Characterization of super duplex stainless steel SAF2507 deposited by directed energy deposition

Abstract

Super duplex stainless steel SAF2507 is characterized by good mechanical properties and corrosion resistance. However, the combination of high strength and corrosion resistance requires the balanced ratio of austenite and ferrite in microstructure. At the same time, the presence of other secondary phases such as intermetallic phases (σ, χ), carbides (M7C3, M23C6) and nitrides (π, CrN, Cr2N) is strongly undesirable. Additive manufacturing of duplex stainless steel has not been described in detail yet and the existing studies deal with the duplex stainless steels deposited by powder bed methods primarily. This study investigates the properties of steel SAF2507 prepared by the directed energy deposition (DED) method in the as-built state and after post-manufacturing solution annealing at 1100 °C for 60 min followed by water quenching and compares them with the duplex stainless steel additively manufactured by powder methods and conventional ways of preparation. Microstructure, phase composition and mechanical properties of both materials were studied in detail. The material after DED contains of about 28 wt% austenite, which is much more compared to additively manufactured duplex stainless steels by powder bed. The as-built state contained allotriomorphic grain boundary austenite (GBA), Widmanstätten austenite (WA) and intragranular austenite (IGA). Coarsening of austenite occurred during solution annealing, while ferrite's grains size decreased. The solution annealed material achieves approximately the desired ratio of austenite-ferrite 50:50. Yield strength decreased slightly during solution annealing from 680 MPa to 540 MPa, whereas elongation and notch toughness increase. The ductile to brittle transition temperature (about −100 °C) did not change significantly during solution annealing.