Formation mechanism and causes of metallographic defects in special parts of ductile iron

When the carbon content is increased to solve the shrinkage cavity and porosity defect, the top graphite floating and the center graphite abnormal distortion will appear in the thick part of the casting due to the increase of carbon equivalent. The morphology of graphite, the number of graphite balls and the roundness of nodular cast iron have important influence on its physical and chemical properties. The mechanical properties of nodular cast iron will be decreased if graphite floating and broken graphite defects appear.

The theoretical study on the effect of carbon equivalent on graphite floating defects shows that the two main causes of graphite floating defects are high carbon equivalent and thick casting wall. Carbon equivalent (CE) refers to the increase or decrease in the amount of carbon converted from the elements that have an impact on the actual C content at the eutectic point of materials. In practical engineering application, only Si and P elements are considered in the calculation, which is expressed by CE.

In the formula, C, Si and P are the contents of carbon, silicon and phosphorus in molten iron (%).

It can be determined by the theory of high carbon content in the same kind of cylinder. In this case, it is easy to produce graphite floating defects. At the same time, different Si content has corresponding effects on the graphite morphology and mechanical properties. The more serious graphite floating defects mainly concentrated on the upper surface of the top 400mm thick part of the pouring position. The other slight graphite floating positions are the same as the slag inclusion defects. The defects are distributed in black layer and the color difference is obvious. It is difficult to show the floating defects of graphite by non-destructive testing method. Generally, according to the practical experience, the comparison analysis is carried out by referring to ut detection and color comparison card. In the actual inspection, qualitative analysis is made on the defect of the machined surface where the color difference is greater than grade 3 and serious ultrasonic detection bottom wave attenuation is found.

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