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Features of metal cementation. Chemical-thermal treatment of steel. Carburizing steel at home Carburizing steel with graphite at home

The cementation process is based on the principle of chemical and thermal metal processing. The whole point of the procedure is to saturate the steel surface with the required amount of carbon under certain temperature conditions.

Several years ago, this procedure was almost impossible to implement at home. Today this is possible using graphite media or their analogues. The main thing is desire and some knowledge.

General information about the process

First of all, you need to understand basics of heat treatment become.

The features of metal carburization include the following factors:

1. Thanks to the procedure, case-hardened steels become stronger, which increases wear resistance and strength material;
2. The operating properties of the metal are changed by heating the products in a liquid, gas or solid medium, which improves its characteristics;
3. Parts can be heated to different temperatures; there is no limited constant or exact recommendations. At home, the cementation process takes place at a temperature of 500 degrees Celsius. In industrial conditions using professional equipment, the heating temperature in the furnace reaches more than 1300 degrees Celsius. You should know that the temperature is chosen taking into account the concentration of impurities and carbon.
4. Professionals recommend at home cement low carbon types of steel (approximately 0.2%). For example, a blade from an inexpensive kitchen knife made of steel or small parts.
5. Carbon penetrates into the structure of steel rather slowly. Therefore, cementation of the knife blade in a home procedure occurs at a rate of no more than 0.1 ml per hour. In order for the same blade to withstand heavier loads, it is necessary to strengthen the layer with a thickness of up to 0.8 ml per hour. It is also important to understand that carburizing a knife or small shaft in a home workshop will take minimum eight hours. In this case, you should maintain a certain temperature in the oven so as not to disturb the temperature regime.
6. During the process of carburization, not only the properties of the metal change, but also its phase composition and the atomic lattice. In general, the surface obtains the same characteristics as when hardened, but it is possible to control it over a narrow temperature range in order to avoid various material defects.

Carry out carburization of stainless steel a little more complicated, but at the same time this will qualitatively improve the characteristics of this type of metal.

In what environment is steel carburization possible?

The hardening process takes place under various environmental conditions:

• in solid;
• in gaseous;
• in an electrolyte solution;
• in the form of a special paste;
• in a fluidized bed.

Most often it is carried out in a home workshop carburization of steel with graphite. This greatly simplifies the process, since there is no additional need to worry about the strong seal of the oven.

In industrial production, gas is most often used, since this method reduces the time spent on hardening.

Types of metal that can be processed

There are three main groups of metal used for hardening:

1. Steel with non-hardening core. This group includes the following grades of steel suitable for cementing - 20, 15 and 10. These parts are small in size and are used for use in domestic conditions. During hardening, austenite is transformed into a ferrite-pearlite mixture.
2. Steel with a weakly hardened core. This group includes metals of such grades as 20Х, 15Х (low-alloy chromium steels). In this case, an additional ligation procedure is performed using small doses of vanadium. This ensures a fine grain, which results in a more ductile and ductile metal.
3. Steel with a highly hardened core. This type of metal is used for the manufacture of parts with complex configurations or large cross-sections that can withstand various shock loads and are exposed to alternating current. During the hardening process, nickel is introduced or, if it is deficient, manganese is used, while grain is crushed small doses titanium or vanadium.

In general, the process of carburizing steel is necessary to improve the wear resistance and strength of parts.

The most common parts that are carburized are shafts, axles, knife blades, bearing parts and gears.

How does steel carburization occur in a solid environment at an enterprise and in a home workshop?

The mixture for solid cementation is prepared from barium, calcium with charcoal and sodium carbonate. It is better to take coal from oak or birch and divide it into small fractions, no more than ten millimeters. To remove excess dust, charcoal is recommended sift. The salts are also ground to a powder and passed through a sieve.

There are two methods for preparing the mixture:

1. Wood charcoal is poured with salt, which is previously dissolved in water. The resulting mixture is dried, its humidity should be no more than 7%.
2. Dry coal and salt are thoroughly mixed to eliminate the possibility of stains appearing during chemical and thermal treatment.

At the same time, the first method is considered to be of higher quality. Since it guarantees that the mixture will come out uniform, and the result will be without stains or streaks. The finished mixture is also called carburizer.

The process of solid cementation itself takes place in special boxes, where the mixture is poured in the required quantity. Ideally, the boxes match the size and shape of the product being processed. Since in this case the time spent on heating the container is reduced, and the quality of the cementation layer improves. To avoid gas leakage, the cracks are covered with special refractory clay and everything is covered with a tight-fitting lid.

It should be noted that it is economically profitable to produce containers that fit perfectly when it comes to a conveyor procedure. If you need to harden one or two parts, then it is better to choose a container universal shape- square, round or rectangular.

Boxes are selected from low-carbon or heat-resistant steel.

The process of cementation in a solid mixture proceeds as follows:

• parts that need to be hardened are evenly placed in boxes filled with solid carburizer;
• the oven is heated to 900-1000 degrees and containers with products are served into it;
• The boxes are heated at temperatures from 500 to 700 degrees. This heating is called through heating. A signal that the oven has heated up to the desired temperature is the uniform color of the hearth plate; there are no longer dark areas under the drawers;
• the temperature is raised to 900 or 1000 degrees Celsius.

It is at this temperature that the diffuse changes in the structure of parts at the atomic level.

At home, it is quite difficult to heat the oven to the desired temperature and maintain the entire temperature regime from start to finish. Wherein everything is possible. It should be remembered that the effectiveness of home cementation is much lower than industrial cementation.

Case hardened steels using gas

For the first time, steel cementation with gas was carried out at the Zlatousovsky plant under the vigilant leadership of P. Anosov. This effective method was developed by V. Prosvirin, S. Ilyinsky and N. Minkevich.

The essence of the process is quite simple - the metal is cemented under the influence of carbon-containing gas (natural, artificial or generator) in a hermetically sealed closed oven.

The most accessible and frequently used gas is a composition obtained from the decomposition of petroleum products.

It is made in the following way:

• Kerosene is poured into a special steel container and heated until the process of pyrolysis - the decomposition of kerosene into a mixture of several gases;
• approximately 60% of this gas is modified and made suitable for cementation.

A mixture of modified gas and pure pyrolysis gas is used for cementation. The need to modify part of the gas is due to the fact that from using pure pyrolysis gas the steel is insufficiently carburized, and some parts may have a little soot deposited on them, which is difficult to remove.

The process of carburizing steel with gas is carried out in special continuous conveyor furnaces. Or they use unique stationary units.

First, the part is placed in the furnace, its muffle. The installation is closed and the furnace is heated to 950 degrees. Then pre-prepared gas is supplied.

It is almost impossible to carry out this procedure at home.

At the same time, it has several advantages over the solid processing method:

• less time is spent on preparing raw materials for cementation;
• more favorable and safe working conditions for workers;
• acceleration of the hardening process by reducing the holding time of products.

The most important thing when carburizing steel is a well-organized process and high-quality equipment and raw materials. The solid method can be implemented at home if you have a stove, carburizer and metal molds. As well as certain skills and abilities associated with this steel hardening process.

Chemical-thermal treatment of steel. Carburizing steel at home

Chemical-thermal treatment of steel | Metal processing – steel and non-ferrous metals

This treatment of the metal changes not only its structure, but also the chemical composition of its surface. Thanks to this, the part can have a binder core that can withstand impact loads, and high hardness and resistance to external influences. There are several methods of chemical-thermal treatment, but in a small workshop (especially at home) only cementation can be performed.

Cementation is the saturation of the surface layer of steel with carbon without air access, in an environment (carburizer) that has a significant carbon content. Parts made of low-carbon steels are usually cemented, which are ground after hardening the surface layer. Carburizers for carburizing the surface layer of steel can have different compositions, but the simplest is this, %:

Sodium carbonate or barium carbonate (for critical parts)………10Calcium carbonate……………………………..3Charcoal…………………………………..87

The carburizer can be prepared from sodium carbonate (soda 6-10%) and sawn horn or peat coke (90-94%). Small or single parts are cemented in a paste, which consists of the following components,%:

Carbon black……………………………………….28 Soda ash………………………3.5 Yellow blood salt………………………..1.5 Spindle oil… …………………………….67or Dutch soot……………………………30Soda ash……………………….10Spindle oil………………………… ……40 Dextrin (glue) ……………………………..20

A good paste for cementation can be prepared from the artistic paint “Gas Soot” (sold in tubes), to which soda ash is added piece by piece. Places on parts that do not need cementation are protected with anti-cementation coatings. The simplest coating is fireproof clay with the addition of (10%) asbestos waste. Mix the mixture with water. This coating is easy to use, %: talc-50, kaolin - 25, water - 25. This coating is diluted to the desired density with liquid glass or office silicate glue. Place the parts in cementation boxes after the coating has dried. Cement it like this. In a metal box with a lid, a layer of 30-40 mm of carburizer is poured onto the bottom and prepared (coated) parts are placed on it so that the distance between them, as well as between them and the walls of the box, is about 10-15 mm. Sprinkle the parts with carburizer on top with a layer of 30-40 mm, close the lid, coat its edges with refractory clay and dry. If paste is used for cementation, then spread the part with a layer thicker than 3-4 mm, put it in a box, close it and also coat the edges of the lid with refractory clay . After the clay has dried, the box is placed in an oven and kept (temperatures 930-950 °C) for 1.5-3 years (with a solid carburizer 7-8 years). In this case, the cementation layer reaches 1 mm. Sometimes holes are made in the cementation box and 1-2 pieces of soft steel wire with a diameter of 3-4 mm (probe) are inserted into them. The cracks are well coated with fireproof clay. After some time, the probe is removed, the hole is covered with clay, and the probe is recut and the depth of cementation is determined, i.e. feasibility of further warming up. After carburization, the parts are cooled together with the box, then heated to a temperature of 760-780 °C. and hardened. Simplified cementation. Small parts made of low-carbon steels are cemented using the following methods: yellow blood salt (ferric potassium cyanide). For carburization, the part is heated, sprinkled with salt and heated again until the salt melts (850 °C), then removed and hardened. At the same time, the cementation layer is relatively small - about 0.15 mm. If it is necessary to increase it, the part is sprinkled with salt and heated at the specified temperature for 1 hour, then immediately hardened with cast iron filings. This is the old blacksmith method of “strengthening in the forge.” The part heated to white color, without removing it from the furnace, is cleaned with a wire brush and sprinkled with cast iron filings, from which carbon transfers to the surface of the part. Cleansing and powdering are repeated several times. It is advisable to sprinkle the heated product with charcoal at the same time as the sawdust. After this, the product is hardened with a reducing burner flame. If the gas burner is not given enough oxygen, the carbon present in acetylene does not burn completely and can become part of the metal at the welding site, i.e. local shallow carbonization occurs. When simultaneously using steel wire as an additive, which is used in machines for welding crank journals and other shafts, the thickness of the carbon layer can be increased to 1-2 mm. See also heat treatment of steel.

ometals.ru

Steel carburization technology. Hardening at home video.

An important way to improve the performance properties of steel is chemical thermal treatment (CHT), which involves exposing the workpiece to a reagent at an elevated temperature. In a cold state, steel is characterized by low chemical activity, even the formation of rust occurs quite slowly. To increase the reaction rate, the steel is heated to a high temperature. According to van't Hoff's rule, an increase in temperature by 10° doubles the rate of a chemical reaction. High temperatures allow steel to be processed in a time acceptable for industrial production.

The choice of processing method depends on the chemical composition of the steel

During processing, the workpiece is in an environment of a chemically active substance. It forms a layer on the surface of the workpiece with special properties that differ from the main part. In most cases, chemical treatment can improve the mechanical properties of the material, its strength, hardness and wear resistance.

The most popular chemical treatment processes are carburization, which involves saturation with carbon, and nitriding, which involves treatment with nitrogen. Nitrocarburization, which combines nitriding and carburization, is also highly effective. Processing with other elements is rare. The choice of processing method depends on the chemical composition of the steel, the carbon content and alloying substances.

Chemical treatment is one of the final operations in the technological cycle of production of steel parts. It is performed after forming operations, pressure processing and cutting. As a result of CHT, the part acquires increased strength of the surface layer, and processing such a part becomes much more difficult. CHT is performed in such a way that warping of the surface is minimal and there is no need to level the surface by cutting off the top layer. After chemical treatment, only surface grinding is performed.

Cementation of steel products

Steels subjected to carburization can be divided into three groups according to their chemical composition:

Structural steels, unalloyed or low alloyed: 15, 18, 20, 20Х, 20ХФ, 20ХМ, 18ХГ, 20ХН. These steels are low cost but cannot be hardened by conventional methods due to their insufficient carbon content, so saturating the surface is a simple way to increase their strength.

Steels alloyed with titanium 18KhGT, 25KhGT, 30KhGT, 20KhNT, 20KhGNTR. They form a particularly strong compound – titanium carbide.

High-alloy structural steels: 12Х2Н4А, 20Х2Н4А, 18Х2Н4ВА, 18Х2Н4МА. The most durable and critical parts are made from them.

The development of carburization technology depends on the group to which the steel belongs. To obtain high surface hardness, from 58 HRC and above, the processed part is subjected to hardening followed by low tempering. Thus, the core of the part remains “soft” and viscous, with a hardness of 35-40 HRC with high surface hardness. Thanks to this, the processed part can be used under bending loads, which are dangerous for fragile products.

Methods for performing cementation

Gas cementation is the most popular. It is performed in shaft-type furnaces, that is, installations where the parts being processed are located below floor level. This design is associated with several process features. Firstly, this makes it easier to ensure the oven is airtight. The second reason is that the gases affecting the metal have a higher density than air. Gas carburizing is used in factories that mass produce hardened parts.

The temperature during gas carburization is 920-950 °C. The duration of treatment depends on the required depth of the cemented layer. The average speed of carbon diffusion is 0.15 – 0.18 mm/h. Typically, a hardened layer with a thickness of 0.8 - 1.5 mm is required, that is, the process duration can reach 10 hours. The carbon content in the surface layer after treatment reaches one percent. The carburization process and subsequent heat treatment can be carried out in three different modes, depending on the degree of responsibility of the parts. Non-critical parts are processed in 2 heats, parts with increased requirements – in 3, and with particularly high requirements – in 4, using double hardening.

In small-scale production, the cementation method using a solid active substance, which includes charcoal and coke, as well as process catalysts, is quite popular. For processing, the part is placed in a steel container. Several parts are placed so that they do not touch each other or the container.

The temperature of this method is slightly higher than with gas carburization. A disadvantage of the process is that it is difficult to control surface carbon saturation, which can lead to uneven hardness.

samara-metall.ru


In one of our previous materials, we presented a review of a video on making a knife from a saw. Such a knife has a blade thickness of 3 mm, and if you heat it to the required temperature and cool it sharply for hardening, then most likely the blade will twist into a propeller, or the knife will simply burst. Therefore, we offer a video review on hardening such a knife in graphite. According to the author of the idea, hardening in graphite has the advantage that heating is required only on the cutting edge of the blade.

We will need: - a direct current source; - a little salt; - water; - powdered graphite; - a small piece of rag; - a metal profile.


Graphite can be obtained from brushes used in construction tools, as well as from AA batteries. We also note that it is advisable to use a non-synthetic rag. As for the metal profile, it will be used to make the body, and if desired, it can be replaced with a corner.
We take the profile and connect it to the positive contact of the DC source.
Then take some salt and dissolve it in water.
After dissolving the salt in water, wet a rag with it. This is necessary in order to create a kind of fuse between the metals in order to avoid accidental contact with the knife on the metal workpiece, since in this case a direct arc will occur, as a result of which the metal of the knife will deteriorate.
Wring out the rag and place it in the metal profile.

Pour graphite onto a rag.
We connect the blade to the negative wire, after which we can begin hardening. Become the author of the site, publish your own articles, descriptions of homemade products and pay for the text. Read more here.

usamodelkina.ru

Chemical-thermal treatment of steel

Thanks to this treatment, not only the structure of the metal changes, but also the chemical composition of its upper layer and the part can have a viscous core that can withstand shock loads, high hardness and wear. Of the existing methods of chemical-thermal treatment of steel, only carburization can be performed in a small workshop. Carburization is the carburization of the steel surface. This process is most often applied to products made from low-carbon steels containing no more than 0.2% carbon and some alloy steels. Parts intended for cementation are first cleaned. Surfaces that are not subject to carburization are covered with special protective anti-cement coatings.

The first composition of the simplest coating: fireproof clay with the addition of 10% asbestos powder, water. The mixture is diluted to the consistency of thick sour cream and applied to the desired areas of the surface of the product. After the coating has dried, further cementation of the product can be carried out.

2nd composition of the coating used: kaolin - 25%, talc - 50%: water - 25%. This mixture is diluted with liquid glass or silicate glue.

Cementation is done after the coating has completely dried.

The substances that make up the coating are called carburizers. They are solid, liquid and gaseous.

In a small home workshop, it is more convenient to carry out cementation using paste. This is carburization in a solid carburizer. The paste contains: soot - 55%, soda ash - 30%, sodium oxalate - 15%, water to form a creamy mass. The paste is applied to the desired areas of the product and allowed to dry. Then the product is placed in an oven, kept at a temperature of 900-920°C for 2-2.5 hours. When using such a paste, cementation ensures a thickness of the carburized layer of 0.7-0.8 mm.

Liquid carburization is also possible in a small workshop if there is a bathtub furnace, in which tools and other products are carburized. The composition of the liquid includes: soda - 75-85%, 10-15% sodium chloride, 6-10% silicon carbide. The bath furnace is filled with this composition and the product or tool is immersed. The process takes place at a temperature of 850-860°C for 1.5-2 hours; the thickness of the carburized layer reaches 0.3-0.4 mm.

Gas cementation is carried out in a mixture of hot gases containing methane and carbon monoxide in special chambers at a temperature of 900-950°C and only under production conditions. After carburization, the parts are cooled together with the furnace, then hardened at 760-780°C with final cooling in oil.

Heating the workpiece is a responsible operation. The quality of the product and labor productivity depend on the correctness of its implementation. You need to know that during the heating process the metal changes its structure, properties and characteristics of the surface layer and as a result of the interaction of the metal with atmospheric air, scale is formed on the surface; the thickness of the scale layer depends on the temperature and duration of heating, the chemical composition of the metal. Steels oxidize most intensively when heated above 900°C; when heated to 1000°C, oxidation increases 2 times, and at 1200°C - 5 times. Chrome-nickel steels are called heat-resistant because they practically do not oxidize. Alloy steels form a dense, but not thick layer of scale, which protects the metal from further oxidation and does not crack during forging. When heated, carbon steels lose carbon from a surface layer of 2-4 mm.

This threatens the metal with a decrease in the strength and hardness of the steel and hardening deteriorates. Decarburization is especially harmful for small-sized forgings followed by hardening.

Carbon steel blanks with a cross-section of up to 100 mm can be quickly heated and therefore they are placed cold, without preheating, in a furnace where the temperature is 1300°C. To avoid cracks, high-alloy and high-carbon steels must be heated slowly.

When overheated, the metal acquires a coarse-grained structure and its ductility decreases. Therefore, it is necessary to refer to the iron-carbon diagram, which defines the temperatures for the start and end of forging. However, overheating of the workpiece can, if necessary, be corrected by heat treatment, but this requires additional time and energy. Heating the metal to an even higher temperature leads to burnout, which disrupts the bonds between grains and such metal is completely destroyed during forging. Overburning is an incorrigible marriage. If the workpiece is forged by heating to a temperature below Tn, this will lead to the formation of cracks.

At a temperature 20-30°C above the temperature Tk, crystallization occurs in the metal and the structure remains fine-grained. At this stage it is necessary to finish forging.

When forging products from low-carbon steels, less heating is required than when forging a similar product from high-carbon or alloy steel.

So. When heating metal, it is necessary to monitor the heating temperature, heating time and temperature at the end of heating. As the heating time increases, the scale layer grows, and with intense, rapid heating, cracks may appear. It is known from experience that on charcoal a workpiece 10-20 mm in diameter is heated to forging temperature in 3-4 minutes, and workpieces with a diameter of 40-50 mm are heated for 15-25 minutes, monitoring the color of the heat.

www.kefa.ru

Hardening and carburizing of metals - Craftsman

Hardening aims to give steel a special hardness, characteristic of steel heated above 700 °C and quickly cooled. During hardening operations, proper heating of the metal (no burnout) and uniform rapid cooling are of great importance. When heating metal, excessive oxidation of the surface should be avoided. It is best to coat heated steel with a special composition that contains carbon. This carbon passes into steel (carburization) and gives it special hardness. 1. Baths for hardening according to Shen. Used by an experienced hand, a water bath is the cheapest means of hardening metals. You just need to make sure that the water bath is at the same temperature for a long time, preferably 27°. With warmer water the metal becomes brittle, with hot water it becomes insufficiently hard. It is most advantageous to establish the correct temperature for each type of product by trial experiment and then maintain it during operation. 2. A special method of hardening steel. As you know, steel can be given such hardness by special hardening that it will cut glass like a diamond. But not everyone knows that there is a simple way to impart steel of such hardness. An awl, knife blade or other tool should be heated to a bright red glow and immediately immersed in ordinary sealing wax for just one second. The operation of immersion in sealing wax must be repeated several times, each time choosing a fresh place in the sealing wax for immersion until the steel has cooled and will no longer enter the sealing wax. Then the hardening process is considered complete. All that remains is to clean off the stuck particles of sealing wax. When using a steel tip or blade hardened in this way, it is advisable to moisten it with turpentine each time. 3. Compositions for carburizing hardenable steel. a) A good composition for carburizing hardened steel may be the following: take 1 crushed glass, 200 table salt, 8 animal coal, 2 charcoal, 2.5 rye flour, 25 rosin and 1200 yellow blood salt, grind all the components into powder and knead in alcohol until a thick dough is obtained. This composition is used to coat steel objects before hardening. It is especially suitable for tools such as files, etc. b) Instead of the above recipe, you can use the following. Take 700 rosin, 300 potassium ferric syneride (yellow blood salt), 100 copper sulfate and 100 linseed oil. These components, starting with rosin, are boiled in a pot with constant stirring (according to Bruckert) until a residue of 1000 remains (in this way 200 evaporate). The mass is poured into boxes where it hardens. To harden an instrument, it is heated to a cherry-red heat and stuck into a mass, which immediately becomes soft under the influence of the heated instrument. Good steel is heated again and then immersed in cold water, causing the steel to become very elastic. Steel of poorer quality needs to be immersed 2-3 times in a row, each time before heating it, into the hardening mass. 4. Hardening of files. The files are sprinkled with a mixture of 5 horn flour, 5 powdered charcoal, 2 powdered table salt and 1 ferrous potassium (yellow blood salt).

www.sdelaysam.info

sovmasteru.ru

Carburizer: production, features, application

In the process of smelting cast iron and steel, carbon is introduced into the melt or applied to the metal surface at the time of its pouring. For this purpose, special carbon-containing materials are used, which are called carburizers (or carburizers). The carburizer gives iron alloys additional strength and hardness, reduces their viscosity and ductility, and also prevents oxidation of castings during cooling.

Artificial crushed graphite of various brands is actively used as carburizers.

Carburizing agent production

The production technology is based on deep heat treatment of the source material, as a result of which it acquires a perfect (or close to perfect) crystal lattice with ordered atoms. The process takes place at high temperatures (2400-2600 °C) in industrial conditions without air access.

The raw materials for production are:

1. Electrode fight.
2. Graphite shavings remaining after processing of shaped products.
3. Petroleum coke, pitch coke.
4. Cinders of graphite electrodes.
5. Graphite scrap.
6. Anthracite.

Carburizers are characterized by a particularly low nitrogen content, because at a mass fraction above 0.009% it forms structural defects - gas pores or cracks.

Features of choice

The main indicators that determine the feasibility of using a material of a particular brand are the following:

• mass fraction of carbon: the purity of the chemical composition has a direct effect on the degree of saturation of the melt. Materials subjected to graphitization are characterized by the lowest content of impurities: when the structure of atomic layers is ordered, the ash content also decreases;
• the degree of assimilation of carbon by liquid metal: largely depends on the type of melting unit used, but the rate of dissolution of carburizing reagents, their chemical and mineralogical composition, and the content of volatile substances are also important. Crushed graphite dissolves in melts quickly and evenly even at relatively low temperatures;
• fraction size: dusty and small particles can be carried away by convective air currents; in addition, they are the most susceptible to oxidation. Large fractions dissolve slowly. To correctly calculate this parameter, one should proceed from the volume of steel (cast iron) being processed, as well as the intensity of mixing;
• ash content: ash consists of various chemical elements that have a negative effect on the absorption of carbon by the liquid metal. Mineral impurities are the first to react, thereby delaying the carburization process. The percentage of ash largely depends on the composition of the feedstock, the temperature of its processing and the size of the particles.

Materials of natural origin have a higher ash content (compared to artificial ones).

Application area

Carburizers are designed to produce steel and cast iron of a certain chemical composition with specified properties. The main advantages of using artificial crushed graphite for their production:

• reducing the cost of final products;
• optimization of the smelting process;
• deep, complete and uniform dissolution of carbon in the metal;
• ease of transportation and storage;
• oxidation resistance.

In addition to steel enrichment, carburizing agents are widely used for foaming slags, in the manufacture of carbon-graphite products and materials, and also serve as fillers for graphite plastics.

doncarb.com

Many methods have been accumulated for processing metal structures. One of them is steel case hardening; what this is, how to do it at home, can be understood by studying the basic technological techniques of the process.

General information

In the old days, axes made of raw iron were placed in a sealed clay pot filled with coal and placed in an oven for several days. The outer layers were saturated with carbon under the influence of heat. The tool received a hard surface while maintaining a soft, plastic core. The essence of the technology was preserved, the technique was developed, became widespread and was divided into several branches.

The carburization method is described as a method of processing metal using high temperatures in the environment of certain chemicals (chemical-thermal treatment); the medium can be liquid, gas or solid. Chemical components release free carbon when heated. The surface of the heated metal absorbs gas atoms, changing its structure (diffuse saturation occurs to a depth of 0.5 to 2 mm).

Cementation is applied to parts that are exposed to abrasion and experience vibration and shock during operation. The purpose of such heat treatment is to change (strengthen) some characteristics of the surface layer of a metal product:

1. The layer is strengthened, its properties such as hardness and wear resistance are improved; at the same time, the deeper layers retain their original properties (viscosity and elasticity). The surface resists abrasion well, the core is able to withstand dynamic loads.
2. After processing, the object acquires a hardness similar to that obtained using the classical hardening method (fire and mechanical stress).

The following features of the method are distinguished:

• When organizing the cementation process, it is important to maintain time and temperature intervals. The optimal atomic carbon density appears when the temperature is maintained between 850 and 950°C.
• Diffuse saturation occurs at a low speed; this is its peculiarity. The surface absorption of gas atoms flows at a speed of 0.1 mm/hour (the value may vary slightly depending on the environment and temperature). Considering that the expected layer thickness starts from 0.8 mm, it is easy to calculate that the part will acquire useful properties in at least 8 hours.
• The method is found effective for alloy (tool) and low-carbon steels, where the proportion of carbon in the composition is limited to 0.2-0.25%, and they are capable of absorbing an additional number of gas atoms. Machine-building, construction and reinforcing steel grades 20x, 40x are allowed. Carbon steels are not processed in this way.
• Cementation technology allows the use of several media. Hardening techniques have been developed in the presence of a solid and gas carburizer (a carbonaceous substance capable of dividing carbon). Surface carburization is possible in a fluidized bed, in electrolyte solutions and in pastes.

The most common in production cycles are gas and solid carburizers.

Gas cementation

In mechanical engineering, the technology of saturating the top layer of steel products with carbon in an atmosphere of carbon-containing gases is widespread. It is known that such production is convenient for mass processing of parts, since:

1. Adjustment of gas density is allowed; thereby forming a carbon layer with the desired properties.
2. The complete heat treatment cycle (cementation, hardening, washing and tempering) takes place in one place - in a shaft (cementation) furnace.
3. The process is economical, mechanized and automated.
4. Boxes with a carburizer do not require heating, which reduces the time of carburization.
5. The rate of carburization of parts increases by 2–3 times (compared with other methods), and the uniformity of the layer is higher.
6. The temperature of the gas mixture of hydrocarbons (methane and carbon monoxide) is adjusted to 900-950°C.
7. After carburization, the technological chain is completed by tempering (hardening).

In a solid carb (solid)

Charcoal is used as a carbon donor medium; as an option - peat coke, coal semi-coke. The mixture is supplemented with activators (sodium carbonate, barium or calcium).

For high-quality saturation, the coal is crushed to particles 3-10 mm in size, and then sifted to get rid of dust. Activators are also crushed and sifted, trying to give the salts the appearance of a fine powder.

The steel carburization process takes place in several stages:

• Items, cleared of emulsion and oil, are loaded into a steel box with a carburizer, which should completely cover them. They must not be allowed to touch the walls of the box and each other.
• The container is placed in the oven. Its tightness is ensured by a ground-in lid, clay or sand seal.
• Starting with preheating, the temperature is increased to technological 900-950°C.
• An accelerated version is possible (at 980°C), which reduces the saturation time by 2 times, but causes the formation of a carbide network (arising due to too high carbon saturation). To eliminate it and correct the structure, additional multi-stage processing (normalization) is carried out.

Cementation of steel at home is carried out in a solid medium or using graphite. Both options are available and do not require special knowledge. Arrangement of the room for the first method is simplified as much as possible, since the oven does not need to be highly airtight.

This condition is quite achievable in a home workshop. Despite the obvious advantages, the method also has disadvantages: labor intensity and low productivity.

Before firing, a solid mixture (carburizer) is prepared. It consists of a mixture of charcoal with carbon dioxide salts of barium, sodium or calcium. The salts are first ground to a powder and then sifted to ensure homogeneity. The share of charcoal in the mixture is 70-90%, the rest is salt.

The mixture is created in one of two ways:

1. Salt and charcoal are thoroughly mixed. If the mixture is not homogeneous enough, different parts of the surface of the part will absorb different amounts of gas during carburization. As a result, spots form on the product, signaling an insufficient carbon concentration; the surface quality will be uneven.
2. Coal is soaked in salt dissolved in water. Then it is dried to obtain a mixture whose moisture content does not exceed 7%. This composition is more homogeneous and better suited for use at home.

Stages of cementation in a solid medium:

• The product is cleaned of contaminants and placed in a metal box, filled with solid carburizer (powder mixture). It is necessary to ensure that a gap of 2-2.5 cm is maintained between the walls and it. The size of the box must correspond to the shape of the object; this will reduce the warm-up time and improve the quality of the cemented layer.
• The box is covered with a fitted lid, its edges are coated with clay for tightness (against gas leakage).
• The container is placed in a preheated oven. Cementation starts at a temperature of 850-920°C.
• Carbon atoms are absorbed by the hot top layer.

Hard carburization of the steel allows repeated use of the carburizer. To re-anneal the used carburizer, it is enough to add up to 30% of the fresh mixture.

For most critical machine parts (shafts, piston pins, gears, shares, spindles), a cemented layer depth of 0.6 to 2 mm is considered sufficient. The holding time to acquire sufficient hardness can vary from 6 to 20 hours.

Products after carburization in a solid carburizer are strong, but fragile. To get rid of an undesirable property, parts are subjected to heat treatment (hardening) with heating to 840-850°C, followed by tempering (heating to 780-800°C), which relieves internal stresses.

Graphite

Cementation of steel at home can be carried out in a slightly different way, without heating in a furnace.

You can check the hardness of a blade (knife, chisel) using a file, which perfectly sharpens a tool that is not hardened to the required degree. You can fix the problem and increase the strength of the edge yourself, using simple equipment and spending a little time on it.

For home use, we can use the method of carburizing metal using graphite powder as a substance with good electrical conductivity. When hardening in graphite, heating occurs only along the cutting edge.

To organize a workplace you will need:

1. Graphite powder, crushed into dust (gives smaller sparks).
2. Power supply (step-down transformer); For comfortable work with a graphite electrode, 6-12 V is enough.
3. The wires are of sufficient size.
4. Metal substrate (pallet, corner or piece of profile).
5. An object on which it is first desirable to remove nicks (with fine sandpaper).

• Graphite powder is poured onto a metal tray (it can be obtained by grinding off a graphite brush from an electric motor or from a battery)
• The positive contact is connected to the substrate, and the negative wire is connected to the object requiring hardening.
• Voltage is supplied to the transformer.
• The object (blade) must be moved over the graphite layer with smooth movements; in this case, the circuit closes and small sparks jump between the blade and the powder.
• The blade heats up during the process; it should not touch the substrate. In contact with the pan, a short circuit (arc) may burn through the edge.

Limitations of the method:

1. It is difficult to achieve uniform heating in the powder, and, therefore, acceptable quality for a product of noticeable size. Carburizing the part with graphite carbon is suitable for cementing the cutting edge of garden tools (shovels) and knives. This method is not recommended for critical parts.
2. Theoretically, high-quality cementation occurs at a speed of about 0.1 mm/hour. The speed can be increased by increasing the temperature, but this also causes the resulting brittleness.

In this way you can cement a shovel, scythe, drill, screwdriver, and lawn mower blades.

Cementation is also carried out in less common carburizers.

In paste

The method can be used at home. The paste must be applied to the object and allowed to dry. Afterwards, the item is placed in an oven and kept at 900-950°C for the estimated time. This method can achieve a thickness of the carburized layer of 0.7-0.8 mm.

The paste consists of:

• soot, 55%;
• soda ash (yellow salt), 30%;
• sodium oxalate, 15%.

The ingredients are mixed in water until it forms a paste.

In electrolyte solution

The method is based on the anode effect and is suitable for small objects.

1. The object is immersed in a furnace-bath with a solution preheated to operating temperature (from 450 to 1050°C, on average 850-860°C). The required voltage is 150-300 V.
2. In 1.5-2 hours, the metal surface is saturated with carbon to a depth of 0.3-0.4 mm.

The standard solution contains:

• soda 75-85%;
• sodium chloride 10-15%;
• silicon carbide 6-10%.

Video: carburization (hardening).

Fluidized bed cementation

An industrial method that takes place in a special installation (fluidized bed furnace). The basis of the method is to obtain a pseudo-liquid state of a granular substance (corundum) in a mixture of hot gases (exogas). The temperature is distributed evenly throughout the entire volume of the furnace, which reduces the deformation of objects and their warping.

The processing of the product is not completed by cementation; It is recommended to heat treat (temper) or grind it. To achieve the required level of strength during carburization and consolidate its hardness, it is necessary to correctly observe the conditions of the technological process.

In one of the previous materials, we presented an overview of the manufacturing video. Such a knife has a blade thickness of 3 mm, and if you heat it to the required temperature and cool it sharply for hardening, then most likely the blade will twist into a propeller, or the knife will simply burst. Therefore, we offer a video review on hardening such a knife in graphite. According to the author of the idea, hardening in graphite has the advantage that heating is required only on the cutting edge of the blade.

We will need:
- direct current source;
- a little salt;
- water;
- graphite in powder form;
- a small piece of rag;
- metallic profile.

Graphite can be obtained from brushes used in construction tools, as well as from AA batteries. We also note that it is advisable to use a non-synthetic rag. As for the metal profile, it will be used to make the body, and if desired, it can be replaced with a corner.

We take the profile and connect it to the positive contact of the DC source.

Then take some salt and dissolve it in water.

After dissolving the salt in water, wet a rag with it. This is necessary in order to create a kind of fuse between the metals in order to avoid accidental contact with the knife on the metal workpiece, since in this case a direct arc will occur, as a result of which the metal of the knife will deteriorate.

Wring out the rag and place it in the metal profile.

Pour graphite onto a rag.

We connect the blade to the negative wire, after which we can begin hardening.

One of the frequently used methods of chemical-thermal treatment of metal is steel carburization, which can be carried out in different environments at fairly high temperatures.

1 Steel carburization process - general information

Chemical-thermal treatment of steel is understood as the process of heating products in a liquid, gas or solid medium in order to change their chemical composition, which is achieved by saturating the surface layer of the objects being processed with carbon. This change significantly increases the wear resistance and hardness of the parts. Moreover, their core remains viscous.

The carburization process produces expected results when processing low-carbon steels that have a carbon content of less than 0.2 percent. The surface layer of the product is saturated when it is heated to a certain temperature (from 850 to 950 ° C) in a specially selected environment that can easily release active carbon.

Under these conditions, not only the chemical composition of the part is modified, but also its microstructure, as well as the phase composition. The surface of the product becomes hardened; in fact, it acquires characteristics similar to those obtained after. In this case, it is very important to correctly select the steel holding time and carburization temperature.

Cementing steel is a fairly lengthy process. As a rule, the rate of saturation of the surface and the acquisition of special properties by it is approximately 0.1 millimeter per 60 minutes of exposure. Most parts require a hardened layer of more than 0.8 mm, which means the process will take at least 8 hours. Now carburization is carried out in the following environments (they are called carburizers):

• in gas;
• in pasty form;
• in solid;
• in electrolyte solutions;
• in a fluidized bed.

Most often, carburization is used in gas and solid carburizers.

2 Cementation of steel in a solid environment - at home and at the enterprise

A solid carburizer is made from a mixture of sodium carbonate, barium or calcium with charcoal (birch or oak), which is crushed into small fractions (from 3 to 10 mm), and then sifted to remove dust. Salts also need to be ground to a powder and passed through a sieve.

The mixture itself is prepared using two methods:

• dry salt and coal are mixed as thoroughly as possible to eliminate the possibility of staining during the chemical-thermal treatment of steel;
• charcoal is poured with salt, which is previously dissolved in water, then the resulting composition is dried (the moisture content of the finished mixture should be no more than 7%).

The second method is considered to be many times better than the first, as it guarantees a uniform mixture to saturate surfaces with carbon. Charcoal in the finished carburizer is from 70 to 90%, the rest is calcium carbonate and barium carbonate.

Hard cementation is carried out in boxes with a carburizer. It is optimal if the boxes are made according to the shape of the products that are supposed to be processed, since in this case the quality of the cemented layer improves, and the time it takes to warm up the “container” is reduced. To prevent gas leakage, the boxes are covered with clay (fireproof) and covered with tight-fitting lids.

Let us note that “containers” of a special shape (for a specific type of product) are economically feasible to produce and use only when many parts are subjected to chemical-thermal treatment. More often, standard-shaped (square, round and rectangular) boxes with different geometric parameters are used, which allows you to select them depending on the number of products and the size of the oven.

The material for the boxes is low-carbon or (which is better) heat-resistant steel. And the processing process itself in a solid carburizer is carried out according to the following scheme:

• products that need to be saturated with carbon are placed alternately with the prepared mixture in boxes;
• the oven is heated to 900–950 °C, and the “container” is fed into it;
• the boxes are heated at a temperature of 700 to 800 degrees (such heating is called through heating), a sufficient level of heating is indicated by the uniform color of the hearth plate (without dark places under the box);
• raise the temperature in the oven to 900–950 °C.

At this temperature, active carbon (its atoms) diffuses into the crystalline structure of the metal. Theoretically, even cementation of a house is possible; there are many craftsmen who carry out this process on their own. But the efficiency of “home” carburization is low due to the duration of processing and the need to ensure a high process temperature.

3 Gas carburization is the best option for mass processing of products

The theoretical foundations of such cementation were developed by S. Ilyinsky, N. Minkevich and V. Prosvirin, and were first carried out at the Zlatoust plant under the leadership of P. Anosov. The process is carried out in an environment of carbon-containing gases (generator, artificial, natural) in completely sealed heating furnaces. The most popular artificial gas is a composition that is obtained from the decomposition of petroleum products. It is made as follows:

• Kerosene is fed into a heated steel container, pyrolysis takes place (decomposition of kerosene into a mixture of gases);
• a certain volume of pyrolysis gas (about 60%) is cracked (its composition is modified).

A combination of cracked gas and pure pyrolysis gas is used to carry out the chemical-thermal carburization process. The need to obtain cracked gas is due to the fact that when using only a pyrolysis composition, the depth of cementation of steel is insufficient, and, in addition, a lot of soot settles on the parts, which is not so easy to remove.

The gas carburization process is carried out in continuous conveyor furnaces (in methodical ones) or in stationary units. The products that want to be strengthened are placed in the furnace muffle, the installation is closed, heated to 950 degrees, and then the prepared gas is supplied. The advantages of this procedure compared to processing parts in a solid carburizer: