Chunky Graphite in Ferritic Spheroidal Graphite Cast Iron: Formation, Prevention, Characterization, Impact on Properties: An Overview

Abstract

Ferritic spheroidal graphite cast iron (SGI) materials have 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 such as, for example, chunky graphite. 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 fairly limited. Addressing this status, the article is providing a current overview on the subject. Existing theories on formation and growth mechanisms of chunky graphite are briefly reviewed. Furthermore, from a metallurgical point of view, causes for the appearance of chunky graphite as well as preventive measures are concisely summarized. Particular attention is paid to the morphology of chunky graphite and how it can be characterized by destructive and nondestructive techniques. Special emphasis was laid on providing a comprehensive overview on the impact of chunky graphite on strength, ductility, fatigue limit, fatigue crack growth rate as well as fracture toughness of ferritic SGI materials based on experimental data. Moreover, conclusions for the assessment of castings affected by chunky graphite are drawn.

Effect of Antimony on Graphite Growth in Ductile Iron

“…Possible reasons are long solidification times, high amounts of Si and the presence of Ce in the melt. 1 According to References 2-4, small amounts of Pb, Bi or Sb in a defined stoichiometric ratio with Ce can neutralize the detrimental effects of Ce on graphite growth by forming intermetallic phases in the melt. That is why mainly in large ductile iron castings very small amounts of Sb between 20 and 40 ppm are added to the cast iron melt to avoid the formation of chunky graphite.…”

Recent Developments in Understanding Nucleation and Crystallization of Spheroidal Graphite in Iron-Carbon-Silicon Alloys

“…There are many forms of so-called degenerated graphite, including dendritic, exploded, chunky, and spiky. They all have damaging effects on the mechanical properties of the iron, as summarized in an excellent review for chunky graphite (CHG) by Baer [13].…”

“…This trend is the result of the continuous progress in understanding the processing -structureproperties paradigm of ductile iron, to include its solidification and its subsequent solid-state transformation during cooling or heat treatment. Review work on the solidification of cast iron has been periodically published, [6][7][8][9], with some addressing nucleation [10,11], while others focus on growth [12,13]. We also note Prof. Campbell's work [14] that will be discussed in more detail later in this paper.…”

“…Maximum tolerable Si content to avoid chunky graphite, Si is in mass%, and the elements on the abscissa are in ppm, 3.40% ≤ C ≤ 3.77%, reproduced from[13,141] with permission from Springer.…”

“…Influence of element ratio parameter on the risk of chunky graphite formation in SG iron, M-thermal modulus, 3.47% ≤ C ≤ 3.75%, reproduced from[13,142] with permission from Springer.…”