Quantification of δ-ferrite and austenite retention in laser-deposited martensitic stainless steel

Abstract

Type 420 martensitic stainless-steel powder was clad onto a CF3M austenitic stainless steel substrate using laser direct-energy-deposition (L-DED). Through comprehensive characterization and numerical simulation, the evolution of microstructures, particularly the retention of δ-ferrite and austenite, was investigated. In the clad fusion zone, the primary phase to solidify was δ-ferrite, followed by austenite resulting from the peritectic reaction. The segregation patterns developed during rapid solidification from the L-DED process exerted a significant effect on the stability of the austenite in the room temperature microstructures. Due to the fast cooling rate in the solid state, the time available for transformation of the δ-ferrite to austenite regarding the segregation patterns was limited, and the transformation was incomplete. The room temperature microstructure was therefore comprised of δ-ferrite distributed in the dendrite core regions, surrounded by martensite, which was further surrounded by a small fraction of retained austenite in the interdendritic regions. The retained austenite region was enriched with segregated alloying elements, which pushed the martensite start temperature Ms below room temperature. The retention of both δ-ferrite and interdendritic austenite was proved to have caused a softened fusion zone with a lowered martensite fraction.