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

• Published in: Additive manufacturing Vol. 25; pp. 187 - 195
• Main Authors: Alnajjar, Michella, Christien, Frédéric, Wolski, Krzysztof, Bosch, Cedric
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-01-2019; Elsevier
• Subjects: Engineering Sciences; Ferrite; Martensite; Phase transformation; Selective laser melting; Stainless steel; Phase transformation; Ferrite; Selective laser melting; Martensite; Stainless steel
• ISSN: 2214-8604; 2214-7810
• DOI: 10.1016/j.addma.2018.11.004

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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.