Influence of the Polarity on the Cleanliness Level and the Inclusion Types in the ESR Process

This paper describes the effect of different electrical power modes on the cleanliness level and the inclusion types in the ESR material X38CrMoV5-l. Therefore selected ingots, which were remelted in an open laboratory ESR plant, were investigated with SEM (combined with EDX) and with a conventional light microscope according to ASTM E45, method D. For these investigations, ingots with two DC polarities (cathodic and anodic) and with low frequency (1 Hz) AC power supplies were remelted under (apart from the polarity) comparable remelting conditions. The results show, that the cleaning effect of both DC polarities is lower than the one with a low frequency AC operation, whereas the cathodic DC polarity results in better cleanliness levels than the anodic one. In addition the oxygen content of the ingots shows a strong increase when remelting with the anodic DC polarity. A strong relation between the type of the electric power supply and the inclusion types in the ESR ingots could be found. The most pronounced differences occur with the oxide-type inclusions. Thereby, the amount of the originally dominating (Al, Mg)-spinell-type-oxides is further increased when remelting with DC-, but rather eliminated in favor of Al₂O₃-type inclusions when DC+ polarity is used. Regarding the oxysulfide inclusions, the examined electrical set-ups lead to Al-oxysulfides as the main part, while Mg- and Ca-oxysulfides are decreased respectively eliminated. Furthermore, a significant reduction of sulfides, especially CaS was found. A comparison with industrial ESR ingots indicates that the results gained with the laboratory ESR plant are in good agreement with or even slightly better concerning the cleanliness than larger scale 50Hz-AC remelted ingots.