A Study On Factor Influencing the Microstructure and Shrinkage Porosity Formation in Compacted Graphite Iron
Sammanfattning: During the last decades, an intensive effort has been dedicated to introduce compacted graphite iron (CGI) for truck engine components as its intermediate graphite morphology provides an increased tensile property compared to lamellar cast iron (LGI) and a better thermal conductivity compared to nodular cast iron (SGI). However, the narrow production technology window in this cast iron grade requires increased technological discipline. The microstructure that develops during the solidification process determines the final properties of the cast material. But, due to lack of knowledge regarding the mechanisms behind the microstructure and defect formation in CGI, it is difficult to produce sound cast components. Shrinkage porosity formation is one of the main challenging problems. The scope of the present work is to characterize the microstructure of CGI cast components and investigate the influencing parameters and mechanisms related to a shrinkage porosity formation.The influence of the graphite nodularity fraction in CGI on the shrinkage porosity has been studied. It was found that the tendency for shrinkage pore formation increased with an increased fraction of graphite nodules.Pore surface investigation has revealed the existence of dendritic networks with different morphologies inside the shrinkage pores being covered with a graphite film. Furthermore, no dendrite was detected inside the gas pores. Based on these observations, these pores were suggested to be formed at different time periods during the solidification process.Investigation of the levels of gas (hydrogen and nitrogen) in the CGI melts produced with different charge materials before and after casting, revealed that the gas content was increased after mold filling. Shrinkage pores were observed in the components produced based on wet and nitrogen enriched charge material. The hydrogen level was not found to be critical, but the nitrogen level in the components containing porosities was found to be above the solubility limit. Also, the percent nodularity was found to be very low in these samples. Therefore, it was concluded that the excess of nitrogen has contributed to the shrinkage pore formation.The eutectic colony sizes and distributions were also studied in CGI components containing different shrinkage porosity levels. It was revealed that the shrinkage porosity propensity increased with an increased number of eutectic colonies of smaller sizes in the microstructure.To further enhance the understanding of the solidification sequence, the microstructure and the microsegregation behavior of the alloying elements were studied. The austenite grain boundaries in the samples solidified under normal cooling conditions. In addition to CGI, the microsegregation of LGI and SGI have been investigated. The contribution of the MnS particles in graphite nucleation has been demonstrated in LGI as well as the MgS particles in CGI. The ability of the used etching reagent to reveal the silicon microsegregation has been demonstrated by an accurate Si segregation measurements. Based on the silicon propensity tosegregate inside austenite, the primary austenite dendrites, and the eutectic colonies were revealed. Also the existing graphite nodules both in CGI and SGI were differentiated with respect to the size and the surrounding Si segregation. These data provided indications regarding the mechanism and chronology of solidification. In general, a good correlation was found between EMPA and color etching results. However, in some cases it was observed that sensitivity of the color etching might not be adequate to detect the size of the eutectic colonies.
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