Swelling Resistance of Additively Manufactured Grade 91 Steel Produced with Integrated Thermal Processing

The objective of this work is to evaluate the effectiveness of integrated thermal processing on swelling resistance of Wire-Arc AM DED produced Grade 91 steel. We hypothesize the microstructure of the integrated thermally processed Grade 91 will result in less or equal swelling to traditional Grade 91 at high dpa, demonstrating the recovery of performance without post-build heat treatments. Operational conditions of fourth generation (Gen. IV) concept nuclear reactors will place significant demands on their structural materials and irradiation induced swelling is a concern for several proposed reactor types. Thus, any material solutions must be scalable, sustainable, and low cost with reliable and accelerated development. Nuclear materials development can be accelerated by innovative materials processing such as additive manufacturing (AM), combined with computational modeling and high-throughput characterization. Variable solidification parameters and temperature gradients inherent to AM as layers progressively build can change microstructures and impart significant residual stresses, distortion, weakening, or cracking. These variations are compounded in high temperature nuclear materials: specific microstructural features (precipitates, phases) which impart swelling resistance can degrade across an AM print volume. Ferritic/martensitic (FM) materials (e.g., Grade 91) possess sufficient Cr and C content that they respond to heat treatments – rapid cooling from the austenitization temperature results in a martensitic microstructure. These steels are nominally processed through subsequent treatments to obtain tempered martensite along with MX and M2X carbo-nitrides, and M23C6 carbide precipitates. AM deposition without additional heat treatments creates brittle untempered martensite which severely reduces ductility. Integrated thermal processing during the AM layer-by-layer build offers the possibility to produce a tempered structure without off-line processing. The proposing team seeks use, through the Nuclear Science User Facilities, of the Michigan Ion Beam Laboratory (MIBL) for dual ion irradiation of three AM process variants of Grade 91 steel with a traditionally prepared Grade 91 to 200 dpa at 460°C, of the Michigan Center for Materials Characterization (MC2) for TEM lamella preparation and characterization of dislocation loops, nano-oxides, precipitates, and cavities. There are 4 specimens irradiated in one dual ion irradiation experiment, and thus, in total, the proposed experiments will require an estimation of about 80 hours for dual ion irradiations, 40 hours for lamella preparation and 64 hours for post-irradiation examination with transmission electron microscopy. The outcome of this work will provide quantitative analysis of the irradiated microstructure including dislocation loops, cavities, and any secondary precipitate phases. The availability of this dataset will support ongoing development activities in determining large scale AM fabrication methods of FM steels for advanced reactor applications.