Which of the following cooling curves (shown in schematic) in an eutectoid steel will produce 50% bainitic structure? 
To determine which cooling curve will produce a 50% bainitic structure in eutectoid steel, we must understand the concept of Time-Temperature-Transformation (TTT) diagrams. These diagrams show the transformation of austenite to different microstructures like pearlite, bainite, and martensite, depending on the cooling rate and temperature.
From the given TTT schematic:
- The curve labeled P passes directly through the pearlite region, indicating it would transform primarily to pearlite before reaching the bainitic region.
- The curve labeled Q intersects the 50% line within the bainitic region, suggesting that on cooling, it forms 50% bainite. This is the desired outcome for the question.
- The point identified in the bainitic region signifies the start of bainite transformation and completion at 50%, which makes this curve correct for achieving 50% bainitic structure.
- The curve labeled R indicates transformation mainly to martensite as it cools rapidly past the bainitic region without sufficient time for bainite formation.
- The curve labeled S also results in martensite due to even faster cooling, bypassing intermediate transformations.
Thus, the correct answer is Q since it represents a cooling curve that intersects the 50% transformation line specifically within the bainitic region, thus producing 50% bainite in the microstructure.
Important Questions from Heat Treatment Quenching Critical Cooling Rate
- Which of the following statements is/are CORRECT when Ni is added as an alloying element to a low alloy steel?
- In a Jominy end-quench test of the eutectoid plain-carbon steel, which of the following represents the sequence of microstructures observed from the quenched end of the specimen?
- Which one of the following is the correct decreasing sequence of Quenching Power for quenchants used in heat treatment of steels?
- Which one of the following techniques does NOT require quenching to obtain final case hardness?
The microstructures of a quenched steel tempered at three temperatures $T_1 < T_2 < T_3$ for a fixed time are schematically illustrated. The solid circles represent cementite particles in ferrite matrix; $\bar{r}_1, \bar{r}_2$ and $\bar{r}_3$ are average radii of cementite particles, and $V_1, V_2$ and $V_3$ are volume fractions of cementite at temperatures $T_1, T_2$ and $T_3$, respectively.
If the cementite in steel is more noble than ferrite, then which one of the three microstructures will have the highest corrosion rate when exposed to an aqueous solution of $3.5 \text{ wt.\% NaCl}$?
