the melt. However, as its size increases, it will float to the surface as slag or be entrapped in the casting as an inclu- sion. Magnesium sulfide is of lower density and easily floats to the surface of the melt, but it is of low stability and easily forms magnesium oxide, releasing sulfur into the melt. The sulfur reacts with the magnesium dissolved in the melt. These reactions form a cycle that contributes to the fading of the magnesium’s effect. When a rare earth metal, such as selenium, is used as a spheroidizing agent, the fading is lessened, and the tendency to form slag is reduced. Category II: Segregation-type spheroidizing elements,
such as titanium, antimony, lead,vanadium, bismuth, etc., gather around the eutectic phases as the eutectic reac- tion happens, which causes the deformation of graphite. Magnesium has little capability to resist the interference of elements; however, rare earth metals have a strong capability to resist this interference, even with a residual content of cerium of 0.008%. Terefore, the addition of rare earth metals will reduce the fading of magnesium’s spheroidizing capability and make it more difficult to form slag and inclusions.
Measures to Reduce the Use of Rare Earth Metals
Te excess use of
spheroidizing agents with rare earth met- als results in a waste of resources and causes casting quality problems, such as the increase of under- cooling of the melt, inoculation difficulty and the deformation of graphite nodules. To reduce the use
of rare earth metals, consider removing
浮渣，也可能被卷入铸件内部成为夹渣。同时，硫化镁 （MgS）的密度低，易于上浮到铁液表面，但其稳定 性较差，与氧接触后会形成氧化镁，将硫释放回铁液、 再次与铁液中的镁反应。这种反应的不断发生，是单用 镁作球化剂时球化易于衰退的主要原因之一。用稀土（ 铈）作球化剂时，衰退现象就不这么明显，形成夹渣的 倾向也较小。
另一类是偏析型反球化元素，如钛、锑、铅、钒、铋 等，在铸铁发生共晶转变时富集于共晶团之间，使石墨 的形状畸变。镁抗干扰元素的能力差，而稀土元素抑制 这类干扰元素有害作用的能力很强，铸铁中残留铈量为 0.008%时，就能有效地控制干扰元素的反球化作用。 所以，以镁为主的球化剂中加入稀土，可减缓球化衰 退，而且形成夹渣的倾向也较小。
三．铸造行业中减少稀土合金用量的措施
看来，节约稀土合金是我们今后必须面对的问题。实 际上，目前我国铸 造行业中，球化剂中 稀土用量过多的情况 是普遍存在的，不但 浪费资源，对铸件质 量也有很多负面的影 响，如：球化剂中稀 土用量过多，会使铁 液的过冷度增大，孕 育处理的难度大；还 会导致石墨球畸变。 要切实减少稀土的 用量，不能只从节约的 方面考虑，而且还必 须提高对稀土作用的 认识。在可用、可不用 的条件下，就应该不 用；在必须采用的条件 下，也应该尽可能地减 少用量。当然，不用或 减少用量，都要以确 保铸件质量为前提，因
Rare earth metals have a strong capability to resist deleterious elements, even with a residual con- tent of cerium of 0.008%.
镁抗干扰元素的能力 差，而稀土元素抑制这 类干扰元素有害作用的 能力很强，铸铁中残留 铈量为0.008%时，就能 有效地控制干扰元素的 反球化作用 。
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FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION Fall 2011
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