Jose Alberto, who wanted to distinguish isothermal martensite from bainite. In the case of bainite, the temperatures are such that it is thermodynamically necessary for carbon to partition into the austenite during the nucleation stage, though as the nucleus grows, it accelerates and grows without diffusion. In the case of isothermal martensite, there is no diffusion of carbon even during nucleation or growth. We normally think of martensite as athermal, with the volume fraction increasing with the undercooling below the M_S temperature. However, this is because the reaction is fast and we can only monitor the evolution of the volume fraction by very special, high time-resolution techniques. But if we do careful measurements, then all martensite is isothermal, but very fast. On the other hand, if we work close to the M_S temperature, and the alloy composition is suitable to cause slow growth, then isothermal martensite can also be seen using conventional techniques such as dilatometry. All this means that there is no difference between bainite and martensite in interstitial-free steels.