Metallurgy Question:
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When austenitized, carbon steel is quenched, why is the BCC crystal elongated into a body-centered tetragonal crystal structure?

Answer:

Martensite crystals ideally have planar interfaces with the parent austenite. The preferred crystal planes of the austenite on which the martensite crystals form are designated habit planes, which vary according to alloy composition. In steels, the parent phase is usually austenite with a face-centered cubic (fcc) crystal structure, but the crystal structure of the product phase may be body-centered cubic (bcc). Under special conditions, steels undergo martensitic transformations in which the crystal structure of the product phase reverts to that of the parent. Most medium-carbon and high-carbon steels form martensite with a bct crystal structure, because carbon atoms occupy only one of the three possible sets of octahedral interstitial positions.

Allotropy means the property by which certain elements (like Fe) may exist in more than one crystal structure. Iron exists in two allotropic forms: BCC and FCC. In other words at 700°C (1290°F) it undergoes an allotropic transformation from FCC to BCC (in quenching, i.e. iron has FCC structure above this temperature and BCC structure below that).

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