Phase evolution of AISI 321 stainless steel during directional solidification

The phase evolution of AISI 321 stainless steel was studied by directional solidification and quenching techniques. Two interfaces, solid/liquid and the peritectic reaction interface, were found to exist in the directional solidification structure. With increasing growth velocity the solid/liquid interface changed in the sequence of planar, cellular, dendritic and the primary phase changed from austenite to ferrite. The phase and morphology selection was verified by the interface response functions (IRFs) and the maximum growth temperature criterion. The ferritic island banding structure was observed, not only in the austenite cellular primary growth condition (3 μm s⁻¹), but also in the dendritic ferrite primary growth one at relatively low growth velocity (5 μm s⁻¹). It is deemed that the former resulted from the nucleation of ferrite in the continuous matrix of austenite phase, yet the latter is the residual primary ferrite attributed to the growth of austenite. Both of them do not come from the nucleation near the solid/liquid interface.