Part II – Microstructural and mechanical characterization of additively manufactured components of 18Ni300M steel with gas and water-atomized powders: a comparative study between aged PBF-LB and conventional material focused on the fatigue behavior

Maraging 300 (18Ni300M) steel is a ferrous alloy renowned for its good combination of mechanical strength and ductility, making it ideal for aerospace and defense applications. A previous study demonstrated that the atomization process has a significant influence on the monotonic mechanical properties and microstructure of as-built components produced by powder bed fusion with a laser beam (PBF-LB) technique. In Part II, the tensile and high-cycle fatigue behavior of 18Ni300M steel aged at 480 ºC for 3 h was investigated across two processing routes: conventional vacuum induction melting followed by vacuum arc remelting (VIM/VAR), and PBF-LB using gas-atomized (GA) and water-atomized (WA) powders. The research included tensile and rotary bending fatigue tests, transmission electron microscopy (TEM) for nanoparticle analysis, and electron backscatter diffraction (EBSD) for identifying secondary microcrack paths. The results showed that aging increased the mechanical strength of both GA and VIM/VAR samples, albeit with a reduction in ductility. In contrast, the WA condition exhibited compromised plasticity, as oxide inclusions acted as critical-size defects. The VIM/VAR specimens demonstrated the highest fatigue resistance, followed by the GA condition, which was limited by nano-oxide formation and process-induced defects. The WA specimens displayed the lowest fatigue performance due to a greater number of defects exceeding the critical defect size predicted by Murakami’s model. These findings highlight the influence of powder characteristics, processing routes, and microstructural features on the mechanical properties of an ultra-high-strength steel intended for structural applications.