Solidification pathway and ferrite-to-austenite massive transformation in austenitic stainless steels processed by laser powder bed fusion
Léo Monier, M. Véron, Jean‐Jacques Blandin, Guilhem Martin, Flore Villaret, Arthur Després
Abstract
Austenitic stainless steels fabricated by laser powder bed fusion notoriously exhibit a variability of as-built microstructure (grain size, grain boundary character, texture) which is problematic for nuclear applications. Several hypotheses have been proposed in the literature to explain this phenomenon. Here, we examine the hypothesis that this variability would result from a change in solidification mode, from the F/MA mode (solidification in ferrite followed by massive transformation in austenite) to the A mode (solidification in austenite). We investigate two 316 L steels with slight differences in compositions but large differences in as-built microstructures. The first steel shows features associated with the A solidification mode, with columnar grains, regular grain boundaries and partitioning in α-stabilizers in the intercellular regions. In the second steel, although the final microstructure is also fully austenitic, both characteristics of A and F/MA solidification mode are found at the melt pool scale. The characteristics of mode A solidification are identified preferentially near the melt pool boundaries, while features associated with the mode F/MA solidification appear more frequently in the melt pool interiors. The latter regions exhibit slight Ni partitioning (γ-stabilizer), which is considered a typical signature of mode F/MA solidification. By compiling data from the literature, we show that the two different types of microstructures can be discriminated based on the Cr eq vs. Ni eq welding diagram. For a high content of α -stabilizers, the microstructure will be typical of mode F/MA solidification, while for a high content of γ -stabilizer, it will exhibit the features associated with mode A.