Size and Type of Inclusions in Fe–Cr–Co Heat–resistant Steel and Elevated-temperature Strength under the Effect of Electromagnetic Stirring

Abstract

We fabricated the Fe-18Cr-2Ni-1Mo-0.2C-0.2Mn-0.1Si heat-resistant steels containing 3 to 12% Co by casting in the presence of a rotary electromagnetic stirring (EMS). The effect of EMS on the Fe-Cr-Co steels was evaluated by the size and type of inclusions and the examination of mechanical properties. The morphology and composition of inclusions were observed, and the formation mechanism of the inclusions was analyzed through thermodynamic calculation using Thermo-Calc. The results showed that there were mainly two types of inclusions generated in the solidification process, including the Al 2 O 3 corundum and the MnO. Cr 2 O 3 spinel. The application of EMS during solidification has shown a significant effect on the exclusion of large inclusions. Both the number density and the size of inclusions decreased under the effect of EMS. The mechanical properties at elevated temperatures were investigated considering the effect of cobalt addition and EMS. The strength of the steel increased with increasing cobalt content. At 800°C, an increase in cobalt from 3 to 12% resulted in a substantial strength increase concomitant with an increase in elongation rate. EMS reduced the ultimate tensile strength but enhanced the ductility of the steels. These effects were discussed relating to an early suppression of the δ-ferrite formation and the exclusion of large inclusions.