2CO → CO2 + C,
at high temperature back reaction takes place
CO2 + C → 2CO,
sometimes being a link in the direct reduction process, since direct reduction is largely carried out by means of carbon monoxide.
The decomposition of carbon monoxide at a temperature of 400–600° can be one of the links in indirect reduction, since the decomposition product, black carbon, being an active reducing agent, interacts directly with iron oxides (in pores or on the surface of ore pieces) already at 540–580° . Meanwhile, carbon in less active forms, for example, in the form of graphite or coke, does not reduce iron oxides at low temperatures. At temperatures of the order of 600-700°, black carbon already vigorously interacts with ferrous oxide in a reaction;
FeO + C → Fe+CO.
Summing up this reaction by the decay reaction
2CO → CO2 + C,
we get the indirect reduction reaction:
FeO + CO → Fe + CO2.
Thus, the decomposition of carbon monoxide can be a link in indirect reduction. However, the decomposition of carbon monoxide is not an indispensable condition for the process of indirect reduction, which also develops independently of the decomposition of CO.
Active black carbon, which reduces iron at a temperature of 600-700°, on the one hand, increases the indirect reduction, and on the other hand, reduces it, since the gas is enriched with carbon dioxide, which inhibits the reduction of carbon monoxide. Therefore, it is difficult to establish whether indirect reduction is facilitated or worsened during the development of the reaction
2CO → CO2 + C.
The enrichment of the gas with carbon dioxide in the calculations gives an apparent increase in the degree of indirect reduction. But this increase in recovery does not yet mean that most of the iron was recovered indirectly. An increase in indirect reduction is actually possible only if the released black carbon reduces iron at moderate temperatures, and the increased amount of CO2 does not inhibit gas reduction at the same temperatures.
It has now been proven that the processes of decay and the recovery process often proceed simultaneously and simultaneously.
The decomposition of carbon monoxide is associated with the formation of iron carbides (Fe3C) and other carbides, and, consequently, the dissolution of carbon in iron, i.e., the production of cast iron.
It is difficult to imagine the formation of carbide by the direct interaction of solid iron with carbon. Due to the difficulty of ensuring close contact between solid reactants, these reactions cannot develop on an appreciable scale. Iron carbide can be obtained by directly exposing iron to carbon monoxide:
Previously, it was believed that carbon monoxide interacts directly with FeO by the reaction
3FeO + 5CO → Fe3C + 4CO2,
which allows a higher CO2 content in the equilibrium gas mixture than reaction (II, 21). This process should have been carried out already in the upper zones of the furnace, before the appearance of the first portions of reduced iron. However, laboratory studies by G.I. Chufarov and A.N. Kulikov showed that in the presence of ferrous oxide, CO does not decompose into CO2 and C, while it is sponge iron freshly reduced at low temperatures that is a catalyst capable of causing the decomposition of carbon monoxide, which gives the carbon necessary for the formation of carbide.
Carbide from iron in the presence of carbon monoxide can be obtained at a relatively low temperature, as soon as metallic iron appears. The process is noticeably accelerated with increasing temperature, and already at 550-650° iron can completely transform into carbide even in an atmosphere of top gas if the process is sufficiently long. At a temperature of 1000°, the carbide decomposes into iron and carbon (annealing carbon), which is deposited in the mass of the reduced metal. Thus, a mixture of metallic iron with annealing carbon is gradually formed, which then dissolves in the metal when it is melted. Other ways of carbide formation are possible, but the one described above is the most probable.
As experimental and computational studies show, in an equilibrium gas mixture of the reaction
3Fe + 2CO ⇔ Fe3C + CO2
contains less CO2 than for the reaction
2CO ⇔ CO2 + C.
Therefore, the equilibrium curve for this case goes above the theoretical curves shown in Fig. 52.
The final carbon content of cast iron depends on the stability of the carbides. With weak carbides, cast iron contains less carbon, since carbon from the decomposition of carbides floats to the surface of cast iron and is removed from it.
The stability of carbides is determined by the presence of other impurities in cast iron: manganese and chromium can themselves give strong carbides with carbon, as a result of which their presence increases the carbon content in cast iron. On the contrary, silicon and phosphorus form very fragile carbides, and, in addition, they themselves are able to combine with iron to form strong silicides and phosphides, destroying carbides and displacing carbon from them. Therefore, the more silicon and phosphorus in cast iron, the less carbon it contains.
Cast iron, containing only 4.3% carbon, has the lowest melting point in comparison with other iron-carbon alloys - 1130-1135 °. The eutectic points of cast irons containing, in addition to carbon, other elements turn out to be different.
The following formula is recommended, taking into account the influence of the most important impurities of cast iron on its carbon content:
where C, Si, P and Mn are the percentages of the corresponding elements in cast iron, the formula does not take into account the sulfur content in cast iron; it is assumed that it is present within the normal range. With an increased sulfur content, the formula can be supplemented with a member (-0.0325).
According to formula (11.22), it can be calculated that in ordinary open-hearth iron (0.8% Si, 2% Mn, 0.2% P) contains about 4.1% C, in foundry (3% Si, 1% Mn , 0.2% P) - about 3.5%, in ferrosilicon (10% Si) - about 1.5-2%; in ferromanganese (80% Mn) - 6.5-7% and in mirror cast iron (20% Mn) - about 5% carbon.
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Cementite- this is a chemical compound of composition Fe 3 C - iron carbide. It is known that the solubility of carbon in iron is low: about 0.02% C dissolves in α-iron at eutectoid temperature, and at room temperature - about 0.008%. In - iron dissolves up to 2.14% C at a temperature of 1147 ° C, at room temperature - iron does not exist (about austenite at room temperature is written in the article "How to see austenite?" on this site). Therefore, in the structure of steel and cast iron, carbon is in a bound state in the form of cementite or in a free state in the form of graphite.
Now we are talking about cement. The melting point of cementite is about 1250 o C. Cementite has a hardness of about 800 HB and very low ductility. Such properties of cementite are associated with its complex crystal lattice (Fig. 1).
Figure 1. Scheme of the crystal lattice of cementite (A.P. Gulyaev. Metal Science)
We are primarily interested in the fact that cementite is a phase of iron-carbon alloys. If we proceed only from the iron-carbon state diagram, then cementite is distinguished primary, secondary and tertiary. The position of these types of cementite is shown in the state diagram (Fig. 2) in different colors.
Figure 2. Cementite on an iron-carbon state diagram.
Primary cementite is formed in hypereutectic cast iron directly during crystallization (Fig. 3). Long elongated crystals are precipitates of primary carbide. The area on the state diagram in which only primary cementite is present is colored pink.
Figure 3. Hypereutectic chromium iron
Cementite in the composition of ledeburite also belongs to primary cementite (Fig. 4, 5). The cementite of ledeburite in hypoeutectic iron is also primary, since ledeburite is formed during the crystallization of the melt.
Figure 4. Dendrites of primary cementite (a) and ledeburite in half cast iron(b)
Figure 5. Ledeburite
secondary cementite is formed during cooling of austenite (green area on the state diagram), since below 1147 ° C, the concentration of carbon in austenite decreases, and cementite is formed from the "liberated" carbon. At the eutectoid temperature, austenite decomposes, therefore, below 727 ° C, there is always secondary cementite on the phase diagram. At room temperature, we see secondary cementite in pearlite, as well as along grain boundaries in hypereutectoid steel (Fig. 6).
Figure 6. Secondary cementite and perlite in hypereutectoid steel
Tertiary cementite is formed when the ferrite is cooled (blue section on the state diagram). Unlike other types of cementite, it is difficult to observe, because it is released in small quantities, and when the concentration of carbon increases, it combines with perlite cementite. Usually tertiary cementite exudes along the grain boundaries(Fig. 7). Sometimes he himself is not visible, but decorates the border of the grain, and it is better visible.
 |
 |
 |
| A | b | V |
Figure 7. Tertiary cementite in steel 08Yu (a), carbon steel(b,c)
Isolation of tertiary cementite is the result aging. The fact is that the concentration of carbon in ferrite changes with temperature. The saturation limit of ferrite with carbon is shown by the PQ line (Fig. 8). The concentration of carbon in ferrite changes with temperature - at 723 ° C, about 0.03% C dissolves in ferrite, and at room temperature - up to 0.008% C. If the steel is heated above the PQ line, and then quickly cooled (i.e., quenched), then as a result of quenching, we get a supersaturated solid solution of carbon in α - iron. This solution will not change over time. And at room temperature, and at elevated temperatures, it will decay - grow old. The hardening phase formed during such aging is tertiary cementite.
Figure 8. Region of tertiary cementite precipitation (marked in blue) on the iron-carbon state diagram.
So:
. primary cementite crystallizes from the melt;
. secondary cementite is formed upon cooling of austenite;
. tertiary cementite is formed when ferrite is cooled.
In steels and cast irons, cementite can take on a variety of forms. Secondary cementite in steels can have different morphologies. Usually, cementite has the form of plates in the composition of perlite (Fig. 9a). Then it can be distinguished from ferrite only with special etching. After a certain annealing, pearlite transforms, and cementite takes the form of rounded inclusions. This granular perlite(Fig. 9 b). An article about granular perlite is on this site.
In steels where there are alloying elements, cementite does not have the formula Fe 3 C. Cementite can form substitutional solid solutions. Carbon atoms can be replaced by other non-metals - nitrogen, oxygen, iron atoms - by atoms of alloying elements - chromium, nickel, tungsten, etc. In this case, it is formed alloyed cementite, whose formula is written as Me 3 C. The letter M denotes metal atoms that partially replace iron. In most steels, alloyed cementite. We deal with cementite steels mainly in the following cases. After normalization, steel cementite enters into pearlite (secondary cementite, Fig. 6); sometimes tertiary cementite can also be seen (Fig. 7). After quenching and tempering, carbides are present in the steel; their composition is different; but one way or another, carbides of composition Me 3 C must be attributed to secondary cementite. On fig. 10 shown carbides in steel R18 after hardening and tempering. Still there you can see the remains of a cast structure (“skeleton” in the center of the frame); it is a carbide that has crystallized from a liquid; it can be taken as primary cementite. It was not eliminated by further processing. This is a structural defect that leads to a decrease in properties.
Figure 9. Morphology of cementite in steel: a - lamellar cementite, b - globular.
Figure 10. Primary cementite and carbides in R18 steel
In white cast iron, primary cementite is present, similar in structure to that shown in Figs. 3 and 4. In gray cast irons (after the graphitization operation), a small amount of cementite can sometimes remain (Fig. 11a, the inclusion of cementite is marked by an arrow), as well as cementite pearlite, which does not disintegrate during annealing (Fig. 11 b). In Fig. 12, white "skeletons" are primary cementite in austenitic cast iron; white small inclusions across the field are also segregations of cementite (or carbides, because it is alloyed cast iron), which can be called secondary.
Figure 11. Structure of gray cast iron.
Figure 12. Cementite in austenitic cast iron.
CEMENTITE- a chemical compound (iron carbide) in iron-carbon alloys, and corresponding to the maximum carbon content. The chemical formula of cementite is Fe 3 C, the concentration of carbon is 6.67% (by mass) (by mass).
As follows from the iron-carbon state diagram, as a phase component, cementite exists in iron-carbon alloys already at very low carbon contents (hundredths of a percent) and its amount increases with increasing carbon content. In this case, cementite is included in the structural component of perlite (a mixture of ferrite and cementite) that exists in steel, along with ferrite. As the carbon content increases, the proportion of perlite in the structure increases and, accordingly, the amount of cementite increases. With a carbon content of 0.8% (eutectoid steel), the structure is entirely perlite. With a further increase in the carbon content in steel, in addition to perlite, excess cementite appears. Up to a carbon content of 1.7%, iron-carbon alloys are called steels, at higher concentrations up to a maximum of 6.67% - cast irons.
In the process of heat treatment in steels, cementite is formed upon cooling and decomposition of a solid solution (austenite), in cast irons - directly upon cooling from a liquid state. The corresponding structural component of cementite and austenite is called ledeburite with a total carbon content of 4.3%. With a further increase in the proportion of carbon during cooling, cementite (primary) and ledeburite are released from the liquid during cooling. In cast irons containing austenite, upon cooling, pearlite transformation occurs, which also leads to the release of cementite.
Cementite has high hardness and brittleness; therefore, iron-carbon alloys containing a lot of cementite are not amenable to plastic deformation.
Due to the different mechanisms of cementite formation, its microstructure can be very different for alloys with different carbon contents after various heat treatments, and crystal sizes can vary from hundredths to several mm.
The crystal structure of cementite, determined by X-ray diffraction analysis, is rhombic. Its elementary cell, i.e. the minimum configuration of atoms, the parallel transfer of which can fill the space, is a rectangular parallelepiped with different sizes along all three axes and a certain arrangement of iron and carbon atoms in the cell.
In alloy steels, compounds can occur with a chemical formula similar to that of cementite, but with some of the iron atoms replaced by atoms of the alloying element. Such compounds are called special carbides.
Figure 3 - Diagram of the state of the iron-carbide-iron system
Components: iron and carbon.
1) Liquid phase. In the liquid state, iron readily dissolves carbon in any proportions with the formation of a homogeneous liquid phase.
2) Ferrite - a solid solution of carbon incorporation in α-iron with a bcc (body-centered cubic) lattice.
3) Austenite (γ) - a solid solution of carbon incorporation in γ-iron with an FCC (face-centered cubic) lattice.
4) Cementite (Fe3C) - a chemical compound of iron with carbon (iron carbide), with a complex rhombic lattice, contains 6.67% carbon.
Mechanical mixtures:
1) Perlite (P) - a mechanical mixture (eutectoid, that is, similar to a eutectic, but formed from a solid phase) of ferrite and cementite, containing 0.8% carbon. Perlite can be lamellar and granular (globular), which depends on the form of cementite (plates or grains) and determines the mechanical properties of perlite.
2) Ledeburite (L) - a mechanical mixture (eutectic) of austenite and cementite containing 4.3% carbon. Ledeburite is formed when a liquid melt solidifies at 1147°C. Since austenite transforms into pearlite at a temperature of 727°C, this transformation also includes austenite, which is part of ledeburite. As a result, at temperatures below 727°C, ledeburite is no longer a mixture of austenite with cement, but a mixture of pearlite with cementite.
Alloy grade: U18A.
5. Show graphically the annealing mode to obtain pearlitic ductile iron. Describe the structural transformations that occur during annealing and the mechanical properties of cast iron after heat treatment
Malleable cast irons are obtained from white cast irons by graphitizing annealing (languishing).
Figure 4 - Graph of annealing modes of white cast iron for malleable
Annealing is carried out in two stages. First, white cast iron castings are heated for 20-25 hours to a temperature of 950-970 °C. During holding (15 h) at this temperature, the first stage of graphitization occurs, i.e. the decomposition of cementite, which is part of ledeburite (A + Fe 3 C), and the establishment of a stable austenite + graphite equilibrium. As a result of the decomposition of cementite, flaky graphite is formed. Then the castings are slowly cooled (over 6-12 hours) to a temperature of 720 °C. During cooling, secondary graphite is released from austenite and graphite inclusions grow. Upon reaching a temperature of 720 ° C, a second long exposure is given, during which the decomposition of cementite, which is included in perlite, into ferrite and graphite occurs. The second stage of graphitization lasts about 30 hours, and after its completion, the cast iron structure consists of graphite and ferrite. The fracture of ferritic cast iron is velvety black due to the large amount of graphite.
If the second stage of graphitization is not carried out, then ductile iron with a graphite + pearlite structure is obtained. The fracture of such cast iron is light.
Annealing is a long 70...80 hours and expensive operation. IN Lately, as a result of improvements, the duration was reduced to 40 hours.
Various measures are taken to speed up the annealing of white cast iron to malleable cast iron: cast iron is modified with aluminum (rarely boron or bismuth), the heating temperature is increased before casting, quenching is carried out before annealing, the temperature of the first graphitization stage is increased (up to 1080 ° C), etc.
In terms of mechanical and technological properties, malleable cast iron occupies an intermediate position between gray cast iron and steel. The disadvantage of ductile iron compared to ductile iron is the limitation of the wall thickness for casting and the need for annealing.
Ductile iron castings are used for parts operating under shock and vibration loads. Ferritic cast irons are used to make gearbox housings, hubs, hooks, brackets, clamps, couplings, flanges. Perlitic cast irons, characterized by high strength and sufficient ductility, are used to make forks of cardan shafts, links and rollers of conveyor chains, and brake shoes.
Bibliography
1. Zaplatin, V.N. Fundamentals of materials science (metalworking): Proc. manual for NGOs / V.N. Zaplatin. - M.: Academy, 2008 - 200 p.
2. Zaplatin, V.N. Reference manual for materials science (metalworking): Proc. manual for NGOs / V.N. Zaplatin. - M.: Academy, 2007. - 135 p.
Cementite- chemical compound of carbon with iron ( iron carbide), Fe 3 C - one of the structural components of iron-carbon alloys (cast iron, steel), contains 6.67% C.
Cementite is an unstable compound and certain conditions breaks down to give free carbon as graphite. Cementite capable of forming substitutional solid solutions. Carbon atoms (C) can be replaced by non-metal atoms - N, O; iron atoms (Fe) - metals Mn, Cr, W and others. Such a solid solution based on the cementite lattice is called alloyed with cementite. The usual designation of alloyed cementite is M 3 C, where the letter M refers to iron and other metals that replace iron atoms in the cementite lattice.
ICM (www.website)
Cementite Fe 3 C has a rhombic structure. Grating periods: a=0.45244±0.0005 nm, b=0.50885±0.0005 nm, c=0.67431±0.0005 nm; subsequent studies confirmed this structure and gave close values of the periods. Neutron diffraction analysis also confirmed rhombic structure of cementite. Transition temperature cementite from the ferromagnetic to the paramagnetic state is 215°C.
The figure shows a cell crystal lattice of cementite. The crystal structure of cementite is very complex. cementite crystal consists of a series of octahedrons, the axes of which are located to each other at certain angles. Inside each octahedron is a carbon atom. In the figure, carbon atoms are highlighted in green: each carbon atom is surrounded by eight iron atoms; each iron atom is bonded to three carbon atoms.
ICM (www.website)
Properties of cementite
Cementite has metallic properties (electrical conductivity, metallic luster, etc.) due to the fact that both iron and carbon are positively ionized in the lattice, that is, both metal and carbon behave like a metal in a compound. Cementite brittle, has high hardness (HB over 800); t pl =1250°C; Cementite has an extremely low, almost zero ductility. These properties of cementite are probably a consequence of the complexity of the structure of the crystal lattice of cementite.
At low temperatures, cementite is weakly ferromagnetic. Magnetic properties of cementite, according to various sources, loses at a temperature of 215-217 °.
Melting point of cementite
According to Gulyaev A.P. the melting point of cementite is about 1600°.
ICM (www.website)
According to the calculated data, the virtual melting point of cementite is estimated to be 1200-1450°. It is possible that cementite undergoes incongruent decomposition at temperatures of 1250-1300°.
Primary cementite
There are primary, secondary and tertiary cementite. Primary cementite released from the liquid. Primary cementite is released only during the quenching of alloys containing up to 5.5% (by mass) of carbon. Form of primary cementite: long large plates.
secondary cementite
Secondary cementite is released from austenite - a γ-solid solution. During cooling, the separation occurs along the ES line (Fe-C diagram). Form of secondary cementite: cementite network, cementite along grain boundaries.
Tertiary cementite
Tertiary cementite precipitates from ferrite. Tertiary cementite form: plates and veins, as well as precipitates in the form of needles in ferrite grains. With more rapid cooling, part of the carbon remains in solid solution; the release of tertiary cementite is suppressed.
Other forms of existence of cementite(according to Howe): perlite cementite, ledeburite cementite, Stead cementite, granular cementite, special carbides.
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