Furthermore, martensite, which is characterised by an acicular appearance, forms progressively over a temperature range as the temperature falls; if the temperature is held constant after the start no further action takes place. Martensite formation produces an expansion related to the carbon content. The mechanical properties of martensite depend on the carbon content; low carbon martensites (less than O.OS%C) have reasonable ductility and toughness, high carbon martensites have no ductility or