Strengthening 316L stainless steel fabricated by laser powder bed fusion via deep cryogenic treatment

Additive manufacturing (AM), laser powder bed fusion (L-PBF) in particular, enables the rapid creation of 316L stainless steel components with complex geometries, thereby overcoming the limitations associated with traditional manufacturing methods. To further optimize the performance of these components, deep cryogenic treatment (DCT)—an extension of traditional heat treatment involving exposure to temperatures below 143 K—was employed to modify the microstructure of L-PBF fabricated 316L stainless steel. Experimental results showed that DCT with a duration below 15 min strengthened 316L stainless steel without obviously compromising ductility. Specifically, a 5-min DCT increased yield strength by 12.3% compared to the as-fabricated part. Microstructure analysis demonstrated that the strength enhancement was primarily attributed to the formation of nanotwins with thicknesses ranging from 20 to 30 nm. However, when DCT duration exceeded 15 min, softening rather than strengthening occurred. This softening was associated with a decrease in dislocation density, disruption of cellular substructure, and dissolution of nanoparticles, which are the key features of as L-PBF fabricated 316L stainless steel. Additionally, prolonged DCT caused the thickness of nanotwins to increase to sub-micron sizes, reducing their contribution to strengthening.