On the Microstructure and the Thermal Conductivity in Compacted Graphite Iron

Licentiate thesis, 2009

Compacted graphite iron (CGI) is a cast iron material that has gained much attention due to its attractive material properties which are intermediate between those of grey and ductile cast iron. In the automotive industry compacted graphite iron has become an interesting alternative material to grey cast iron in components like cylinder blocks or cylinder heads. The reason for this interest is the higher strength found in CGI. However, one disadvantage is the lower thermal conductivity of CGI compared to grey cast iron. The aim of the present work is to investigate how chemical composition and cooling rate affects the microstructure in compacted graphite iron and trying to relate the microstructure to the thermal conductivity. The effect of an austempering heat treatment process on some material properties in CGI, grey and ductile cast iron is also investigated. The austempering heat treatment was shown to have a significant influence on many of the material properties investigated. The hardness was increased, while the thermal conductivity was reduced, for all three graphite morphologies observed. Scuffing resistance increased considerably for the austempered grey iron but was slightly reduced for the other two austempered morphologies. The ultimate tensile strength in bending was reduced in the austempered grey iron and CGI samples but slightly increased in the ductile iron sample. It was confirmed that alloying elements like magnesium increased nodularity, while copper and tin promoted formation of pearlite in CGI. Silicon had a ferrite promoting effect as well as an inoculating effect and chromium and molybdenum promoted formation of carbides. The thermal conductivity was highly affected by the fraction of ferrite as well as nodularity and addition of carbon and silicon. Based on these results a mathematical model was developed for calculation of the thermal conductivity at various temperatures.