Effect of aging temperature on microstructure evolution and strengthening behavior of L-PBF 18Ni(300) maraging steel

This work investigated the microstructure evolution with aging temperature of laser-powder bed fusion (L-PBF) 18Ni(300) maraging steel at different scales. Important issues of cell structure changes, precipitation sequence, and austenite reversion were evaluated. The dominant strengthening factor in each state was elucidated based on yield strength modeling. The as built fully martensitic matrix consists of dislocation cell structures, and its yield strength comes from the martensite lath and solid solution strengthening with comparable contributions. Aged at 440 °C, the cell walls kept stable and massive ω particles prominently strengthened matrix by shearing mechanism. At 490 °C, the cell walls were partially destroyed and austenite reversion just began along the lath boundary. Densely distributed η-Ni3Ti particles produced the peak strength dominantly by the Orowan mechanism, but the shearing process may also exist meanwhile. At 540 °C, cell walls further degraded and reverted austenite formed along the lath boundaries and around some retained cell walls. Its strength decreased mildly mainly due to the wider inter-particle spacing, but was still maintained quite high by Laves-Fe2Mo and η-Ni3Ti particles via the Orowan process. The reverted austenite with limited fraction had a minor role in its strength decrease, but improved ductility significantly.