Additive Manufacturing Potentials of High Performance Ferritic (HiperFer) Steels

Featured Application Empowering metal additive manufacturing by unveiling the economic potentials of novel, low-cost alloys with improved mechanical properties (e.g., in comparison to conventional AM alloys, such as AISI 316L) and capable of in-build heat-treatment. Improvement of the quality of AM-processed material via the implementation of microstructural mechanisms for actively counterbalancing AM-process-related manufacturing flaws (pores, cavities). In the present study, the first tailored steel based on HiperFer (high-performance ferrite) was developed specifically for the additive manufacturing process. This steel demonstrates its full performance potential when produced via additive manufacturing, e.g., through a high cooling rate, an in-build heat treatment, a tailored microstructure and counteracts potential process-induced defects (e.g. pores and cavities) via "active" crack-inhibiting mechanisms, such as thermomechanically induced precipitation of intermetallic (Fe,Cr,Si)(2)(W,Nb) Laves phase particles. Two governing mechanisms can be used to accomplish this: (I) "in-build heat treatment" by utilizing the "temper bead effect" during additive manufacturing and (II) "dynamic strengthening" under cyclic, plastic deformation at high temperature. To achieve this, the first HiperFer(AM) (additive manufacturing) model alloy with high precipitation kinetics was developed. Initial mechanical tests indicated great potential in terms of the tensile strength, elongation at rupture and minimum creep rate. During the thermomechanical loading, global sub-grain formation occurred in the HiperFer(AM), which refined the grain structure and allowed for higher plastic deformation, and consequently, increased the elongation at rupture. The additive manufacturing process also enabled the reduction of grain size to a region, which has not been accessible by conventional processing routes (casting, rolling, heat treatment) so far.