Characterization of Ferritic Ductile Cast Irons Modified by W and Nb Additions

The aim of this study was to develop ferritic ductile cast irons as alternative exhaust manifold materials to commercial SiMo alloy using elements like W and Nb in place of Mo. In the compositional design, Thermo-Calc software was used to follow all the phase transformations at equilibrium and obtained results were compared with the commercial SiMo alloy. Calculations revealed several alloy carbides such as M6C and MC depending on the presence of Mo, W and Nb within the composition. The calculated critical transformation temperatures shifted to higher ones by the usage of W and Nb in place of Mo and higher Si. The designed compositions were then cast and their microstructural features were characterized by several metallurgical analyses. The examinations on the cast alloys showed that; (i) microstructural features i.e. ferritic matrix, graphite, carbides were present within the solidified structure as predicted, (ii) graphite morphology varied according to the alloying element and Nb strongly deteriorated graphite morphology by changing it from spheroidal to irregular spheroidal and vermicular