17-4 PH is a martensitic stainless steel of interest for additive manufacturing (AM) applications because of its combination of high strength and corrosion resistance. In the wrought form this steel has a fully martensitic structure that can be strengthened by precipitation of fine Cu-rich FCC precipitates upon aging. When produced via AM, its microstructure depends strongly on the powder feedstock, particularly the gas used during powder atomization, typically argon or nitrogen. Argon-atomized powder costs significantly more than that produced with nitrogen.
However, nitrogen is a strong austenite stabilizer in iron and steel, resulting in an as-built microstructure of AM 17-4 PH that contains up to 90% retained austenite.
Conventional annealing heat treatments reduce the retained austenite to around 10% to 15%, even after cryogenic treatment, resulting in mechanical behavior that differs significantly compared to cast/wrought 17-4 PH.
This work examines alternative post-build processing of nitrogen gas-atomized AM 17-4 PH to minimize or eliminate the retained austenite in the final microstructure, producing a material that more closely resembles its cast/wrought counterpart.
Eric Lass, Carelyn Campbell, Maureen Williams, National Institute of Standards & Technology (NIST)
https://www.nist.gov/people/eric-lass
