CRITICAL-TEMPERATURE FOR GROWTH OF MARTENSITE

M(s) may be defined as the temperature below which the formation of martensite starts upon cooling. It may also be useful to define M(g), the temperature below which martensite can grow if it is already nucleated. In order to analyze the mechanism of martensite formation, it is essential to know the difference M(g) - M(s). We have tried to evaluate M(g) - M(s) for an Fe-C alloy with a decarburized surface zone in order to induce nucleation. The samples were studied by means of electron microprobe, serial sectioning and optical microscopy. The results indicate that M(g) is surprisingly close to M(s). The possibility that M(s) is controlled by growth rather than nucleation is discussed.