Effect of chemistry on martensitic phase transformation kinetics and resulting properties of additively manufactured stainless steel

Here we investigate the effect of chemistry, including initial powder chemistry and spatial chemical variations due to vaporization during fabrication, on strain-induced martensitic phase transformation in 304L stainless steel components fabricated by directed energy deposition (DED) additive manufacturing (AM). The austenite stability was altered by mixing pre-alloyed 304L stainless steel powder with Fe powder, which promoted martensitic phase transformation and resulted in an increase in ultimate tensile strength and elongation to failure over pure 304L walls deposited by DED AM. The chemical composition variation with position, due to spatial variations in thermal history, was quantified, showing that austenite stabilizing elements Cr, Mn, and Ni were preferentially vaporized during deposition. We present a martensitic transformation kinetics equation that describes the evolution of martensite volume fraction with respect to plastic strain as a function of chemistry. This allows for the prediction of strain induced martensite evolution as a function of strain, nominal chemistry, and heterogeneous chemistry due to vaporization within additively manufactured components. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.