Papers by Author: Christian Redl

Abstract: During hot extrusion, tools experience cyclic thermo-mechanical loads that can lead to materials degradation and failure. For a process optimization and study of the occurring damage mechanisms, the finite element method (FEM) is an appropriate means. Local inelastic strains result from the interaction of the applied temperature and stress loading and can be computed by suitable inelastic constitutive equations. Stress amplitudes and dwell times during extrusion result in creepfatigue damage. A lifetime consumption model sums increments of a damage variable over time and defines materials failure as the accumulation of the resulting damage variable to a critical value. The predominant failure mechanism, i.e. creep or fatigue, can be found by the investigation of the damage rate over several cycles. A comparison of both a creep dominated (copper extrusion) and a fatigue controlled (aluminium extrusion) lifetime consumption in an extrusion die is shown with the hot work tool steel Böhler W300 ISOBLOC in comparison with W400 VMR (both ~ EN 1.2343).

Abstract: Residual stresses are of great importance during the entire production cycle of high-grade steels. The use of modern tools based on the finite element method is steadily increasing to optimize heat treatment processes. As for industrial purposes it is often not possible to measure the entire set of material data a sensitivity analysis shows the relative influence of material properties related to phase transformation on the residual stresses during hardening. Subsequently the application of the numerical heat treatment model is shown on two examples: The magnitudes of residual stresses during the quenching of a forged bar in different quenching media are compared. The paper concludes with a numerical simulation of the heat treatment of a die used for extrusion processes. Phase distribution and residual stresses after gas quenching of the tool are presented.