This defect has been observed in all gray solidified cast iron alloys, i.e. nodular, flake, vermicular graphite cast iron, irrespective of the molding or casting process. The defect occurs in particular in case of thin wall thicknesses, at edges or in corners.
Edge hardness or chill is already visible to the naked eye in the fracture structure. Ledeburitic carbides are then clearly visible in the metallographic specimen. In these areas, the structure reveals chill which gradually changes into normal gray structure in areas with thicker wall thicknesses (Figures 1 and 2). These chill areas are characterized by a high hardness compared to the matrix and are thus hard to process which may lead to rejection of the entire casting.
The chill areas as a result of which the eutectic was formed from austenite and cementite are referred to as ledeburite. In actual castings, the chill area changes into a mottled area, before the desired gray solidification occurs (Figures 3 and 4). The structure is here partly white, according to the meta-stable system, and partly gray, according to the stable system.
The iron-carbon phase diagram reveals that the eutectic temperature of the stable system is higher than the temperature of the meta-stable system. Accordingly, if a cast iron melt cools down very rapidly (low wall thickness, rapid heat dissipation of the mold, for example, in gravity die castings) or if the melt has a poor balance of nuclei (for example, due to extensive overheating of the melt) it may occur that crystallization does not start until a high level of supercooling is reached, i.e. below the temperature of the meta-stable system. The melting heat released during the eutectic reaction can lead to an increase of the temperature above the meta-stable eutectic temperature and the solidification (Fig. 5).
The chill is thus produced at the start of the eutecticsolidification in contrast to the formation of boundary grain carbides where chill and carbide formation occurs at the end of solidification (s. Carbides). As the degree of saturation (or carbon equivalent decreases and the cooling rateincreases, the tendency of the cast iron for chill (edge hardness) increases. As a consequence, the degree of saturation of the cast iron must be adjusted to the wall thickness of the casting and the casting process. Accordingly, for example, the EN-GJL-300 cast iron print may not be used for castings with wall thickness below 10mm if edge hardness or chill is to be avoided.
The highest risk of edge hardness obviously occurs in those casting processes where the heat can be rapidly dissipated through the mold. For example, this is the case with the gravity die casting process where metallic permanent molds are used. The high cooling rate in this process leads to very fine cementite components which significantly differ from the surrounding structure due to their distinct demarcation.
As the cooling conditions are often determined by the construction and the molding process, the cause of edge hardness often originates in an inadequate balance of nuclei in the melt. Consequently, a lack of or insufficient inoculation has an extremely negative impact, especially in case of thin-walled castings.
Measures for prevention (acc. to S. Hasse, FT&E):
1. Adjustment of a chemical composition of the iron according to the wall thickness (Sc, CE).
2. Targeted addition of nucleus for graphite crystallization, e.g. by adding silicon. This promotes the solidification temperature according to the stable system, as the equilibrium temperature of the meta-stable system is shifted towards lower temperatures (s. Carbides).
3. The inoculation of the melt as described in section 2 should be carried out as late as possible, i.e. shortly before or during casting.
4. Avoidance of carbide-stabilizing elements in the melt, i.e. review of the charge materials, if required, reduce the steel scrap proportions (cupola furnaces) or improve carburization in the electric furnace process.
5. Avoidance of excessive overheating of the liquid iron.
6. Reduce cooling rate, if necessary (preheat dies).
7. Keep magnesium contents as low as possible in nodular graphite cast iron.
8. Reduce the alloy element content or only use such elements if they are absolutely necessary in order to achieve certain properties or certain combinations of properties.
Chill wedge test piece