IMPROVEMENT OF ACCURACY IN DETERMINING FLOW-STRESS IN HOT UPSETTING TESTS

Knowledge of correct flow stress curves of steel at high temperatures is of essential importance for reliable plasto-mechanical simulations in materials processing and for an effective planning and designing of industrial hot forming schedules. Error considerations of flow curve determinations allow an estimation of the quality of flow curves. The experiments are performed on a computer controlled servo-hydraulic test maschine. Flow stress errors are calculated as a function of geometry deviations (due to thermal expansion, scaling, predeformation and inaccuracies in specimen geometry), temperature and strain rate deviations. The influence of friction and initial microstructure on flow curves have to be considered. Metal flow can be improved by using a suitable specimen geometry and lubricants. Thermal treatment before testing has to provide a microstructure, similar to the real structure of the material. Different initial microstructures (from varying thermal treatments) influence the overall magnitude of the flow stress curve. Using the flow stress data in a rolling simulation programme the effects on forming loads and torques are analysed.