Warm deformation processing maps for the plain eutectoid steels

The ductility of eutectoid steels can be improved if the fully lamellar microstructure could be converted to nano/ultrafine ferrite + cementite microstructures by utilizing warm deformation. In this research, the constitutive flow behavior of a eutectoid steel has been characterized in the temperature range 620-770 °C at strain rates in the range 0.01-10 s−1, aided with microstructural characterization of deformed specimens. Various deformation mechanisms including spheroidization, operating during warm deformation of the steel have been identified and their domains/sub-domains in temperature - strain rate space delineated through construction of a power dissipation map based on the principles of dynamic materials modeling. An instability map showing a regime characterizing the manifestation of instabilities in the temperature - strain rate space was also plotted based on Kumar's criterion. Superposition of the instability map on the power dissipation map yielded the processing map for the steel identifying the deterministic domains for various mechanisms, in particular the safe processing window for defect-free processing of the steel. The results show that the pearlitic microstructure exhibits several deformation mechanisms within these warm working conditions. Dynamic spheroidization through cementite fragmentation occurred in the range 640-720 °C and 0.01-0.1 s−1 with a power dissipation efficiency of 21-30%. Additionally, a novel rapid microstructural conversion technique for dynamic spheroidization in eutectoid steel at 730-740 °C and strain rate of 0.1-1 s−1 has been demonstrated. In the instability regime, various microstructural defects such as flow localization, lamella sliding, lamella kinking, lamella cracking and prior austenite grain boundary cracking appeared.