I = Impact resistant cast iron

The Type K box of 1980 was the first to be cast in impact resistant cast iron, a technology where the presence of spheroidal graphite particles increases the ductility of the iron giving it greater impact and fatigue resistance. In grey cast iron, used prior to the 1980s, graphite is present in flakes rather than spheres and the resulting casting is more brittle and prone to impact shattering. Modern boxes are now cast in ductile cast iron.
The Letter Box Study Group
2016-02-10T16:07:21+00:00
The Type K box of 1980 was the first to be cast in impact resistant cast iron, a technology where the presence of spheroidal graphite particles increases the ductility of the iron giving it greater impact and fatigue resistance. In grey cast iron, used prior to the 1980s, graphite is present in flakes rather than spheres and the resulting casting is more brittle and prone to impact shattering. Modern boxes are now cast in ductile cast iron.