Role of pearlite colonies on the dynamic flow stress of low carbon steel

The dynamic deformation of low carbon steel is examined in two different microstructures; the as-received coarse grained material composed of α-ferrite and pearlite phases, and a heat treated microstructure in the form of carbide particles dispersed within a ferrite matrix. Testing in the dynamic regime is completed using a Split Hopkinson Pressure Bar at strain rates of 102-104 s−1 with testing temperatures of room temperature, 300, 500 and 650 °C. The correlation between strain rate and corresponding flow stress showed a distinct transition delineating the quasi-static regime as dominated by the thermally activated flow stress and that of the dynamic regime. The strain rate and basic characteristics of the thermal and athermal stresses, are studied to determine the mechanisms of deformation as a function of the microstructure, strain rate and temperature. The relative influence of short and long range barriers on the flow stress components is studied in a microstructure in which the pearlite colonies, an effective long range flow stress barrier, are removed. Separation of flow stress components is utilized in a constitutive equation based on thermal activation theory to predict flow stress as a function of strain, strain rate, temperature and explicit microstructure variables including pearlite volume fraction and grain size. Results of this model are compared with those obtained experimentally for both the as-received and heat treated materials.