lower than baseline silica by 56%. Another trend that can be observed from the thermal expansion of the blends is that the cristobalite phase transition induced at a lower temperature when compared to the baseline silica sand sample. At the 2,372-2,462F (1,300-1,350C) core surface temperature, the silica and zircon blend samples are already going through a secondary expansion. Tough a steep con- traction can be seen after the alpha-beta phase transition at 1,063F (573C), the large secondary expansion at a lower temperature negates the strain on the surface of the core at higher temperatures as both the surface and sub-surface of the core are expanding, thereby reducing the network of cracks on the surface of the core. Silica with 7.5% chromite has an expansion profile similar
to the zircon blends, with slightly higher temperature of cristobalite transition. Te secondary expansion seen after the cristobalite transition is also lower. Silica with 10% chromite shows sudden steep contraction at 2,012F (1,100C) while silica with 30% chromite shows a small secondary expan- sion at 1,652F (900C) before a steep contraction at the same temperature. Tis steep contraction for the 10% and 30% chromite samples and the relatively smaller secondary expan- sion after the cristobalite transition may lead to the sand grains fusing to each other in steel castings.
Surface Viscosity and Specific Heat Capacity Results
Te sintering temperature and the peak viscosity at sinter- ing temperature for each sample are shown in Table 2, along with the associated specific heat capacity at 2,192F (1,200C). Baseline silica has a sinter temperature of 2,619.3F (1,437.4C) with a peak viscosity of 5.030 x 108 Pa.s (5.03 x 1011 cP). Te sinter temperature of both the zircon and chromite blends decreases with increasing amounts of the aggregates. However, with the zircon blends, the peak viscosity increases with increas- ing amounts, while the peak viscosity in the case of the chromite blends decreases with increasing amounts.
Casting Quality Analysis Te baseline silica casting obtained is shown in Fig. 6. Te
casting exhibits several veins along the surface, which is typical of silica sand castings. No penetration defects are visible. More veins are formed along the thicker sections of the casting, where the metal takes longer to solidify. Tis would enable the cores to reach higher temperatures while the metal is still in its liquid form. Silica with 10% zircon does not display any veining
defects. Tough the alpha-beta transition peak expansion for silica with 10% zircon is similar to baseline silica, the early inducement of the cristobalite transition, the second- ary expansion and higher viscosity at sintering temperature leads to lower strain on the surface of the core, thereby reducing the veining defect. However, silica with 20%, 30% and 40% zircon display
slight veining and penetration defects at the thicker casting sections. Silica with 7.5% chromite (Fig. 7) displays no veining or penetration defects. Silica with 7.5% chromite displays a lower peak expansion at the alpha-beta phase transition temperature when compared to silica sand. Also, the cristobalite phase transition is induced approximately
֨。Պबਸ਼ी方Շ诱Јچ温Ѻ更֨לਈ物合混，ઈ৲ ℉（
硅，Јچ温面੮ਯङ）℃
管ـ。胀膨次ид੧进经已就ս样物合混砂ਸ਼锆չ砂 ŜबՊեѫӟ现 ś完੧进Јچ温）℃ ℉（ ֨ ਈ是还胀膨次и幅םङ੧进Јچ温Ѻ较Ѹ，ঢ়收速迅 չ面੮ਯ（用ҁ起Љব张ङЈچ温高更֨面੮ਯ҅ל 。量数ङৠ়裂面੮ਯдصӗ৲ђ，）胀膨֮面੮Ј 混砂ਸ਼锆Њӑە胀膨ङ砂硅ङ砂矿铁铬 խ有 ਸ਼ी方。Պ转ਸ਼ी方现ӟѫЈچ温高ऋ֨，ѷঝ物合 铬铁矿 有խ。Ѻ更چ३胀膨次иङӱب观եՊ转 ङ然突现ӟЈچ温）℃ ℉（ ֨ѫ砂硅ङ砂 ℉ ֨ѫ砂硅ङ砂矿铁铬 剧烈收ঢ়，৲խ有 ش现ղեФ此֨并，ঢ়收烈剧现ӟЈچ温）℃ （ ֨ս样ङ砂矿铁铬 չ 有խ。胀膨次иङ֠ਸ 胀膨次иङش较ثबչঢ়收烈剧ङ现ӟեՊ转ਸ਼ी方 。起一֨合熔лबӄў铸钢֨ট砂成造ਈՕ
果গ؟热ѼҿЉ性॰面表 ，қّ性ডङЈچ温ৈ烧֨ҿՃچ温ৈ烧ङս样՟
,（ ֨ҿգ连
Оچ温ৈ烧ङ砂硅ӕׂ。ॐ۱ ^ Оқّ性ড，）
੮ײ容热ѽӀङЈچ温）) ℉（ ^
6G Y（
I6）。随व砂数量增加，锆ਸ਼砂չ铬铁矿砂混合物ङ ডҿ，ઈ৲物合混砂ਸ਼锆йث，是Ѹ。Ѻ降چ温ৈ烧 物合混砂矿铁铬йث৲，高升৲加增量数ҿ随қّ性 。Ѻ降৲加增量数ҿ随қّ性ডҿ，ઈ৲
铸件ઔ量Ӣ析 现ղ面੮沿ў铸。ॐ۱ ֣ײў铸砂硅ӕׂङ得ੂ
现Շ未ӹऩ。点特型Ӂङў铸砂硅是这，়ਉЗךӟ 此，ך更়ਉङ成形ӣ部Լ较ў铸。陷缺性透渗ѾѠ לਈਯ砂得҅这。֡凝间时ङ长更要需ً金ہ液ङה 。چ温ङ高更ӱ达Јӑەङہ液О然依ً金֨ 管ـ。陷缺়ਉѾѠ现ղ未砂硅ङ砂ਸ਼锆 խ有
砂硅ӕׂЊқّ胀膨Պ转Ŝ խ有 锆ਸ਼砂ङ硅砂ś 、Շઇ早更Պ转ਸ਼ी方ङ成造Јچ温ৈ烧֨Ѹ，ѷঝ 降چ३ব张ङ面੮ਯ得҅都چডङ高更չ、胀膨次и 。成形ङ陷缺়ਉдصӗ৲进，Ѻ 较֨砂硅ङ砂ਸ਼锆 Ѹ是，խ有 、 չ
有խ。陷缺透渗Њ়ਉङش微ӟ现ղюӣ部ў铸Լ ）未ղ现ӟѠѾਉ়或 ֣（砂硅ङ砂矿铁铬 渗透缺陷。较硅砂৲ઈ，խ有 铬铁矿砂ङ硅砂
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FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION March 2017 ℃
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