Eliminating Veining Defects
Simulation software and lab testing methods were used to pinpoint why the defect occurs and how to avoid it.
A GLOBAL CASTING MAGAZINE STAFF REPORT 消除脉纹缺陷
仿真模拟软件和实验室测试方法用于确定缺陷产生机理以及防止措施 一篇来自GLOBAL CASTING MAGAZINE工作人员的报道
along primary parting lines. Veining can occur in any alloy but is primarily seen in either ferrous-based or copper-based metals. Although various studies have examined the defect, the basic causes have been elusive. Researchers at the University of Northern Iowa Metal Casting Center recently conducted studies to determine the mechanisms that cause veining defects in iron and steel cast- ings. Advanced testing methods along with computer casting simulations were used to evaluate various sand mixtures for the propensity to form casting veins. Te results were shared in the paper, “Causes and Solutions to Veining Defects in Iron and Steel Castings,” by Jerry Tiel and Sairam Ravi of the University of Northern Iowa, Cedar Falls, Iowa.
C
Question What sand factors contribute to veining defects and how
can the defect be avoided?
Background According to previous research, the veining defect in cores and molds stems from a tensile stress exerted by a combination of contracting sand at the mold metal interface and subsurface expanding sand. Tis situation is made possible by the loss of sand volume after reaching 1,063F (573C). Te stress is created by an imbalance in the temperature at various points in the sand at differing distances from the liquid metal heat source that cre- ates differences in the thermal expansion and related strain. When the forces exerted on the mold or core’s surface
exceed the high temperature strength at the surface, a tensile mode failure cracks the sand and allows liquid metal to enter. Tis differentiates the defect from the scabbing and buckling defects that exhibit overlap of sand surfaces from increased surface sand volume. Much of the veining defect research has emphasized the
determination and development of sand additives to reduce or eliminate the defect. Engineered sand additives work by caus- ing two effects. Te first takes advantage of a high tempera- ture phase change that occurs in sand at approximately 1,598F (870C). Four main phases of quartz impact veining (Table 1). Te first phase is alpha quartz, which is stable from room
asting veining or finning defects appear as projections in the form of veins generally occur- ring perpendicular to the casting surface, either singly or in networks. Tey are not situated
线。各种合金铸件都会有脉纹，但是主要是黑色金属或 者铜合金。尽管各种研究都检测过缺陷，但基础成因还 难以找到。
和 问题
型砂对于形成脉纹缺陷有什么样的影响？如何避免此 类缺陷产生？
背景
根据之前的研究，芯子和砂型中的脉纹缺陷源于铸型 金属表面的收缩砂粒和表层以下膨胀的砂粒的共同作用 产生的拉应力。这种情形可能是由于达到573℃后砂体 积缩小造成的。这种应力引起型砂的热膨胀差异和相应 的应变，是由于距离液态金属热源远近不同，型砂中各 点温度不均衡而造成的。
当施加在铸型或者砂芯表面的力超过砂子表面的高温 强度时，受拉伸模式失效驱使，砂产生裂纹，同时液态 金属进入。这一点可以将脉纹缺陷与由于表面砂体积增 加，砂在表面层叠引起的夹砂、沟槽缺陷区分开。 许多脉纹缺陷的研究都强调研发砂添加剂以减少或消 除缺陷。工程用砂添加剂通过两种效果起作用。首先是 利用在近870℃时砂中发生的高温相变。石英的4个相对
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FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION September 2014
北爱荷华大学金属铸件中心的研究人员，近来开展了 铸铁和铸钢件中产生脉纹缺陷决定机理的研究。研究人 员采用了先进的测试方法与计算机仿真模拟，评估各种 砂混合物形成铸件脉纹的倾向。美国爱荷华州的北爱荷 华大学的Jerry Thiel 和Sairam Ravi编写的《铸铁和铸 钢件的脉纹缺陷的成因及解决办法》论文中分享了这个 研究成果。
推测的一样，铸件的脉纹或者飞翅缺陷通常 在铸件表面的垂直方向，单独或者成网状以 刺的形式出现。它们的位置不沿着主要分型
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