of other types, which, in the end, leads to the destruction of the pipe.
This type of corrosion is usually characteristic of the pipe walls of high-pressure boilers. It usually occurs in the high-temperature part of the pipes, where the circulation is unstable and the pipe material experiences alternating stresses.
MECHANICAL CORROSION PREVENTION
The operation of the evaporator must be monitored to prevent entrainment of salts, which will cause them to appear in the boiler water, as described above. The condition of the condenser pipes should be monitored to prevent leaks leading to seawater entering the system.
In order not to exceed the steam capacity of the boiler, which can lead to a violation of circulation and contact of the pipe walls with steam, the boiler must be operated at specification modes.
To prevent the flame from touching the pipes, the walls of which are in contact with steam, it is very important to monitor the shape and direction of the flame and the fuel spray.
CHEMICAL TREATMENT
Acid corrosion can be prevented by maintaining the correct alkalinity of the boiler water. The correct dosage of alkaline preparations such as caustic soda (liquid concentrated alkali) will keep the alkalinity within the recommended limits and prevent acid corrosion.
Please note: Alkalinity can be measured directly in "ppm" or indirectly in pH. Maintain the alkalinity or pH within the recommended limits according to your water treatment program.
Alkaline corrosion most often occurs in high-pressure boilers (60 kgf/cm2 or more) in the presence of free caustic alkali. Drew ULTRAMARINE coordinated phosphate - pH, used in high pressure boilers, does not allow free hydroxyl ions (OH-) in the boiler water. Maintaining a balance of water treatment products minimizes free caustic alkalinity concentrations.
The use of the Drew ULTRAMARINE boiler water treatment program coordinated phosphate - pH helps to reduce hydrogen embrittlement, primarily through the action of phosphates and maintaining the pH in the boiler water.
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Chemical processing in the production of semiconductor devices covers the main technological steps listed below.
Chemical treatment with phosphates and tannins does not affect silicate solutions.
Bath feeder LTSH-18. Chemical treatment is carried out in tanks filled with linseed oil. The electrodes are then placed in chlorination tanks. Chlorination of graphite in cold water continues throughout the day. This completes the process of chemical treatment of anodes.
Chemical treatments before flowering and immediately after flowering in the fight against pests of buds and leaves (currant bud mites, sawflies, moths, aphids), generative organs (gooseberry moth) and skeletal parts (glass and borer), as well as in the fight against mealy dew and anthracnose.
Chemical treatment consists in successive etching in solutions of hydrochloric and nitric acids of a certain concentration at certain temperatures with intermediate washings in water and must be carried out carefully and carefully to avoid significant damage to the rectifier.
Chemical treatment begins in a solution of silver nitrate, and the picture is very clear. The sulfide film formed during additional processing reagent Na, regardless of the chemical composition, should cause the same effect as the oxide film during thermal etching.
Chemical treatment is carried out by introducing a chemical reagent into an intermediate block with OA, which is equipped with hydraulic agitators or combined systems of hydraulic and mechanical agitators.
Chemical treatment (phosphating, etching with passivation of the coated surface) is necessary to obtain high-quality coatings, since it improves the adhesion of the polymer coating to the metal base.
Influence of heat treatment on the properties of fibers from poly-d-benzamide obtained in the presence. Chemical processing is widely used to modify aliphatic polyamides. As far as aromatic polyamides are concerned, this trend has not yet been sufficiently developed, although the available data indicate the fruitfulness of chemical treatment.
Chemical treatment is also accompanied by the removal of a certain thickness of the surface layers of the matrix material. Usually this treatment includes degreasing operations, alkaline or acid pickling, and sometimes a combination of both, surface passivation. After each of the above operations, flushing is mandatory. Reagents for chemical processing are selected individually for each matrix. Technological parameters of the chemical treatment process, including the concentration of etchants, temperature and processing time, are determined experimentally from the conditions for providing required quality surface layers, maintaining this quality for some time (including the interval between the operations of chemical treatment and diffusion welding) and removing the surface layers of the matrix of a given thickness. The latter condition is due to the fact that foils of small thickness (0 007 - 0 1 mm) are usually used as the matrix, and the removal of a layer of several microns from the surface can subsequently significantly change the ratio of the matrix and the hardener in the composite material.
Strength diagram of a welded joint of structural steel 45 steel 45 from the degreasing method. welding mode. 1000 C, t5 min., P20 - 4 mm Hg. Art., P2 kg / mm2. Chemical treatment and washing - surfaces allow to stabilize the strength characteristics of the connection. Moreover, the actions of different environments are different. When degreasing, for example, with carbon tetrachloride, the strength increases by 14%, compared with wiping with acetone.
Chemical treatment with gaseous chlorine or an aqueous solution of chromium peak, which are strong oxidizing agents, gives good results, but is not technologically advanced due to the difficulty in controlling the degree of processing, the use of harmful substances and the problem of their removal after use, and the multi-stage process.
Chemical processing and methods for obtaining and refining gold and platinum.
Schematic representation of anodes with a smooth (a and rough (b) surface. Chemical treatment is the etching of the anode foil in solutions that corrode aluminum. The most commonly used options for etching solutions are: a) 400 - 600 el b) 250 - 600 cm3 HCl and 0 15 - 125 g CuCl2 per 1000 cm3 H2O at 65 - 85 C; c) 200 - 300 cm3 HC1 and 150 - 200 cm3 HNOS per 1000 cm3 H2O at 90 - 95 C.
Schematic representation of anodes with a smooth (a) and rough (b) surface. Chemical treatment makes it possible to obtain an increase in the Cd of anodes up to 8–10 times compared to smooth anodes. making it difficult to wash off the chloride ions remaining in the pores after etching.
Chemical treatment can be used alone or in combination with ultrasonic treatment. A significant disadvantage of chemical cleaning of substrates is the need to control the purity of the cleaning solution to prevent contamination of the substrate with substances previously dissolved in the cleaning solution. A prerequisite for the final flush is the constant renewal of the washing medium.
Chemical treatments do not completely free plants from lilac mites. After the cessation of chemical treatments, the infestation of lilacs with mites is restored to the original level in 1-2 years. Therefore, other methods of dealing with it were also tested.
Chemical treatment (etching) of pipelines for systems consists of the following operations: a) etching in an acid solution; b) washing with running water; c) neutralization; d) washing hot water; e) drying; e) lubrication with engine oil; g) capping the ends of the pipes with wooden plugs.
Chemical treatments make it possible to save about 2 5 quintals of grain per hectare.
Chemical machining is less labor intensive and can be more productive than mechanical milling.
Chemical treatment is used most often to protect parts from corrosion (oxidation, phosphating); as a primer for paints and varnishes (phosphating); for coloring metals; in order to improve the run-in of rubbing surfaces.
Chemical treatment, depending on the type of device and the stage of the technological process, can be performed by various methods and is a mandatory operation preceding all thermal operations. After mechanical treatment, chemical treatment is carried out to remove the mechanically disturbed semiconductor layer and clean the surface.
Chemical treatment may not give the desired effect if the pesticide is washed off by rain from the plants, or after treatment, new batches of bugs arrive on these arrays, or the air temperature drops sharply.
Chemical treatment is the main means of bringing the properties of the drilling fluid in accordance with specific geological and technical conditions. It is considered appropriate to use more expensive reagents if the desired effect is achieved with small, infrequently introduced directed additives that are environmentally safe and combine well with other materials.
Chemical processing, unbalance compensation, as well as alteration (except for profiling calibration and Bakelite impregnation) of high-speed circles are not allowed.
Chemical treatments and microbiological preparations are used differentially according to the cotton-growing zones.
Chemical treatment aims to create polar groups (OH, CO, etc.) on the surface due to the oxidation of the surface, capable of chemical or adsorption interaction with adhesives. For polyolefins, such reagents are various chromium compounds, potassium permanganate, concentrated sulfuric acid, and other oxidizers. In particular, good results will be obtained by processing polyethylene for 1 - 10 minutes at a temperature of 70 - 100 C in a mixture of: 50 g K2Cr207, 880 g 98% H2SO4 and 70 g H2O, followed by washing in water.
Chemical treatment has shown that on corn crops there is no need to frequently carry out mechanical loosening of row spacings. The use of herbicides, especially simazine and atrazine, allows a full transition to corn cultivation without manual labor.
Chemical treatment is not widely used, since the application of this method is complicated by the large amount of liquor discharged by the pulp mills and, consequently, the large consumption of reagents, as well as the large volume of the resulting sludge.
Chemical treatment in a solution of nickel sulfate at pH Zch-6 pre-etched workpieces is the most important operation in technological process preparation of steel for enamelling. A nickel film is formed on the surface of the steel, which acts as an adhesion agent during the firing of soil or enamel that does not contain NiO. However, under production conditions favorable results are not always achieved and therefore this treatment is not used in all plants.
Chemical processing varies greatly depending on the material being processed.
Chemical treatment in a solution of nickel sulfate at pH 3-6b of pre-etched workpieces is the most important operation in the technological process of preparing steel for enameling. A nickel film is formed on the surface of the steel, which acts as an adhesion agent during the firing of soil or enamel that does not contain NiO. However, favorable results are not always achieved under production conditions and therefore this treatment is not always applied.
Chemical treatment with pesticides should be carried out only after a preliminary examination and determination by a plant protection specialist of the appropriateness of such treatment. Do not apply pesticides to areas that do not need it. The introduction of pesticides into the soil, the processing of plants should be carried out taking into account their background content in the soil so that the total amount of the drug does not exceed the maximum allowable amounts. Processing with pesticides is carried out in a timely manner with the obligatory observance of consumption rates and frequency of application of the drug.
The chemical treatment should ensure good mixing of the sample and indicator. It consists in dissolving or decomposing the sample and may involve converting the sample element and the indicator into the same chemical compound. The following two features should be noted.
Chemical treatment can be effectively used both in combination with ultrasonic and independently.
Chemical processing is usually used for products of complex shape, in particular, large products, when other methods of processing are not applicable. It involves immersing an article (LDPE or HDPE) in an etching solution such as chromic acid, permanganate, sulfuric acid, or chlorosulfonic acid. Infrared spectroscopy studies reveal significant surface chemical changes in the case of LDPE, but not HDPE or PP.
Dependence of the ratio of the filtration parameters of the crust of a 4% suspension of bentonite for water (G7K 10 8 105 s / m and for aqueous solutions of reagents and electrolytes on the concentration. / - GPAA. 2 - CMC-500. 3 - USHR. 4 - Na2SiO3. 5 - NaCI.6 - KCI.7 - CaCI2.8 - MgCI2.|Dependence of the filtration index of crusts / 7K on the concentration of polymeric reagents that treated the original crusts before filtering the 20% suspension of kaolin.7 - GPAA.2 - metas.3 - CMC-500 Chemical treatment makes it possible to modify the surface of the pore channels in the filter cake.The determining role in this case is played by the activity of the structure-forming phase.
Chemical treatment is based on the oxidation of mercury to its oxide or chloride. The method based on the interaction of mercury with a 20% aqueous solution of ferric chloride is considered one of the simplest and most reliable. The surface to be treated is abundantly moistened with a solution and rubbed several times with a brush to better emulsify mercury, and then left to dry completely.
Chemical treatment consists of degreasing and surface etching operations.
Chemical processing of materials in the production of semiconductor devices is usually carried out aqueous solutions acids and alkalis and organic solvents. As we approach the final assembly operations of the instrument, the purity of water and chemicals should be given more and more attention.
Chemical treatment of waste should be used in conjunction with other possible treatments, such as separation.
Chemical processing of veneer is carried out in baths (Fig. 46), equipped with coils for supplying steam and compressed air. The veneer is placed in the bath in such a way that the alkali solution freely penetrates to its surface.
Bath for chemical treatment of veneer. Chemical processing of veneer is carried out in baths (Fig. 103), equipped with coils for supplying steam and compressed air. The veneer is placed in the bath in such a way that the alkali solution freely penetrates to its surface.
The chemical treatment of glass aims to eliminate the defects present on the surface layer. This is done by etching the surface, most often with hydrofluoric acid to a depth of 50 to 150 microns, by treating it with organosilicon liquids, which, as it were, heal defects.
Chemical processing of alcohol is an auxiliary operation that helps to purify alcohol from impurities that are difficult to separate by the rectification method. Chemical cleaning is designed to eliminate acids, esters, aldehydes and unsaturated compounds from raw materials.
Chemical treatments give a quick return with the least investment of time and money, but it is advisable to use them when all other methods do not give a positive effect. This approach is due to the fact that the chemicals used for plant protection, especially under the condition of their unreasonable use, contaminate food products and environment. Caution in the use of chemical preparations should also be shown because the effect of many of them on warm-blooded organisms has not yet been sufficiently studied.
Secondary chemical treatment is to maintain the properties of the solution obtained during the primary treatment. Changes in the properties of the flushing fluid during drilling, determined by the nature of the influence of passable rocks on the fluid, the degree of mineralization of groundwater, and a number of other factors, may require multiple secondary processing. The interval through which it is necessary to carry out additional secondary processing is determined by the intensity of the change in the properties of the solution.
Chemical treatment of the reservoir involves the injection of solutions of chemically active reagents (hydrochloric acid, clay acid, etc.) into it and their chemical interaction with the mineral skeleton of the reservoir and the substance filling the pore space.
Chemical processing of lands or their chemical activation consists in their prolonged heating while mixing with dilute hydrochloric or sulfuric acid.
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Chemical treatment of metals consists in the formation of oxide and other compounds on the surface of metals under the action of special chemical solutions. The quality of the resulting films depends on the chemical composition of the solution, the temperature regime of processing, the exposure time of the parts in the solution, and the quality of surface preparation before processing.
The essence of the chemical treatment of metals is the formation of oxide and other compounds on the surface of metals under the influence of special chemical solutions.
The method of chemical processing of metals has been known since time immemorial, its origins date back to the ancient Egyptians, who used etching for decorative purposes long before our era. Since the 18th century, chemical treatment has been widely used in engraving art, and then in the printing industry.
In the chemical processing of metals (etching, oxidation, anodizing, etc.), the product should be immersed in the solution and removed from it only with the help of special devices or the right tool.
Other methods of chemical treatment of metal - fluorination, nitriding, etc. A heat-resistant electrical insulating film on the surface of Al, Ni, Cu, Mg, Cr can be obtained by the action of gaseous fluorine on a metal at an elevated temperature. In this case, a film of aluminum fluoride A1F3 is obtained on aluminum, a film of copper fluoride CuF2 is obtained on copper, and the corresponding fluorides are obtained on other metals. Thus, an A1F3 film 1 μm thick can be formed in a fluorine flow at 500 C. Aluminum fluoride can also be obtained by treating aluminum with hydrogen fluoride.
Galvanic coatings and chemical treatment of metals are widely used in the repair of equipment, technical equipment and tools.
A number of coatings obtained by chemical processing of metal include protective coatings that form directly on the surface of the metal. The formation of protective oxide films on the surface of metal products in technology is called oxidation. Some processes have special names. So, for example, the processes of applying oxide films to steel are sometimes called burnishing, and the electrochemical oxidation of aluminum is called anodizing.
A number of coatings obtained by chemical processing of metal include protective coatings formed directly on the surface of the metal. The formation of protective oxide films on the surface of metal products is technically known as oxidation. Some processes have special names. So, for example, the processes of applying oxide films to steel are sometimes called burnishing, and the electrochemical oxidation of aluminum is called anodizing.
The category of coatings obtained by chemical processing of metal includes protective coatings formed as a result of chemical interaction directly on the surface of the metal.
However, acid etching remains the main type of chemical treatment of metals, widely used in metallurgical, machine-building and other enterprises.
Thus, in solutions for electrochemical and chemical processing of metals, a stationary potential is established on the electrode in the absence of current.
In table. 4 shows the main dimensions of typical baths for the electrolytic and chemical processing of metals, developed by NIIKHIMMASH.
Direction of scientific research: development of adhesives for elastomers and applied research in this area; products for the chemical processing of metals.
A matte surface or a peculiar texture, a pattern that remains after oxidation, can be obtained by mechanical or chemical processing of the metal, and in some cases by their sequential application. A decorative pattern is obtained by knurling, embossing, turning using a diamond tool. A peculiar crystalline texture is revealed when metal is etched after its mechanical and thermal treatment. Aluminum parts are subjected to re-crystallization annealing for 30 min at 500 - 550 C followed by cooling in air. After that, anodic etching of parts is carried out in an electrolyte containing 150 g/l NaCl and NMO3 each, at a current density of 20–30 A/dm2 for 5–10 min. In this case, the texture of the metal, formed as a result of the previous processing, is revealed.
Chemically resistant ceramics (Table 42) are used for the manufacture or lining of various containers in which the chemical treatment of metals is carried out. Chemically resistant ceramics are characterized by low porosity, complete impermeability to liquids, sufficiently high mechanical strength, and satisfactory thermal stability. Chemically resistant ceramics are subdivided into lining, packed, and apparatus ceramics.
Chemical-thermal treatment of metals, simultaneous thermal and chemical effects on the metal in order to change the composition, structure and properties of the surface layer of the material being processed. Products from various grades of steel, cast iron, pure metals, alloys based on nickel, molybdenum, tungsten, cobalt, niobium, copper, and aluminum are subjected to chemical-thermal treatment. As a result of chemical-thermal treatment, it is possible to obtain hardening of the surface layers of these materials (increase in hardness, fatigue strength, wear resistance), change in physicochemical and other properties (corrosion, fractional). This type of treatment differs from surface hardening in that the surface of the workpieces is preliminarily saturated with various elements: carbon - carburizing, nitrogen - nitriding, carbon and nitrogen - nitrocarburizing, cyanidation, boron - borating, aluminum - aluminizing, silicon - siliconizing, chromium - chromium plating . Penetrating into the main metal lattice, the atoms of the corresponding element form a solid solution of interstitial or substitution, or a chemical compound.
The chemical-thermal treatment process proceeds in three successive stages. At the first stage, active atoms are formed in a saturating medium near the surface or directly on the surface of the metal. This process is due to dissociation, which consists in the disintegration of molecules and the formation of active atoms of the diffusing element (for example, the dissociation of carbon monoxide, accompanied by the release of elemental carbon, or the dissociation of ammonia with the release of nitrogen).
Next, adsorption (sorption) of the formed active atoms of the diffusing element with the surface of the steel product occurs and chemical bonds are formed with the metal atoms. Adsorption is a complex process that proceeds on the saturation surface in an unsteady manner. There are physical (reversible) adsorption and chemical adsorption (chemisorption). During chemical-thermal treatment, these types of adsorption are superimposed on each other. Physical adsorption leads to the adhesion of the adsorbed atoms of the saturating element (adsorbate) to the formed surface (adsorbent) due to the action of van der Waals forces of attraction, and it is characterized by an easy reversibility of the adsorption - desorption process. During chemisorption, an interaction occurs between the atoms of the adsorbate and the adsorbent, which is close to chemical in nature and strength.
The next stage of chemical-thermal treatment is diffusion, that is, the movement of adsorbed atoms in the lattice of the treated metal. The diffusion process is possible only when the diffusing element can be dissolved in the material being processed, and occurs at a sufficiently high temperature to provide the energy necessary for the process to proceed. The depth of diffusion, to which the element penetrates deep into the material, increases with increasing temperature (according to the exponential law) and with increasing duration of the process (according to the parabolic law). The diffusion layer, which is formed, for example, during the chemical-thermal treatment of parts, by changing the structural-energy state of the surface, has a positive effect not only on the physico-chemical properties of the surface, but also on their bulk properties. The power of the diffusion flow, i.e., the number of active atoms formed per unit time, depends on the composition and state of aggregation of the saturating medium, which can be solid, liquid or gaseous, the interaction of individual components with each other, temperature, pressure and chemical composition of the material being processed.
The thickness of the diffusion layer, and, consequently, the thickness of the hardened layer of the surface of the product, is the most important characteristic of chemical-thermal treatment. The higher the concentration of the diffusing element on the surface, the greater the layer thickness. The higher the temperature of the process, the greater the rate of diffusion of atoms, and consequently, the thickness of the diffusion layer increases. Grain boundaries are areas where diffusion processes are facilitated due to the presence of a large number of defects in the crystal structure. If the solubility of the diffusing element in the metal is low, then predominant diffusion along the grain boundaries is often observed.
The concentration of the diffusing element on the surface of the metal and alloy, as well as the structure and properties of the diffusion layer, depend on the method of chemical-thermal treatment. Chemical-thermal treatment gives products increased wear resistance, heat resistance, corrosion resistance, fatigue strength.
