Prediction of ductile fracture in skew rolling processes

Abstract

This paper begins with a review of the literature regarding the modelling of skew rolling processes, with a special focus on the problem of material fracture. Numerical modelling is performed to compare stresses and strains in the cross sections of bars produced by skew rolling with the use of two and three rolls. It has been found that the two-roll skew-rolled bars are more prone to the occurrence of fracture in their axial zone. It is proposed that cracking in skew rolling processes be predicted with the use of classical fracture criteria. The accuracy of crack prediction by these criteria depends on the correct determination of the critical damage. The critical damage in skew rolling processes can be determined by two tests: rotary compression of a tapered specimen (when a process is performed with two tools) or rotary compression of a disc-shaped specimen in a cavity between the tools (when a process is performed with three tools). Six fracture criteria and a new test of rotary compression of a tapered specimen are employed to investigate fracture in a two-roll skew rolling process for producing cylindrical bars made of 100Cr6 grade steel. It has been found that stress-based criteria are more suitable for modelling fracture due to their lower sensitivity to temperature variations in the axial zone of rolled bars.