It is difficult to imagine modern construction, engineering, mechanical engineering and other important industries without the use of the main metal alloys made of steel and cast iron. Their production exceeds all others tenfold.
If we consider steel and cast iron from the point of view of a science such as metal science, then the central figure is the state diagram of iron-carbon alloys, which allows you to get detailed ideas about the composition and structural transformations in these materials. And also get acquainted with their phase composition.
Discovery story
For the first time, there are certain (special) points in alloys (steels and cast irons), the great metallurgist and inventor Dmitry Konstantinovich Chernov pointed out (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 on 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 a science as metallography begins its life.
The diagram of iron-carbon alloys is the result of the hard work of scientists from several countries of the world. All letter designations of the main points and phases in the diagram are international.
Chart concept
A graphical representation of the processes occurring in an alloy with a change in temperature, concentration of substances, pressure is called a state diagram. It allows you to volume and visually see all the transformations occurring in the alloys.
Iron-carbon Chart Elements
Brief information about each of these elements.
Iron is a silver gray metal. Specific gravity - 7, 86 g / cm3. 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 interstitial solutions.
Iron has the ability, being initially solid, to be in several states, which in metal science are usually called "alpha" and "gamma". This quality is called polymorphism. More on this later in the article.
Carbon is non-metal. If it acts as graphite, then the melting point is 3500 ° C. If like diamond - 5000 ° C. The carbon density is 2.5 g / cm 3 . It also has polymorphic properties.
In iron-carbon alloys, this element forms a solid solution, in the composition of which there is a ferrum called cementite (Fe 3 C). Also forms graphite in cast irons.
Iron-Carbon Alloy Chart
As a result of the interaction of the components of the diagram with each other, cementite is obtained - a chemical compound.
As a rule, when studying the diagram by metalworking students, all stable compounds are considered as components, and the graphic image itself is examined in parts.
Also in the classroom, a cooling curve is constructed using the iron-carbon diagram: the percentage of carbon is selected, and then it is necessary to determine which phase corresponds to which temperature in the diagram.
To do this, in addition to the diagram itself, draw a coordinate system (temperature-time). And starting from maximum degrees, move gradually downward, depicting a curve and sections of the transition of one phase to another. In this case, it is necessary to name them and indicate the type of crystal lattice.
Next, we consider in more detail the graphical image of the iron-carbon state diagram.
Firstly, it has two forms (parts):
- iron cementite;
- iron graphite.
Secondly, alloys in which the main "actors" is ferrum and carbon are conventionally divided into:
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% is cast iron. For this reason, the diagram is divided into two phases.
Polymorphic Transformations
More details about each phase are a bit lower 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 volume 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 the atoms is lower than in the bcc.
With the transition of α-ferrum to γ-ferrum, the volume of the substance becomes smaller. The reason for this is the crystal lattice - its appearance. Because the fcc 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, then the volume of the alloy increases.
When the temperature reaches 1392 ° C (but less than the melting point of iron 1539 ° C), the α-ferrum turns into a δ-ferrum, but this is not its new form, but only a variety. In addition, δ-ferrum is an unstable structure.
Properties of technically pure iron
Magnetic properties of iron at various temperatures:
- less than 768 ° C - ferromagnetically;
- more than 768 ° - 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;
- elongation of 50%;
- relative narrowing - 80%;
- high modulus of elasticity.
Iron carbide
Graphical view of the constituent part of the iron-carbon: Fe3C diagram. The substance is called iron carbide, or cementite. It is characteristic of him:
- The carbon content of 6.67%.
- The specific gravity is 7.82%.
- The crystal lattice has a rhombic shape, consisting of octahedrons.
- Melting occurs at a temperature of ≈1260 ° .
- Low ferromagnetic properties at low temperatures.
- Hardness - 800 HB.
- Plasticity is almost zero.
- Iron carbide forms substitutional solid solutions in which carbon atoms are replaced by non-metal atoms (nitrogen) and iron atoms by metals (chromium, tungsten, manganese). This solid composition is called alloyed.
As noted above, cementite is an unstable phase, and graphite is stable. Since the first substance is an unstable compound, decomposing under certain temperature conditions.
In the iron-carbon diagram, there are such states:
- liquid phase;
- ferrite;
- austenite;
- cementite;
- graphite;
- perlite;
- ledeburit.
Let's consider each of them in detail.
Liquid phase
Ferrum in a liquid state dissolves carbon well. This is regardless of what proportion they are in percentage. The result is a homogeneous liquid mass.
Ferrite
It is a solid solution of carbon incorporation in α-ferrum. A small amount of impurities may also be included. But ferrite has almost the same qualities as pure iron. If you look at the structure under a microscope, you can see the polyhedral grains of light tone.
It happens:
- low temperature (at a temperature of 727 ° , 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;
- elongation of 50%;
- has good magnetic properties (up to a temperature of 768 ° C).
Austenite
This is a solid solution of carbon incorporation in γ-ferrum. There may also be a small amount of impurities. In the crystal lattice, carbon is located in the center of the fcc cell. When examining the structure of austenite under a microscope, it is visible as light grains of a polyhedral shape with twins.
It has the following characteristics:
- The solubility of carbon in the γ-ferrum 2.14% (at a temperature of 1147 ° C).
- The hardness of austenite is 180 HB;
- Elongation - 40-50%;
- Good paramagnetic qualities.
Cementite and its forms
Present in such phases: C1, C2, C3 (primary, secondary and tertiary cementite).
As for the physicochemical parameters of these three states, they are approximately equal. The mechanical properties are affected by the size of the particles, their number and location.
The diagram also shows that:
- C1 is formed from a liquid state (under a microscope, it is visible as large-sized plates);
- C2 - from austenite (located around its grains in the form of a grid);
- C3 - from ferrite (located at the boundaries 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 enlarged, this structure has 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 is a mixture of austenite and cementite. It is formed from the liquid phase upon cooling to a temperature below 1147 ° C.
Cast iron
Alloys in the iron-carbon diagram that contain more than 2.14% carbon are called cast irons. They are highly fragile. The cross section of such 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%. During crystallization, a mixture is formed consisting of austenite and cementite, collectively called ledeburite. The phase composition is constant.
At a carbon concentration of less than 4.3% (pre-eutectic cast iron), austenite is released from the solution during crystallization. Next, C2 stands out from it. And at 727 ° C, austenite turns into perlite. The structural state of such cast iron is as follows: large areas of dark perlite.
In hypereutectic white cast iron (carbon of more than 4.3%) upon cooling, structuring occurs with the formation of C1 crystals. Further, the transformations are carried out already in the solid state. The structure is ledeburite, which is the background for dark-tone perlite fields. And large strata are C1.
conclusions
It is impossible to achieve absolute equilibrium, both physical and chemical, except under special laboratory conditions.
In practice, the equilibrium can be close to absolute, but under certain conditions: it is enough to slowly increase or decrease the temperature of the alloy, which will be maintained for a long time.