Approach for Qualifying Laser Powder Bed Fusion 316H in Section III, Division 5

Efforts to include laser powder bed fusion (LPBF) in Section III, Division 1 for use in light water nuclear reactors are well underway, however, there are several challenges with qualifying a new manufacturing technique for use in advanced nuclear reactors, where time dependent properties are critical. This work focuses on LPBF 316H as a demonstration case as to how additively manufactured material can be qualified for high temperature reactors, as well as a test case for demonstrating accelerated qualification techniques. Variations in materials properties have been observed in the LPBF 316H testing and efforts are underway to understand these variations through variables such as the manufacturing equipment, location of manufacturing, processing parameters, specimen geometry, and powder chemistry. Microstructure and defect studies have provided a technical basis for understanding process-microstructure-property correlations and developing methods and limitations for the qualification approach. In addition to manufacturing parameters, post-build heat treatments were found to have a significant effect on the material performance. Examination and down-selection of heat treatments (as-built, stress-relieved, solution annealed, and hot isostatic pressed) has been a critical goal of the preliminary materials testing program. LPBF 316H tensile results have shown that the material has a higher yield strength than wrought 316H, even after being aged up to 10,000 hours. The ultimate tensile strength may fall below the average wrought properties, though it is within the scatter of the wrought data. Higher temperature heat treatments increased the ductility but reduced the strength. Anisotropy (build direction vs. transverse direction) was also found to be reduced with the more aggressive heat treatments. The creep results have shown that LPBF 316H steel has similar rupture times, but lower rupture strains compared to the wrought materials, particularly in the cases of the lower temperature heat treatments. Fatigue results were comparable to conventional fusion-welded 316H properties, although creep-fatigue results generally show a significant decrease in cyclic life, beyond what was expected based on wrought creep-fatigue design models. In some mechanical properties, batch variation was observed to be more impactful than heat treatment. Additional testing is still needed to qualify the material and manufacturing method.