On the understanding of microstructural evolution during hot deformation of n-ODS-18Cr ferritic steel containing heterogeneous microstructure

The objective of the present work is to investigate the microstructural evolution during hot deformation of nano oxide dispersion strengthened (n-ODS) Fe – 18Cr ferritic steel over a wide range of temperatures 1273–1573 K and strain rates 10⁻² – 10 s⁻¹. Electron backscatter diffraction analysis was performed extensively on deformed samples to investigate the restoration mechanisms operating in the material. Small angle X-ray scattering (SAXS) analysis was performed on the deformed samples to investigate the nanoprecipitate characteristics. Flow curves obtained in the current study exhibit initial strain hardening followed by flow softening for all the studied deformation conditions. An inflection point was noticed in the plot between strain hardening rate $\left(\theta ={\left(\frac{d\sigma }{d\epsilon }\right)}_{T,\dot{\epsilon }}\right)$ and flow stress for almost all test conditions except the ones corresponding to 1573 K and at 10⁻² and 10⁻¹ s⁻¹, which is an obvious indication for the occurrence of dynamic recrystallization (DRX) in the n-ODS-18Cr steel. Microstructural analysis revealed the occurrence of continuous DRX and it is responsible for the grain refinement in the present steel. Grain growth was dominant in the samples deformed at 1423 K (and above) and 10⁻² s⁻¹. SAXS analysis revealed considerable coarsening of nanoprecipitates with bimodal particle distribution in the sample deformed at 1573 K and 10⁻² s⁻¹. It showed that the average size of fine nanoprecipitates increased to ~6.2 nm from 3.8 nm and coarse precipitates of size 20–30 nm.