Phenomenon
Fragmented graphite is a type of distorted graphite frequently found in large-section (wall thickness ≥ 100 mm) ductile iron castings, particularly at hot spots. On the macroscopic fracture surface, dense, 1-3 mm in size dark spots are visible in the central region of the slowly cooling casting. The presence of fragmented graphite results in a porous texture, deteriorating mechanical properties, especially a significant reduction in plasticity.
Under an optical microscope, fragmented graphite appears isolated and often accompanied by well-formed spheroidal graphite. However, deep etching of the sample followed by scanning electron microscopy reveals that the fragmented graphite possesses its own eutectic clusters. Within a eutectic cluster, the fragmented graphite is interconnected, and because it forms during slow solidification, the eutectic cluster grows significantly larger than that of spheroidal graphite, exhibiting a generally spherical geometry. Because these graphites are very small and highly branched, the metallic matrix accompanying the fragmented graphite eutectic clusters is often ferrite.
Causes of Formation
The mechanism of fragmented graphite formation is not yet fully understood.
Scanning electron microscopy observations show that molten iron has an erosive effect on fragmented graphite. Initially, fragmented graphite eutectic clusters are formed. Later, due to the very slow solidification process, the resulting eutectic clusters are large. Because these fragmented graphite fragments are frequently and finely branched, the connections at their ends are loose. Under the thermal convection of molten iron, graphite near the eutectic cluster boundaries may be eroded, forming free fragments. Furthermore, larger fragmented graphite fragments, under thermal convection, break into even smaller fragments, thus detaching from the eutectic clusters and floating at the cluster boundaries.
Secondly, due to the slow solidification, the precipitated graphite spheres are much larger than typical primary graphite spheres. When these graphite spheres exceed a certain size, the amount of iron inclusions within them increases. As these graphite spheres grow further in the molten iron, internal stress is generated due to size changes. The increasing internal stress caused by growth, exceeding a certain value, causes the graphite spheres to begin to fracture into fragments. During solidification, molten iron convection further reduces these fragments, and they are further propelled into the interdendritic space by the thermal turbulence of the molten iron, forming nuclei for the crystallization of fragmented graphite.
Prevention
1. Chemical Composition
The carbon equivalent has the greatest impact on chemical composition. In thick-section ductile iron, the carbon equivalent should be increased as much as possible without causing graphite flotation. Studies using magnesium treatment show that changes in carbon equivalent significantly affect graphite morphology; as the carbon equivalent increases, the number of graphite spheres increases, while the number of non-spherical graphite decreases. Therefore, for hypoeutectic ductile iron, slow cooling causes distortion of spheroidal graphite; for eutectic ductile iron, even with slow cooling, the graphite remains spherical; and for hypereutectic ductile iron, the graphite is not only round but also fine. However, when the spheroidizing agent contains rare earth elements, fragmented graphite is easily formed. Therefore, a carbon equivalent mass fraction of 4.2% to 4.4% is recommended.
Furthermore, fragmented graphite is closely related to silicon content; increasing silicon content will promote the formation of fragmented graphite. Therefore, in thick-section ductile iron, a lower silicon content should be used as much as possible. For example, for pearlitic ductile iron, the maximum silicon content mass fraction should not exceed 2.4%.
Excessive rare earth elements will lead to an increase in fragmented graphite. Production practice shows that if the residual rare earth content exceeds the residual magnesium content, fragmented graphite will inevitably appear in thick-section ductile iron. Therefore, the residual rare earth element content must not exceed 0.03% by mass. Furthermore, in thick-section ductile iron, the slow solidification process leads to magnesium evaporation loss; therefore, the residual magnesium content must be controlled at a higher level, with a residual magnesium content not less than 0.05% by mass.
For austenitic ductile iron with a nickel content exceeding 20% by mass, fragmented graphite often appears. To address this, antimony can be added at a mass fraction of 0.002% to 0.008% to overcome the occurrence of fragmented graphite.
2. Inoculation
For thick-section ductile iron, fragmented graphite can be prevented when the number of graphite nodules per square millimeter exceeds 60.
Delayed inoculation is also effective in increasing the number of graphite nodules in thick-section ductile iron. In-mold inoculation can double the number of graphite nodules.
Using long-acting and efficient inoculants, such as those containing barium and cerium, can maintain fine and uniform spheroidal graphite in thick-section ductile iron with a solidification time of up to 3 hours. Using FeSi75 with a barium mass fraction of 1%–2% can provide long-lasting inoculation.
For thick-section ductile iron, using coarse-grained (e.g., 3–5 mm) or agglomerated inoculants is beneficial in overcoming fragmented graphite. However, excessive inoculation can also lead to fragmented graphite formation; therefore, the mass fraction of inoculant added for late inoculation should not exceed 0.1%.
3. Trace Elements
In thick-section ductile iron, adding appropriate amounts of trace elements such as antimony and bismuth (antimony and bismuth are naturally interfering with spheroidization) along with cerium not only does not interfere with graphite spheroidization but can also eliminate fragmented graphite.
In the absence of cerium and other trace elements, adding 0.002% antimony by mass can result in very round graphite in the center of a 200mm cross-section ductile iron. The antimony recovery rate is 80%–85%, and most of the antimony in recycled materials can also be recovered. When using recycled materials or other furnace charges with unclear compositions, the antimony by mass is prone to exceed 0.005%. Therefore, when adding antimony, 0.01%–0.05% cerium by mass should be added simultaneously to counteract the destructive effects of antimony and other interfering elements.
4. Process Measures
The most effective process measure is to use a metal mold or chill. Practice shows that using a metal mold can significantly shorten the solidification time, thus reducing the probability of fragmented graphite. For a cylindrical casting with a diameter of 300 mm, the solidification time in a sand mold is 120 minutes, while the solidification time in a metal mold can be shortened to 60 minutes; for a cylindrical casting with a diameter of 200 mm, the solidification time in a sand mold is 60 minutes, while the solidification time in a metal mold is shortened to 30 minutes. In this way, no fragmented graphite will appear.
Post time: Nov-14-2025