Fig.7. This casting was produced in a mold with a silica blend with 7.5% chromite.
խ֨是ў铸本 ֣ 有 铬铁矿ङ硅 砂混合物模ӀИф 生ङ。
of specialty aggregates to silica sand. Te addition of two types of zircon and chromite were studied. Te zircon sand utilized was a special domestic zircon sand which exhibited unique chemical properties not seen in other zircon sands. Te use of this special zircon sand emulated the thermal expansion effects of commercial engineered sand additives while improving the sands refractory value. High temperature physical properties of the various blends were evaluated and test step-cone castings were poured and ana- lyzed for most blends.
Discussion of Results Figure 3 shows the thermal expansion results for the base-
line silica sand sample. Te sample can be observed to have a steady expansion leading to the alpha-beta phase transition at 1,063F (573C). A peak expansion of 0.01228 in./in. was observed at this temperature. After this transition, a steady contraction is seen leading to the cristobalite phase transition at higher temperature, where a secondary expansion can be observed. Te beta quartz-beta cristobalite phase transition occurs at approximately 2,642F (1,450C). A graphical representation of the temperature pro-
file of a step-cone core at 5% solid fraction can be seen in Fig. 4. The temperature of the core surface at the thicker metal sections is already at 2,372-2,462F (1,300- 1,350C). At this temperature range, the baseline silica sand sample is contracting after the alpha-beta phase transition. However, the subsurface, is still at a tem- perature range of 752-1,112F (400-600C) where the silica sand is expanding leading to the alpha-beta phase transition. This combination of contracting sand on the surface with expanding sand directly beneath the surface creates tensile failures that fill with liquid metal to form the defect classified as veining. Figure 5 shows the expansion results for the silica with
zircon blends samples. Te peak expansion for silica with 10% zircon is similar to baseline silica sand. However, from 20% zircon onwards, a reduction in the alpha-beta phase transition peak expansion can be seen with silica with 40% zircon having the lowest peak of 0.005 in./in., which is
ङ国ф锆ਸ਼砂，੮现ӟЉգйҿѕ锆ਸ਼砂ङ独特化学性 砂३工Џ商Ї赶לਈ果效胀膨热ङ砂ਸ਼锆殊特य़这。ૅ 合混գЉثմы९ू。چ火৳砂硅֑改时գ并，剂加添 ，物合混ӣ部םث针并，ѳછд੧进性ً理物温高ङ物 浇注并ӣ析дડ验级步锥铸ў。
গ果讨论 ，ӱب观Օ。果ৈ胀膨热ङս样砂硅ӕׂдॐي ֣
Јچ温）) ,（ ֨ৄ最并，胀膨定६לਈս样 ӱ达ս样ӱب观Օ还Јچ温થ。ՊबŜ 实现ś 现ӟӱب观ਈ，եՊ转થ完੧进֨。қّ胀膨ङ OT OT 观并，Պबਸ਼ी方੧进Јچ温高更֨ѫեФ，ঢ়收定६ ѫЈچ温）℃ ℉（ 约֨。胀膨次и现ӟӱب ӟ现Ŝीਸ਼ӱŜ方ीਸ਼转Պ。 金Լ较。ॐ۱ ֣ײ֣٢ӣچ温ਯ轮级数ӣब֡ ℉（
ӱ达已چ温面੮ਯङהӣ部ً
Ŝ ś֨ѫս样砂硅ӕׂ，ӄ֠ਸچ温થ֨。）℃ ֨然依面੮Јҿ，是Ѹ。ঢ়收եՊब （
℉
膨ѫ然依砂硅ङӣ部这，ӄ֠ਸچ温ङ）℃ 胀，फਙś ŜबՊ。੮面Їङ收ঢ়砂连գ੮面Ј方ङ膨 ਉд成形并，来进ҭ填ً金ہ液得҅，়裂д成形砂胀 ়ঝ缺陷。 。果ৈ胀膨砂硅ङս样物合混砂ਸ਼锆有խдॐي ֣
。ѷঝ砂硅ӕׂЊ果ৈқّ胀膨硅ङ砂ਸ਼锆 խ有 ّ胀膨ՊबŜ Ѹ是，ђխ有 锆ਸ਼砂ङ硅砂开始，ś ّ胀膨ҿ，砂硅ङ砂ਸ਼锆 有խ৲，Ѻ降۱有就қ Ѻ砂硅ӕׂ较ਙ甚， қ竟达ӱ最Ѻङ OT OT ս样砂硅ӕׂ较，是势趋一Րङӱب观胀膨热物合混。
March 2017
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