Cryogenic mechanical properties of 316L stainless steel fabricated by selective laser melting

Abstract

Abstract 316L stainless steel fabricated by selective laser melting (SLM) has a wide range application in complex structural parts servicing at cryogenic temperature due to its good corrosion resistance and extremely low ductile-brittle transition temperature. In present work, the as-built and heat-treated microstructure and cryogenic mechanical properties of the SLM-fabricated 316L stainless steel were investigated. Due to the extremely high cooling rate in SLM process, the SLM-fabricated 316L stainless steel has unique microstructure, consisting of the heterogeneous columnar grain structure with a weak texture and intragranular cellular structure attaching with high dislocation network. The cryogenic yield, ultimate tensile strength, and elongation of as-built horizontal specimens are 840 MPa, 1510 MPa and 35%, those of as-built vertical specimens are 785 MPa, 1400 MPa and 37%. After annealing treatment, the yield strength decreases and the elongation increases, but the ultimate strength changes little. The high ultimate strength and the low yield ratio of SLM-fabricated 316L stainless steel is attributed to the martensite transformation during cryogenic deformation owing to its low stacking fault energy at low temperature. By comparing the martensite content of specimens with different strains and tensile directions, it was found that the grain texture is the dominant factor for the anisotropy of strength and ductility of specimens. The cellular structure is also in favor of the generation of shear band to increase the nucleation sites of α′ martensite, and finally promoting the occurrence of martensite transformation, but the promoting effect of cellular substructure on martensite transformation is limited compared with the influence of the grain texture.