Wire Arc Additive Manufacturing of Nuclear Steels

Wire arc additive manufacturing (WAAM) presents an opportunity to tailor the microstructures of engineering alloys to obtain desired mechanical properties under elevated temperatures and irradiation. In particular, the composition can be tailored through the optimization of the processing atmosphere, also known as the shielding or cover gas. Traditionally, a cover gas is a high purity inert gas such as Ar. The composition of the cover gas influences on the chemical reactions that occur in the molten pool during solidification.

This work explores optimizing the composition of the processing atmosphere during WAAM to increase MX carbonitride precipitation in the Grade 91 steel during solidification and subsequent heat treatments. This is completed through optimization of the carbon and nitrogen composition in the cover gas, which allows for superior control of the carbon and nitrogen content in the resulting alloy. Due to sharp thermal gradients, it is possible to trap solutes such as carbon and nitrogen during the rapid solidification encountered during WAAM and precipitate a fine distribution of carbonitrides. Following WAAM, heat treatments are also explored to further optimize the microstructure.