Extremely low temperature mechanical behavior of in-situ oxide containing 304L stainless steel fabricated by laser powder bed fusion

Abstract

This study investigates the mechanical properties and microstructure of SS304L stainless steel (SS) fabricated via laser powder bed fusion (LPBF) under controlled oxygen levels (0.2%) at both room and cryogenic temperatures (77 K and 4 K). Experimental results show that the LPBF SS304L exhibits significant improvements in yield strength (YS), with an increase of ≈336 MPa at room temperature and up to ≈398 MPa at 4 K compared to wrought SS304L. Additionally, the current LPBF SS304L demonstrates an extra ≈64 MPa YS strengthening over previous LPBF SS304L data at room temperature. These strength enhancements are primarily attributed to oxide dispersion hardening, promoted by the controlled oxygen level, alongside grain boundary strengthening and dislocation hardening, without significant ductility loss. Furthermore, strain-induced martensitic transformation (SIMT) was absent at room temperature and reduced at cryogenic temperatures compared to wrought SS304L, likely due to high dislocation density and nitrogen-stabilized austenite. A jerk flow observed at 4 K is attributed to adiabatic heating from plastic deformation, consistent with the low thermal conductivity. Finite element simulations reveal a short residence time (0.0137 s) for molten material during the LPBF process, with oxide particles forming predominantly through heterogeneous nucleation at the melt pool surface, and uniformly distributed by Marangoni convection. These findings provide key insights into developing LPBF parameters for enhanced mechanical performance of SS304L for cryogenic and ambient temperature applications.