Evaluation of an Additively Manufactured Ferritic-Martensitic Steel for Advanced Nuclear Applications

Abstract

A multidisciplinary investigation is presented in which the first known Additively Manufactured (AM) ferritic/martensitic (FM) steel alloys were evaluated for suitability as fast reactor structural components. As AM becomes more mainstream, it offers new possibilities in improving the design and cost of metal parts, especially those with weldability and workability limitations. However, questions remain about AM’s ability to reliably produce the types of high performance ferritic alloys that require carefully tailored microstructures. Laser-based AM produces heating patterns that interfere with the phase transformations from which wrought FM steels derive their ductility, high strength, and creep resistance. Additionally, study of irradiation effects on AM materials is immature. To address these questions, this dissertation presents an analysis of AM Grade 91 steel, an alloy with established pedigree in the nuclear and fossil fuel sectors, and whose ASME code case establishment was the first in a family of creep strength enhanced FM steels. Material from the first known successful AM build of Grade 91 steel was prepared, heat treated, analyzed using microstructural characterization techniques, subjected to a range of mechanical testing (to 600 °C), and irradiated up to 100 dpa with 5 MeV Fe2+ ions.

Among the most salient findings were that i) AM material that was subjected to a prescribed normalization heat treatment developed a uniform microstructure and martensite fraction similar to wrought material, ii) normalized and tempered AM material had a similar distribution of carbide precipitates, but finer grain structure than wrought material, iii) AM material was slightly harder and less ductile than wrought material at room temperature, but at 300 °C and 600 °C, their mechanical strength/ductility were virtually the same, iv) AM heat treated material directly built and tested without heat treatments had an unpredictable and heterogeneous microstructure, but that when tensile tested, demonstrated extremely high strength and unexpectedly high ductility, especially at high temperatures, and iv) AM material showed less radiation-induced hardening, due to its fine grain structure. Indications are that AM Grade 91 steel may well be suitable for advanced nuclear applications, and additional research leading to a path forward for certification should be pursued.