On the role of cellular microstructure in austenite reversion in selective laser melted maraging steel

Abstract

Cellular microstructure is a unique feature in alloys fabricated by selective laser melting (SLM). Abundant efforts have been made to reveal the formation mechanism of cellular microstructures and its influences on mechanical performances, while its potential role in microstructure architecting during post-heat treatment is rarely explored. In this work, we investigated the features of cellular microstructures in an SLM-fabricated 18Ni(300) steel and revealed how this microstructure influences austenite reversion upon aging. Segregation of Ti and Mo is experimentally detected at cell boundaries. It is interestingly found that a distinctive reverted austenite network forms rapidly along cell boundaries during aging, whereas much less austenite is found in conventionally treated 18Ni(300) steels. The rapid austenite reversion in SLM-fabricated material proceeds mainly via the growth of retained austenite on cell boundaries while the nucleation and growth of new austenite grains is negligible. Phase-field simulations suggest austenite grows in a fast, partitionless manner along cell boundaries where the chemical driving force for austenite reversion is substantially enhanced by Ti and Mo segregations, but in a sluggish, partitioning manner towards cell interiors. Contrary to conventional views that austenite fraction should be confined to avoid strength reduction, current SLM-fabricated 18Ni(300) steel containing ∼13% cellular austenite is found to have higher tensile strength compared to its counterparts with negligible austenite. The design of austenite also shows its potential to enhance fracture toughness. The current study demonstrates that cellular structures could substantially alter austenite reversion behavior, providing a new route for microstructure architecting in additively manufactured steels.