Formation of Austenite in Additively Manufactured and Post-Processed Duplex Stainless Steel Alloys

The additive manufacturing of duplex stainless steels has been limited by the inability to maintain a balanced ferrite/austenite microstructure. In order to investigate the impact of the complex thermal histories inherent to the additive manufacturing process on austenite fractions and morphology, a laser-based directed energy deposition process was used to fabricate lean (UNS S32101), standard (UNS S32205), and super (UNS S32507) duplex structures. In these structures, the austenite phase fractions ranged from 16.1 ± 1.1 pct in the lean, to 38.5 ± 1.6 pct in the standard, and 58.3 ± 0.1 pct in the super duplex stainless steel grades. While the overall austenite levels were comparable to those found in wrought alloys, the austenite fractions increased with build height as preheating from previously deposited material promoted the ferrite to austenite transformation. Of the austenite morphologies observed in each of the duplex stainless steel grades, intragranular austenite was dominant, comprising between 55 and 76 pct of the austenite present within each build. The intragranular austenite formed during reheating and its formation was enhanced by the presence of submicron inclusions which originated from the powder feedstock and served as heterogenous nucleation sites. After post-process hot isostatic pressing heat treatment, the austenite morphology became more similar in appearance to that observed in the wrought condition. The overall austenite fractions in the post-processed lean (28.2 ± 0.7 pct), standard (57.6 ± 0.2 pct), and super (66.5 ± 0.3 pct) duplex grades increased over their respective as-deposited conditions and became more uniform with changes in build height.