Stress Relief in WAAM SS316L Flow-Formed Tube

The development of a preformed cylinder made by wire and arc additively manufacturing (WAAM) from 316L stainless steel (SS316L) and extended to a thin tube via flow-forming operation was introduced in previous work. It was shown that the material can tolerate high plastic deformation in the as-received condition and did not require any heat treatment (HT) 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 might affect the final product. For most engineering applications these residual stresses must be reduced or eliminated by appropriate HT. In this work, different HT on the final tube were investigated. The stress-relief heat treatments were conducted in air and vacuum environments, within the temperature range of 400–800 °C. The representative test samples were rings made of the above flow-formed tube. In an attempt to minimize the variation in the mechanical properties of the produced tube, a typical high reduction flow-formed tube was sliced into rings. Part of the rings were sectioned to evaluate the residual stresses left after each HT. Other rings were tested via the ring expansion technique to evaluate the impact of each HT on the ultimate strength of the material. It is shown that the variation in the HT temperatures directly influenced the residual stresses, the thermal treatments reduced the material’s ultimate strength by about 20% with respect to the post deformation state. It was shown that even further strengthening can be gained by the HT up to 600 °C while the residual stress decreases. Full stress relief is achieved in 800 °C with some strength reduction.