Additively Manufactured Nitrogen-Atomized 17-4 PH Stainless Steel with Mechanical Properties Comparable to Wrought
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ve manufacturing (AM) of metallic materials is a potentially transformative technology because it provides a method for producing complex, three-dimensional parts with virtually unlimited design freedom. However, to date, only a handful of alloys are available for use in AM processes, and many of these materials, although weldable in wrought form, produce significant challenges when applied to AM. Of these materials, precipitation-hardenable martensitic stainless steels, specifically 17-4, are of great interest for AM applications because of their unique combination of high strength and corrosion resistance. However, successful
ERIC A. LASS, MARK R. STOUDT, and MAUREEN E. WILLIAMS are with the Materials Science and Engineering Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, 20899. Contact e-mail: [email protected] Manuscript submitted November 21, 2018.
implementation of these materials for AM-produced components has proven difficult. A significant amount of nitrogen, a well-known FCC/austenite stabilizer in steels, remains in N2-gas-atomized 17-4 powder feedstock.[1–6] As a result, components built from such powder contain a large volume fraction of retained austenite (RA). The as-built material exhibits a yield point phenomenon in tension and compression, with an upper yield strength of 500 to 600 MPa.[1,5,6] This is significantly lower than the yield strength of wrought 17-4 (W17-4), which is typically around 1000 MPa, but can be as low as 760 MPa. Yielding is followed by a region of constant stress with increasing strain, followed by significant work hardening, An increase in the ultimate tensile strength and elongation to failure is also commonly observed in AM17-4 compared to W17-4. This is likely a result of the strain-induced transformation of austenite to martensite,[1] akin to transformation-induced plasticity in TRIP steels. Argon gas-atomized 17-4 powder provides one alternative to N2-atomized material.[1,3,7–9] However, Ar-atomized powder is more expensive. More importantly, Ar is insoluble in steel, resulting in large amounts of trapped-gas porosity in the AM product, which is detrimental to both mechanical and corrosion behavior.[8–10] Water-atomized powder is sometimes used in traditional powder metallurgy applications,[11–13] though the powder properties are not well-suited for AM, and its investigation as feedstock for AM is limited.[14] Both Ar- and water-atomized AM17-4 material can exhibit the same unusual stress–strain behavior as observed in N2-atomized material in the as-built condition,[7–9,14] a result of the rapid solidification conditions yielding a fraction of RA in the as-deposited microstructure. However, cast/wrought thermal processing protocol, consisting of homogenization at 1423 K (1150 °C) for 1 hour, followed by solutionization at 1323 K (1050 °C) for 30 minutes (i.e., condition A (CA)), eliminates such behavior. Thermal processing of N2-atomized AM17-4 reduces but does not eliminate this behavior, and yield strength remains
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