The influence of constraint rolls on temperature evolution and distribution in radial ring rolling

Lohmar, J.; Cleaver, C. J.; Allwood, J. M.

Producing nearer to net-shape hot rolled rings whilst meeting demanding product property requirements is a major industrial challenge, especially in high value materials such as nickel-based superalloys. Recent process innovations propose additional tooling to improve control over the geometric accuracy, but it is not known what influence this will have on the temperature distribution and product properties. To study this, a set of 24 thermally-coupled numerical simulations was conducted for a key material test case, Inconel 718, with a furnace temperature of 1030 °C. The thermal parameters used in the model were validated against an a newly-conducted full industrial trial, and both average surface temperature prediction and rolling tool temperature were found to be accurate to within ±10 °C. The set of simulations focused on the production of two target ring geometries; for each target ring, four different rolling scenarios were developed, covering a realistic range of machine operating parameters. All eight of these scenarios were simulated with three different tooling set-ups, using two, four and six constraint rolls. For a typical rolling scenario with two constraint rolls, the average surface temperature was found to drop by 79 °C after transfer to the machine, and then by a further 54 °C during rolling. Trebling the number of constraint rolls doubled the temperature drop to 113 °C. By comparison, a factor 2.7 increase in rolling time led to a factor 3.0 increase in temperature loss to 166 °C. Simulations for the second, slenderer, target geometry predicted comparable surface temperature drops, but increased temperature loss in the core of the ring. In Inconel 718 regions of the ring that drop below 900 °C experience strongly retarded dynamic recrystallization, rendering them prone to damage. Trebling the constraint rolls was found to increase the area vulnerable to damage by 0–20%, across the 8 scenarios. These predictions suggest that the influence of constraint rolls could to some extent be mitigated by reasonable reductions in rolling or transfer time. However, if used to produce slenderer, nearer to net-shape rings with larger overall reductions and longer process times then temperature and product property control will be difficult.