Effect of the cellular structure on austenite distribution and cryogenic mechanical properties of maraging stainless steel processed by laser powder bed fusion

The cellular structure, a distinctive feature of steels fabricated by laser powder bed fusion (L-PBF), can enhance strength and ductility in austenitic steel. Its effect in maraging stainless steel undergoing austenite-martensite transformation is more complex. This study compares the microstructure and mechanical properties of L-PBFed Fe-Cr-Ni-Co-Mo maraging steel with and without the cellular structure. The as-built steel shows a ∼0.52 μm cellular structure with Cr and Mo segregation at cellular walls, which disappears after homogenization. Direct solution-aging of the as-built (775) specimen yields 22.6 % retained austenite with an island-like morphology, whereas the homogenized (1775) specimen has 18.6 % with a lath-like morphology. This difference results from elemental segregation, which stabilizes austenite at cellular walls and disrupts lath-like austenite continuity. At 23 °C, the 1775 specimen shows higher strength and elongation than the 775 specimen. At −196 °C, this advantage increases, with yield strength of 1697 vs. 1606 MPa, ultimate strength of 1737 vs. 1658 MPa, elongation of 26 % vs. 16 %, and impact energy of 44 vs. 7 J. The island-like austenite in the 775 specimen provides limited crack resistance, causing more secondary cracks. In contrast, the lath-like austenite with heterogeneous shells in the 1775 specimen undergoes greater strain-induced martensitic transformation, producing a stronger transformation-induced plasticity (TRIP) effect. Eliminating the cellular structure via homogenization treatment is essential for optimizing L-PBF maraging steels.