Understanding the Microstructure of Annealed Hypereutectoid Steel

The question asks about the microstructure of annealed hypereutectoid plain carbon steel. Let's break down the terms and the process involved.

• Plain carbon steel: This is an alloy primarily composed of iron and carbon.
• Hypereutectoid steel: This refers to steel containing more carbon than the eutectoid composition, which is approximately 0.76% carbon. Hypereutectoid steels typically have carbon content between 0.76% and 2.14%.
• Annealing: This is a heat treatment process that involves heating the steel to a specific temperature, holding it there, and then cooling it very slowly, usually in the furnace. The main goal of annealing is to soften the steel, improve ductility, relieve internal stresses, and produce a coarse lamellar pearlite structure. For hypereutectoid steel, full annealing involves heating above the upper critical temperature line ($A_{cm}$) in the phase diagram.

Phase Transformations During Annealing of Hypereutectoid Steel

When hypereutectoid steel is heated above its $A_{cm}$ temperature, the structure becomes entirely austenite ($\gamma$ phase).

During the slow cooling process (annealing) from the austenite phase, the following transformations occur:

1. As the temperature drops below the $A_{cm}$ line, excess carbon in the austenite starts precipitating out in the form of cementite ($Fe_3C$). This cementite preferentially forms along the austenite grain boundaries because these are high-energy sites that facilitate nucleation. This is known as proeutectoid cementite. The amount of proeutectoid cementite increases as the carbon content increases above the eutectoid composition.
2. This precipitation of proeutectoid cementite continues until the temperature reaches the eutectoid temperature ($A_1$, approximately 727°C). At this point, the remaining austenite has reached the eutectoid composition (about 0.76% carbon).
3. Upon cooling through the eutectoid temperature, this remaining austenite transforms into pearlite. Pearlite is a lamellar (layered) structure consisting of alternating plates of ferrite ($\alpha$ phase) and cementite ($Fe_3C$).

Therefore, the final microstructure of annealed hypereutectoid plain carbon steel consists of two main constituents:

• Proeutectoid cementite ($Fe_3C$) located primarily at the boundaries of the original austenite grains.
• Pearlite colonies that fill the areas within these grain boundaries (where the austenite was).

Evaluating the Options

Let's consider the given options based on this understanding:

• Option 1: Pearlite with grain boundary cementite - This description perfectly matches the microstructure expected from the slow cooling (annealing) of hypereutectoid steel.
• Option 2: Full pearlite - This is the characteristic microstructure of slowly cooled eutectoid steel (approximately 0.76% carbon), not hypereutectoid steel.
• Option 3: Ferrite and pearlite - This is the characteristic microstructure of slowly cooled hypoeutectoid steel (less than 0.76% carbon), where proeutectoid ferrite forms before the eutectoid transformation.
• Option 4: Tempered martensite - Martensite is formed by rapid quenching, not slow cooling like annealing. Tempered martensite is the result of heating martensite to improve ductility, which is a different heat treatment process.

Based on the phase transformations during the annealing of hypereutectoid steel, the microstructure will be pearlite within the grains and cementite along the grain boundaries.