Determination of recrystallisation phenomenon in type 316 stainless steel VIM-VAR ingots during cogging operations

This study is focused on understanding the effects of key process parameters on the onset of recrystallisation of coarse, elongated, and dendritic grains of the as-cast microstructure during ingot-to-billet conversion, consisting of successive cogging, upsetting, and reheating operations, in type 316 austenitic stainless steel. The microstructural characteristics of the initial as-cast ingot (e.g., size, orientation and morphology of grains), and the complex thermo-mechanical path have significant impact on recrystallisation behaviours, which make the existing material models inefficient of precise microstructure prediction. These investigations aim at exploring the recrystallisation mechanisms and their dependency on the forging process parameters. Cogging trails were conducted on VIM-VAR stainless steel ingots and microstructural observations were made to identify the recrystallisation mechanisms. Finite element simulations coupled with constitutive material models were developed in parallel, to enable microstructurally informed prediction of the cogging process. Thus, static and discontinuous dynamic recrystallisations occur in succession after reaching a threshold level of deformation during cogging. Grain growth occurred in the statically recrystallised grains during reheating. The importance of the reheating and the associated static recrystallisation on the efficiency of the billet conversion process in terms of grain refinement is thus highlighted.