Metastability mediated grain size control in PH 17-4 stainless steel fabricated using Laser-Powder Bed Fusion (LPBF)

Abstract

Varying the chemical composition and cooling rates during additive manufacturing (AM) can enable the formation and, in some cases, the retention of metastable phases affecting the solidification pathways. Altering the solidification pathways directly affects the microstructure and in turn the mechanical properties of the parts. In this study, we show that modifying the solidification pathway through the deliberate retention of metastable austenite in PH 17–4 stainless steel (SS) leads to significant grain refinement (one order of magnitude smaller grains) and improvement in tensile strength (30% higher ultimate tensile strength) of parts printed using Laser-Powder Bed Fusion (LPBF). Nitrogen (N2)-atomized feedstock powder containing higher concentrations of austenite-stabilizing elemental nitrogen was used to print parts with retained austenite. Parts absent of retained austenite printed using argon-atomized feedstock powder were used for comparison of microstructure and tensile properties. The grain refinement has been attributed to the “crowding effect” observed due to the simultaneous growth and coexistence of metastable austenitic phase with the stable ferrite. We also show that employing a three-step heat treatment procedure can eliminate the unwanted yield point behavior associated with the softer austenite while preserving the superior tensile properties in N2-atomized samples.