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Table of Contents:
Introduction
We strongly recommend you to go through Iron-Iron Carbide Phase Diagram- Part 1 before getting into this one, as they are connected. A short recap of the last article is also presented here as follows: The iron-iron carbide diagram is a diagram between iron and carbon at different temperatures, compositions, and phases of iron. There are 3 horizontal lines at 727 deg, 910 deg, and 1147 deg Celcius. Each horizontal line represents an isothermal reaction. i.e. Peritecticc, Eutectic, Eutectoid reactions.
Equilibrium Cooling and Non-Equilibrium Cooling
In the equilibrium cooling process, the cooling rate is very slow, similar to natural cooling.
It is uniform throughout the composition of the phase and there is no diffusion of solute. Equilibrium cooling gives us a perfect microstructure.
In the non-equilibrium cooling process, the cooling rate is higher, mainly for industrial processes.
Transformations in hypo-eutectoid steels:
In Hypo-eutectoid steels, the Carbon percentage is less than 0.76%.
Let us consider 0.3% of the Carbon amount as an example to understand the transformations. Consider at 0.3%
Carbon and 940 deg. you will find in the figure, that it falls in the region of Austenite (FCC). If you look at the figure, you will see a microstructure, of only gamma structures.
Later, when you start to cool it slowly or naturally, the microstructure remains the same until you reach line or point A3. At this point A3, ferrite starts forming along the grain boundaries of austenite because it is easy to nucleate. Then you get a microstructure of alpha and gamma.
The percentage of ferrite keeps on increasing. The carbon from the ferrite comes out of the solution and dissolves in austenite, as ferrite is stable only at lower carbon percentages. This ferrite is called pro eutectoid ferrite.
When the equilibrium cooling continues, austenite is not stable below 727, hence it gets converted to perlite. Pearlite is a mixture of ferrite and cementite, which is called the Eutectoid reaction. Pearlite has a lamellar structure.
Hypo eutectoid steels are mostly used in building structures. Hypo eutectoid steels have pro eutectoid ferrite which makes them soft and ductile, conversely, with the presence of cementite, it is hard and strong.
Transformations in Hyper eutectoid steels:
Hyper-eutectoid steels have a Carbon Percent from 0.76% to 2.03%.
Keeping the approach similar to the previous section let us consider, 1% of Carbon steel, at 1500 deg. Observing the diagram one can see that this falls in the Austenitic zone. Similar to the section above we will start cooling it in equilibrium. cool slowing.
Only see a microstructure, of gamma structures.
Cooling it slowly, it touches the line at point B. The formation of Cementite starts in Austenite.
As it starts cooling below the line, Cementite starts to form in Austenite. This cementite is pro eutectoid cementite is formed and the microscopic structure is as shown in the figure.
When the temperature reaches 727 deg. Austenite is unstable and gets transformed. Pro eutectoid cementite makes the structure harder. Austenite transforms to pearlite. There is a new creation of cementite on the border of the austenitic grain boundary. Cementite has more carbon percent than austenite. In this way, there is a diffusion of the carbon percentage from the neighboring alpha ferrite. Cementite pulls all the carbon leaving alpha ferrite behind.
Pro eutectoid cementite is hard and pearlite is lamellar, hence hypereutectoid steels are used in the industries for high strength.
Lever Rule
The amount of phase present is inverse of its length.
Let's apply the Lever Rule to the examples we saw above.
Conclusion
In the equilibrium cooling process, the cooling rate is very slow, similar to natural cooling.
The microstructure and the properties of the steel depend very much on the cooling method, temperature, and carbon percent.
The amount of phase present is inverse of its length.
As the properties of the steels change the application varies greatly.
Short Video
Stay tuned for more content on this topic :) Seems like a few more threads.