Effect of heat treatment on microstructural heterogeneity and mechanical properties of maraging steel fabricated by wire arc additive manufacturing using 4% nitrogen shielding gas

In this paper, a maraging steel was fabricated by wire arc additive manufacturing (WAAM) using Ar 94% + O2 2% + N2 4% ternary mixture. The three thin-walls were studied as as-deposited, direct ageing, solid-solution ageing specimens, respectively. The evolution of microstructure, strengthening mechanism of nano -precipitates and the mechanical properties were systematically revealed. The results show that the microstruc-ture of as-deposited sample is mainly composed of martensite, delta-ferrite and retained austenite. A small amount of martensite was reverted to austenite in aged specimens and delta-ferrite remained. A small number of epsilon-copper particles precipitated on martensite lath owing to interlayer transient ageing in the as-deposited sample, and then grow up and disperse in the ageing sample. The Cr2N formed by the direct reaction of N and Cr atoms in the molten pool is dissolved after solid-solution. The heterogeneity of microstructure and preferential growth of texture result in large anisotropy of mechanical properties in as-deposited and ageing samples. Solid-solution heat treatment not only eliminates this anisotropy, but also greatly improves the elongation, which is attrib-uted to the elimination of delta-ferrite in the microstructure. In addition, the twins of martensite lath formed during solution can enhance the strength of martensite but deteriorate its impact toughness to 42 J.