Austenitized-Carbon steel-Tetragonal crystal structure

Question

When austenitized, carbon steel is satiated, why BCC crystal elongated in the body-centered tetragonal crystal structure?

Answer 1

Martensite crystals ideally enfold the planar interfaces with parent austenite. The preferred crystal planes of austenite on which martensite crystals form are designated habit planes, which differ along with alloy composition. In steels, the parent phase is basically austenite with the face-centered cubic (fcc) crystal structure, however the crystal structure of the product phase might be body-centered cubic (bcc). Under special situations, steels endure martensitic transformations in which crystal structure of product phase reverts to that of the parent. Most medium-carbon and high-carbon steels form martensite with a bct crystal structure, since carbon atoms occupy only one of the three probable sets of octahedral interstitial positions.