Post-build tempering response was investigated for AISI Type 431 martensitic stainless steel produced via laser-based Directed Energy Deposition (DED-LB) at 350-650 °C. Phase evolution and stored strain were quantified using electron backscatter diffraction (EBSD) and x-ray diffraction (XRD) with Rietveld refinement, then correlated with hardness and build-oriented tensile properties. A wrought AISI Type 431 hardened-and-tempered bar served as a benchmark reference. The as-deposited DED-LB microstructure comprised highly strained lath martensite containing δ-ferrite pockets, retained austenite, and Si-Mn-rich oxide inclusions inherited from the water-atomized feedstock, resulting in inherently brittle mechanical behavior. A low-temperature austenite reversion window was identified, with austenite peaking at approximately 6.7% following tempering at 350 °C and preferentially nucleating at δ-ferrite/martensite interfaces. This condition produced a substantial ductility increase, with elongation rising from 1.2% in the as-deposited state to 10.2% while sustaining tensile strength above 1180 MPa. Tempering at 450 °C induced secondary hardening, achieving a peak hardness of 606 HV5, accompanied by reduced elongation as austenite content declined. Above 550 °C, austenite decreased below 1%, transitioning toward a predominantly tempered martensitic microstructure and wrought-like ductility. The ductility evolution across the tempering series is governed by matrix recovery rather than inclusion content. The austenitization-free, single-step tempering route provides a systematic and industry-relevant thermal processing strategy for tailoring strength–ductility trade-offs and mitigating build-induced residual strain sensitivity in additively manufactured martensitic stainless steels.
Hatem et al. (Wed,) studied this question.
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