Synchrotron based investigation of anisotropy and microstructure of wire arc additive manufactured Grade 91 steel

Abstract

Additive manufacturing (AM) rapidly produces complex shapes crucial for energy technologies and engineering designs. In this work, the microstructure and mechanical properties of additive manufactured Grade 91 (modified 9Cr–1Mo) ferritic/martensitic (FM) steel were investigated. Computational thermodynamics, synchrotron X-ray diffraction, scanning electron microscopy, and microhardness testing were utilized to quantify microstructure (crystallographic phases, microstrain), lattice parameters, and mechanical properties as functions of the build directions and post-build heat-treatment. The as-built microstructures were found to contain a large fraction of microstrain due to two-dimensional defects, and a low-carbon martensite phase (maximum value 38.4 vol%), as quantified from the X-ray diffraction analysis. It was shown that a post-build heat-treatment can effectively remove this martensitic phase and promote the formation of desired microstructures with dispersed M23C6 carbides which was consistent with Thermo-Calc predictions. From the X-ray diffraction analysis, an ∼82.5% reduction in microstrain, a 61.5% increase in the coherent grain size and complete elimination of body-centered tetragonal (BCT) phase demonstrated that an appropriate heat-treatment of AM built FM steel yields microstructures and hardness comparable to conventionally processed steel. However, based on lattice parameter and hardness map statistics, heat-treatment on the as-manufactured steel did not eliminate anisotropy.