Introduction: Austenitic stainless steels, such as 316, are used in a wide variety of power generation applications due to a favorable combination of corrosion resistance, creep strength, and microstructural stability. Large scale additive manufacturing processes, such as wire arc directed energy deposition (WA-DED), are attractive for producing large components for power generation due to high deposition rates and a range of deposition modes depending on the gas metal arc welding (GMAW) heat source. Although there is great potential to rapidly manufacture new or replacement power generation components, there is very little known about the creep resistance of WA-DED 316 stainless steels. In this work, we aim to evaluate the creep performance of WA-DED using several welding feedstocks derived from 316 and compare the results to wrought properties.
Experimental Procedures: A Fronius TSPi400 GMAW heat source utilizing the cold metal transfer (CMT) process was used to create WA-DED builds using the commercially available austenitic stainless steel feedstocks 316L, 316LSi, 16-8-2, and 316H. Builds were solution annealed at 1040 °C for 1 hour followed by water quenching. Creep specimens were machined from the solution annealed builds and constant load creep tested at 650, 750, and 825 °C to failure. Creep testing parameters were designed to result in lives ranging from 100 - 1000 hours based on data for wrought 316H.
Results and Discussion: WA-DED samples of 316H exhibited creep lives equivalent to the mean performance of wrought 316H at 650, 750, and 825 °C. There was not a statistically significant difference in creep performance of WA-DED 316H between samples tested parallel or perpendicular to the build direction. WA-DED samples of 316L, 316LSi and 16-8-2 exhibited shorter creep lives than that of WA-DED or wrought 316H at 650 °C. Interestingly, 16-8-2 did showed much lower creep lives compared to 316H, although 16-8-2 was specifically designed for welding wrought alloys such as 316H. In fact, 316LSi exhibited longer creep lives than 16-8-2 suggesting that although this variant of 316L was not designed for high temperature service, increased solid solution strengthening from elevated silicon and nitrogen contents result in considerable increases in creep strength and thus longer creep lives.
Conclusions: In this work, we demonstrate that WA-DED samples made of commercially available feedstocks have creep performance comparable to wrought counterparts. WA-DED of 316H showed creep performance equivalent to wrought 316H at 650, 750, and 825 °C with negligible anisotropic with respect to the build direction. The unexpectedly superior performance of 316LSi compared to 16-8-2 and 316L suggest that further improvements in creep strength can be obtained with new alloys designed specifically for WA-DED that incorporate greater degrees of solid solution strengthening.