A new pore closure concept for the manufacturing of heavy rings

In order to avoid pores in heavy rings, a new forming concept is presented. The proposed manufacturing route is based on an analysis regarding the current manufacturing of a ring associated with serious pore closure problems. It is concluded that the present method results in small strains close to the outer surface of the ring. Thus, the likelihood for remaining pores in this region is considered to be high. This apprehension was confirmed both by ultrasonic testing and metallographic analysis. The forming route starts with heavy upsetting of a cylindrical billet followed by piercing in a closed die. The penetrating punch is furnished with an upper flat part that is used for upsetting at the end of the process, forcing the material to fill the cavity between the die and the upper tool. The remaining bottom disk is then sheared off in a separate tool. The new concept starts with a lighter upsetting, resulting in a higher “pan cake” of smaller diameter. This workpiece is then moved to a closed die of the same diameter as that of the material. The diameter of the piercing punch is not changed. Because of the small die diameter, the workpiece is, after piercing, considerably higher than in today’s production. The so-obtained workpiece is placed into a die equal to the original one with the exception of a bottom hole. A new mandrel, with a flat upper part and a lower part meant for shearing off the thin bottom disk, will be inserted to fit the recently generated cylindrical hole. At the beginning of the operation, the bottom disk is cut away as the punch moves through the bottom hole and after that, during the same stroke, the high ring will be heavily upset to its final geometry. During the upsetting the ring is supported by the mandrel. If the ring is too high during upsetting, there is a risk for the material to loose contact with the punch, which might result in instability and a fold. Applying the new concept, the strains close to the envelope surface became approximately 160% larger. The present analysis is carried out with an FE-code Q-form making it possible to trace different material elements of the initial billet throughout the different manufacturing steps. The results are presented in form of hydrostatic pressure as a function of the effective strain. Theoretically determined workpiece shapes, based on the conditions of today’s production are in good agreement with those obtained from full-scale experiments. The present work also includes comparisons with another analysis of similar character, published by the authors. In that work, the manufacturing of two other types of rings were analysed, one with pore closure problems and one without.