OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 gies applied to the mold wall [35, 36]. Liquid iron begins to cool and shrink immediately after pouring. The density of the liquid increases and the specifi c volume decreases, which leads to shrinkage of the liquid. This shrinkage can be compensated for by risers. According to [39], in iron, solidifi cation then begins at the liquidus temperature (TL) with the formation of dendrites that grow inward from the walls of the cup until eutectic solidifi cation begins (zone 1 in fi gure 6). Dendritic shrinkage can continue even after solidifi cation has begun (TE_start), since the amount of eutectic formed is initially small (zone 2). As long as the supply channel is open and the permeability of the loose dendritic region is suffi ciently high, the shrinkage is compensated by the fl ow from the risers. After reaching maximum supercooling (TE_low), the rapid formation of eutectic shifts the emphasis of solidifi cation from the predominance of dendritic shrinkage (zones 1 and 2) to the predominance of graphitic expansion (zones 3 and 4). The expansion of graphite may or may not continue until the end of solidifi cation [39]. In zone 3, suffi cient expansion of the graphite compensates for the compression of the liquid and dendrites. In zone 4, when the amount of eutectic formed and therefore graphite decreases, there is a risk of micro-shrinkage (microporosity) as the expansion of graphite may become insuffi cient to compensate for the shrinkage. In principle, both LG and SG cast irons are close to eutectic in composition and should exhibit expansion during solidifi cation, hence should not be prone to cavity formation or shrinkage of porosity. Although this is true for gray cast iron, conventionally produced nodular iron is subject to shrinkage porosity. For nodular cast iron, the melting zone is much larger and its permeability is much less than that of fl ake graphite cast iron. This, according to [39], limits the fl ow from the riser and reduces the cooling rate. Due to the limited growth of graphite at the end of solidifi cation, austenite shrinkage predominates, which causes a decrease in the specifi c volume and leads to uncompensated shrinkage in the last solidifi cation zone. This eff ect and the signifi cant release of gas from the solidifying liquid lead to the formation of porosity. Experimental linear displacement analysis (LDA) and thermal analysis (TA) devices have been used by a number of researchers [7–21] to measure the amplitude of the expansion/contraction eff ects occurring during the solidifi cation of cast iron. An extensive literature review on various methods was provided in [35]. A setup for thermal analysis was previously presented by us in [7]. In addition to this, a stand was developed that includes two parallel molds for pouring specimens (mold sizes 200 mm and 30 mm), a cooling module (0.72 cm) and a deformation recording module. The high-speed interface simultaneously records temperature and linear displacement data. The results of preliminary experiments are shown in fi gure 7. The morphology of graphite has a marked infl uence on the initial expansion value: it increases from fl ake graphite (LG) through vermicular graphite (CG) to nodular graphite (NG), respectively. In the same way, sensitivity to shrinkage increases, and the connection between two parameters is obvious: initial expansion - level of shrinkage. These experiments also demonstrated the importance of accurately estimating contraction/expansion events and their relationship to cooling curve events, respectively. Several key parameters have been identifi ed that correlate with the specifi c behavior of our inoculants as it relates to graphite release and shrinkage sensitivity of gray cast iron. These were: depth of eutectic supercooling, recalescence and maximum recalescence rate, temperature of the end of solidifi cation, maximum initial expansion and the total integral from the fi rst derivative of the compression curve to the end of pre-pearlite compression. Subcooling at the end of solidifi cation relative to the metastable (carbide) Fig. 7. The results of the study of the infl uence of graphite morphology on the initial expansion (compression curve) and the tendency to shrinkage (Гф1 – lamellar, Гф5 – vermicular)
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