Cast iron and its types. Grey, ductile and malleable cast irons

Alloys of iron with carbon, in which the carbon content is more than 1.7% are called cast irons.

Cast irons differ in structure, manufacturing methods, chemical composition and purpose.
According to the structure, cast irons are gray, white and malleable. According to the manufacturing methods - ordinary and modified.
According to the chemical composition, cast irons are distinguished not alloyed and alloyed, i.e., those that contain special impurities.

Gray cast iron

1. ferrite graphite,
2. ferrite-derlite-graphite and
3. perlite-graphite.

If gray cast iron is cooled rapidly after melting, it becomes bleached, that is, it becomes very brittle and hard. Gray cast iron performs several times better in compression than in tension.

Gray cast iron welds quite well with the use of preheating and as a filler material of special cast iron rods with a high content of carbon and silicon. Welding without preheating is difficult due to bleaching of cast iron in the weld zones.

white cast iron

Welding of white cast iron is very difficult due to the formation of cracks during heating and cooling, and also because of the heterogeneity of the structure formed at the welding site.

malleable iron

With the American method, languishing is carried out in sand at a temperature of 800-850°C. In this case, carbon from a chemically bound state passes into a free state in the form of graphite, located between the grains of pure iron. Cast iron acquires viscosity, which is why it is called malleable.

With the European method, castings are languished in iron ore at a temperature of 850-950 °. At the same time, carbon from a chemically bound state from the surface of the castings passes into iron ore and in this way the surface of the castings is decarburized and becomes soft, which is why cast iron is called malleable, although the core remains brittle.

In the designations of ductile iron grades, after the letters, a number is written showing the average value of the tensile strength at break in kg / mm2, and then a number showing the elongation in%.

For example, KCh37-12 denotes malleable cast iron, with a tensile strength of 37 kg/mm2 and an elongation of 12%.
Welding of malleable iron is difficult due to bleaching of the iron in the weld area.

modified cast iron

The modification consists in the fact that during the melting of cast iron, a certain amount of additives are added to the liquid metal, which contribute to the release of carbon in the form of graphite during solidification and cooling. This modification process, with the same chemical composition of cast iron, significantly improves the mechanical properties of cast iron and is very important. The designation of grades of modified cast iron is similar to the designation of grades of gray cast iron.

Among the most common types of cast iron are gray and white. What is each of them?

What is gray cast iron?

The corresponding type of cast iron is one of the most common in the field of mechanical engineering. This metal is characterized by the presence of lamellar graphite in the thin section. Its content in gray cast iron can vary. The larger it is, the darker the metal becomes at a break, and also the softer the cast iron. Castings from the considered type of metal can be produced in any thickness.

The main features of gray cast iron:

1. minimum relative elongation - as a rule, not exceeding 0.5%;
2. low impact strength;
3. low plasticity.

Gray cast iron contains a small percentage of bound carbon - no more than 0.5%. The rest of the carbon is presented in the form of graphite - that is, in a free state. Gray cast iron can be produced on a pearlitic, ferritic, as well as mixed - ferrite-pearlitic - basis. In the metal under consideration, as a rule, there is a significant percentage of silicon.

Gray cast iron is fairly easy to work with cutting tools. This metal is used in the casting of products that are optimal in terms of compressive strength. For example, various support elements, batteries, water pipes. The use of gray cast iron is also widespread in mechanical engineering - most often in the manufacture of parts that are not characterized by shock loads. For example, cases for machine tools.

What is white cast iron?

This type of cast iron is characterized by the presence of carbon, which is almost completely represented in the structure of the metal in a bound state. The metal in question is hard and at the same time quite brittle. It is resistant to corrosion, wear, temperature effects. White cast iron is quite difficult to work with hand tools. At a break, this metal has a light shade, a radiant structure.

The main scope of white cast iron is further processing. As a rule, it is converted into steel, in many cases - just the same into gray cast iron. In industry, its use is not very common due to its brittleness and difficulty in processing.

The percentage of silicon in white cast iron is significantly less than in gray cast iron. In the metal under consideration, there may also be a higher concentration of manganese and phosphorus (note that their presence is largely determined by the chemical composition of the ore from which cast iron is smelted). Actually, an increase in the amount of silicon in the metal is accompanied by a decrease in the volume of bound carbon in its structure.

Comparison

The main difference between gray cast iron and white cast iron is that in the first there is a small percentage of bound carbon, in the second, on the contrary, there is mainly bound carbon. This feature predetermines the difference between the considered metals in the aspect:

• hardness;
• break colors;
• wear resistance;
• fragility;
• workability with hand tools;
• scope of application;
• percentage of bound and free carbon;
• percent silicon, manganese, phosphorus.

To more clearly study what the difference between gray and white cast iron is in these aspects, a small table will help us.

Table

1. DEFINITION

Cast iron is commonly called iron-carbon alloys containing carbon under normal crystallization conditions above the limit of solubility in austenite and eutectic in the structure. In accordance with the state diagram of iron-carbon alloys, cast iron is alloys containing more than 2% carbon. The eutectic in the structure of these alloys, depending on the conditions of its formation, can be carbide or graphite.

The above definition, which underlies the classification of conventional iron-carbon alloys, is not always sufficient.

Indeed, carbide eutectics are present not only in cast irons, but also in high-alloy steels containing little carbon (less than 2%), for example, in high-speed steels. The issue of graphite eutectic is also complicated, since secondary and eutectoid graphite are not isolated separately. By structure alone, it can be difficult to correctly distinguish graphitized cast iron from graphitized steel. Therefore, it is often necessary to resort to additional definitions. In particular, a characteristic feature of cast iron is better casting and worse plastic properties compared to steel, which is a consequence of the high carbon content (significantly higher solubility limit in austenite). The generally accepted boundaries between cast iron and steel with a carbon content of 2% or more are conditional, regardless of the degree of alloying and the nature of the structure.

The structure of cast iron remains the most important classification feature, since it determines its basic properties. The structure of graphitized cast irons consists of a metal base penetrated by graphite inclusions. The latter have a very favorable effect on the wear resistance and cyclic toughness of cast iron.

The most important classification features also include mechanical properties (and for cast iron special purpose and special properties), composition of castings, production technology, design of castings and their areas of application.

The strength properties of cast iron are determined by the nature of the metal base and the degree of weakening of this base by graphite inclusions. The latter include primarily the quantity, shape, and nature of the distribution of graphite inclusions.

2. CLASSIFICATION BY CHEMICAL COMPOSITION

In cast iron, in addition to iron and carbon, silicon, manganese, phosphorus and sulfur are contained (as usually determined permanent impurities). Cast irons also contain small amounts of oxygen, hydrogen and nitrogen.

According to the chemical composition, cast irons are divided into unalloyed and alloyed.

Cast irons are considered unalloyed, in which the amount of manganese does not exceed 2% and silicon 4%. In the presence of these elements in large quantities or with the content of special impurities, cast irons are considered alloyed. It is generally accepted that in low-alloy cast irons the amount of special impurities (Ni, Cr, Cu, etc.) does not exceed 3%.

With low and moderate alloying, attempts are made to improve the general properties of cast iron—the uniformity of the structure, the preservation of strength and elasticity when heated to relatively low temperatures—300–400°C, an increase in wear resistance, an increase in strength, and so on.

With medium, high and high alloying, cast iron acquires special properties, since the composition of solid solutions and carbides changes significantly. In this case, the change in the nature of the metal base acquires the greatest significance. By alloying, martensite, acicular troostite and austenite can be obtained directly in the cast state. This improves corrosion resistance, heat resistance and changes the magnetic properties.

3. CLASSIFICATION ACCORDING TO THE STRUCTURE AND CONDITIONS OF FORMATION OF GRAPHITE

According to the degree of graphitization, the forms of graphite and the conditions for their formation, the following types of cast irons are distinguished:

b) half-chat,

c) gray with lamellar graphite,

d) high-strength with nodular graphite and

e) malleable.

The nature of the metal base of cast iron is determined by the degree of graphitization, the state of alloying and the type of heat treatment.

According to the degree of graphitization, white cast iron is almost non-graphitized, half cast irons are slightly graphitized, and the rest of the cast irons are significantly graphitized (Fig. 1).

Fig. 1. Scheme for classifying cast irons according to the degree of graphitization, type of fracture, form and conditions for the formation of graphite

In white and half cast irons, the presence of ledeburite is mandatory, and in highly graphitized cast irons, ledeburite should not be present.

The structure of cast iron in one casting can be different and belong to different types of cast iron; sometimes they even deliberately achieve different structures in different layers, for example, in the production of bleached rolling rolls and crushing balls. The outer layers are made of white cast iron, the transition layers are made of hollow cast iron, the core is made of highly graphitized cast iron.

Let us consider in more detail the main features of the listed cast irons.

A) White cast iron. White iron is called iron, in which almost all of the carbon is in a chemically bound state. White cast iron is very hard, brittle and very difficult to process with cutters (even hard alloys).

Rice. 2. Structure of white cast iron (ledeburite, perlite and secondary cementite)

On fig. 2 shows the microstructure of an unalloyed white hypoeutectic iron, consisting of ledeburite, perlite and secondary cementite. In alloyed or heat-treated cast irons, troostite, martensite, or austenite may be present instead of pearlite.

White cast iron castings are of limited use due to their high hardness and brittleness. They are used as wear-resistant, corrosion-resistant and heat-resistant.

White cast iron is called because the type of fracture in it is light-crystalline, radiant (Fig. 3).

Rice. 3. Type of fracture of white cast iron.

b) Half cast iron. Half cast iron is characterized by the fact that, along with carbide eutectic, graphite is also present in the structure. This means that the amount of bound carbon exceeds its limiting solubility in austenite under real conditions of solidification.

The structure of half cast iron is ledeburite + perlite + graphite. In alloyed and heat-treated cast irons, martensite, austenite, or acicular cane can be obtained.

Half cast iron is called because the type of fracture in it is a combination of light and dark areas of the crystalline structure. Half cast iron is hard and brittle; the use of half cast iron products is limited. Most often, this structure is found in chilled castings as a transition zone between the chilled layer and the graphitized part.

V) Gray cast iron (SC). Gray cast iron is the most common engineering material. The main difference between gray cast iron is that graphite in the plane of the section has a lamellar shape (Fig. 4). When the plates are very dispersed, the graphite is called dispersed or pointy. Obtaining a lamellar form of graphite does not require heat treatment or mandatory modification.

Lamellar graphite is distinguished by the degree of isolation, the nature of the location, the shape and size of the plates

Rice. 4 . Lamellar graphite (rectilinear). x100

Rice. 5. Lamellar graphite, colonies of a high degree of isolation. x100.

On fig. 5 shows lamellar graphite, located in colonies of a high degree of isolation, and in fig. 6 small degree of isolation. The last graphite (dispersed) is located between the dendrites and is called interdendritic point. In FIG. & shows interdendritic lamellar graphite, and in fig. 8 rosette graphite.

Rice. 6. Lamellar graphite, colonies of a small degree of isolation. x100.

Rice. 7. Interdendritic graphite. x100.

Rice. 8. Rosette graphite. x100.

Rice. 9. Swirled graphite. x100.

Rice. 10. Structure of gray cast iron (sorbitol, graphite and phosphides) x400.

Rice. 11. Pearlite-ferritic gray cast iron. x100.

Rice. 12. Nodular graphite. x400.

Rice. 13. Pearlite high strength. x400.

Rice. 14. Pearlite-ferritic ductile iron. x100.

Rice. 15. Ferritic ductile iron. x200.

Graphite in fig. 4 is called rectilinear, or large: in contrast to the swirling one shown in fig. 9.

Graphite inclusions are divided into ten groups according to the predominant length of the sections on the thin section, indicated below.

The type of fracture in gray cast iron largely depends on the amount of graphite - the more graphite, the darker the fracture.

Gray cast iron castings are produced in any thickness.

Due to the strong weakening effect of graphite plates, gray cast iron is characterized by an almost complete absence of relative elongation (less than 0.5%) and a very low impact strength.

Due to the fact that gray cast iron, regardless of the nature of the metal base, has low ductility, for the most part they strive to obtain it with a pearlite base, since perlite is much stronger and harder than ferrite. A decrease in the amount of pearlite and an increase due to this amount of ferrite lead to a loss of strength and wear resistance without increasing ductility. Alloying gray cast iron and obtaining an austenitic base also do not give great plasticity.

Rice. 16. Flaky and crab-shaped graphites.

Rice. 17. Ductile iron with a ferritic base.

On fig. 10 shows the structure of pearlite-graphite gray cast iron, and fig. 11 structure of pearlitic-ferritic gray cast iron with approximately equal amounts of pearlite and ferrite.

G) Ductile cast iron with nodular graphite (HF). The fundamental difference between ductile iron and other types of cast iron lies in the spherical shape of graphite (Fig. 12), which is obtained mainly by introducing special modifiers (Mg, Ce) into liquid cast iron. Therefore, ductile iron is often called magnesium, although it is called "high-strength" in GOST. The size and number of graphite inclusions are different.

The globular form of graphite is the most favorable of all known forms. Nodular graphite weakens the metal base less than other forms of graphite. The metal base of ductile iron is, depending on the required properties, pearlitic (Fig. 13), pearlite-ferritic (Fig. 14) and ferritic (Fig. 15). By alloying and heat treatment, it is possible to obtain an austenitic, martensitic or acicular-troostite base.

Ductile iron castings, as well as gray iron castings, can be produced in any thickness.

e) Malleable cast iron (KCh). The main difference between ductile iron is that the graphite in it has a flaky or spherical shape. Flaky graphite can be of various compactness and dispersion (Fig. 16 L, B, C, D), which affects the mechanical properties of cast iron.

Industrial ductile iron is produced mainly with a ferritic base; however, it always has a pearlite border. In recent years, cast irons with a ferrite-pearlitic and pearlitic base have become widely used. Cast iron with a ferritic base (Fig. 17) has great ductility.

The fracture in ferritic ductile iron is black-velvety; with an increase in the amount of pearlite in the structure, the fracture becomes significantly lighter.

Accordingly, cast irons can be classified according to the nature of the charge, the method of melting and the method of processing liquid iron.

The state of the mold and the nature of the pouring into it also have a great influence on the properties of cast iron. According to the method of producing castings, cast iron can be divided into chill casting (grinding the structure due to accelerated cooling), centrifugal (dense structure), reinforced (hardening of castings), etc.

A significant change in properties is achieved by heat treatment of castings. With the help of heat treatment, it is possible to change the degree of dispersion of the metal base and its nature up to its transformation into needle-troostite and martensitic. Up to a certain limit, the amount of bound carbon can be changed, and during chemical-thermal treatment, the composition of cast iron can also be changed in the surface layers. According to the type of heat treatment, castings can be divided into annealed, normalized, improved, surface-hardened, nitrided, etc.

6. CLASSIFICATION BY TYPE OF CASTINGS AND THEIR FIELDS OF APPLICATION

Cast iron castings according to the types of castings and their areas of application can be divided into machine, cylinder, automobile, bearing, chilled iron mill rolls, etc.

Of the above classifications, the clearest is the classification by structure, the least clear is the classification by types of castings, since cast irons with the same structure and the same composition can be suitable for various types of castings and engineering industries.

It is necessary to distinguish the main (determining) signs of classification - the shape of graphite from clarifying signs, which include the nature of the metal base, the method of manufacture, etc. basis, how it is obtained (by modification or heat treatment), whether it is alloyed and with what it is alloyed.

In mechanical engineering, castings from gray, ductile and high-strength cast iron are used. These cast irons differ from white cast irons in that all or most of their carbon is in the free state in the form of graphite (and in white cast iron, all carbon is in the form of cementite).

The structure of these cast irons consists of a metal base similar to steel (pearlite, ferrite) and non-metallic inclusions - graphite.

Grey, malleable and ductile irons differ from each other mainly in the form of graphite inclusions. This determines the difference in the mechanical properties of these cast irons.

At gray cast iron graphite (when viewed under a microscope) has the form of plates.

Graphite has low mechanical properties. It breaks the continuity of the metal base and acts as a notch or a small crack. The larger and straighter the forms of graphite inclusions, the worse the mechanical properties of gray cast iron.

Main difference high-strength cast iron is that the graphite in it has a spherical (rounded) shape. This form of graphite is better than lamellar, since in this case the continuity of the metal base is much less disturbed.

Malleable cast iron obtained by long-term annealing of white cast iron castings, as a result of which flake-shaped graphite is formed - annealing carbon.

The mechanical properties of the considered cast irons can be improved by heat treatment. At the same time, it must be remembered that significant internal stresses are created in cast irons, therefore, cast iron castings should be heated slowly during heat treatment in order to avoid cracking.

Cast iron castings are subjected to the following types of heat treatment.

Low temperature annealing. To relieve internal stresses and stabilize the dimensions of cast iron castings from gray cast iron, natural aging or low-temperature annealing is used.

The older way is natural aging , in which the casting after complete cooling undergoes a long aging - from 3-5 months to several years. Natural aging is used when the required annealing equipment is not available. This method is now almost never used; produce mainly low-temperature annealing. To do this, the castings, after complete solidification, are placed in a cold oven (or a furnace with a temperature of 100-200 ° C) and together with it slowly, at a rate of 75-100 ° C per hour, are heated to 500-550 ° C, at this temperature they withstand 2-5 hours and cool-give to 200 ° C at a rate of 30-50 ° per hour, and then in air.

Graphitizing annealing.

When casting products, partial chilling of gray cast iron from the surface or even over the entire section is possible. To eliminate the chill and improve the machinability of cast iron, high-temperature graphitizing annealing is carried out with holding at a temperature of 900-950 ° C for 1-4 hours and cooling the products to 250-300 ° C together with a furnace, and then in air. With such annealing in the chilled areas, cementite Fe 3 C decomposes into ferrite and graphite, as a result of which white or half cast iron turns into gray.

Normalization.

Castings of a simple shape and small sections are subjected to normalization. Normalization is carried out at 850-900 ° C with an exposure of 1-3 hours and subsequent cooling of the castings in air. With such heating, part of the carbon-graphite is dissolved in austenite; after cooling in air, the metal base acquires a troostite pearlite structure with higher hardness and better wear resistance. For gray cast iron, normalization is used relatively rarely; quenching with tempering is more widely used.

hardening.

It is possible to increase the strength of gray cast iron by hardening it. It is produced with heating up to 850-900 ° C and cooling in water. Both pearlitic and ferritic cast irons can be hardened. The hardness of cast iron after hardening reaches HB 450-500. The structure of hardened cast iron contains martensite with a significant amount of residual austenite and graphite precipitates. An effective method of increasing the strength and wear resistance of gray cast iron is isothermal hardening, which is performed similarly to steel hardening.

Ductile irons with nodular graphite can be subjected to flame or high-frequency surface hardening. Cast iron parts after such treatment have a high surface hardness, a ductile core and good resistance to impact loads and abrasion.

Alloyed gray cast irons And high-strength magnesium cast irons sometimes subjected to nitriding. The surface hardness of nitrided cast iron products reaches HV600-800°C; such parts have high wear resistance. Good results are obtained by sulfiding cast iron; for example, sulfided piston rings run in quickly, resist abrasion well, and their service life increases several times.

Vacation.

To remove hardening stresses, tempering is performed after hardening. Parts designed to work on abrasion undergo low tempering at a temperature of 200-250 ° C. Cast iron castings that do not work on abrasion are subjected to high tempering at 500-600 ° C. When tempering hardened cast irons, the hardness decreases significantly less than when tempering steel. This is due to the fact that in the structure of hardened cast iron there is a large amount of residual austenite, and also because it contains a large amount of silicon, which increases the tempering resistance of martensite.

For annealing on malleable cast iron, white cast iron is used with approximately the following chemical composition: 2.5–3.2% C; 0.6-0.9% Si; 0.3-0.4% Mn; 0.1-0.2% P and 0.06-0.1% S.

There are two methods of annealing for malleable iron:

graphitising annealing in a neutral environment, based on the decomposition of cementite into ferrite and annealing carbon;

decarburizing annealing in an oxidizing environment based on carbon burning.

Annealing for ductile iron according to the second method takes 5-6 days, therefore, at present, ductile iron is obtained mainly by graphitization. Castings, cleaned of sand and sprues, are packed in metal boxes or stacked on a pallet, and then annealed in methodical, chamber and other annealing furnaces.

The annealing process consists of two stages of graphitization. The first stage consists in uniform heating of the castings to 950-1000°C with a holding time of 10-25 hours; then the temperature is lowered to 750-720°C at a cooling rate of 70-100°C per hour. At the second stage, at a temperature of 750-720 ° C, an exposure of 15-30 hours is given, then the castings are cooled together with the furnace to 500-400 ° C and at this temperature they are taken out into the air, where they are cooled at an arbitrary rate. With such a stepwise annealing in the temperature range of 950-1000 ° C, the decomposition (graphitization) of cementite occurs. As a result of annealing according to this mode, the structure of malleable iron is ferrite grains with inclusions of annealing carbon nests - graphite.

Pearlitic ductile iron is obtained as a result of incomplete annealing: after graphitization at 950-1000 ° C, the cast iron is cooled together with the furnace. The structure of pearlitic ductile iron is composed of pearlite and annealed carbon.

To increase the viscosity, pearlitic ductile iron is subjected to spheroidization at a temperature of 700-750°C, which creates the structure of granular pearlite.

To speed up the annealing process for malleable cast iron, white cast iron products are quenched, then graphitized at 1000-1100 ° C. The acceleration of graphitization of hardened cast irons during annealing is explained by the presence of a large number of graphitization centers formed during quenching. This makes it possible to reduce the annealing time of hardened castings to 15–7 hours.

Thermalmalleable iron processing.

To increase strength and wear resistance, malleable cast irons are subjected to normalization or hardening with tempering. Normalization of ductile iron is carried out at 850-900°C with holding at this temperature for 1-1.5 hours and cooling in air. If the blanks have increased hardness, they should be subjected to high tempering at 650-680°C with a holding time of 1-2 hours.

Iron-carbon alloys containing more than 2% are called cast irons. carbon. Cast iron has lower mechanical properties than steel, but is cheaper and well cast into complex shapes. There are several types of cast iron. white iron, in which all carbon (2.0 ... 3.8%) is in a bound state in the form of Fe 3 C (cementite), which determines its properties: high hardness and brittleness, good wear resistance, poor machinability by cutting tools. White cast iron is used to produce gray and ductile iron and steel. Gray cast iron contains carbon in a bound state only partially (not more than 0.5%). The rest of the carbon is in the cast iron in a free state in the form of graphite. Graphite inclusions make the break color gray. The darker the fracture, the softer the cast iron. The formation of graphite occurs as a result of the heat treatment of white cast iron, when part of the cementite decomposes into soft ductile iron and graphite. Depending on the predominant structure, gray cast iron is distinguished on a pearlitic, ferritic or ferritic-pearlitic basis. With slow cooling of iron-carbon alloys, graphite is released. Gray cast iron widely used in mechanical engineering, as it is easy to process and has good properties. Depending on the strength, gray cast iron is divided into 10 stamps (GOST 1412). Gray cast irons with low tensile strength have a fairly high compressive strength. Gray cast irons contain carbon - 3,2…3,5 % ; silicon - 1,9…2,5 % ; manganese - 0,5…0,8 % ; phosphorus - 0,1…0,3 % ; sulfur - < 0,12 % . Given the low resistance of gray iron castings to tensile and shock loads, this material should be used for parts that are subjected to compressive or bending loads. In the machine tool industry, these are basic, body parts, brackets, gears, guides; in the automotive industry - cylinder blocks, piston rings, camshafts, clutch discs. Gray iron castings are also used in electrical engineering, for the manufacture of consumer goods. The properties of gray cast iron depend on the cooling regime and the presence of certain impurities. For example, the more silicon, the more graphite is released, and therefore the cast iron becomes softer. Gray cast iron has a moderate hardness and is easily machined with cutting tools. Gray cast iron used in construction. Pearlitic gray cast irons have the best strength properties and wear resistance. Structural elements that work well in compression are cast from gray cast iron: columns, support pads, shoes, tubing, heating batteries, water and sewer pipes, floor slabs, gears and other parts. When marking gray and modified cast iron, for example SCH12-28, the first two digits indicate the tensile strength, the next two - the flexural strength.

The finished cast iron contains about 93% iron, up to 5% carbon and a small amount of impurities of silicon, manganese, phosphorus, sulfur and some other elements that have passed into cast iron from gangue.

13. Cast iron with lamellar and flaky graphite inclusions. Methods of obtaining, properties, marking. Gray cast irons - 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 carbon or most of it is graphitized in the form of lamellar graphite, and the carbon content in the form of cementite is no more than 0.8%. Gray cast irons have good technological and strength properties, which determines their wide application as a structural material.

Grey, high-strength, malleable cast irons are characterized by the fact that all or part of their carbon is in a free state in the form of graphite, evenly distributed in the metal base.

They have different forms of graphite separation. According to the structure of the metal base, these cast irons can be:

a) ferritic (from ferrite and graphite);

b) ferrite-pearlitic (from ferrite, perlite, graphite);

c) pearlitic (from perlite, graphite).

Thus, their structure is a metal base, similar to hypoeutectoid and eutectoid steel, penetrated by graphite inclusions.

The graphitization of cast iron is significantly affected by the number of elements present in it, the presence of graphite crystallization centers and the cooling rate.

All elements introduced into cast iron are divided into graphite-forming (C, Si, Al, B, Br, etc.) and carbide-forming (Mn, Cr, V, W, Ti, Mo, etc.).

The cooling rate has a significant effect on the graphitization of cast iron. The lower the cooling rate, the more complete the graphitization processes.

In gray cast irons, graphite is present in the form of plates (flakes).

The properties of gray cast irons with the same metal base depend on the size, quantity and distribution of graphite inclusions. They can be considered as cracks, pores, internal cuts that violate the integrity of the metal base.

The more graphite in cast iron, the coarser its inclusions and the less they are isolated from each other, the lower the quality of cast iron. With an increase in the amount of perlite with the same form of graphite inclusions, the mechanical properties (strength, hardness) of cast iron increase.

Gray cast irons are marked with letters: C - gray and H - cast iron, after the letter there are numbers indicating the amount of tensile strength.

Ductile irons obtained by annealing castings made of white cast iron. During the annealing process, cementite, which is part of the structure of white cast iron, decomposes into iron, and graphite, which has a flaky shape (during the solidification of castings - ordinary gray cast iron - graphite does not take this form). The flaky form of graphite improves the plastic properties of cast iron: such cast iron is not allowed under impact and bending.

Depending on the structure of the metal base, pearlitic, ferritic-pearlitic and ferritic malleable cast irons are distinguished. The last of them is the most plastic, its hardness is minimal. Ductile iron is marked with letters: K - malleable, H - cast iron and numbers. The first two digits -  2, the second - relative elongation.