Processing of High Interstitial Austenitic Steel with Powder Bed Fusion‐Laser Beam/Metal: Evolution of Chemical Inhomogeneity and Microstructural Features during Postprocessing

Abstract

Powder bed fusion‐laser beam/metal (PBF‐LB/M) additive manufacturing provides a high potential to overcome the poor machinability of nickel‐free high interstitial alloy austenitic (HIA) steels. Therefore, this study focuses on the PBF‐LB/M processability of HIA X40MnCrMoN21‐18‐2 and the effect of postprocessing on microstructure and chemical homogeneity. Samples are fabricated on a laboratory and industrial PBF‐LB/M machine and subsequently postprocessed by conventional solution annealing or hot isostatic pressing (HIP). The influence of the processing steps on the microstructure and on the chemical composition is evaluated by scanning electron microscopy, X‐ray diffraction, transmission electron microscopy, atom probe tomography, and electron backscatter diffraction. The commercially available HIA powder exerts good processability, both by optimized and predefined PBF‐LB/M parameters. Loss of Mn and N is detected after PBF‐LB/M processing. Chemical homogenization but no further change in composition occurs during postprocessing. The as‐built microstructure shows segregation of elements (N, Mo, Cr, Mn) in intercellular spaces. A thermodynamic calculation confirms that N approaches a para‐equilibrium state in the PBF‐LB/M as‐built condition, while C does not. Porosity can be reduced by thermomechanical posttreatment with HIP. At the same time, HIP partially recrystallizes the microstructure, while (Mn + Cr)2SiO4 type oxides delay recovery and recrystallization of the microstructure.