Regular Article

Experimental and ab initio study of the influence of a compound modifier on carbidic ductile iron

School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, PR China

^* e-mail: hgfu@bjut.edu.cn

Received: 22 June 2018
Accepted: 6 November 2018

Abstract

To improve the morphology of carbides in carbidic ductile iron, a compound modifier consisting of 0.1% Nb + 0.1% Ti + 0.1 wt.% Y was added to the base ductile iron with chemical composition of 3.72% C, 2.77% Si, 0.51% Mn, 0.99% Cr and balance Fe (wt.%). The effect of this compound modifier on the microstructures of carbidic ductile iron was studied. Also, first-principles calculations were carried out to better understand the modification mechanisms. The results showed that the maximum diameter of spheroidal graphite nodules decreased from 58 to 34 µm after the addition of compound modifier, and continuous carbide networks changed into a broken network. The roundness of graphite nodules decreased slightly, and the percent nodularity of the graphite nodules and the number of carbides decreased by 3 and 1.8%, respectively. Compounds with higher melting point are formed thanks to the compound modifier which acts as heterogeneous core, and the remaining Ti and Nb elements can be selectively attracted by (010) surface of Fe₈Cr₄C₄. Furthermore, Cr elements can be easily replaced by Ti and Nb in the carbides to form more stable Fe₈Cr₃TiC₄ and Fe₈Cr₃NbC₄, which can prevent the continuing growth of carbide on the Fe₈Cr₄C₄ (010) crystal surface and break the continuous network M₃C. Y atoms cannot be directly adsorbed onto Fe₈Cr₄C₄ (010) surfaces. They combine first with oxygen in the ductile iron to form Y₂O₃. The work of adhesion of the interface between a Y₂O₃ (100) and a Fe₈Cr₄C₃ (010) is predicted to be 0.3 J/m². The addition of Y element is found to have a positive effect on breaking up the continuity of the carbide network.