Cast iron began to be used many decades ago. This material has special performance characteristics that differ from those characteristic of steel. The production of cast iron, despite the emergence of a large number of different alloys, is established in many countries. In order to determine the properties of cast iron, one should consider the features of its chemical composition, on which certain physical qualities depend.

The chemical composition of cast iron is important factor, which largely determines the mechanical properties of the resulting castings. In addition, many properties are influenced by the mechanisms of primary and secondary crystallization.

Considering the chemical composition of cast iron, it should be noted that, in addition to iron and carbon, it necessarily includes the following elements:

1. Silicon (concentration no more than 4.3%). This element has beneficial effect on cast iron, making it softer and improving its casting properties. However, too high concentration can make the material more susceptible to plastic deformation.
2. Manganese (no more than 2%). By adding this element to the composition, the strength of the material significantly increases. However, too high a concentration can cause the structure to become brittle.
3. Sulfur is a harmful impurity that can significantly impair the performance of the material. As a rule, the concentration of sulfur in cast iron does not exceed 0.07%. Sulfur causes cracks to appear when the composition is heated.
4. Phosphorus is contained in the composition in a concentration of less than 1.2%. An increase in the concentration of phosphorus in the composition causes cracks to appear when the composition cools. Besides this, this element causes deterioration of other mechanical qualities.

As with many other compounds, the most important of chemical elements cast iron is carbon. The type of material depends on its concentration and type. The structure of cast iron can vary significantly depending on the production technology used.

Physical properties

Cast iron has become widespread due to its attractive physical properties:

1. The cost of the material is significantly lower than the cost of other alloys. That is why it is used to create a wide variety of products.
2. Considering the density of cast iron, we note that this indicator significantly lower than that of steel, making the material much lighter.
3. The melting point of cast iron can vary slightly depending on its structure, but in most cases it is 1,200 degrees Celsius. Due to the inclusion of various additives in the composition, the melting temperature of cast iron can significantly increase or decrease.
4. When choosing a material, many pay attention to the fact that the color of cast iron may differ slightly depending on the structure and chemical composition.

The boiling point of cast iron also largely depends on the chemical composition. In order to consider physical properties material, attention should be paid to each of its varieties. A different structure and chemical composition cause different physical and mechanical properties to be imparted.

Production technology

Cast iron has been smelted for several decades, due to its unique performance qualities. The large number of varieties of alloys determines the application of special marking rules. Marking of cast iron is carried out as follows:

1. Foundries are designated by the letter L.
2. Gray has become widespread; the combination of letters “SCH” is used to designate it.
3. Malleable is designated KCH.
4. Extreme or white is designated by the letter P.
5. Anti-friction or gray indicate ASF.
6. Alloy cast irons can have a wide variety of chemical compositions and are designated by the letter “C”.

The cast iron production technology involves several stages that make it possible to obtain the required structure. Considering the process of producing cast iron, we note the following points:

1. Production is carried out in special blast furnaces.
2. Alloyed and heat-resistant cast iron can be obtained by using iron ore as a raw material.
3. The technology is presented in the reduction of iron oxides ore. As a result of the restructuring of the crystal lattice and changes in the structure, the output is a material called cast iron.
4. Considering production methods, we note that the features of the technology also lie in the materials used - cokes. By coke we mean natural gas or thermal anthracite, acting as fuel.
5. The production of cast iron involves tempering iron in solid form using a special furnace. At this stage, liquid cast iron is obtained.

Equipment for the production of cast iron can vary significantly. In addition, the production technology used largely determines what kind of material will be obtained. An example is the production of ductile iron, which involves giving the structure an unusual shape.

Types of cast iron

There is quite large number varieties of the material in question. The classification of cast iron largely depends on the structure and chemical composition. Highlight the following types cast iron:

1. . This type of material is characterized by low ductility and high viscosity, as well as good machinability. Carbon is contained in the form of graphite. Area of ​​application – mechanical engineering; production of wear parts. As practice shows, phosphorus concentration can vary over a fairly wide range: from 0.3 to 1.2%. Due to its special chemical composition, the material has high fluidity and is often used in artistic casting. Anti-friction cast iron is relatively low in cost, which also determines its wide distribution.
2. . Due to the fact that in this composition carbon is presented as cementite, the structure is characterized by extreme fragility and increased hardness, as well as low casting properties and poor machinability. It is worth considering that white cast iron is used for conversion into steel or for the manufacture of malleable iron. Very often it is called the limit.
3. Half-type is characterized by increased wear resistance, which is associated with the distribution of carbon into a cementite and free base. Often this type of material is used in mechanical engineering and machine tool building.
4. Alloy. In order to give special properties Cast iron is also alloyed. Alloy cast iron has increased wear resistance, corrosion resistance due to the inclusion of nickel and chromium, as well as copper. Such versions of cast iron get their name depending on how the alloying element was used in their manufacture.
5. High-strength cast iron is produced by adding liquid gray cast iron various elements, for example, magnesium and calcium. As a result of alloying, the shape of graphite changes - it resembles a sphere and does not change the crystal lattice. It is worth considering that in its properties this metal resembles carbon steel and is used mainly in the manufacture of various wear-resistant parts.
6. Malleable. It is obtained by melting white cast iron, which should be heated to high temperature and maintain it in this condition. In some cases, alloying elements are added to impart special properties to the composition. The main properties include high viscosity and an increased degree of plasticity. Widespread in the engineering industry.
7. Special. It is an alloy that contains a large amount of manganese and silicon. It is often used to remove oxygen from steel during its production or remelting, thereby lowering the melting point.

Each type of cast iron has its own special structure and chemical composition, which determine the scope of application.

Application

Due to its special physical and mechanical qualities, the use of cast iron has become possible in a wide variety of areas:

1. For the production of various parts in the mechanical engineering industry. For many years, this alloy has been used in the manufacture of a wide variety of parts for internal combustion engines. At the same time, automakers change the basic properties of the material by alloying it, which is necessary to achieve unique qualities. In addition, brake pads made of this alloy have become widespread.
2. Cast iron products can withstand low temperatures. Therefore, the material is used in the production of equipment and tools that are used in harsh climatic conditions.
3. Cast iron is valued in the metallurgical field. This is due to the low cost, which largely depends on the carbon concentration and the characteristics of the resulting structure. High castability also makes the material more attractive. The resulting products are characterized by high strength and wear resistance.
4. Over the past few decades, the alloy in question has been widely used in the manufacture of sanitary equipment. This is due to high anti-corrosion abilities, as well as the possibility of obtaining products by ourselves various shapes. Examples include cast iron bathtubs and radiators, various pipes, radiators and sinks. Despite the emergence of materials that could replace cast iron, such products are very popular. This is due to the fact that they retain their original appearance throughout long period operation.
5. The alloy is also used for the manufacture of various decorative elements, which is associated with high casting qualities. An example is a railing grid, various figurines and much more.

In addition, the scope of application depends on the following properties of the material in question:

1. Some brands have high strength, which is characteristic of steel. That is why the material is used even after the advent of modern alloys.
2. Cast iron products can retain heat for a long period. At the same time thermal energy can spread evenly throughout the material. These qualities began to be used in the manufacture of heating radiators or other similar products.
3. It is generally accepted that cast iron is an environmentally friendly material. That is why it is often used in the manufacture various dishes, for example, a cauldron.
4. High resistance to acid-base environments.
5. Highly hygienic, since all contaminants can be easily removed from the surface.
6. The material under consideration is characterized sufficiently long term service provided that the operating instructions are followed.
7. Included in chemicals cannot cause harm to health.

In conclusion, we note that for a long time open technology production of the material in question has remained virtually unchanged over the years. This is due to the fact that a large volume of molten alloy could be obtained at relatively low cost. Today, material is often produced from scrap, which makes it possible to further reduce the cost of the resulting product.

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

Cast irons differ in structure, manufacturing methods, chemical composition and purpose.
The structure of cast iron is gray, white and malleable. According to manufacturing methods - ordinary and modified.
Based on their chemical composition, cast iron is divided into unalloyed and alloyed, i.e., those that contain special impurities.

Gray cast iron

Gray cast iron is most widely used in mechanical engineering for casting various machine parts. It is characterized by the fact that the carbon in it is in a free state in the form of graphite. Therefore, gray cast iron can be easily processed with cutting tools. When broken, it has a gray and dark gray color. Gray cast iron is produced by slow cooling after melting or heating. The production of gray cast iron is also facilitated by an increase in the content of carbon and silicon in its composition.
The mechanical properties of gray cast iron depend on its structure.
The structure of gray cast iron is:

1. ferrite-graft,
2. ferrite-derlite-graphite and
3. pearlite-graphite.

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

Gray cast iron can be welded quite well using preheating and as a filler material for special cast iron rods with increased content carbon and silicon. Welding without preheating is difficult due to the bleaching of cast iron in the weld areas.

White cast iron

White cast iron is used in mechanical engineering in significantly smaller quantities than gray cast iron. It is an alloy of iron and carbon, in which carbon is in the form chemical compound with iron. White cast iron is very brittle and hard. He doesn't give in machining cutting tools and is used for casting parts that do not require processing, or is subjected to grinding with abrasive wheels. In mechanical engineering, white cast iron is used, both ordinary and alloyed.

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

Malleable iron

Malleable cast iron is usually obtained from white cast iron castings by simmering them for a long time in furnaces at a temperature of 800-950°C. There are two methods for producing malleable cast iron: American and European.

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

In the European method, castings are simmered in iron ore at a temperature of 850-950°. In this case, 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 decarbonized and becomes soft, which is why cast iron is called malleable, although the core remains brittle.

In the designations of malleable cast iron grades, after the letters a number is written indicating average value tensile strength in kg/mm2, followed by a number indicating elongation in %.

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

Modified cast iron

Modified cast iron differs from ordinary gray cast iron in that it more carbon is in the form of graphite than in gray cast iron.

The modification consists in the fact that when cast iron is melted, a certain amount of additives is added to the liquid metal, which promotes the release of carbon in the form of graphite during solidification and cooling. This modification process at the same chemical composition 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.

Gray cast iron has low mechanical characteristics. St. in tensile tests. Graphite inclusions play the role of stress concentrators. The hardness and strength in compression tests, which depend on the properties of the metal base, are quite high in cast iron. Gray cast iron with flake graphite has a number of advantages. It allows you to obtain cheap castings, because at low cost it has good fluidity and low shrinkage. Fur. The properties of gray cast iron depend on the metal base, as well as the shape and size of graphite inclusions. The most durable are gray cast irons based on pearlite, and the most ductile are gray cast irons based on ferrites. Gray cast iron is produced by adding substances to molten metal that promote the decomposition of cementite and the release of carbon in the form of graphite. For gray cast iron, the graphitizer is silicon. When about 5% silicon is introduced into the alloy, gray cast iron cementite almost completely disintegrates and a structure of a plastic ferrite base and graphite inclusions is formed. With a decrease in silicon content, cementite, which is part of pearlite, partially disintegrates and a ferrite-pearlite structure with graphite inclusions is formed. With a further decrease in silicon content, a pearlite-based gray cast iron structure with graphite inclusions is formed.

Inclusions of graphite make the chips brittle, so cast iron is well processed by cutting. Thanks to the lubricating effect of graphite, cast iron has good anti-friction properties. Cast iron has high damping properties and dampens vibrations and resonant vibrations well. Marked gray cast iron with the letters SC and numbers characterizing the value of tensile strength during tensile tests. Nr, SCH10 contains (3.5...3.7)% C, (2.2...2.6)% Si, (0.5...0.8)% Mn, P<0,3% и S<0,15%, d В =100МПа, твёрдость <190НВ. SCH35 d V =350MPa, hardness<275НВ.

Gray cast iron - This is cast iron. Gray cast iron comes into production in the form of castings. Gray cast iron is a cheap construction material. It has good casting properties, is easy to cut, resists wear, and has the ability to dissipate vibrations under vibration and variable loads. The property of damping vibrations is called damping capacity. The damping capacity of cast iron is 2-4 times higher than that of steel. High damping strength and wear resistance have led to the use of cast iron for the manufacture of frames of various equipment, crankshafts and camshafts of tractor and automobile engines, etc. The following grades of gray cast iron are produced (the numerical values ​​of hardness NV are indicated in brackets): SCh 10 (143-29), SCh 15(163-229), SCh 20(170-241), SCh 25(180-250), SCh 30(181-255), SCh 35(197-269), SCh 40(207-285), SCh 45 (229-289).

According to physical and mechanical characteristics, gray cast iron can be divided into four groups: low strength, high strength, high strength and with special properties.

Alloy gray cast iron It has a fine-grained structure and a better graphite structure due to the addition of small amounts of nickel and chromium, molybdenum and sometimes titanium or copper.

Modified gray cast iron has a uniform structure over the cross-section of the casting and a smaller swirling form of graphite. Modifiers - ferrosilicon, silicoaluminum, silicocalcium, etc. - are added in an amount of 0.1 -0.3% by weight of cast iron directly into the ladle during its filling.

Gray and white cast iron differ sharply in properties. White cast iron very hard and brittle, difficult to process with cutting tools, they are melted into steel and are called pig iron. Part of the white cast iron is used to produce malleable cast iron.

White cast irons are used as wear-resistant structural materials. In such cast irons, all carbon is in a bound state with carbide-forming elements (chrome, manganese, boron, titanium). With the introduction of 5-8% Cr, a cementite-type carbide (Fe,Cr) 3 C is formed, and with a content of more than 10% Cr, complex and hard carbides (Fe, Cr) 7 C 3 and (Fe, Cr) 23 C 6 are formed. To give cast iron greater viscosity, heat or corrosion resistance, nickel is introduced into its composition.

1. DEFINITION

Cast iron is usually called iron-carbon alloys containing carbon under normal crystallization conditions above the solubility limit 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 given definition, which forms the basis for the classification of conventional iron-carbon alloys, is not always sufficient.

In fact, carbide eutectic is found not only in cast iron, but also in high-alloy steels containing little carbon (less than 2%), for example, in high-speed steels. The issue with graphite eutectic is also complicated, since secondary and eutectoid graphite are not distinguished separately. Based on 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 (a much higher solubility limit in austenite). The generally accepted boundaries between cast iron and steel with a carbon content of 2% or more are arbitrary, 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 with graphite inclusions. The latter have a very positive effect on the wear resistance and cyclic viscosity of cast iron.

The most important classification characteristics also include mechanical properties (and for special-purpose cast irons, special properties), composition of castings, production technology, design of castings and areas of their 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 primarily include the number, shape and nature of the distribution of graphite inclusions.

2. CLASSIFICATION BY CHEMICAL COMPOSITION

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

Based on their chemical composition, cast irons are divided into unalloyed and alloyed.

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

With low and moderate alloying, they strive to improve the general properties of cast iron - uniformity of structure, preservation of strength and elasticity when heated to relatively low temperatures - 300-400 °, increased wear resistance, increased strength, etc.

With medium, increased 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 becomes most important. By alloying, martensite, acicular troostite and austenite can be obtained directly in the cast state. This increases corrosion resistance, heat resistance and changes magnetic properties.

3. CLASSIFICATION BY STRUCTURE AND CONDITIONS OF GRAPHITE FORMATION

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

b) half-hearted,

c) gray with flake graphite,

d) high-strength with spherical graphite and

d) 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 iron is slightly graphitized, and the remaining cast iron is significantly graphitized (Fig. 1).

Fig. 1. Scheme of classification of cast irons according to the degree of graphitization, type of fracture, shape and conditions of graphite formation

In white and half cast irons the presence of ledeburite is obligatory, but in significantly graphitized cast irons there should be no ledeburite.

The structure of cast iron in one casting can be different and belong to different types of cast iron; sometimes even special efforts are made to obtain different structures in different layers, for example, in the production of bleached rolling rolls and crushing balls. The outer layers consist of white cast iron, the transition layers of half-cast iron, the core of highly graphitized cast iron.

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

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

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

In Fig. Figure 2 shows the microstructure of unalloyed white hypoeutectic cast iron, consisting of ledeburite, pearlite and secondary cementite. In alloyed or heat-treated cast irons, instead of pearlite, there may be troostite, martensite or austenite.

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

It is called white cast iron because its fracture pattern 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, the structure also contains graphite. This means that the amount of bound carbon exceeds its limiting solubility in austenite under real solidification conditions.

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

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

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

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

Rice. 4. Flake graphite (straight). x100

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

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

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

Rice. 7. Interdendritic graphite. x100.

Rice. 8. Rosette graphite. x100.

Rice. 9. Swirl graphite. x100.

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

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

Rice. 12. Nodular graphite. x400.

Rice. 13. High-strength pearlite. x400.

Rice. 14. Pearlite-ferritic high-strength cast iron. x100.

Rice. 15. Ferritic ductile iron. x200.

Graphite in Fig. 4 is called straight, or large: in contrast to the vortex shown in Fig. 9.

According to the predominant length of the sections on the thin section, graphite inclusions are divided into ten groups 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 very low impact strength.

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

Rice. 16. Flaky and crab-shaped graphites.

Rice. 17. Malleable cast iron with a ferritic base.

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

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

The spherical form of graphite is the most favorable of all known forms. Nodular graphite is less likely than other forms of graphite to weaken the metal base. Depending on the required properties, the metal base of high-strength cast iron can be pearlitic (Fig. 13), pearlitic-ferritic (Fig. 14) and ferritic (Fig. 15). By alloying and heat treatment, an austenitic, martensitic or acicular-troostite base can be obtained.

Castings from high-strength cast iron, like gray cast iron, can be produced in any thickness.

d) Malleable cast iron (DC). The main difference between malleable cast iron is that the graphite in it has a flake or spherical shape. Flaky graphite comes in different compactness and dispersion (Fig. 16 L, B, C, D), which affects the mechanical properties of cast iron.

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

The fracture of ferritic malleable cast iron is black and 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 smelting method and the method of processing liquid cast iron.

The properties of cast iron are also greatly influenced by the condition of the mold and the nature of the pouring into it. According to the method of producing castings, iron casting can be divided into chill casting (refinement of 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. Using heat treatment, you can change the degree of dispersion of the metal base and its character, up to its transformation into acicular-troostite and martensitic. Up to a certain limit, the amount of bound carbon can be changed, and with chemical-thermal treatment, the composition of cast iron can be changed in the surface layers. Based on the type of heat treatment, castings can be divided into annealed, normalized, improved, surface-hardened, nitrided, etc.

6. CLASSIFICATION BY TYPES OF CASTINGS AND AREAS OF THEIR APPLICATION

Cast iron castings, according to the types of castings and their areas of application, can be divided into machine tool, cylinder, automobile, bearing, rolling rolls made of bleached cast iron, etc.

Of the above classifications, the most clear 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 branches of mechanical engineering.

It is necessary to distinguish the main (defining) features of classification - the form of graphite from clarifying features, which include the nature of the metal base, manufacturing method, etc. For example, it is not enough to say gray cast iron (lamellar graphite), it is necessary to clarify which gray cast iron is based on metal basis, how it is obtained (by modification or heat treatment), whether it is alloyed and with what it is alloyed.

It was first mastered in China back in the 10th century, after which it became widespread in other countries of the world. The basis of cast iron is an alloy of iron with carbon and other components. A distinctive feature is that cast iron contains more than 2% carbon in the form of cementite, which is not found in other metals. A prominent representative of such an alloy is white cast iron, which is used in mechanical engineering for the manufacture of parts, in industry and in everyday life.

Appearance

The alloy has a white color when fractured and a characteristic metallic luster. The structure of white cast iron is fine-grained.

Properties

In comparison with other metals, iron-carbon alloy has the following characteristics and properties:

• high fragility;
• increased hardness;
• high resistivity;
• low casting properties;
• low machinability;
• good heat resistance;
• large shrinkage (up to 2%) and poor filling;
• low impact resistance;
• high wear resistance.

The metal mass has great corrosion resistance in hydrochloric or nitric acid. If there are free carbides in the structure, then corrosion will occur when cast iron is placed in sulfuric acid.

White cast irons, which contain a lower percentage of carbon, are considered more resistant to high temperatures. Due to the increased mechanical strength and toughness that appear when exposed to high temperatures, the formation of cracks in castings is minimized.

Compound

Iron-carbon alloy is considered a cheaper material compared to steel. White cast iron contains iron and carbon, which are in a chemically bonded state. Excess carbon, which is not present in the solid solution of iron, is contained in a combined state in the form of iron carbides (cementite), and in alloyed cast iron in the form of special carbides.

Species

Depending on the amount of carbon content in white cast iron is divided into the following types:

1. Hypoeutectic contains from 2.14% to 4.3% carbon and, after complete cooling, acquires the structure of pearlite, secondary cementite and ledeburite.
2. Eutectic contains 4.3% carbon and has a structure in the form of a light background of cementite, which is dotted with dark pearlite grains.
3. Hypereutectic has from 4.3% to 6.67% carbon in its composition.

Application

Based on the above properties, we can conclude that it makes no sense to practice thermal and mechanical treatment of white cast iron. The alloy found its main application only in the form of casting. Consequently, white cast iron obtains its best properties only if all casting conditions are met. This processing method is actively used if it is necessary to produce massive products that must have high surface hardness.

In addition, white cast iron is annealed, resulting in malleable cast iron, which is used for the production of thin-walled castings, for example:

• automobile parts;
• products for agriculture;
• parts for tractors, combines, etc.

The alloy is also used for the manufacture of slabs with a ribbed or smooth surface, and is also actively used for gray cast iron.

The use of white cast iron in agriculture as a structural metal is quite limited. Most often, iron-carbon alloy is used for the manufacture of parts for hydraulic machines, sand throwers and other mechanisms that can operate under conditions of increased abrasive wear.

Bleached cast irons

This alloy is considered a type of white cast iron. It is possible to achieve a chill of 12-30 mm by rapidly cooling the surface of the iron-carbon alloy. Material structure: the surface part is made of white, gray cast iron in the core. Wheels and balls for mills are made from this material, which are mounted in machines for processing sheet metal.

Alloying elements of the alloy

Specially introduced alloying substances added to the composition of white cast iron can impart greater wear resistance and strength, corrosion resistance and heat resistance. Depending on the amount of added substances, the following are distinguished:

• low alloy alloy (up to 2.5% excipients);
• medium alloyed (from 2.5% to 10%);
• highly alloyed (more than 10%).

Alloying elements can be added to the alloy:

• chromium;
• sulfur;
• nickel;
• copper;
• molybdenum;
• titanium;
• vanadium,
• silicon;
• aluminum;
• manganese.

Alloyed white cast iron has improved properties and is often used for casting turbines, blades, mills, parts for cement and conventional furnaces, pumping machine blades, etc. The iron-carbon alloy is processed in two furnaces, which allows the material to be brought to a certain chemical composition:

• in a cupola;
• in electric melting furnaces.

Castings made of white cast iron are annealed in furnaces to stabilize the required dimensions and relieve internal stress. The annealing temperature can increase to 850 degrees. The heating and cooling process must be done slowly.

The marking or designation of white cast iron with impurities begins with the letter H. Which alloying elements are contained in the alloy can be determined by the subsequent letters of the marking. The name may contain numbers that indicate the amount in percentage terms of additional substances that are contained in white cast iron. If the marking contains the designation Ш, this means that the alloy structure contains spherical graphite.

Types of annealing

To form white cast iron, industry uses rapid cooling of the alloy. Today, the following main types of carbon alloy annealing are actively used:

• softening annealing is used primarily to increase the ferrite content of cast iron;
• annealing to relieve internal stresses and minimize phase transformations;
• graphitizing annealing, as a result of which it is possible to obtain;
• normalization at a temperature of 850-960 degrees, resulting in graphite and perlite, and also increases wear resistance and strength.

Additional information

Today it has been proven that there is no direct relationship between wear resistance and hardness of a carbon alloy. Only due to the structure, namely the arrangement of carbides and phosphides in the form of a regular network or in the form of uniform inclusions, increased wear resistance is achieved.

The strength of white cast iron is most strongly influenced by the amount of carbon, and the hardness depends on the carbides. The greatest strength and hardness are those cast irons that have a martensitic structure.