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. 

Thermal conductivity, hardness and strength are all highly important material properties, affecting the performance and life expectancy of components operating at elevated temperatures. The main purpose of this work has been to increase the knowledge and understanding concerning how mechanical and physical properties are affected by temperature and microstructure in compacted graphite irons. This was accomplished by investigating the whole chain from how a certain microstructure can be achieved, how that microstructure affects mechanical and physical properties, how those properties were related and how they can be estimated.

By investigating how specific alloying elements affected the microstructure it was possible to confirm the pearlite promoting effects of copper and tin, the carbide stabilizing effects of chromium and molybdenum and magnesium’s ability to alter the compactness of the graphite particles. An ausferritic metal matrix could be attained by performing an austempering heat treatment or by increased solidification rate on samples highly alloyed with molybdenum. Increasing the content of ferrite improved the thermal conductivity, while increased content of free carbide, ausferrite or nodularity reduced the thermal conductivity. Ferrite containing high amount of dissolved silicon had a negative influence on thermal conductivity values but also a strengthening effect. Thermal conductivity values in CGI generally showed a maximum at about 300 °C but a large content of ferrite resulted in quite temperature stable values below 300 °C. Tensile strength parameters such as ultimate tensile strength and yield strength were temperature stable for temperatures up to 300 °C before the values rapidly decreased at higher temperatures. The values of Young’s modulus continuously decreased with increasing temperature and seemed to increase slightly with increasing content of free carbide and nodularity. Increasing nodularity also increased the values of the strength parameters. Contradictory linear relationships between strength or hardness and thermal conductivity were found highlighting the problem in optimizing both these properties. Linear regression models based on five key parameters were created to describe thermal conductivity values and hardness values, where the model describing thermal conductivity included temperature. Stress values for plastic deformations were possible to approximate with good accuracy by using constituent equations such as the Hollomon and Ludwigson equations.