FEM-Simulation of Drawing Out in Open Die Forging Henrik Overstam and Magnus Jan

Department of Technology, Orebro University, Orebro/Sweden

Experiments and finite element simulations were pertormed in order to study how the bite ratio influences the closing of inner flaws such as voids and pipes in the open die forging process. Square rolled blooms of carbon steel with a transversal hole in the centre were forged with a constant height reduction but with different bites in order to study the closing of voids during the process. Corresponding finite element simulations were performed in full 3D with full thermo-mechanical coupling. Also the influence of the friction and of the temperature gradient in the workpiece were studied. The elimination of an artificial defect by forging was successfully simulated by the finite element method. It is established that the closure of voids is highly dependent on the bite ratio. The closure of voids under the edge of the tool is however not im- proved by a higher bite ratio. There is a slight tendency that the situation under the tool edge is worse when the bite ratio is increased.

Keywords: open die forging, upsetting, defects, porosity, voids, metal working, computer simulation, finite element method,  FEM.

Introduction

Open die forging is the oldest metal working process, but it is still important [1-3]. The aim of the forging is twofold; the product shall be given the desired geometrical form as well as optimum properties for the product. Open die forg- ing is the only process possible to use for heavy products with uneven cross sections. It is not realistic to design a rolling mill for finished products thicker than about 250 mm if a sound centre is needed. Forging is thus also used for heavy bars and plates. If lighter parts are produced in small numbers, the die cost for closed die forging makes open die forging competitive. In a rolling mill the roll diameter and the reduction give the length of contact between the roll and the workpiece. In a forging press the length of contact and the reduction are independent. This greater flexibility of open die forging compared to rolling makes it more suitable for working of material with inferior workability. Another difference between rolling and open die forging may be mentioned, rolling is a stationary process, all cross sections will be worked in the same way.

Open die forging

Open die forging is performed in presses or hammers. The hammers are energy-restricted and the presses force-re- stricted machines. The time of contact between the work- piece and the die is much shorter in a hammer compared to a press. The cooling down of the workpiece is thus lower in the hammer. This will be an advantage for forging materials with inferior ductility and a narrow temperature span for forging. For heavy parts, i.e. over 25 tons, presses are the only realistic alternative; heavy hammers would release en- ergies detrimental to the surrounding [2]. For a 1200 mm in- got a forging press of 20 MN is required, or a hammer with a weight of 37 tons [3].

The forging process must eliminate the voids in the mate- rial. Heavy draughting may be used, but this will elongate the inclusions and cause poor material properties in the transverse direction. Sulphides are most detrimental, equiv- alent to voids. The material properties in the longitudinal di-rection are improved by the draughting up to a saturation level, where all the voids are eliminated. The material prop- erties in the transversal direction will obtain a maximum and deteriorate with further increasing draughting. This was shown in early investigations by Korschan and Maurer [4] 1931 (3 tons ingots) and Coupette [5, 6] 1941 (12 tons in- gots). Coupette studied different types of ingots with differ- ent degrees of segregation. The material properties in the transverse direction were substantially deteriorated by heavy segregations. A reduction of the sulphur content im- proved the properties in the transverse direction, especially at larger draughting.