Effect of bright annealing on stainless steel 304 formability in tube hydroforming

Effects of bright annealing (BA) on enhancing formability of stainless steel 304 tube in a tube hydroforming (THF) process were studied. The tube material was the metastable austenitic stainless steel 304 with an initial thickness of 0.5 mm and an outer diameter of 31.8 mm. Both FEA and experimental results showed that THF process of the investigated part alone failed to achieve desired tube expansion of the diameter of 50.8 mm without severe fracture. Thus, a heat treatment process, also known as bright annealing (BA), which caused little to none oxide on the surface of annealed tube, was considered. Initially, effects of different annealing parameters such as temperature and holding time on the material formability were investigated using tensile tests. Stress–strain responses of various conditions were compared. As a result, an annealing process consisted of heating at the temperature of 1,050 °C, holding for 30 min, and rapidly cooling by purging N2 gas was identified. This annealing should be applied intermediately after a pre-forming step. With the aid of the BA process, tube deformation was significantly increased and the required tube expansion could be therefore attained. In addition, strain-induced martensitic transformation occurred during the forming process was examined by X-ray diffraction (XRD) method. The amounts of martensite taking place in tubes (pre-forming, post-forming, and after annealing) were determined and discussed.