Fig. 2. Shown is the general layout of the dilatometer used in the study.
stress induced at the surface of the core or mold through expansion. Along with the cristobalite transformation and associated expansion, the sintering point of the sand decreases. Unfortunately, the reduction of veining with the use of iron oxide may come at the high cost of room tem- perature tensile strength.
Procedure Te lack of data on the elevated temperature performance
of unbonded sand is most likely due to the lack of the avail- ability of appropriate equipment and methods to allow the thermal expansion of unbonded sand to be measured. To address the problem of measuring the thermal expansion of foundry sands, the University of Northern Iowa’s Metal Casting Center designed and built an updated dilatometer, capable of accurately measuring the thermal expansion char- acteristics of heterogeneous foundry sands. Te dilatometer was designed robustly to provide the temperature range and degree of accuracy required. Termal expansion tests were run on bonded sand samples utilizing the university’s high temperature aggregate dilatometer (Fig. 2). An experimental method to measure surface softening
of small solid particles heated to high temperatures uses a dilatometer to measure the rate of deformation of a granular material and calculate the viscosity. Te calculated viscosity is useful in describing the sintering characteristics of a granular material. An extension to the this method was developed to better understand the surface softening of sand particles. Te surface viscosity measurement is based on the compac- tion of the sand particles while under a compressive load and constant heating rate. Sand particles, being porous, initially expand with temperature but subsequently contract due to softening and sintering at intergranular contact points. Tis behavior results from surface softening and deformation where the compressive load is concentrated. Since veining defects are common in silica sand cores and/or
molds, a casting simulation software model was created based on the expansion and contraction of silica sand as it was heated up to a high temperature to simulate surface strain. Step cone iron and steel castings produced in molds con- sisting of various additives were tested and compared with the predictions of the casting simulation software.
Results and Conclusions Results from the tests showed that two forces act on the sur-
face of the sand that can either contribute to or reduce the vein- ing defect. Te linear expansion of bonded silica sand causes the volume of sand to increase sharply until 1,063F (573C), where
图2. 研究中采用的膨 胀仪的总体设计图
膨胀。这一增长模仿了铸 铁铸造温度下的工程用砂 添加剂的效果，并通过膨 胀的方法，减少型芯或模 型表面拉应力诱导脉纹缺 陷。随着方石英相变和相 应膨胀的进行，型砂的烧 结点降低。不幸的是，使
用氧化铁来减少脉纹缺陷可能会给室温抗拉强度带来 很大的损失。
过程
无粘结剂型砂高温性能数据的缺乏，很大程度上可 能是由于缺乏用于测量无粘结剂型砂热膨胀的合适的 设备和方法。为了解决铸造用砂的热膨胀测量问题， 北爱荷华大学金属铸造中心设计并建造了一个最新的 膨胀仪，能够准确测量多种铸造用砂的热膨胀参数。 膨胀仪设计的很全面，可提供所需的温度范围或者准 确的温度值。使用该大学的高温膨胀仪对已粘结的砂 样进行热膨胀测试的照片见图二。
使用膨胀仪来测量颗粒材料变形率和计算粘度的一 种试验方法，用于测量加热到高温时小的固态颗粒的 表面软化情况。计算出的粘度对于描述一种颗粒材料 的烧结性能是有用的。这种方法的延伸可以更好地理 解砂粒表面的软化。表面粘度的测量是基于砂粒在压 紧状态下，并处于压应力和稳定的升温速率状态。砂 粒变成多孔状态，开始时随着温度升高而膨胀，但随 后因为软化和晶间烧结而收缩。这种反应是由于表面 压应力集中，表面软化和变形而引起的。 脉纹缺陷在硅砂砂型和砂芯中普遍存在，因此，研 发了基于硅砂的膨胀和收缩的铸造模拟软件模型，通 过将硅砂加热到高温来模拟表面应变。 添加各种添加剂的铸型生产的塔轮铸铁和铸钢件， 与铸造模拟软件预测的结果进行了测试和比较。
结果和结论 测试结果表明，2股力量作用于砂表面，要么增加 52 |
FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION September 2014
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