What are called steel and cast iron. Cast iron. Properties of cast iron. Cast iron is an alloy of iron and carbon. History of the use of cast iron in ancient China

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

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

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

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

Types of cast iron

The grade of a cast iron alloy is determined by the amount of carbon and other substances in its composition.

This approach allows us to distinguish the following types of this material:

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

White cast iron

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

White cast iron is divided into the following groups:

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

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

Gray cast iron

The carbon in this grade of cast iron is in the form of plates. Gray cast iron also contains components such as:

• silicon up to 0.8%;
• manganese up to 0.3%, etc.

To produce castings from this material, molds made of cast earth or steel are used. Such forms are called chill molds. The key area of ​​use of gray cast iron is mechanical engineering. It is used to make structures that operate when there are no shock impacts, for example, wheel V-belt drives, bearing cups, etc. A cast iron alloy of this type is marked as follows: SCh 32 - 52. The first number shows the tensile strength, the second the bending limit.

As part of this material, carbon has a flocculent shape. The chemical composition of this material includes up to 1.4% silicon, 1% manganese, etc. Malleable cast iron is made from white cast iron.

To do this, it is subjected to heat treatment, that is, it is heated and kept in this state for the time specified by the technology. This operation is called languishing. Malleable cast iron is marked as CN 45 - 6. The first number indicates tensile strength, the second elongation as a percentage.

As part of this cast iron, carbon has a spherical shape. To produce cast iron of this type, modification is used, that is, magnesium is introduced into the melt. It ensures the formation of carbon in the form of spherical inclusions. This solution made it possible to bring cast iron of this grade closer to carbon steels in a number of properties. Its casting parameters are greater than those of other brands of cast iron alloys, with the exception of gray.

Cast irons of this class are used in the production of such parts as pistons, crankshafts, and components of braking systems.

High-strength cast iron is marked as follows - HF - 45-5. The first number indicates the tensile strength, the second the percentage elongation.

Features of the production of malleable cast iron

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

Cast iron of the BC grade, which is the main product of malleable iron, does not have very good casting parameters. In particular, it has reduced fluidity, a large amount of shrinkage during cooling, and it is prone to the formation of various casting defects. These are the reasons why during production it is necessary to overheat the metal and take measures to combat casting defects. The production of malleable cast iron can be carried out with the obligatory consideration of shrinkage and changes in the dimensions of the workpieces during simmering. Thin workpieces have maximum shrinkage, thick ones have minimal shrinkage. The simmering operation is performed at 1350 - 1450 degrees Celsius.

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

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

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

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

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

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

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

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

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

Types of malleable cast iron

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

• ferritic;
• pearlite;
• ferritic-pearlitic.

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

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

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

Main characteristics of the metal

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

The difference in the structure of malleable cast iron is also reflected in the final properties of products obtained from it. For example, workpieces made from ferritic cast iron have lower hardness than those made from pearlitic material, but at the same time the former have increased ductility. Graphite in the form of flakes provides high strength parameters to finished parts with relatively good ductility. Products made of KCh cast iron can be deformed at room temperature and humidity. It was this property that determined the name of this material – malleable. In fact, this is a conditional name and does not mean that finished parts are obtained from it using forging equipment. Casting is used to produce products. The main property of this material is that there is no stress in it.

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

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

Properties of malleable cast irons

The basic property of the KCh cast iron alloy is that it contains carbon inclusions in different forms, which determines its strength and ductility. Cast iron with a low amount of carbon (decarbonized), in fact, is the only material from structural cast iron alloys that is well welded and is used to produce welded metal structures. For welding, either gas protection or butt technology is used. This grade of cast iron lends itself to pressing, embossing and quite simply fills voids and gaps. Parts made from a malleable ferritic cast iron alloy are subjected to cold processing, while parts made from pearlitic alloy are heated.

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

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

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

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

Scope of application

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

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

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

The basis for making cast iron or steel is iron. In nature, it is a metal with a silvery tint and does not have sufficient hardness. This metal is practically not used in industry, and various iron alloys are widely used.

Cast iron and steel are alloys of iron and carbon, but the quality of the metal will depend on the content of these elements and impurities.

Cast iron

Cast iron is a primary product of metallurgy. Its composition contains more than 2% carbon and a significant amount of impurities that affect the properties of the metal: manganese, phosphorus, silicon, sulfur, alloying additives.

Cast iron is a brittle metal; it can easily be broken into fragments upon impact, so it is less practical to process and use. The type of carbon contained in cast iron affects its properties, therefore several types of cast iron are distinguished:

A grey, soft metal with a low melting point;

White, with increased hardness, but brittle;

Malleable, a secondary product of white cast iron;

Highly durable.

The density of cast iron is 7000 kg/m3.

Steel

The percentage of carbon in the alloy should not exceed 2%, and iron should not be less than 45%. The remaining 53% may contain various alloying additives and impurities that allow you to change its properties.

There are a large number of varieties and classifications. Depending on the number of connecting elements, they are distinguished:

Low alloy;

Medium alloyed.

Also distinguished by the amount of carbon:

Low carbon;

Medium carbon;

High carbon.

The quality of the metal is affected by the presence of non-metallic inclusions (oxides, sulfides, phosphides) and there is a classification by quality.

The general characteristic is that it is a metal with good strength, wear resistance, hardness, and is suitable for various types of processing. Steel density is 7700 – 7900 kg/m3.

Despite the large number of varieties of cast iron and steel, we can highlight the main differences between these metals:

Steel has greater strength, ductility and hardness;

It is more plastic, therefore it lends itself well to processing (stamping, forging, rolling, welding), cast iron products are made by casting;

Cast iron has a lower cost;

Steel has high thermal conductivity, quality is improved by hardening, and cast iron, due to the porosity of the metal, is able to retain heat;

Alloys have different specific gravity.

Metallurgy supplies the market with hundreds of varieties of both alloys, which have their own characteristics and characteristics, but the essential components of these metals are iron and carbon. Therefore, steel and cast iron can be combined into the group of iron-carbon alloys.

Cast iron– an alloy of iron (Fe>90%) with carbon (C from 2.14% to 6.67%).
Carbon can be contained in cast iron in the form of graphite (C) or cementite (Fe3C).
Cast iron also contains impurities of silicon, manganese, phosphorus and sulfur.
Cast irons with special properties also contain alloying elements - chromium, nickel, copper, molybdenum, etc.

Cast iron is the most widely used material for the manufacture of cast parts used under relatively low stresses and low dynamic loads. The advantages of cast iron over steel are high casting properties and low cost. Cast irons are also better at cutting than most steels (except for automatic steels), but they are poorly weldable and have less strength, rigidity and ductility.

Depending on the state of carbon in cast iron, there are:
white cast iron
gray cast iron(GOST 1412 - "Cast iron with flake graphite for castings")
malleable iron(GOST 1215 - "Muctile iron castings")
ductile iron(GOST 7293 - "Nodular cast iron for castings")

White cast iron

In white cast iron, all carbon is in a bound state in the form of cementite Fe3C.
White cast iron has high wear resistance and hardness, but it is brittle and poorly processed by cutting, so they find limited use in mechanical engineering and are mainly used for processing into steel.
Based on carbon content, gray cast iron is divided into:
Hypoeutectic with carbon content from 2.14% to 4.3%
Eutectic with carbon content 4.3%
Hypereutectic with carbon content from 4.3% to 6.67%.

In gray, malleable, and high-strength cast irons, all or most of the carbon is in the form of graphite of various shapes (they are also called graphite).

Gray cast iron

In the structure of gray cast iron, graphite is plate-shaped.
Gray cast irons contain: 3.2-3.5% carbon, 1.9-2.5% silicon, 0.5-0.8% manganese, 0.1-0.3% phosphorus and less than 0.12% sulfur .
Castings of gray cast iron parts are made in molds - earthen or metal molds.
Gray cast iron is widely used in mechanical engineering. Due to the low mechanical properties of gray cast iron castings and the ease of production, they are used for the manufacture of parts for less critical purposes, parts that operate in the absence of shock loads. In particular, they are used to make covers, pulleys, machine beds and presses.
An example of gray cast iron designation: SCh32-52. The letters indicate gray cast iron (GC), the first number indicates the tensile strength (32 kgf/mm2 or 320 MPa), the second number indicates the bending strength.

Malleable iron

In the structure of malleable cast iron, graphite is flake-shaped.
Malleable cast irons contain: 2.4-3.0% carbon, 0.8-1.4% silicon, 0.3-1.0% manganese, less than 0.2% phosphorus, no more than 0.1% sulfur.
Malleable cast iron is obtained from white cast iron by heating and holding for a long time. This procedure is called graphitizing annealing or simmering.
An example of the designation of malleable cast iron: KCH45-6. The letters indicate malleable cast iron (CC), the first number is the tensile strength (45 kgf/mm2 or 450 MPa), the second is the relative elongation in% (6%).

Ductile iron

Ductile iron contains nodular graphite.
It has the highest strength properties.
Ductile iron contains: 3.2-3.8% carbon, 1.9-2.6% silicon, 0.6-0.8% manganese, up to 0.12% phosphorus and no more than 0.3% sulfur.
High-strength cast iron is produced by modifying (i.e. introducing a modifier additive - magnesium) the liquid melt. Modifiers promote the formation of spherical graphite inclusions, due to which the mechanical properties of such cast iron approach those of carbon steels, and the casting properties are higher (but lower than those of gray cast iron).
High-strength cast iron is used to make critical parts for mechanical engineering - pistons, cylinders, crankshafts, brake pads. Pipes are also made from high-strength cast iron.
An example of the designation of high-strength cast iron: VC45-5. The letters indicate high-strength cast iron (DC), the first number indicates the tensile strength (45 kgf/mm2 or 450 MPa), the second indicates the elongation in%.

Cast iron is an alloy of iron with carbon (content more than 2.14%).
Carbon in cast iron may be contained in form of cementite and graphite.
IN cast iron.
Cast iron V

Cast iron is an alloy of iron with carbon containing more than 2.14% (maximum solubility point carbon in austenite on the phase diagram).
Carbon in cast iron may be contained in form of cementite and graphite.
IN depending on the shape of graphite and the amount of cementite, they are distinguished: pale, colorless, malleable and high-strength cast iron.
Cast iron hold permanent impurities (Si, Mn, S, P), and V in individual events also alloying elements (Cr, Ni, V, Al, etc.).
Usually, cast iron fragile.

Malleable cast iron obtained by long annealing of white cast iron, V As a result, flake-shaped graphite is formed.
The metal base of this cast iron: ferrite and less commonly pearlite.
Malleable cast iron got its name due to its increased plasticity and viscosity (despite the fact that it is not subjected to pressure treatment).
Malleable cast iron has increased tensile strength and increased impact resistance.
From malleable cast iron they produce parts of complex shapes: car rear axle housings, brake pads, tees, angles, etc.

Including small resistance castings from gray cast iron tensile and impact loads, this material should be used for parts that are subject to compressive or bending loads.
IN in machine tool building these are basic, body parts, brackets, gears, drives;
V automotive industry - cylinder blocks, piston rings, camshafts, clutch discs.
Gray castings cast iron is also used in electrical engineering, for the manufacture of consumer goods.

Carbon in cast iron may be in form of cementite, graphite or V the same time in the form of cementite and graphite.
The appearance of a permanent phase - graphite V cast iron may occur in as a result of its direct separation from a weak (solid) solution or due to the disintegration of pre-formed cementite (with slow cooling of the molten cast iron cementite can undergo decomposition ResS - > Fe + GC with the formation of ferrite and graphite).
Formation process in cast iron(steel) graphite is called graphitization.

By carbon content of cast iron are divided into hypoeutectic - 2, 14 ...
4.3% C, eutectic - 4.3% C and hypereutectic - 4.3 ...
6.67% C carbon.
Hypoeutectic cast iron, including 2, 14 ...
4.3% C, after final cooling they have the structure of perlite, ledeburite (perlite + cementite) and secondary cementite.
Eutectic cast iron(4.3% C) at temperatures below + 727 ° C consists only of ledeburite (perlite + cementite).
Hypereutectic, which cannot be canceled 4, 3...
6.67% C, at temperatures below + 727 ° C, consist of primary cementite and ledeburite (perlite + cementite).
In practice, the most widely used are hypoeutectic cast iron, including 2, 4 ...
3.8% C carbon.
Solid meaning carbon content in cast iron is determined by its technological characteristics during casting - ensuring good fluidity.
Fluidity is the ability of metals and alloys V in the molten state, fill the mold cavity, accurately reproducing the outlines and dimensions of the casting.
Enlarged carbon content of cast iron above 3.8% C leads to a sharp increase in hardness and brittleness.
Fluidity is determined by a spiral test, and its value is determined by the length of filling part of the spiral.
Shrinkage is a reduction in the linear and volumetric dimensions of metal submerged V figure during its crystallization and cooling.

In industry, types of cast iron are marked with the following type: conversion cast iron- P1, P2;
conversion cast iron for castings (processing - foundry) - PL1, PL2, pigment phosphorous cast iron- PF1, PF2, PF3, high-quality conversion cast iron- PVK1, PVK2, PVK3;
cast iron with lamellar graphite - SCh (numbers after the letters “SCh” mean the value of tensile strength V kgf/mm) ;
antifriction cast iron anti-friction gray - AChS, anti-friction high-strength - AChV, anti-friction malleable - AChK;
cast iron with spherical graphite for castings - HF (the numbers after the letters “HF” mean temporary tensile strength V kgf/mm and relative elongation (%);
cast iron alloyed with special properties - Ch.

Iron-carbon alloys with a carbon content of more than 2% are conventionally called cast iron, regardless of the degree of alloying. The exception is some tool steels and high-silicon cast irons, for example, silal, which, depending on the grade, contains from 1.6 to 2.5% C. The accepted distinction between the cast iron area and the steel area coincides with the maximum solubility of carbon in γ-iron.

The properties of cast iron are determined by the quantity, shape and nature of the distribution of structural components. The phase composition of cast iron depends on the chemical composition, smelting conditions and crystallization conditions of cast iron.

Iron-carbon phase diagram

The iron-carbon phase diagram in the concentration range from iron to cementite is shown in Fig. 1. Line ABCD is the liquidus of the system, line AHJECF is the solidus.

The three horizontal lines in the diagram (HJB, ECF and PSK) indicate the occurrence of three invariant reactions. At 14850 (line HJB), the peritectic reaction LB+FN→AJ occurs. As a result of the peritectic reaction, austenite is formed. This reaction occurs only in alloys containing carbon from 0.1 to 0.5%. At 11300 (horizontal ECF), the eutectic reaction LC→AE+C occurs. As a result of this reaction, a eutectic mixture is formed. A eutectic mixture of austenite and cementite is called ledeburite. This reaction occurs in all alloys of the system containing more than 2% carbon. At 7230 (horizontal PSK) the eutectoid reaction AS→FR+C occurs. The transformation product is a eutectoid mixture. A eutectoid mixture of ferrite and cementite is called pearlite.

All alloys containing more than 0.02% carbon, i.e., almost all industrial iron-carbon alloys, undergo pearlite (eutectoid) transformation. Thus, the iron-carbon diagram characterizes the occurrence of eutectic, eutectoid and peritectic transformations in these alloys.

The appearance of the iron-carbon diagram (in its pre-cementite part), i.e. the arrangement of the lines on the diagram, is quite definite and well-established. Only the coordinates (i.e., the temperature and concentration of the most characteristic points) are refined.

The coordinate values ​​of the points on the iron-carbon diagram are presented in Table 1.

Rice. 1. Iron – carbon diagram

Table 1.

Characteristic points on the iron-carbon diagram

Components and phases of iron-carbon alloys

The main components of iron-carbon alloys are iron, carbon and cementite. Iron is a transition metal with a silvery-light color. It has a high melting point - 15390±50 C. In the solid state, iron can be found in two modifications. Polymorphic transformations occur at temperatures of 9110 C and 13920 C. At temperatures below 9110 C, α-Fe with a body-centered cubic lattice exists. In the temperature range 9110÷13920 C, γ-Fe with a face-centered cubic lattice is stable. At temperatures below 7680 C, iron is ferromagnetic, and above it is paramagnetic. The Curie point of iron is 7680 C.

Iron of technical purity has low hardness (80 HB) and strength (tensile strength - σ = 250 MPa) and high ductility characteristics ( relative elongation – δ=50%). Properties may vary within certain limits depending on the grain size.

Iron is characterized by a high modulus of elasticity, the presence of which is also manifested in alloys based on it, providing high rigidity of parts made from these alloys. Iron forms solid solutions with many elements: with metals - substitution solutions, with carbon, nitrogen and hydrogen - interstitial solutions.

Carbon is a non-metal. It has a polymorphic transformation, depending on the conditions of formation, it exists in the form of graphite with a hexagonal crystal lattice (melting point - 35000C, density - 2.5 g/cm3) or in the form of diamond with a complex cubic lattice with a coordination number of four (melting point - 50000C ).

Since iron, in addition to forming the chemical compound Fe3C with carbon, has two allotropic forms, the following phases exist in the system: liquid phase, cementite, ferrite, austenite.

Liquid phase. In the liquid state, iron readily dissolves carbon in any proportions to form a homogeneous liquid phase.

Cementite is a chemical compound of iron and carbon (iron carbide), containing 6.67% carbon. Does not experience allotropic transformations. The crystal lattice of cementite consists of a series of octahedra, the axes of which are inclined to each other. The melting point of cementite has not been precisely established (1250, 15500C). At low temperatures, cementite is weakly ferromagnetic, losing its magnetic properties at a temperature of about 2170C.

Cementite has high hardness (more than 800 HB, easily scratches glass), but extremely low, almost zero, ductility. Such properties are a consequence of the complex structure of the crystal lattice. Cementite capable of forming substitutional solid solutions. Carbon atoms can be replaced by non-metal atoms: nitrogen, oxygen; iron atoms - metals: manganese, chromium, tungsten, etc. Such a solid solution based on a cementite lattice is called alloyed cementite.

Cementite– the compound is unstable and under certain conditions decomposes with the formation of free carbon in the form of graphite. This process is of great practical importance in the formation of the structure of cast iron.

Iron-carbon alloys also contain phases: primary cementite (C I), secondary cementite (C II), and tertiary cementite (C III). The chemical and physical properties of these phases are the same. The mechanical properties of alloys are influenced by differences in the size, quantity and location of these precipitates. Primary cementite is released from the liquid phase in the form of large lamellar crystals. Secondary cementite is released from austenite and is located in the form of a network around austenite grains (when cooled, around pearlite grains). Tertiary cementite is released from ferrite and is located in the form of small inclusions at the boundaries of ferrite grains.

Ferrite has a variable limiting solubility of carbon: minimum – 0.006% at room temperature (point Q), maximum – 0.02% at a temperature of 7270C (point P). Carbon is located in lattice defects. At temperatures above 13920C there is high-temperature ferrite with a limiting carbon solubility of 0.1% at a temperature of 14990C (point J).

The properties of ferrite are close to those of iron. It is soft (hardness - 130 HB, tensile strength σв = 300 MPa) and plastic (relative elongation δ = 30%), magnetic up to 7680C.

Austeniteγ-Fe (C) is a solid solution of interstitial carbon in γ-iron. At the center of a face-centered cubic cell is a carbon atom. Austenite has a variable limit solubility of carbon: minimum – 0.8% at a temperature of 7270C (point S), maximum – 2.14% at a temperature of 11470C (point E). Austenite has a hardness of 200÷250 HB, is plastic (relative elongation – δ=40÷50%), and paramagnetic. When other elements are dissolved in austenite, the properties and temperature limits of existence may change.

Microstructure of cast irons

Obtaining a particular structure of cast iron depends on many factors: the chemical composition of cast iron, the technology of smelting and out-of-furnace metal processing, the rate of crystallization and cooling of the melt in the mold, and, consequently, the thickness of the casting wall, the thermophysical properties of the mold material, etc. The structure of the metal base of cast iron can be can also be changed by heat treatment. Table 2 shows the most common structures and structural components of cast iron and some of their properties.

Table 2.

Structures and structural components of cast iron

Classifications of cast irons

Classification of cast irons by chemical composition

In addition to iron and carbon, cast iron contains certain amounts of silicon, manganese, phosphorus and sulfur as permanent impurities. Of these, phosphorus and sulfur are considered harmful impurities.

According to their chemical composition, cast irons are divided into unalloyed, low-, medium- and high-alloy. Cast irons containing up to 2% manganese and up to 4% silicon, up to 0.1% chromium and up to 0.1% nickel are considered unalloyed. If these elements are present in large quantities or if they contain special impurities, cast iron is considered alloyed.

In low-alloy cast irons, the amount of special impurities (nickel, copper, chromium, etc.) usually does not exceed 3%; in medium-alloy cast iron, the amount of alloying impurities is 7-10%, and in high-alloy cast iron it significantly exceeds 10%.

By low alloying of cast iron, they strive to improve its general properties, obtain a homogeneous structure, increase the tensile strength and elasticity while maintaining these properties when heated, improve hardness and wear resistance, antifriction, etc. With medium and high alloying, the composition of solid solutions and carbides changes significantly, due to which the change in the nature of the metal base becomes most important.

Classification of cast irons according to structure and conditions of graphite formation

According to the degree of graphitization, the forms of graphite and the conditions of their formation, the following types of cast iron are distinguished: white, half, gray, malleable and high-strength with nodular graphite (see diagram Fig. 2). The nature of the metal base of cast iron is determined by the degree of graphitization and alloying, as well as the type of heat treatment.

According to the degree of graphitization, white cast iron can be considered the least or not at all graphitized, half cast iron can be considered partially graphitized, and the remaining cast irons can be considered significantly graphitized.

Rice. 2. Cast iron classification scheme

In white and half cast iron, the presence of ledeburite (a mechanical mixture of a solid solution of carbon in iron and iron carbide) is required, but in significantly graphitized cast iron there should be no ledeburite.

White cast iron is cast iron in which all the carbon is in a chemically bonded state. White cast iron is very hard, brittle and very difficult to cut. The microstructure of unalloyed white hypoeutectic cast iron consists of ledeburite, pearlite and secondary cementite. In alloyed or heat-treated cast iron, martensite or even austenite can be obtained instead of pearlite. White cast iron is used for the manufacture of wear-resistant, corrosion-resistant and heat-resistant parts. In addition, castings made of white cast iron of the appropriate composition are used to produce parts from malleable cast iron by graphitizing annealing. White cast iron is called so because its fracture pattern is light crystalline, radiant. What is characteristic of half cast iron is that, along with ledeburite, it also contains graphite.

The structure of half cast iron is pearlite-ledeburite with graphite. In alloyed or heat-treated cast irons, austenite, martensite or bainite can be obtained instead of pearlite.

Half cast iron is so called because its fracture pattern is a combination of light (white) and dark (graphitized) areas. Half cast iron is hard and brittle. In bleached cast iron parts, the surface layers have the structure of white cast iron, and the core has the structure of graphitized cast iron. Between the surface layers and the core there is a zone of half-cast iron.

Gray cast iron is the most common engineering material. Gray cast iron is marked with the letters C - gray and H - cast iron. The letters are followed by numbers indicating the average tensile strength (kgf/mm2) and relative deformation.

The main distinguishing feature of gray cast iron is the absence of an unacceptable amount of cementite and ledeburite and the fact that the graphite in the plane of the polished section has a lamellar shape. When the graphite plates are very dispersed, it is called dotted. Lamellar forms of graphite can be straight and have varying degrees of vorticity. To obtain a lamellar form of graphite, heat treatment and special modification are not necessary. Elimination of graphite inclusions of undesirable forms and combinations is achieved by modifying graphitizing additives. The type of fracture of gray cast iron largely depends on the amount of graphite: the more graphite, the darker the fracture of cast iron.

Gray cast iron is characterized by an almost complete absence of relative elongation (up to 0.5%) and very low impact strength. This feature of gray cast iron is a consequence of the very strong weakening effect of flake graphite on the metal base.

Since gray cast iron, regardless of the nature of the metal base, has very low ductility, they strive to obtain a pearlitic metal base in it, since pearlite is much stronger and harder than ferrite. Reducing the amount of pearlite and increasing the amount of ferrite in the structure due to this leads to a loss of strength and wear resistance without increasing ductility.

In alloyed and heat-treated cast irons, austenite, martensite or bainite can be obtained instead of pearlite. Inclusions of secondary and eutectic cementite are mostly undesirable. The fundamental difference between high-strength cast iron is the spherical shape of graphite, which is obtained by introducing special modifiers into liquid cast iron.

The spherical form of graphite is the most favorable of all known forms. Nodular graphite weakens the metal base less than other forms. The metal base of this cast iron is usually pearlitic, pearlitic-ferritic and ferritic, depending on the required properties. By alloying and heat treatment it is also possible to obtain an austenitic, martensitic or bainite base.

A certain amount of flake graphite may be allowed in the structure of high-strength cast iron, provided that its properties satisfy the required grade. Irregular (distorted) shapes of spherical graphite are also allowed. High-strength cast iron is marked with the letters HF, followed by numbers that show the average value of tensile strength (kgf/mm2).

The main difference between malleable cast iron is that the graphite in it is obtained by annealing white cast iron and has a flake or spherical shape. The spherical shape is obtained through special modification or decarburizing annealing. Flaked graphite comes in different compactness and dispersion, which significantly affects the mechanical properties of cast iron.

Malleable cast iron is produced not only with ferritic, but also with ferritic-pearlitic and pearlitic metal base.

Cast iron with a ferritic base has the greatest ductility, which is why it is most often used. 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 lighter.

Malleable cast iron is marked with the letters KCH and numbers. The first two digits indicate the tensile strength (kgf/mm2), the second - the relative elongation (%).

Classification of cast irons by properties

Cast iron can be classified according to its mechanical and special properties. Based on their mechanical properties, cast iron castings are divided into hardness, strength and ductility.

Table 3.

Classification of cast iron by properties.

GOST 7769-82 “Cast iron for casting with special properties” provides nine brands of white wear-resistant cast irons: low-alloy chromium brands Chx3t, highly alloyed chromium brands Chkh9N5, WHO16, ChH16M2, WH22, ChH29D2, ChH32, Glorified manganese of the CG7X4 and Low-Loire Disheated nickel brand Chn4x2. The first letter stands for "cast iron". The numbers indicate the content of the alloying element, indicated as a percentage after the corresponding letter. If there is no number after the letter, then the content of the corresponding alloying element is 1%. Other alloyed special cast irons are marked in the same way, except for anti-friction ones, where the first letter means “anti-friction”. You may also encounter the following terms: “nomag” (non-magnetic cast iron), “niresist”, “silal”, “nikrosilal” (corrosion-resistant), “chugal” (heat-resistant) and some others.

In terms of magnetic properties, cast irons currently used can be divided into ferromagnetic and paramagnetic. In turn, ferromagnetic cast irons can be divided into magnetically soft and magnetically hard. This division is very arbitrary, since under no circumstances can cast iron be a soft or hard magnetic material in the true sense. Magnetic-soft include gray, malleable and high-strength cast irons.

General characteristics of gray cast iron

Gray cast iron is obtained directly through the process of crystallization with slow cooling, while the graphite has a lamellar shape. Depending on the degree of graphitization, different structures of the metal base (matrix) of gray cast iron can be obtained: gray pearlitic cast iron with a P+G structure; gray ferritic-pearlitic cast iron with structure F+P+G; gray ferritic cast iron with F+G structure.

The mechanical properties of gray cast iron as a structural material depend both on the properties of the metal base (matrix) and on the number, geometric parameters and nature of the distribution of graphite inclusions. The fewer these inclusions and the smaller they are, the higher the strength of cast iron. The metal base in gray cast iron provides the greatest strength and wear resistance if it has a pearlite structure. Gray cast iron with a ferritic base has the least strength. The relative tensile elongation of gray cast iron, regardless of the properties of the metal base, is practically zero (δ≤0.5%).

Gray cast irons modified with ferrosilicon and silicocalcium have the highest mechanical properties. Modification - the addition of non-melting crushed particles to the melt - ensures the grinding of graphite inclusions.

Ferritic and ferritic-pearlite gray cast irons are used for lightly loaded parts of agricultural machines, cars, and tractors. Cast irons with a pearlite base, which have a very high ability to dampen mechanical vibrations (high damping capacity), are used for casting the frames of machine tools and mechanisms, as well as for the manufacture of diesel cylinders, parts of internal combustion engine blocks (piston rings, rods).

Microstructure of gray cast irons

When examining a microsection of gray cast iron through a microscope, inclusions of lamellar graphite are clearly visible (Fig. 3). The size and location of graphite inclusions are affected by the cooling rate, temperature and holding time of molten cast iron before casting, the chemical composition of cast iron, and the introduction of certain impurities (modifiers) into cast iron. For example, the cooling rate influences in such a way that, with otherunder equal conditions, graphite is formed the larger theslower cooling. The greater the overheating of the liquidcast iron and the longer the holding time, thegraphite inclusions become smaller .

Rice. 3. Inclusions of lamellar graphite. Unetched sections(x100):

A)straight;b)swirled; c) rosette, d) interdendritic

The metal base in gray cast irons is very similar to the microstructure of steels and, depending on the amount of fixed carbon, can be ferritic, ferritic-pearlitic and pearlitic.

Rice. 4. Ferritic gray cast iron - ferrite and flake graphite;

A)

Rice. 5. Ferritic-pearlitic gray cast iron – ferrite + pearlite + flake graphite: a) microstructure (x500); b) microstructure diagram

Rice. 6. Pearlitic gray cast iron - pearlite + lamellar graphite:

A)microstructure (x500); b) microstructure diagram

Thus, the following types of gray cast iron structures are possible: ferrite + flake graphite – ferritic gray cast iron (Fig. 4). Ferrite + pearlite + lamellar graphite – ferritic-pearlitic gray cast iron (Fig. 5). The ratio of the amount of ferrite and pearlite in the structure of cast iron can be different depending on the chemical composition and cooling conditions. Perlite + flake graphite – pearlitic gray cast iron in Fig. 6.

Rice. 7.Microstructure of gray cast iron with phosphide eutectic:

perlite + plawebbed graphite + phosphide eutectic(x500)

At elevated phosphorus concentrations in gray cast iron there is phosphide eutectic (Fig. 7), which spreads completely or partially along the grain boundaries.