Table 1. Phase Transformation Temperatures of Quartz 图1. 石英相变温度
Phase Transition ➞相变
alpha-quartz to beta-quartz α石英到β石英
beta-quartz to beta-tridymite β石英到β鳞石英
beta-tridymite to beta-cristobalite β鳞石英到β方石英
beta-quartz to beta-cristobalite β石英到β方石英
Temperature ➞温度
573C (1,063F) 870C (1,598F) 1,470C (2,678F) 1,470C (2,678F)
temperature up to around 1,063F (573C). Te second phase is beta quartz. Tis phase of silica sand is less stable than alpha quartz, and there is a decreased viscosity from a solid indicat- ing some surface softening. Tis change occurs regardless of binder type. Losses in volume during this stage can range from 50 to 100% of the original length of the sample. If there is sufficient softening on the surface of the sand
grains to create a liquid, tridymite will be formed. Materi- als such as sodium, lithium or aluminum can force the phase change that is associated with linear change three times as great as the sands original alpha/beta expansion. One Engineered Sand Additive (ESA) that contains lithium has been shown to force tridymite transformation on sands resulting in high tem- perature increases in volume. Tis resulting increased volume of up to 12% has been shown to reverse surface strain and thereby effectively eliminate veining defects in iron castings. Steel castings are poured at higher temperatures and
therefore the sand temperatures at the mold metal interface are higher. As the sands treated with ESA are heated above 1,922F (1,050C), they soften and lose volume (Fig. 1). Tis loss of volume at higher temperatures mimics the strain induced in plain silica sand setting up high surface stresses that cause cracking and veining in steel castings. Although very effective in iron castings, the tridymite phase change and resulting increase in volume occur at too low of a temperature to prevent veining in steel castings. Looking at the expansion of silica sands which have additions of iron oxide, it can be seen that although they experience softening, they do not change phase from beta quartz to beta tridymite. Te loss of volume continues with increased temperatures until a change to the fourth phase of silica. Tis phase consists of the change from beta quartz to
beta cristobalite and is associated with a vol- ume change up to 14.7% occurring around 2,678F (1,470C). Tis increase mimics the effect of ESA at iron casting tempera- tures and reduces veining by reducing the tensile
Density Change (Volume) ➞密度改变（体积）
2.65 to 2.53 (+4.7%) 2.53 to 2.25 (+12.44%) 2.25 to 2.20 (+2.27%) 2.53 to 2.20 (+14.71%)
Linear Change ➞线性改变
+1.56% (0.0156 in./in.) +3.99% (0.0399 in./in.) +0.75% (0.0075 in./in.) +4.74% (0.0474 in./in.)
脉纹缺陷产生影响（见表一）。第一个是α相，其在室 温下到573℃是稳定的；第二个是β相，此相的硅砂没 有α相的稳定，从固态开始，其涂料粘度降低，显示出 一些表面软化。这样的变化与粘结剂的种类无关。这个 阶段的体积缩减量能达到样品的初始长度的50-100%。 如果砂粒表面充分软化，将形成鳞石英。钠、锂、铝 等材料能够以3倍于砂最初的α/β相扩展的线性变化的方 式影响相变。一种含锂的工程用砂添加剂（ESA）促使 砂中鳞石英的形成，导致高温下体积增长。由此导致的 体积膨胀达到12%，与表面应变起相反的作用，因此有 效地消除脉纹缺陷。
铸钢件在更高的温度下浇注，这样铸型内的砂温和金 属液界面的温度更高。当加入工程用砂添加剂的砂加热 到超过1050℃时，他们软化同时体积变小（见图一）。 在更高温度下的体积缩减与应变诱导的在普通硅砂中建 立高表面应力情况类似，会导致裂纹和脉纹的产生。尽 管在铸铁件中，鳞石英相变导致体积增加很有效果，但 是对于铸钢件，由于温度太低，而不能避免脉纹缺陷。 回顾加入氧化铁的硅砂的膨胀，尽管它们经历了软化， 但没有经历由β相向鳞石英相的相变。随着温度升高， 体积一直在缩小，直到第四个石英相的产生。 这个阶段是β相向方石英相的转变而且关联着在 1470℃时14.7%的体积
Fig. 1. Transformation tem- peratures of silica phases are graphed.
图1. 石英相变温度 September 2014
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