Multiscale characterisation study on the effect of heat treatment on the microstructure of additively manufactured 316L stainless steel

Abstract

Additively manufactured 316L components are known to exhibit complex three-dimensional microstructures on multiple length-scales. The effect of heat treatment on the stability of grains and dislocation cell structures within the microstructure has previously been investigated but there are appreciable disagreements surrounding the nature of the texture that is present in the specimens, and there is also limited published work on the compositional homogeneity of specimens, particularly on the atomic scale, after heat treatments. In this study, we investigate the effect of applying two different heat treatments to 316L components produced by laser powder bed fusion before characterising them using a combination of electron microscopy and atom probe tomography. The importance of using multiple characterisation techniques, which span from the nanometre to micrometre scale, in addition to carefully and accurately describing analysis methods when investigating the evolution of the microstructure of additively manufactured 316L steels is demonstrated. Our EBSD (Electron Back Scatter Diffraction) results show the presence of a strong texture in the build direction of the samples, and a reduction in morphological texture as a result of heat treatment. TEM (Transmission Electron Microscopy) results indicate a dissolution of the dislocation cell structure that forms during solidification. Atom probe tomography was used to investigate compositional homogeneity in the samples and indicated that there are regions enriched in Cr, Mn, Mo, and Ni in the As-Printed samples that are likely associated with the dislocation cell walls. The atom probe results also reveal the presence of impurities, such as Co, which were not detected in the feedstock powder.