To Study Iron-Iron Carbide Equilibrium Phase Diagram.

Theory:


Like every metal, iron has its definite melting and solidification temperature. Alloying elements influences these temperatures. Iron is an allotropic metal, as depending upon temperature iron can exist in more than one type of lattice structure. The temperature at which the allotropic changes take place in iron is influenced by alloying elements. The most important of which is carbon. This inherent alloying element plays important role in properties of steel and cast iron-the fundamental engineering materials.

Different solidification temperatures and plotted in a form temperature v/s weight percent carbon contact are incorporated and plotted in a form of temperature v/s weight percent carbon graph. This graph is known as Iron-Iron Carbide equilibrium diagram. The portion of iron-carbon alloy system that is of interest is shown in fig. 1. This is the between pure iron and interstitial compound iron carbide, Fe,-C, containing 6.67 % carbon by weight.

This is not true equilibrium as the compound iron carbide will decompose in iron and carbon (graphite). This decomposition will take a very long time at room temperature and even at 700`C it takes several years to form graphite. Therefore iron-iron carbide diapharm, even though it technically represents metastable conditions, it can be considered as representing equilibrium changes, under conditions of relatively slow heating and cooling.

Significance:


 1.  It distinguishes steels and cast irons.

 2.  One can know the melting, solidifying and allotropic change temp. from the diagram.

 3.  Solubility of alloying element (carbon) at different temperature and in various phases can be known.

 4.  It gives rough idea about post-solidification structure and properties, with varying carbon content.

 5.  It is very useful in heat treatment processes.

The diagram shows three horizontal lines, which indicate isothermal reactions. This is observed during solidification.

     

 

Fig: Iron-carbon diagram

 

1) Peritectic reaction:

 This is observed at 1498`C. It occurs in 0.18 % carbon steel. The reaction is as follows:

Solid l + Liquid → Solid 2

D + L     → Y

Where, d=delta iron (ferrite), L=Liquid steel (molten) and Y=Gamma iron (austenite).

The steels having carbon less than0.18% may be termed as hypoperitectic steels while those containing more than 0.18% are termed as hyperperitectic steels. All steels containing carbon in the range of 0.10 to 0.55 % show paratactic reaction. However, commercial heat treatment is not done in delta region.

2). Eutectic reaction:

 It is observed only in cast iron containing 4.3 % carbon at 1147`C during cooling. The reaction is as follows:

Liquid → Solid l + Solid 2

L → Y + Fe3C (iron carbide or cementite)

Cast iron contains carbon in the range of 2 to 6.67 %. Cast irons having carbon less than 4.3 % are called hypereutectic cast irons. While those containing than 4.3 % are hypereutectic cast irons.  At 4.3 % carbon, when eutectic reaction undergoes, the liquid gets transformed into austenite and cementite. This transformed matrix is known as ledeburite.

3) Eutectoid reaction:

This reaction is observed at 723`C. It occurs in 0.8 % carbon steel. The steels containing carbon in the range of 0.008 to 2 % show eutectoid reaction. The steels having carbon less than 0.8 % are called as hypoeutectoid steels. The reaction is as follows.

Solid → Solid l + Solid 2

Y → a + Fe3C    where, a=alpha iron (BCC)

This means austenite gets transformed into ferrite into ferrite and cementite. This mixture is known as pearlite.

This diagram shows the following existing phases which can be explained as follows:

Ferrite:

In iron-iron carbide syatem, ferrite exists in two forms as delta and alpha ferrite. Delta (d) ferrite (BCC) is defined as an interstitial solid solution of carbon in delta region. Alpha iron exists at room temp, which is interstitial solid solution of carbon in alpha region. The solubility of carbon in alpha iron is maximum 0.025 % at 723 `C and 0.008 % only at room temp. Under the microscope, ferrite is seen as homogeneous polyhedral grains.

It is soft and ductile phase. It can not be heat treated. However, it can be hardened by cold working. It is magnetic upto 786`C and then it becomes non-magnetic. Average mechanical properties are: Tensile strength = 40000 psi, Elongation = 40 % in 2 inch, Hardness is less than HRB 10 or HRC zero.

Austenite:

It is interstitial solid solution ofm carbon in Y(gamma) iron (FCC). It is very weak in magnetic property. The maximum solubility of carbon in austenite is 2 % at 1147`C. This phase is stable only upto 723`C. during cooling. Below 723`C, it decomposes to ferrite and cementite. It is ductile and soft phase. Usually steels are hot worked in austenite region. The grain size of steel at room temp is determined by austenite grain size. Austenite transforms to various phases as pearlite, bainite and martensite. These newly formed phases show properties, which are not present in austenite, e.g. martensite is harder than austenite.

Austenite phase is observed at room temp only in special alloy steels such as austenite stainless steels. For almost all heat treatments, the steels are initially heated in austenite region. Average mechanical properties are: T.S. = 150000 psi, Elongation = 10 % in 2 inch, hardness = HRC 40 approx. Toughness is high.

Pearlite:

It is the eutectoid mixture containing 0.8 % carbon and is formed at 723 % `C on very slow cooling. It is a very fine platelike or lamellar mixture of ferrite and cementire. The white ferrite background or matrix, which makes of most of the eutectoid mixture contains the plates of cementite. 100% pearlite is observed in 0.8% carbon steels. It shows better strength and hardness. Average mechanical properties are:  Tensile strength = 120,000 psi, Elongation = 20% in 2 inch, Hardness = HRC 20 approx or BHN 250 to 300.

Cementite:

It is an intermetallic compound of iron and carbon. It has a fixed chemical formula, as Fe3 , Cementite has carbon content of 6.67% by weight. It is the hardest (BHN 700) and most brittle phase. It is found with ferrite in pearlite. Usually it appears at grain boundaries in high carbon steels. It crystal structure is orthorhombic. It dissolves only at high temp. It has melting point around 1550 ºC. It possessed low tensile strength (5000 psi approx.) but high compressive strength. Under certain conditions, cementite decomposes to from free carbon called as graphite.

Ledeburite: It is an eutectic consisting of austenite and cementite. It is observed in cast irons constaining 4.3% carbon at 1147 ºC.

Critical temperatures:

These are defined as the temperatures at which a phase change occurs during heating and cooling. Various critical temperatures are given as follows:

A1: During heating at this temp; pearlite transforms to austenite. This transformation occurs at a constant temp. of 723 ºC called as eutectoid temp. It has importance in various annealing processes. It is called as lower critical temp. line.

A2: This line indicates Curie temperature. During heating, the ferrite which is magnetic becomes non-magnetic above this line.

A3: This line, in hypoeutectoid steels shows the completion of ferrite to austenite transformation during heating. The decline from the temp axis with increasing carbon content. It starts at 910 ºC with zero percent carbon and ends at 727 ºc with 0.8% carbon. For various heat treatments, the hypoeutectoid steels should be heated above this line. This line represents upper critical temperature for ferrite.

Acm: At this line, in hypereutectoid steels the cemenmtite to austenite transformation is  completed during heating. This line shows increasing slope from 727 ºc to 1148 ºC.It starts at 727 ºC with 0.8% carbon and ends at 1148 ºC with 2% carbon For certain Heat treatments, which involve dissolution of cementite, hypereutectoid steels are heated above this line. This line. This represents upper critical temperature for cementite.

During heating or cooling at faster rates, the transformation occurs at higher or lower temperature respectively. This shifts the equilibrium of critical temperature lines. These lines are denoted by letter C; from French word chauffage – means heating and by letter; from the French word – refroidssement – means cooling e.g. AC, AC during heating , Ar1, Ar3 – during cooling.

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