A lecture given by Dong-Woo Suh, at the Adventures in the Physical Metallurgy of Steels (APMS) conference held in Cambridge University. Explains how the fact that the bainitic ferrite unit cell does not have cubic symmetry explains the reluctance of the carbon to partition into austenite. Some remarkable discoveries have been made recently regarding the thermodynamic stability of carbon in ferrite, whose unit cell does not have cubic symmetry, but rather, is tetragonal. A combination of ab initio and phase stability calculations indicate that the change in symmetry leads to a dramatically modified phase diagram for the Fe-C system. The implications of this to both academic phenomena such as the reluctance of carbon to partition from ferrite into austenite, and to products of technological importance (such as the quench and partitioning process) are described.