Strain rate sensitivity behaviour of a chrome-nickel austentic-ferritic stainless steel and its constitutive modelling

Abstract

In the present investigation, the plastic flow curves and work softening behaviour of a dual phase Fe–Cr–Ni alloy during hot deformation (low to intermediate temperature range, 948 K (675°C) to 1 248 K (975°C)) along with concurrent microstructural development were investigated. The flow stress increased with the increase in strain rate and decreased with the increase in deformation temperature. The single peak characteristic appearing in all the flow curves indicated that dynamic recrystallization (DRX) was the dominant softening mechanism in the later stage of deformation. The critical strain for DRX initiation was c = 0.632p and the peak strain (p) were expressed through the Zener-Hollomon parameter (Z). For flow stress modelling, an Arrhenius type constitutive model was established to predict the flow stress behaviour during hot deformation. The results showed that the calculated flow curves agreed reasonably well with the experimental results. The microstructural analysis using optical microscopy indicated that all the deformed structures exhibited elongated grains similar to that of parent microstructure and some equiaxed grains (resulting from DRX in the austenite phase). The fraction of equiaxed grains (in austenite) increased with the deformation temperature. At low Z, the ferrite phase accommodates the strain and dynamic recovery was the prominent restoration process. At high Z, austenite controlled the deformation mechanism and DRX was the likely cause for microstructural refinement. The iso-strain rate sensitivity (m) contour map was used to determine the optimum regime of high temperature workability.