374F (190C) lower and the peak viscosity is about twice as much as baseline silica. Tis would prevent failure on the surface of the core. However, in the silica with 10% chromite and silica with


30% chromite castings a large extent of fused sand on the castings which increases with increasing amounts of chro- mite. Looking at the expansion and viscosity, we can see that silica with higher amounts of chromite undergoes a sudden contraction at around (2,012F (1,100C) unlike the steady contraction seen in silica with 7.5% chromite. Also, the peak viscosity decreases and the sinter temperatures drop by as much as approximately 212F (100C) for the higher chro- mite content samples. Table 3 displays the veining ranking for baseline


silica and the various blends. Silica with 10% and 30% chromite could not be evaluated due to the large extent of fused sand on the castings. A lower content of the specialty aggregates display better performance when compared to the higher content. Baseline silica has a high veining index, as expected. Silica with 10% zircon and 7.5% chromite both display no indica- tions of veining defects.


Conclusion Te quality of metal castings relates to the high tempera-


ture performance of the refractory aggregates used. Tis high temperature performance is determined by the thermal volume stability and resistance to high temperature softening. Silica sand with its low cost and abundant availability


does have limitations not easily overcome. Its high rate of expansion through phase transformations results in casting defects. Te use of molds and cores produced entirely from specialty sands like chromite and zircon are expensive and in many applications do not warrant the increased cost. Te use of sand blends of inexpensive silica sand with higher cost specialty sands has been shown to have applications where the quality of the casting can be improved without the associated high cost of 100% specialty sand cores. As little as 10% of specialty sands can improve the quality of the final casting by reducing the extent of veining defects, according to the research results. Te effect of blending silica sand and specialty sands highly depends on the thermal input of the metal and the mass of the mold that determines the heating rate of the mold and associated cooling rate of the casting. Te chemical reaction between the base sand and the


specialty sand must be accurately determined, as was the case of silica and chromite sand blends. Higher heat inputs in the larger metal sections caused the mixture to fuse, leading to casting defects. Lower percentages of chromite sand improved casting


quality in the test casting. Tis illustrates the importance of comparing the high temperature physical properties of the sand blend with the specific casting application before practic- ing the technology. ■


This article is based on the paper “The Use of Specialty Sand Blends to Reduce Veining Defects in Steel Castings” (16-060) which was presented at CastExpo16.


ਈ还，时գ。қّ胀膨ङѺ更ӟ现ղЈچ温ՊबŜ ś֨ बਸ਼ी方Շ诱Јچ温ङ）) ,（ 约ѺӹФ较֨ל ੮ਯ止防לਈ这。ҕиङ砂硅ӕׂ是约چডқّ৲，Պ 面故障ङ形成。 Ѹ是，խ有 铬铁矿砂ङ硅砂չխ有 铬铁矿


砂ङ硅砂铸ў随व铬铁矿砂խ量增加，ҿ铸ўЇङ熔砂 Њ，ӱरљՕћ我，چডЊӑە胀膨йث。加增ѫЭ量 խ有 铬铁矿砂ङ硅砂ङ६定收ঢ়Љգ，铬铁矿砂խ ,（ )）Ј经Է突然收 约֨要砂硅ङ高更量 ѫЭچডҿ，ս样ङ高更量խ砂矿铁铬йث，时գ。ঢ় ℉（ ℃）。 约降Јѫ还چ温ৈ烧，Ѻ降 йं。级ঈ়ਉङ物合混գЉչ砂硅ӕׂдॐي ੮


խ有 铬铁矿砂ङ硅砂չխ有 铬铁矿砂ङ硅砂ङ Ѻ。ѳછ੧进ҿث法无此֜，砂熔ङ量ם֨存面੮ў铸 。ਈ性ङױ更ӟ现ղ砂य़特量խ高比לਈ砂य़特ङ量խ 有խ。数指়ਉङ高更व有ઈ৲期预较砂硅ӕׂ


锆ਸ਼砂ङ硅砂չխ有 铬铁矿砂ङ硅砂未ղ现ѠѾਉ ়缺陷。


গ论 ਈ性温高થ。Ҽबਈ性温高ङ砂火৳用۱Њ量ૅў铸


ՈӐй热ѽ॥६定性չ高温软化৳Չ性。 ङ服ұљ难व有实ेѸ，ئЙ源来用ՕЌѺ本成砂硅


。陷缺造铸成造ѫ实े率胀膨高ङ间期Պब֨ҿ。Ӳ限 Ӏ模ङ成Ӳ）砂ਸ਼锆չ砂矿铁铬ײ（砂य़特ंҶ完用҅ 本成ङӟ高ચґਈЉ并И用应ךક֨，贵昂ӣ十ਯչ 量。现已有应用依据੮明，҅用խ有成本较高特य़砂ङ 用采йंѫЉЌ，量ૅў铸֑改לਈ物合混砂硅贵昂非 特य़砂ਯ造成बҼ成本很高。ू९ৈ果੮明，ю


়ਉѺ降，本成ў铸ৄ最进改לਈ就砂य़特ङ ҅用 йӐՈЇچ३ם很֨果效合混砂य़特Њ砂硅。چ३陷缺 率速温升ङӀ模व定Ӑҿ，量ૅӀ模Ճљҵ输热ङً金 չ铸ўङबҼӒ却速率。 Ն学化ङ生фѫ间Ф砂य़特չ砂ӕׂӟ定ेेত要


较面截ً金果ײ。此ײ是Э物合混砂矿铁铬չ砂硅，应 ৲ђ，化熔物合混成造ѫ，高更ر量热ङҵ输ӄҿ，ם ф生铸造缺陷。 Э这。量ૅङў铸ડ测֑改לਈ砂矿铁铬ङ量խص较


性理物温高ङ物合混砂ر，ӹ૪实й用术技ر֨д明પ ૅգ特定铸造应用ब比较ङ重要性。■


物合混砂य़特用҅Ȕ文ખङॐي 本文依据ङ是)GYZ+^VU 降Ѻ钢铸ўӄङਉ়缺陷ȕ（


）。 March 2017 FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION | 51


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