Effect of Chromium on the Solidification Process and Microstructure of Vermicular Graphite Cast Iron

Abstract

The paper presents the results of studies of the effect of chromium concentration on the solidification process, microstructure and selected properties of cast iron with vermicular graphite. The vermicular graphite cast iron was obtained by an Inmold process. Studies covered the cast iron containing chromium in a concentration at which graphite is still able to preserve its vermicular form. The effect of chromium on the temperature of eutectic crystallization and on the temperature of the start and end of austenite transformation was discussed. The conditions under which, at a predetermined chromium concentration, the vermicular graphite cast iron of a pearlitic matrix is obtained were presented, and the limit concentration of chromium was calculated starting from which partial solidification of the cast iron in a metastable system takes place. The effect of chromium on the hardness of cast iron, microhardness of individual phases and surface fraction of carbides was disclosed.

Experimental Simulation of Directional Crystallization of SiMo Cast Iron Alloyed with Al and Cr

“…The presently applied intermediate Al and Cr additions into SiMo-type DCI (alloy 3: 1.48% Al–0.09% Cr; and alloy 2: 1.87Al–1.01% Cr), which were subjected to directional crystallization at a 9 mm/h retraction rate, showed the presence of a significant amount of coarse compact and vermicular graphite, i.e., like in other SiMo-type alloys with Al addition [ 19 , 20 , 24 ]. Increasing the retraction rate, as in the case of sample 2.1, from 9 mm/h to 30 mm/h caused significant refinement of graphite particles.…”

“…The most spectacular effect of the highest introduced alloying additions was a change in graphite particle shape, which was especially eye-catching in the case of the largest one, i.e., from a compacted type to a lath-like structure. This could only be attributed to the more than doubling of Al addition as Cr is tied up in pearlite formation, i.e., in Fe 3 C-type carbides [ 20 ].…”

“…% boosts precipitation of M 6 C carbides at ferrite grain boundaries (simultaneously, Si-stabilizing ferrite has to be added to counteract the development of pearlite) and allows the tensile strength of these alloys to rise from ~400 MPa to ~600 MPa, even as the elongation falls from ~8% to ~4% [1][2][3]. These relatively good plastic properties, being unusual for cast iron, are achieved by graphite spheroidization in the ferritic matrix caused by the addition of Mg [4][5][6]. As in the case of most metallic materials, their tensile strength lowers with the rise in temperature, i.e., in the case of SiMo51, its R m at RT equals ~610 MPa, but at 800 • C, it falls down to ~53 MPa, limiting its application potential.…”

“…This effect is only slightly alleviated at higher alloying additions [ 19 ]. The Cr diminishes eutectic temperature but strongly promotes perlite formation; therefore, it is generally advised to keep its concentration in DCI at a low level (<0.05%) [ 17 , 20 ].…”

“…This observation was supported later on in an investigation using SiMo45-6 cast into a wedge-shaped mold, which showed that the volume of carbides and pearlite precipitates is more closely related to segregations during solidification than to cooling rates at the eutectoid temperature [ 22 ]. The presence of Cr in cast iron generally increases the amount of pearlite and contributes to the formation of M 7 C 3 carbides [ 20 , 23 ], while additions of up to ~2.5% Al supports graphitization, simultaneously promoting ferrite phase [ 24 ]. Starting even from small Al additions, it forms oxides, which contribute to improving the inoculation of the iron melt with (MnX)S compound [ 25 ].…”