In situ study for thermal stability comparison of cast and selective laser melted SS316L stainless steel using high-temperature confocal scanning laser microscopy

We have compared the thermal stability of cast and wrought (C&W) samples of SS316L, with its additively manufactured (AM) counterpart. Selective laser melting process (SLM) was used to produce samples of 316L austenitic stainless steel (ASS), which is extensively used in household goods, aerospace, nuclear industries, and other industries for its high ductility, weldability, moderate yield strength, and corrosion resistance. Typically AM samples have a non-equilibrium microstructure, due to the rapid heating and cooling that occurs during AM process. Our interest is to study the evolution of the as-deposited (SLM) microstructure at various temperatures and holding times. This was done by using a High Temperature-Confocal Scanning Laser Microscopy (HT-CSLM) technique. For comparison, we have also used a C&W sample that was cold-rolled to 50% reduction (Rolled-C&W). The samples were heated to 850°C, 1000°C, 1300°C, and 1400°C temperatures at a 200°C/min rate and hold for 100, 100, 200, and 180 seconds, respectively. Significant differences were observed in the microstructure that occurred in the rolled-C&W and SLM samples during heating, with the grain structure in the SLM sample being more stable than the rolled-C&W. In rolled-C&W alloy, recrystallization started at around 850°C with fine grains becoming visible, but not in SLM sample. The SLM sample started recrystallizing at around 1150°C. In rolled-C&W specimens, twin boundaries start to form around 1350-1360°C, but not in SLM specimens. There is complete twin formation at 1400°C and thick grain boundaries in rolled-C&W, whereas SLM specimens lack twining but show thick grain boundaries. We further explore the reasons underlying the stability of AM samples, in terms of stored dislocation density, and also segregation of alloying elements to various defects such as grain boundaries or cell walls.