INVESTIGATION OF THE MICROSTRUCTURE OF AISI 321 STAINLESS STEEL AFTER LASER SURFACE MELTING

Abstract

The aim of the present paper is to investigate the microstructure of laser-melted surface layers of austenitic steel for biomedical applications. The surface of prismatic specimens from AISI 321 stainless steel was treated by continuous CO₂ laser. Three modes of laser processing were used, ensuring surface melting. The microstructure was observed by OM and SEM, while the chemical composition was investigated by EDX analysis. It was found that the microstructure of as-delivered steel was two-phase and relatively inhomogeneous in morphology and chemical composition. It consisted of austenite with grain sizes between 20 - 150 μm, relatively large amount of striped δ-ferrite and spherical carbides along the grain boundaries. After laser melting, the microstructure remained two-phase (δ-ferrite and austenite), but became more homogeneous in morphology and composition. Different dendrites morphology in the particular regions of the molten layer was confirmed - fine equiaxed dendrites on the top surface and columnar at the bottom of the molten pool. Delta-ferrite is located in the interdendritic areas and in larger amounts in the transition zone between the molten.