Microscopic characterisation of ε-Cu interphase precipitation in hypereutectoid Fe-C-Cu alloys

Abstract

During isothermal pearlitic transformation in medium or high carbon copper steels the decomposition of austenite leads, apart from the formation of proeutectoid phases and pearlite, to the precipitation of the ε-Cu phase. At temperatures close to that of the eutectoid of the system interphase precipitation of ε-Cu occurs within proeutectoid ferrite (medium carbon steels), within grain boundary proeutectoid cementite (high carbon steels) and within both pearlitic ferrite and cementite. As the temperature of the isothermal pearlitic transformation is lowered the formation of copper supersaturated pearlitic ferrite occurs while within proeutectoid cementite (grain boundary or Widmanstatten) and pearlitic cementite interphase precipitation of ε-Cu continues. This study of the isothermal pearlitic transformation in the Fe-Cu-C system revealed that interphase precipitation of ε-Cu always occurs on moving cementite/austenite interphase boundaries but ε-Cu interphase precipitation only occurs on moving ferrite/austenite boundaries at temperatures close to the eutectoid temperature range of the system.