Inclusion control in steel castings

“Non-metallic inclusions are mainly comprised of oxides, sulfides, and nitrides, and are formed in liquid steel during the melting and refining process, as a result of reoxidation, worn-out refractories, or entrained slag. The notch toughness of high strength steels is particularly susceptible to the type, number, size, and distribution of non-metallic inclusions. High manganese and aluminum austenitic steels, or Fe-Mn-Al steels, have gained much interest in the military and automotive sector because of their excellent combinations of high strength and toughness. However, these steels are subject to both oxide bifilms and aluminum nitride, AlN, inclusions which form during melting and casting. During the casting process, the gating system plays an important role in determining the casting quality and the overall level of inclusions and bifilm defects. Two solutions to control inclusions during mold filling include molten metal filtration and design of novel “naturally pressurized” gating systems that control metal flow into the casting cavity and reduce air entrainment. However, the use of filters and these naturally pressurized gating systems add an additional cost and generally reduce casting yield. In the current study, the effects of filtration and gating design on reduction of bifilms and inclusions in Fe-Mn-Al steel were determined using two novel mold designs. A parallel gating study involving an aluminum deoxidized, cast composition of SAE 8630 was also performed. The results of this study showed that ceramic foam filters were more effective at removing solid oxide bifilms and aluminum nitride inclusions from Fe-Mn-Al steel castings than the naturally pressurized gating systems. Future studies should be directed at developing naturally pressurized gating systems with filters”