It’s hard to imagine modern construction,machinery, engineering and other major industries without the use of major metal alloys of steel and cast iron. Their production exceeds all the rest dozens of times.
If we consider steel and cast iron from the point of viewsuch a science as metallography, the central figure is the iron-carbon alloy state diagram, which allows you to get a detailed understanding of the composition and structural transformations in these materials. And also get acquainted with their phase composition.
History of discovery
For the first time that in alloys (steels and cast iron)There are certain (special) points, the great metallurgist and inventor pointed out - Dmitry Konstantinovich Chernov (1868). It was he who made an important discovery about polymorphic transformations and is one of the creators of the iron-carbon state diagram. According to Chernov, the position of these points in the diagram is directly dependent on the percentage of carbon.
And what is most interesting, it is from the moment of this discovery that such science as metallography begins its life.
The diagram of iron-carbon alloys is the result of the hard work of scientists in several countries of the world. All letter designations of the main points and phases in the diagram are international.
Concept chart
Graphic representation of the processes occurring inalloy with a change in temperature, concentration of substances, pressure, called a state diagram. It allows you to volume and clearly see all the transformations occurring in the alloys.
Iron-Carbon Chart Elements
Summary of each of these elements.
Iron is a silver gray metal. Specific weight - 7, 86 g / cm3. It has a melting point of 1539 ° C.
The interaction of iron and other metals produces compounds called substitution solutions. If with non-metals, for example with carbon or hydrogen, then - with implantation solutions.
Iron has the ability, being originallysolid, to be in several states, which are called “alpha” and “gamma” in metallurgy. This quality is called polymorphism. About this later in the article.
Carbon is non-metal. If it acts as graphite, then the melting point is 3500 ° C. If it is like a diamond, it is 5000 ° C. The density of carbon is 2.5 g / cm3. It also has polymorphic properties.
In iron-carbon alloys, this element forms a solid solution, which contains a ferrum, called cementite (Fe3C) Also forms graphite in cast iron.
Iron-carbon alloy diagram
As a result of the interaction of the components of the diagram with each other, it turns out cementite - a chemical compound.
As a rule, when studying a diagram by metal students, all stable compounds are considered as components, and the graphic image itself is examined in parts.
Also in the classroom, the cooling curve is constructed according to the iron-carbon diagram: the percentage of carbon is selected, and then it is necessary to determine which phase corresponds to which temperature on the diagram.
To do this, besides the diagram itselfdraw the coordinate system (temperature-time). And starting from the maximum degrees, move gradually downwards, depicting the curve and areas of transition from one phase to another. It is necessary to call them and indicate the type of crystal lattice.
Next, we consider in more detail the graphical image of the iron-carbon state diagram.
First, it has two forms (parts):
- iron cementite;
- iron graphite.
Secondly, the alloys, in which the main "actors" are ferrum and carbon, are conventionally divided into:
- become;
- cast iron.
If the carbon in the alloy is less than or equal to 2.14% (point E in the diagram), then it is steel, if more than 2.14%, it is cast iron. For this reason, the diagram is divided into two phases.
Polymorphic transformations
More details about each phase below in the article. In short, the implementation of the main transformations occurs at special temperatures.
The state of iron is designated as α-ferrum (at a temperature of less than 911 ° C). The crystal lattice is a volumetric face-centered cube. Or bcc. The distance between the atoms of such a lattice is quite high.
Iron acquires a gamma modification, that is, is designated as γ-ferrum (911-1392 ° C). The crystal lattice is a face-centered cube (FCC). In this lattice, the distance between atoms is lower than in the bcc.
In the transition α-ferrum in γ-ferrum the volume of mattergetting smaller. The reason for this is the crystal lattice - its appearance. Because the fc lattice has a more ordered state of atoms than bcc.
If the transition is carried out in the opposite direction - from γ-ferrum to α-ferrum, the volume of the alloy increases.
When the temperature reaches 1392 ° C (butless than the melting point of iron 1539 ° C), then α-ferrum turns into a δ-ferrum, but this is not its new form, but only a variety. In addition, the δ-ferrum is an unstable structure.
Properties of technically pure iron
Magnetic properties of iron at various temperatures:
- less than 768 ° С - ferromagnetic;
- more than 768 ° С - paramagnetic.
A temperature point of 768 ° C is called the point of magnetic transformation, or the Curie point.
Properties of technically pure iron:
- hardness - 80 HB;
- temporary resistance - 250 MPa;
- yield strength - 120 MPa;
- relative elongation of 50%;
- relative narrowing - 80%;
- high modulus of elasticity.
Iron carbide
Graphic view of the component of the diagram iron-carbon: Fe3C. The substance is called iron carbide, or cementite. It is typical for him:
- Carbon content of 6.67%.
- Specific weight - 7.82%.
- The crystal lattice has a rhombic form, consisting of octahedra.
- Melting occurs at a temperature of ≈1260 ° C.
- Low ferromagnetic properties at low temperatures.
- Hardness - 800 HB.
- Plasticity is almost zero.
- Карбид железа образует твердые растворы substitutions in which carbon atoms are replaced by non-metal atoms (nitrogen), and iron atoms - metals (chromium, tungsten, manganese). This solid composition is called alloyed.
As noted above, cementite isunstable phase, and graphite - stable. Since the first substance is an unstable compound, disintegrating under certain temperature conditions.
In the diagram iron-carbon there are such states:
- liquid phase;
- ferrite;
- austenite;
- cementite;
- graphite;
- perlite;
- ledeburit.
Consider each of them in detail.
Liquid phase
Ferrum in the liquid state dissolves carbon well. This is regardless of what proportion they are in percentage content. The result is a homogeneous liquid mass.
Ferrite
Is a solid solution introducing carbon intoα-ferrum. A small amount of impurities may also be included. But ferrite has almost the same quality as pure iron. If we examine the structure under a microscope, then we can see polyhedral grains of a light tone.
It happens:
- low-temperature (at a temperature of 727 ° C, the solubility of carbon is 0.02%);
- high-temperature (at 1499 ° C, the solubility of carbon is 0.1%), or it is called δ-ferrum.
Ferrite properties:
- hardness - 80-120 HB;
- temporary resistance - 300 MPa;
- relative lengthening - 50%;
- has good magnetic properties (up to 768 ° C).
Austenite
This is a solid solution introducing carbon intoγ-ferrum. Can also be in a small amount of impurity. In the crystal lattice, carbon is in the center of the fcc cell. When examining the structure of austenite under a microscope, it is visible as light grains of polyhedral shape with twins.
It has the following characteristics:
- The solubility of carbon in γ-ferrum 2.14% (at a temperature of 1147 ° C).
- 180 austenite hardness;
- Lengthening - 40-50%;
- Good paramagnetic quality.
Cement and its forms
Present in the following phases: C1, C2, C3 (primary, secondary and tertiary cementite).
As for the physico-chemical indicators of these three states, they are approximately equal. The mechanical properties are influenced by the size of the particles, their number and location.
You can also see from the diagram that:
- C1 is formed from a liquid state (under a microscope, it is visible as a plate of large size);
- C2 - from austenite (located around its grains in the form of a grid);
- Ц3 - from ferrite (located at the borders of ferritic grains in the form of small particles).
Perlite and Ledeburite
A mixture of ferrite and cementite is called perlite. It is formed during the decomposition of austenite (at a temperature of less than 727 ° C). When magnified, this structure is in the form of plates or grains.
Perlite with a gradual decrease in temperature is present in all alloys with a carbon content of 0.02–6.67%.
Ledeburite - a mixture of austenite and cementite. It is formed from the liquid phase when cooled to a temperature below 1147 ° C.
Cast iron
Сплавы на диаграмме железо-углерод, которые contain carbon more than 2.14%, called cast iron. They are highly brittle. The cross section of such a cast iron has a light tone, and therefore it is called white cast iron.
In the diagram, this is point C, called eutectic,with a corresponding carbon content of 4.3%. When crystallization forms a mixture consisting of austenite and cementite, collectively referred to as ledeburit. The phase composition is constant.
When the carbon concentration is less than 4.3%(hypoeutectic cast iron) during crystallization austenite is released from the solution. Next, it stands out for C2. And at 727 ° C, austenite turns into perlite. The structural state of such a cast iron is as follows: large portions of dark tone pearlite.
В заэвтектическом белом чугуне (углерода более 4.3%) upon cooling, structuring occurs with the formation of C1 crystals. Further transformations are carried out in the solid state. The structure is a ledeburite, which is the background for dark-colored perlite fields. And large strata is C1.
conclusions
To achieve absolute equilibrium, both physical and chemical, is impossible, except in special laboratory conditions.
In practice, the equilibrium can be approximated toabsolute, but under certain conditions: it is sufficient to slowly raise or lower the temperature of the alloy, which will last for a long time.
