Question

Using the Iron-Carbon diagram, describe the microstructural phases present in a hypereutectoid steel slowly cooled from the austenite region to just below the eutectoid temperature.

Accepted Answer

When a hypereutectoid steel, which is an iron-carbon alloy containing between 0.76 and 2.14 weight percent carbon, is slowly cooled from the austenite region, distinct microstructural phases develop. Initially, in the high-temperature austenite region, the steel consists entirely of austenite, a solid solution of carbon in face-centered cubic (FCC) iron, known as gamma-iron. Austenite can dissolve a relatively high amount of carbon. As this hypereutectoid steel begins to cool from the austenite region, it eventually crosses the Acm line on the Iron-Carbon diagram. The Iron-Carbon diagram is a phase diagram illustrating the stable phases of iron-carbon alloys as a function of temperature and carbon content. Upon crossing the Acm line, the solubility limit of carbon in austenite is exceeded for the steel&#x27;s specific carbon content. Since the steel is hypereutectoid, the excess carbon forms a new phase called cementite. This cementite, also known as iron carbide (Fe3C), is an interstitial compound of iron and carbon, which is very hard and brittle. This cementite precipitates from the austenite, often along austenite grain boundaries, and is referred to as proeutectoid cementite because it forms above the eutectoid temperature. As proeutectoid cementite forms, it depletes the surrounding austenite of carbon, causing the carbon concentration within the remaining austenite to progressively decrease. The cooling continues until the steel reaches the eutectoid temperature, which is 727 degrees Celsius (1341 degrees Fahrenheit). At this specific temperature, the remaining austenite, having continuously lost carbon to form proeutectoid cementite, now possesses the eutectoid composition of 0.76 weight percent carbon. Upon reaching and cooling just below the eutectoid temperature, this eutectoid-composition austenite undergoes a eutectoid transformation. During this transformation, the austenite simultaneously decomposes into two new phases: ferrite and more cementite. Ferrite, also known as alpha-iron, is a solid solution of carbon in body-centered cubic (BCC) iron, which has very low carbon solubility and is relatively soft and ductile. The two phases, ferrite and cementite, form a characteristic lamellar, or layered, microstructure called pearlite. Therefore, just below the eutectoid temperature, the final microstructure of a slowly cooled hypereutectoid steel consists of two primary components: the proeutectoid cementite that formed above the eutectoid temperature, appearing as distinct networks or regions, and pearlite, which is the lamellar aggregate of ferrite and cementite that resulted from the transformation of the remaining austenite.

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Using the Iron-Carbon diagram, describe the microstructural phases present in a hypereutectoid steel slowly cooled from the austenite region to just below the eutectoid temperature.