Solidification of superfine graphite iron revealed by interrupted solidification experiments

Abstract

The tensile strength of near-eutectic grey iron can be increased from 230-300 to 300-345 MPa, without a significant increase in hardness, through 0.3-0.4%Ti addition to low sulphur (<0.01%S) iron. This is due to the combination of higher primary austenite/eutectic ratio and the precipitation of superfineinterdendritic-graphite (SIG), characterised by a fine (10-20 μm) and highly branched fibrous structure. To reveal the influence of the %Ti on graphite shape evolution during solidification and its relationship to the solid fraction, quenching experiments at successive solidification stages were carried out on hypoeutectic alloys with 0.18% and 0.32%Ti. The graphite shape factors were measured, and their evolution as a function of the titanium content and the solid fraction was analysed. SEM was used to evaluate the change in graphite shape during early solidification, as well as its nucleation and growth. The correlation between the oxygen in the melt and SIG formation was also explored. It was concluded that nucleation of graphite in SIG irons occurs on carbon rich regions at the austenite/liquid interface and, sometimes, on titanium carbides. Solidification velocity-undercooling curves were used to explain the transition from lamellar (type-A), to interdendritic (type-D), to SIG, and then to coral graphite.

Niobium addition effect in molds at last cooling step on EN-GJL250 gray cast iron: Microstructural changes and electrochemical behavior

“…The source of these additions is ferroalloys. The originality of this work concerns the niobium addition in the last step of cooling of the melt cast iron in comparison with the traditional method where the addition of elements is made in the melt [5,14] . slopes plots) in a 3.5 M NaCl solution.…”

“…Since the carbide formation energy is much smaller than that of the sulfides, and due to its high affinity to carbon, niobium may first form carbides. This leads to a local impoverishment of carbon in the liquid, which may favor the formation of graphite type D [5] . Effectively, given the high affinity of niobium for carbon, Nb 2 C and NbC carbides are formed in the skin near the outer walls, revealed by XRD analysis point by point with a step of 2 mm (Fig.…”

“…This arrangement of graphite is of type (B). According to some authors [5,14] , in cast iron whose chemical composition is near to eutectic, this type of graphite is formed with low supercooling and growth rate in large eutectic cells. At 1 wt.% of niobium (Fig.…”

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I t is well established that the variety of applications of cast iron is attributed to the presence of chemical elements in the solid solution as interstitial or substitutional form, such as so-called addition elements Ni, Cu, Ti, Cr, V and Nb [1][2][3][4] , and inoculation elements Al, Si, Ca, Sr, Mn, Zr, P, Sn and Ce [5][6][7] .In the foundry, additions are done by controlling the

Abstract:The purpose of this study was to determine the impact of niobium addition as an inoculation element on the microstructure and electrochemical properties of EN-FGL250 gray cast iron. Niobium additions are in a powder form and have a 0.5 mm particle size at dfferent proportions of 1wt.% and 3wt.%.

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“…I t is well established that the variety of applications of cast iron is attributed to the presence of chemical elements in the solid solution as interstitial or substitutional form, such as so-called addition elements Ni, Cu, Ti, Cr, V and Nb [1][2][3][4] , and inoculation elements Al, Si, Ca, Sr, Mn, Zr, P, Sn and Ce [5][6][7] .…”