20 / 2025-03-30 21:13:42

Transformation behavior of additively manufactured 17-4PH at sub zero temperatures

sub-zero treatment,heat treatment,dilatometry,17-4PH stainless steel,additive manufacturing

Additively manufactured (AM) parts fabricated through Laser Powder Bed Fusion (LPBF) often require a modified heat treatment to accommodate for the differences in microstructure and composition as compared to their conventional counterparts. The unconventional microstructure is a consequence of the very fast cooling rate inherent to the LPBF process. 17-4PH stainless steel (17-4PH) is a precipitation hardening steel which hardens through the aging of a martensitic matrix during aging treatment. Stainless steel parts fabricated through LPBF typically result in a relatively high nitrogen content. This is due to i) higher nitrogen content in the N2 gas-atomized powder and ii) additional uptake from reaction with the “inert” N2 cover gas during the LPBF process. An enhanced nitrogen content in 17-4PH will unavoidably lower the martensite start temperature and lead to a higher fraction of retained austenite. Controlled sub-zero treatment can accomplish near complete transformation into martensite; previous ex-situ analysis of sub-zero treatments determined that a combination of boiling nitrogen (-196 °C) and extended hold in dry ice (-78 °C) produced the greatest hardness, suggesting a significant degree of isothermal martensite transformation. In this paper, dilatometry is used to track the phase transformations in LPBF and nitrogen-enriched wrought 17-4PH to further examine the martensite formation behavior at sub-zero temperatures. A variety of temperatures and hold times are applied to study the kinetics of the martensite transformation. The extent of martensite transformation is additionally examined through XRD analysis and optical microscopy.