Effect of heat treatment on microstructure and mechanical properties of low-cost high strength steel by plasma arc-based directed energy deposition

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A low-cost high strength steel thin-walled structure was manufactured by plasma arc-based directed energy deposition (PA-DED), and the heat treatment process had been investigated. Martensite, ferrite, and nano-precipitated phases (Ni 3 Ti and Ni 3 Mo) existed in the as-deposited (AD) sample. Differently, the microstructure of the solution + aging (SAT) samples was mainly lath martensite and had more nano-precipitated phases. Post-deposition heat treatments were conducted at solution temperatures of 900 °C, 1000 °C, 1100 °C, and 1150 °C for 1 h, followed by aging at 450 °C for 6 h. Solution treatment eliminated columnar grains and elemental segregation, forming equiaxed austenite that transformed into lath martensite upon cooling. Aging promoted uniform precipitation of intermetallic compounds (IMCs), significantly enhancing strength via the Orowan mechanism. Higher solution temperatures led to coarser martensite and reduced ductility, while lower temperatures (900–1000 °C) retained residual austenite and anisotropy. These findings demonstrated that SAT effectively homogenized microstructure, eliminated anisotropy, and primarily enhanced strength through precipitation strengthening.