Ermittlung von Umformgrenzen beim Freiformschmieden

Determination of forming limits during open die forging The fundamentals of determining the forming limits during open die forging have been investigated in the research project “Vermeidung von Oberflächenfehlern beim Freiformschmieden” (“Prevention of surface defects during open die forging processes”) which was supported by the AiF (“Arbeitsgemeinschaft industrieller Forschungsvereinigungen”). The industrially produced material was analysed in the form of cast ingots of two high‐alloyed steels (1.2367 and 1.6957) by compression, tension and torsion tests. The tests showed that the sampling point has a small influence on the materials’ plasticity for the present dimensions of the ingots. Furthermore transformation and precipitation behaviour of the both steels have been determined and additional metallographic investigations have been made to find out reasons of variation of measurements for low forming temperatures. Forming limit diagrams are generated to predict the forming limits of open die forging processes. For this purpose, compression tests and numerical simulations of the compression tests and forging processes were made for crack‐critical temperatures. The points in time of crack initiation of the samples during the compression tests were determined by the acoustic emission analysis and show a high variation of measurements. Thus the forming limit diagrams also have high scatter, an unique forming limit cannot be determined. Instead of that a maximum allowed height‐reduction was determined and checked in open die forging tests in the laboratory under industrial conditions. The results show that the determined limits are very safe because no cracks were generated during the forging of both materials.