Fig. 6-7. The core dimensions at the adjacent liquid’s liquidus temperature, left, and the core dimensions at the adjacent liquid’s solidus temperature.
core surface undergoes the cristobalite phase transition leading to a large secondary expansion. Silica with 20% zircon displays
a trend similar to the 10% zircon sample with slightly lower expan- sion at the thinner sections of the casting. This is due to the slightly lower peak expansion seen in the 20% zircon sample at the alpha-beta phase transition. Better dimensional accuracy can be seen in the thicker sections of the casting. Silica with 30% zircon displays
lower expansion of the casting through the 1.5–3.5-in. (3.81 - 8.89 cm) core step when compared to baseline silica sand. Silica with 40% zircon displays
similar results in the 4- and 3.5-in. (8.89 cm) core section as 30% zircon. However, in the 3-in. (7.62 cm) and 2.5-in. (6.35 cm) section, the cast- ing contracts more in the 40% zircon sample due to the sample having the lowest alpha-beta peak expansion, and subsequently, a lower contraction after the alpha-beta transition.
Prediction of Final Casting Dimensions
The high temperature properties of molding materials have a large effect on the quality of the finished casting. It also has been shown that the thermal expansion of molding aggregates is one such property that
needs to be addressed. This can cause changes in the dimensions of the cores and molds and thus finished castings. Te versatility of the casting process is
demonstrated by the ability of the metal in its liquid state to conform to the vol- ume of its container. By understanding the heat transfer of the molding aggre- gate and its effect on the temperature of the cores and mold, we can estimate the dimensional changes that take place dur- ing the casting process. Te temperature of the core already
increased above 2,372F (1,300C) on the surfaces of the 1–2.5 in. step dimen- sions. Te temperature of the adjacent metal is above the liquidus temperature of the steel used in the experiment. Te thermal expansion of the sand increased the dimension of the core to 0.005 in. at the 2.5 in. step while the metal is still in its liquid state.
Once the liquid metal solidifies, it
contracts in its solid state according to the coefficient of thermal expansion (CTE) of the respective metal. Tis solid state contraction for the ASTM A128 WCB steel was measured at 7.229 X 10-6
in/in. F. Te austenite
transformation is visible at around 1,250F (676C). Te dimensions of the core at each diameter step were determined using the simulation core displacement results at the liquidus temperature of the adjacent metal (Fig. 6-7). Tis
event occurred at multiple time steps during the cooling of the casting relative to the core and casting sec- tion size. Tis was the first attempt at determining the critical tempera- ture (temperature at which solid state contraction following the published CTE occurs).
Te published shrink rule of 0.250
in./ft. of dimension fails to accurately predict the final casting dimension. Te predicted dimension of +0.008 in. at the 2.5-in. step shows a similar trend to the actual casting dimen- sions but underestimates the effect of the expansion of the core. As the casting cools past the liqui- dus temperature, the core temperature continues to increase. Tis increase of core temperature continues to affect the dimensions of the expand- ing sand, resulting in increases in the core’s diameter. Te accurate predic- tion of final casting dimension must determine at which point the metal obtains its final form and at that point contracts as is predicted by the CTE of the solid metal. By moving the core measurement further down the cooling of the metal to the solidus temperature, the predicted dimen- sions of 0.025 in. at the 2.5 in. step of the internal sections of the casting closely follow the actual measure- ments obtained by the CMM. Tis leads us to believe the final
casting dimensions depend on the core July 2016 MODERN CASTING | 39
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