Deformation induced austenite formation in as-cast 2101 duplex stainless steel and its effect on hot-ductility

The microstructural evolution during hot deformation of 2101 grade lean duplex stainless steel and its effect on hot-workability have been investigated by hot-compression testing using Gleeble® simulator over the range of deformation temperature of 800-1100 °C. Besides the dynamic recovery of δ-ferrite matrix and deformation of large austenite (γ) regions, fine γ-islands (<8 µm in size) were observed to form inside the δ-matrix. The density of those islands reached the highest value at deformation temperature of 900-1000 °C and increased significantly with the increase in applied true strain from 0.25 to 0.8. Such γ-islands are expected to form either by dynamic strain induced δ to γ transformation or by γ to δ strain-induced transformation followed by rapid precipitation of γ on heterogeneous nucleation sites (sub-grain boundaries, deformation bands etc.) present in the δ-matrix. The average size of the islands decreased with the decrease in deformation temperature. As the precipitated γ-islands follow Kurdjumov-Sachs orientation relationship with the δ-ferrite matrix, formation of such islands is detrimental to the hot-workability of the duplex stainless steel. Those islands not only restrict the plastic flow in δ-matrix but also provide favourable path for crack propagation through the δ.