Elevated temperature tensile and creep performance of wire arc directed energy deposited austenitic stainless steels

Abstract

Wire arc directed energy deposition (WA-DED) has been emerging as a promising solution to enable the rapid manufacturing of new and replacement parts of complex and large structural components in power generation applications, reducing manufacturing costs derived from long lead times and equipment. However, there is very limited work available in the literature about the long-term stability of microstructures and mechanical properties produced by WA-DED and therefore, they are not well understood. In this work, we evaluate the influence of WA-DED on high temperature mechanical properties of commercially available austenitic stainless steels wire feedstocks needed to deploy AM for power generation applications. A study of the impact of thermal aging at 650 °C on the formation of embrittling phases in wire feedstocks of 316L, 316LSi, 316H, and 16-8-2 was undertaken to understand their tensile behavior at room and elevated temperatures. Results revealed that secondary phases formed at grain boundaries (likely σ, but also possibly η and Laves) caused insignificant changes in tensile performance in 316L, 316LSi, 316H after aging, while samples of 16-8-2 formed significant amounts of ferrite and/or martensite after aging, increasing tensile strength but reducing ductility when tested at room temperature. These findings highlight the need for developing of wire feedstock designs that promote stable compositions with respect to ferrite and/or martensite formation, in addition to embrittling phases, to guarantee microstructure and mechanical property stability for high temperature WA-DED components with long service lives. Additional work was performed to understand the creep behavior of WA-DED stainless steels for power generation applications under elevated temperature service conditions. Negligible influences of sample orientation or build strategy were observed on long- and short-term creep strength, ductility, or life at 650, 750, and 825°C in WA-DED 316H. Furthermore, the creep performance of WA-DED 316LSi and 316H were found to be similar to wrought 316H, while samples of 16-8-2 and 316L show lower creep lives.