How high-strength cast irons are obtained

How the structure of malleable cast iron is formed

Cast iron is an alloy of iron and carbon (from 2.14% to 4-5% carbon), used in industry, plumbing and heating, and used in households. Cast iron is cheaper than steel (also an alloy of iron and carbon), has better casting properties, greater thermal inertness, and therefore is widely used in various branches of mechanical engineering.

Scheme of annealing white cast iron for malleable.

Depending on the manufacturing technology, two different carbon forms are formed in the alloy structure: graphite or cementite. The presence of one or another type of carbon inclusion determines the type of cast iron and its properties. Gray cast iron contains free carbon (graphite) and is castable. It is characterized by sufficient ductility that allows it to be machined.

White cast iron, containing fixed carbon (cementite), is characterized by high hardness and subsequent wear resistance; it is fragile and poorly processed by mechanical cutting. It is the basis for obtaining a malleable appearance that combines the properties of strength and ductility. What processing leads to the transformation of white cast iron into malleable cast iron and during which technological operations the structure of malleable cast iron is formed?

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Types of alloys: white and gray

Diagram of the microstructure of ductile cast iron.

The structure of white cast iron is formed due to rapid cooling during solidification. With this technology, carbon dissolved at high temperatures does not have time to separate into a separate structural component and remains in a bound form (cementite or iron carbide Fe3C). Its presence determines the properties of hardness, wear resistance and brittleness.

Since the cooling rate plays a decisive role in the formation of the structure, the thickness of the castings is important. If the cross-section is too large (more than 50 - 60 mm), it is difficult to adjust the required cooling rate and obtain the required graphite-free structure throughout the entire thickness.

White alloys are often called pig alloys, since they are not used in themselves, but serve as an intermediate alloy, which is either annealed into malleable iron (DC) or melted into steel.

The technology for producing gray cast iron involves slow cooling during solidification plus additional modification with silicon in the amount of 1-3% (silicon enhances graphitization), which allows dissolved graphite to separate out in the form of separate inclusions.

The structure of cast iron with the resulting graphite inclusions forms a lower hardness of the material (than with cementite) and allows it to be processed by cutting. The shape and dispersion of graphite, the structure of the metal base determine the properties and types of cast iron alloy material: gray (SG), high-strength (HF).

Malleable cast iron: properties and structure

Diagrams of cast iron microstructures.

The malleable type of cast iron is not processed by pressure and is not forged. The name “malleable cast iron” is associated with increased ductility and toughness.

To obtain an optimal combination of strength and ductility, the technology of long-term annealing of castings (from 50 to 120 hours) at a temperature of 900 - 1050ºC is used. The thickness of the casting walls should be less than 50 mm. In this case, the existing cementite inclusions disintegrate into free carbon and ferrite (iron).

The process of cementite decomposition, called graphitization, may or may not be complete. With complete decomposition, the structure of cast iron is freed from all cementite inclusions, which first dissolve in austenite (a high-temperature modification of iron) and then precipitate in the form of graphite. Complete graphitization of cast iron and transformation of the cast material into malleable cast iron occurs with prolonged exposure and slow cooling.

Scheme for producing malleable cast iron.

Smooth cooling ensures the flaky form of carbon in the structure of malleable cast iron. Unlike the lamellar form of graphite, which is a stress concentrator and a source of destruction, ensuring the fragility of cast iron, the flake form does not weaken the structure of the metal alloy. Flaked graphite forms the ductility and toughness required for ductile cast iron.

A structure with spherical carbon inclusions, characteristic of a high-strength alloy, will be more plastic. High-strength cast irons are produced from gray cast iron by modification (alloying) with alkaline earth metals (additives of magnesium, cerium).

Thus, as a result of long-term annealing and slow cooling of thin-walled castings, a structure is formed, which is characterized by a soft ferrite base and compact graphite inclusions.

This cast iron structure is characterized by good wear resistance, sufficient impact strength, can be easily machined and is therefore widely used in various industrial fields.

Thick-walled castings (more than 50 mm) form not only flake-like, but also lamellar graphite inclusions. Such a structure provides a poorer range of mechanical properties of a malleable material.

Annealing: technology and phase transformations

Diagram of isothermal transformations of austenite in malleable cast iron with 2.9% C; 0.88% Si, 0.36% Mn and 0.09% S.

According to the resulting structure, the malleable type of material is divided into white-core (pearlite) and black-core (ferritic) material. Pearlitic is harder and more wear-resistant, while ferritic is more durable and tough. The structure of malleable cast iron (ferrite or pearlite + graphite inclusions) is determined by the characteristics of annealing, temperature and holding time (simmering) in the furnace.

The properties of the finished material depend on the structural components and their shape. For proper annealing to obtain the required properties, the structure of cast iron must not contain any free graphite deposits and must be “bleached” over its entire cross-section. To do this, the amount of silicon content in the alloy, which promotes graphitization, is limited.

Perlite base

Main types of cast iron.

White-core (pearlitic) malleable cast iron is produced by decarburizing annealing of castings in iron ore powder. The popularity of this technology in the past is explained by the high carbon content in cupola castings (3.4 - 3.6%).

Modern cupola melting produces lower carbon content (up to 3%).

For pearlitic malleable cast iron, white iron castings are used, containing carbon in the amount of 3.0 - 3.6%, technological additives of silicon, manganese, phosphorus, and sulfur. They are placed in boxes and covered with fresh ore powder or scale. During annealing, an oxidizing environment is formed in iron ore, and the carbon partially burns out (oxidizes). The surface layer at a depth of up to 2 mm is completely decarbonized.

The resulting cast product has strength to tensile and fatigue loads, close to that of steel, and increased wear resistance. The casting after decarburization develops different properties along the section: lower hardness on the surface and higher hardness in the core.

Heating is performed in one stage: up to 1000 ºC, followed by a long exposure (from 60 to 100 hours) and slow continuous cooling with the furnace. The formed “white-core” structure consists of perlite, which has a silvery white color when broken.

In Image 2 below, the structure of pearlitic malleable material corresponds to photo “a”.

Ferritic base

Cold arc welding of cast iron.

Black-hearted (ferritic) malleable cast iron (photo “b” in the above image) is annealed without the presence of ore.

To protect against oxidation, the castings are covered with sand or chamotte, possibly with steel shavings. This type of annealing is called graphitizing.

To obtain ferritic malleable cast iron, the structure of the original casting and the chemical composition of the alloy are important. Ferritic black-core material is annealed from white, containing carbon in an amount of 2.4 - 2.8%, as well as the addition of silicon, manganese, sulfur and phosphorus. Such low-carbon alloys melt at elevated temperatures (compared to medium-carbon alloys), so a duplex process is used to melt them.

At the first stage, at a temperature of 900 - 1050 ºC, cementite carbon disintegrates (holding time 10-15 hours). At the second stage, at 720 - 760 ºC, pearlite decomposes with the release of free ferrite and graphite, holding time is 25 - 30 hours.

Thus, the structure of cast iron after two stages of annealing contains ferrite and free flake graphite. The fracture of ferrite grains has a dark gray color, which is why such cast irons are called black-hearted. Black-hearted malleable cast iron is characterized by good viscosity, which makes it possible to process it mechanically (on a cutting machine). The casting density and low casting stresses allow the casting of ductile parts into thin-walled parts with wall thicknesses from 4 to 40 mm.

In terms of mechanical and casting properties, the malleable type of material is better than other types of alloys, but worse than steel.

The ferritic structure has low hardness and wear resistance, but is characterized by good toughness and strength. This material replaces steel in non-critical components.

Source: https://moyakovka.ru/izdeliya/struktura-kovkogo-chuguna.html

Forging iron

Malleable cast iron is a type of cast iron obtained by heat treating white cast iron. A distinctive feature of malleable cast iron is the presence of graphite in flake form.

Which cast iron is called malleable

It must be understood that malleable cast iron is not cast iron obtained by forging. Products made of malleable cast iron can become deformed at high humidity even at room temperature. This property of malleable cast iron predetermined its name. Malleable cast iron is produced by casting. An interesting and important feature of malleable cast iron is the absence of internal stresses.

Types of cast iron

Let us recall that all cast irons are divided into the following groups:

• white;
• gray (GOST 1412);
• malleable (GOST 1215);
• high-strength (GOST 7293).

In white cast iron, carbon is present in the form of cementite. White cast irons have high hardness and wear resistance. Due to its high hardness, white cast iron is very difficult to process with metal-cutting equipment.

In gray cast iron, carbon is present in lamellar form. Gray cast irons are not as hard as white ones. Their main area of ​​application is in structures that do not experience shock loads.

In malleable cast iron, graphite is present in flake form. Ductile iron is used to make products that operate under high shock and vibration loads.

In ductile iron, graphite is present in spherical form. High-strength cast iron is produced by modifying it with magnesium, which ensures the formation of carbon in the form of balls. High-strength cast irons are similar in properties to carbon steels. Pistons, crankshafts, and various components of braking systems are made from high-strength cast iron.

Preparation of malleable cast iron

As mentioned above, malleable cast iron is obtained by heat treatment of white cast iron followed by simmering (holding at a certain temperature).

Since white cast irons have poor casting properties, during the production of malleable cast irons it is necessary to take measures aimed at reducing casting defects. For this purpose, white cast iron is overheated, and during casting, its shrinkage is taken into account, as well as changes in the dimensions of the workpieces during simmering, which is carried out at a temperature of 1350-1450 ° C.

The process of simmering malleable cast iron is carried out in special workshops, where blanks made of cast iron alloys are placed in pots that can hold up to 300 castings.

Malleable cast iron obtains maximum strength if it undergoes an annealing process in pots made of white cast iron alloyed with chromium.

Malleable cast iron is produced in electric muffle furnaces, which can flexibly regulate the temperature in the simmering mode, while the products of fuel combustion do not come into contact with the pots and the stacked workpieces.

Ductile iron grades

Malleable cast iron is marked with CN 45 - 6. The first number is the tensile strength, the second is the elongation as a percentage.

The main physical and technical parameters of a malleable cast iron alloy are standardized in GOST 1215-79.

The specific brand of CP directly depends on the conditions under which the simmering was carried out. After this operation, three classes of cast iron are obtained:

• Ferritic class contains ferrite and flake graphite:
  • CC 30-6
  • CC 33-8
  • CC 35-10
  • CC 37-12
  • The pearlite class contains pearlite and flake graphite:
    • CC 45-7
    • CC 50-5
    • CC 56-4
    • CC 60-3
    • CC 65-3
    • CC 70-2
    • CC 80-1.5
    • The ferrite-pearlite class contains ferrite, pearlite and flake graphite.

Structure of Ductile Iron

Compact graphite inclusions, which are the main feature of the microstructure of ductile cast iron, determine its high strength and ductility. Low carbon ductile iron is the only type of cast iron that can be welded. It is well pressed, hammered, easily filling gaps and voids.

Composition of malleable cast iron:

• C – 2.4-3.0%
• Si – 0.8-1.4%
• Mn – 0.3-1.0%
• P – up to 0.2%
• S – up to 0.1%

After going through the simmering process, malleable cast iron contains austenite and graphite.

With slow cooling, the cementite that is part of pearlite decomposes, and the structure takes on the appearance of ferrite and graphite (ferritic malleable cast iron).

Rapid cooling produces pearlitic ductile iron because there is no second stage of decomposition.

Application of Ductile Iron

The use of ductile iron products is due to its mechanical properties, which are between steel and gray cast iron. On the one hand, malleable cast iron has high fluidity, wear resistance, and good anti-corrosion properties. On the other hand, ductile cast irons are high-strength, which allows them to be used in the production of pipeline fittings for gas and water.

At low temperatures, malleable cast iron becomes quite brittle and is susceptible to shock loads.

Products made of malleable cast iron are widely used in mechanical engineering, automotive industry, and railway transport.

The most widely used are ferrite castings, which are cheaper to produce. Gearbox housings, hubs, hooks, brackets, clamps, couplings, and flanges are made from ferritic malleable cast iron.

Pearlitic malleable cast iron is used for the manufacture of parts operating in units under high loads. Perlite cast iron is used to make driveshaft forks, links and rollers of conveyor chains, and brake pads.

Malleable cast iron is used to produce castings with a thin wall, the size of which can range from 3 to 40 mm.

Source: http://mining-prom.ru/rud/zhelez/kovka-chuguna/

Gray malleable high-strength cast iron

The reliability and durability of a product in modern mechanical engineering largely depends on the properties of the structural materials used. Over 80% of mechanical engineering parts of various weights and complexity are made from iron-based alloys. Depending on the carbon content, iron-based alloys are divided into steels and cast irons .

Unlike steel, in cast iron, under certain conditions, part of the carbon is released in the form of graphite rosettes. In the cross-section of such a socket, only individual plates are visible. Therefore, isolated inclusions of graphite are noticeable on a polished section of cast iron. The structure of the matrix is ​​most often ferrite-pearlite or pearlite. This type of cast iron is called gray.

Typically, gray cast iron contains between 2.5% and 3.6% carbon. It contains silicon and manganese in certain quantities. Sulfur and phosphorus are constantly present as impurities.

The strength of cast iron is determined by the presence of plate-shaped graphite in its structure. Such graphite inclusions significantly weaken the matrix. Under the influence of load, stress arises in the metal with the highest concentration at the ends of graphite inclusions. Microcracks appear in these places. Gray cast iron has relatively low strength and breaks without plastic deformation.

Cast iron is a casting alloy.

The cooling conditions of cast iron after filling the mold have a decisive influence on the formation of its structure. In thin sections of the casting, where the cooling rate during the crystallization period is high, a white cast iron . Carbon in it is in the form of cementite, graphite is absent. In the remaining sections, a gray cast iron structure is formed.

The chemical composition also affects the structure. As the content of manganese and sulfur increases, the chill zone increases. An increase in the content of graphitizing elements - carbon and silicon - reduces the tendency of cast iron to whiten.

To obtain castings with specified properties, it is necessary in each specific case to take into account both the chemical composition and the cooling rate of the cast iron in the mold.

Gray cast iron

Despite its relatively low mechanical properties, gray cast iron is widely used. Because it is easy to process, has increased damping capacity, as well as anti-friction properties. Because the graphite of cast iron holds the lubricant and itself serves as a lubricant. The mating cast iron parts move easily relative to each other.

Gray cast iron with small additions of chromium and nickel acquires good elastic properties. A piston ring made of such cast iron, after removing the load, returns to its original dimensions.

Gray cast iron has high fluidity. At real pouring temperatures, the length of a spiral cast iron sample is almost twice as long as a steel one, which makes it possible to produce castings of complex configurations.

Gray cast iron is characterized by low volumetric shrinkage during crystallization, allowing in many cases to dispense with installation and profit.

The most common unit for smelting gray cast iron is a cupola with a dripper, in which metal accumulates, as well as its composition and temperature are averaged. To reduce the tendency of cast iron to chill, it is modified by introducing silicon-containing additives into the liquid metal.

Modification allows you to equalize the properties of the metal in different sections of the casting. This can be seen from the example of measuring the hardness of cast iron. Unmodified and modified.

The chill depth on a wedge sample of modified cast iron is significantly less than that of unmodified cast iron. The shape of graphite inclusions also changes as a result of modification.

In addition to cupola furnaces, electric furnaces . They allow metal to be smelted at a higher temperature, which is important for subsequent, out-of-furnace processing of cast iron. Molds for producing castings from gray cast iron are made by compacting the molding sand in flasks. Cores are installed into the mold cavity to perform the internal configuration of the casting.

In mass production, automatic flaskless molding lines , including installation of cores using a core layer.

The metal of the mold is also poured automatically. Gray cast iron castings are made not only in sand molds, but also in metal ones. To produce castings shaped like bodies of rotation, the centrifugal casting method is widely used. At the same time, labor productivity increases, molding materials are not consumed, and there is no gating system.

Gray cast iron is a generally recognized construction material. It is used for the manufacture of various parts operating under conditions of static loads, vibration, and increased friction.

Malleable iron

It is known that such car parts as wheel hubs and differential housings experience dynamic loads. Can cast iron be used to make them? It is possible if its plasticity is significantly increased. Malleable cast iron has this property , in which the graphite has a flake-like rather than a lamellar shape. Compared to gray cast iron, ductile iron has a lower concentration of graphitizing elements - carbon and silicon.

Malleable cast iron is superior to gray cast iron in terms of strength and ductility. Changes in the chemical composition have led to a decrease in fluidity and an increase in shrinkage during solidification, which requires the installation of profits even on small castings. The production of malleable iron usually uses the duplex process.

Cast iron is smelted into a cut, then transported in a transfer ladle and poured into an electric induction furnace, where it is heated before pouring to increase fluidity.

The technological process for producing castings from malleable cast iron is similar to producing castings from gray cast iron. Automatic molding lines are becoming increasingly common. Metal is poured into molds on a conveyor belt. The castings produced must have a white cast iron structure throughout their entire cross-section.

To obtain the structure of malleable cast iron, they are subjected to graphitizing annealing in thermal furnaces. During the holding period, white cast iron cementite decomposes and flake-shaped graphite inclusions are formed. After heat treatment, the castings are straightened on special presses.

The need to use long-term heat treatment and straightening significantly increases the complexity of manufacturing parts made of malleable cast iron. The forged steel blank of an engine camshaft is noticeably different from the finished part.

The cast billet is much closer in configuration to it, which greatly reduces the complexity of machining. The same applies to crankshafts, critical parts. To replace forged blanks with cast ones, an alloy is needed that would combine the mechanical properties of steel with the technological and operational properties of cast iron.

Ductile iron

High-strength cast iron has such properties which, during crystallization, spherical graphite inclusions are formed. Compared to gray cast iron, high-strength cast iron is characterized by a higher content of carbon and silicon. As well as a low concentration of sulfur. The mechanical properties of cast iron are determined by testing samples specially manufactured in accordance with GOST.

In ductile iron, the nodular graphite, less than the flake graphite in gray, weakens the matrix and significantly reduces the stress concentration under load.

The strength of cast iron with spherical graphite is close to steel. Castings made from high-strength cast iron can be processed just as well as those made from gray cast iron. At the same time, the required accuracy and surface cleanliness are achieved.

Ductile iron has high airtightness . Cylinders of gas engine compressors are made from it, which can withstand pressures of up to 100 atmospheres during testing.

At the same time, high-strength cast iron is prone to the formation of shrinkage cavities , which requires the installation of profits to feed massive parts of the castings.

For the smelting of high-strength cast iron, induction crucible furnaces are widely used, in which cast iron of the desired composition and temperature sufficient for subsequent modification is obtained.

Magnesium, cerium, and yttrium are used as modifiers in the form of pure metals or alloys. To prevent rapid floating and increase the time of contact with the melt, the modifier is covered with steel sheets. The metal is then released from the furnace into a ladle.

This technology increases the absorption of modifiers in cast iron and ensures process stability.

To reduce the tendency of cast iron to chill, it is additionally modified with ferrosilicon. Molds for large mass castings are mainly made on large shaking tables.

The molds are assembled and poured on a special platform. During the crystallization of high-strength cast iron under the influence of modifiers in the melt, multiple branching of graphite plates and the formation of spherical inclusions occurs. If the amount of modifier is insufficient or its uneven distribution in cast iron, ordinary flake graphite can form.

To stabilize the structure and ensure uniformity of the physical and mechanical properties of high-strength cast iron, large castings of complex shape are subjected to heat treatment. For example, normalization.

After machining, the parts are sent to the inspection area. Critical parts undergo flaw detection. Replacing a number of steel parts that experience high shock loads and pressure during operation with parts made of high-strength cast iron significantly reduces the cost of production of certain types of engineering products.

About 50% of crankshafts for engines for various purposes are made from high-strength cast iron. The operational and casting properties of cast iron have ensured their widespread use in various branches of mechanical engineering. More than two-thirds of the cast billets used by industry in our country are obtained from them.  

Source: https://kovkapro.com/metalloobrabotka-obschie-svedeniya/chugun-seryy-kovkiy-vysokoprochnyy/

Where is ductile iron used?

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An alloy of iron and carbon is called cast iron. We will devote the article to malleable cast iron. The latter is contained in the alloy structure either in the form of graphite or cementite. In addition to the above components, cast iron contains impurities based on the following chemicals - silicon, manganese, etc.

Alloying components can be added to cast iron alloys, which have a significant impact on their technical parameters.

Cast iron is used in the production of products by casting, for example, machine tool housings, which operate under small static and dynamic, including multidirectional loads.

Unlike steel, cast iron has good casting parameters and a low price. In addition, this raw material is better processed on metal-cutting equipment than most steel alloys. But, on the other hand, cast iron alloys, regardless of type, are welded with certain difficulties. In addition, cast iron has low parameters of strength, hardness, and brittleness.

White cast iron

In this alloy, carbon is collected in the form of cementite. This grade of material is wear-resistant and has good hardness parameters. At the same time, it is quite poorly processed on metal-cutting equipment.

White cast iron structure

White cast iron is divided into the following groups:

• hypoeutectic with carbon concentration from 2.14% to 4.3%;
• eutectic - 4.3%;
• hypereutectic from 4.3% to 6.67%.

In other grades of cast iron, the carbon is in the form of graphite.

Features of the production of malleable cast iron

The production of KCh cast iron has a number of subtleties that are determined by casting characteristics and other properties.

Ductile iron production

Cast iron of the BC grade, which is the main product of malleable iron, does not have very good casting parameters. In particular, it has reduced fluidity, a large amount of shrinkage during cooling, and it is prone to the formation of various casting defects.

These are the reasons why during production it is necessary to overheat the metal and take measures to combat casting defects. The production of malleable cast iron can be carried out with the obligatory consideration of shrinkage and changes in the dimensions of the workpieces during simmering. Thin workpieces have maximum shrinkage, thick ones have minimal shrinkage.

The simmering operation is performed at 1350 - 1450 degrees Celsius.

Annealing (simmering) is a basic step in the production of cast iron. It is produced in separate workshops called languid. The preparations are placed in pots made of steel or cast iron alloys of different grades for simmering. Up to 300 castings can be placed in a pot, based on the fact that up to 1,500 kg should be per cubic meter.

Malleable cast iron gains its greatest strength in pots made from white cast iron with chromium additions and a minimum amount of phosphorus. The consumption of pots is measured by weight; it can range from 4 to 15% of the weight of the workpieces. That is why increasing their durability plays a big role in determining the cost of finished malleable cast iron.

To avoid warping of the finished castings, placing the blanks in pots must be done with special care. They are laid as tightly as possible; to enhance the effect, the workpieces are sprinkled with sand or ore. These materials protect the workpieces from deformation and excess oxidation.

Electric furnaces are used to produce malleable cast iron. This is due to the fact that during the simmering process it must be possible to regulate the temperature, a sharp rise during heating and a rapid decrease at the stage of its graphitization. In addition, it will not be superfluous to be able to adjust the air mixture in the oven.

Most of the furnaces used to produce malleable cast iron are muffle furnaces. That is, the products of fuel combustion do not come into contact with the pots in which the workpieces are placed.

Castings made from malleable iron go through a cleaning operation several times, and after annealing, feeders are removed and straightened. The first cleaning is carried out to remove residual molding sands. For cleaning, sandblasting equipment or special tumbling drums are used. Removal of feeder residues occurs using emery cloth.

Malleable cast iron defects

The most common defects in malleable cast iron are the following:

• shrinkage cavities;
• underfilling;
• cracks, etc.

Some defects cannot be corrected by further heat treatment. It should be noted that the production of malleable cast iron requires strict compliance with all GOST requirements, technological rules and regulations. Only in this case can we talk about obtaining high-quality malleable cast iron, which can be used to replace other, expensive materials - steel, non-ferrous metals.

Types of malleable cast iron

The grade of cast iron alloy KCh is directly related to the conditions under which annealing is carried out. After this operation, three classes of cast iron are obtained:

The first contains in its chemical composition ferrite and carbon of a flocculent structure. The second includes pearlite and graphite with a flocculent structure. The third contains ferrite, pearlite and flake-like carbon.

Types of malleable cast iron

Malleable pearlitic cast iron results from rapid cooling of the workpiece while it is in the decomposition zone. In this case, in addition to ferrite, the structure of cast iron will contain pearlite. It will persist even with further cooling of the workpiece to a temperature lower than 727 degrees.

That is, we can say that the structure of cast iron is strictly related to the annealing temperature conditions and the presence of alloying components.

Main characteristics of the metal

The key parameters of cast iron are determined by the amount of carbon, which has the form of graphite, and the presence of silicon. Pearlitic malleable cast iron alloy contains two more constituent elements - chromium and manganese.

Characteristics of Ductile Iron

The difference in the structure of malleable cast iron is also reflected in the final properties of products obtained from it. For example, workpieces made from ferritic cast iron have lower hardness than those made from pearlitic material, but at the same time the former have increased ductility. Graphite in the form of flakes provides high strength parameters to finished parts with relatively good ductility.

Products made of KCh cast iron can be deformed at room temperature and humidity. It was this property that determined the name of this material – malleable. In fact, this is a conditional name and does not mean that finished parts are obtained from it using forging equipment. Casting is used to produce products.

The main property of this material is that there is no stress in it.

Microstructure of ductile iron

The mechanical properties of ductile cast iron are between gray cast iron and steel. That is, cast iron of this type has high fluidity, resistance to wear, corrosion, and aggressive substances. In addition, this material has high strength properties. Thus, a part with a wall thickness of 7–8 mm can withstand working pressure of up to 40 atm. This allows it to be used for the manufacture of pipeline fittings for gas and water.

We must not forget that at low temperatures, cast iron becomes very brittle and is very susceptible to impact.

Properties of malleable cast irons

The basic property of the KCh cast iron alloy is that it contains carbon inclusions in different forms, which determines its strength and ductility.

Cast iron with a low amount of carbon (decarbonized), in fact, is the only material from structural cast iron alloys that is well welded and is used to produce welded metal structures. For welding, either gas protection or butt technology is used.

This grade of cast iron lends itself to pressing, embossing and quite simply fills voids and gaps. Parts made from a malleable ferritic cast iron alloy are subjected to cold processing, while parts made from pearlitic alloy are heated.

The cast iron used in production is made from a white cast iron alloy by annealing it. The structure obtained after performing this operation may have a ferritic or pearlitic form.

One of the advantages of a malleable cast iron alloy is that it has uniform cross-sectional properties; in addition, it is well processed on turning-milling machines.

The main physical and technical parameters of a malleable cast iron alloy are standardized in GOST 1215-79. The marking of this material is based on permissible tensile and elongation values. The hardness of the material is determined by the structure, and the strength parameters and ductility are determined by the presence of graphite.

It must be understood that the properties of the material are affected not only by the shape, but also by the amount of graphite contained in the alloy. Malleable cast iron reaches its maximum strength characteristics in the presence of fine pearlite and a small amount of graphite. The maximum ductility and toughness of cast iron of this class is achieved in the presence of ferrite and the same amount of graphite.

Scope of application

Malleable cast iron has found its application in mechanical engineering for the production of machine tools, individual car parts, structures and mechanisms used in railway transport, etc.

Most often, ferrite castings are used, which are somewhat cheaper than all others. Perlite castings are used for the manufacture of parts that are used for products and assemblies operating under increased loads.

Malleable cast iron is used to produce castings with a thin wall; its size can range from 3 to 40 mm.

Source: https://moy-instrument.ru/masteru/gde-primenyaetsya-vysokoprochnyj-chugun.html

Malleable iron

An alloy of iron and carbon is called cast iron. We will devote the article to malleable cast iron. The latter is contained in the alloy structure either in the form of graphite or cementite. In addition to the above components, cast iron contains impurities based on the following chemicals - silicon, manganese, etc.

Cast iron

Alloying components can be added to cast iron alloys, which have a significant impact on their technical parameters.

Cast iron is used in the production of products by casting, for example, machine tool housings, which operate under small static and dynamic, including multidirectional loads.

Iron casting

Unlike steel, cast iron has good casting parameters and a low price. In addition, this raw material is better processed on metal-cutting equipment than most steel alloys. But, on the other hand, cast iron alloys, regardless of type, are welded with certain difficulties. In addition, cast iron has low parameters of strength, hardness, and brittleness.

Composition and structure, as well as features of cast iron production

Cast iron is an important product of ferrous metallurgy, on which the efficiency of many industries depends. We will consider the main features and methods of its production in this article.

The production of iron and steel is an important industry in the life of the country. And first, we’ll talk about the composition of the first metal.

Cast iron is made from iron ore. After production, cast iron has the following composition:

• Iron, which contains carbon.
• Manganese impurity.
• Sulfur impurity.
• Phosphorus impurity.
• Silicon impurity.

The percentage of carbon in cast iron is approximately 2.14%.

There are several types of cast iron: white and gray. Each type has an individual composition and structure.

This video will tell you about the composition of cast iron and steel, as well as the nuances of their production:

Due to the characteristics of its composition, white cast iron has a light shade. Consider the composition of white cast iron:

• Cementite (carbon is found here in this state).
• Perlite.
• Ledeburite.
• Silicon.
• Phosphorus.
• Manganese.
• Sulfur.

Gray view

Gray cast iron is missing one component (ledeburite). Gray cast iron consists of:

• Graphite (some of the carbon is present here in this form).
• Perlite.
• Silicon.
• Phosphorus.
• Manganese.
• Sulfur.

Carbon and other components

• The most important element in cast iron is, of course, carbon. Depending on its quantitative content, different types of material are obtained.
• After carbon, silicon comes in second place. Its percentage affects the softness, fluidity and casting properties of the material.
• Thanks to an element such as manganese, cast iron gains strength.
• The presence of phosphorus in the material makes it capable of quickly forming cracks in cold castings. In addition, this impurity significantly reduces the mechanical properties of cast iron. Phosphorus makes cast iron hard and very durable. But such cast iron is not used for the production of parts that require increased strength.
• The most harmful impurity is sulfur. Its presence negatively affects the refractoriness and fluidity of the material.

If you break white and gray cast iron, you can see completely different structures.

Visually, the structure of gray cast iron can be represented as a metal matrix with graphite crystals. The matrix can have a completely different appearance. There are:

• Ferritic type (there is no bound carbon in the structure).
• Ferrite-pearlite type (there is bound carbon in an amount of up to 0.8%).
• Pearlite type (carbon amount 0.8%).

Structures are affected by temperature. In the case of rapid cooling, a pearlite structure is obtained, and in the case of slow cooling, a ferritic structure is obtained.

Graphite

Depending on the form of graphite, they are distinguished:

• Malleable cast iron (graphite crystals are flaky).
• Ductile iron (graphite crystals are spherical).
• Ordinary gray cast iron (graphite has a lamellar shape).

Graphite can be incorporated into the structure of gray cast iron in various ways. Depending on this, cast iron is:

• With a nest-like structure.
• With a needle-like structure.
• With a plate-like rectilinear structure.
• With a lamellar swirl structure.

If we compare white and gray cast iron, then gray is the most used. White cast iron is difficult to cut and is difficult to cast. In addition, it is characterized by hardness and fragility.

Chemical composition

Cast irons can be manufactured according to their intended purpose. Depending on the purpose and specific chemical composition, cast irons are:

1. High strength . High-strength cast iron is produced by introducing special additives into gray cast iron (liquid state). It is used for very important parts. Ductile iron is often used to replace steel.
2. Malleable . Malleable cast iron is obtained from white cast iron. Heat treatment is used for production. Malleable cast iron has good toughness, high ductility, increased resistance to impact and stretching.
3. Alloyed . Alloy cast iron contains almost all elements. That is, it contains titanium, chromium, nickel, and sulfur. The material is wear-resistant, hard and durable. This type of cast iron is mainly used for the production of machine parts with high mechanical properties. Depending on the predominant element, cast irons are nickel, chromium and titanium.
4. Special (ferroalloys). Special cast iron contains a high content of several elements: silicon and manganese. Such cast irons are mainly used for melting steel and make it possible to remove harmful impurities (oxygen) from steel.

Next, foundry, blast furnace and other processes in the production of cast iron are considered, and the source materials for this are also indicated.

This video will tell you about the production of iron and steel:

To produce cast iron, a huge blast furnace is required. The overall dimensions of this furnace are impressive: height 30 meters, internal diameter 12 meters.

• The widest part of the blast furnace is called steam.
• The lower part is called the forge (hot air enters the oven through it).
• The uppermost part of a blast furnace is called the shaft. It has an upper hole, which is called a Kalashnik. The Kalashnik is closed with a special bolt.

The operation of a blast furnace is based on counterflow. Air is supplied from below, and materials (coke, fluxes and ore) are charged from above. Coke is needed to heat, melt and reduce ore. Without flux, slag cannot form. And ore is the main raw material for the production of cast iron.

In addition to the blast furnace, for the production of cast iron you will definitely need:

• trolleys,
• special crane,
• bunkers,
• conveyors
• and more, in the conditions of a metallurgical plant.

Set of substances

The composition of iron ore includes ore substance and waste rock, which consists of: sandstone with an admixture of clay substances, quartzite, limestone and dolomite. Ore matter refers to iron carbonates, oxides and silicates.

The ore may contain varying amounts of ore matter. Depending on this, she can be rich or poor. Low-grade ore is sent for beneficiation, and rich ore is immediately used in production.

Iron ores come in several types:

1. Brown iron ore . The composition includes iron in the form of aqueous oxides. The total percentage of iron is 25-50%. The waste rock of such iron ore can be siliceous-alumina and differ in clay content.
2. Red iron ore . The material has a second name - hematite. Iron in the structure of this iron ore is presented in the form of anhydrous oxide. Such iron ore contains very few harmful impurities, and the iron itself contains 45-55%. The color of the ore ranges from dark red to dark gray.
3. Magnetic iron ore . Iron is presented in the form of iron oxide and makes up 30-37% of the total amount. This ore is black or dark gray in color. Gang rock is a siliceous mass with other oxides.
4. Spar iron ore . The second name is siderite. The color of this substance is dirty gray or yellowish-white. This material is easy to restore. Ironstone oxidizes quite easily and turns into brown limestone. Iron in spar iron ore is presented in the form of carbon dioxide salt.

Manganese ores are used in the iron production process. They are added to the charge to increase the amount of manganese in the cast iron.

Read below about what substances are raw materials for the production of cast iron.

Calculation

To produce one ton of cast iron you will need:

• Three tons of iron ore.
• One ton of fuel coke.
• Flux (amount depends on the type of ore).
• Twenty tons of water.

Next, the technology of cast iron production is considered.

Technologies

Cast iron production technology consists of the following stages:

1. Ore preparation . Its essence is to re-sort the ore according to the size of the pieces and chemical composition. Large-sized ore is crushed into smaller pieces, and fractions in the form of dust or small particles, on the contrary, are agglomerated. The agglomeration process is based on agglomeration, during which the charge is sintered and a porous product is formed. Also at this stage, the procedure for enriching low-grade ores is carried out. During processing, most of the waste rock is removed and the iron content increases.
2. Fuel preparation . The coke is screened. This process removes unnecessary fines that could cause heat loss during the smelting process.
3. Preparation of fluxes . The flux is also crushed and fines are sifted out. After this, all materials are loaded into the oven.
4. Blast furnace production process . The blast furnace is filled with coke, then with an agglomerator (ore sintered with flux) and again with coke. The temperature for smelting is maintained by injecting heated air. In the forge, coke burns and carbon dioxide is formed. The CO2 passes through the coke and forms CO. Carbon monoxide reduces the bulk of the ore. When reduced, the iron becomes hard. It gradually moves into the hotter part of the blast furnace, where it dissolves carbon within itself. The result is cast iron. Liquid cast iron flows into special ladles, from where it is poured into molds or mixing containers intended for it.

The blast furnace operates in continuous mode. Simultaneously with the production of cast iron, manganese, silicon and other impurities are restored.

The final section of the article will briefly talk about the production and producers (manufacturers) of cast iron in Russia and countries around the world.

Blast furnace production of cast iron

The world's producers of cast iron account for about 900 million tons of products per year. If we compare production volumes, the leaders are: China (550 million tons of pig iron per year), then Japan (already 69 million tons), after Russia (45 million tons), India (29 million tons per year) and South Korea (27 million tons).

• In China, cast iron and steel are produced at five plants: Baotous, Baoshan, Shungan, Anshan and Wuhan.
• The largest enterprises in Russia in the field of iron processing are considered to be Magnitogorsk, Chelyabinsk Combine, Ural Steel LLC, Cherepovets Iron and Steel Works, Novokuznetsk Iron and Steel Works and others.

Blast furnace production of cast iron is discussed in detail in this video:

Source: http://stroyres.net/metallicheskie/vidyi/chyornyie/chugun/proizvodstvo-i-syire.html

Calculation of the charge for producing high-strength cast iron HF50 using the selection method | Agency Lite++

In the article below, technical director of the Casting++ Agency O.N. Vinogradov. describes in detail the methodology for calculating the charge for producing high-strength cast iron with nodular graphite grade VCh50 according to GOST 7293-85.

There are a wide variety of methods for producing high-strength cast iron; for calculations we choose the “sandwich process”, as the most unfavorable due to the low absorption of magnesium and, as a consequence, the high specific consumption of the spheroidizing modifier, which introduces about 1% silicon into the liquid cast iron. With such a high percentage of silicon introduced, it is very difficult to ensure that the entire amount formed during the production process is used in the charge and the return of high-strength cast iron with a high silicon content.

Technological

The production of ductile iron by the “sandwich process” is a two-stage process: at the first stage, the “original cast iron” is smelted in a melting furnace; at the second stage, the “original cast iron” is subjected to spheroidizing modification in a “ladle reactor”.

The calculation of the charge is carried out in the reverse order: the number of elements introduced by the modifier is calculated and, knowing the required chemical composition of the ductile iron, the composition of the “initial cast iron” is determined, and at the second stage, the number of charge components for smelting the “initial cast iron” is calculated.

Initial data

• Cast iron will be smelted in an IST-0.4 induction melting furnace with a capacity of 400 kg;
• the yield is 70.0%;
• return of own production – 25.0%;
• waste and irretrievable losses – 5.0;
• the consumption of charge materials to obtain 1 ton of suitable casting is 1428.57 kg.

To produce cast iron, the enterprise has the following charge materials:

1. High-quality pig iron grade PVK3 (GOST 805-95), the following composition, %: C – 4.4; Si – 0.4; Mn – 0.5; S – 0.02; P – 0.02.
2. Scrap steel grade 1A, the following composition,%: C – 0.2; Si – 0.15; Mn – 0.6; S – 0.05; P – 0.04.
3. Return of own production, the following composition, %: C – 3.7; Si – 2.2; Mn – 0.7; S – 0.01; P – 0.07.
4. Ferrosilicon grade FS45, the following composition,%: C – 0.1; Si – 45; Mn – 0.4; S – 0.02; P – 0.04.
5. Ferromanganese grade FMn75, the following composition,%: C – 0.5; Si – 1.8; Mn – 75; S – 0.02; P – 0.1.
6. Broken graphite electrodes, the following composition,%: C – 95; S – 0.02.

Modifiers

1. Spheroidizing – complex modifier of the FSMg7 brand with the following composition, %: Si – 50; Mg – 8.
2. Graphitizing – ferrosilicon with barium grade FS65Ba17 of the following composition,%: Si – 65; Ba – 17.

A comment

Unlike other metals and alloys, the mechanical properties of the casting material depend to a much greater extent on the rate of cooling and crystallization of the casting, its modulus, the thickness of the casting wall and the production method, therefore GOST 7293-85 “Nodular cast iron for castings. Marki" gives only the recommended chemical composition and deviation in the chemical composition is not a rejection sign.

Reference data 

1. The waste of elements during melting in an induction melting furnace of high frequency is: C – 5%; Si – 3%; Mn – 10%; S – 0%; P – 0%.
2. The assimilation of carbon from scrap graphite electrodes during melting in a high-frequency induction melting furnace is 90%.
3. Recommended chemical composition of cast iron grade VCh50 according to GOST 7293-85: C – 3.2-3.7%; Si – 1.9-2.9%; Mn – 0.3-0.7%; S – up to 0.02%; P – up to 0.1%.

Manufacturer's request

• Use the entire volume of the return from your own production as part of the charge, i.e. – 25.0%.

At the first stage, a table for calculations is built and all currently available data and reference materials are entered into it (Table 1).

Table 1. Calculation of the composition of the charge for producing high-strength cast iron grade VCh50

№	Name of material	Brand	Mass fraction of elements in charge materials, %	Soda material in the charge, %	Mass fraction of elements in cast iron, %	Massage dumps, kg
C	Si	Mn	S	P	C	Si	Mn	S	P
1	High -quality pig iron	PVK3	4,4	0,4	0,5	0,02	0,02
2	Steel scrap	1A	0,2	0,15	0,5	0,05	0,04
3	Return	3,7	2,2	0,7	0,01	0,07	25
4	Ferrosilicon	FS45	0,1	45	0,4	0,02	0,04
5	Ferromanganese	FMn75	0,5	1,8	75	0,02	0,4
6	Breakdown of graphite electrodes	95	0,02
7	TOTAL:	100%	400
8	Frenzy	5	3	10	—	—
9	TOTAL:
10	Required composition of the “initial cast iron”
11	Complex modifier	FSMg7	50
12	Ferrosilicon with barium	FS65Ba17	65
13	Required composition HF50
14	Recommended chemical composition VCh50 (GOST 7293-85)	3,2-3,7	1,9-2,9	0,3-0,7	up to 0.02	up to 0.1

At the second stage, from the content range of HF50 elements proposed by GOST 7293-85, specific content values ​​are selected for each element, to which we will try to adjust the composition of high-strength cast iron. The data is entered in line 12 (Table 2).

Table 2. Calculation of the composition of the charge for producing high-strength cast iron of the VCh50 grade

№	Name of material	Brand	Mass fraction of elements in charge materials, %	Soda material in the charge, %	Mass fraction of elements in cast iron, %	Massage dumps, kg
C	Si	Mn	S	P	C	Si	Mn	S	P
1	High -quality pig iron	PVK3	4,4	0,4	0,5	0,02	0,02
2	Steel scrap	1A	0,2	0,15	0,5	0,05	0,04
3	Return	3,7	2,2	0,7	0,01	0,07	25
4	Ferrosilicon	FS45	0,1	45	0,4	0,02	0,04
5	Ferromanganese	FMn75	0,5	1,8	75	0,02	0,4
6	Breakdown of graphite electrodes	95	0,02
7	TOTAL:	100%	400
8	Frenzy	5	3	10	—	—
9	TOTAL:
10	Required composition of the “initial cast iron”
11	Complex modifier	FSMg7	50
12	Ferrosilicon with barium	FS65Ba17	65
13	Required composition HF50	3,6	2,5	0,5

Source: https://on-v.com.ua/novosti/texnologii-i-nauka/raschet-shixty-dlya-polucheniya-vysokopro/