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Iron – Cementite Phase Diagram

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Iron – Cementite Phase Diagram Module 23 Iron Carbon System I Lecture 23 Iron Carbon System I 1 NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | | | Keywords : Ferrite (), Austenite (), Ferrite (d): three different forms of iron, solubility of carbon in different forms of iron, cementite, graphite, Fe – Fe3C meta‐stable phase diagram, pearlite, ledeburite, hypo‐eutectoid steel, hyper‐eutectoid steel, hypo‐eutectic white cast iron, hyper‐eutectic white cast iron Introduction The last five modules were devoted to the solidification behavior of binary alloys. This is best represented by its phase diagram. We are now familiar with a wide range of simple and complex phase diagrams. However while introducing this we only considered hypothetical cases. Now let us consider a specific binary alloy of considerable importance. We are going to talk about iron ‐ carbon system. Iron can exist in two different crystalline forms. These are BCC and FCC. This is known as allotropy. Carbon too has several allotropic forms. However the form which is of relevance to this system is graphite. It has a hexagonal structure. This is considered to be the most stable state of carbon. An atom of carbon atom is significantly smaller than that of iron. Therefore it can be accommodated within the interstitial sites of iron lattice resulting in an interstitial solid solution. Solubility carbon is likely to be limited. Apart from this iron has a strong affinity to form carbide. The most common form is cementite. Its chemical formula is Fe3C. This is truly a meta‐stable. However the rate of decomposition of Fe3C into Fe and C is extremely slow. In most of the common grades iron – carbon alloy excess carbon is present as cementite. Let us first look at the Fe‐Fe3C meta‐stable phase diagram and in a later module we shall discuss about iron – graphite phase diagram as well. The alloys belonging to this system is popularly known as steel or cast iron. We would soon learn about the difference between the two. Solidification of pure iron: Fe : crystal structure T Liquid Solubility of carbon in Fe = 1539 f(structure, temperature) BCC 1394 FCC Where is the carbon Slide 1 910 located in iron lattice? Paramagnetic BCC 770 Ferromagnetic 2 time NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | | | The sketch in slide 1 is a typical cooling curve of pure iron. Solidification begins with nucleation and growth of crystals of iron at 1539°C. It is BCC (body centered cubic). At 1394°C it transforms into FCC (face centered cubic) structure. This is stable till 910°C where it again transforms into BCC. Each of these transformations appears as steps on the cooling curve. Apart from this there is another transformation which may not get detected by thermal analysis. This is the transformation from paramagnetic to ferromagnetic state. It occurs at 770°C. This is known as its Curie temperature. The property which is most sensitive to detect it, is magnetic permeability. The three different forms of iron are known as ferrite (), stable until 910°C, austenite (), stable from 910°‐1394°C and ferrite (), stable from1394°– 1539°C. Note that the BCC form of iron is known as ferrite. Therefore in order to distinguish between the two, the high temperature form is termed as delta ferrite. If carbon atoms are introduced into iron these are likely to occupy the interstitial sites because the atoms carbon are much smaller than those of iron atoms. The interstitial sites in BCC and FCC are shown in slide 2. The solubility of carbon in iron is a function of temperature and crystal structure. Solubility of carbon is higher in BCC or FCC? Interstitial sites in iron lattice c c Slide 2 b b a a BCC FCC Lattice site Interstitial site Slide 2 shows the lattice sites occupied by iron atoms and possible interstitial sites for carbon atoms in both BCC & FCC structures. The interstitial sites shown here are known as octahedral sites. The sketches in slide 2 show only the positions of iron atoms and the interstitial sites in one unit cell. The sites located within the cell belong exclusively to a unit cells but those lying on the faces, the edges or the corners are 3 shared by the neighboring unit cells as well. BCC has eight corner sites for iron atoms. Each of these is shared by 8 neighboring unit cells. The contribution of corner site is thus 1/8. The atom at the centre belongs exclusively to this unit cell. Therefore the number of iron atoms / unit cell = 8 x (1/8) + 1 = 2. Let NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | | | us use the same approach to estimate the number of interstitial sites /unit cell for BCC structure. The sketch in slide 2 shows the locations of interstitial sites. There are 6 sites at the centers of 6 faces. Each face is shared by two unit cells. There are 12 sites at each of the 12 edges. Each of these is shared by 4 neighboring cells. Therefore the number of interstitial sites / unit cell in BCC crystal = 12 x (1/4) + 6 x (1/2) = 6. Note that it is 3 times the number of Fe atoms in a unit cell. Look at the sketch for FCC unit cell in slide 2. Count the number of lattice sites. Follow the same approach to show that the number of Fe atoms / unit cell = 6 x (1/2) + 8 x (1/8) = 4. In the same way the number of interstitial sites / unit cell in FCC structure = 1 + 12 x (1/4) = 4. Note that in FCC structure the ratio of the number of interstitial site to the number of lattice sites = 1. Now that we know about the possible sites carbon atoms could occupy is it possible to guess which of these is likely to have higher solubility? F D C F O A B D Fig 1 A O E C B E (a) (b) Fig. 1(a): Shows the lattice and the interstitial sites in a BCC unit cell. Circles are the lattice sites. The black dots are the interstitial sites. In a lattice several of such cells are stacked one after the other. F represents the body center site in a unit cell just above the one shown in the sketch. The figure ABCDEF formed by joining the lattice sites as illustrated is an octahedron. There is an interstitial site at its center marked as O. All the interstitial sites are identical. Note that AB = BC = CD = DA = = the lattice parameter. The other edges of the octahedron AE = BE = CE = DE = AF = BF = CF = DF = √3/2. It is not a regular octahedron. The interstitial site in BCC structure is not symmetrical. The gap along EF is shorter than that along AC or DB. It is 2√3 /2. The lattice parameter of BCC iron called ferrite is 0.286nm. The shortest gap is equal to 0.038nm. Fig. 1(b): Shows the lattice and the interstitial sites in a FCC unit cell. Circles are the lattice sites. The black dots are the interstitial sites. In a lattice several of such cells are stacked one after the other. The figure ABCDEF formed by joining the lattice sites as illustrated is an octahedron. There is an interstitial site at its center marked as O. All the interstitial sites are identical. Note that AB = BC = CD = DA = AE = BE = CE = DE = AF = BF = CF = DF = √ , where = the lattice parameter. It is a regular octahedron. The 4 interstitial site in FCC structure is symmetrical. The gap along EF is equal to that along AC or DB. It is 2√2 /2. The lattice parameter of FCC iron called austenite is 0.362nm. The interstitial gap is equal to 0.106nm. It is much larger than the interstitial gap in ferrite. NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | | | FCC has higher packing density than BCC. Yet the solubility of carbon is FCC is higher. This is because the interstitial sites in FCC are bigger than that in BCC. There are two types of interstitial sites known as octahedral and tetrahedral. In FCC the tetrahedral sites are surrounded by four close packed atoms. The gap is extremely small, whereas in BCC the four atoms are more widely spaced. The tetrahedron is not symmetric. The shortest dimension (size) of tetrahedral sites in BCC lattice was derived in module 5. This is nearly same or marginally lower (0.036nm) than that of the octahedral site (0.038nm). The number of such sites is much more than that of the octahedral sites. Although the packing density in BCC is lower than that of FCC, the gaps are equally distributed between two types of interstices making these significantly smaller than that of FCC. This is why the solubility of carbon which is an interstitial solute is significantly lower in ferrite (BCC) than that in austenite (FCC). Apart from this the interstitial site in BCC is asymmetric. When a carbon atom goes into the interstices the atoms along one of the axes are pushed further apart. This results in a tetragonal distortion. We shall talk about it in one of the subsequent modules. Phases in iron – carbon binary system: Iron can exist in three different crystalline forms each having limited solubility of carbon. The stability of these depends on temperature and composition. The two high temperature forms of iron are ferrite which is BCC (stable above 1394°C) and austenite (stable above 910°C) which is FCC.
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