Mechanical Properties and Microstructure of the 316L Steel Produced by Different Methods

Abstract

The 316L stainless steel meets all health, strength, and quality standards and is an irreplaceable material in the manufacture of medical equipment. The study focused on the 316L austenitic stainless steel, manufactured with the conventional technique in accordance with ASTM A276/A276M–17 Condition A (samples rolled and annealed at 1050°C with water cooling) and with the selective laser melting (SLM) technique (as-printed starting samples). Unlike conventional manufacturing techniques, SLM offers significantly greater design freedom. An AxioMat 200M optical microscope was employed to analyze the microstructure in different lighting modes, and Kalling’s and Marble’s reagents were used to reveal the structure. The 316L steel produced conventionally mainly consisted of austenite (microhardness of 239 kg/mm²), and substantial cross- sectional grain heterogeneity was established in the test sample. Twins and an atypical multidirectionally oriented dense acicular structure in the area of individual grains (microhardness of 260‒286 kg/mm²) and a unidirectional loose structure (microhardness of 317‒328 kg/mm²) were observed. The microstructure of the 316L steel produced with the SLM technique mainly consisted of austenite (microhardness of 268 kg/mm²). The boundaries of the primary austenite grains were revealed with Marble’s reagent, and arc-shaped structures of the melt bath were established. Kalling’s reagent revealed an atypical multidirectionally oriented intragranular substructure, located primarily between the tops of next-layer tracks in areas where previous-layer tracks overlapped (longitudinal microhardness of 239–251 kg/mm² and cross-sectional microhardness of 286–317 kg/mm²). Elongated columnar grains were found using differential interference contrast microscopy. The average ultimate strength of the steel samples produced with the conventional technique was higher than that of the samples produced with SLM by 4.63%, yield strength by 1.53%, relative elongation by 8.27%, and relative contraction by 18.36%. The lower level of properties and greater spread of their values for the SLM steel were due to the presence of elongated grains and anisotropy relative to the buildup direction. The actual level of properties shown by the SLM steel in the starting state meets the regulatory requirements.