Wire-arc directed energy deposition super martensitic stainless steel with excellent strength and plasticity

Abstract

Super martensitic stainless steel (SMSS) is wildly used in hydroelectric, petrochemical, and nuclear power industries relying on high strength and sound weldability. While the low ductility and high strength make it difficult to be deformed and machined. The wire-arc directed energy deposition (DED) process allows a high deposition rate and can produce high-quality parts. In this study, the SMSS thin-wall parts were fabricated through wire-arc DED with the cold metal transfer technique. This research aims to investigate the microstructural evolution regulation and mechanical performance improvement mechanism of the additive manufacturing SMSS parts and achieve good comprehensive performance. The microstructure of the as-deposited wall consists of full lath martensite with columnar and fine equiaxed grains in periodic alternation layer by layer. By regulating the temperature between each layer, the overall wall's microstructure is kept consistent. The yield strength and ultimate tensile strength of the as-deposited wall are 1046 and 903 MPa with total elongation of 9 % and impact energy of 16 J. Thus, the toughness and plasticity needed to be improved by heat treatment. Based on the energy dispersive spectroscopy (EDS) technique and diffraction pattern analysis using transmission electron microscopy (TEM), the phases formed during the intercritical tempering process were identified as tempered martensite and reversed austenite. The enhancement of toughness is associated with austenitic transformation-induced plasticity. The amount of reversed austenite is 29.5 % at 600 °C and then disappeared at 700 °C. The best comprehensive mechanical properties were obtained at a temperature of 550 °C with an ultimate tensile stress of 927 MPa, yield stress of 850 MPa, impact energy of 33 J, and elongation of 18 %, comparable to the rolled parts.