Ferrous metallurgy products are widely used in many sectors of the national economy, and ferrous metal is always in demand in construction and mechanical engineering. Metallurgy has been successfully developing for a long time, thanks to its high technical potential. Cast iron and steel products are most often used in production and in everyday life.

Cast iron and steel both belong to the group of ferrous metals; these materials are alloys of iron and carbon that have unique properties. What are the differences between steel and cast iron, their main properties and characteristics?

Steel and its main characteristics

Steel represents deformed alloy of iron and carbon, which is always up to a maximum of 2%, as well as other elements. Carbon is an important component because it gives strength to iron alloys, as well as hardness, thereby reducing softness and ductility. Alloying elements are often added to the alloy, which ultimately results in alloyed and high-alloy steel, when the composition contains at least 45% iron and no more than 2% carbon, the remaining 53% being additives.

Steel is the most important material in many industries; it is used in construction, and as the technical and economic level of the country grows, the scale of steel production also increases. In ancient times, craftsmen used crucible melting to produce cast steel, and this process was low-productivity and labor-intensive, but the steel was of high quality.

Over time, the processes for producing steel changed, the crucible process was replaced by the Bessemer and open hearth method obtaining steel, which made it possible to establish mass production of cast steel. Then they began to smelt steel in electric furnaces, after which the oxygen-converter process was introduced, which made it possible to obtain especially pure metal. Depending on the number and types of connecting components, steel can be:

• Low alloy
• Medium alloyed
• High alloy

Depending on carbon content it happens:

• Low carbon
• Medium carbon
• High carbon.

The composition of the metal often includes non-metallic compounds - oxides, phosphides, sulfides; their content differs depending on the quality of the steel; there is a certain quality classification.

The density of steel is 7700-7900 kg/m3, and the general characteristics of steel consist of such indicators as strength, hardness, wear resistance and suitability for various types of processing. Compared to cast iron, steel has greater ductility, strength and hardness. Due to its ductility, it is easy to process; steel has a higher thermal conductivity, and its quality is improved by hardening.

Elements such as nickel, chromium and molybdenum are alloying components, each of which gives steel its own characteristics. Thanks to chromium, steel becomes stronger and harder, and its wear resistance increases. Nickel also imparts strength, as well as toughness and hardness, and increases its anti-corrosion properties and hardenability. Silicon reduces viscosity, and manganese improves weldability and hardening properties.

All existing types of steel have melting temperature from 1450 to 1520 o C and are strong, wear-resistant and deformation-resistant metal alloys.

Cast iron and its main characteristics

The basis for the production of cast iron is also iron and carbon, but unlike steel, it contains more carbon, as well as other impurities in the form of alloying metals. It is fragile and breaks without visible deformation. Carbon here acts as graphite or cementite and due to the content of other elements Cast iron is divided into the following types:

The melting point of cast iron depends on the carbon content in it; the more of it there is in the alloy, the lower the temperature, and also its fluidity when heated increases. This makes the metal non-plastic, fluid, and also brittle and difficult to process. Its melting point is from 1160 to 1250 o C.

Cast iron firmly entered our lives many years ago. It is relatively easy to produce and widely used in various fields. To have a clear understanding of this material, you need to know its features, disadvantages, advantages, chemical composition, properties, structure of cast iron and its alloys, their production and scope of application.

So, let's find out which iron-carbon alloys are called cast irons.

Concept

Cast iron is an iron-carbon alloy containing carbon, that is, it means a material that consists of an alloy and carbon. The percentage of carbon in cast iron is more than 2.14%. The latter element can be included in cast iron in the form of graphite or cementite.

This video talks about the features of cast iron:

Varieties

There are white and gray cast iron.

• The carbon in white cast iron is in the form of iron carbide. If you break it, you can see a white tint. White cast iron is not used in its pure form. It is added to the process of producing malleable iron.
• At a fracture, gray cast iron has a silvery tint. This type of cast iron has a wide range of uses. It lends itself well to processing with cutters.

In addition, cast irons are high-strength, malleable and with special properties.

• High strength Cast iron is used to increase the strength of the product. The mechanical properties of such cast iron allow this to be done perfectly. High-strength cast iron is obtained from gray cast iron by adding magnesium to the mass.
• Malleable Cast iron is a type of gray. The name does not mean that this cast iron is easily forged. It has increased plasticity properties. It is obtained by annealing white cast iron.
• They also distinguish half-hearted cast iron. Some of the carbon in it is in the form of graphite, and the remaining part is in the form of cementite.

Special Features

The peculiarity of cast iron lies in the process of its production. The average melting point of different types of cast iron is 1200ºC. This value is 300 degrees less than that of steel. This is due to the very high carbon content. Carbon and do not have a very close relationship with each other.

When the smelting process takes place, carbon cannot be completely incorporated into the iron lattice. As a result, cast iron takes on the property of brittleness. It cannot be used for the manufacture of parts that will be subject to constant load.

Cast iron is a ferrous metallurgy material. Its characteristics are often compared to steel. Products made of steel or cast iron are widely used in our lives. Their use is justified. After comparing the characteristics, we can say the following about these two materials:

• The cost of cast iron products is lower than the cost of steel ones.
• Materials vary in color. Cast iron is a dark matte material, while steel is light and shiny.
• Cast iron is easier to cast than steel. But steel is easier to weld and forge.
• Cast iron is less durable than steel.
• Cast iron is lighter in weight than steel.
• Steel has a higher carbon content than steel.

Advantages and disadvantages

Cast iron, like any material, has positive and negative sides.

The advantages of cast iron include:

• Carbon in cast iron can be in different states. Therefore, this material can be of two types (gray and white).
• Certain types of cast iron have increased strength, so cast iron is sometimes placed on the same line as steel.
• Cast iron can maintain temperature for quite a long time. That is, when heated, the heat is evenly distributed throughout the material and remains in it for a long time.
• In terms of environmental friendliness, cast iron is a clean material. Therefore, it is often used to make dishes in which food is subsequently prepared.
• Cast iron is resistant to acid-base conditions.
• Cast iron has good hygiene.
• The material has a fairly long service life. It has been noticed that the longer cast iron is used, the better its quality.
• Cast iron is a durable material.
• Cast iron is a harmless material. It is not capable of causing even slight harm to the body.

The disadvantages of cast iron include:

• Cast iron will rust if it is exposed to water for a short time.
• Cast iron is an expensive material. However, this minus is justified. Cast iron is very high quality, practical and reliable. Items made from it are also high quality and durable.
• Gray cast iron is characterized by low ductility.
• White cast iron is characterized by brittleness. It is mainly used for smelting.

Properties and characteristics

1. Physical. These characteristics include: specific gravity, coefficient of linear expansion, actual shrinkage. Specific gravity varies depending on the carbon content of the material.
2. Thermal. The thermal conductivity of a material is usually calculated using the displacement rule. For solid cast iron, the volumetric heat capacity is equal to 1 cal/cm 3 * o C. If the cast iron is liquid, then it is approximately 1.5 cal/cm 3 * o C.
3. Mechanical. These properties depend on the base itself, as well as on the size and shape of the graphite. Gray cast iron with a pearlite base is considered the most durable, and the most ductile is with a ferritic base. The maximum reduction in strength is observed with the “plate” shape of graphite, and the minimum – with the “ball” shape.
4. Hydrodynamic. Viscosity in cast iron varies depending on the presence of manganese and sulfur. It also increases sharply when the temperature of cast iron passes the point where solidification begins.
5. Technological. Cast iron has excellent casting properties, resistance to wear and vibration.
6. Chemical. According to the electrode potential (in decreasing order), the structural components of cast iron are arranged in the following form: cementite - phosphide eutectic - ferrite.

Differences between cast iron and steel in chemical composition and properties

The properties of cast iron are affected by special impurities.

• Thus, the addition of sulfur can significantly reduce fluidity and reduce refractoriness.
• The addition of phosphorus simultaneously makes it possible to create a product of complex shape, but does not give it increased strength.
• The admixture in the form makes the melting point not so high and significantly improves the casting properties. Different percentages of silicon create different types of cast iron, from pure white to ferritic.
• Manganese worsens casting and technological properties, but increases strength and hardness.

The video below will show you how to weld cast iron using electric welding:

Structure and composition

If we consider cast iron as a structural material, then it is a metal cavity with graphite inclusions. The structure of cast iron is mainly pearlite, ledeburite and ductile graphite. Moreover, for each type of cast iron these elements predominate in different proportions or are absent altogether.

According to the structure of cast iron there are:

• perlite,
• ferritic and
• ferritic-pearlitic.

Graphite is present in this material in one of the forms:

• Globular. Graphite takes on this shape when magnesium is added. The spherical shape of graphite is characteristic of high-strength cast irons.
• Plastic. Graphite is similar to the shape of petals. In this form, graphite is present in ordinary cast iron. This cast iron has increased ductility properties.
• Flaky. Graphite acquires this shape by annealing white cast iron. Graphite is found in flake form in malleable cast iron.
• Vermicular. The named form of graphite is found in gray cast iron. It was developed specifically to improve ductility and other properties.

Metal production

in special blast furnaces. The main raw material for producing cast iron is. The technological process consists of reducing the iron oxides of the ore and obtaining another material as a result - cast iron. The following fuels are used to make cast iron: coke, natural gas and thermal anthracite.

Once the ore is reduced, the iron is in a solid form. Next, it is lowered into a special part of the furnace (steam), where carbon is dissolved in the iron. The output is liquid cast iron, which falls into the lower part of the furnace.

The price of cast iron (per 1 kg) depends on the amount of carbon in it, the presence of additional impurities and alloying components. Approximately the price of a ton of cast iron will be 8,000 rubles.

Areas of use

• It is used for the production of parts in mechanical engineering. Engine blocks and crankshafts are mainly made from cast iron. The latter require advanced cast iron, to which special graphite additives are added. Due to the resistance of cast iron to friction, it is used to make excellent quality brake pads.
• Cast iron can operate smoothly even at extremely low temperatures. Therefore, it is often used in the production of machine parts that will have to work in harsh climatic conditions.
• Cast iron has proven itself well in the metallurgical field. It is valued for its relatively low price and excellent casting properties. Products made from cast iron are characterized by excellent strength and wear resistance.
• A large variety of plumbing products are made from cast iron. These include sinks, radiators, sinks and various pipes. Cast iron bathtubs and heating radiators are especially famous. Some of them still serve in apartments today, although they were purchased many years ago. Cast iron products retain their original appearance and do not require restoration.
• Thanks to its good casting properties, cast iron produces real works of art. It is often used in the manufacture of artistic products. For example, such as beautiful openwork gates or architectural monuments.

Are you choosing a bath? Don't know which is better, cast iron or steel? Then this video will help you:

It differs from steel in its composition by its higher carbon content, in its technological properties - better casting qualities, low ability to plastic deformation (under normal conditions it cannot be forged). Cast iron is cheaper than steel.

Cast irons are classified according to the following indicators:

• state of carbon:

- white cast iron- all carbon is in a bound state in the form of carbide;

- gray cast iron- carbon is largely or completely in a free state in the form of lamellar or fibrous (vortex) graphite;

- ductile iron- carbon is largely or completely in a free state in the form of spherical graphite;

- malleable iron- obtained by annealing white cast iron castings. All or a significant portion of the carbon is in a free state in the form of flake graphite (annealed carbon);

• structure:

- ferritic;

- ferrite-pearlite;

- pearlitic;

• chemical composition:

- unalloyed;

- alloy- special purpose.

Thus, cast iron (except white) differs from steel in the presence of graphite inclusions in the structure (Fig. 1), and cast irons differ from each other in the shape of these inclusions.

Rice. 1. Cast iron classification according to the structure of the metal base and the shape of graphite inclusions: A - ferrite; b - ferrite and pearlite; V- perlite; / - lamellar; 2- swirling; 3 - flaky; 4- spherical.

The mechanical properties of cast iron depend on the structure and mainly on the shape, quantity, size and nature of the distribution of graphite inclusions. Graphite inclusions determine the technological and operational properties of cast iron. The presence of graphite inclusions facilitates the processing of cast iron parts by cutting due to brittle chips. Graphite increases wear resistance and imparts good anti-friction properties to cast iron through its own “lubricating” effect. Cast iron has low sensitivity to various surface defects, cuts, grooves, etc., since graphite inclusions themselves are stress concentrators, and adding a few more to them does not have a significant effect on the overall strength of the material. Unlike a metal base, graphite does not transmit elastic vibrations well, so cast iron has a high damping capacity, which allows it to dampen vibration and resonant vibrations.

The hardness of cast iron depends little on the shape of the graphite inclusions and is determined by the structure of the metal base. Ferritic cast irons have a hardness of ~150 HB, and ferrite-pearlite cast iron has a hardness of ~200 HB; pearlite ~250 HB.

Impurities in cast iron

Regular industrial cast iron contains the same impurities , as carbon steel, i.e. manganese, silicon, sulfur and phosphorus, but in larger quantities. These impurities significantly affect the conditions of graphitization and, consequently, the structure and properties of cast iron.

Silicons has a particularly strong effect on the structure of cast iron, enhancing graphitization. The silicon content in cast iron varies widely: from 0.3-0.5 to 3-5%. By changing the silicon content, it is possible to obtain cast irons that are completely different in properties and structure - from low-silicon white to high-silicon ferritic (gray with lamellar or high-strength with nodular graphite).

Manganese unlike silicon, it prevents graphitization, or, as they say, promotes the bleaching of cast iron.

Sulfur also contributes to the bleaching of cast iron, but at the same time worsens its casting properties (in particular, it reduces fluidity). Therefore, the sulfur content in cast iron is limited: the upper limit for small castings is 0.08%; for larger ones (when slightly worse fluidity can be tolerated) - up to 0.1-0.12% S.

Phosphorus has virtually no effect on the graphitization process. However, phosphorus is a useful admixture in cast iron, as it improves fluidity.

White cast iron

Cast iron received this name from the type of fracture, which has a matte white color. All the carbon in this cast iron is in a bound state in the form of cementite. White cast irons, depending on the carbon content, can be hypoeutectic (perlite + ledeburite), eutectic (ledeburite) and hypereutectic (primary cementite + ledeburite). These cast irons are distinguished by their high hardness (450-550 HB) due to the presence of a large amount of cementite in them. Therefore, they are very fragile and are not used for making machine parts. White cast iron castings are used for the subsequent production of malleable cast iron using graphitizing annealing. Subsequently, it is used for the manufacture of parts with increased fatigue strength: crankshafts and camshafts, valve seats, oil pump gears, disc brake calipers, etc.

Bleached cast irons have surface layers (12-30 mm) with the structure of white cast iron, and a core with the structure of gray cast iron. The high surface hardness of such a casting increases its resistance to abrasion. Therefore, bleached cast iron is used for the manufacture of sheet mill rolls, wheels, brake pads and many other parts that operate under conditions of increased wear.

Gray cast iron

Cast iron got its name from the type of fracture, which is gray in color. Gray cast iron contains graphite in its structure. The structure of cast iron consists of a metal base and graphite (in the form of plates), and its properties depend on these two components.

Compared to paper, graphite has low mechanical properties, so to some approximation we can assume that the places it occupies are voids and cracks. With an increase in the number of voids, the mechanical properties of cast iron sharply deteriorate. Under tensile stresses, fracture centers easily form at the ends of graphite inclusions. Cast iron behaves much better under compression and bending.

Gray cast irons are alloys of complex composition containing iron, carbon, silicon, manganese and impurities such as sulfur and phosphorus. The latter partially dissolves in ferrite (~0.3%) and, in addition, enters into the ternary eutectic (Fe-C-P) with a melting point of 950 °C. This significantly improves the casting properties of cast iron.

Sulfur is a harmful impurity that reduces the mechanical and casting properties of cast iron and increases the tendency for cracks to form in them.

Silicon is included in the composition of gray cast iron (1-3%) as the main chemical element and increases the release of graphite during solidification and decomposition of released cementite.

Manganese (0.2-1.1%) has a positive effect on the mechanical properties of cast iron, but complicates the graphitization process or promotes its bleaching. Thus, we can say that the degree of graphitization directly depends on the amount of carbon (2.2-3.7%) and silicon (1-3%) in cast iron.

In small quantities, chromium, nickel and copper can enter gray cast iron from the ore, which also affect the graphitization condition. The number of graphite inclusions and the structure of the base affect the properties of gray cast iron.

Based on the structure of the metal base, gray cast irons are divided into three groups:

1) gray pearlite with a pearlite + graphite structure (the amount of bound carbon is ~0.8%).

2) gray ferrite-pearlite with the structure ferrite + pearlite + graphite (the amount of bound carbon is less than 0.8);

3) gray ferritic with a ferrite + graphite structure (all carbon in the form of graphite).

The mechanical properties of gray cast iron depend on the properties of the metal base and its quantity, shape and size of graphite inclusions (voids).

Markinggray cast iron

According to GOST 1412-85, the designation of cast iron includes a combination of letters and numbers, for example SCH15. SCH stands for gray cast iron, the numbers indicate the value of tensile strength. The standard provides for the following grades of cast iron: SCh10; SCH15; SCh18; SCh20; SCh21; SCh24; SCH25; SCh30; SCh35; SCh40; SCH45.

The values ​​of the indicators of some gray cast irons are given in table. 1.

Table 1. Mechanical properties of some gray cast irons

The presence of graphite contributes to the grinding of chips during cutting and has a lubricating effect, which increases the wear resistance of cast iron.

Ferritic gray cast iron grades SCh10 and SCh15 are used for lightly and moderately loaded parts: covers, flanges, flywheels, calipers, brake drums, clutch drive discs, etc.

Ferritic-pearlite gray cast iron grades SCh20 and SCh25 are used for parts operating under increased static and dynamic loads: engine cylinder blocks, cylinder pistons, clutch drums, machine beds, etc.

Pearlitic cast iron is used for casting frames of powerful machine tools and mechanisms. Perlite gray modified cast irons are often used. Such cast irons are obtained by adding special additives to liquid cast iron before casting - ferrosilicon (0.3-0.6% by weight of the charge) or silico-calcium (0.3-0.5% by weight of the charge). Such cast irons include cast irons of the SCh40 and SCh45 grades, which have higher mechanical properties due to the refinement of the shape of graphite inclusions. These cast irons are used for the manufacture of pump casings, compressors and hydraulic drives.

For parts operating at elevated temperatures, alloyed gray cast iron is used, which additionally contains chromium, nickel, molybdenum and aluminum.

Malleable iron

Malleable cast iron is called malleable because it can be subjected to pressure treatment, although cast iron is not forged, and cast iron parts are produced only by casting due to the fact that malleable cast iron has higher ductility compared to gray cast iron.

Malleable cast iron is produced by graphitizing annealing of white hypoeutectic cast iron castings. Malleable cast iron should not contain a large amount of manganese, since during annealing it interferes with the graphitization process, as well as a large amount of carbon and silicon, which makes it difficult to obtain castings from white cast iron, because during crystallization, graphite begins to precipitate in the form of plates. Therefore, the chemical composition of white cast iron annealed into ductile cast iron has content restrictions: 2.5-3.0% C; 0.7-1.5% Si; 0.3-1.0% Mn; less than 0.12% S; less than 0.18% R.

The thickness of the casting section should not exceed 40-50 mm, since with a greater thickness, lamellar graphite is formed in the core, which makes cast iron unsuitable for annealing.

Annealing is carried out in two stages. At the first stage, white cast iron castings are slowly heated to a temperature of 930-1050 ° C and maintained for 15 hours at this temperature. In this case, the cementite included in the high-temperature ledeburite decomposes, and flake-like graphite is formed from the released carbon.

At the second stage, the castings are cooled to a temperature of 700-760 °C, corresponding to the eutectoid transformation, and maintained at this temperature for up to 30 hours, or cooled very slowly. In this case, the cementite included in the perlite decomposes. After the end of the second stage, the structure of cast iron consists of ferrite and flake graphite. This type of cast iron is called ferritic malleable cast iron.

If the cooling was not slow enough in the region of temperatures corresponding to the eutectoid transformation, or the exposure at the second stage of graphitization was insufficient, then the decomposition of cementite included in pearlite will not occur completely. In this case, the structure of cast iron will consist of ferrite, pearlite and flake graphite. This type of cast iron is called ferrite-pearlite malleable cast iron.

If cooling in the temperature range was accelerated, then the decomposition of cementite included in perlite will not occur. In this case, the structure of cast iron will consist of perlite and flake graphite. This type of cast iron is called pearlitic malleable cast iron.

Marking. Malleable cast iron according to GOST 1215-79 is marked with the letters “KCH” and two numbers: the first indicates tensile strength; the second is relative elongation (in%).

The values ​​of the mechanical properties of some malleable cast irons are given in table. 2.

Table 2. Mechanical properties of some ductile cast irons

Ductile iron

High-strength cast iron is called cast iron with a spherical graphite obtained through the process of crystallization of the casting. This form of graphite inclusions has a smaller surface compared to plate-like and flake-like ones with the same volume, and reduces stress concentration.

The spherical form of graphite is obtained by introducing magnesium, or magnesium with nickel, or ferrosilicon into liquid cast iron.

Under the influence of modifiers, graphite takes on a spherical shape during crystallization. Cast irons with spherical graphite have higher mechanical properties compared to other cast irons. High-strength cast irons are similar in properties to cast carbon steel, but have better casting properties, are easy to cut, and retain high wear resistance. To increase ductility and toughness, castings made of high-strength cast iron are subjected to heat treatment: annealing, normalization, injection and tempering. Simultaneously with the increase in ductility, heat treatment reduces residual stresses in castings, which increases their performance.

Marking. High-strength cast iron according to GOST 7293-85 is designated similarly to malleable cast iron: with the letters “HF” and numbers: the first indicates the value of tensile strength, the second - relative elongation (in%).

The standard provides for the following grades of cast iron: VCh35-22; VCh40-15; HF45-10; VC50-7; HF60-3; VC70-2; HF80-2; HF 100-2. Chemical composition of high-strength cast iron: 3.2-3.6% C; 1.6-2.9% Si; 0.3-0.7% Mn; no more than 0.02% S; no more than 0.1% R. High-strength cast irons on a ferritic basis (VCh35-22, VCh40-15, VCh45-10) have δ from 22 to 10%, 140-225 HB; on a pearlite basis (VCh50-7, VCh60-3, VCh70-2, VCh80-2, VCh100-2) - δ from 7 to 2%, 153-360 HB.

The high strength and ductility of high-strength cast irons make it possible to use them for the manufacture of crankshafts for automobile diesel engines and other parts operating in friction units under increased loads.

Anti-friction cast irons

Antifriction cast irons are special gray and high-strength cast irons with increased antifriction properties. These cast irons have a low coefficient of friction, depending on the ratio of ferrite and pearlite in the base, as well as the amount and form of graphite. In pearlitic cast irons, high wear resistance is ensured by a metal base consisting of thin pearlite and uniformly distributed phosphorus eutectic in the presence of isolated deposits of lamellar graphite.

Castings made of antifriction cast iron (GOST 1585-85) are used for the manufacture of parts operating in bearing friction units.

Marking. There are the following grades of antifriction cast iron: AChS1; ASF2; ASFZ; ASF1; AChV2; ACC1; ABC2. The letters "ACS" indicate anti-friction gray cast iron; “AChV” - anti-friction high-strength cast iron; "AChK" - anti-friction malleable cast iron.

Antifriction gray cast irons - pearlitic cast iron AChS-1 and AChS-2 and pearlitic-ferritic cast iron AChS-3 - have a low coefficient of friction, depending on the ratio of ferrite and pearlite in the base, as well as on the amount and form of graphite. In pearlitic cast irons, high wear resistance is ensured by a metal base consisting of thin pearlite and uniformly distributed phosphorus eutectic in the presence of isolated deposits of lamellar graphite.

Anti-friction gray cast iron is used for the manufacture of sliding bearings, bushings and other parts that operate under friction with metal, often in the presence of a lubricant. Parts working in tandem with hardened or normalized steel shafts are made from cast iron of the AChS-1 and AChS-2 grades, and for working in tandem with thermally untreated shafts, AChS-3 cast iron is used.

Anti-friction high-strength (with nodular graphite) cast irons are manufactured with a pearlite structure - AChV-1 and ferrite-pearlite (50% pearlite) - AChV-2. AChV-1 cast iron is used for work in friction units with increased peripheral speeds in conjunction with a hardened or normalized shaft.

The main advantage of anti-friction cast irons compared to anti-friction bronzes is their low cost, and the main disadvantage is poor run-in, which requires precise mating of the rubbing surfaces.

CAST IRON

Cast iron -- these are iron-carbon alloys containing more than 2% carbon and solidifying to form eutectic. Unlike steel, cast iron has low ductility. However, due to their high casting properties, sufficient strength and relative cheapness, cast iron has found wide application in mechanical engineering.

Cast iron is smelted in blast furnaces, cupola furnaces and electric furnaces. Cast irons smelted in blast furnaces are pig iron, special cast iron (ferroalloys) and foundry cast iron. Pipeline and special cast irons are used for subsequent smelting of steel and cast iron. Cast iron is melted in cupola furnaces and electric furnaces. About 20% of all cast iron produced is used to make castings.

CLASSIFICATION OF CAST IRONS

The casting and mechanical properties of cast iron depend on how close its composition is to eutectic. To assess this, two indicators are used:

Eutecticity degree S E -- the ratio of the concentration of carbon C in cast iron to its concentration in the eutectic, taking into account the influence of silicon and phosphorus:

where 4.26 is the concentration of carbon in the eutectic of the “iron-graphite” system (see Fig. 7.1), Si and P are the content of these elements in cast iron, %.

Carbon equivalent defined as:

C eq = C + 0.3(Si + P)

Cast irons are divided into: hypoeutectic (S uh< 1, C эв < 4,2-4,3), eutectic (S e 1, S eq 4.2-4.3) and hypereutectic (S e > 1, C eV > 4.2-4.3).

Cast irons can behave differently during crystallization and further cooling (Fig. 1): either in accordance with the metastable state diagram Fe--Fe 3 C (white cast irons in which carbon is present in the form of Fe 3 C), or in accordance with stable Fe--C diagram (gray cast irons in which carbon is present in the form of graphite).

In the presented diagrams (Fig. 1), in addition to the common lines AC, AE, GS, the other lines do not coincide. In the Fe--C system, graphite eutectic (austenite--graphite) contains 4.26% C and is formed at 1,153 ° C. Along line E " S " in the temperature range 1,153-738 ° C, secondary graphite is released. The eutectoid transformation occurs at 738 ° C with the formation of a eutectoid (ferrite + graphite). The use of the Fe--C and Fe--Fe 3 C diagrams is not fundamentally different from each other.

The probability of cementite formation from the liquid phase is much higher than graphite. Any process is determined by thermodynamic and kinetic conditions. The driving force of the graphitization process is the desire of the system to reduce the supply of free energy. Cementite is a thermodynamically less stable phase than graphite. However, the difference between the formation temperatures of cementite and graphite is small, and with relatively slight supercooling, crystallization of cementite rather than graphite will occur.

Graphite is formed only at low cooling rates in a narrow temperature range, when the degree of supercooling of the liquid phase is low. With accelerated cooling and when liquid cast iron is supercooled below 1,147 ° C, cementite forms.

Graphitization of cast irons

Graphitization is the process of precipitation of graphite during the crystallization or cooling of cast iron. Graphite can be formed both from the liquid phase during crystallization and from the solid phase. According to Fe--C diagram below line C " D " primary graphite is formed along line E " C " F " -- eutectic graphite, along line E " S " -- secondary graphite and along the P line " S " TO " -- eutectoid graphite.

Graphitization of cast iron and its completeness depends on the cooling rate, chemical composition and the presence of graphitization centers.

The influence of the cooling rate is due to the fact that the graphitization of cast iron proceeds very slowly and includes several stages:

· formation of graphitization centers in the liquid phase or austenite;

· diffusion of carbon atoms to graphitization centers;

· increased release of graphite.

During graphitization of cementite, stages of preliminary decomposition of Fe 3 C and dissolution of carbon in austenite are added. The slower the cooling of cast iron, the greater the development of the graphitization process.

Depending on the degree of graphitization, cast irons are distinguished white, gray And half.

White cast iron -- are obtained with accelerated cooling and with supercooling of liquid cast iron below 1,147 °C, when, due to structural and kinetic features, a metastable Fe 3 C phase will form, rather than graphite. White cast irons, containing fixed carbon in the form of Fe 3 C, are characterized by high hardness, brittleness and are very difficult to machine. Therefore, they are not used as a structural material, but are used to produce malleable cast iron by graphitizing annealing.

Gray cast iron -- are formed only at low cooling rates in a narrow temperature range, when the degree of supercooling of the liquid phase is low. Under these conditions, all or most of the carbon is graphitized in the form of flake graphite, and the carbon content in the form of cementite is no more than 0.8%. Gray cast iron has good technological and strength properties, which determines its widespread use as a structural material.

Half cast irons -- occupy an intermediate position between white and gray cast iron, and in them the main amount of carbon (more than 0.8%) is in the form of Fe 3 C. Cast iron has the structure of perlite, ledeburite and lamellar graphite.

Industrial cast irons contain 2.0-4.5% C, 1.0-3.5% Si, 0.5-1.0% Mn, up to 03% P and up to 0.2% S. The strongest positive effect on graphitization is caused by silicon. By changing the silicon content, it is possible to obtain cast irons with different structures and properties. (Fig. 2) approximately indicates the boundaries of structural regions depending on the silicon and carbon content at a content of 0.5% Mn and a given cooling rate (for a casting wall thickness of 50 mm).

Manganese inhibits graphitization, increasing the susceptibility of cast iron to bleach. Sulfur is a harmful impurity. Its whitening effect is 5-6 times higher than manganese. In addition, sulfur reduces fluidity, promotes the formation of gas bubbles, increases shrinkage and the tendency to crack. Phosphorus does not affect graphitization and is a useful admixture, increasing the fluidity of gray cast iron due to the formation of low-melting (950-980) ° C phosphide eutectic.

Rice. 2. Structural diagram: 1 -- white cast iron; 2 -- half cast iron; 3, 4, 5 -- gray cast iron on pearlitic, ferrite-pearlitic and ferritic base, respectively

Thus, by adjusting the chemical composition and cooling rate, it is possible to obtain the desired cast iron structure in castings.

Classification of gray cast iron

Gray cast iron can be considered as a structure that consists of a metal base with graphite inclusions. The properties of cast iron depend on the properties of the metal base and the nature of the graphite inclusions.

The metal base can be: pearlite, when 0.8% C is in the form of cementite, and the rest of the carbon is in the form of graphite; ferrite-pearlite, when the amount of carbon in the form of cementite is less than 0.8% C; ferritic, when carbon is practically in the form of graphite.

Depending on the shape of graphite inclusions, gray cast irons are classified into:

· cast iron with lamellar graphite;

· cast iron with flake graphite (malleable cast iron);

· cast iron with nodular graphite (high-strength cast iron);

· cast iron with vermicular graphite.

Figure 3 shows a generalized classification of cast irons according to the structure of the metal base and the shape of graphite.

The microstructure of cast iron is shown in Fig. 7.4.

Rice. 3.

Rice. 4. Various forms of graphite in cast iron: a) flake graphite; b) flake graphite; c) spherical graphite; d) vermicular graphite. H 200

Compared to the metal base, graphite has low strength. Therefore, graphite inclusions can be considered discontinuities (voids) in the metal base, and cast iron can be considered as steel permeated with graphite inclusions, weakening its metal base. At the same time, the presence of graphite also determines a number of advantages of cast iron: good fluidity and low shrinkage; good machinability (graphite makes chips brittle); high damping properties; antifriction properties, etc.

When classifying, cast irons with special properties are allocated to a separate group. As a rule, these cast irons are alloyed and are divided according to their purpose into the following types: anti-friction, wear-resistant, heat-resistant, corrosion-resistant, heat-resistant.

Surely many have encountered cast iron products in everyday life or at work. This material has good strength and excellent casting properties.

Cast iron is a steel, or more correctly, an iron-carbon alloy, consisting of iron and carbon, which has a volume from 2.14% to a maximum of 6.67% and can be included in the composition as cementite or graphite. Cast iron, by definition, is an engineering material that is inexpensive and easy to manufacture and serves as the basis for steel smelting. Its production refers to complex chemical processes occurring at certain stages of production.

Main characteristics and composition

In addition to iron and carbon, this alloy includes additional impurities that affect its properties. The diverse composition of cast iron provides it with high hardness, fluidity, and increases fragility. It includes: sulfur, silicon, manganese, phosphorus. Due to the incoming carbon, the cast iron alloy has high hardness, but at the same time the malleability and ductility of the substance are reduced. To give the metal special characteristics, certain additives are added. The following alloying components are used: nickel, vanadium, as well as chromium and aluminum. The cast iron formula consists of an iron-carbon base with additional inclusions. It has a density of about 7.2 g/cm3, which is a fairly high value for metal compounds.

Cast iron consists of several components, which is why the properties of its variations can differ significantly. In addition to carbon and iron, the composition includes up to 2% manganese, 1.2% phosphorus, 4.3% silicon and up to 0.07% sulfur. Silicon is responsible for the fluidity state, significantly improves casting qualities, and also makes it softer. Manganese is used to enhance strength. The addition of sulfur reduces the refractoriness and reduces its fluidity. In addition, it has a harmful effect, manifested in the appearance of cracks on hot castings (red brittleness). The presence of phosphorus reduces the mechanical properties, but allows the casting of objects of complex shapes.

The structure of cast iron looks like a metal base with graphite inclusions. Depending on the type, it includes perlite, flake graphite, and ledeburite. These elements determine its characteristics and are present in varying quantities or completely absent.

The melting point ranges from a minimum of +1160 °C to a maximum of +1250 °C. It has high anti-corrosion properties and actively counteracts both dry (chemical) and wet corrosion. Thanks to him, stainless steel was born - a steel alloy with a high content of chromium component.

Application area

Cast iron is widely used in mechanical engineering for casting various parts. Used for the manufacture of crankshafts and engine blocks. In addition, high-quality pads are produced that are highly resistant to friction. They are used at low temperatures, where cast iron is used exclusively due to its high performance properties. These qualities are used in the production of various machine elements that use a cast iron alloy for operation in harsh climates. This material is widely used by metallurgists due to its excellent casting characteristics and low price. Cast products have high wear resistance and increased strength.

Many plumbing parts are also made from a cast iron base. These are radiators, heating radiators, pipes, bathtubs, various sinks with sinks. Many products still serve today, although they were installed several decades ago. These items retain their original appearance for many years and do not require restoration work. In addition, cast iron cookware is considered one of the most convenient when preparing many dishes.

Varieties

According to its characteristics, cast iron alloy is divided into conversion and foundry. The first is used in steel smelting using the oxygen-converter method. This species is characterized by a reduced amount of manganese and silicon. Cast iron foundry material is used to produce numerous parts. Samples of products made from this base can be seen in the corresponding photos.

A special variety includes nickel-chromium alloys (nihards). These include low-carbon and high-carbon cast iron. The first is characterized by increased strength, and the second by increased wear resistance. The main varieties are white and gray alloys. These materials differ in carbon content as well as properties. In addition, malleable, alloyed and high-strength types are actively used.

Grey

Gray cast irons have low ductility, viscosity, and are easy to cut during processing. They are used in the manufacture of non-critical parts, as well as elements subject to wear. Gray cast iron contains carbon in the form of graphite, perlite or ferrite-perlite. Its amount is about 2.5%, which provides high strength to the products. Cases of various industrial equipment, gears, brackets, and bushings are made from the gray alloy. A material containing a high amount of phosphorus (about 0.3 - 1.2%) has good fluidity and is used in artistic casting.

White

Contains a large amount of carbon (over 3%), presented in the form of cementite or carbide. The white color at the fracture site of this material gave its name to the connection. An alloy of this type has increased fragility and brittleness, which significantly narrows the scope of use. Based on it, parts of simple shapes are produced to perform static functions without exposure to significant loads. The technical characteristics of the white material can be improved by adding alloying components. For this, nickel, chromium, and much less often aluminum or vanadium are used. The brand with such additives is called “sormite”. It is used as a heating element in a variety of devices. Sormite has stable characteristics at temperatures not exceeding +900 °C. This material serves as the basis for the manufacture of ordinary household bathtubs.

Malleable

This type is obtained from white by casting with further heat treatment. In this case, long-term annealing is used, during which cementite disintegrates, forming graphite. This process is called graphitization with the formation of carbon flakes in the structure. Graphite acquires this shape through prolonged annealing. This has a positive effect on the metal base, which becomes more integral, ductile and viscous.

Malleable cast iron works well at low temperatures and is not very sensitive to cuts. Used in the manufacture of elements operating under continuous friction. In addition, the malleable alloy serves as the basis for products of very complex configurations: angles, brake pads, tees, automobile housings for rear axles and other structures. Improved properties are achieved by adding boron, tellurium, and magnesium.

High strength

It has increased strength and is used to produce critical products, and in some cases even replaces steel. This high-strength cast iron is obtained by adding special impurities (cerium, calcium, yttrium, magnesium) to the gray form. Gears, pistons, crankshafts and other parts are made from it. High thermal conductivity makes it possible to cast elements for heating units, as well as pipelines.

Alloy

Alloyed cast iron alloy contains additional impurities. The composition includes high levels of titanium, nickel, chromium, as well as zirconium, vanadium, molybdenum, aluminum and other elements. They impart high strength, hardness, and wear resistance. Alloyed materials are used in the production of parts of mechanisms that interact with gaseous and aggressive environments, as well as those operating under the influence of aqueous solutions.

Advantages of metal

This alloy is classified as a material produced by ferrous metallurgy. It is often compared to steel when determining certain characteristics. An item made of cast iron has a low cost compared to its steel counterpart. In addition, cast iron elements have less weight and strength. These properties of cast iron are significantly expanded through the use of various additives to the alloys. Its parameters have the following positive qualities:

• environmentally friendly material that is used in the production of household items, including dishes;
• resistant to acid-base environment;
• hygienic;
• ability to maintain temperature for a long time;
• some types have strength comparable to steel;
• duration of operation, during which its quality indicators only improve;
• Completely harmless to the body.

Production

The production of cast iron alloy is a material-intensive and costly process. Smelting one ton of material will require about 900 liters of ordinary water and about 550 kg of coke. The melting point is about +1200 °C, which requires specific melting equipment. To obtain mass, ore is required, where the mass fraction of iron contained is over 70%. Depleted ore rocks are not used due to economic inefficiency.

The material is smelted in special blast furnaces. There, iron ore goes through a full technological cycle, starting with the reduction of ore oxides and ending with the production of a cast iron alloy. Casting the material requires fuel: coke, thermoanthracite, and natural gas. At the end of the reduction process, the iron in solid form is placed in a special part of the furnace to dissolve the carbon in it. After the interaction, a cast iron mass is obtained, which falls down in liquid form. Unmelted impurities are pushed to the surface and subsequently removed. This slag is used to produce numerous materials. After removing unnecessary particles from the melt, additives are added to obtain certain grades of cast iron alloys.