Cast irons, their varieties and properties. The concept of cast iron, its features and nuances of use

"Iron casting" has been known to mankind since time immemorial. Nowadays, it is widely used in many areas of the national economy and is called cast iron. And if an ordinary person may not know about crankshafts, gearbox housings, wheel hubs, fittings, then everyone knows about frying pans, cast irons, radiators, bathtubs, grates and other cast iron products.

The metallurgical industry produces various simple and special types of cast iron, each of which has its own scope.

Cast iron features

Cast iron is an iron-carbon alloy smelted using fuel from magnetic, red or brown iron ore, with the addition of special inorganic substances - fluxes (fluxes).

Many people do not see the fundamental differences between steel and cast iron, mistakenly assuming that they are one and the same.

Both products of metallurgy are alloys - they consist of several components, one of which is iron.

Cast iron is the raw material for steel production.

Technological properties:

• for steel - deformation (stamping, rolling, forging);
• for cast iron - foundry.

Presence of carbon:

• steel - 0.02 - 2.14%;
• cast iron - 2.14 - 6.67%.

External differences:

• cast iron dark and matte;
• steel is silvery and shiny.

Various physical characteristics

• higher casting qualities;
• easily processed by cutting;
• has less weight;
• lower melting point.

The disadvantages of cast iron include:

• low plasticity;
• fragility;
• poorly amenable to forging and welding.

Cast iron has a low cost, it is cheaper than steel.

Additives and impurities

All supplied pig iron is regulated by GOSTs in terms of its chemical composition and impurity content. Cast iron, in addition to iron, has some “ingredients” that affect the final product and add certain features:

• carbons - increase the hardness of the alloy;
• silicon - improves casting qualities;
• manganese - gives strength;
• sulfur - "thickens", limits the fluidity of cast iron.
• phosphorus causes cracking in the cold state and reduces mechanical parameters.

In order to improve the starting material, cast iron is alloyed, that is, various alloying additives are introduced that change the physical and / or chemical properties.

Alloy additives:

• zirconium;
• aluminum;
• molybdenum;
• titanium;
• vanadium;
• copper;
• chromium.

Cast irons with a high content of silicon and manganese in the composition are classified as alloyed.

Cast iron classification

The metallurgical industry produces various types of cast iron. The grade depends on the forms of graphite or cementite involved in the alloy and other components.

Gray cast iron (MC)

Designated by the letters SC. On the cut it is grayish-black, which is due to the presence of graphite, this natural color. The composition also contains various impurities, including silicon. This type of cast iron, freely cuttable and often used in the engineering industry for "minor" parts, becomes fluid when phosphorus is added. Let's apply to all types of molding, including art.

white cast iron

On the cut, it is light due to the presence of iron carbide. It is further processed into malleable iron and steel. Therefore, the variety is called redistribution. Properties - brittleness and hardness, poorly processed, not suitable for independent use. Hard, poorly processed, brittle - such properties make it unsuitable for independent use.

malleable iron

Designation - KCh. With prolonged annealing, white cast iron is converted into malleable.

Properties - not amenable to pressure treatment, but at the same time it has increased impact resistance and tensile strength. Ductile iron is suitable for the manufacture of parts of a complicated configuration.

high strength

Marked with the letters HF. Obtained by introducing special additives into gray liquid cast iron to give graphite a spheroidal shape. The high-strength type of cast iron is used for the manufacture of critical parts - gears, crankshafts, pistons, which must have high wear resistance.

The release form of conversion and foundry types - special forms - ingots. Modern technologies make it possible to obtain semi-finished products, square, sheet, lamellar, bar blanks of cast iron varieties.

Depending on the purpose and chemical composition, the following types of cast iron are distinguished:

1. ferroalloys
2. doped.

They have names corresponding to additive metals:

• zirconium;
• chromic;
• vanadium;
• copper;
• titanium.

Alloyed species are most in demand in the production of units, mechanisms, assemblies and parts operating in particularly unfavorable environments and conditions.

Cast iron, characterized by an increased percentage inclusion of ferromanganese or ferrosilicon, is classified as special - ferroalloys. They are added in steelmaking to release oxygen - deoxidation.

Alloy cast irons include:

1. Antifriction;
2. Heat resistant;
3. Heat resistant;
4. Corrosion resistant.

Antifriction types are marked with the first letters ACH. For example, ASF is anti-friction gray cast iron. You can also see the marking AChV - anti-friction ductile iron and AChK - anti-friction malleable.

Heat-resistant grades include: ChN19KhZSh.

To corrosion-resistant: marking ChNHT, ChN1MHD

They are also called special cast irons.

Production

The technology of industrial extraction of iron from iron-containing raw materials and the production of cast iron is quite laborious and complex. It makes no sense to describe all the chemical and technological processes and delve into the terminology. You can study the issue if you wish in the sources on metallurgy.

Cast iron is smelted from magnetic, red, brown iron ore, at metallurgical plants, in special blast furnaces. The fuel is coke, which can be partially replaced by fuel oil or gas.

The ore is pre-treated before entering the blast furnace. In addition to ore and fuel, fluxes are used for smelting - limestones necessary for the formation of slag and the removal of sulfur from the melt.

Methods of preparation depend on the quality of the ore - these are crushing, sorting, agglomeration, enrichment and others.

Having gone through all the complex processes, the ore turns into a charge, which is continuously loaded into a blast furnace.

Hot air enriched with oxygen and natural gas is supplied through the tuyeres in the lower part, which burns under the influence of high temperatures, forming oxygen dioxide. Rising higher, the gas combines with oxygen and with carbon that has not yet been burned, transforming into carbon monoxide CO. It reacts with iron oxides, “taking away” oxygen from them.

The result is an almost pure metal. Molten iron mass flows into the furnace. Fireproof residues also flow down.

Finished cast iron is poured at regular intervals into special ladles.

While the smelting process is going on in the furnace, the hole through which the cast iron is released is clogged with a special stopper from a refractory mass. To release the metal, a hole is punched in the cork. Through special channels in the floor of the workshop, the stream of molten metal flows with a “red drain”.

Liquid slag is also discharged from the furnace through another channel.

A sample is taken from each melt. The metal is poured into a special mold and analyzed. All processes are automated. Operators follow them.

And to a simple layman, a blast furnace seems to be a giant test tube in which the “mystery” of turning iron ore into cast iron takes place.

Advantages of "iron casting"

Cast irons, like any materials, have certain pros and cons when using various products from them - spare parts for cars, machine parts, plumbing equipment and other products.

Advantages

• environmental friendliness;
• the ability to maintain temperature;
• high heat transfer;
• resistance to temperature extremes;
• resistance to acids and alkalis;
• corrosion resistance;
• some types are of increased strength, which makes it possible to compare with steel;
• wear resistance;
• durability.

Flaws:

• fragility, care should be taken when assembling;
• large weight of products;
• rusts on prolonged contact with water.

The basis of ferrous metallurgy in our country is the production of iron, steel and rolled products. The largest consumers of "iron casting" are such strategically important industries as metalworking, engineering, construction, transport, light industry, chemical and others.

Iron casting does not give up its position in the production of consumer goods - cast iron boilers, frying pans, ducklings, fences. Skillful craftsmen from this metal create real works of art - fireplace grates, fences, benches, railings, decorating them with openwork cast-iron lace.

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.

(Polish stal, from German Stahl) - a deformable (ductile) alloy of iron with carbon (and other elements), characterized by a eutectoid transformation. The carbon content in steel is not more than 2.14%, but not less than 0.022%. Carbon gives iron alloys strength and hardness, reducing ductility and toughness.

Given that alloying elements can be added to steel, steel is an alloy of iron containing at least 45% iron with carbon and alloying elements (alloyed, high-alloyed steel).

In ancient Russian written sources, steel was referred to by special terms: "Otsel", "Kharolug" and "Uklad". In some Slavic languages ​​today steel is called "Ocel", for example in Czech.

Steel is the most important structural material for mechanical engineering, transport, construction and other sectors of the national economy.

Steels with high elastic properties are widely used in machine and instrument making. In mechanical engineering, they are used for the manufacture of springs, shock absorbers, power springs for various purposes, in instrumentation - for numerous elastic elements: membranes, springs, relay plates, bellows, stretch marks, suspensions.

Springs, springs of machines and elastic elements of devices are characterized by a variety [source not specified 122 days] of shapes, sizes, various working conditions. The peculiarity of their work is that under large static, cyclic or shock loads, residual deformation is not allowed in them. In this regard, all spring alloys, in addition to the mechanical properties characteristic of all structural materials (strength, ductility, toughness, endurance), must have a high resistance to small plastic deformations. Under conditions of short-term static loading, resistance to small plastic deformations is characterized by the elastic limit, under long-term static or cyclic loading - by relaxation resistance

Classification

Steels are divided into structural and tool steels. A variety of tool is high-speed steel.

According to the chemical composition, steels are divided into carbon and alloyed; including by carbon content - into low-carbon (up to 0.25% C), medium-carbon (0.3-0.55% C) and high-carbon (0.6-0.85% C); Alloy steels according to the content of alloying elements are divided into low-alloyed, medium-alloyed and high-alloyed.

Steels, depending on the method of their production, contain different amounts of non-metallic inclusions. The content of impurities underlies the classification of steels by quality: ordinary quality, high quality, high quality and extra high quality.

According to the structure, steel differs in austenitic, ferritic, martensitic, bainitic or pearlitic. If the structure is dominated by two or more phases, then the steel is divided into two-phase and multi-phase.

Steel characteristics

Density - 7700-7900 kg/m³.

Specific gravity - 75537-77499 n / m³ (7700-7900 kgf / m³ in the MKGSS system).

Specific heat capacity at 20 °C - 462 J/(kg °C) (110 cal/(kg °C)).

Melting point - 1450-1520 °C.

The specific heat of fusion is 84 kJ/kg (20 kcal/kg).

Thermal conductivity coefficient - 39 kcal / (m h ° C) (45.5 W / (m K)). [source not specified 136 days]

Coefficient of linear thermal expansion at approx. 20 °C:

steel St3 (grade 20) - (1/deg);

stainless steel - (1/deg).

Tensile strength of steel:

steel for structures - 38-42 (kg / mm²);

silicon-chromium-manganese steel - 155 (kg / mm²);

engineering steel (carbon) - 32-80 (kg / mm²);

rail steel - 70-80 (kg / mm²);

An alloy of iron with carbon (usually more than 2.14%), characterized by a eutectic transformation. Carbon in cast iron can be contained in the form of cementite and graphite. Depending on the shape of graphite and the amount of cementite, there are: white, gray, malleable and high-strength cast irons. Cast irons contain permanent impurities (Si, Mn, S, P), and in some cases also alloying elements (Cr, Ni, V, Al, etc.). As a rule, cast iron is brittle. World production of pig iron in 2007 amounted to 953 million tons (including in China - 477 million tons).

Types of cast iron

white cast iron

In white cast iron, all carbon is in the form of cementite. The structure of such cast iron is perlite, ledeburite and cementite. This cast iron got its name because of the light color of the fracture.

Gray cast iron

Gray cast iron is an alloy of iron, silicon (from 1.2-3.5%) and carbon, also containing constant impurities of Mn, P, S. In the structure of such cast irons, most or all of the carbon is in the form of lamellar graphite. The fracture of such cast iron due to the presence of graphite has a gray color.

malleable iron

Ductile iron is obtained by long-term annealing of white iron, which results in the formation of flaky graphite. The metal base of such cast iron: ferrite and less often perlite.

Ductile iron

Ductile iron has nodular graphite in its structure, which is formed during the crystallization process. Spheroidal graphite does not weaken the metal base as much as lamellar graphite, and is not a stress concentrator.

half cast iron

In cast iron, part of the carbon (more than 0.8%) is contained in the form of cementite. The structural components of such cast iron are perlite, ledeburite and lamellar graphite.

Classification

Depending on the carbon content, gray cast iron is called hypoeutectic (2.14-4.3% carbon), eutectic (4.3%) or hypereutectic (4.3-6.67%). The composition of the alloy affects the structure of the material.

Depending on the state and content of carbon in cast iron, they are distinguished: white and gray (according to the color of the fracture, which is determined by the structure of carbon in cast iron in the form of iron carbide or free graphite), high-strength with nodular graphite, malleable cast iron, cast iron with vermicular graphite. In white cast iron, carbon is present in the form of cementite, in gray cast iron - mainly in the form of graphite.

In industry, varieties of cast iron are labeled as follows:

pig iron - P1, P2;

pig iron for castings - PL1, PL2,

conversion phosphorous cast iron - PF1, PF2, PF3,

conversion high-quality cast iron - PVK1, PVK2, PVK3;

cast iron with lamellar graphite - SCH (the numbers after the letters "SCH" indicate the value of the tensile strength in kgf / mm);

anti-friction cast iron

anti-friction gray - ASF,

antifriction high-strength - AChV,

antifriction malleable - AChK;

nodular cast iron for castings - HF (numbers after the letters "HF" mean tensile strength in kgf / mm and relative elongation (%);

cast iron alloyed with special properties - Ch.

3. Blast furnace,

blast furnace - a large metallurgical, vertically located shaft-type furnace for smelting cast iron, ferroalloys from iron ore raw materials. The first blast furnaces appeared in Europe in the middle of the 14th century, in Russia around 1630.

Description

The blast furnace is a structure with a height of up to 35 m, the height is limited by the strength of the coke, which holds the entire column of charge materials. The charge is loaded from above, through a typical loading device, which is also the gas seal of the blast furnace. In the blast furnace, rich iron ore is restored (at the present stage, reserves of rich iron ore have been preserved only in Australia and Brazil), sinter or pellets. Sometimes briquettes are used as ore raw materials.

The blast furnace consists of five structural elements: the upper cylindrical part - the top, which is necessary for loading and efficient distribution of the charge in the furnace; the largest in height expanding conical part - the mine, in which the processes of heating materials and the reduction of iron from oxides take place; the widest cylindrical part - steam, in which the processes of softening and melting of reduced iron take place; tapering conical part - shoulders, where a reducing gas is formed - carbon monoxide; the cylindrical part - the hearth, which serves to accumulate liquid products of the blast-furnace process - cast iron and slag.

In the upper part of the hearth there are lances - holes for supplying blast heated to a high temperature - compressed air enriched with oxygen and hydrocarbon fuel.

At the level of the lances, a temperature of about 2000 °C develops. As you move up, the temperature decreases, and at the tops it reaches about 270 ° C. Thus, different temperatures are set in the furnace at different heights, due to which various chemical processes of the transition of ore into metal take place.

Processes taking place in the furnace

In the upper part of the hearth, where the oxygen supply is high enough, the coke burns, forming carbon dioxide and releasing a large amount of heat.

C + O 2 \u003d CO 2 + Q

Carbon dioxide, leaving the zone enriched with oxygen, reacts with coke and forms carbon monoxide - the main reducing agent of the blast-furnace process.

Rising up, carbon monoxide interacts with iron oxides, taking away oxygen from them and reducing them to metal:

Fe 2 O 3 + 3CO \u003d 2Fe + 3CO 2

The iron obtained as a result of the reaction flows down in drops over the hot coke, being saturated with carbon, resulting in an alloy containing 2.14 - 6.67% carbon. This alloy is called cast iron. In addition to carbon, it includes a small proportion of silicon and manganese. In the amount of tenths of a percent, the composition of cast iron also includes harmful impurities - sulfur and phosphorus. In addition to cast iron, slag is formed and accumulated in the furnace, in which all harmful impurities are collected.

Previously, slag was tapped through a separate slag taphole. At present, both pig iron and slag are tapped through the Pig-iron tap-hole at the same time. The separation of pig iron and slag takes place already outside the blast furnace - in the chute, using a separating plate. The pig iron separated from the slag is poured into pig iron ladles and transported to the steelmaking shop.

Today, there is almost no sphere of human life where cast iron is not used. This material has been known to mankind for quite a long time and has proven itself excellently from a practical point of view. Cast iron is the basis of a great variety of parts, assemblies and mechanisms, and in some cases even a self-sufficient product capable of performing the functions assigned to it. Therefore, in this article we will pay close attention to this iron-containing compound. We will also find out what types of cast iron are, their physical and chemical features.

Definition

Cast iron is a truly unique alloy of iron and carbon, in which Fe is more than 90%, and C is not more than 6.67%, but not less than 2.14%. Also, carbon can be found in cast iron in the form of cementite or graphite.

Carbon gives the alloy a sufficiently high hardness, however, at the same time, it reduces malleability and ductility. As a result, cast iron is a brittle material. Also, special additives are added to certain grades of cast iron, which can give the compound certain properties. The role of alloying elements can be: nickel, chromium, vanadium, aluminum. The density index of cast iron is 7200 kilograms per cubic meter. From which we can conclude that the weight of cast iron is an indicator that cannot be called small.

History reference

Iron smelting has long been known to man. The first mention of the alloy dates back to the sixth century BC.

In China, in ancient times cast iron was produced with a rather low melting point. Cast iron began to be produced in Europe around the 14th century, when blast furnaces were first used. At that time, such iron casting was used for the production of weapons, shells, and parts for construction.

On the territory of Russia, the production of cast iron actively began in the 16th century and then expanded rapidly. During the time of Peter I, the Russian Empire was able to bypass all the states of the world in terms of iron production, but after a hundred years it began to lose ground again in the ferrous metallurgy market.

Cast iron has been used to create a variety of works of art since the Middle Ages. In particular, in the 10th century, Chinese masters cast a truly unique figure of a lion, whose weight exceeded 100 tons. Starting from the 15th century in Germany, and after that in other countries, cast iron casting became widespread. Fences, lattices, park sculptures, garden furniture, tombstones were made from it.

In the last years of the 18th century, iron casting was most involved in the architecture of Russia. And the 19th century was generally called the "cast iron age", since the alloy was very actively used in architecture.

Peculiarities

There are various types of cast iron, but the average melting point of this metal compound is about 1200 degrees Celsius. This figure is 250-300 degrees less than required for steel smelting. This difference is associated with a rather high carbon content, which leads to its less close bonds with iron atoms at the molecular level.

At the time of smelting and subsequent crystallization, the carbon contained in cast iron does not have time to completely penetrate into the molecular lattice of iron, and therefore cast iron eventually turns out to be quite brittle. In this regard, it is not used where there are constant dynamic loads. But at the same time, it is excellent for those parts that have increased requirements for strength.

Production technology

Absolutely all types of cast iron are produced in a blast furnace. Actually, the smelting process itself is a rather laborious activity that requires serious material investments. One ton of pig iron requires about 550 kilograms of coke and almost a ton of water. The volume of ore loaded into the furnace will depend on the iron content. Most often, ore is used, in which iron is at least 70%. A lower concentration of the element is undesirable, since it would be economically unprofitable to use it.

First stage of production

The smelting of iron occurs as follows. First of all, ore is poured into the furnace, as well as coking products, which serve to pressurize and maintain the required temperature inside. In addition, these products during the combustion process actively take part in the ongoing chemical reactions as iron reducing agents.

In parallel, a flux is loaded into the furnace, which serves as a catalyst. It helps the rocks to melt faster, which promotes the rapid release of iron.

It is important to note that the ore is subjected to special pre-treatment before being loaded into the furnace. It is crushed in a crushing plant (small particles melt faster). It is then washed to remove metal-free particles. After that, the raw material is fired, due to this, sulfur and other foreign elements are removed from it.

Second stage of production

Natural gas is fed into the furnace loaded and ready for operation through special burners. The coke heats up the raw material. In this case, carbon is released, which combines with oxygen and forms an oxide. This oxide subsequently takes part in the recovery of iron from the ore. Note that with an increase in the amount of gas in the furnace, the rate of the chemical reaction decreases, and when a certain ratio is reached, it stops altogether.

Excess carbon penetrates the melt and joins with the iron, eventually forming pig iron. All those elements that have not melted are on the surface and are eventually removed. This waste is called slag. It can also be used to produce other materials. The types of cast iron obtained in this way are called foundry and pig iron.

Differentiation

The modern classification of cast irons provides for the division of these alloys into the following types:

• White.
• Half.
• Gray with lamellar graphite.
• High-strength with nodular graphite.
• malleable.

Let's look at each type separately.

white cast iron

Such cast iron is one in which almost all of the carbon is chemically bonded. In mechanical engineering, this alloy is not used very often, because it is hard, but very brittle. Also, it cannot be machined with various cutting tools, and therefore is used for casting parts that do not require any processing. Although this type of cast iron allows grinding with abrasive wheels. White cast iron can be both ordinary and alloyed. At the same time, welding it causes difficulties, since it is accompanied by the formation of various cracks during cooling or heating, and also due to the inhomogeneity of the structure formed at the welding point.

White wear-resistant cast irons are obtained due to the primary crystallization of a liquid alloy during rapid cooling. Most often they are used to work in dry friction conditions (for example, brake pads) or for the production of parts with increased wear resistance and heat resistance of mills).

By the way, it got its name due to the fact that the appearance of its fracture is a light-crystalline, radiant surface. The structure of this cast iron is a combination of ledeburite, perlite and secondary cementite. If this cast iron is alloyed, then pearlite is transformed into troostite, austenite or martensite.

half cast iron

The classification of cast irons will be incomplete if this type of metal alloy is not mentioned.

This cast iron is characterized by a combination of carbide eutectics and graphite in its structure. In general, the full-fledged structure has the following form: graphite, pearlite, ledeburite. If the cast iron is subjected to heat treatment or alloying, then this will lead to the formation of austenite, martensite or acicular troostite.

This type of cast iron is quite brittle, so its use is very limited. The alloy itself got its name because its fracture is a combination of dark and light areas of the crystalline structure.

The most common engineering material

Gray cast iron GOST 1412-85 contains about 3.5% carbon, from 1.9 to 2.5% silicon, up to 0.8% manganese, up to 0.3% phosphorus and less than 0.12% sulfur.

Graphite in such cast iron has a lamellar shape. It does not require any special modification.

Graphite plates have a strong weakening effect and therefore gray cast iron is characterized by a very low impact strength and an almost complete absence of relative elongation (the indicator is less than 0.5%).

Gray cast iron is well machined. The structure of the alloy can be as follows:

• Ferrite-graphite.
• Ferrite-pearlite-graphite.
• Perlite-graphite.

Gray cast iron performs much better in compression than in tension. It also welds quite well, but this requires preheating, and special cast iron rods with a high content of silicon and carbon should be used as filler material. Without preheating, welding will be difficult, since the cast iron will bleach in the weld zone.

Gray cast iron is used to produce parts that operate in the absence of shock loading (pulleys, covers, beds).

The designation of this cast iron occurs according to this principle: SCh 25-52. Two letters indicate that this is gray cast iron, the number 25 is an indicator of tensile strength (in MPa or kgf / mm 2), the number 52 is the tensile strength at the moment of bending.

Ductile iron

Nodular cast iron is fundamentally different from its other "brothers" in that it contains nodular graphite. It is obtained by introducing special modifiers (Mg, Ce) into the liquid alloy. The number of graphite inclusions and their linear dimensions can be different.

What is good about spheroidal graphite? The fact that such a form minimally weakens the metal base, which, in turn, can be pearlitic, ferritic or pearlitic-ferritic.

Due to the use of heat treatment or alloying, the cast iron base can be acicular-troostite, martensitic, austenitic.

Ductile iron grades are different, but in general terms, its designation is as follows: VCh 40-5. It is easy to guess that HF ​​is high-strength cast iron, the number 40 is an indicator of tensile strength (kgf / mm 2), the number 5 is relative elongation, expressed as a percentage.

malleable iron

The structure of malleable iron is the presence of graphite in it in flaky or spherical form. In this case, flaky graphite can have different fineness and compactness, which, in turn, has a direct effect on the mechanical properties of cast iron.

In industry, ductile iron is often produced with a ferritic base, which provides greater ductility.

The fracture appearance of ferritic ductile iron has a black velvety appearance. The higher the amount of perlite in the structure, the lighter the fracture will become.

In general, it is obtained from white cast iron castings due to prolonged languishing in furnaces heated to a temperature of 800-950 degrees Celsius.

Today, there are two ways of making ductile iron: European and American.

The American method consists in languishing the alloy in sand at a temperature of 800-850 degrees. In this process, graphite is located between grains of the purest iron. As a result, cast iron becomes viscous.

In the European method, castings are languishing in iron ore. The temperature at the same time is about 850-950 degrees Celsius. Carbon passes into iron ore, due to which the surface layer of the castings is decarburized and becomes soft. Cast iron becomes malleable, while the core remains brittle.

Ductile iron marking: KCh 40-6, where KCh is, of course, malleable iron; 40 - indicator of tensile strength; 6 - elongation, %.

Other indicators

As for the division of cast irons by strength, the following classification is applied here:

• Normal strength: σv up to 20 kg/mm ​​2 .
• Increased strength: σv \u003d 20 - 38 kg / mm 2.
• High strength: σv = 40 kg/mm ​​2 and above.

According to plasticity, cast irons are divided into:

• Non-plastic - elongation less than 1%.
• Low-plastic - from 1% to 5%.
• Plastic - from 5% to 10%.
• Increased plasticity - more than 10%.

In conclusion, I would also like to note that the properties of any cast iron are quite significantly affected even by the shape and nature of the pour.

The carbon in cast iron may be in the form of cementite (Fe3C) or graphite. Cementite is light in color, has high hardness and is difficult to machine. Graphite, on the contrary, is dark in color and quite soft. Depending on which form of carbon prevails in the structure, there are: white, gray, ductile and high-strength cast irons. Cast irons contain permanent impurities (Si, Mn, S, P), and in some cases also alloying elements (Cr, Ni, V, Al, etc.).

white cast iron- a type of cast iron in which carbon in the bound state is in the form of cementite, in a fracture it has a white color and a metallic luster. There are no visible inclusions of graphite in the structure of such cast iron, and only a small part of it (0.03-0.30%) is detected by fine methods of chemical analysis or visually at high magnifications. White cast iron castings have wear resistance, relative heat resistance and corrosion resistance. The strength of white cast iron decreases and the hardness increases with increasing carbon content.

White cast iron is very hard, almost impossible to machine, and therefore is not used for the manufacture of parts, but is used for conversion into steel and for the manufacture of parts from ductile iron. Such cast iron is also called pig iron.

Gray cast iron- an alloy of iron, silicon (from 1.2-3.5%) and carbon, also containing permanent impurities Mn, P, S. In the structure of such cast irons, most or all of the carbon is in the form of lamellar graphite. The fracture of such cast iron due to the presence of graphite has a gray color. A separate type (group of grades) of gray cast iron is ductile cast iron with globular (spherical) graphite, which is achieved by modifying it with magnesium (Mg), cerium (Ce) or other elements.

Gray cast iron is characterized by high casting properties (low crystallization temperature, fluidity in the liquid state, low shrinkage) and serves as the main casting material. It is widely used in mechanical engineering for casting machine and mechanism frames, pistons, and cylinders.

The high brittleness inherent gray cast iron due to the presence of graphite in their structure, makes it impossible to use them for parts that work mainly "in tension" or "in bending"; cast irons are used only when working "in compression".

Gray cast iron is marked with the letters СЧ, after which the guaranteed value of the tensile strength in kg / mm² is indicated, for example СЧ30. Ductile irons are marked with the letters HF , after which the strength is indicated and, through a dash, the relative elongation in percent, for example VCh60-2.

malleable iron- the conditional name of soft and ductile cast iron, obtained from white cast iron by casting and further heat treatment. Long-term annealing is used, which results in the decomposition of cementite with the formation of graphite, that is, the process of graphitization, and therefore such annealing is called graphitizing.

Ductile iron, like gray iron, consists of a steel base and contains carbon in the form of graphite, but the graphite inclusions in ductile iron are different from those in ordinary gray iron. The difference is that the inclusions of graphite in malleable iron are in the form of flakes, which are formed during annealing, and are isolated from each other, as a result of which the metal base is less separated, and the cast iron has some toughness and ductility. Because of its flaky shape and method of preparation (annealing), graphite in ductile iron is often referred to as annealed carbon. Malleable cast iron got its name from its increased ductility and toughness (although it is not subjected to pressure treatment).

Ductile iron has high tensile strength and high impact resistance. Parts of complex shape are made from ductile iron: rear axle housings of cars, brake pads, tees, squares, etc.

Ductile iron is marked with two letters and two numbers, for example KCh 370-12. The letters KCh mean ductile iron, the first number is the tensile strength (in MPa) at break, the second number is the relative elongation (in percent), characterizing the ductility of the cast iron.

Ductile iron- cast iron, having spheroidal graphite inclusions. Spheroidal graphite has a smaller ratio of its surface to volume, which determines the greatest continuity of the metal base, and hence the strength of cast iron.

Ductile iron is most often used for the manufacture of critical products in mechanical engineering, as well as for the production of high-strength pipes (water supply, sewerage, gas and oil pipelines). Ductile iron products and pipes are characterized by high strength, durability, and high performance properties.