In the heat of the furnace

Hot forming is the controlled plastic deformation of heated metals into useful shapes. During heating, the metal oxidises and a layer of oxide – scale – can grow on the surface, before the material-deformation step. The oxide scale will influence the heat transfer and friction between the tool and the work-piece during hot-forming operations, modifying the process (in respect of both its rheology and tribology) and, therefore, the characteristics of forged component. Thus if the process is to be correctly set up and optimised, an estimate is needed of the growth and evolution of the scale. These phenomena cannot be neglected when the process is numerically simulated by FEM (finite element methods) – the difference between conditions at the tool-work-piece interface in the presence and in the absence of a scale layer needs to be explicitly recognised. In spite of the long history of investigation of oxide scale, there are still many aspects of this phenomenon that limit the use of relevant knowledge in simulating the forming process. In a physical experiment, we investigated the growth of scale, obtaining a simple heuristic model to include in numerical simulation of the forming process. There were two control variables, temperature and time. Thickness of oxide scale was the response variable. Then, having fitted the experimental data to the model, we used FEM simulation to study the effect on heat transfer and, therefore, on temperature distribution when different scale layers, as predicted by the experimental model, were present on the surface of a work-piece to be forged.