Effects of Secondary-Phase Formation on the Electrochemical Performance of a Wire Arc Additive Manufactured 420 Martensitic Stainless Steel under Different Heat Treatment Conditions.

This study aims to investigate the effects of annealing, quenching, and tempering (Q&T) heat treatments on the microstructure, crystallographic orientation, and electrochemical performance of a wall shaped 420 martensitic stainless steel part fabricated by wire arc additive manufacturing technology. The formation of a martensitic matrix with delta ferrite in the as-printed sample, islands of spherical chromium carbides embedded in a ferritic matrix in annealed sample, and intergranular chromium-rich carbides along the primary austenite grain boundaries in addition to intra-lath Fe-rich carbides in the quenching and tempering heat treated sample were detected. To characterize the corrosion performance of the fabricated samples, open circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy tests were performed on all samples in aerated 3.5 wt.% NaCl electrolyte at room temperature. The corrosion morphology of the as-printed sample was characterized by localized corrosion attacks adjacent to the delta ferrite phase, while severe pitting occurred in the annealed sample due to the high susceptibility of ferritic matrix-carbide interface to pitting. In contrast to the as-printed and annealed sample, the electrochemical performance of the quenched and subsequently tempered samples was found to be significantly improved, ascribed to elimination of the chromium depleted regions adjacent to the delta ferrite phase, and enhanced protectiveness of the passive film on the alloy's surface.

On the Post-Printing Heat Treatment of a Wire Arc Additively Manufactured ER70S Part.

Wire arc additive manufacturing (WAAM) is known to induce a considerable microstructural inhomogeneity and anisotropy in mechanical properties, which can potentially be minimized by adopting appropriate post-printing heat treatment. In this paper, the effects of two heat treatment cycles, including hardening and normalizing on the microstructure and mechanical properties of a WAAM-fabricated low-carbon low-alloy steel (ER70S-6) are studied. The microstructure in the melt pools of the as-printed sample was found to contain a low volume fraction of lamellar pearlite formed along the grain boundaries of polygonal ferrite as the predominant micro-constituents. The grain coarsening in the heat affected zone (HAZ) was also detected at the periphery of each melt pool boundary, leading to a noticeable microstructural inhomogeneity in the as-fabricated sample. In order to modify the nonuniformity of the microstructure, a normalizing treatment was employed to promote a homogenous microstructure with uniform grain size throughout the melt pools and HAZs. Differently, the hardening treatment contributed to the formation of two non-equilibrium micro-constituents, i.e., acicular ferrite and bainite, primarily adjacent to the lamellar pearlite phase. The results of microhardness testing revealed that the normalizing treatment slightly decreases the microhardness of the sample; however, the formation of non-equilibrium phases during hardening process significantly increased the microhardness of the component. Tensile testing of the as-printed part in the building and deposition directions revealed an anisotropic ductility. Although normalizing treatment did not contribute to the tensile strength improvement of the component, it suppressed the observed anisotropy in ductility. On the contrary, the hardening treatment raised the tensile strength, but further intensified the anisotropic behavior of the component.