New Experimental Technique for Nodularity and Mg Fading Control in Compacted Graphite Iron Production on Laboratory Scal
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TRODUCTION
ONE of the critical issues of CGI and SGI production is the control of nucleation and growth mechanisms of graphite.[1] The nucleation of graphite is commonly assumed heterogeneous in main graphite nucleation theories.[2] This concept assumes that graphite particles nucleate on a pre-existing inclusion in the liquid.[3–7] In the case of lamellar graphite iron (LGI), these inclusions are complex sulfides (Mn,X)S that at the same time have nucleated on complex oxides of Al, Si, Zr, Mg, and Ti.[3–5] On the other hand, graphite in SGI and CGI is observed to have similar nuclei, formed by complex Mg silicates (MgO.SiO2) which are nucleated on the external layer of MgS and CaS sulfides.[6] The subsequent growth of graphite nuclei is primarily affected by two factors: the presence of surface-active impurities in the melt, and the cooling rate during solidification of the alloy.[1] The
JUAN CARLOS HERNANDO, BJO¨RN DOMEIJ, DANIEL GONZA´LEZ, JOSE´ MANUEL AMIEVA, and ATTILA DIO´SZEGI are with the Department of Materials and Manufacturing, Jo¨nko¨ping University, Gjuterigatan 5, 551 11, Jo¨nko¨ping, Sweden. Contact e-mail: [email protected] Manuscript submitted March 28, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
effect of the latter is widely accepted; higher cooling rates promote the formation of SGI. The influence of impurities can be divided into two categories: (a) reactive impurities leading towards a SGI transition such as Mg, Ce, Ca, Y, and La, called compacting or spheroidizing elements; and (b) surface-active impurities favoring lamellar graphite formation, such as S, O, Al, Ti, As, Bi, Te, Pb, and Sb, called anti-compacting or anti-spheroidizing elements.[1] The presence of these impurities influences the predominant growth direction of graphite.[8,9] In the case of LGI, dominant growth occurs along the A-axis, while in SGI it happens along the C-axis. CGI is in an intermediate situation since dominant growth direction changes continuously between the A-axis and the C-axis.[8,9] The surface-active impurities are absorbed in the prismatic face of the hexagonal graphite lattice, creating a non-faceted interface, that requires low driving forces to grow, i.e., low undercooling, like in the case of LGI, while for SGI the faceted interface requires larger undercooling to grow.[2,4,7] Previous experiments show that a completely impurities-free melt would solidify as SGI, indicating that the preferential shape for graphite is spheroids in the total absence of active surface impurities.[9] However, the amount of impurities needed to promote the formation of LGI is extremely low,[2] being the presence
of O and S the most detrimental to that effect due to their strong tendency to reduce the surface energy.[1,8,9] The presence of dissolved oxygen in the melt has been shown to have a direct impact in the nodularity of the iron.[10–12] The formation of CGI and SGI, therefore, requires the reduction of the amount of dissolved O and S that are regularly present in the materials used as base alloys. The practi
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