The martensite transformation in carbon steels

Martensite formed in plain carbon steels containing less than 1.4% carbon has been studied by direct examination of thin foils in the electron microscope. It was found that the martensite in low-carbon steels forms predominantly as long needles parallel to $\langle111\rangle_M$, while in the high-carbon steels it forms as plates internally twinned on a fine scale. The existence of these narrow twins in the high-carbon martensites enables the transformation mechanism to be derived uniquely. This mechanism, which is largely in agreement with those previously proposed, explains all the observed experimental data. The observed change in the morphology of the martensite from needles to plates as the carbon content increases suggests an explanation of the change in hardness of quenched steels with change in the percentage carbon.