Achieving high strength with low residual stress in WAAM SS316L using flow-forming and heat treatment

Preformed cylinder made of wire and arc additive manufacturing (WAAM) using 316 L stainless steel (SS316L) was used for a flow-forming operation to achieve a high-strength thin-walled cylinder. It was shown that the material can undergo large plastic deformation in the as-built condition without any heat treatment before the flow-forming operation. However, the flow-forming's high reduction in tube thickness and length increase generates large amounts of residual stresses that must be reduced or eliminated by appropriate heat treatment. In this work, different heat treatments on the final tube were investigated within the temperature range of 400–800 °C in air and in vacuum. The variation in the heat treatment temperatures was observed to directly influence the residual stress values from very high initially down to almost zero at the highest temperature. It was observed on the other hand, that up to ca. 20% strengthening could be gained by heat treatments up to 600 °C, while the residual stress decreased, in comparison to the post-deformation state, as in the case of anneal-hardening. It is only past this phase that the “classical” full stress relief, with corresponding strength reduction, was achieved at 800 °C. [Display omitted] •WAAM and bulk SS316L cylinder was prepared by flow-forming and sliced to rings.•Ultimate and residual stresses were calculated from the rings after heat treatment (HT).•HT for decreasing the residual stress while increasing the ultimate stress was found.•Microstructural examination revealed nano-twins as the strengthening mechanism.•Anneal-hardening is also suspected to be strengthening mechanism.