What is metal carburization

All about metal carburization using steel as an example

Cementation, carried out in various environments and exclusively under the influence of high temperatures, is a very common method of chemical-thermal treatment of metal, successfully used for decades.

Preparing parts for cementation

The essence of the cementation process

The meaning of any method of chemical-thermal treatment of metals, which includes steel carburization, is that the product is heated to a high temperature in a special medium (liquid, solid or gaseous). This effect leads to a change in the chemical composition of the metal - the surface of the workpiece is saturated with carbon, ultimately becoming harder and more wear-resistant. What is important is that the core of the processed parts remains viscous.

It is possible to achieve the desired effect after such an impact on the metal only if low-carbon steels containing no more than 0.2% carbon are subjected to processing. In order to carry out cementation, the product is heated to a temperature of 850–950 degrees Celsius, and the composition of the medium is selected so that it releases active carbon when heated.

If carburization is carried out skillfully, it is possible not only to change the chemical composition of a metal product, but also to transform its microstructure and even phase composition.

As a result, it is possible to significantly strengthen the surface layer of the part, giving it characteristics similar to those of hardened steel.

In order to achieve such results, it is necessary to correctly select the parameters of chemical-thermal treatment of the metal - heating temperature and exposure time of the processed product in a special environment.

Equipment for vacuum carburizing of steel

This technological operation is quite time-consuming, since the process of saturating the surface layer of steel with carbon is very slow (0.1 mm in 60 minutes).

Considering the fact that the hardened surface layer for most products must be at least 0.8 mm, it can be calculated that it will be necessary to spend at least 8 hours to perform metal carburization.

The main types of media for performing metal carburization (or, as they are correctly called, carburizers) are:

• gaseous media;
• electrolyte solutions;
• pasty media;
• fluidized bed;
• solid media.

The most common are gaseous and solid carburizers.

Dependence of the thickness of the cemented layer on time and processing temperature

Carrying out carburization of steel in a solid environment

Most often, to carry out carburization of metal in a solid medium, a mixture consisting of sodium carbonate, barium or calcium and birch or oak charcoal (70–90%) is used. Before this, all components of such a mixture are crushed to a fraction of 3–10 mm and sifted, which is necessary to remove too small particles and dust.

After the components of the mixture for chemical-thermal treatment of metal are prepared, they can be mixed in several ways.

• The components of the mixture (salt and coal) are thoroughly mixed in a dry state. If this requirement is neglected, then after the completion of the cementation process, stains may form on the surface of the product.
• The salt is dissolved in water and the resulting solution is poured over the charcoal, after which it is dried until the humidity reaches no more than 7%.

It should be noted that the second method is preferable, as it allows you to obtain a mixture with a more uniform composition.

Charcoal carburizer

Both in industrial and at home conditions, carburization of steel products is carried out in boxes in which a carburizer is poured. To improve the quality of the surface layer of the metal being processed, as well as to reduce the time it takes to warm up the boxes, it is best to manufacture them as close as possible to the dimensions and shapes of the parts.

Optimal conditions for steel carburization can be created by eliminating the leakage of gases formed in the carburizer during the heating process. To do this, the boxes, which must have tight-fitting lids, are carefully coated with refractory clay before being placed in the oven.

Naturally, it is advisable to use specially made boxes only in industrial conditions. For carburizing metal at home, boxes of standard sizes and shapes (square, rectangular, round) are used, selecting them depending on the number of parts to be processed and the internal dimensions of the furnace.

The optimal material for such boxes is heat-resistant steel, but containers made of low-carbon alloys can also be used. The technological process of carburizing metal products is as follows.

Visual representation of the change in structure after cementation

• The parts prepared for processing are placed in boxes, sprinkled with layers of carburizer.
• The filled boxes, coated with refractory clay, are placed in a preheated oven.
• The so-called through heating of the boxes with parts is performed, during which they are heated to a temperature of 700–800 degrees Celsius. The fact that the boxes have warmed up well can be judged by the color of the hearth plate: there should be no dark spots on it in places of contact with the container.
• The temperature in the oven is raised to 900–950 degrees Celsius. It is at these values ​​that steel is carburized.

The high temperature and special environment in which the metal is located contribute to the diffusion of active carbon atoms into the steel crystal lattice. It should be noted that carburizing steel is possible at home, but often does not achieve the desired effect. This is explained by the fact that the carburization process requires long exposure of the part at high temperature. As a rule, this is difficult to achieve at home.

Cementation of parts in a gas environment

The authors of this technology are S. Ilyinsky, N. Minkevich and V. Prosvirin, who, under the leadership of P. Anosov, first used it at the plant in Zlatoust. The essence of this technology is that the metal parts being processed are heated in an environment of carbon-containing gases, which can be of artificial or natural origin. The most commonly used gas is the gas produced during the decomposition of petroleum products. This gas is obtained in the following way:

• a steel container is heated and kerosene is fed into it, which upon evaporation decomposes into a mixture of gases;
• the composition of some part (60%) of the resulting gas is modified (cracking).

The resulting mixture is used to perform chemical-thermal treatment of steel.

If steel carburization is carried out using only pyrolysis gas, without adding cracked gas, then the depth of the carburized layer will be insufficient. In addition, in this case, a large layer of soot will settle on the surface of the workpiece, which can take a lot of time and effort to remove.

Furnaces used to perform gas carburization of metal must be hermetically sealed. At modern production enterprises, two main types of such furnaces are used: methodical and stationary. The process of cementation itself in a gas environment is as follows. The parts to be processed are placed in an oven, the temperature of which is adjusted to 950 degrees Celsius. Gas is supplied to the heated furnace and the parts are kept in it for a certain time.

Compared to steel carburization using a solid carburizer, this technology has a number of significant advantages:

• providing better conditions for service personnel;
• high speed of achieving the required effect due to the fact that parts can be kept in a gas environment for a shorter amount of time (in addition, time is not required to prepare a solid carburizer).

Cementing steel at home

In what other environments can steel carburization be carried out?

Certain grades of carbon, low-carbon and alloy steels, in particular 15, 20, 20KhGNR, 20Kh, 20Kh2N4A, 18Kh2N4VA, 20G, 12KhN3A, etc., can undergo carburization in other environments.

Electrolytic solution

In such an environment, only small parts can be carburized. This method is based on the anodic effect, due to which the metal surface is saturated with carbon contained in the electrolyte solution.

In order for the solution to contain a sufficient amount of active carbon, glycerin, acetone, sucrose and other substances are added to it. Before placing a steel part into the solution, it is heated to a temperature of 450–1050 degrees Celsius (depending on the metal being processed and the size of the part).

To heat the solution, an electric current with a voltage of 150–300 V is used.

fluidized bed

Cementation of steel using this technology is carried out in an environment of a hot gas flow formed by the passage of methane and endogas through a layer of heated finely ground (0.05–0.2 mm) corundum.

Paste compositions

To carburize the metal surface using this technology, special pastes are used, consisting of yellow salt, wood dust and soot. Before processing, the part is coated with such a paste and dried, and only then heated to a temperature of 910–1050 degrees Celsius using high-frequency currents.

Regardless of the technology used to carburize the steel, after its completion it is recommended to temper the metal.

Source: http://met-all.org/obrabotka/himicheskaya/vse-o-tsementatsii-metalla-na-primere-stali.html

Steel cementation at home: what it is and how to do it

The carburization process is based on the principle of chemical and thermal treatment of metal . 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 .

The first step is to understand the basics of heat treating steel.

The features of metal carburization include the following factors:

1. Thanks to the procedure, case-hardened steels become stronger, which increases the wear resistance and strength of the 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. cementing low-carbon at home . 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 at least 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 cementation, not only the properties of the metal change, but also its phase composition and 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.

Carburizing stainless steel is a little more difficult , but at the same time it 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.

steel is carburized with graphite in a home workshop . 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.

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 small doses of titanium or vanadium are added to crush the grain.

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, it is recommended to 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 of a 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 regime that 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. At the same time, 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 furnace .

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 occurs - 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 the use of pure pyrolysis gas on steel results in insufficient carburization, and a little soot may settle on some parts, 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.

Source: https://tokar.guru/metally/stal/cementirovanie-stali-i-cementaciya-metalla-v-domashnih-usloviyah.html

Steel Cementation

Metal cementation is a type of heat treatment of metals using additional chemical action. Atomic carbon is introduced into the surface layer, thereby saturating it. Saturation of steel with carbon leads to strengthening of the enriched layer.

Steel Cementation

Carburization process

The purpose of steel case hardening is to improve the performance characteristics of the part. They must be hard and wear-resistant on the outside, but the internal structure must remain sufficiently viscous.

To achieve these requirements, a high temperature is required, an environment that releases free carbon. The carburization process is applicable to steels with a carbon content of no more than two tenths of a percent.

To carburize the outer surface layer, the parts are heated using a furnace to a temperature in the range of 850C - 950C. At this temperature, the release of carbon is activated, which begins to penetrate into the intercrystalline space of the steel lattice.

Cementation of parts is a fairly lengthy process. The carbon incorporation rate is 0.1 mm per hour. It is not difficult to calculate that the 1 mm required for long-term operation can be obtained in 10 hours.

Effect of cementation duration on layer depth

The graph clearly shows how the time duration depends on the depth of the carbonized layer and the heating temperature.

Technologically, steels are carburized in various environments, which are commonly called carburizers. Among them are:

• solid medium;
• liquid medium;
• gas environment.

Surface layer obtained by cementation

Steels for carburization are usually alloyed or with low carbon content: 12ХН3А, 15, 18Х2Н4ВА, 20, 20Х and the like.

Cementation methods

Cementation has become widespread in the processing of gears and other parts operating under shock loads. The high hardness of the working surfaces ensures a long service life, and the sufficiently viscous middle allows you to compensate for shock loads.

Many carburizing methods have been developed. The most commonly used are the following:

• in a solid environment;
• in liquid;
• in gas;
• in a vacuum.

A mixture of charcoal (birch, oak) and carbonic acid salts with calcium and other alkali metals is used as a solid carburizer. The amount of charcoal can reach 90%. To prepare the mixture, the components are crushed to improve the carbon yield. The particle size should not exceed 10 mm. There should also be no microscopic particles in the form of dust and crumbs, so the mixture is sifted.

Cementation of steel in a solid environment

To obtain the finished mixture, two methods are used. The first is to thoroughly mix salt and coal in a dry state. The second method is to obtain a solution from salt. To do this, it is diluted in water, and then charcoal is generously moistened with this solution. The coal is dried before being placed in the oven. Its humidity should not exceed 7%. Obtaining a carburizer using the latter method is of higher quality.

The mixture is poured into boxes. Then the parts are placed in them. To prevent the outflow of gas produced during heating, the boxes are sealed. The tightly closing lid is additionally coated with fireclay clay.

Boxes are selected depending on the shape of the part, their number and the volume of the filled mixture. They are usually rectangular and round. The material for making boxes can be either heat-resistant or low-carbon steel.

The technological process of steel carburization can be presented in the following order:

• Parts intended for cementation are placed in metal boxes and evenly sprinkled with coal.
• The boxes are sealed and fed into a preheated oven.
• Initially, it is warmed up to temperature readings of the order of 700C - 800C.
• Warmth control is carried out visually. The drawers and the hearth plate have a uniform color without dark spots.
• Next, the temperature in the furnace is increased to the required 850C-950C. In this range, diffusion of the insertion of carbon atoms occurs.
• The length of time the parts are kept in the oven directly depends on the required layer thickness.

How does the cementation process occur in a gas environment?

Cementation of steel in a gas environment is carried out during mass production of parts. The depth of cementation does not exceed 2 mm. The gases used are natural or artificial gases containing carbon. Typically, gas obtained from the breakdown of petroleum products is used.

Cementation of steel in a gas environment

It is obtained in most cases by heating kerosene. More than half of the gas is modified and cracked.

Active carbon with this processing method is obtained by decomposition, and the formula has the following form:

2СО=СО2+С

CH4=2H2+C

If pyrolysis gas was used without a modified one, then the resulting enriched layer of metal will be insufficient. In addition, pyrolysis gas creates abundant soot.

Furnaces for this cementation method must be sealed. Usually they use stationary ovens, but as an option they use methodical ones.

Steel carburization and the technological process can be presented in the following order:

The products to be carburized are placed in an oven. The temperature rises to about 910C - 950C. Gas is supplied to the furnace. Exposure in a gas environment for a certain time.

The duration of thermal exposure is 15 hours at a temperature of 920C with a resulting layer of 1.2 mm. To speed up the production process, the temperature is raised. Already at 1000C it is possible to obtain the same carburized layer in 8 hours.

Recently, a method of carrying out the process in an endothermic environment has found widespread use. During active carburization, a significant carbon potential is maintained in the gas environment by introducing natural gas (propane, butane or methane). For this period, the concentration of gas from petroleum products is set at 1%.

The process of cementation in a liquid medium

The liquid medium is molten salts. Metal carbonates are used as salts, although the metals must be alkaline with a low melting point. The temperature of cementation with this method is 850C. The process occurs when parts are immersed in a melt bath and kept there.

Cementation of steel in a liquid medium

Cementation in a liquid medium is characterized by a small saturated layer, which does not exceed 0.5 mm. Accordingly, it takes up to 3 hours. Among the advantages, it should be noted: the processed parts have minor deformation, and hardening is also possible without an intermediate step.

How does the carburization process occur in a vacuum?

The insufficient pressure created in the furnace significantly reduces the processing time. Steel carburization and the technological process can be presented in the following order:

• With this method, the parts are placed in a cold oven.
• After sealing the heating chamber, a vacuum is created in it.
• Then it is heated to the required temperature.
• An exposure is carried out, which takes up to an hour. During this time, the temperature equalizes and contaminants that interfere with carburization fall off the surface of the heated parts.
• Hydrocarbon gas is then supplied into the chamber under pressure. Due to this, the active phase of enrichment of the surface layer occurs.
• At the next stage, the diffusion introduction of carbon occurs. At this stage, vacuum pressure is again created in the chamber.
• In a short period of time, the required carburized layer is not obtained, so the process is repeated until the required depth is achieved. Usually the result is obtained in three stages.
• Cooling to ambient temperature occurs in a furnace under the influence of inert gases at different pressures.

Vacuum Carburizing Furnace

The process is completely computerized. The gas supply, temperature, and pressure are monitored by a program responsible for the entire technological process. Among the advantages it should be noted:

• carbon regulation;
• the absence of oxygen prevents the formation of oxides;
• gas penetrates even into holes of minimal diameter;
• alternation of processes occurs under equal conditions;
• full automation; shortened terms.

The process of cementation with pastes

When performing one-time work, it is more rational to use pastes for cementation. The paste contains: soot with charcoal dust. The thickness of the applied paste layer must be increased eightfold to obtain the required saturated layer.

After application, the composition is dried. High frequency induction furnaces are used for the carburization process. The process temperature reaches 1050C.

How does the cementation process occur in an electrolytic solution?

The process is in many ways similar to electroplating. The workpiece is placed in the heated electrolyte solution. The supplied current causes the production of active carbon and promotes its penetration into the surface of the steel workpiece.

Small-sized parts are processed in this way. Parameters for cementation: voltage – 150-300V, temperature 450-1050C.

Properties of metal after processing

After carburization, the hardness of the carburized layer reaches: 58-61 HRC on alloy steels and 60-64 HRC on low-carbon steels. Prolonged exposure of steel to high temperatures causes a change in the structure of the metal.

Steel structure after carburization

To correct large grains of metal, parts after carburization are subjected to reheating and hardening, followed by tempering or normalization.

Hardening is carried out at a temperature not exceeding 900C. In the metal, grain refinement occurs due to the production of pearlite and ferrite.

Instead of hardening, alloy steels are subject to normalization. After through heating, martensite forms in the middle of the part. The heating of the part depends on the grade of steel from which it was made.

Heat treatment modes for steel after carburization

The final phase is low-temperature tempering, which eliminates surface stresses and deformations caused by high-temperature treatment.

Disadvantages of cementation

As mentioned above, the main disadvantage after carburization remains a change in the structure of the metal. In this regard, additional processing is required, which increases the time of the already long cementation process.

To carry out the work, trained and highly qualified personnel are required. Among the disadvantages, the need to prepare the carburizer should be highlighted.

In conclusion, it is worth noting that carburization allows the use of steels with low carbon content for the manufacture of critical parts with a long service life, which significantly reduces the final cost.

To protect surfaces not intended for cementation, pastes, copper plating, or increased processing tolerances are used.

Source: https://stankiexpert.ru/spravochnik/materialovedenie/cementaciya-stali.html

Features of metal carburization: process technology, choice of working environment

During the manufacture of metal structures, it is necessary to resort to various methods of chemical-thermal treatment. Among them, steel carburization is quite common. This method is notable in that it can be used in different environments at relatively high temperatures of the working environment.

Metal carburization process - general information

Chemical-thermal treatment or carburization is a procedure in which products are exposed to high temperatures when placed in a liquid, gas or solid medium, which is done to give them a changed chemical composition.

Moreover, this effect ensures the saturation of the surface layer of the processed objects with carbon. Thanks to this treatment, it is possible to provide products with high wear resistance characteristics and increased hardness.

It is noteworthy that the core of these parts retains its initial viscosity.

The effectiveness of the carburization method is observed provided that the work is carried out with low-carbon steels, in which the proportion of carbon does not exceed 0.2%. Heat treatment ensures the saturation of the surface layer of the parts, and for this they are placed in a specially selected environment that can easily release active carbon, where the temperature is maintained in the range from 850 to 950 degrees Celsius .

The creation of such processing conditions makes it possible to change, in addition to the chemical composition of the elements being processed, the microstructure along with the phase composition.

The positive effect of such treatment is to increase strength; as a result, the characteristics of such a part do not differ from products that have undergone a hardening operation.

To achieve the best results, special attention should be paid to proper calculation of the time during which the part should be kept in the created environment, as well as the selection of carburization temperature.

The peculiarity of steel carburization is that this procedure takes quite a lot of time . Most often, the process of saturating the surface and giving it special properties occurs at a speed of about 0.1 mm per one full hour of exposure.

Many elements require the creation of a hardened layer more than 0.8 mm thick, which suggests that at least 8 hours will have to be spent on this processing.

Currently, metal cementation technology involves the use of several media:

• gas;
• pasty;
• hard;
• electrolyte solutions;
• fluidized bed

Typically, when choosing a medium for metal processing, gas and solid carburizers are used.

Cementation of metal in a solid medium

The material for the solid carburizer is a mixture of sodium, barium or calcium carbonate with charcoal, which must be used in crushed form, presented in fractions of about 3-10 mm in size . Moreover, this base must be sifted to remove dust. A mandatory procedure for salts is grinding to give them a powdery state, after which this mass is sifted through a sieve.

To create a mixture, two main methods can be used:

• The main components are dry salt and coal, which must be thoroughly mixed with each other, thereby minimizing the risk of staining during chemical-thermal treatment of steel;
• Salt, previously mixed with water, must be poured onto the prepared charcoal until dissolved. Next, the mass created on the basis of these components must be placed for drying, and it is optimal when the moisture content of the mixture does not exceed 7%.

Of these methods, the second is the most preferable due to its higher quality. This is manifested in the fact that it can be used to create a more uniform mixture to saturate the surface with carbon. In the composition of the finished carburizer, charcoal accounts for about 70-90%, and the rest is occupied by calcium carbonate and barium carbonate .

To carry out solid carburization, boxes are used where the carburizer is placed. It is best to use a box that matches the shape of the items being processed. The fact is that this will help improve the quality of the cemented layer, while it will be possible to reduce to a minimum the time required to warm up the container. It is important to ensure that there are no gas leaks: this problem is solved by covering the boxes with clay and then covering them with airtight lids.

An important point is that resorting to the considered option of creating specially shaped containers for direct use makes sense in cases where it is necessary to process a large number of parts using the chemical-thermal method.

The most widely used boxes are those that have a standard shape , which differ in geometric dimensions.

This makes it possible to select the most optimal option from them, which takes into account to the greatest extent the number of processed products and the dimensions of the furnace.

Typically, boxes are made from low-carbon or heat-resistant steel. Moreover, when processing parts using a solid carburizer, adhere to the following scheme:

• Parts that need to be saturated with carbon should be placed alternately in a box filled with a pre-prepared mixture;
• Next, prepare the oven for work, for which it is heated to a temperature of 900-950 degrees, then the work container is placed there;
• The operation itself to warm up the box is performed at a temperature of 700 to 800 degrees. You can determine that the boxes have warmed up sufficiently by looking at the hearth plate, which should have a uniform color;
• At the final stage, the oven temperature is increased to 900-950 degrees Celsius.

The creation of the specified temperature regime provides conditions for the penetration of diffusion into the crystalline structure of the metal of active carbon.

Theoretically, this method can also be used for chemical-thermal treatment of buildings , and individual craftsmen are able to cope with this task on their own.

However, in terms of efficiency, such treatment carried out at home is characterized by rather low efficiency, the reason for which is the long processing time and the need to create a high temperature regime.

Gas cementation

The authors of theoretical materials that reveal the essence of such cementation are S. Ilyinsky, N. Minkevich and V. Prosvirin. At the same time, the first experience of practical implementation took place at the Zlatoust plant, where P. Anosov supervised all work.

sealed heating furnaces serve as the main working equipment . Among the known artificial gases, the composition that is the result of the decomposition of petroleum products is most often used. Its manufacturing technology involves several stages:

It is necessary to take a steel container, heat it and fill it with kerosene, then begin the pyrolysis process, which involves the decomposition of kerosene into a mixture of gases;

A certain part of the pyrolysis gas (approximately 60%) is subjected to cracking, the essence of which is to change the composition.

A mixture of cracked gas and pure pyrolysis gas acts as the basis by which chemical-thermal treatment is carried out, providing carbon enrichment. necessary to produce cracked gas for the reason that in the case of using one pyrolysis composition, the depth of steel cementation turns out to be small, and the processed parts are covered with a large amount of soot, which is difficult to remove.

Continuous conveyor furnaces or stationary units are used as equipment for gas carburization.

Parts that need to be given more durable characteristics are placed in the furnace muffle, and after closing, the temperature inside is brought to 950 degrees. Next, they begin to supply prepared gas there.

Among the advantages of this procedure , which differs from the processing of products using a solid carburizer, it is necessary to highlight the following:

• creating more comfortable conditions for staff;
• reduction in the time required to complete processing, which is achieved by reducing the aging period of parts and eliminating the need for lengthy preparation of a coal-based carburizer.

Cementation in less popular carburizers

In a situation where it is necessary to subject steels 20, 15, as well as alloy steels with low carbon content to chemical-thermal treatment, the following carburizers can be used.

Electrolyte solution

The essence of this method comes down to the use of the anodic effect, due to which it is possible to enrich small-sized products with carbon using multicomponent electrolytes. Processing these parts requires creating a temperature regime in the range of 450-1050 degrees and of 150-300 V. A mandatory operation is the introduction of sucrose, acetone, glycerin, as well as individual substances containing carbon into the electrolyte.

fluidized bed

In its structure, it has the appearance of an upward flow of methane and endogas, which “pierces” small particles of corundum distributed on the furnace gas distribution grid.

Pastes

The use of such a carburizer for processing products involves creating a layer of paste on the surface of a steel part that needs to be enriched with carbon, its subsequent drying and heating with high or industrial frequency current . It is worth noting that carburization alone should not complete the processing of the part. The operation recommended here is heat treatment of the steel in the form of tempering. A positive effect is also achieved by grinding metal.

Conclusion

Quite often, certain structures need to be given increased strength characteristics so that they can successfully perform the task that is assigned to them. This can be solved by using various processing methods, among which metal carburization is quite effective. At the same time, to obtain the desired result, it is necessary to take into account the important features of this process.

In addition to the proper selection of the working environment, for metal carburization it is important to strictly follow the technology for carrying out such processing .

After all, the slightest mistake can negatively affect the chemical composition of the product, which in the future can reduce the service life of the structure in which it will be used.

For this reason, it is important to pay attention to every point, avoiding deviations from current norms and rules regarding the chemical-thermal treatment of metal parts.

Source: https://stanok.guru/metalloobrabotka/himicheskaya-obrabotka/osobennosti-cementacii-metallov.html

What is steel carburization and how to do it at home?

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.

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).

Source: https://metmastanki.ru/tsementatsiya-stali-chto-eto-kak-sdelat

Cementation - Chemistry

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.

Cementation of steels

Introduction

Cementation is the process of saturating the surface layer of steel with carbon. There are the following types of carburization: solid, gas carburization, paste carburization, liquid carburization. The purpose of carburization is to obtain a hard, wear-resistant surface, which is achieved by enriching the surface layer with carbon to a concentration of 0.8–1.2% and subsequent hardening with low tempering. Cementation and subsequent heat treatment simultaneously increase wear resistance and endurance limit.

For carburization, steels with a carbon content of 0.1–0.3% of various alloys are usually used. The choice of such steels is necessary so that the core of the product, which is not saturated with carbon during carburization, retains high toughness after hardening the steels.

When carburizing in a solid carburizer, the products are placed in boxes and covered with carburizer, often charcoal.

When heated, the carbon in charcoal combines with oxygen in the air to form carbon monoxide, which, in turn, reacts with iron to produce atomic carbon. This active carbon is absorbed by austenite and diffuses deep into the product.

To speed up the cementation process, process catalysts are added to charcoal (coke): barium carbonate (BaCO3) and soda ash (Na2CO3) in an amount of 10–40% by weight of coal.

Cementation regimes

Gas carburization is the main process in mass production, while solid carburization is used in small-scale production. The depth of carburization, depending on the purpose of the product and the composition of the steel, is usually in the range of 0.5–2.00 mm. Cementation is carried out at 910–950 ºС or to speed up the process at 1000–1050 ºС. With increasing temperature, the time to reach a given cementation depth decreases.

Thus, with gas carburization, a carburized layer 1.0–1.3 mm thick is obtained at 920 ºС in 15 hours, and at 1000 ºС in 8 hours. To prevent strong growth of austenite grains, hereditarily fine-grained steels (HMZS) are subjected to high-temperature carburization. . Also, overheating after carburization can be corrected by subsequent complete recrystallization of the alloy during quenching from reheating.

The carbon concentration in the surface layer of the product is usually 0.8–1.5%.

Cementation is an intermediate operation, the purpose of which is to enrich the surface layer with carbon. The required hardening of the surface layer of the product is achieved by hardening after carburization.

Hardening should not only strengthen the surface layer, but also correct the overheating structure that occurs due to many hours of exposure of steel at a high carburization temperature.

After carburization, critical products are subjected to double hardening (hardening with reheating). This is done to reduce metal warping, and also so that heating for re-hardening corrects all discontinuities in the microstructure of the metal obtained during carburization - for example, large grains from overheating.

Re-hardening is carried out with heating to 850–900 °C (above point A3) so that complete recrystallization of the steel occurs. In carbon steel, due to the shallow depth of hardenability, the core of the product after hardening consists of ferrite and pearlite. Instead of first hardening, normalizing can be applied to carbon steel. In through-hardening alloy steel, the core of the product consists of low-carbon martensite. This structure provides increased strength and sufficient viscosity of the core. When gas carburizing, one hardening with carburizing heating is often used after cooling the product to 840–870 ° C, but more often the cooling temperature depends on the specific grade of steel. This scheme reduces the total time of the carburization process, but does not correct the structural defects obtained during high-temperature carburization. The final operation of heat treatment of cemented products in all cases is low tempering at 160–180 ºС and converting the hardening martensite in the surface layer into tempered martensite, which relieves stress.

Advantages of gas carburization over carburization in a solid carburizer

The gas carburization process has a number of advantages compared to carburization in a solid carburizer:

— the productivity of the process increases compared to carburization in a solid carburizer, since there is no need to spend time on packaging and warming up the boxes;

— the required production area and the amount of labor are reduced;

— the need for heat-resistant material is reduced;

— it becomes possible to regulate the process to obtain a cemented layer of a given depth and saturation;

— deformation of parts is reduced due to more uniform heating to operating temperature;

— sanitary and hygienic working conditions are improved;

— it becomes possible to harden parts immediately after carburization;

— allows you to completely mechanize and automate the process.

Disadvantages of gas carburization process:

— the need for more complex and expensive equipment;

— the need for more qualified labor;

— the complexity of operating the equipment due to the need to ensure the tightness of the furnace, uniform circulation of gases, etc.;

— complex safety requirements.

Liquid cementation

Liquid cementation is performed in molten salts, usually salts consisting of alkali metal carbonates. This mixture is melted in a bath and carburization is carried out by immersing the parts in the melt. The process is carried out at 850°C for 0.5 - 3.0 hours, while the milking depth is within 0.2 - 0.5 mm. The main advantage of the process is the possibility of direct hardening from a carburization bath and small deformations of the processed products.

In conditions of individual and small-scale production, cementation from pastes has found some application. In this case, a coating containing soot (33 - 70%), wood dust (20 - 60%), yellow blood salt (5 - 20%) and other components is applied to the treated surface. Organic, organomineral and inorganic adhesives are used as binding materials. The thickness of the coating should be 6 - 8 times greater than the required thickness of the cemented layer.

Currently, the most promising cementation method is carburization in an endothermic atmosphere with controlled carbon potential.

During gas cementation in an endothermic atmosphere, at the beginning of the process (during the active period of saturation), a high carbon potential of the atmosphere is maintained by adding unprocessed hydrocarbon gas (methane or propane-butane) to the endothermic atmosphere.

During the diffusion period, the carbon potential of the atmosphere is set to 0.8 - 1.0% and the amount of added hydrocarbon gas decreases sharply.

Vacuum carburization

All of the above methods of cementation have one big drawback - the long process time. Significant acceleration of the process can be achieved as a result of the use of high-temperature carburization at pressure below atmospheric pressure, with vacuum carburization. (Follow the link for more information on vacuum carburizing). The vacuum carburizing process has a number of advantages over traditional carburizing methods.

— the ability to effectively regulate the carbon distribution profile in the cemented layer and its microstructure;

— absence of oxygen-containing components in the atmosphere, which eliminates internal oxidation of parts;

— better penetration of carburizing gas into small-diameter holes, which ensures uniform cementation of internal cavities;

— high repeatability of the results of processes taking place under the same conditions;

— obtaining a light surface of parts after cementation;

— lack of gas preparation plants and instruments for monitoring carbon potential;

— reduction of specific consumption of electricity and process gas;

— greater equipment mobility (starting and stopping takes a few minutes);

— reduction in the duration of the process as a result of carrying it out at high temperatures and changes in atmospheric potential;

— improving production standards and working conditions.

The first information about the vacuum carburization process dates back to the early 70s, when specialists from the Hayes company (USA) first carried out vacuum carburization in modernized VCQ furnaces.

During vacuum carburization, parts are loaded into a cold chamber, then the furnace is started, and further control of all technological parameters (temperature, gas flow, pressure, duration of carburization and diffusion periods) is carried out using a program entered into the control computer. First, the furnace is evacuated, followed by stepwise heating to the carburization temperature.

Then the charge with the parts is kept at a constant temperature to equalize the temperature inside the charge and remove contaminants from the surface of the steel that prevent the penetration of carbon. The duration of exposure at temperature ranges from 20 to 60 minutes. (depending on the cross-section of the parts). Next, a reaction gas is supplied into the chamber, which is used as hydrocarbons such as methane, propane, butane or acetylene.

Gas pressure and flow depend on the type of gas, chamber volume and surface area of ​​the parts. The gas pressure can be in the range of 4 - 400 mbar, and the flow rate in the range of 500 -5000 Nl/h. At the same time, they try to enrich the surface zone with carbon as much as possible so that the carbon concentration in this zone reaches higher values ​​than the specified values ​​for the final processed part. The carburization stage is followed by a diffusion stage of the process.

To avoid further carburization during the diffusion stage, the furnace is evacuated again at the end of the carburization stage. Next, a little nitrogen is pumped into the furnace (until the pressure in the furnace reaches 2 mbar) in order to reduce the effect of sublimation (weathering, volatilization) in a vacuum of carbon and alloying elements from the surface of the parts during the diffusion stage.

The carburization and diffusion stages are alternated until the required carburization depth and carbon concentration profile are achieved. The optimal vacuum carburizing process consists of three carburization stages and three diffusion stages.

At the next stage, the furnace and charge with parts are cooled to the workshop temperature and, depending on the design of the furnace, this can occur both in the chamber itself using inert gas (nitrogen, argon or helium) at different pressures, and in the oil of the quenching tank. After the furnace reaches the workshop temperature, the computer control is turned off and the charge is unloaded using a loader.

Some features of the technology - instead of a conclusion

Sometimes during grouting it is necessary to protect certain surfaces. For this, 3 main methods are used: protection with tolerances, copper plating of the surface, protection with pastes.

Cementation is widely used in mechanical engineering to increase the hardness and wear resistance of products while maintaining the high viscosity of their core. The specific volume of the hardened carburized layer is larger than the core, and therefore significant compressive stresses arise in it. Residual compressive stresses in the surface layer, reaching 400–500 MPa, increase the endurance limit of the product.

Low carbon content (0.08–0.3%) ensures high core viscosity due to incomplete hardenability. High-quality steels 08, 10, 15 and 20 and alloy steels 12KhNZA, 18KhGT, etc. are subjected to carburization.

The hardness of the surface layer for carbon steel is 60–64 HRC, and for alloy steel – 58–61 HRC; the decrease in hardness is explained by the formation of an increased amount of retained austenite.

plastic packaging

Source: https://HeatTreatment.ru/cementaciya-stalej