Ductile cast irons: Microstructure influence on the fatigue initiation mechanisms

Abstract

In the last decades, the combination of high mechanical performances and low production costs increased the industrial interest on ductile cast irons. These grades are often used for applications where the fatigue resistance can be a critical issue (eg, machine frames for the wind‐power industry or crankshaft used in trucks) and the investigation of the main damaging mechanisms during both the crack initiation and the crack propagation stage could offer new perspectives about these alloys. Ductile cast irons can be considered as a natural composite, being characterized by graphite elements (nodules) embedded in a more or less ductile matrix (ranging from fully ferritic to pearlitic, from martensitic to austempered). In this work, the fatigue crack initiation mechanisms were investigated considering different matrix microstructure and the presence of a mechanical properties gradient in the graphite nodules.

Characterisation of the damaging micromechanisms in a pearlitic ductile cast iron and damage assessment by acoustic emission testing

“…A step‐by‐step testing procedure was used to allow the periodic SEM analysis of the specimen under in situ conditions. The tension was applied and kept on the specimens using a tensile holder . The deformation of the specimens was controlled by a linear variable differential transformer (LVDT).…”

“…The tension was applied and kept on the specimens using a tensile holder. 2,4,10 The deformation of the specimens was controlled by a linear variable differential transformer (LVDT). Two miniature load cells (10 kN each) were used to measure the applied load.…”

“…In the last decades, the industry encouraged the experimental investigation of ductile cast irons (DCIs) . These materials are commonly used for a variety of critical engineering systems such as pipelines and wind turbines . DCIs were obtained in the '40s by adding different elements, such as magnesium or cerium , during the preparation/treatment of cast irons.…”

“…1 These materials are commonly used for a variety of critical engineering systems such as pipelines and wind turbines. 2,3 DCIs were obtained in the '40s by adding different elements, such as magnesium or cerium, during the preparation/treatment of cast irons. The addition produced nodular graphite elements embedded within the iron matrix, instead of the elongated flakes (lamellae), typical of grey cast irons.…”

“…The graphite nodules were considered in the past as mechanical voids, and the matrix-nodule debonding was assumed as the primary mechanism involving the nodules. 6,7 Recent studies 2,8,9 proved that the damage evolution of DCIs is quite more complex. The damaging micromechanisms might either start within the nodules or affect them in the early stages of deterioration.…”