Comparison of Maraging Steel Micro- and Nanostructure Produced Conventionally and by Laser Additive Manufacturing

TLDR

Maraging steels are fabricated by additive manufacturing such as LMD and SLM, yet the impact of these processes on retained austenite, reversed austenite, and precipitate nanostructure remains largely unstudied. The study used microhardness, optical microscopy, EBSD, EDS, and APT to characterize the as‑produced microstructure and its evolution during ageing heat treatment. Retained austenite is present in as‑LMD and as‑SLM parts but not in conventionally produced material; as‑LMD parts show higher hardness (except near the surface) due to early precipitation from cyclic re‑heating, while in the peak‑aged state SLM and LMD hardness is slightly lower than conventional because of retained/reversed austenite formed during ageing.

Abstract

Maraging steels are used to produce tools by Additive Manufacturing (AM) methods such as Laser Metal Deposition (LMD) and Selective Laser Melting (SLM). Although it is well established that dense parts can be produced by AM, the influence of the AM process on the microstructure—in particular the content of retained and reversed austenite as well as the nanostructure, especially the precipitate density and chemistry, are not yet explored. Here, we study these features using microhardness measurements, Optical Microscopy, Electron Backscatter Diffraction (EBSD), Energy Dispersive Spectroscopy (EDS), and Atom Probe Tomography (APT) in the as-produced state and during ageing heat treatment. We find that due to microsegregation, retained austenite exists in the as-LMD- and as-SLM-produced states but not in the conventionally-produced material. The hardness in the as-LMD-produced state is higher than in the conventionally and SLM-produced materials, however, not in the uppermost layers. By APT, it is confirmed that this is due to early stages of precipitation induced by the cyclic re-heating upon further deposition—i.e., the intrinsic heat treatment associated with LMD. In the peak-aged state, which is reached after a similar time in all materials, the hardness of SLM- and LMD-produced material is slightly lower than in conventionally-produced material due to the presence of retained austenite and reversed austenite formed during ageing.