Transformation law of microstructure evolution and mechanical properties of electron beam freeform fabricated 321 austenitic stainless steel

Abstract AISI 321 austenitic stainless steel deposition was manufactured by electron beam freeform fabrication, which was to analyze the changes in microstructure evolution and mechanical properties along deposited direction, and to explore their underlying causes in view of the change in thermal process of deposited layers. The microstructure was transformed from columnar crystals to dendrites along fabricated direction. Both δ-ferrite and acicular α′-martensite were formed within the deposition due to the non-equilibrium solidification. More than one kind of M23C6 particles precipitated inside δ-ferrite. The content of all phases varied in different regions, corresponding to the increased density of δ-ferrite but decreased density of α′-martensite along the fabricated direction. Transformation law of microstructure evolution was analyzed. The change in heat dissipation condition in different deposited layers and the component fluctuation especially for Cr and Ni were the main reasons for the microstructural variation. The fabricated 321 austenitic stainless steel showed the decent mechanical properties, of which the maximum microhardness of 198 HV and the maximum tensile strength of 765 MPa. Considering the decrease in α′-martensite, both microhardness and tensile strength were decreased along fabricated direction.