DAN GHEORGHE, M. SCHWARTZ, ION CIUCA, ROBERT CIOCOIU A COMPARATIVE MICROSTRUCTURE EVOLUTION OF AISI H21 AND INCONEL 718 IN CYCLIC HEATING The AISI H21 steel used for dies in copper hot extrusion softened only after several runs. To improve production parameters a new material was necessary for the die, and, as a first choice, Inconel 718 was preferred. Prior to adopting this material for new extrusion dies an experimental program with temperature exposure cycles was performed so as to study microstructure evolution within AISI H21 and Inconel 718. The samples were studied by optic and scanning electron microscopy (SEM) and were also submitted to X-ray diffraction (XRD) and microhardness tests. The results showed carbide coalescence, which led to softening, followed by a re-precipitation in the steel, while, in Inconel 718, the precipitation (in small amounts) of delta (ä) phase was observed.Using these results and combining them with practical experience with steel, we could predict that the Inconel 718 die performance is such that at least a double quantity of copper could be extruded per die.