Sub-zero Treatment of 17-4PH Stainless Steel Manufactured by Laser Powder Bed Fusion (LPBF) - Tom Bell Young Author Award Candidate

Additively manufactured (AM) metals require a modified heat treatment to accommodate for slight differences in composition, namely nitrogen pickup, caused by powder atomization and fabrication of parts using N2 as “inert” gas. 17-4PH stainless steel (17-4PH) is a precipitation hardening steel which hardens through the formation of small Cu precipitates in the martensitic matrix during aging. In Laser Powder Bed Fusion (LPBF) manufacturing of 17-4PH, N2-gas atomized powder and N2 cover gas can result in significant nitrogen pick-up and thus lead to a large fraction of retained austenite. This is due to the strong austenite-stabilizing effect of (dissolved) nitrogen which lowers the martensite start temperature of the steel. The large fraction of retained austenite will strongly influence the precipitation kinetics in the aging step, as precipitation occurs much faster in martensite than in austenite. Consequently, this leads to insufficient hardening of the material. Introducing a sub-zero treatment could promote a complete martensite transformation. In the present work, controlled sub-zero treatment of solution heat treated nitrogen containing LPBF fabricated 17-4PH is performed. The subsequent aging response of nitrogen containing martensite is investigated and compared with conventional wrought nitrogen free 17-4PH. The phase transformations are recorded in-situ with dilatometry in addition to ex-situ hardness, optical microscopy, SEM, and XRD analysis to characterize the final microstructure.