The Influence of Post-Build Microstructure on the Electrochemical Behavior of Additively-Manufactured 17-4PH Stainless Steel

The repetitive rapid solidification that occurs during additive manufacturing (AM) generates microstructures that deviate significantly from those observed in wrought materials with the same nominal composition.  The result is a heterogeneous microstructure with significant variations in the local composition, and phases that are improbable in traditional wrought materials.  Most AM studies focus on creating a reproducible build process, with little emphasis on the development of appropriate post-build heat treatments necessary to relieve residual stress and homogenize the microstructure.  However, consideration of the full life-cycle performance of AM parts is a critical element for part qualification and the influence of the microstructural inhomogeneities is a critical issue.  The microstructural inhomogeneities in AM materials promote variability in the mechanical performance, but the relationship between the post-build microstructure and the corrosion resistance is not as well understood.  Since variability in the corrosion resistance of any stainless steel can be a performance-limiting factor, this must be assessed.  SS17-4PH is an industrially important alloy for applications that require high-strength and good corrosion resistance.  This alloy also performs well during AM processing.  In wrought form, SS17-4PH is typically a two-phase structure consisting of delta-ferrite stringers in a martensite matrix that can be heat treated to achieve a specific strength, but the as-built AM microstructure is often a mixture of alpha-ferrite and austenite that does not readily transform to martensite.  The results of previous analyses indicated the pitting resistance of AM-produced SS17-4PH is slightly better than wrought SS17-4PH.  This could be due to the presence of nitrogen in the powder.  This presentation describes a series of experiments designed to determine whether the nitrogen has a beneficial influence on the electrochemical behavior of SS17-4PH.  The experimental design, the microstructures, and results for the wrought and AM-produced materials will be presented, compared, and discussed.