ANS:B - pearlite

The eutectoid composition of carbon steel at room temperature forms pearlite. Eutectoid steel contains approximately 0.8% carbon by weight. When this steel is cooled from a high temperature to room temperature, it undergoes a phase transformation known as eutectoid transformation. During this transformation, the austenite phase (which exists at high temperatures) decomposes into two phases: ferrite (a solid solution of carbon in iron) and cementite (an intermetallic compound of iron and carbon with the chemical formula Fe₃C). The microstructure resulting from the eutectoid transformation consists of alternating layers of ferrite and cementite, which collectively form a lamellar structure known as pearlite. Therefore, pearlite is the phase that predominates at room temperature in eutectoid carbon steel. To clarify further, the options given are as follows:

• Cementite: A phase composed primarily of iron and carbon, with the chemical formula Fe₃C. It is a component of pearlite.
• Pearlite: A lamellar structure consisting of alternating layers of ferrite and cementite, which forms during the eutectoid transformation in steel.
• Martensite: A metastable phase formed by rapid cooling of austenite, characterized by a highly strained and distorted lattice structure.
• Ferrite: A solid solution of carbon in iron, which is one of the phases formed during the eutectoid transformation in steel.