Page 209 - Trenchless Technology Piping Installation and Inspection

Pipe and Pipe Installation Considerations 173

FIGURE 4.31 A sample of internal tuberculation in unlined cast iron potable
water pipe. (Source: 3M Corrosion Protection Products.)

Also referred to as gray cast iron, it accounts for a very large portion
of buried water-piping material throughout North America even
today. Cast iron is a very strong, but brittle material. Early unlined
installations of cast-iron pipe has lasted more than a century in some
cases on account of the sheer thickness of the walls of the pipes. The
thick walls played a sacrificial role as they slowly corroded over the
years. Internal tuberculation in unlined cast-iron pipes has caused
severe hindrance to flow in most cases. Figure 4.31 presents a sample
of internal tuberculation in unlined cast-iron potable water pipe.

Manufacturing The principal raw material used in producing ductile
iron pipe is recycled ferrous material, including scrap steel; scrap iron,
and other ferrous materials obtained from shredded automobiles,
appliances, and others. While ductile iron is very similar in basic
chemical makeup to gray cast iron, ductile iron is instead produced by
treating molten low-sulfur base iron with magnesium under closely
controlled conditions. The startling change in the metal is character-
ized by the free graphite in ductile iron being deposited in nodular
form, instead of flake-form as in gray iron. With the free graphite in
nodular form, the continuity of the metal matrix is at a maximum,
accounting for the formation of a far stronger, tougher ductile mate-
rial. Ductile iron is roughly twice the strength of gray iron (ductile
iron has greater yield strength than ASTM A36 carbon steel) and
further surpassing gray iron in ductility and impact characteristics.