A double ring is a prestressing tool made of two hardened steel rings that are assembled by press fitting. A double ring is about 40% stronger than a 1-ring system but is approx. half as strong as a stripwound container. The stiffness is comparable with the SC 200 container as the ring is made of tool steel. Specific calculations are required in each case.
The purpose of a double ring is the same as a stripwound container and can in general be applied for both cold, warm, and hot working applications. A double ring is cheaper than a stripwound container, but it is at the expense of reduced strength capability, and consequently a shorter service life of both the forming die and the double ring itself. Minimum diameter of a double ring can be approx. ø50 mm.