Relationship between structure and thermal fatigue in cast iron

Abstract

Thermal fatigue of a material is determined by rupture stress, the elasticity modulus, heat conductivity, and thermal expansion. In addition to thermal expansion, one has to consider also the volume changes as a result of phase transformations. It is known that high rupture stress and high heat conductivity result in high resistance to thermal fatigue. A high Young's modulus and high thermal expansion give low resistance to thermal fatigue. Cast iron is a composite material, consisting mostly of graphite, ferrite, and cementite. The graphite can occur in a number of different morphologies. It can be spherical, as in ductile cast iron, it can be flakelike, as in flake cast iron, but it can also be rodlike, as in vermicular or undercooled graphite. Many of the properties important for thermal fatigue are influenced by the shape of the graphite. By using various models to explain the properties of composite materials, the changes in the properties of cast iron as a function of graphite shape are analysed. The analytical results are compared with experimental results. It is shown that the elasticity modulus and thermal expansion are lowest for flake graphite and that thermal conductivity is highest for this material. The conclusion is that grey cast iron has a better resistance to thermal fatigue than vermicular as well as nodular cast irons, in spite of its lower rupture stress.MST/783