Graphite Cold Iron Microstructure Highlights

Graphite Cold Iron Microstructure Highlights

Graphite offers unique advantages in iron-based materials.graphite cold iron It counteracts solidification shrinkage, increases machinability by acting as a lubricant and reduces vibrations when cutting. It also lowers the weight of products compared to steel and improves ductility, hardenability and impact resistance. However, the full potential of graphite in iron-based materials will not be realised without the understanding of its microstructure. This article presents three microstructural highlights on graphite formation, namely secondary graphite in ductile cast irons, graphitization of high silicon medium carbon steels and void formation during dissolution of graphite.

The first highlight focuses on a type of graphite known as vermicular-type graphite, which is commonly observed in heavy sections of nodular iron castings.graphite cold iron It appears as a worm-like structure under the microscope after the metallic matrix is etched away. Its morphology is attributed to a short diffusion distance between branches of the graphite flakes, making it difficult for them to move in the same direction in response to thermal loading. This causes the out-of-plane displacement U3 (as shown in Figure 4a) to be much larger for the graphite than the metallic matrix.

It is known that a substantial proportion of the thermal residual stresses in nodular cast irons is generated by interfacial debonding between the graphite phase and the metallic matrix. This is due to mismatch of thermal expansion between the two phases, and is generally exacerbated by a higher CTE for the graphite than the matrix.

Another issue is the poor ductility of nodular irons. This is partly attributed to a low crystallographic shear strength of the pearlite eutectoid phase, but may also be caused by short diffusion distances between graphite filaments and ferrite. Increasing the graphite content, by using additives or a more porous melt, can significantly improve the ductility of nodular irons.

Graphite cold iron has the potential to address both of these issues, as it contains small graphite spheroids within a pearlitic matrix. The spheroids prevent the graphite from diffusing as fast as the iron, and thus inhibits the growth of the eutectoid phase. The spheroids can also be used to refine the ausferrite during austempering, which can help increase the ductility of spheroidal graphite cast iron.

Finally, the micrograph in Figure 8 shows an ausferritic matrix with small voids at former graphite positions. These voids form as a result of the several orders of magnitude lower diffusion rate for interstitial carbon, compared to the rate for iron and substitutional solutes. This phenomenon also occurs in spheroidal graphite cast steels during the austempering process, which is why this type of cast iron is pre-quenched before the austempering heat treatment.

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