ASK Chemicals
0519 ASK sand drying in progess

Preventing Flaky Graphite Formation in CGI Castings

May 1, 2019
Compacted graphite iron is gaining importance for ferrous foundries, and requiring its own core and mold coating solutions to ensure the finished parts solidify according to the processing requirements.

Q: Can we prevent flake-type graphite formation in Compacted Graphite Iron with a refractory coating?

A: Yes, a correctly engineered refractory coating can enhance the results of casting CGI.

To explain the problem in general, note that compacted graphite iron (CGI, also referred to as vermicular graphite iron, or VG iron) is a lightweight cast iron that offers greater tensile strength, stiffness, and fatigue strength than gray iron. It is gaining in popularity as an alternative to gray iron and aluminum for diesel and gas engine manufacturers seeking to reduce weight, noise, and emissions for their designs. CGI makes it possible to cast more compact, thinner wall castings that nevertheless are sufficiently rigid to perform under the high-pressure stress inherent to diesel or gas engines.

In addition to engine blocks, CGI may be selected for casting cylinder heads, exhaust manifolds, or brake discs.

The major problem for manufacturers working with CGI is the "flake"-like graphite structures that forms after casting, which may result in material that is significantly less machinable than comparable gray iron castings — 25-40% reduced mechanical properties than gray iron, by some reports.

The CGI graphite structure is often compared to coral, with lower levels of discontinuities and stress concentration effects in the metal, which enhances the material strength but reduces its machinability.

The flakes are a primary cause of the mechanical properties of CGI. The sharp flake edges make it possible for cracks to form and the smooth flake surfaces cause cracks to proliferate by delaminating the flake/metal interface. Even a small amount of flake graphite may reduce the material’s tensile strength and elasticity. Fatigue strength and impact toughness also are reduced by the formation of flake graphite.

The operating demands and stresses on CGI engine blocks and heads requires highly repeatable quality in the material microstructure, providing high strength for durability and mechanical properties. In order to achieve this consistent microstructure and metallurgical properties, the level of flake graphite formation needs to be strictly controlled.

To accomplish this, a properly designed refractory coating blend that insulates the core and provides consistent cooling rates will promote good nodularity in compacted graphite iron castings. This type of coating also can act as a barrier to prevent sulfur pick-up from some sand binder systems, which is known to reduce the volume of flake graphite formation.

ASK Chemical’s highly engineered water-based refractory coating, ISOCOTE BPG-TM, is proven to accomplish these critical performance requirements.

ISOCOTE BPG-TM also contains a drying indicator that will change the color of the coating from blue (wet) to light yellow (dry) when used on most types of cores and molds. It must be noted that the initial color of the sand may have some effect on the final color of the coating. It is common for this coating to revert to a blue color if moisture is present.
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