Discuss the iron carbon diagram and various allotropies of steel. Or Draw a neat sketch of iron carbon equilibrium diagram and explain the following reactions- (i) eutectic reaction (ii) eutectoid reaction (iii) peritectic reaction.Ripunjay Tiwari
Ans. Fig· 1.1 shows an Iron-carbon equilibrium diagram since any iron
carbon alloy in the molten state may be considered as a solution of iron carbide
Fe3C in iron. Therefore, it is also called as iron-iron carbide equilibrium diagram.
An equilibrium phase or constitutional diagram is a graphic representation of
the effects of temperature and composition upon the phase present in an alloy.
An equilibrium diagram is constructed by plotting temperature along y-axis
And percentage composition of the alloy along the x-axis. This diagram shows ranges of temperatures and composition within which the various phase changes are stable and also the boundaries at which the phase changes occur.
Iron-carbon equilibrium diagram indicates the phase changes that occur
during heating and cooling and the nature and amount of structural components
that exist at any temperature. Besides, it establishes a correlation between the
microstructure and properties of steel and cast irons and provides a basis for
the understanding of the principles of heat treatment.
An iron-carbon equilibrium diagram forms a basis for differentiating among
iron (0.008% C or less), hypoeutectoid steels (0.008 to 0.8% C), hypereutectoid
steels (0.8 to 2.0% C), hypoeutectic cast irons (2 to 4.3%C) and hypereutectic
cast iron above 4.3% carbon.
The iron-carbon equilibrium diagram has a peritectic point J, an eutectic
point C and an eutectoid point S.
Peritectic reaction equation may be written as
The horizontal line at 1500°C shows the peritectic reaction .
The eutectic reaction takes place at 11 30°C and its equation may be
Eutectic point is at 4.3% carbon. Eutectic mixture is not usually seen in
the microstructure, because austenite is not stable at room temperature and
must undergo another reaction during cooling.
The eutectoid reaction is represented by the horizontal line of723°C and
point S marks the eutectoid point. The eutectoid equation may be written as
All alloys represented in region above ABCD are completely liquid. Alloys
containing carbon upto 4.3% when cooled to temperature on the curve ABC,
solid crystals begin to form. ABC is thus called liquidus for these alloys. While
alloys containing more than 4.3% carbon starts to solidify along the line (‘I)
liquidus for these alloys. These alloys completely solidify when cooled to a
temperature of 1130°C, out alloys containing carbon upto 4.3% solidify
completely along the line HJEC called solidus.
Total transformation which takes place in the carbon equilibrium diagram
can be divided into two types of transformations –
(i) Primary transformation – Transformation from the liquid to the
(ii) Secondary transformation -Transformation in the solid state.
(i) Primary Transformation – In this transformation liquid alloys
of various carbon content are cooled just below the eutectic temperature 1130°C
so that they convert into solid.
Consider cooling of an alloy containing 0.8% carbon started to cooled
from a temperature of 1600°C, shown by point m on the diagram above the
liquidus line ABC, it will remain liquid until it is cooled to a temperature t1 on
liquidus. At this point, crystal of austenite of composition m begins of precipitate
from the liquid alloy. When temperature is decreased further the amount of
austenite increases along the line HJE. At temperature t2 percentage of carbon
on solidus curve HJE will be 0.8% same as of original alloy, the alloy solidifies
completely and contains only austenite.
Consider another alloy containing 1.7% carbon shown by point non the
diagram. Similar process as discussed above will take place. The austenite
crystals will begin to precipitate at temperature t3 on liquidus. Liquid become
richer in carbon as austenite separating at eutectic temperature ( 1130°C) last
drop of liquid containing 1.7% carbon will solidify. Solidified alloy consists