In this article we will discuss about:- 1. Introduction to the Iron-Carbon Equilibrium Diagram 2. Phases in Fe-Fe3C Diagram 3. Critical Temperatures 4.Transformations and Microstructures of Slowly Cooled Steels 5. Methods Used to Distinguish between Free-Ferrite and Free-Cementite 6. Limitations.
Contents:
- Introduction to the Fe-Fe3C Equilibrium Diagram
- Phases in Fe-Fe3C Diagram
- Critical Temperatures in Fe-Fe3 C Equilibrium Diagram
- Transformations and Microstructures of Slowly Cooled Steels in Fe-Fe3C Equilibrium Diagram
- Methods Used to Distinguish between Free-Ferrite and Free-Cementite in Fe-Fe3C Equilibrium Diagram
- Limitations of Fe-Fe3C Equilibrium Diagram
1. Introduction to the Fe-Fe3C Equilibrium Diagram:
Carbon is the most important alloying element in iron which significantly affects the allotropy, structure and properties of iron. The study of Fe-C system is thus, important, more so as it forms the basis of commercial steels and cast irons, and many of the basic features of this system influence the behaviour of even the most complex alloy steels. Steels may have incidental elements, or intentionally added alloying elements, which modify this diagram, but if modifications are interpreted cautiously, then this diagram acts as a guide.
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The ability to interpret this diagram is important for proper appreciation of phase changes. Fe-C diagram actually provides a valuable foundation on which to build knowledge of large variety of both plain carbon and alloy steels.
Conventionally, the complete Fe-C diagram should extend from 100% Fe to 100% carbon, but it is normally studied up to around 6.67% carbon as is also illustrated in Fig. 1.22, because iron alloys of practical industrial importance contain not more than 5% carbon. Thus, this diagram is only just a part of the complete Fe-C equilibrium diagram.
Iron forms a compound with carbon called cementite, when the carbon content becomes more than the solubility limits of iron. Though, carbon in the form of graphite should from as it has lower free energy than cementite, yet cementite forms because the formation of cementite is more probable kinetically, i.e. it is easier to form it as only 6.67% C has to diffuse to segregate to form cementite, whereas 100% segregation of carbon is required to nucleate graphite.
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Cementite, chemical formula Fe3C has a fixed carbon content of 6.67%, i.e., when iron has 6.67% carbon, then 100% cementite is obtained. Thus, this diagram (see bottom most horizontal line in (Fig. 1.22) which has components iron and cementite (100% Fe on one end to 100% cementite on the other end of the diagram) can rightly be called as iron-cementite diagram.
Iron-Cementite diagram is not a true equilibrium diagram, since equilibrium means no change of phase with time, however long it may be. Graphite is more stable form of carbon. Cementite is a metastable phase, which decomposes to graphite if given long periods of time. Graphitisation, however, rarely occurs in steels and may take years to form. Thus, cementite, though a metastable phase, can be taken to be practically stable.
Thus, Fe – Fe3C diagram even though represents metastable conditions, can be assumed to represent equilibrium state relevant to the behaviour of most steels in practice. In cast irons, the high carbon content and the high silicon additions promote graphite formation and thus, in cast irons (except white cast iron) transformations are based much more on Fe-graphite diagram. Fe-C diagram is strictly valid only at one atmospheric pressure. Conventionally, the compositions are marked in weight %.
2. Phases in Fe-Fe3C Equilibrium Diagram:
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(i) Alpha Ferrite, or in Short Commonly Called Just Ferrite:
Ferrite is an interstitial solid solution of carbon in alpha iron and thus, is BCC in structure. It derives its name from Latin word ‘ferrum’ meaning iron. The maximum solubility of carbon in ferrite is 0.02% at 727C (point T in Fig. 1.22), which decreases with the fall of temperature to negligible amount at 0°C (< 0.00005% at 20°C). It is soft and ductile phase.
Ferrite is ferromagnetic at low temperatures but loses its magnetic properties with the rise of temperatures with major loss at Curie temperature, 768°C and above this temperature it becomes non-magnetic (Paramagnetic). Fig. 1.21 (b) illustrates micro- structure of ferrite showing polyhedral grains. Each grain has difference in orientation of atoms a unit cell of one grain is outlined.
(ii) Austenite:
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It is an interstitial solid solution of carbon in gamma-iron and has FCC structure. It derives its name from ‘Sir Austen’. The maximum solubility of carbon in austenite is 2.11% at 1147°C (point Q in Fig. 1.22), which decreases to 0.77% carbon at 727°C. Austenite is soft, ductile tough and malleable (FCC structure) and non-magnetic (paramagnetic). Steels are commonly rolled and forged above about 1100°C when they are in austenitic state due to its high ductility and malleability, which is also due to its FCC structure.
The mechanical properties are given in Table 1.9. It is stable above 727°C in plain carbon steels but can be obtained at room temperature, say by adding elements like Ni or Mn in steels and the micro-structure is illustrated in Fig. 1.21 (a) with one unit cell outlined of a grain of polyhedral structure.
(iii) Delta-Ferrite:
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It is an interstitial solid solution of carbon in delta iron having BCC structure. It has maximum solubility of carbon of 0.09% at 1495°C. It is a high temperature phase and is a high temperature manifestation of a-ferrite.
(iv) Cementite, Iron Carbide, Fe3C:
It is an interstitial intermediate compound having a fixed carbon content of 6.67%. It has a complex orthorhombic crystal structure with 12 iron atoms and 4 carbon atoms per unit cell. It is a compound with high hardness (~ 800 VPN), which easily scratches the glass. It is brittle phase with low tensile strength and high compressive strength. It is slightly ferromagnetic up to 210°C and paramagnetic above it. The melting point is around 1227°C.
Cementite can form substitutional solid solution, i.e., its carbon atoms can be substituted by non-metallic elements like N or O. Its iron atoms can be substituted by atoms of Mn, Cr, W, etc., when it is called alloyed cementite and can be represented by (Fe, M)3 C where M stands for the symbol of the metal. Cementite is a metastable compound and decomposes under certain conditions to form free carbon i.e., graphite.
3. Critical Temperatures in Fe-Fe3 C Equilibrium Diagram:
Steels also experience the arrest of temperatures during cooling, or heating, when the transformations, both, phase as well magnetic, take place in them. The temperatures at which the transformations occur (arrests occur) in the solid state are called critical temperatures, or critical points. Similar symbols are used, as for pure iron, to denote them.
The critical points, during heating the steels, are:
A0:
The 210°C is the Curie temperature (magnetic to non-magnetic change on heating) of cementite as indicated by the dotted line, A0 in Fig. 1.22.
A1:
Addition of carbon in amounts more than 0.02% in iron, results in this critical point due to the eutectoid invariant transformation at constant temperature of 727°C, where pearlite changes to austenite (of 0.77 % C) on heating and vice versa. Heat is evolved during cooling to cause thermal arrest and in fact, if a steel of carbon more than 0.6 % is cooled in darkness, the heat evolved at A1 may raise the temperature of sample to such an extent that a red glow is, again momentarily, observed before the cooling is resumed. This A1 thermal arrest point is also called the recalescence point, or carbon point. Though, A1 owes its existence to carbon, it is unaffected by the carbon content as is illustrated by the horizontal line TUT’ in Fig. 1.22. A1 is, also called, lower critical temperature.
A2:
It is called, the Curie temperature (of ferrite), where ferromagnetic ferrite on heating changes to paramagnetic, i.e. at 768°C. It is shown as a horizontal line at 768°C to point V (Fig. 1.25). The loss of ferromagnetism of ferrite (i.e., A2) in Fe-C alloys of higher carbon than point V (≈ 0.5%C) follows the line VUT, depending on the carbon of the alloy, though the change is to paramagnetic austenite then.
A3:
It is the temperature at which ferrite just starts forming from austenite, on cooling a hypoeutectoid steel, or last traces of free ferrite changes to austenite, on heating. Thus, it is the temperature corresponding to γ + α/γ phase boundary for hypoeutectoid steels and is a function of carbon content of the alloys, as it decreases from 910°C at 0 % C to 727°C at 0.77 % C. It is also called the upper critical temperature of hypoeutectoid steels.
The temperature interval between A1 and A3 is called the critical range in which the austenite exists in equilibrium with ferrite. Fig. 1.25 illustrates that curve UV is common (on heating) both for the disappearance of ferrite (A3) as well as for disappearance of ferromagnetism (A2) and thus, this part of the curve represents A3,2 temperatures For hypereutectoid steels, A2, A3 coincide with the eutectoid temperature, and thus, to the right of 0.77% C, the lower critical temperature is often designated as A3,2,1.
Acm:
It is the temperature, in a hypereutectoid steel, at which proeutectoid cementite just starts to form (on cooling) from austenite. It represents the temperature of γ/γ + Fe3C phase boundary and, is a function of carbon. The variation of this temperature with carbon is represented by QU as in Fig. 1.22. Acm line gives composition of austenite in equilibrium with cementite.
Acm line illustrates that solid solubility of carbon in austenite decreases very rapidly from a maximum of 2.11 % at 1147°C to a maximum of 0.77% at 727°C, due to greater stability of cementite at lower temperatures. The extra carbon precipitates from austenite as proeutectoid cementite in hyper eutectoid steels (also called secondary cementite in cast irons). Separation of cementite from austenite (on cooling) is also accompanied with the evolution of heat.
Acm line is much steeper than A3 line, which though, means that the amount of proeutectoid cementite in commercial steels is very small, but it also means that heating, too much high temperatures, has to be done to dissolve this cementite for complete homogenisation of austenite. This heating to high temperatures is not desirable in practice.
4. Transformations and Microstructures of Slowly Cooled Steels in Fe-Fe3C Equilibrium Diagram:
In Fe-Fe3C diagram (Fig. 1.22), ABCD is a liquidus, above which every alloy is in liquid state. AOPQCRD is a solidus below which every alloy is completely solid. To understand the transformations, which take place, consider the slow cooling of some alloys from liquid state to room temperature. Fig. 1.27 illustrates part of the Fe-Fe3C diagram along with the thermal cooling curves of some steels which are discussed below. A vertical line, indicating the composition of the steel, is drawn to discuss it.
1. Hypoeutectoid Steels:
Consider the slow cooling of Fe-0.4 % C steel under equilibrium conditions from say 1600°C to room temperature. At a temperature ‘a’ in Fig. 1.27, the solidification begins with the formation of solid δ-ferrite. As cooling continues, more δ-ferrite continues to form till temperature B is reached (1495°C).
At this instant (before the peritectic reaction takes place), the amount of phases are:
At this temperature, the peritectic reaction occurs, but the amount of liquid is more than required for complete peritectic reaction (δ-ferrite/liquid = 4.5).
Thus, when this reaction is completed, the amounts of phases present are:
As further cooling takes place, liquid changes to solid austenite until temperature C is attained, at which solidification is completed, i.e., at temperature, C, the solid is composed entirely of grains of austenite (single phase) of carbon 0.4%. As the cooling continues, austenite persists until a temperature corresponding to N is reached, which is the A3 temperature of the alloy and then, ferrite begins to form at the grain boundaries of austenite.
The proeutectoid ferrite (also cementite in hyper eutectoid steels) always forms at grains boundaries of austenite during slow cooling. As the atoms, at the grain boundaries, are not at lattice sites but are in a metastable state, i.e. high energy state, these can easily form the ferrite there. Ferrite continues to be formed with decreasing temperature until the eutectoid temperature is reached.
At this instant, proportions of phases are:
This 50.67 % of austenite (of 0.77 % C) at the eutectoid temperature must undergo eutectoid reaction to give a mixture of ferrite and cementite, called pearlite in amount 50.67 %. Thus, the 0.4 % C steel has 49.33 % of pro-eutectoid ferrite and 50.67 % of pearlite. See the schematic cooling in Fig. 1.25 and the schematic microstructures of this steel.
This pearlite or any pearlite is composed of the following proportion of ferrite and cementite:
As already said, the ferrite which forms prior to the eutectoid reaction is called proeutectoid ferrite. The ferrite which is incorporated in pearlite is called eutectoid ferrite, whereas, the cementite present in pearlite is called eutectoid cementite.
In this alloy (0.4 % C steel), the proportions of ferrite and cementite (just after the eutectoid reaction) can be obtained directly by applying the lever rule as:
The curve TS’ in Fig. 1.25 illustrates that the solid solubility of carbon in ferrite decreases with the fall of temperature, i.e. from a maximum of 0.02 % C solubility at 727°C, it decreases to < 0.00005 % at 20°C. Thus, any ferrite, whether free, or, eutectoid ferrite in the steel, when cooled from 727°C (eutectoid temperature after the eutectoid reaction is complete) to room temperature, or say, 20°C leads to the precipitation of small amount of cementite called tertiary cementite.
The maximum amount of this tertiary cementite precipitates in an alloy having 100 % ferrite at 727°C, i.e. in 0.02 % C steel, and the amount is (at 20°C):
In most of the commercial steels and cast irons, the amount of tertiary cementite is much less than this value and for most purposes, it is neglected. Thus, we shall be neglecting this, while discussing the microstructures of steels and cast irons. Thus, in most cases, the micro-structure obtained just after the eutectoid reaction is taken to be the micro-structure at room temperature. The importance of tertiary cementite is discussed in quench ageing of steels.
A 0.4 % carbon steel has approximately 50 % ferrite and 50 % pearlite in micro-structure under slow cooling conditions.
2. Eutectoid Steel (0.77 % C Steel):
Eutectoid steel (Fig. 1.27) when cooled from say, 1600°C, starts solidifying at temperature J with the formation directly of solid austenite from liquid and is completed at point K, when the alloy has in micro-structure, grains of austenite only (carbon 0.77 %). Further cooling of this alloy to point U or eutectoid temperature takes place without any change. This alloy, then, at 727°C, undergoes eutectoid reaction to give 100% pearlite.
The amount of eutectoid ferrite and eutectoid cementite (i.e. in pearlite) has been calculated in equations 1.22 and 1.23. Thus, eutectoid steel shows 100% pearlite in micro-structure as shown schematically in Fig. 1.25.
3. Hyper-Eutectoid Steels:
Hyper eutectoid steels contain carbon between 0.77% to 2.11 %. Consider cooling of say, 1.2 % carbon steel from molten state say, from 1500°C. Solidification begins at a temperature d in Fig. 1.27 with the formation of solid austenite. With continued cooling, solid austenite continues to form until temperature e is reached at which, solidification is completed. At this temperature, solid steel is composed entirely of grains of austenite of composition 1.2 % carbon. This phase persists, with continued cooling, until a temperature corresponding to w is reached, which is the Acm temperature of the steel, and cementite begins to form.
Cementite continues to form as a network along grain boundaries of austenite with the decreasing temperature (Fig. 1.25), until the eutectoid temperature is reached. Equations 1.26 and 1.27 give the amount of austenite and the pro-eutectoid cementite respectively in 1.2 % C steel at the temperature, 727°C just reached.
The austenite (of 0.77 % C) must then undergo eutectoid reaction to produce pearlite. Thus, a 1.2 % C steel has 7.29 % of pro-eutectoid cementite and 92.71 % pearlite the schematic micro-structure.
5. Methods Used to Distinguish between Free-Ferrite and Free-Cementite in Fe-Fe3C Equilibrium Diagram:
When the carbon content of steels is much away from the eutectoid carbon, then distinction can easily be made between hypoeutectoid steel and the hypereutectoid steel. Nital is the common etching reagent. Under light microscope, proeutectoid cementite appears white and proeutectoid ferrite, also, appears white, but cementite is present as network at the grain boundaries of pearlite (at room temperature), whereas ferrite is present as grains (equiaxed polyhedral grains) with grain boundaries in between ferrite grains (if etched properly).
Commercial steels have carbon up to 1.5% and thus, the amount of proeutectoid cementite is less than 10 % in hyper eutectoid steels. It is probable that microstructures of these slowly cooled hyper-eutectoid steels may appear similar to those of some of the slowly cooled hypoeutectoid steels, which contain proeutectoid ferrite in amounts less than 10 % (that is, have carbon between 0.7 and 0.77%), because the proeutectoid phases (cementite in former and ferrite in latter) are present at the grain boundaries of pearlite and appear white under optical microscope due to etching with nital.
One or more of the following methods could be used to distinguish between the free-ferrite and free-cementite:
1. Shapes of the Phases:
Proeutectoid ferrite appears as grains which are quite wide, polyhedral and the grain boundaries in between neighboring ferrite grains (if etched properly) can be seen. The films of proeutectoid cementite generally are much thinner, have irregular outlines and bounded by sharp lines. These are present as network of needles, or platelet. Cementite looks much brighter and sharp because of its hardness and etching characteristics with nital. See Fig. 1.29 (a) and (b).
If surface topography of polished and etched (nital) samples of hypoeutectoid steel and hyper-eutectoid steel, is examined by focusing the piece and by bringing slightly out of focus (or focussing the top of the relieved plate of cementite), the cementite appears to stand erect above the plane of etching as illustrated in Fig. 1.30 (a) and (b).
2. Relative Hardness:
Cementite is very hard (~ 800 VPN) and ferrite is very soft (~ 95 VPN), micro hardness testing can be done to distinguish between ferrite and cementite. However, a simple test can be done.
Make a scratch on the polished and etched surface of the steel and then, examine the point of the scratch where it enters the white proeutectoid phase from the pearlite. See Fig. 1.31. If the scratch widens on entering, it is the soft phase ferrite, and thus, the steel is hypoeutectoid steel, but if it thins in white phase, then the white phase is much harder than pearlite, that is, it is cementite of the hypereutectoid steel.
3. Special Etchants:
Nital etching causes cementite as well as ferrite to look white under microscope, making their detection difficult in some, steels. Classically, a sodium picrate solution used either boiling (which is inconvenient), or electrolytically, darkens cementite but not ferrite, though the cementite plates in pearlite are also darkened, i.e. the pearlite colonies are darkened.
See Fig. 1.32 microstructures of hypereutectoid steel, (a) etched with nital, (b) etched with sodium picrate solution. Another etchant based on sodium thiosulphate and ammonium nitrate (developed by Beraha) colours ferrite but not cementite [Fig. 1.32 (c)]
6. Limitations of Fe-Fe3C Equilibrium Diagram:
Actual heat treatment cycles use cooling rates much faster than equilibrium cooling rates by which the Fe-Fe3C diagram has been drawn and thus put serious limitations on the interpretations from the Fe-Fe3C diagram below A1 Temperature.
Though Fe-Fe3C diagram provides a base, but has little significance in the heat treatment of steels because:
(i) Fe-Fe3C diagram represents behavior of steels under equilibrium conditions, whereas the actual heat treatments of steels are normally under non-equilibrium conditions.
(ii) The diagram does not indicate the character of transformation of austenite such as to bainite, or martensite.
(iii) The diagram does not indicate the presence of metastable phases like martensite, or bainite.
(iv) It does not indicate the temperature of start of martensite Ms, or bainite, Bs.
(v) It does not indicate the kinetics of the transformation of austenite to martensite, bainite, or even pearlite.
(vi) It does not indicate the possibilities of suppressing the pearlitic, or bainitic transformations.