Effect of feed and roller contact start point on strain and residual stress distribution in dome forming of steel tube by spinning at an elevated temperature

Tube spinning, without mandrel, is a common process used for manufacturing pressure vessels, e. g. CNG (Compressed Natural Gas) capsules for automotive industry and fire extinguishers. The process is carried out at an elevated temperature for forming a dome on thick wall steel tube ends. Two of the most important control parameters in the process are the roller contact start point (RCSP) and spinning feed (pitch), both of them highly affecting the process time and deformation behavior of the tubes and therefore success and quality of the product. In this article, using a three-dimensional dynamic explicit finite element model, the effects of these parameters are investigated on circumferential, axial and radial (thickness) strains of the formed tube in three thickness layers of the tube wall. The model is also verified by experiment. While circumferential strain is shown to be independent of the feed, axial and thickness strains are highly affected by both the feed and roller contact start point. It is shown that when the roller contact start point distance from the free end of the tube increases, there is a risk of indentation instead of normal bending behavior. It is also shown that axial strain has an inverse relation with feed, i.e. decreasing the feed results in further elongation of the tube. On the other hand, thickness strain increases by increasing the feed, so bigger thicknesses are expected in domes manufactured by higher feeds. In addition, it is shown that increasing the feed results in a decrease of the equivalent strain. The amount of residual stress (regardless of the temperature change) increases with increasing feed and its distribution is more uniform for higher distances of the contact start point from the free end.