灰铁和球铁在冷却和凝固过程中的体积变化,不同于工业上其它重要的金属和合金。其根本的区别在于,冷却至共晶温度或高于共晶温度时液体就停止收缩,进一步冷却和凝固则出现膨胀,这种膨胀一直持续到近于凝固完全结束为止。但是,最后凝固的一部分液体将再次收缩(二次收缩)。无论是灰铁还是球铁,体积变化的这种模式并不是一成不变的,它至少取决于三项因素:(a)冷却速度;(b)与化学成份有关的石墨化倾向;(c)与各种工艺参数有关的冶金质量。虽然可应用常规的冒口设置方法,然而其它更为经济的方法,也可用于灰铁件和球铁件的生产。所有这些方法都是利用膨胀来补偿二次收缩的。有三种不同的方法可供采用:(a)压力冒口;(b)无胃口设计;(c)卸压冒口。在用压力冒口吋,液态收缩得到补偿,而一旦膨胀开始,铸件和冒口系统之间的连接处就凝结。只要浇注温度不高,液态膨胀和凝固膨胀可补偿二次收缩,因此冶金质量良好的大断面铸件,无需设置任何冒口,就能成为健全的铸件。除了其它要求之外,这种方法要求使用刚度好的铸型。当不得不用刚度较差的铸型(如湿砂型)来浇注中等厚度和比较厚的灰铸铁和球铁铸件吋,恰当地应用卸压冒口方法,就能使铁水反馈到冒口而适当减少一部分膨胀,但仍有足够的膨胀用以补偿二次收缩。 The volumetric changes of gray iron and ductile iron during cooling and solidification are different from other important metals and alloys in the industry. The fundamental difference is that the liquid stops shrinking when it cools to or above the eutectic temperature, and further cooling and solidification expands until it is almost complete. However, a portion of the liquid that is finally solidified will contract again (secondarily contract). Whether gray iron or ductile iron, the mode of volume change is not static and at least depends on three factors: (a) the rate of cooling; (b) the tendency of the graphitization associated with the chemical composition; (c) Metallurgical quality related to process parameters. While conventional riser setup can be applied, other, more economical, methods can be used for the production of gray iron and ductile iron. All of these methods utilize expansion to compensate for the secondary contraction. There are three different methods available: (a) pressure risers; (b) appetite-free designs; and (c) pressure relief risers. In pressure risers, liquid shrinkage is compensated, and once the expansion begins, the junction between the casting and the riser system condenses. As long as the pouring temperature is not high, liquid expansion and solidification expansion can compensate for secondary contraction, so good quality metallurgical castings of large sections, without setting any riser, can become a sound castings. Among other things, this method requires the use of a rigid mold. When it is necessary to cast gray and ductile iron castings of medium and thick thickness with less rigid molds, such as wet sand, the proper use of the relief riser method allows the hot metal fed back to the riser to be properly reduced A part of the expansion, but still enough expansion to compensate for the second contraction.