A microscopic examination of the precipitation phenomena occuring during the isothermal pearlitic transformation in high carbon-copper nickel steels

During isothermal pearlitic transformation of high carbon austenite, the formation of proeutectoid phases precedes that of pearlite. Widmanstatten cementite is the major proeutectoid phase that forms at all temperatures. Within this constituent the precipitation of ε-Cu always occurs, together with the formation of stabilized coarse austenite entrapments. Within pearlitic cementite interphase precipitation of ε-Cu occurs at all transformation temperatures, while within pearlitic ferrite a banded interphase precipitation of ε-Cu occurs only at temperatures close to the eutectoid point. As the temperature of the isothermal pearlitic transformation is lowered the formation of non-partitioned pearlitic ferrite is possible and ε-Cu precipitation only forms within this ferrite as a result of ageing occurring during prolonged isothermal holding. It was concluded that interphase precipitation of ε-Cu always occurred on the migrating austenite/cementite interphase boundaries, but ε-Cu interphase precipitation occurred on the migrating austenite/ferrite boundaries only at temperatures close to the eutectoid point.