Microstructure Development in Duplex Stainless Steels from Additive Manufacturing with Coaxial Directed Energy Deposition and Heat Treatment

Abstract Duplex stainless steels are widely used in offshore oil and gas, maritime, process, and nuclear industries as they deliver high corrosion resistance, toughness, and ductility, but they are sensitive to formation of brittle microstructures during additive manufacturing. This study has explored the microstructural development during heat treatment for three duplex stainless steel grades: 2205, 2209, and 2509, built by wire-fed coaxial directed energy deposition. The as-built and heat-treated microstructures (solution annealed at $$1020 \, {\text{to}}\,1170~^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>1020</mml:mn> <mml:mspace/> <mml:mtext>to</mml:mtext> <mml:mspace/> <mml:mn>1170</mml:mn> <mml:mmultiscripts> <mml:mspace/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:mrow> </mml:math> C) have been studied and compared. The 2209 and 2509 grades produced more homogeneous microstructures with higher austenite fractions in the as-built condition than 2205. To avoid sigma phase precipitation, 2209 and 2509 had to be solution annealed at higher temperatures than conventional duplex and super duplex. The experimental findings were compared with volume fraction estimates from thermodynamic calculations. For 2209, both as-built and heat-treated materials were evaluated through hardness and Charpy impact toughness testing, revealing significantly improved toughness after heat treatment. Optimal heat treatments produced limited grain growth and fine-grained slightly spheroidized Widmanstätten austenite morphology without any detrimental phases. The presented results offer valuable insights for optimizing microstructure through heat treatment of additively manufactured duplex stainless steel.