Sub-zero Celsius Transformation of Retained Austenite into Martensite in Laser Powder Bed Fusion Fabricated Precipitation Hardening Steel AISI 630

Abstract Laser Power Bed Fusion (LPBF) fabricated parts often require a modified heat treatment to accommodate for slight differences in composition due to the additive manufacturing process. Powders used for LPBF fabrication of the precipitation hardening 17-4PH stainless steel are often spray-atomized using N 2 cover gas, resulting in a significant nitrogen uptake which lowers the martensite start (M s ) temperature of the steel. Sub-zero treatments can promote martensite transformation in such alloys. In this work, wrought 17-4PH samples were high temperature solution nitrided (HTSN) to achieve nitrogen contents comparable to LPBF 17-4PH. Sub-zero dilatometry was used to track martensite formation behavior at sub-zero temperatures in the HTSN wrought and LPBF 17-4PH. Samples were further characterized through XRD, hardness, and optical microscopy. It was found that martensite morphology changes from lath to predominantly twinned with decreasing M s (higher nitrogen), and thermally activated martensite formation was recorded on heating. The HTSN wrought samples also exceeded the standard peak hardness for 17-4PH, demonstrating that sub-zero treatments are effective at producing a hard martensitic microstructure for both HTSN and AM parts produced with N 2 -atomized powders.