This experiment proposes to further understanding of the relationship between microstructure and irradiation-assisted stress corrosion cracking susceptibility in austenitic stainless steels.
Previous work has demonstrated that additively-manufactured stainless steels are susceptible to IASCC. This experiment seeks to utilize the unique microstructures generated by the different additive manufacturing processes to allow for examination of crack initiation behavior in new microstructural environments. This analysis can provide insights into the cracking mechanism.
Samples will be generated using three different fabrication methods. One method will be conventional fabrication – the steel has been hot rolled, cold worked, and machined. The second method will be one of AM methods, binder jet printing and sintering using a powder-bed printing system (Invent+, ExOne). The third method will also be one of AM methods, selective laser sintering using a laser powder-bed fusion system (M290, EOS).
The samples to be tested will be in the form of SSRT samples, of a design provided by the University of Michigan. This sample design is compatible with both the accelerator stage fixtures and the CERT loading system at the University of Michigan. The specimens are to be irradiated at conditions intended to represent pressurized water reactor (PWR) neutron irradiation. The irradiation parameters are to be 5 dpa at 360°C and 2 MeV. After irradiation, the slow (10E-7) strain rate tests will be conducted, with half the irradiated samples tested in inert environments, and half the samples tested in simulated PWR conditions.
The post-test fracture surface examinations will be examined via light optical microscopy and scanning electron microscopy, with chemical composition data gathered by energy-dispersive x-ray spectroscopy. The crack lengths and orientations will be noted, as well as any microstructural features in the vicinity of the crack initiation (grain boundaries, inclusions, or other discontinuities). Metallography of the fractured samples will be conducted to examine in cross section the interaction of the cracks with the microstructure of the additively manufactured steel.
Overall these data will be used to draw conclusions about the interaction of the PWR water environment with specific microstructural features present in the additively manufactured steels. These data will then also be compared to existing literature data for conventionally-fabricated steels to draw more general conclusions about the IASCC mechanism.
Specifically, it will be expected that residual stresses, subgrain boundaries and structures, and chemical segregation due to the additive manufacturing process will result in altered crack initiation behavior. Distinguishing the microstructural features associated with crack initiation in the additively manufactured steel will allow for new insights into the IASCC mechanism.