Iron Carbon Phase Daigram
The iron-carbon (Fe-C) phase diagram is important in engineering as it provides the basis for understanding all cast irons and carbon steels and their heat treatment. In case of pure iron, the low temperature form of iron is called ferrite (or alpha-iron), with a BCC (body-centred cubic) structure. On heating pure iron changes to austenite (or gamma-iron) at 910 deg C, and switches to FCC structure. Pure austenite is stable up to 1,394 deg C, when it changes back to BCC structure delta-iron, before melting at 1,538 deg C. A key characteristic of the iron-carbon system is the extent to which iron dissolves carbon in interstitial solid solution, forming single phases. This is where the changes between BCC and FCC are significant. The interstitial holes are larger in FCC than in BCC. This leads to low solubility of carbon in BCC ferrite and delta-iron, and much higher solubility in FCC austenite.
The iron–carbon phase diagram s shown in Fig 6. The diagram shows on the right side actually shows two diagrams namely (i) the stable iron-graphite diagram (red lines), (ii) and the metastable Fe-Fe3C (iron- cementite) diagram. Cementite is metastable, and the true equilibrium is to be between iron and graphite (C). Although graphite occurs extensively in cast irons, it is normally difficult to get this equilibrium phase in steels. The stable condition normally takes a very long time to develop specially in the low temperature and low carbon range. Hence, the normal equilibrium diagram which is normally used is the metastable Fe-Fe3C diagram since it is relevant to the behaviour of the majority of steels in practice.
The details of the stable and metastable phase diagrams of the iron-carbon system, especially on the iron-rich side, are known much better than any other binary systems with similar complexity. However, there are still substantial areas where the phase diagram has not been well established such as in the temperature, composition, and pressure ranges not related directly to ironmaking and steelmaking.
There are some important metallurgical phases and micro-constituents in the iron-carbon system. In the Fe–Fe3C system, carbon is an interstitial impurity in iron. It forms a solid solution with alpha (alpha ferrite), gamma (austenite), and delta (delta ferrite) phases of iron. These are important phases in Fe – Fe3C phase diagram. Between the single-phase fields, there are found regions with mixtures of two phases, such as ferrite and cementite, austenite and cementite, and ferrite and austenite. At the highest temperatures, the liquid phase field can be found and below this are the two-phase fields of liquid and austenite, liquid and cementite, and liquid and ferrite. In heat treatment of steels, the liquid phase is always avoided. At the eutectic point (4.26 % carbon), liquid alloy on cooling gets directly converted into austenite and cementite without any two-phase field. Similarly, at the eutectoid point (0.76 % carbon), austenite phase on cooling gets directly converted into ferrite and cementite without any two-phase field. Some important boundaries at single-phase fields have been given special names which facilitate the understanding of the diagram. Main phases of iron and steels in equilibrium are the following phases.
Ferrite or alpha-iron phase – It is a stable form of iron at room temperature. It is relatively soft low temperature phase and is a stable equilibrium phase. It transforms to FCC austenite (gamma phase) at 910 deg C. Ferrite is a common constituent in steels and has a BCC structure, which is less densely packed than the FCC structure. It is soft, and fairly ductile. It is magnetic below 768 deg C. It has low strength and good toughness.
Austenite or gamma iron phase – Austenite is a high temperature phase. It is a solid solution of carbon in the FCC iron. Hence, it has FCC structure, which is a close packed structure. It is a non-magnetic and ductile phase. It transforms to BCC delta ferrite at 1,394 deg C. It is not stable below the eutectoid temperature (727 deg C) unless cooled rapidly. Austenite has good strength and toughness.
Delta ferrite phase – It is solid solution of carbon in BCC iron. It is stable only at temperature higher than 1,394 deg C. It melts at 1,538 deg C. It has paramagnetic properties.
Cementite – It is Fe3C or iron carbide. It is an inter-metallic compound of iron and carbon. It has a complex ortho-rhombic structure and is a meta-stable phase. It is a hard, brittle phase. It has low tensile strength, good compression strength and low toughness. It decomposes (very slowly, within several years) into alpha ferrite and C (graphite) at the temperature range of 650 deg C to 700 deg C
On comparing austenite with ferrite, the solubility of carbon is more in austenite with a maximum value of 2.14 % at 1,148 deg C. This high solubility of carbon in austenite is extremely important in heat treatment. When solution treatment in the austenite followed by rapid quenching to room temperature allows formation of a super-saturated solid solution of carbon in iron. The ferrite phase is restricted with a maximum carbon solubility of 0.025 % at 727 deg C. Since the carbon range available in common steels is from 0.05 % to 1.5 %, ferrite is normally associated with cementite in one or other form. Similarly, the delta-phase is very restricted and is in the temperature range between 1,394 deg C and 1,538 deg C. It disappears completely when the carbon content reaches 0.5 %.
Alloy of eutectoid composition (0.76 % C) when cooled slowly, forms pearlite, which is a layered structure of two phases namely alpha‐ferrite and cementite. Pearlite is the ferrite-cementite phase mixture. It has a characteristic appearance and can be treated as a micro-structural entity or micro-constituent. It is an aggregate of alternating ferrite and cementite lamellae which degenerates (spheroidizes or coarsens) into cementite particles dispersed with a ferrite matrix after extended holding below 727 deg C. It is a eutectoid and has a BCC structure. It is a partially soluble solution of iron and carbon. Mechanically, the pearlite has properties intermediate to soft, ductile ferrite and hard, brittle cementite. It has high strength and low toughness.
Hypo-eutectoid alloys contain pro-eutectoid ferrite (formed above the eutectoid temperature) along with the eutectoid pearlite which contain eutectoid ferrite and cementite. Hyper-eutectoid alloys contain pro-eutectoid cementite (formed above the eutectoid temperature along with pearlite which contain eutectoid ferrite and cementite.
In case of non-equilibrium solidification of iron-carbon system some additional type of micro-structures can also be formed. Some of these microstructures are given below.
Bainite – It is a phase between pearlite and martensite. It is a hard meta-stable micro-constituent and consists of non-lamellar mixture of ferrite and cementite on an extremely fine scale. Upper bainite is formed at higher temperatures and has a feathery appearance. Lower bainite is formed at lower temperatures and has an acicular appearance. The hardness of bainite increases with decreasing temperature of its formation. It has good strength and toughness.
Martensite – It is a very hard form of steel crystalline structure. It is named after the German metallurgist Adolf Martens. It is formed by rapid cooling and is hard and brittle. It is a body-centered tetragonal (BCT) form of iron in which some carbon is dissolved. It is formed during quenching, when the face centered cubic lattice of austenite is distorted into the body centered tetragonal structure without the loss of its contained carbon atoms into cementite and ferrite. It is super saturated solution of carbon atoms in ferrite. It is a hard metastable phase. It has lath morphology when carbon is less than 0.6 %, plate morphology when carbon is more than 1 %, and mixture of those in between. It is having high strength and hardness and low toughness.
Sorbite / troostite – Structures of the lower pearlite stage with very fine flakes are referred to as sorbite and troostite. These are the transformation structures of the pearlite stage which correspond to the increasing cooling rates. However, it changes the structure ratio and the formation of pearlite with regard to flake distance. The structure cannot be seen under an optical microscope.
Widmanstatten ferrite – It is obtained when hypo-eutectoid plain carbon steel is cooled down rapidly form a temperature above the A3 temperature. Because of the fast cooling, there is little time available for the ferrite crystals to nucleate not only on the grain boundary but also within the large austenite grains. The crystals quickly grow into some preferred crystal direction inside the grain and hence become longish. The structure is either in the form of needles (laths) or plates which tend to align along the same direction within one grain.
There are several temperatures and critical points in the Iron-carbon diagram which are important both from the basic and the practical point of view. These are the temperatures when during cooling, or heating, the transformations of phase as well magnetic transformation take place in them. The temperatures at which the transformations occur in the solid state are called critical temperatures, or critical points. Major temperatures and critical points are given below.
A0 temperature – It is the Curie temperature when the magnetic to non-magnetic change of cementite occurs on heating. The structure can develop defects such as dislocations, faults, and vacancies. Cementite is metallic and ferromagnetic with a Curie temperature of around 210 deg C. When alloyed, metallic solutes substitute on to the iron sites, smaller atoms such as boron replace carbon at interstitial sites.
A1 temperature – It is the temperature (727 deg C) when the eutectoid transformation takes place. At this temperature pearlite changes to austenite on heating and vice versa.
A2 temperature – It is called the Curie temperature of ferrite (768 deg C). At this temperature, ferromagnetic ferrite on heating changes to paramagnetic. At this temperature, no change in microstructure is involved.
A3 temperature – It is the temperature at which ferrite just starts forming from austenite, on cooling hypo-eutectoid steel or last traces of free ferrite changes to austenite, on heating. Hence, it is the temperature corresponding to gamma + alpha / gamma phase boundary for hypo-eutectoid steel and is a function of carbon content of the steel, as it decreases from 910 deg C at 0 % carbon to 727 deg C at 0.76 % carbon. It is also called the upper critical temperature of hypo-eutectoid steels. The temperature interval between A1 and A3 temperatures is called the critical range in which the austenite exists in equilibrium with ferrite
Acm temperature – It is the temperature, in a hypereutectoid steel, at which pro-eutectoid cementite just starts to form (on cooling) from austenite. It represents the temperature of gamma / gamma + Fe3C phase boundary and, is a function of carbon. Acm line shows that solid solubility of carbon in austenite decreases very rapidly from a maximum of 2.14 % at 1,148 deg C to a maximum of 0.76 % at 727 deg C, because of the higher stability of cementite at lower temperatures. The extra carbon precipitates from austenite as pro-eutectoid 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.
A4 temperature – It is the temperature at which austenite transforms to delta iron. The lowest value for this temperature is 1,394 deg C which is in case of pure iron. This temperature increases as the carbon percent is increased.
Ms temperature – It is the temperature at which transformation of austenite to martensite starts during cooling.
Mf temperature – It is the temperature at which martensite formation finishes during cooling. All of the changes, except the formation of martensite, occur at lower temperatures during cooling than during heating and depend on the rate of change of temperature.
Austenite- ferrite transformation – Under equilibrium conditions, pro-eutectoid ferrite is formed in iron-carbon alloys containing up to 0.76 % of carbon. The reaction occurs at 910 deg C in pure iron, but takes place between 910 deg C and 727 deg C in iron-carbon alloys. However, by quenching from the austenitic state to temperatures below the eutectoid temperature, ferrite can be formed down to temperatures as low as 600 deg C. There are pronounced morphological changes as the transformation temperature is lowered, which normally apply in general to hypo-eutectoid and hyper-eutectoid phases, although in each case there are variations because of the precise crystallography of the phases involved. For example, the same principles apply to the formation of cementite from austenite, but it is not difficult to distinguish ferrite from cementite morphologically.
Austenite-cementite transformation – There are different morphologies of cementite which are formed at progressively lower transformation temperatures. The initial development of grain boundary allotriomorphs is very similar to that of ferrite and the growth of side plates or Widmanstatten cementite follows the same pattern. The allotriomorph has a shape which does not reflect its internal crystalline symmetry. This is because, it tends to nucleate at the austenite grain surfaces, hence forming layers which follow the grain boundary contours. The cementite plates are more rigorously crystallographic in form, despite the fact that the orientation relationship with austenite is a more complex one. As in the case of ferrite, most of the side plates originate from grain boundary allotriomorphs, but in the cementite reaction more side plates nucleate at twin boundaries in austenite.
Austenite-pearlite reaction – Pearlite is the most familiar microstructure in the iron carbon phase diagram. It was discovered by Sorby, who correctly assumed it to be a lamellar mixture of iron and iron carbide. It is a very common constituent of a wide variety of steels, where it provides a substantial contribution to strength. Lamellar eutectoid structures of this type are widespread in the metallurgy of steels. These structures have much in common with the cellular precipitation reactions. Both types of reaction occur by nucleation and growth, and are, hence, diffusion controlled. Pearlite nuclei occur on austenite grain boundaries, but it is clear that they can also be associated with both pro-eutectoid ferrite and cementite. In commercial steels, pearlite nodules can nucleate on inclusions.
Post a Comment
0Comments