Additive Manufacturing of 17-4 PH Stainless Steel: Post Processing Heat Treatment to Achieve Uniform Reproducible...

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 precipitates upon aging. When fabricated via additive manufacturing (AM), specifically laser powder-bed fusion, 17-4 PH 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 studied in comparison to that of conventionally produced wrought 17-4 PH with the intention of creating a more uniform, fully martensitic microstructure. The recommended stress relief heat treatment currently employed in industry for post processing of AM 17-4 PH steel is found to have little effect on the as-built, dendritic 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 destroyed, resulting in a microstructure containing about 90 % martensite with 10 % retained austenite.