球墨铸铁中的奥氏体枝晶与缩松

Austenite Dendrite and Shrinkage Porosity in the Nodular Iron

采用着色腐蚀技术 ,金相显示了球墨铸铁缩松区中奥氏体枝晶组织形貌 ,分析了球铁缩松的形成机制。研究表明 ,奥氏体枝晶对缩松缺陷的类型及形成机制具有显著影响 ;宏观缩松常常出现在枝晶晶簇的间隙 ,产生于共晶凝固前期树枝晶骨架形成以后 ,是异地凝固收缩造成的对热节中心 (厚壁处 )铁水抽吸流动的结果 ;显微缩松是于凝固末期 ,晶簇间隙中的凝固收缩得不到补偿而产生的微小孔洞 ;枝晶数量增多 ,形态趋于发达 ,液态金属异地抽吸作用增强 ,易于形成宏观缩松 ;反之 ,枝晶数量减少 ,形态粗壮 ,倾向于形成显微缩松 ;共晶石墨化膨胀有利于消除缩松 ,而不是缩松形成的原因。

The morphology of austenite dendrites in shrinkage porosity in nodular iron is displayed by the color etching technique and the formation mechanism of shrinkage porosity has been analyzed. It reveals that austenite dendrite has remarkable effect on the defect type and formation mechanism of shrinkage porosity, and after dendrite framework has formed, the macro-porosity which appears in the gap of clusters of dendrites arise from that the melt in thermal center be drew by the shrinkage of solidification in the other area at the earlier stage of eutectic solidification. In the latest stage of eutectic, macro-porosity will form because the shrinkage of solidification in the gaps of clusters of crystals is not compensated. Moreover, more and developed dendrites will result in increasing of drawing function of the melt from thermal center to earlier solidification area through passageway in dendrite framework and it is liable to forming macro-porosity. On the contrary, under less and stronger dendrite there has a tendency to forming micro-porosity. Besides, the graphitization expand is beneficial to eliminate the shrinkage porosity but is not the reason to cause it.