Research on Abnormal Grey-speckle Structure of Ductile Iron
Author of the article:DENG Han;YANG Zhigang;WANG Rui;CHEN Lu
Author's Workplace:CRCC Qishuyan locomotive&rolling stock technology research institute Co.,Ltd.,Changzhou 213011,China
Key Words:ductile iron black spot segregation
Abstract:
Under certain conditions,most of the thickness of nodular cast iron was prone to strip gray spot defects,which would significantly reduce the hardness of the material.The microstructure and microregion composition of abnormal gray spots were analyzed by SEM and EDS.The results show that carbon equivalent of less than 4.3%,composition segregation and thick and relatively closed casting structure are the main causes of this defect.Under these conditions,slow-cooling dendrites are easy to form.Si elements are isolated and enriched in the slow-cooling austenite branchlets to promote the formation of ferrite.Mn and Cu are separated and enriched near and outside the dendrite to promote the formation of pearlite.The hardness difference of the two kinds of matrix makes great color difference after processing and forms the macroscopic gray spot morphology.