Eutectoid Reaction |Iron Cementite Phase Diagram Part 2| Heat Treatment Lecture1| Invariant Reaction

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Heat Treatment Lecture 1-Part 2
Invariant Reactions
In a Binary alloy system, the invariant reaction is the reaction that occurs when three phases are in equilibrium.
If we apply Gibb’s phase rule to a binary alloy system
Degree of Freedom(F) = No. components(C) - No. Phases (P) + 1
Degree of Freedom(0) = No. components(2) - No. Phases (3) + 1
So we came to know that the invariant reactions have zero degrees of freedom.
Eutectoid Reaction
The lowest temperature invariant reaction occurs at 723 oC and this reaction is known as Eutectoid Reaction.
Every invariant reaction is always represented by a horizontal line.
As we have three invariant reactions.
So, we have three horizontal lines.
Eutectoid reaction is the invariant reaction in which three solid phases came into equilibrium with each other.
In this reaction, a single solid phase transforms on cooling into two new solid phases.
Let's take an alloy of eutectoid composition (0.8%C) and cool it down from a temperature within the Gamma-phase region.
Initially, the alloy is composed of 100% Austenite, so the composition of each grain is 0.8%C.
Nothing happens until the invariant temperature (723 oC) is reached.
At this stage, Austenite has to be transformed into Ferrite and cementite
But the problem is that ferrite can’t dissolve 0.8% C in its solution so it has to reject this carbon out of its solution.
As the triple junction zones and the grain boundaries are the high energy regions so, the transformation starts at the triple junction zones and then at the grain boundaries.
Carbon atoms diffuse away from the 0.022-wt% ferrite regions and to the 6.70-wt% cementite layers,
The layered structure known as the pearlite extends from the grain boundary into the unreacted austenite grain.
The layered pearlite forms because carbon atoms need diffuse only minimal distances with the formation of this structure.
The temperature becomes constant until the entire transformation gets completed.
Upon crossing this temperature, the Austenite would transform into 100% pearlite.

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