Effect of heat treatments on the microstructure and mechanical properties of an ultra-high strength martensitic steel fabricated via laser powder bed fusion additive manufacturing

Abstract

A newly developed ultra-high strength martensitic steel, AF9628, has generated academic and industrial attention due to its combination of high yield strength and ductility at a low cost. Several studies have demonstrated that this steel can be successfully additively manufactured (AM) using laser powder bed fusion (LPBF). However, as-printed parts have displayed microstructural inhomogeneity, anisotropy, and a discrepancy in mechanical properties as compared to the traditionally processed material. There is currently no work detailing the effects of post processing heat treatments on the microstructure or mechanical properties of as-printed AF9628. In the interest of producing robust near-net-shape parts with complex geometries, this work explores the effects of LPBF and post processing heat treatments on the microstructure and mechanical properties of AF9628. Parts fabricated by LPBF were subjected to three different quench and temper heat treatment schedules. Quench and temper treatments displayed refined equiaxed prior-austenitic grain structures, reducing grain size by up to 45%. Significant variability was observed in Vickers hardness measurements of as-printed samples due to tempered and untempered bands of martensite alternating throughout the microstructure caused by thermal cycling during printing. Heat treatments were observed to increase the overall hardness of the material and reduce variability. As-printed samples displayed a tensile strength of 1.41 GPa, 10% elongation, and a Charpy impact toughness of 28 J. Quench and temper heat treatments were observed to increase tensile strengths to 1.66 GPa, but reduce the elongation to 7.6% and impact toughness to 24 J. Understanding the effects of these post-processing treatments is expected to allow for property optimization of AF9628 additively manufactured parts.