Formation of annular austenitic ring between outer ferrite layer and solid globule in a semi-solid processed SS 304L

• A low carbon variety 18Cr–8Ni SS was rapidly quenched from its semi-solid state.
• A unique microstructure that differs from the cast steel was obtained.
• The microstructure showed inner core, intermediate γ-ring and outer ferrite layer.
• Thermodynamically stable phase for solidification was identified by DSC experiments.
• Mechanism for the formation of intermediate γ-ring and outer ferritic layer is proposed.

The microstructure of an austenitic SS 304L rapidly quenched from its semi-solid state shows a unique annular austenitic ring in between the core of each globule and its ferritic outer layer. On the basis of experimental results and microstructural analysis, it is proposed that the ring is formed as a result of preferential austenitic phase nucleation in a small quantity of liquid entrapped between adjacent solid globules during rapid quenching, in spite of the fact that ferrite is the thermodynamically stable phase for the alloy.