Evidence of austenite by-passing in a stainless steel obtained from laser melting additive manufacturing

Abstract Microstructural characterization was carried out on AISI 17-4 PH stainless steel fabricated by selective laser melting (SLM) in an argon environment. Conventionally, this steel exhibits a martensitic structure with a small fraction of δ ferrite. However, the combined findings of x-ray diffraction and electron backscatter diffraction (EBSD) proved that SLM-ed 17-4 PH steel has a fully ferritic microstructure, more specifically  δ ferrite. The microstructure consists of coarse ferritic grains elongated along the build direction, with a pronounced solidification crystallographic texture. These results were associated to the high cooling and heating rates experienced throughout the SLM process that suppressed the austenite formation and produced a “by-passing” phenomenon of this phase during the numerous thermal cycles. Furthermore, the energy-dispersive X-ray spectroscopy (EDS) measurements revealed a uniform distribution of elements without any dendritic structure. The extremely high cooling kinetics induced a diffusionless solidification, resulting in a homogeneous elemental composition. It was also found that the ferritic SLM-ed material can be transformed to martensite again by re-austenitization at 1050 °C followed by quenching.