Effects of Post-processing Heat Treatment on the Microstructure of Additively Manufactured 17-4 PH Stainless Steel

17-4 precipitation hardenable (PH) stainless steel is a useful material when a combination of high strength and good corrosion resistance up to about 315 °C is required.  In the wrought form this steel has a fully martensitic structure that can be strengthened by precipitation of fine Cu-rich FCC phase upon aging.  When fabricated via additive manufacturing (AM), specifically laser powder-bed fusion, 17-4 PH steel exhibits a dendritic structure containing a substantial fraction of nearly 50 % of retained austenite along with BCC/martensite and fine niobium carbides preferentially aligned along interdendritic boundaries.  The effect of post-build thermal processing on the material microstructure is investigated with the intent of producing a more uniform microstructure, with more predictable properties.  Simple stress-relief annealing heat treatment has little effect on the as-built, dendritic microstructure.  Further, the solutionization heat treatment at 1050 °C used for wrought 17-4 product reduces the amount of retained austenite, but is unable to fully remove the microsegregation found in the as-built microstructure.  It is found that by implementing the recommended homogenization heat treatment regimen of Aerospace Materials Specification (AMS) 5355 for CB7Cu-1, a casting alloy analog to 17-4 PH, the dendritic solidification structure is eliminated including all reminant microsegregation, resulting in a microstructure containing about 90 % martensite with 10 % retained austenite.