Fig. 9. This diagram illustrates the reaction process in the mold material-coating-melt system.
图9. 该表说明了造型 材料-涂层-熔体体系 内的反应过程。
Te fewer alloy elements the melt contains, the more severe the casting defect is. Te chemically bound organic binders (furan and phenolic resin) decompose as a result of pyrolysis. Te free carbon instantly reacts with oxygen, forming CO. Since a reduced atmosphere prevails within the enclosed mold, some free carbon remains. Te car- bon reduces SiO2
is
formed. Te silicon can also be used as a reducing agent, which means that SiO(g)
4. It is therefore extremely important to use systems with fewer reducing agents. Te SiO2
as described in equation 3 and SiO(g) can form as described in equation component can be reduced
by using a coating containing no or just a small amount of silicon. Tis minimizes the severity of the casting defect but does not eliminate it completely. When inorganic binder systems are used instead of organic binder systems, the cast- ing defect is eliminated but problems associated with the severe sintering of the mold material arise. On the basis of the experiments conducted and the available literature, Figure 9 shows the process by which the reactions resulting in the defect could potentially take place. Te reaction products—amorphous and fibrous silicon—
were examined and evaluated in terms of both the mold material and cast. Carbon-like structures were identified in these phases. Tese phases were formed in the system at temperatures of less than 1,000C because the casting temperature is around 1,350C and the casting stays in the mold for another three to eight days. Following unpacking, the temperature is around 300C–400C, which means the casting still needs to be air-cooled before being processed any further. Tis means that the casting defect that takes the form of a white film can occur through the process described. Following processing, the surface takes on a pitted appearance because the casting defect can be easily removed from the surface. Tese pits/holes that mark the surface and are formed when the casting is solidified arise due to either the defor- mation of any gas that has developed or its penetration into the casting surface. Gas develops on the mold and core side when the binders are combusted. Te gas can penetrate the casting surface only when the excess gas pressure exceeds the resistance on the metal side. Tis resistance is a function of the casting height and specific weight of the liquid melt. Tis can lead to the formation of large holes on the casting surface. Te smaller surface deformation arises due to the mechanical penetration of the gas in the liquid melt at the interface between the metal and mold material. ■
显：合金元素越少，铸件缺陷越严重。化学有机粘 结剂（呋喃和酚醛树脂）受热分解。游离碳会立即 与氧气发生反应，生成CO。由于密闭砂型内以还 原环境为主，因而会剩余一些游离碳。如反应式3 所述，碳会还原SiO2
，形成SiO(g) 还原剂，即也可按反应式4形成SiO(g)
。硅也可以用作 。因此，使
用还原剂较少的体系极其重要。可以通过使用不含 硅或只含有少量硅的涂料来还原SiO2
铸件缺陷的严重程度，但不能完全消除缺陷。使用 无机粘结剂体系取代有机粘结剂体系时，铸件缺陷 被完全消除，但是又会出现造型材料严重烧结的问 题。
在这些实验和现有文献的基础上，图9显示了导致 缺陷可能发生的反应过程。
反应产物——无晶和纤维状硅——均已根据造型 材料和铸件进行了检测和评估。在这些相内发现了 类似碳的结构。在温度低于1000℃时体系内会形成 这些相，因为铸造温度为约1350℃，而铸件会在砂 型内停留3至8天。开箱后，温度为约300℃-400℃ ，因而铸件在进一步处理前仍需用空气冷却。也就 是说，表现为白膜的铸件缺陷可能在上述过程中出 现。经过处理后，因为可以轻易地从表面清除这层 白膜，但是铸件表面就会呈麻面。
表面上的这些孔/洞形成于铸件凝固时，受所生 成气体的变形或其渗入铸件表面的影响而产生。粘 结剂氧化时，砂型和砂芯一侧会产生气体。只有当 额外的气体压力超过金属侧的阻力时，气体才可能 渗入铸件表面。该阻力是铸件高度与铁水具体重量 之间的函数。这可能导致在铸件表面形成较大的孔 洞。较小的表面变形是由于金属和造型材料接触面 的铁液内气体的机械渗透引起的。 ■
。这最小化了
December 2014
FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION | 67
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