Flow stress analysis for determining the critical condition of dynamic ferrite transformation in 6Ni-0.1C steel

In order to clarify the occurrence of dynamic ferrite transformation in a 6Ni-0.1C steel, the stress-strain behavior in uniaxial compression was analyzed for a wide range of temperatures and strain rates. Significant softening of flow stress for austenite was observed at lower temperatures at a constant strain rate, which seemed to correspond with the occurrence of dynamic transformation to ferrite. Analysis of the maximum stress in the stress-strain curves indicated that dynamic ferrite transformation occurred above a certain value of the Zener-Hollomon parameter (Z). The critical deformation condition (ZC) for the occurrence of dynamic transformation was determined. Increasing the amount of softening resulted in an increase in the fraction of ferrite, and the maximum flow stress came close to the flow stress of ferrite. Microstructural observations revealed that the specimens exhibiting softening consisted of ferrite grains with typical characteristics of deformation microstructure, such as a change in crystal orientation within the ferrite grain, inhomogeneity in ferrite morphology and dislocation substructures inside the grains. All these characteristics confirmed the occurrence of ferrite transformation during deformation, i.e. dynamic ferrite transformation.