(a) What is the distinction between hypoeutectoid and hypereutectoid steels?
(b) In a hypoeutectoid steel, both eutectoid and proeutectoid ferrite exist. Explain the difference between them. What will be the carbon concentration in each?
The Correct Answer and Explanation is :
(a) Distinction Between Hypoeutectoid and Hypereutectoid Steels
Hypoeutectoid steels are steels that contain less than 0.8% carbon, which is the eutectoid composition. In these steels, the microstructure consists primarily of ferrite (α-phase) and pearlite (which forms at the eutectoid composition, 0.8% carbon). When cooled from the austenite phase (γ-phase), hypoeutectoid steels solidify as a mixture of proeutectoid ferrite and pearlite.
On the other hand, hypereutectoid steels have more than 0.8% carbon, and their microstructure includes cementite (Fe₃C) along with pearlite upon cooling. These steels will form proeutectoid cementite (Fe₃C) in addition to the pearlite structure. The higher carbon content causes cementite to form before the pearlite eutectoid reaction occurs.
(b) Difference Between Eutectoid and Proeutectoid Ferrite in Hypoeutectoid Steels
In hypoeutectoid steels (those with less than 0.8% carbon), the microstructure consists of both eutectoid ferrite and proeutectoid ferrite.
- Eutectoid Ferrite is the phase that forms when the steel reaches the eutectoid composition of 0.8% carbon. At this composition, pearlite forms as a lamellar structure of alternating ferrite and cementite. The eutectoid reaction occurs at around 727°C where austenite transforms into pearlite (which is a mixture of ferrite and cementite).
- Proeutectoid Ferrite is the ferrite that forms before the steel reaches the eutectoid composition. In hypoeutectoid steels (those with carbon concentrations below 0.8%), proeutectoid ferrite forms first as the steel cools from the austenitic phase. The proeutectoid ferrite is present in the microstructure as a phase that exists in excess of the eutectoid transformation.
The carbon concentration in each phase is:
- Eutectoid Ferrite: The carbon content of eutectoid ferrite is essentially zero, as it is almost pure iron.
- Proeutectoid Ferrite: The proeutectoid ferrite has a lower carbon concentration than the eutectoid composition. The carbon concentration in proeutectoid ferrite is roughly 0.02%, making it almost pure iron as well.
I’ll generate an image showing the phase diagram for hypoeutectoid steel, with the eutectoid and proeutectoid phases illustrated.
Here is the phase diagram for hypoeutectoid steel, illustrating the distinctions between eutectoid and proeutectoid ferrite, as well as the other phases like pearlite and austenite. It shows the temperature and carbon concentration relations at different phases.
