On thermal and mechanical behavior of cast irons at elevated temperatures

Casting is a production process that lends itself superbly to creation of complex shapes in many scales. To get every sliver of performance from a casting, one needs to combine the knowledge of shaping methods to a deep understanding of properties and behaviour. Cast irons in particular are widely used in thermally loaded applications, where both mechanical and physical properties are required. Spheroidal graphite cast irons hold potential in becoming a replacement for lamellar graphite alloys with certain combination of microstructure and alloying. In this work, thermal and mechanical properties of select cast iron types are explored at elevated temperatures, from room temperature (RT) to around 400 °C. A Transient Plane Source-measurement system is used to characterize thermal conductivities of lamellar, compacted and spheroidal graphite irons. Secondly, the effects of silicon alloying in the range of 1.5 to 4.3 w-% and different ferrite-pearlite fractions to spheroidal graphite thermal conductivity at elevated temperatures are tested and models for estimation purposes are made. Mechanical property side of this work concentrates on evaluating the occurrence of Dynamic Strain Aging (DSA) in ferritic to pearlitic microstructures of spheroidal graphite cast iron. DSA testing consisted of tensile and cyclic load cases with various strain rates and amplitudes from RT to 450 °C. The results in this thesis indicate that the used TPS measurement system is capable of characterizing cast irons at elevated temperatures, when care is taken in analysis and certain operator related challenges are considered. Additionally, spheroidal graphite cast iron thermal conductivities can be optimized or estimated with the chosen test parameters as the contribution of graphite in spheroids is relatively minimal. The mechanical property experiments show that DSA related phenomena are evident in all tested ferritic and pearlitic microstructures with certain combination of strain rates and temperatures. As DSA effects include negative strain rate sensitivity, ductility minimums and positive temperature dependence for flow stress and work hardening, it should be considered in analysis of components that are used in the range of 200-400 °C. This work fills gaps in knowledge regarding cast irons at elevated temperatures. However, taking the outlined approaches further remains a topic for future research. Additional testing is required to cover the full production window of spheroidal graphite cast irons to be fully usable in production simulation, optimization tools and local property approaches