Application of the Monkman-Grant law to the creep fracture of nodular cast irons with various matrix compositions and structures

Creep behaviour of nodular cast irons with three different matrix compositions and microstructures has been investigated up to fracture in the elastic regime for temperatures between 650 °C and 900 °C. The elastic stress levels were chosen taking into account the non-linear dependence between elastic stresses and corresponding strains. Results show that austenitic cast irons are more creep resistant than ferritic ones. However, all materials obey to a single Monkman-Grant law. Fracture of samples with short life times is dominated by the plastic straining of the matrix independently of their metallurgical state. Creep fracture of long life time samples is controlled by diffusion mechanisms like cavity nucleation on the grain boundaries. It is shown that the damage growth in secondary and tertiary creep regimes can be represented by a single parameter whatever the creep mechanisms and the metallurgical properties of nodular cast irons.