Eutectoid Steel Phase Transformations During Cooling

Eutectoid steel undergoes phase transformations when cooled from its austenite state. The resulting microstructure depends heavily on the cooling rate.

Continuous Cooling Outcomes

• Slow Cooling: Results in the formation of pearlite, which is a lamellar structure of ferrite and cementite.
• Rapid Cooling (Quenching): Leads to the formation of martensite, a hard, brittle phase formed by a diffusionless transformation.
• Intermediate Cooling: Typically produces a mixture of phases, often pearlite and bainite. Bainite forms at temperatures between pearlite and martensite formation.

Reasoning for Non-Formation of Fully Bainitic Structure

During a standard continuous cooling process for eutectoid steel, achieving a structure composed exclusively of bainite (fully bainitic) is generally not observed. While bainite can form, especially at intermediate cooling rates, it usually appears alongside pearlite. The transformation kinetics represented on a Continuous Cooling Transformation (CCT) diagram show that very slow cooling favours pearlite, very rapid cooling favours martensite, and intermediate rates often yield a combination. Fully bainitic structures are more typically associated with specific interrupted cooling processes like austempering, rather than simple continuous cooling.

Therefore, among the given options, fully bainitic is the structure that typically does not form as the sole product during continuous cooling of eutectoid steel.