Tailoring microstructure of wire arc additively manufactured C-Mn-Si steel with post process heat treatment

Wire + arc additive manufacturing (WAAM), a promising technology to produce complex parts, was used to produce steel components with a custom-designed cored-metal wire. The chosen chemical composition of the cored wire is known to produced advance high-strength steel. The as-printed microstructure consists of martensite, retained austenite, and different morphological phases of ferrite, including quasi-polygonal, acicular, and Widmanstatten. Nanoindentation shows the highest hardness value for martensite, followed by acicular ferrite and then quasi-polygonal ferrite. However, a considerable spatial microstructural heterogeneity was observed in as-printed samples due to thermal cycling and chemical segregation. A higher fraction of harder martensite phase was observed at the bottom of the printed coupon, and the phase fraction of martensite decreased due to heat accumulation along the deposition height. Microstructural heterogeneity is reflected in the inhomogeneous distribution of nanohardness. Tempering of the printed sample results in carbide precipitation in martensite phases, leading to a decrease in the martensite hardness, but the heterogeneity is conserved. Both quench hardening and quenching and partitioning (Q&P) treatment were found to homogenize the microstructures. The quench-hardened samples' microstructure was comprised of lath martensite, while the Q&P treatment led to formation of tempered martensite and retained austenite.