Limits to making L-shape ring profiles without ring growth

A novel ring rolling process is proposed to flexibly produce shaped rings without circumferential ring growth, potentially saving material and energy as well as reducing upstream and downstream processing requirements. In this paper, six circumferential constraint rolls are used constrain circumferential growth and enable L-shape profiles to be developed through axial material flow, via a compressive hoop stress. Process limits were studied in 22 experiments on lead rings and a set of axisymmetric thermally coupled simulations on a high value engineering material, Inconel 718. Profile depths of 75 % of the original wall thickness were achieved in a range of rings and operating conditions, and material savings of up to 60 % demonstrated over rectilinear rolling. There was no evidence of cracking or void formation, unlike processes where under-deformed regions are stretched circumferentially and are vulnerable to cracking. In several cases a non-circular ring shape developed, limiting the achievable profile depth especially for small wall thicknesses, large reductions in thickness per pass, or large profile heights. The constraint roll forces when this 'collapse' occurs was studied and an upper bound predicted by a plastic hinge model. The thermal simulations showed that in all except 4 cases reheats would be required to keep within safe temperature bounds, thus suggesting an optimum reduction in thickness per pass to avoid both excessive cooling and collapse.