Evaluation of deformation and recrystallization behavior in oxide dispersion strengthened 18Cr ferritic steel

Abstract

This paper presents the results of an experimental study aimed to obtain an ultrafine equiaxed grain distribution in 18Cr oxide dispersion strengthened (ODS) ferritic steel through cold working and annealing starting with an initial columnar grain structure with a predominant α-fibre texture in a product consolidated from the alloy powders during extrusion at high temperatures. Deformation along the extruded direction (ED) resulted in the retention of α-fibre texture, while deformation in the transverse direction (TD) showed a shear banded structure with a reduced percentage of α-fibre texture. Differential Scanning Calorimetry (DSC) analysis of the deformed steel established the occurrence of two significant events during heating namely recovery and recrystallization, whose temperatures were influenced by the heating rate. The recovery and recrystallization domains have been distinctly observed at 1350 K and 1420 K respectively at a low heating rate of 7 K min−1. The resultant microstructure showed very coarse elongated grains interspersed with regions of ultrafine (<1 μm) equiaxed grains, due to the incomplete recovery. The deformed steel was subjected to a two step heat treatment designed based on the above inputs with an aim to reduce the microstructural anisotropy in longitudinal direction. The microstructure of the heat treated steel showed randomization of the initial <1 1 0>//ED α-fibre texture, which improved further with repeated deformation and two step heat treatment cycles. A gradual increase in hardness during the above cycles was observed reflecting the increase in dislocation density which offers the propensity to achieve an ultrafine grained microstructure.