Note that the grey cast iron is heat treated, and note that they come out black. If you heat cast iron to a red color and slowly cool will cause the graphite flakes to form round carbon nodes, or nodular cast iron. The exterior color is black. From the wiki article.

[i]Malleable iron starts as a white iron casting, that is then heat treated at about 900 °C. Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles rather than flakes. Due to their lower aspect ratio, spheroids are relatively short and far from one another, and have a lower cross section vis-a-vis a propagating crack or phonon. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems faced by grey cast iron. In general, the properties of malleable cast iron are more like mild steel. There is a limit to how large a part can be cast in malleable iron, since it is made from white cast iron.

A more recent development is nodular or ductile cast iron. Tiny amounts of magnesium or cerium added to these alloys slow down the growth of graphite precipitates by bonding to the edges of the graphite planes. Along with careful control of other elements and timing, this allows the carbon to separate as spheroidal particles as the material solidifies. The properties are similar to malleable iron but parts can be cast with larger sections.</i>

I suspect that the heat treat method described is not enough to make the iron fully malleable or nodular iron, but somewhere between. Still very hard but more flexible than plain grey iron. Poor heat treatment may also account the very soft and ductile rings some have experianced.