in what state and form is carbon found in gray cast iron?

In what state and form is carbon found in gray cast iron?

What is cast iron?
Cast iron is an alloy of iron and carbon obtained by smelting iron ore in blast furnaces. Cast iron contains from 2 to 5% carbon. How are blast furnace cast irons divided depending on their chemical composition and purpose? Depending on the chemical composition and purpose, blast furnace cast iron is divided into pig iron, foundry cast iron and special cast iron.

How are cast irons separated depending on the nature of the carbon-iron compound?

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Depending on the nature of the combination of carbon with iron, cast iron is divided into white and gray.

What is characteristic of white cast iron?

The carbon in white cast iron is present in the form of cementite (FeO), so it is very hard and is practically not processed, but is used for conversion into steel and for other purposes. The fracture of white cast iron has a matte white color.

What is the state of carbon in gray cast iron?

In gray cast iron, all or part of the carbon is in a free state in the form of graphite, which gives it a gray or dark gray color when broken. Gray cast iron is the main foundry material; it has quite satisfactory mechanical properties, is cheap, has high casting properties and is well processed by cutting tools.

What is the designation for gray cast iron?

Gray cast iron is designated by two letters and two two-digit numbers separated by a dash. The letters SCH mean gray cast iron, the first two-digit number is the tensile strength, the second is the bending strength. For example, SCh 18-36 is gray cast iron, the tensile strength is 18, and the bending strength is 36.

How is modified cast iron different from regular gray cast iron?

Modified cast iron differs from ordinary gray cast iron in that it has improved mechanical and casting properties. It is obtained by adding graphitizing additives (ferrosilicon, elico-calcium or silicoaluminum) to liquid gray cast iron. Grades of modified cast iron: SCh 28-48, SCh 32-52, etc.

How is high-strength cast iron produced?

Ductile iron is produced from gray cast iron by ladle addition before casting into magnesium molds. The result is nodular cast iron, which has high mechanical and casting properties.

Grades of high-strength cast iron according to GOST 7293-54: VCh 45-0, VCh 45-5, VCh 40-10, VCh 50-1.5, VCh 60-2. The letters VCh mean high-strength cast iron, the first two digits indicate tensile strength , and the latter - the value of relative elongation during tension.

How is malleable cast iron obtained?

Malleable cast iron is produced by long-term annealing of white cast iron in special furnaces, after which graphite is formed in the cast iron instead of free cementite.

Grades of malleable cast iron according to GOST 1215-59: KCh 30-6, KCh 33-8, KCh 35-10, KCh 37-12, etc. The letters KCh mean malleable cast iron, the first two digits are the tensile strength, and the last are relative tensile elongation.

What parts are made from gray cast iron in crane construction?

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Malleable iron

Malleable iron

– soft and viscous cast iron, obtained from white casting and further heat treatment. Graphitizing annealing is used - long-term annealing, as a result of which cementite decomposes with the formation of graphite.

Malleable cast iron, like gray cast iron, consists of a steel base and contains carbon in the form of graphite, but in the form of flakes that are obtained during annealing (annealing carbon) and are isolated from each other, as a result of which the metal base is less separated and the cast iron has viscosity and ductility .

In composition, white cast iron annealed into ductile iron is hypoeutectic and has a ledeburite-cementite (secondary)-pearlite structure. To obtain the ferrite-carbon annealing structure, ledeburite cementite, secondary cementite and eutectoid cementite, that is, included in pearlite, must be decomposed during the annealing process. The decomposition of ledeburite cementite and secondary cementite (partially) occurs at the first stage of graphitization, which is carried out at a temperature above critical (950-1000 ° C); decomposition of eutectoid cementite occurs at the second stage of graphitization, which is carried out by holding at a temperature below critical (740-720 °C), or with slow cooling in the critical temperature range (760-720 °C).

Chemical composition of cast iron

Cast iron is an alloy of iron and carbon, in which the percentage of carbon is not less than 2.14% but not more than 4.5%. Carbon is part of cast iron in the form of cementite or graphite. If the percentage of carbon content is less than 2.14%, the alloy is called steel.

It is known that cast iron alloy was first produced in China in the 6th century. The secret of its production came to Europe in the 14th century, and in Russia its composition was brought to perfection only in the 17th century. Over all this long time, the formula of cast iron has not changed.

The highest quality material was produced at the Demidov brothers foundry, located in the Urals.

Over the centuries, it not only has not lost its relevance, but has also acquired an even wider range of applications.

Introduction

Cast iron is an alloy of iron with carbon and other elements containing more than 2.14% C.

In metallurgical production, cast iron is smelted in blast furnaces. The resulting cast iron is divided into: conversion, special (ferroalloys) and foundry. Converting and special cast irons are used for subsequent processing into steel. Cast iron (about 20% of all cast iron) is sent to machine-building plants for use in the manufacture of cast parts (castings).

Structural unalloyed cast iron for the production of castings in mechanical engineering has the following chemical composition,%: 2.0 - 4.5 C; 1.0 - 3.5 Si; 0.5-1.0 MP; impurity content: no more than 0.3% S; no more than 0.15% S.

The widespread use of cast iron in industry is due to the optimal combination of various properties: technological (casting, machinability), operational (mechanical and special) and technical and economic indicators.

Alloy features

The main feature of cast iron is hidden in the process of its manufacture. The fact is that for different types of this alloy the melting point reaches 1200ºC, while for steel it is 1500ºC. This factor is affected by too high a carbon content. Iron and carbon atoms do not have very close bonds with each other.

When smelting occurs, the carbon atoms cannot be completely embedded in the molecular lattice of the iron, causing the cast iron alloy to become brittle. In this regard, it is not used in the production of parts that will be constantly subject to load.

This material belongs to the ferrous metallurgy industry and its characteristics are similar to steel. Products made of cast iron and steel are widely used in everyday life, and it is entirely justified.

If we compare the characteristics of these metals, we can draw the following conclusions:

What is cast iron

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

Concept

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

This video talks about the features of cast iron:

Varieties

There are white and gray cast iron.

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

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

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

Special Features

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

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

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

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

Advantages and disadvantages

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

The advantages of cast iron include:

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

The disadvantages of cast iron include:

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

Metal composition and structure

Cast iron as a structural material is represented by a metal cavity with graphite inclusions. Its main components are pearlite, ledeburite and plastic graphite. It is interesting that in different types of alloys these elements are present in different proportions or may be completely absent.

According to its structure, cast iron alloy is divided into the following varieties:

In this case, graphite may be present in it in one of the following forms:

Manufacturing technologies

As you know, cast iron is produced in special blast furnaces. The main raw material for its production is iron ore. The manufacturing process consists of the reduction of iron ore oxides and the resulting production of another material - cast iron. For its production, fuels such as coke, thermoanthracite, and natural gas are used.

To produce one ton of pig iron, about 550 kilograms of coke and approximately a ton of water are required. The volume of ore loaded into the furnace will depend on the iron content in it. As a rule, ore is used, which contains at least 70% iron. The thing is that it is not economically feasible to use a lower concentration.

The first stage in the production of cast iron is its smelting. Ore is poured into the blast furnace, and then coking coal, which is necessary to pump and maintain the required temperature inside the furnace shaft. During combustion, these components take an active part in the ongoing chemical reactions as iron reducers.

Meanwhile, flux is immersed in the furnace, which acts as a catalyst. By accelerating the melting of rocks, it thereby supports the rapid release of iron. It is important to know that before loading into the furnace, the ore undergoes the necessary pre-treatment. It is crushed in a crushing plant because smaller particles melt faster. It is then washed to remove non-metal particles. Next, the raw material is fired, as a result of which sulfur and other foreign components are extracted from it.

At the second stage of production, natural gas is supplied through special burners into the filled and ready-to-use furnace. Coke is involved in heating the raw materials. Carbon is released, which combines with oxygen to form an oxide. It, in turn, promotes the recovery of iron from ore.

As the volume of gas in the furnace increases, the rate of the chemical reaction decreases. It may even stop completely when a certain gas ratio is reached. Carbon penetrates the alloy and combines with iron to form cast iron. Unmelted elements remain on the surface and are soon removed. Such waste is called slag. It is used to make other materials.

Scope of use

This metal is used in various industries. For example, it is widely used in mechanical engineering for the production of various parts.

Most often this material is used in the production of engine blocks and crankshafts. To manufacture the latter, an improved alloy with the addition of special graphite impurities is required. This metal is resistant to friction, so high quality brake pads are made from it.

In harsh climatic conditions, cast iron alloy is indispensable, as it allows machine parts made from it to operate smoothly even at the lowest temperatures.

It has also proven itself excellent in the metallurgical industry. Its excellent casting properties and relatively low price are highly valued. Products made from it are characterized by very high strength and wear resistance.

A great variety of plumbing products are made from cast iron alloy. These are radiators, sinks, various sinks and pipes. Cast iron bathtubs and heating radiators are widely popular. Their service life is very long. Many apartments still use these products to this day, because they retain their original appearance for a long time and rarely need restoration.

It is also important that the excellent casting properties of cast iron make it possible to make entire works of art from it: such as openwork forged gates and all kinds of architectural monuments.

It is noteworthy that the price for 1 kilogram of cast iron is determined by the amount of carbon in its composition, as well as the presence of various impurities and alloying components. The price of a ton of cast iron is about 8,000 rubles.

Today there is not a single area where this metal is used. Its castings and alloys form the basis of many components, mechanisms and parts. Sometimes it is used as an independent product, perfectly coping with the functions assigned to it. This iron-containing compound is unique in its kind. It remains indispensable to this day.

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Structure and properties of cast irons.

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Iron-carbon alloys containing more than 2.14% carbon are called cast iron. In mechanical engineering, cast iron is one of the main casting materials, which is explained primarily by its good casting and strength properties. It is not subject to pressure treatment. The main factor determining the properties, and, consequently, the scope of application of cast iron, is its structure, which can be varied.

Based on their structure, cast iron is divided into white, gray, malleable and high-strength.

9.1. White cast iron.

White cast iron is called cast iron in which all the carbon is in a chemically bound state in the form of cementite Fe3C, which gives the cast iron a shiny white color.

Phase transformations in these cast irons proceed according to the metastable diagram Fe - Fe3C (see Fig. 23). According to their structure, white cast irons are divided into:

a) hypoeutectic, containing from 2.14 to 4.3 C. They consist of pearlite, ledeburite and secondary cementite, released from austenite grains in the temperature range from 1147° (EC line) to 727° (SK line). Secondary cementite merges with ledeburite cementite and may not be visible on a microsection as an independent structural component (Fig. 51a);

b) eutectic, containing 4.3% C. It consists of eutectic - ledeburite, which is a mechanical mixture of cementite and pearlite (Fig. 51, b);

. c) hypereutectic, containing from 4.3% to 6.67 %

C. They consist of primary cementite, released in the form of large plates and ledeburite (Fig. 51, c).

a B C)

Rice. 51. Structure of white cast iron: a) hypoeutectic b) eutectic c) hypereutectic

The microstructure of white cast iron contains a lot of cementite, so it is very hard and brittle, but resists wear well. It is almost impossible to process by cutting (with the exception of abrasive), so white cast irons do not find direct use in mechanical engineering; they are rarely used, only for the manufacture of parts operating under conditions of increased abrasive wear (parts of hydraulic machines, sand blowers, etc.). Being the main product of blast furnace smelting, this cast iron is used in metallurgy for conversion into steel (pig iron). White cast iron is also used in small quantities to produce malleable cast iron.

9.2. Gray cast iron.

Gray cast iron is called cast iron in which carbon is in the form of graphite, in the form of slightly curved plates or flakes, or branched rosettes with lamellar petals. Due to the large amount of graphite in the structure, such cast iron has a gray color when fractured.

The carbon content in gray cast iron usually ranges from 2.5...4%, with up to 0.83% of carbon in a state chemically bound to iron. In addition to iron and carbon, gray cast iron also contains silicon, manganese, sulfur, phosphorus, etc.

Silicon promotes the graphitization process, reduces shrinkage, silicon is part of ferrite, forming a substitutional solid solution with α-iron.

Manganese increases the tendency of cast iron to retain cementite, and therefore increases the hardness of cast iron.

Sulfur is a harmful impurity in cast iron; it increases their hardness and brittleness 5-6 times more than Mn and significantly impairs casting properties.

Phosphorus in small quantities in cast iron is a useful admixture (unlike steel), it improves the casting properties of gray cast iron, since phosphorus forms the eutectic Fe+Fe2P, which melts at a temperature of 983°C, which is valuable for the production of thin-walled blast. Chemical composition of gray cast iron: 2.5...4% C; 1.0…4.8% Si; 0.5...0.7% Mn; up to 0.12% S; 0.2…0.5% P.

Based on the structure of the metal base, gray cast irons are mainly divided into the following groups;

1. Perlite. The structure is P + PG (lamellar graphite), the metal base is P, and the amount of bound carbon (Fe3C) is equal to the eutectoid concentration of 0.8% (Fig. 52, a).

2. Ferrite-pearlite. The structure is F + P + PG, their metallic base consists of F + P, and the amount of Fe3C is less than the eutectoid concentration (Fig. 52, b).

3. Ferritic. Structure F + PG. Their basis consists of Ф, and Fe3C=0 (Fig. 52, c).

a B C)

Fig. 52. Structure of gray cast iron: a) pearlitic b) ferritic-pearlitic c) ferritic

The mechanical properties of cast iron depend on the properties of the metal base, the number and size of graphite inclusions. When designing machine parts, it should be taken into account that gray cast iron works better in compression than in tension. They are little sensitive to cuts under cyclic loading, absorb vibrations well during vibrations, and have high anti-friction properties due to the lubricity of graphite. Gray cast irons are easy to cut, cheap and easy to manufacture. Along with these positive properties, they have relatively low strength and extremely low ductility.

The grade of gray cast iron consists of the letters SCH (gray cast iron) and a number indicating a 10-fold reduced value (in megapascals) of tensile strength (Table 7).

The strength of cast iron depends significantly on the thickness of the casting wall. The value of σв indicated in the brand corresponds to castings with a wall thickness of 15 mm. As the wall thickness increases from 15 to 150 mm, the strength and hardness of cast iron decreases by almost half.

Graphite, while worsening mechanical properties, at the same time imparts a number of valuable properties to cast iron. It crushes chips during cutting, has a softening effect and, therefore, increases the wear resistance of cast iron and gives them damping ability. In addition, flake graphite ensures low sensitivity of cast iron to surface defects. Thanks to this, the fatigue resistance of cast iron and steel parts is comparable.

According to GOST 1412-85, castings are made from gray cast iron of the following grades: SCh10, SCh15, SCh18, SCh20, SCh25, SCh30, SCh35. The numbers in the brand designation correspond to the minimum tensile strength (σв, kgf/mm2). Cast iron SCh10 is ferritic, and starting from SCh25 and more - pearlitic, intermediate - ferritic-pearlitic.

Ferritic cast irons are used to make mainly non-critical parts, which are mainly subject to the requirements of good machinability rather than strength, for example, plates, weights, troughs, covers, casings, etc.

In the automotive industry, ferritic-pearlite cast iron is used to make crankcases, brake drums, covers, pistons, piston rings, large pulleys, gears, etc.

Perlite - cylinder blocks, liners, flywheels, etc. In machine tool industry, gray cast iron is the main structural material (machine beds, tables and upper slides, spindle heads, columns, carriages, etc.), wear-resistant ones include bleached gray cast iron (0H ), having a thin surface layer with the structure of white cast iron. used for the manufacture of castings of rolling rolls, carriage wheels, etc.

Malleable cast irons.

The name “malleable cast iron” is conditional, since products from it, like any other cast iron, are made not by forging, but by casting. This cast iron received the name “malleable” due to its higher plastic properties compared to gray cast iron.

The principle diagram of the technology for producing parts from malleable cast iron consists of two operations. First, parts are produced by casting from white hypoeutectic cast iron (recommended chemical composition of the alloy poured into molds: 2.4...2.9% C; 1.0...1.6% Si; 0.3...1.0% Mn; ≤ 0 .1% S; ≤ 0.2% P, then the resulting castings are subjected to special graphitizing annealing (simmering).Annealing usually consists of two stages (Fig. 53).

First, white cast iron castings (usually packed in boxes with sand) are slowly heated over 20...25 hours to a temperature of 950...1050°C. And they are kept at the same temperature for a long time (for 10...15 hours). During this period, the first stage of graphitization occurs, i.e. decomposition of cementite, which is part of ledeburite (A + Fe3C), and the establishment of a stable equilibrium of austenite + graphite.

As a result of the decomposition of cementite, flake-like graphite (annealing carbon) is formed.

The metal base of cast iron is formed in the second stage of annealing during the eutectoid transformation. In the case of continuous cooling of the casting (in air) in the eutectoid temperature region (727°C), the austenite disintegrates into pearlite and the graphitization process does not have time to cover the pearlite cementite. Cast iron adopts the structure: lamellar pearlite + flake graphite (CG). It has high hardness, strength and low ductility (HB 235...305, σв = 650...680 MPa, δ = 3.0...15%). To increase ductility while maintaining sufficiently high strength, short-term (2...4 hours) isothermal holding of cast iron or slow cooling at temperatures of 690...650°C is carried out. This is the second stage of annealing, which in this case is annealing onto granular pearlite.

Rice. 53. Annealing schedule for white cast iron for ductile

In mechanical engineering, ferritic malleable cast iron is widely used, characterized by high ductility (δ = 10...12%) and relatively low strength (σв = 370...300 MPa). The ferrite base of cast iron is formed by very slowly passing through the range of 760...720°C or during isothermal holding at 720...700°C. Here austenite and cementite, including pearlite cementite, if pearlite has had time to rejoice, breaks down into ferrite + flake graphite. The flake form of graphite is the main reason for the higher strength and ductility of malleable cast iron compared to gray cast iron (see Table 7).

The duration of annealing in general is 48...96 hours (the duration of stage II is approximately 1.5 times longer than stage I). To reduce the duration of annealing into the melt before pouring it into molds, aluminum (less commonly boron, bismuth, etc.) is introduced (modified), which creates additional artificial centers of graphite formation. According to GOST 1215-79, the following grades of malleable cast iron are produced: KCh30-8, KCh35 -10, KCh37-12, KCh45-7, KCh50-5, KCh55-4, KCh60-3, KCh65-3, KCh70-2, KCh80-1.5. The first two digits correspond to the minimum limit

tensile strength (σв, kgf/mm2); numbers after the dash - relative lengthening (δ, %

)

Malleable cast irons are used for parts operating under shock vibration loads (hubs, brake pads, crankshafts, hooks, gear housings, etc.).

The main disadvantage of obtaining CP is the long annealing of castings and the limitation of their wall thickness (up to 50 mm). In passive parts, as a result of slow cooling during crystallization, lamellar graphite appears (instead of flake-like), which reduces the strength and ductility of cast iron.

Table 7.

Mechanical properties of cast irons.

Gray cast iron (GOST 1412 - 85)

High-strength cast iron (GOST 7293 - 85)

Malleable cast iron (GOST 1215 – 79

9.4. High-strength cast irons.

High-strength cast iron is obtained by modification (microalloying of liquid cast iron with magnesium (0.1...0.5%) or cerium (0.2...0.3%). Moreover, under the influence of magnesium, graphite during the crystallization process takes not a lamellar, but a spherical shape. The microstructure of modified cast iron on a ferritic and pearlite basis is shown in Fig. 54, a, b.

a) b)

Rice. 54. Structure of high-strength cast iron: a) ferritic b) pearlitic

The main reason for the high mechanical properties of high-strength cast iron (Table 7) is the spherical shape of graphite. Nodular graphite, which has a minimum surface area for a given volume, weakens the metal base of cast iron significantly less than flake graphite. Unlike the latter, it is not an active stress concentrator.

According to GOST 7293-85, castings are made from high-strength cast iron of the following grades: VC35, VC40, VC45, VC50, VC60, VC70, VC80, VC100 (the numbers in the designation correspond to the minimum tensile strength σв, kgf/mm2)

High-strength cast iron has high mechanical characteristics and good casting and technological properties. It is used as a new material and as a substitute for steel, ductile and gray cast iron with flake graphite. Compared to steel, it has greater wear resistance, better anti-friction and anti-corrosion properties, better machinability. Due to the lower density of the casting, it is 8...10% lighter than steel. High-strength cast iron, unlike ductile iron, can be used to cast parts of any cross-section, weight and size.

Areas of application: in machine tool industry - calipers, tool holders, heavy faceplates, spindles, levers, etc.; for rolling and press-forging equipment - rolling rolls, beds of rolling mills and forging hammers, chabots, press cross-beams; for other types of equipment - drums of excavator hoists, crankshafts, etc.

9.5. Alloy cast irons.

Requirements for alloy cast irons for castings with increased heat resistance, corrosion resistance, wear resistance or heat resistance are regulated by GOST 7769-82. The grades of alloyed cast irons and their properties are given in Table. 8.

Alloy cast irons are heat treated to provide the required properties and structure.

An important property of alloy cast irons is wear resistance.

Cast irons in accordance with GOST 1585-85 are used as antifriction ones. They are intended for the manufacture of parts operating in friction units with lubrication. The standard defines the grades of antifriction cast irons, their chemical composition, characteristics, purpose, shape, size and distribution of graphite, pearlite dispersion, the nature of the distribution of phosphide eutectic, hardness and maximum operating conditions of parts made from these cast irons. They are based on iron, constant components, %: 2.2-4.3 C; 0.5-4.0 Si; 0.3-12.5 Mn. Allowed impurities, %: 0.1-1 R; 0.03-0.2 S.

The brands of antifriction cast irons, their characteristics and values ​​are presented in table. 9.

Table 8.

Grades and properties of alloy cast iron (GOST 7769-82)

Table 9.

Brands of antifriction cast irons, their properties and purpose

(GOST 1585-85)

The letters in the designations of cast iron grades mean: ACh - antifriction cast iron, C - gray cast iron with flake graphite, B - high-strength cast iron with nodular graphite, K - malleable cast iron with flake graphite. The hardness of castings made of antifriction cast iron (from 100 to 290 HB) depends on the element content and heat treatment conditions.

Limit operating modes of parts made of these cast irons in friction units: specific pressure (50 - 300) 104 Pa (5-300 kgf/cm2), peripheral speed 0.3-10 m/s.

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Date added: 2016-12-16; views: 13988; ORDER A WORK WRITING

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Gray cast iron

Gray cast iron is an alloy of iron and carbon, which, when the metal cools, forms in the form of flake-like or plate-like inclusions. The carbon content of the alloy exceeds 2.14%, which is higher than normal solubility. This is what distinguishes the alloy from steel, in which carbon is completely dissolved and is absent in the form of separate inclusions, the structure of which defines them as graphite.

White cast iron

White (pipe) cast iron

– a type of cast iron in which carbon is in a bound state in the form of cementite, when fractured it has a white color and a metallic sheen. There are no visible graphite inclusions in the structure of such cast iron.

White iron castings are wear-resistant, heat-resistant and corrosion-resistant. Strength decreases and hardness increases with increasing carbon and carbide content.

White cast iron with a martensitic structure has the highest hardness. Cast iron containing 0.7-1.8% boron has particularly high hardness HB 800-850.

White cast iron is a very valuable material for parts operating under wear conditions at very high specific pressures and mainly without lubrication.

The characteristic features of alloyed white cast iron have determined the areas of its use as stainless and magnetic cast iron, as well as cast iron with high electrical resistance.

However, predominantly white cast iron is subsequently converted into steel (80%) and other types of cast iron, and therefore is called pig iron.

Main characteristics

Gray cast iron is the basis of ferrous metallurgy, as it is obtained from the reduction of iron ores using carbon fuel (coke). As a result, in addition to the chemical reaction of reduction of iron oxides, the alloy is additionally saturated with free carbon.

The high free carbon content determines the mechanical properties of gray cast iron. One of the main qualities that allow gray cast iron to be used not only as a pigment metal is its high casting qualities and low shrinkage during solidification. Molten metal has high fluidity, so it can be used to make castings of complex shapes.

Gray cast iron plates

The restriction on the use of products made of gray cast iron is due to the fact that it has low bending strength and high fragility. At the same time, the compressive strength of gray cast iron is very high.

Despite its high fragility, such a characteristic as the wear resistance of cast iron allows it to be used in products operating under friction conditions. Under these conditions, the antifriction properties of the alloy have a strong influence.

Due to the presence of carbon inclusions, welding gray cast iron is practically impossible. There are welding technologies under certain conditions. This is preheating of parts, the use of special high-carbon electrodes, but still, the structure of the weld metal is very different from the base material. The parts to be welded must be cooled slowly to eliminate stress in the weld area.

Cast iron with flake graphite

Rice. 1: Microstructure of gray cast iron with flake graphite, magnification x 500

Gray cast iron with flake graphite (flake graphite iron for casting) is an iron-carbon alloy alloyed with silicon and manganese, in which, during crystallization, carbon is released into a separate phase in the form of flake graphite.

Gray cast iron, with its good casting properties, high cyclic toughness, relatively high fatigue strength, low notch sensitivity, low shrinkage, high yield, good wear resistance, machinability, low production cost, is currently the most common alloy for casting production.

Disadvantages of gray cast iron : low ductility, resistance to impact load application, difficult weldability.

The physical, mechanical and technological properties of castings made of gray cast iron with flake graphite are determined by the microstructure of the casting material, which is formed depending on the chemical composition of the cast iron and the individual components of the charge; rates of crystallization and cooling of castings (shape, design features and wall thickness of castings; physical properties of the material of casting molds and cores, their thickness; pouring temperature, etc.); carrying out modification processes, microalloying and heat treatment.

The microstructure of gray cast iron is composed of a metal matrix (base) and embedded in it are straight or curved (like rose petals) plates of graphite (free carbon), which has low mechanical strength, and the more inclusions of graphite, the larger the size of its inclusions, the more the straighter their shape, the uneven distribution of graphite inclusions over the cross section, the lower the mechanical properties of cast iron.

The main structural components of the matrix:

1. Ferrite is a solid solution of carbon in α-iron, characterized by low mechanical strength (σВ=25-30 kgf/mm2; σТ=12-30 kgf/mm2; δ=30-50%; ψ=60-85%; hardness 80- 100 NV). It is formed from austenite during slow cooling of alloys from the temperatures of the austenitic region. The temperature at which ferrite forms in cast iron is 723°C.
2. Cementite is a chemical compound of carbon with iron (Fe3C is iron carbide, contains 6.67% carbon), the hardest and very fragile component of the structure of cast iron (hardness - 1000 kgf/mm2, elongation during tensile testing is practically not observed), increases hardness cast iron It is formed during cooling of cast iron in accordance with the metastable state diagram of Fe-C (iron-cementite). Depending on the conditions of formation, they are distinguished: primary cementite - released during solidification of the melt, secondary cementite - formed from austenite, and tertiary cementite - resulting from the release of carbon from ferrite. When heated, cementite decomposes into austenite and graphite.
3. Pearlite is a eutectoid mixture of ferrite and cementite. Under conditions close to equilibrium, it is formed as a result of the eutectoid decomposition of austenite during slow cooling: As→Ф+Fe3C. Decomposition occurs at a constant temperature of 723°C. Perlite contains 12% cementite, while all carbon (0.8% volume) is concentrated in cementite. There are lamellar and granular perlite. In lamellar perlite, ferrite and cementite have the form of plates with an interplate distance of 0.5-1 microns. In granular perlite, there are rounded cementite grains against a background of ferrite grains. The structure of perlite strongly depends on the cooling rate - the higher the cooling rate, the finer it is. Fine varieties of perlite are sorbitol (interlamellar distance: 0.2-0.4 µm, hardness: 230-330 HB) and troostite (interlamellar distance: ~0.1 µm, hardness: ~40-45 HRC). The mechanical properties of perlite depend on the distance between the plates - the smaller it is, the higher the tensile strength and yield strength. The hardness of pearlite is about 300 kgf/mm2.
4. Austenite is a solid solution of carbon and alloying elements in γ-iron. The maximum carbon content in austenite is 2.03%. In cast iron, it is stable at temperatures above 723°C. In Fe-C alloys highly alloyed with Cr, Ni or Mn, austenite may be stable at room temperature. Austenite is non-magnetic, has high toughness and plasticity, relatively low strength, and high density compared to other structural components of iron-carbon alloys.
5. Ledeburite is a eutectic mixture of cementite and austenite. Contains 4.3% carbon. Formed at a temperature of 1145°C. Below the eutectoid temperature (723°C), austenite transforms into pearlite and thus at room temperature ledeburite consists of cementite and pearlite.

Various combinations of structural components give gray cast iron a wide range of versatile physical and mechanical properties. The structure and properties of cast iron with flake graphite are largely determined by the graphitization process, which is influenced by the elements present in the cast iron. According to the degree of intensity of impact on the graphitization process, the elements are arranged in the following row:

Si, Al, C, Ti, Ni, Cu, P, Zr | Nb | W, Mn, Cr, V, S, Mg, Ce, Te, B

Elements that promote graphitization of cast iron and the formation of ferrite are located to the left of Nb, and to the right of Nb are elements that promote the formation of carbides and pearlite.

The influence of chemical elements on the properties of gray cast iron:

1. C —promotes the graphitization of cast iron to the greatest extent, reduces strength, increases ductility, and improves casting properties.
2. Si - promotes graphitization, enlarges graphite inclusions, increases mechanical properties (with a content of >3% it reduces ductility), improves casting properties.
3. Mn - desulfurizes and deoxidizes cast iron; inhibits the graphitization process; increases the tendency to bleach, perlite dispersion, mechanical properties (with a content of 0.7 to 1.3%, and with a further increase in content - reduces), increases shrinkage.
4. S - harmful impurity: forms a low-melting eutectic with iron with a melting point of 985 ° C, which, being located at the boundaries of crystals, leads to a decrease in the mechanical properties of cast iron, its fluidity, increased shrinkage, and gives cast iron “red brittleness” (formation of cracks at high temperatures).
5. P - harmful impurity: increases fluidity and fragility (for engineering castings the content is limited to 0.2%, in art castings, where fluidity and not strength come first, the phosphorus content can reach 0.8-1.0%).
6. Ni is an alloying element: it equalizes the mechanical properties in castings with walls of different thicknesses, increases hardness, corrosion resistance and machinability.
7. Cu - promotes graphitization, increases fluidity, increases strength and hardness..
8. Cr - inhibits the graphitization process, grinds graphite, increases perlite dispersion, strength, hardness, reduces ductility and casting properties.
9. Ti - promotes graphitization (with a content of up to 0.05%), with a higher content it inhibits this process and increases mechanical properties.
10. Mg - promotes graphitization (at a content of up to 0.01%), with a higher content it increases chill, and is a strong desulfurizer.
11. Mo is an alloying element: it slows down graphitization, promotes carbide formation, increases hardness (without compromising machinability), and wear resistance.

Standards

The technical characteristics of gray cast iron for the manufacture of castings are regulated by GOST 1412-85 “Cast iron with flake graphite for castings. Stamps."

Marking

Cast iron with flake graphite is marked with the letters SC (the initial letters of the words “gray cast iron”), followed by two numbers indicating the tensile strength σB (in kgf/mm2). For example, the marking SCh20 means gray cast iron with lamellar graphite with a tensile strength of at least 20 kg/mm2.

Classification of cast iron with flake graphite

Depending on the microstructure of the metal matrix, gray cast iron with flake graphite is divided into:

1. Ferritic cast iron (Fig. 2a)
2. Ferrite-pearlitic cast iron (Fig. 2b)
3. Pearlitic cast iron (Fig. 2c)

Rice. 2: Schemes of structures of gray cast iron x 300 [8]: a - ferritic, b - ferrite-pearlitic, c - pearlitic

Mechanical properties

The mechanical properties of the material of castings made of gray cast iron with flake graphite must meet the requirements of GOST 1412-85 given in table. 1.

Table 1: Mechanical properties of gray cast iron according to GOST 1412-85

Note: It is allowed to exceed the minimum value of σB by no more than 100 MPa, unless there are other restrictions in the normative and technical documentation for castings. The tensile strength of cast iron grade SCh10 is determined at the request of the consumer.

The structure of cast iron depends on the wall thickness of cast iron castings. Depending on the thickness of the casting wall, cast iron crystallizes and cools at different rates (the thicker the casting wall, the lower the cooling rate and the more graphite is released in the cast iron structure and the lower the strength characteristics of the casting material). The dependence of the strength characteristics of cast iron on the wall thickness of the castings is given in Table. 2.

Table 2: Approximate data on temporary tensile strength and hardness in the walls of castings of various sections according to GOST 1412-85

Notes:

1. The values ​​of tensile strength and hardness in real castings may differ from those given in the table.
2. The values ​​of tensile strength and hardness in a 15 mm thick casting wall approximately correspond to those in a standard 30 mm diameter workpiece.

Chemical composition

The recommended chemical composition of gray cast iron for castings according to GOST 1412-85 is given in table. 3.

Table 3: Chemical composition of gray cast iron according to GOST 1412-85

Note: Low alloying of cast iron with various elements (Cr, Ni, Cu, P, etc.) is allowed.

Physical properties

Reference data on the physical properties of gray cast iron with flake graphite according to GOST 1412-85, depending on the grade of cast iron, are given in table. 4.

Table 4: Physical properties of cast iron with flake graphite

Chemical composition and structure

The chemical composition of the alloy, in addition to iron and carbon, also includes some silicon content. The properties of the alloy depend on the cooling conditions, since the time of temperature change affects the formation of the internal structure of the material.

With slow cooling, large iron crystals are formed, and metal-carbon compounds acquire a pearlite base. Slow cooling causes an increase in the geometric dimensions of not only iron crystals, but also carbon inclusions, therefore, pearlite metal has high strength, but increased fragility.

Microstructure of gray cast iron

Under conditions of rapid cooling, carbon does not have time to form large graphite inclusions, so the alloy acquires a ferritic structure.

Ferritic gray cast iron is slightly less brittle than pearlitic.

By choosing a cooling mode for a cast workpiece, you can influence the final properties of the material in a certain way, depending on the requirements.

White and high-strength cast iron

White cast irons are characterized by the fact that all their carbon is in a chemically bound state - in the form of cementite. The fracture of such cast iron is dull white. The presence of a large amount of cementite gives white cast iron high hardness, brittleness and very poor machinability with cutting tools.

The high hardness of white cast iron ensures its high wear resistance, including when exposed to abrasive environments. This property of white cast iron is taken into account when making piston rings from them. However, white cast iron is mainly used for casting parts and then annealing them to malleable cast iron.

Malleable cast iron is produced by annealing white cast iron of a certain chemical composition, characterized by a reduced content of graphitizing elements (2.4-2.9% C and 1.0-1.6% Si), since it is necessary to obtain completely bleached cast iron in a casting state. over the entire cross-section of the casting, which ensures the formation of flake graphite during annealing (see figure)

The mechanical properties and recommended chemical composition of high-strength cast iron are regulated by GOST 1215-79. Malleable cast irons are marked with the letters “K” - malleable, “H” _ H Iron and a number. The first group of numbers shows the tensile strength of cast iron, the second - its relative elongation at break. For example, KCH 33-8 means: malleable cast iron with a tensile strength of 33 kg / mm 2 (330 MPa) and an elongation at break of 8%.

A distinction is made between black-heart ductile iron, produced by graphitizing annealing, and ductile iron, produced by decarburizing annealing in an oxidizing environment. In Russia only malleable iron is used. The cast iron matrix can be pearlitic, ferritic, or pearlitic-ferritic, depending on the annealing mode.

To speed up the CN annealing process, various methods are used: the holding temperature is increased during the P 2 period, modified and microalloyed with additions of cast aluminum, boron, titanium or bismuth. All these techniques help to increase the number of crystallization centers and reduce the stability of cementite.

Malleable cast iron is used for the manufacture of critical thin-walled castings of small and medium sizes operating under dynamic variable loads (parts of drive mechanisms, gearboxes,

brake pads, gears, hubs, etc.). However, malleable cast iron is an unpromising material due to complex production technology and the length of the production cycle for making parts from it.

Application

Gray cast iron is widely used in casting products for which high compressive strength is important. This property is important mainly in the manufacture of cast tool beds. The use of the material is limited by the increased fragility of products in the presence of significant bending forces.

Gray cast iron product

Previously, the good casting properties of the material were widely used in the manufacture of various products for household and industrial purposes. A variety of kitchen and household utensils - cast iron pots, frying pans, irons - made by casting with minimal subsequent processing had low cost and ease of production.

Currently, casting is also used to produce highly loaded machine elements where they are not subjected to bending loads. These are pistons and cylinders of internal combustion engines.

High-strength parts cast from gray cast iron have minimal cost and long service life. Without exaggeration, we can say that cast beds and machine bodies are almost eternal compared to other elements of the device.

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