Ferritic spheroidal graphite cast iron materials have gained a remarkable technical potential and economic impact in modern industry. These features are closely related to the question of how the cast materials can be produced without structural defects and graphite degenerations among which chunky graphite is considered most evil. Although the chunky graphite degeneration superficially seems to be well known, its metallurgical background is still controversially discussed, appropriate field-tested nondestructive tools for its quantification in castings are lacking, and the knowledge on its impact on material properties is rather limited. Addressing this status, a comprehensive overview on the subject has been compiled by the author recently [IJMC2019] considering formation and growth mechanisms, metallurgical aspects of appearance and prevention, and with special attention to microstructural characterization and impact on properties.
The present contribution is focused on how to properly understand the specific morphology of chunky graphite and its impact on the fracture behavior under fatigue loading conditions. Challenges and possibilities of the experimental microstructural characterization using destructive and nondestructive techniques are discussed. Special emphasis is laid on the impact of chunky graphite on fatigue properties such as fatigue limit and fatigue crack growth rate of ferritic SGI materials based on available experimental data. Moreover, conclusions for the assessment of castings affected by chunky graphite are drawn.