Daniel Holmgren

Review of thermal conductivity of cast iron

Abstract The existing literature concerning the thermal conductivity of cast iron is reviewed. The thermal conductivity/diffusivity is clearly affected by the amount and morphology of graphite, alloying elements and matrix as well as by temperature. The literature is consistent to some extent, but uncertainties still exist, for instance, about the effects of some alloying elements. Some attempts to model the thermal conductivity of cast iron are discussed. The existing models are based on composite theories. Hence, the accuracy principally depends on how well the morphology and individual properties of the compounds can be determined. In the case of cast iron, the analysis is complex and often based on old and sometimes conflicting experimental data.

Thermal conductivity–structure relationships in grey cast iron

Abstract The thermal conductivity of grey iron cast with different solidification conditions and treated with different inoculants has been evaluated by the laser flash technique. Colour etching has been utilized to reveal the influence of the fraction of primary phase and the eutectic cell diameter. The thermal conditions during solidification together with inoculation largely affect the heat conductivity of grey iron. The results show that an increased fraction of primary austenite depresses the thermal conductivity, while straight, long graphite flakes of type A are favourable with regard to the ability to transfer heat.

Effects of nodularity on thermal conductivity of cast iron

Abstract The thermal transport properties of five predominately pearlitic grades of grey, compacted graphite and spheroidal graphite iron have been investigated by the laser flash technique. Samples have been taken from cylinders cast in controlled thermal environments designed to produce three dissimilar cooling rates. Digital image analysis has been utilised in order to characterise the different graphite morphologies. The results indicated linear relationships between the thermal transport properties and the roundness of the graphite and the nodularity for compacted graphite and spheroidal graphite iron. A pronounced decrease in the thermal conductivity occurred when the lamellar graphite structure was transformed into compacted graphite. The thermal conductivity of compacted and spheroidal graphite iron has been recalculated with good accuracy over a temperature range of 25–500°C by means of regression analysis.

Effects of carbon content and solidification rate on the thermal conductivity of grey cast iron

The thermal conductivity/diffusivity of pearlitic grey irons with various carbon contents was investigated by the laser flash method. The materials were cast in controlled thermal environments producing three dissimilar cooling rates. The cooling rates together with the carbon content largely influence the thermal conductivity of grey iron. Linear relationships exist between the thermal conductivity and the carbon content, the carbon equivalent, and the fraction of the former primary solidified austenite transformed into pearlite. The results show that the optimal thermal transport properties are obtained at medium cooling rates. Equations are given for the thermal conductivity of pearlite, solidified as pre-eutectic austenite, and the eutectic of grey iron. The thermal conductivity of pearlitic grey iron is modelled at both room temperature and elevated temperatures with good accuracy.

Effects of transition from lamellar to compacted graphite on thermal conductivity of cast iron

Abstract Different levels of magnesium were added to a standard grey iron alloy in order to obtain a range of graphite morphologies from lamellar to compacted graphite. The thermal conductivity/diffusivity of samples, solidified at different cooling rates, was investigated by means of the laser flash technique. There is a significant decrease in the thermal conductivity as the morphology transits from lamellar to compacted graphite. The thermal conductivity of grey iron decreases considerably at elevated temperatures, whereas the thermal conductivity of compacted graphite iron is less sensitive to changes in temperature. At increased nodularities, compacted graphite irons exhibit a maximum thermal conductivity at ∼400°C. The influence from the cooling conditions on the thermal conductivity decreases as the morphology alters from lamellar graphite to compacted graphite. The effective thermal conductivity of cast iron is modelled by means of existing models for composites.

Influence of alloying additions on microstructure and thermal properties in compact graphite irons

Abstract Nineteen compacted graphite cast irons were investigated to determine how alloying additions affect the thermal transport properties and microstructure. All melts were based on one chemical composition and alloying elements were added to obtain melts with variation in magnesium, silicon, carbon, copper, tin, chromium and molybdenum. Increasing amounts of magnesium resulted in a further compaction of the graphite particles, reducing the thermal conductivity. Large amounts of silicon resulted in a fully ferritic metal matrix. Silicon also formed solid solution with iron which had a deteriorating effect on the thermal conductivity, i.e. the larger amount of silicon the lower the thermal conductivity. Copper and tin promoted formation of pearlite that had worse thermal properties compared to ferrite. Increasing amount of ferrite generally had a positive influence of the thermal conductivity. Chromium and molybdenum were carbide forming elements and carbides had a negative influence on the thermal conductivity.

Effect of Alloying Elements on Graphite Morphology in CGI

Understanding how alloying elements and amounts affect the shape and size of graphite in compacted graphite cast irons could be of great importance. Some important material properties that are affected by the graphite shape are tensile strength and thermal conductivity. Knowing the effect of alloying additions could be of assistance when trying to optimise material for a specific application. In order to determine how graphite changes depending on alloying additions the microstructure of nineteen CGI materials were investigated. All melts were based on one chemical composition and alloying elements were added to obtain melts with variation in magnesium, silicon, copper, tin, chromium and molybdenum. Some of the more important microstructure features that were analysed are the amount and size of different graphite particles. The result from this analysis should give an indication on what features each alloying element affect and how these features varies with alloying amount.

Regression Model Describing the Thermal Conductivity of Various Cast Irons

The thermal conductivity of various grades of pearlitic cast iron has been modelled with good results by means of regression analysis. The experimental thermal conductivity data, which the modelling is based on, were obtained by the laser flash method. The microstructure was investigated by digital image analysis combined with a colour etching technique. The model developed takes the carbon content, the silicon content, the nodularity as well as the fraction of cementite into consideration. The graphite morphologies of the samples investigated were lamellar, compacted and spheroidal.

Modelling the Thermal Conductivity of Various Cast Irons

The thermal conductivity of five predominately pearlitic grades of lamellar, compacted and spheroidal graphite iron have been modelled by means of existing models based on average field approximations. The model is based on the area fraction of different constituents and the width to length ratio of the graphite. The thermal conductivity of graphite in cast iron is derived by inverse modelling. These data are used in combination with experimental thermal conductivity values for a pearlitic matrix in order to model the thermal conductivity of various cast iron grades with good agreement. The calculations are executed for cast iron from room temperature up to 500°C.