it changes phase from alpha quartz to beta quartz. Upon further heating, the sand loses its volume due to loss of binder volume and softening and rearrangement at the surface of the sand grains. Tis loss of volume at temperatures above 1,063F is the main cause of veining defects. As the temperature of the mold or core surface increases, the length and volume of the sand decrease. Te cooler sand directly beneath the surface increases in volume as it passes through the alpha to beta quartz trans- formation. Te combination of contracting sand on the surface with expanding sand directly beneath the surface creates tensile failures that fill with liquid metal forming the defect classified as veining. Sand additives that reduce veining defects provide liquid on the surface of the sand grain and favor formation of tridymite or cristobalite and greater expansion of the sand. Tis secondary expansion reduces the negative strain at the surface of the core and prevents tensile failure and the associated cracks. Trough the fluxing action of sand additives, the surfaces of the particles adhere to each other, increasing the tensile strength of the sand on the surface of the core or mold. Tis increase in strength reduces the tensile failure on the sand’s surface and reduces veining defects. Te accepted methods of reducing veining defects in cast-
ings are: Using low expansion aggregates including chromite,
zircon, olivine and ceramics to eliminate the difference of expansion rates of surface and subsurface sections of cores and molds. Tese materials generally exhibit linear thermal expansion and little if any phase transformations. Teir refractory value is higher than silica sand, and therefore they show minimal softening and volume loss. Te strain values at the mold metal interface closely match the subsurface strain values and therefore eliminate any mechanical forces that would cause tensile failures and veins. Blends of low expan- sion aggregates with silica sand have been successfully used to reduce or eliminate veining defects. Using sand additives containing fluxes, such as iron oxide and lithium-based products.Fluxes decrease the temperature at which the silica starts to soften and provides liquid on the surface of the grains, increasing the reactivity and lowering the transition temperatures for tridymite and cristobalite. Tese transitions force increases in volume of the subsurface sand and reduce the vein strain on the surface of the core or mold. Tese fluxes can also sinter the sand together at high temperatures, effectively increasing their resistance to tensile failures as shown by increasing viscosities of bonded sands. Using sand additives containing organic materials such
assaccharides or dextrin to provide a slight cushioning effect as the sand goes through the alpha/beta transformation, but mainly to act as a carbon source for a high temperature bond- ing of the
sand.Any available oxygen in the mold cavity is depleted shortly after it fills with liquid metal. In the absence of oxygen, the organic materials break down to primarily car- bon, which bonds to the surface of the sand grains, increasing the viscosity and tensile strength of the surface sand. Tis increase in tensile strength resists the vein strain and reduces veining defects. Often these materials are blended with fluxes and oxides to increase their effectiveness. ■
Tis article is based on a paper (14-030) presented at the 2014 Met- alcasting Congress.
脉纹缺陷，要么减少脉纹缺陷。在573℃时，α向β 相转变，含粘结剂的石英砂的线性膨胀导致砂体积 急速增加。通过进一步加热，由于粘结剂量的损耗 和软化、砂粒表面的重新排列，砂体积减小。在高于 573℃时，砂体积缩小是形成脉纹缺陷的主要原因。 当铸型和砂芯表面温度升高，砂型的长度和体积缩 小。在表面层正下方的温度稍低的砂，经过α向β相转 变，体积增大。面层收缩的砂与背层膨胀的砂相互作 用导致拉伸失效，这样金属液就进入，形成了常见的 脉纹缺陷。减少脉纹缺陷的砂添加剂，形成砂粒表面 的液态层，并利于形成鳞石英或方石英以及砂膨胀。 二次膨胀缩减了芯子表面的消极的拉应力并阻止拉伸 失效及相关的裂纹出现。通过砂添加剂的熔剂作用， 颗粒表面相互连接，增强了铸型或砂芯表面的砂粒的 抗拉强度。强度的增加减少了砂粒表面的拉伸失效及 脉纹缺陷。
在铸件中可用于减少脉纹缺陷的方法是： 使用低膨胀的砂粒，如铬铁矿砂、锆砂、橄榄石 砂、熔融石英或人造砂代替硅砂，以消除铸型及砂芯的 部分面层砂以及背层砂膨胀率的差异。如果发生相变， 这些材料一般不显示出线性膨胀。它们的耐火率高于硅 砂（石英砂），因此软化度和体积损失最小。模型/金 属界面应变值与表层下应变值相匹配对应，因此可以去 除引起拉伸失效和脉纹的机械力。低膨胀颗粒与石英砂 相混合已经成功用于减少或消除脉纹缺陷。 使用包含熔剂的添加剂，比如氧化铁和锂基产品。 熔剂降低温度，在这一温度下石英砂开始软化并在颗 粒表面出现液相，加快鳞石英转为方石英的反应，降 低转变温度。这些转变驱动力增加表层下体积，减少 砂型和模型表面脉纹应变。这些熔剂也能够在高温下 将砂整体烧结，通过增加粘结砂的粘度，有效增加拉 伸失效的阻力。
使用含有有机物的添加剂，比如糖类或糊精，为α 向β相转变时提供一个微小的缓冲效果，但主要还是 起到高温下砂粘结中碳来源的作用。模腔中，在液态 金属充填之后，任何可用的氧很快耗尽。缺氧时，有 机材料主要裂解为碳，将砂粒表面粘结在一起，增加 表层砂的粘度和拉伸强度。拉伸强度的增加阻止脉纹 应变产生，减少脉纹缺陷。这些材料经常与熔剂和氧 化物混合增强效果。 ■
本文摘自发表于2014年铸造大会上的一篇论文（14-030 ）。
September 2014
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